Controlled Deposition of Calcite Crystals on Yttria-Stabilized Zirconia

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DOI: 10.1021/cg101112b

Controlled Deposition of Calcite Crystals on Yttria-Stabilized Zirconia Ceramic Electrets

2011, Vol. 11 166–174

Norio Wada, Miho Nakamura, Wei Wang, Tetsuo Hiyama, Akiko Nagai, and Kimihiro Yamashita* Department of Inorganic Materials, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, Kanda-Surugadai, Chiyoda-ku, Tokyo, 101-0062, Japan Received August 24, 2010; Revised Manuscript Received October 27, 2010

ABSTRACT: We demonstrated the crystallization of calcite on yttria-stabilized zirconia (YSZ) ceramic electrets using diffusion of (NH4)2CO3 vapor into a CaCl2 solution with and without poly(acrylic acid) (PAA). On both negatively and positively charged surfaces in the absence of PAA, the precipitates were calcite crystals with the rhombohedra {10.4} plane oriented parallel to the electrets. This oriented growth was explained by the nucleation theory in the presence of an electric field. The calcite thin films with a fan-like fracture in a cross section formed on the electrets in the presence of PAA. The calcite aggregates consisting of the thin films were made of bundles of calcite needles either having a rough (00.1) top face with a triangular shape or having a faceted rhombohedral {10.4} top face. These calcite needles grew radially from PAA-Ca2þ complex assemblies attached to the electrets with the growth direction being the c-axis. The effects of PAA addition and a surface electric field due to the electrets on calcite crystallization were apparent most remarkably on a negatively charged surface of the electrets. The surface electric field regulates not only CaCO3 supersaturation in its vicinity but also the morphology of the PAA-Ca2þ complex assemblies anchored on the surface of the electret.

Introduction The control of crystallization by magnetic or electric field has been one of the dreams of materials scientists. However, it is at most 30 years ago that electric or magnetic field effect on crystal growth has been recognized as science. Reports on the effect of magnetic field so far have been many,1-6 but much fewer have been on the effect of electric field.7-10 Externally generated electric and magnetic fields, in general, have been used to study the effects of electric and magnetic fields on crystallization. In this case, the effect of these fields reaches throughout the system and thus causes some unfavorable conditions for crystallization, such as the dissociation of water molecules and electrolytic conduction in the solution. Materials that generate electrostatic force (electrets) and magnetic force (magnets) have the advantage to be effective only in a limited area within the system. Electrets, permanently polarized dielectric materials, are equivalent to magnets, which are permanently magnetized magnetic materials. The magnetic fields generated by magnets have been used for crystallization in many cases, but the electric fields generated by electrets have never been used so far. Electrets made from biomaterials, such as calcium phosphate and zirconia, may be less harmful to the body when implanted in the body. Yamashita et al. discovered that hydroxyapatite (HA) electrets had some specific characteristics for bone-like apatite layer growth, namely, growth promotion of apatite on the negatively charged surface but not on the positively charged surface and the c-axis orientation of HA particles.11-15 They also found that the negatively charged surface attracted calcium ions, thus increasing the local nucleation rate of HA from solution. These results have opened up a new strategy for crystal growth through a surface electric field due to electrets. *Corresponding author. E-mail: [email protected]. Tel: þ81-03-5280-8016. Fax: þ81-03-5280-8015. pubs.acs.org/crystal

Published on Web 11/23/2010

Since then, investigations have aggressively been carried out in such areas as the crystal growth of biomaterials and the behaviors of cells and microbes in the limited surface electric fields due to HA electrets.16-24 We have recently pointed out that surface electric fields due to HA electrets, which are bioactive materials, control the orientation of calcite crystals either with or without poly(acrylic acid) (PAA) and also that the electric fields influence not only the nucleation of the deposit but also the subsequent growth.9,10 In addition, we reported that the cooperation of PAA and a self-generating surface electric field caused by polarized HA favored the formation of calcite thin films and acted remarkably on the negatively charged surface of the ceramics.9,10 Calcium carbonate (CaCO3) is one of the most abundant biomaterials on earth and a major component of shells, pearls, corals, etc., which are nanocomposite materials made from organic and inorganic components. Mollusk shells have prismatic layers with calcite crystals and nacreous layers with aragonite crystals. These layers are enclosed in an organic sheath and aligned to form a coherent mineral sheet.25,26 The mechanical strength and toughness of these structures exceed those of any current synthetic ceramics by several orders of magnitude.27 The strength of CaCO3 structures in nature is due to the assimilation of inorganic and organic materials. The formation of CaCO3 films produced in cooperation with insoluble matrixes and soluble organic additives is especially attractive in the simulation study of biomineralization. Therefore, many studies on this cooperation have been reported.28-45 The design and synthesis of inorganic materials through biomimetic methods have increasingly been popular in developing new materials and applications. A novel approach for several kinds of biomaterial fabrication using a self-assembled monolayer with surface charges has been demonstrated. It is well-known that the electric fields due to the self-assembled monolayer make the attractive interaction between ions and r 2010 American Chemical Society

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counter-charged surfaces.30-40 The presence of a surface charge potential assists the reorganization and condensation of clusters at the interface. The electrical nature of crystallization fields with the surface charge has not been fully understood. Hence, to produce biomaterials with novel value, utilizing the power of electric fields of electrets is of particular importance for the strategies involved in the controlled synthesis of a biomaterial, such as calcium carbonate and calcium phosphate. In order to obtain useful knowledge on this issue, in this paper, we examined the precipitation of calcium carbonate in the presence of surface electric fields due to yttria-stabilized zirconia (YSZ) electrets with and without PAA through the diffusion of CO2 vapor from solid (NH4)2CO3 to CaCl2 solution and reported on a controlled deposition of calcite crystals on YSZ electrets. Although it has been well-known that zirconia ceramics such as YSZ have high strength, toughness, and biocompatibility, YSZ ceramics are not bioactive materials. PAA, a soluble additive, is a weak polyacid having a single stranded structure with carboxylic acid but no hydrophobic side chains. It has been suggested that PAA plays many significant roles in the crystallization of calcium carbonate.29,32,34,35,37-45 It functions as a template for epitaxial growth, inhibits crystal growth, and promotes aggregation of crystals by its adsorption. Experimental Section Yttria-stabilized zirconia (YSZ) pellets were prepared according to a method as follows: 3 mol % YSZ powders were pressed at 140 MPa into pellets (10 mm in diameter and 1.0 mm thick). These pellets were sintered at 1400 °C for 1 h in air. The resulting YSZ pellets were polarized by the following procedures: a pellet was sandwiched between two Pt electrodes and polarized at 200 °C for 30 min under an applied electric field of 20 V cm-1. The negatively and positively charged surfaces of polarized YSZ pellets were labeled as the N- and P-surfaces, respectively. The surface of a nonpolarized one was designated as the 0-surface for reference. The stored charges in the YSZ electrets were estimated by measuring the thermally stimulated depolarization current (TSDC). The measurements were done by monitoring the dissipated current density as a polarized specimen was heated at a rate of 5 °C min-1 from room temperature to 650 °C in air. The stored charge was estimated by integrating the current density values resulting from the measurement. The crystallization of calcium carbonate was studied by the system described in our previous papers.9,10 CaCO3 precipitates were prepared from a 5 mM calcium chloride solution (10 mL) supersaturated by adding carbon dioxide generated by the decomposition of ammonium carbonate powder (1.0 g) for the experimental period of 48 h. The nonpolarized and polarized YSZ pellets were immersed in calcium chloride solution. These substrates were suspended by threads in the vessel to prevent deposition of the precipitates formed through homogeneous nucleation. PAA with an average molecular weight of 2.0  103 was added as a soluble additive in the amounts of 2.0, 4.0, and 8.0 mg to the calcium chloride solution (10 mL). The vapor diffusion method, which utilizes thermal decomposition of (NH4)2CO3 in order to generate CO2 slowly, is popular for slow CaCO3 crystallization. However, experiments of this type are very sensitive to the diffusion rate of CO2 and NH3 from the ammonium carbonate. Hence, a drawback of this method is that it depends highly on the experimental setup, particularly on spatial arrangements, and its diffusion rate, which depends on temperature, pH, and the amount. Here, the diffusion rate of CO2 and NH3 vapors from solid ammonium carbonate was controlled through a hole (5 mm) in the lid of the vial containing solid (NH4)2CO3. The pH of the calcium chloride solution with and without PAA was adjusted to 7.2 with NaOH, and the system was kept at 28 °C. The pH of the reactant solution increased gradually upon exposure to (NH4)2CO3. The pH rose to 9.4 in about 0.5 h due to the adsorption of ammonia and then remained at this value throughout the rest of the crystallization

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Figure 1. ATR-FTIR spectra of PAA (;) and PAA-Ca2þ complex ( 3 3 3 ). process due to the buffer effect of an ammonium/ammonium hydroxide solution. Keeping the pH value constant is desirable for studying the effects of organic additives with reactive functional groups on CaCO3 crystallization. The crystal phases and orientation of the CaCO3 precipitates during the growth were examined by thin film X-ray diffraction (TF-XRD) using Ni-filtered Cu KR radiation (λ = 1.5406 A˚). The morphologies of the CaCO3 precipitates were examined by scanning electron microscopy (SEM). The formation of a PAA-Ca2þ complex and the adsorption of PAA were analyzed by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The surface roughness of YSZ ceramics was analyzed using atomic force microscopy (AFM). Contact angle measurements were performed on the 0-, N-, and P-surfaces using distilled and deionized water (1 μL).

Results and Discussion Characteristics of Nonpolarized and Polarized YSZ Ceramics. YSZ ceramics with and without polarization treatment were characterized by TF-XRD and confirmed to be the tetragonal ZrO2 phase. TF-XRD analysis and SEM observations did not reveal any significant differences in the surface properties between nonpolarized and polarized YSZ ceramics. The roughness of these surfaces, which was measured by AFM analysis, ranged from 30 to 40 nm. Based on TSDC measurements, the stored charges in the YSZ electrets were estimated to be 1.0 mC cm-2, which is about 100 times larger than that of HA electrets used in our previous studies,9,10 and the stored charge on the 0-surface was zero. FTIR Analysis of Surfaces of YSZ Ceramics with PAA. The degree of dissociation of the COOH groups of PAA in solution depends on the pH of the solution. In the pH range of 7.2-9.4, the acid groups are mostly in the form of carboxylic ions (-COO-), because the dissociation constant, pK, of PAA is 4.5. Hence, Ca2þ ions in the solution possibly form PAA-Ca2þ complexes through electrostatic forces. In an ATR-FTIR spectrum obtained from the surface of the YSZ electrets, which had been soaked in a PAA solution (2.0 mg, 10 mL) without Ca2þ ions, asymmetrical and symmetrical stretching modes of the -COO- groups of PAA were observed at 1565 and 1417 cm-1, respectively (Figure 1a). On the other hand, an ATR-FTIR spectrum obtained from the surface of the YSZ electrets, which were soaked in a CaCl2 (5 mM, 10 mL)-PAA (2 mg) solution for 2 h without the diffusion of CO2 vapor, showed asymmetrical

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Wada et al. P Table 1. Percentages of (10.4) and (00.6) Peaks and (Ihk.l/I*hk.l) of Calcite Crystals Formed on Nonpolarized and Polarized YSZ Ceramics with and without PAA

Figure 2. X-ray diffraction patterns of calcite continuous film: (a) calcite continuous film formed on N-surface in the presence of 8 mg of PAA; (b) the bar chart of the standard pattern of calcite (JCPS 5-0586).

and symmetrical stretching modes of the -COO- groups at 1553 and 1414 cm-1, respectively (Figure 1 b). These red shifts in the asymmetrical and symmetrical stretching modes confirmed the PAA-Ca2þ complexes attached on the YSZ electrets.37 The ATR-FTIR spectra for nonpolarized YSZ pellets were similar to those of the YSZ electrets obtained either with PAA alone or with CaCl2-PAA. We speculate that the PAA-Ca2þ complex in CaCl2 solutions, being in the pH range 7.2-9.4, has both negatively and positively charged sites in its structure and that the spatial distribution of each charged site should be dependent both on the additional amount of PAA and on the surface electric field caused by the YSZ electrets under our experimental conditions. We believe that the adsorption of the PAA-Ca2þ complexes on the YSZ electret is due to an electrostatic force and that the one on the nonpolarized YSZ ceramics may be due to van der Waals force between YSZ ceramics and the PAA-Ca2þ complexes. We also would like to point out that the morphology of the PAA-Ca2þ complex assembly that adsorbs on the YSZ pellets prior to the calcite nucleation would determine the structure of calcite aggregates formed subsequently. X-ray Diffraction Analysis of Formed Calcite Aggregates. The CaCO3 crystals formed on the N- and P-surfaces with and without PAA at the end of the process were characterized by TF-XRD. The polymorph of CaCO3 crystals formed under all the experimental conditions was only calcite. The TF-XRD profile of the precipitates formed on the N-surface with 8 mg of PAA is shown in Figure 2. This profile indicates that the precipitates consist of calcite crystals with orientation and also shows two strong peaks assigned to the (00.6) and (10.4) peaks of calcite. There was a significant difference in the intensities of the (00.6) and (10.4) peaks under all conditions examined. Therefore, for the quantitative analysis of the orientational uniformity, we normalized the measured intensities of the peaks in these spectra by the standard intensities of the peaks for the randomly oriented calcite powder. The percentage of calcite crystals in the different

crystallographic orientation (%)

amount of PAA (mg/10 mL)

surface

(10.4)

(00.6)

P (Ihk.l/I*hk.l)

0 0 0 2 2 2 4 4 4 8 8 8 10 10 10

0 N P 0 N P 0 N P 0 N P 0 N P

17 41 38 12 28 23 11 13 19 12 7 7 12 12 14

13 8 7 37 50 44 51 54 47 61 73 63 32 37 34

3.2  103 4.2  103 2.4  103 4.3  103 5.1  103 4.0  103 6.0  103 7.4  103 3.0  103 2.0  105 2.2  105 1.8  105 4.7  104 5.6  104 3.7  104

orientations (hk.l) obtained in the range of 20-50° was calculated using the following equation:30 %hk.l = (Ihk.l/I*hk.l)/ P (Ihk.l/I*hk.l)  100, where Ihk.l is the measured intensity of the peaks in the TF-XRD profile of the calcite crystals grown under our experimental conditions, and I*hk.l is the intensity of the peaks of the randomly orientated calcite listed in the Joint Committee of Powder Diffraction Standard (No. 5-0586). The percentages of a (00.6) peak and a (10.4) peak estimated by the above equation are summarized in Table 1. Without PAA, the percentages of the (10.4) peak of calcite crystals formed on the electrets were larger than those of calcite crystals formed on the nonpolarized YSZ substrates. With PAA, the percentage of a (00.6) peak of calcite crystals formed on the nonpolarized and polarized YSZ substrates increased with the additional amount of PAA, while the percentage of a (10.4) peak decreased. The increase due to the additional amount of PAA favors the [00.1] oriented growth of the individual prismatic calcite crystals. In calcite precipitates consisting of the randomly oriented calcite powders, Ihk.l/I*hk.l has a constant value in every (hk.l) reflection. Then the value is regarded as a function of a mass (m) of precipitated calcite. In calcite precipitates of the oriented calcite powders, Ihk.l/I*hk.l has a different value in different (hk.l) reflection, because the ratio is the function of both a mass of precipitated calcite and its percentage of orientation (%hk.l), namely, f(m,%hk.l). The fluctuation of Ihk.l/I*hk.l in a certain (hk.l) reflection, however, is compensated by the increase or P(hk.l) P decrease in that of other (Ihk.l/ reflections because of %hk.l = 100%. Hence, I*hk.l) is considered to bePa function of the amount of the formed calcite crystals. (Ihk.l/I*hk.l) listed in Table 1 indicates that the amount reaches the maximum at 8 mg of PAA and further addition of PAA does not make any difference. The amount of calcite formed on 0-, N-, and P-surfaces increased in the following order: N- > 0- > P-surfaces. Calcite crystallization seemed to promote remarkably on the N-surfaces. These results suggest that the amount of the calcite crystals formed on the substrates may be determined by the balance of the additional amount of PAA and the properties of the electric field due to YSZ electrets. Influence of PAA Addition on Calcite Precipitates. In the absence of PAA, calcite crystals with a well-established rhombohedra morphology with no orientation formed on

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Figure 3. Scanning electron micrographs of calcite crystals formed on 0- and N-surfaces without PAA: (a) 0-surface; (b) N-surface. Scale bar = 2 μm. Table 2. Crystal Density of Calcite Formed on 0-, N-, and P-Surfaces without PAA electric field (V cm-1)

surface

crystal density (cm-2)

0 20 20

0 N P

3.0  105 2.3  106 1.8  105

the 0-surfaces. On the other hand, calcite crystals formed on the N- and P-surfaces were capped with rhombohedral {10.4} planes, which were oriented parallel to the surfaces (Figure 3). This orientation was judged from the fact that parallelograms of calcite crystals showed the angle of 104°, corresponding to the shape of the {10.4} plane. This orientation is also supported by XRD data listed in Table 1. Table 2 gives crystal densities on 0-, N-, and P-surfaces, indicating that these densities increase in the order of N- > 0- > P-surfaces. This tendency was the same as that obtained from the experiment of nonpolarized and polarized HA ceramics, and each crystal density on 0-, N-, and P-surfaces in this study was larger than that obtained in our previous studies of HA ceramics.9,10 This increase in the crystal density can be caused by the large surface electric field due to YSZ electrets in comparison with that due to HA ones and by the difference in the adhesive force between precipitated calcite and YSZ surfaces and the one between precipitated calcite and HA surfaces. An abrasion test using sandpaper indicates that the calcite crystals formed on 0-, N-, and P- surfaces were only weakly bound on the YSZ substrates. Hence, it is concluded that the orientated growth, density, and amount of calcite crystals formed are all dependent on the polarity of the surface electric field and that the adhesion between calcite crystals and YSZ substrates is not improved by the action of the surface electric fields. In the presence of PAA, dramatic changes in the calcite morphologies were observed, namely, the hemispheric aggregates, and these aggregates developed into a continuous or partial thin film through the Volmer-Weber mode (Figure 4). The formation of calcite aggregates occurred distinctively on the N-surfaces. When 8 mg of PAA was added, the amount of calcite crystals formed on the N-surface reached a maximum as listed in Table 1. The thickness of this film was 5 μm. The surface morphology shows a hemispheric structure, and fracture of the films produces a fan-like structure (Figure 5). The aggregates were made up of bundles of calcite needles that were firmly bound to each other. The individual needles were in close contact with each other. The complex structure of its top face was composed of triangular or rhombohedral nanoscale units with external crystal face and the same orientation as shown in enlarged SEM images (Figure 5b,d,f). Its inner structures displayed a layered radial growth structure. The fractures of the calcite aggregates depicted in

Figure 4. Scanning electron micrographs of calcite thin films formed on 0-, N-, and P-surfaces in the presence of 4 mg of PAA: (a) 0-surface; (b) N-surface; (c) P-surface. Scale bar = 10 μm.

Figure 5a,c,e provide further insight into their growth history. Specifically, the calcite needles radially grew from a common point on the substrates and were organized in a hemispheric structure. The fractures also indicated that further growth of the calcite needles was restricted only by a direction approximately perpendicular to the substrate. This means that geometrical selection played an important role at the final stage of the crystal growth of the needles. The final result of this selection is an array of tightly spaced

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Figure 5. Scanning electron micrographs of the surface and fracture morphologies of calcite aggregates formed on 0-, N-, and P-surfaces in the presence of 2 mg PAA: (a, b) 0-surface; (c, d) N-surface; (e, f) P-surface. Scale bar = 1 μm.

calcite needles normal to the surfaces. The fracture of the calcite aggregates has led us to a reasonable assumption that the PAA-Ca2þ complex assembly as a template for the formation of the hemispheric aggregates has a shape of a hemisphere. However, in our previous studies,9,10 we indicated that the PAA-Ca2þ complex assembly as templates, which adsorbed onto HA substrates, had an extended order structure on N-surfaces of them with a small additional amount of PAA, and under other reactant conditions, the one as templates had a hemispheric structure. This difference in HA and YSZ electrets is probably dependent on the magnitude of surface electric field due to each electrets and on the adhesive forces between PAA and HA substrates or YSZ ones, which have different surface characteristics from each other, because HA is a hydroxide and YSZ is an oxide. Therefore, the morphology of the PAA-Ca2þ complex may be determined by the properties of the surface electric field due to an electret, the partially charged distribution in the PAA-Ca2þ complex, and the surface characteristics of a substrate. Considering the morphologies of the top faces and a radial growth structure observed in Figures 5a,c,e, we suggest that the calcite needles radially grow along the c-axis. The direction of the rhombohedron edges of the top faces having welldefined {10.4} faces declares that the c-axis of the needles is perpendicular to the needle base. It was also confirmed from the calcite needles having the triangular top faces with the angle of 60° that the calcite needles have top faces corresponding to the shape of (00.1) face and grow along the c-axis of calcite perpendicular to the needle base. The (00.1) plane of calcite is unstable because of the high interfacial energy associated with the surface charge, and therefore, some of the soluble PAAs and PAA-Ca2þ complexes in solution preferentially adsorb on the (00.1) plane to lower the surface energy and stabilize its surface. Moreover, the calcite hemispheric aggregate that consisted of needles with either of the morphologies of the top faces coexisted with a mosaic mode in the same thin film. The conditions causing the different morphologies of the top faces are presently not clear. However, the adsorption of PAA molecules and PAA-Ca2þ complexes on calcite crystals in the solutions with PAA may determine the final morphology of the top faces. ATR-FTIR spectra, obtained from the calcite aggregates formed on the YSZ electrets, showed the characteristic peaks

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Figure 6. Scanning electron micrographs of the change of surface morphology: (a, c) with 2 mg of PAA addition; (b, d) with 8 mg of PAA addition. Scale bar = 1 μm.

Figure 7. Scanning electron micrograph of calcite needle with sheaf-like morphology. Scale bar = 5 μm.

assigned to those of PAA-Ca2þ complexes (data not shown). This result suggests that the PAA-Ca2þ complexes adsorb on calcite crystals. Increasing the amount of PAA induced a decrease in the size and an increase in the density of the calcite aggregates. It also led to the sequence of morphological changes of the triangular (00.1) top faces and the faceted rhombohedral {10.4} top faces of calcite needles shown in Figure 6, namely, from a regular morphology of triangle or rhombohedra to round morphologies. Some of the mobile PAAs and PAA-Ca2þ complexes in the solution not only act as an adhesive for these needles on each other but also inhibit the growth of these needles through its adsorption. This leads to the aggregation of small calcite needles with different morphologies of their top faces and to the tight adhesive between the calcite needles in the aggregates and further between the calcite aggregates and the YSZ substrates. Based on an abrasion test using sandpaper, the adhesive between calcite aggregates and the YSZ electrets is strong, while the one at 0-surface was weak. In fact, the formed films on the 0-surfaces can easily be peeled off from their surfaces. We conducted another set of experiments to determine the structure of the calcite needles without the polarized substrates.

Article

Calcite aggregates with sheaf-like morphology were formed in a CaCl2 solution with 0.5 mg of PAA. Individual calcite needles consisting of the aggregates were capped with rhombohedral faces in the arrowhead structure (Figure 7). This sheaf-like structure further grew to form two types of structures that were parallel and perpendicular, respectively, to the long axis of the basal face of the needle. The top faces in the arrowhead structure had smooth surfaces, and the well-defined edges intersected at an angle of about 87°. This angle is close to the interplanar angle between the two faces of the rhombohedral {10.4} face (89°). These observations indicate that those two faces are the {10.4} face and that the front surface in the arrowhead structure seems to be the {10.0}. Judging from the direction of the rhombohedron edges of the top faces, we concluded that the calcite needles with the faceted rhombohedral {10.4} top faces have the growth direction of the c-axis. The individual calcite needle with the arrowhead morphology aggregated with an angular spread of the crystallographic c-axis of a few degrees. This morphology is similar to that of the calcite aggregates formed under the conditions where PAA and the polarized YSZ substrates coexist. Although we have no direct observations yet, we believe that the calcite needles with the triangular top faces develop to calcite aggregates with a sheaf-like morphology in the same manner as the calcite needles with the faceted rhombohedral {10.4} top faces. Thus, we conclude that primary calcite needles develop to the bundle of calcite needles with sheaf-like morphology through lateral growth of calcite subunits, which have been nucleated on their own side faces in the presence of PAA, and that the growth direction of the needles is c-axis. Roles of the Surface Electric Field. According to electromagnetism, the electric potential of a point with a distance r from the YSZ electrets of thickness ξ, surface area S, and surface charge density (σ, such that ξ < r, is estimated to be (ξσS cos θ/(4πεr2), where ε is the permittivity of a medium, θ is the angle between the dipole axis, and the direction of a distance r from the surface. Hence, the strength of the surface electric field is large at a local spot in comparison to an electric field with a charged disk. The electric field that occurs on the N-surface attracts Ca2þ ions thus causing its high concentration in this surface vicinity through an electrostatic force. This results in the high local supersaturation of CaCO3 during CO2 diffusion into a calcium chloride solution. It was confirmed by confocal laser scanning microscopy that the attraction of Ca2þ ions occurred on the N-surface of HA electrets on the apatite crystallization.12 On the contrary, an electrostatic interaction between CO32- ions and the P- or N-surfaces is negligible due to the relatively low charge density of the CO32- ions. We believe that a similar attraction occurs under our experimental conditions because the stored charge of YSZ electrets is about 100 times larger than that of HA electrets. This phenomenon leads to a higher concentration of CaCO3 in the N-surface vicinity, supporting the increase in the density and amount of the aggregates on the N-surfaces as listed in Tables 1 and 2. The discussion of Dhanasekaran et al. for the formation of a nucleus in the presence of an electric field is based on an assumption that a stable phase first formed in an unstable environment is a surface two-dimensional nucleus with one monolayer. They indicated that the free energy required for the formation of this nucleus in the presence of an electric field can be estimated on the basis of classical nucleation theory and electromagnetic theory for electric field.46

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Figure 8. Variation of the critical free energy with the orientation of the electric field with respect to the surface of nucleation for various values of an interfacial tension: (;) and ( 3 3 3 ) correspond to values of γ and 3γ/4, respectively.

Figure 9. Change of the critical free energy of heterogeneous nucleation as a function of contact angle.

We shall now briefly describe this nucleation theory. They estimated that the critical free energy for nucleation was ΔG* = β2γ2/{4kT(ln R þ jE2)}, where β is a constant depending on the geometrical shape of the nucleus, γ is the interfacial tension, k is Boltzman’s constant, T is the absolute temperature, R is the supersaturation ratio of the mother phase, E is the electric field assumed to exist in the absence of matter, and j = -v[(1/εs - 1/εl) þ {(εs2 - 1)/εs - (εl2 - 1)/εl} sin2 θ}/(2kT), where v is the volume of the molecule, εs and εl are the dielectric constants of the nucleus and the solution, respectively, and θ is the angle between the direction of the electric field and the nucleation surface.46 Figure 8 shows the relationship between ΔG* and θ. The critical free energy for nucleation decreases as the angle between the direction of the electric field and the nucleation plane approaches 90° and as the interfacial tension of a plane becomes smaller. Based on electromagnetism, we can speculate that the direction of the surface electric field (vector field) due to the disklike YSZ electrets is vertical to the polarized surface. Hence, considering the situation of the surface electric field in the YSZ electrets vicinity as described above, this nucleation theory and the fluctuation theory explain that a (10.4) plane of calcite, which has a minimum interfacial tension, is favorably

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Figure 10. Schematic diagrams of the mechanism of calcite aggregate formation. “A” and “B” show the aggregates of calcite needles having the top of a (00.1) plane and the top of faceted rhombohedral {10.4} plane, respectively. At the initial stage, the crystallization of calcite occurs on the PAA-Ca2þ complex assemblies with a hemispheric structure as a template for its nucleation. At next stage, calcite needles with the growth direction of c-axis grow radially from their assemblies, and the needle-like calcite aggregates with a sheaf-like morphology develop into the hemispheric calcite aggregates through additional growth and agglomeration.

oriented parallel to the surface of YSZ electrets, demonstrating that in the absence of PAA, the surface electric field favors the crystallographic (10.4) orientation of the calcite crystals on the N- and P-surfaces. Classical nucleation theory also point out an importance of the contact angle for heterogeneous nucleation in supersaturated solutions including a substrate. Young’s equation relates the cosine of equilibrium contact angle to the three interfacial tensions by γ1 = γ cos j þ γ2, where the γ parameters are the interfacial tensions of a nucleus, between a nucleus and a substrate, and of a substrate, respectively. When a cap-shaped nucleus with the contact angle j nucleates on substrates according to the classic theory for nucleation, activation energy for nucleation, ΔG*het, is 16πγ3f(j)/ [3(ΔGv)2], where ΔGv is the Gibbs free energy change per unit volume of a nuclear material and ΔGv is kT ln(R), where f(j) is (2 - 3 cos j þ cos3 j)/4, whose function changes monotonically as an increasing function of j in the range of 0 e f(j) e 1. Hence, the equation of ΔG*het indicates that (a) an increase in S decreases ΔG*het, (b) an increase in the contact angle j increases ΔG*het, and (c) ΔG*het is strongly influenced by changes in the interfacial tension of formed nuclei. The activation energy decreases as the contact angle θ becomes smaller as shown in Figure 9. The rate of nucleation, J, is given by J=A exp(-ΔG*het/(kT))=A exp(-16πγ3f(j)/ (3(kT)3 ln2 (R)), where A is a constant, suggesting that J rises rapidly with increase in R and does slowly with decrease in j.

The contact angle values were measured on the six different types of YSZ surfaces using distilled and deionized water (1 μL). The average of the contact angle on the 0-surface showed 101°, while the ones on N- and P-surfaces were 80° and 83°, respectively. The average of the contact angle on the 0-surfaces of nonpolarized YSZ ceramics and N- and P-surfaces of YSZ electrets that were soaked in a CaCl2 solution containing PAA was 88°, 69°, and 76°, respectively. These results indicate that the surface electric field due to YSZ electrets and addition of PAA decreased the contact angle on nonpolarized and polarized YSZ ceramics. This means that a decrease in the activation energy is needed for calcite nucleation and indicates that the calcite nucleation is promoted on the N-surfaces. It, however, does not explain the reason the electric field decreases a contact angle. Although this issue is outside the scope of the present study, further work is being carried out and will be presented in a future paper. Behavior of Oriented Crystallization of Calcite Crystals. In our previous papers, we explained the nucleation and growth model of calcite aggregates by cooperation of PAA and the surface electric field due to HA electrets.9,10 The present model for calcite crystallization on YSZ electrets is similar to the one for HA electrets. It was proposed that calcium complexes with dicarboxylate groups in the PAA, that is, PAA-Ca2þ complexes by a chelating bidentate mode.47 From the results obtained, the model of the Ca2þ coordination to

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PAA, and the fact that the distance between the equivalent carboxy groups (0.502 nm) in the conformation of PAA molecules well agrees with the calcium sites (0.499 nm) on the (00.1) plane of calcite, we propose a possible explanation of the formation of calcite aggregates formed on 0-, N-, and P-surfaces with and without PAA as follows. At the initial stage of the calcite crystallization, PAA-Ca2þ complexes are formed in the CaCl2 solution. Some of the unsatisfied charged sites in these complexes are located on the P- and N-surfaces through an electrostatic force or adsorbed on the 0-surface through van der Waals force and then the PAA-Ca2þ complex assembly with hemispheric structures formed on each of the surfaces. This would reasonably explain the fan-like structure of the fracture observed in the calcite hemispheric aggregates. During the next stage, calcite crystallization occurs at multiple nucleation sites on the PAA-Ca2þ complex assemblies, as templates, and calcite needles with the growth direction of the c-axis grow radially from their assemblies. The needles get together to become needle-like aggregates with sheaf-like morphology and then develop into the hemispheric aggregates with the fracture being a fan-like structure through additional growth and agglomeration. In addition, during their growth process, mobile PAAs and PAA-Ca2þ complexes in solution adsorb on the calcite needles and control their morphology. Finally, calcite aggregates, composed of the needles with the triangular (00.1) top faces or faceted rhombohedral {10.4} top faces, develop to the thin films through growth and coalescence. On the other hand, during the process of the coalescence between calcite hemispheric aggregates, the individual calcite needles grown radically from the hemispheric PAA-Ca2þ complex assemblies come in contact with the ones of the adjacent aggregates at the final stage of their growth. Further growth is restricted only in the direction approximately perpendicular to the substrates, namely, geometrical selection. The final result of this action is formation of an array of tightly spaced calcite crystals. We summarize the formation process of calcite hemispheric aggregates and calcite thin films in Figure 10. Conclusions Both on negatively and on positively charged surfaces of YSZ ceramic electrets without PAA, the precipitates were calcite crystals with the rhombohedral {10.4} plane oriented parallel to the surfaces of the YSZ substrates. The calcite thin films with a fan-like fracture formed on the 0-, N-, and P-surfaces in the presence of PAA. These films were made of bundles of the calcite needles having the top of a (00.1) plane with triangular form or the rhombohedral calcite crystals with three {10.4} top faces. The needles grow in the direction of the c-axis. Calcite crystallization occurs at multiple nucleation sites on the PAA-Ca2þ complex assemblies with a hemispheric order structure as templates. The morphology of the PAA-Ca2þ complex assemblies as template for the calcite nucleation may be determined by the properties of the surface electric field due to XYZ electrets, the partially charged distribution in the PAA-Ca2þ complex, and the surface characteristics of the material of XYZ. The cooperation of PAA and a surface electric field generated by YSZ electrets favored the formation of calcite thin films, and this works most remarkably on negatively charged surfaces. The surface electric fields controlled a supersaturation with calcite in the electret vicinity under CO2 diffusion into a CaCl2 solution and

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decreased the contact angle on the electrets, which led to a decrease in the activation energy for calcite nucleation. The surface electric fields also improve the adhesion between the calcite aggregates and the YSZ electrets in the presence of PAA. The presently obtained results would also suggest new strategies for the controlled crystallization of other types of biomaterials. Acknowledgment. This work was financially supported by a Grant-in-Aid for Scientific Research (Grant No.21923003) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

References (1) Higasitani, K.; Kage, A.; Katamura, S.; Imai, K.; Hatada, S. J. Colloid Interface Sci. 1993, 156, 90. (2) Madsen, H. E. L. J. Cryst. Growth 1995, 152, 94. (3) Wakayama, N. I.; Ataka, M.; Abe, N. J. Cryst. Growth 1997, 178, 653. (4) Yanagiya, S.; Sazaki, G.; Durbin, S. D.; Miyashita, S.; Nakajima, K.; Komatu, H.; Watanabe, K. J. Cryst. Growth 2000, 208, 645. (5) Chibowski, E.; Szczes, A.; Holysz, L. Langmuir 2005, 21, 8114. (6) Knez, S.; Pohar, C. J. Colloid Interface Sci. 2005, 281, 377. (7) Anderson, B. J.; Hallett, J. J. Cryst. Growth 1979, 46, 427. (8) Shichiri, T.; Araki, Y. J. Cryst. Growth 1986, 78, 502. (9) Wada, N.; Nakamura, M.; Tanaka, Y.; Kanamura., K.; Yamashita, K. J. Colloid Interface Sci. 2009, 330, 374. (10) Wada, N.; Tanaka, Y.; Nakamura, M.; Kanamura., K.; Yamashita, K. J. Am. Ceram. Soc. 2009, 92, 1586. (11) Yamashita, K.; Oikawa, N.; Umegaki, T. Chem. Mater. 1996, 8, 2697. (12) Ueshima, M.; Nakamura, S.; Ohgaki, M.; Yamashita, K. Solid State Ionics 2002, 51, 29. (13) Yamashita, K.; Nakamura, S. J. Ceram. Soc. Japan 2005, 113, 1. (14) Kato, R.; Nakamura, S.; Katayama, K.; Yamashita, K. J. Biomed. Mater. Res. 2005, 75A, 652. (15) Iwasaki, T.; Tanaka, Y.; Nakamura, M.; Nagai, A.; Yamashita, K. J. Am. Ceram. Soc. 2008, 91, 3943. (16) Nakamura, S.; Kobayashi, T.; Nakamura, M.; Yamashita, K. J. Biomed. Mater. Res. 2002, 61, 593. (17) S. Itoh, S.; Nakamura, S.; Nakamura, M.; Shinomiya, K.; Yamashita, K. Biomaterials 2006, 27, 5572. (18) S. Makamura, S.; Kobayashi, T.; Nakamura, M.; Yamashita, K. J. Mater. Sci.: Mater. Med. 2009, 20, 99. (19) Okabayashi, R.; Nakamura, M.; Okabayashi, T.; Tanaka, Y.; Nagai, A.; Yamashita, K. J. Biomed. Mater. Res. B 2009, 90, 641. (20) Wang, W.; Itoh, S.; Tanaka, Y.; Nagai, A.; Yamashita, K. Acta Biomater. 2009, 5, 3132. (21) Nakamura, M.; Nagai, A.; Hentunen, T.; Salonen, J.; Sekijima, Y.; Okura, T.; Hashimoto, K.; Toda, Y.; Monma, H.; Yamashita, K. ACS Appl. Mater. Interfaces 2009, 1, 2181. (22) Makamura, S.; Kobayashi, T.; Nakamura, M.; Itoh, S.; Yamashita, K. J. Biomed. Mater. Res. A 2010, 92, 267. (23) Nakamura, M.; Nagai, A.; Tanaka, Y.; Sekijima, Y.; Yamashita, K. J. Biomed. Mater. Res. A 2010, 92, 783. (24) Kumar, D.; Gittings, J. P.; Turner, I. G.; Bowen, C. R.; Bastida-Hidalgo, A.; Cartmell, S. H. Acta Biomater. 2010, 6, 1549. (25) Dauphin, Y. J. Biol. Chem. 2003, 278, 15168. (26) Mann, S. Biomineralization: Principles and Concepts in Bioorganic Materials Chemistry; Oxford University Press: New York, 2001. (27) Calver, P.; Mann, S. J. Mater. Sci. 1988, 23, 3801. (28) Mann, S.; Archibald, D. D.; Didymas, J. M.; Douglas, T. B.; Heywood, R. F.; Meldrum, C.; Reeres, N. J. Science 1993, 261, 1286. (29) Zhang, S.; Gonsalves, K. E. Langmuir 1998, 14, 6765. (30) Aizenberg, J.; Black, A . J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500. (31) Zhu, P. X.; Masuda, Y.; Koumoto, K. J. Colloid Interface Sci. 2001, 243, 31. (32) Hosoda, N.; Kato, T. Chem. Mater. 2001, 13, 688. (33) Masuda, Y.; Koumura, T.; Okawa, T.; Koumoto, K. J. Colloid Interface Sci. 2003, 263, 190. (34) Sugawara, A.; Ishii, T.; Kato, T. Angew. Chem., Int. Ed. 2003, 42, 5299.

174

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(35) Hosoda, N; Sugawara, A.; Kato, T. Macromolecules 2003, 36, 6449. (36) Sato, K.; Kumagai, Y.; Watari, K.; Tanaka, J. Langmuir 2004, 20, 2979. (37) Wada, N.; Suda, S.; Kanamura, K.; Umegaki, T. J. Colloid Interface Sci. 2004, 279, 167. (38) Kotachi, A.; Miura, T.; Imai, H. Chem. Mater. 2004, 16, 3191. (39) Xu, X.; Han, J. T.; Cho, K. Langmuir 2005, 21, 4801. (40) Yu, S. H.; Shu, H. Y.; Xiao, H. G. Langmuir 2006, 22, 6125. (41) Miura, T.; Kotachi, A.; Imai, H. Cryst. Growth Des. 2006, 6, 612.

Wada et al. (42) Pan, Y.; Zhao, X.; Sheng, Y.; Wang, C.; Deng, Y.; Ma, X.; Liu, Y.; Wang, Z. Colloids Surf., A 2007, 297, 198. (43) Payne, S. R.; Heppenstall-Butler, M.; Butler, M. F. Cryst. Growth Des. 2007, 7, 1262. (44) Nishino, Y.; Oaki, Y.; Imai, H. Cryst. Growth Des. 2009, 9, 223. (45) Liu, R; Xu, X.; Cai, A.; Pau, H.; Tang, R; Cho, K. Cryst. Growth Des. 2009, 9, 3095. (46) Dhanasekaran., R.; Ramasamy., P. J. Cryst. Growth 1986, 79, 993. (47) Fantinel., F.; Rieger., J.; Molnar., F.; H€ ubler., P. Langmuir 2004, 20, 2539.