Polymorph Control of Calcium Carbonate on the Surface of

Article pubs.acs.org/crystal

Polymorph Control of Calcium Carbonate on the Surface of Mesoporous Silica Kwang-Min Choi† and Kazuyuki Kuroda*,†,‡ †

Department of Applied Chemistry, Faculty of Science & Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan ‡ Kagami Memorial Research Institute for Materials Science & Technology, Faculty of Science & Engineering, Waseda University, Nishi-waseda 2-8-26, Shinjuku-ku, Tokyo 169-0051, Japan S Supporting Information *

ABSTRACT: The influence of mesoporous silica on the polymorph selectivity in CaCO3 has been investigated. Vaterite is selectively formed from an aqueous solution containing CaCl2 and Na2CO3 by precipitation under the presence of KIT-6-type mesoporous silica. Crystallization time of vaterite from amorphous calcium carbonate (ACC) is much longer on the addition of KIT-6 than that reported previously, indicating the remarkable stabilization of ACC on the surface of mesoporous silica. Other types of silica affect the polymorph selectivity; the addition of amorphous silica gel or assembled silica nanoparticles 12 nm in particle size induced the formation of vaterite as a main phase whereas the presence of assembled silica nanoparticles 30 nm in particle size resulted in the formation of calcite as a main phase with a minor component of vaterite. Therefore, the porous nature of the surfaces of silica greatly influences the polymorph, and a sort of “surface confinement” should play a major role in the selectivity of polymorph.

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INTRODUCTION Calcium carbonate (CaCO3) is one of the most important inorganic materials because it has been used in various industrial applications, including fillers (paints, plastics, rubber, and paper), biomedical implanting, drug delivery, and bone regeneration.1 CaCO3 has three anhydrous crystal polymorphs (rhombohedral calcite, orthorhombic aragonite, and hexagonal vaterite), two hydrated forms (mono- and hexahydrate calcium carbonates), and amorphous CaCO3 (ACC). Among anhydrous crystals, calcite is the most stable phase at room temperature under atmospheric conditions, and aragonite is metastable, while vaterite is thermodynamically unstable at ambient conditions.2 Therefore, aragonite and vaterite transform into calcite.3 Vaterite is a useful material for biomedical and industrial applications because it exhibits unique properties, such as higher solubility, higher dispersion, and smaller specific gravity than aragonite and calcite.4 Vaterite can coexist with aragonite and calcite in biogenic materials, such as fish otoliths,5 ascidian spicules,6 and pearls.7 Mineralization of vaterite has been studied to stabilize the phase in a controlled manner. © 2011 American Chemical Society

Stable vaterite crystals have been reported in the presence of dopamine,8 dendrimers,4,9 bis(2-ethylhexyl)sodium sulfate,10 poly(styrene sulfonate),11 poly(glutamic acid),12 and doublehydrophilic block copolymers.13 On the other hand, aragonite is formed in the presence of poly(vinyl alcohol),12 alginate,14 soluble silicate,15 and Mg ion.16 It is also formed by treating calcium acetate with CO2 at high pressure17 and by precipitating from a mixture of CaCl2 and NaHCO3 under high-power ultrasound irradiation.3 Consequently, organic additives play an important role in preventing the transformation to stable calcite. Hemispherical vaterite forms in alkaline silica−casein sols containing CaCl2 by the diffusion of atmospheric carbon dioxide into the solution.18 The influence of soluble silica species on the polymorph of CaCO3 was also investigated.19 Morphology of CaCO3 (mostly calcite) has been controlled Received: October 3, 2011 Revised: December 5, 2011 Published: December 13, 2011 887

dx.doi.org/10.1021/cg201314k | Cryst. Growth Des. 2012, 12, 887−893

Crystal Growth & Design

Article

Table 1. Fraction of Vaterite under Various Conditions entry

[CaCl2] (mmol)

[Na2CO3] (mmol)

molar ratio (Ca2+/CO32−)

additive

temp (°C)

reaction time (h)

f va

1 2 3 4 5 6 7 8 9 10

1 1 1 1 1 2 1 2 1 1

1 1 2 1 2 1 1 1 1 1

1 1 0.5 1 0.5 2 1 2 1 1

KIT-6 KIT-6 KIT-6 KIT-6 none KIT-6 KIT-6 none none none

25 80 25 25 25 25 80 25 25 80

2 2 2 6 2 2 6 2 2 6

0.93 0.93 0.88 0.69 0.66 0.59 0.50 0.34 0.06 0

a

The f v values are based on Rao’s equation, f v = (I110v + I112v + I114v)/(I110v + I112v + I114v + I104c); I is the intensity of vaterite or calcite; v indicates vaterite, and c indicates calcite.

Figure 1. XRD patterns of KIT-6−CaCO3 composites. The preparation conditions are (a) precipitation at 25 °C for 2 h (entry 1), (b) precipitation at 80 °C for 2 h (entry 2), (c) precipitation 25 °C for 6 h, (entry 4), (d) precipitation at 80 °C for 6 h (entry 7), (e) precipitation at 25 °C for 2 h without the additive (entry 9), and (f) precipitation at 80 °C for 6 h without the additive (entry 10). A, C, and V denotes aragonite, calcite, and vaterite, respectively.

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with silicate anions.20 However, to the best of our knowledge, the influence of “insoluble” silica on the selective CaCO3 mineralization (control of polymorph) has not yet been studied. Additives, irrespective of organic or inorganic, can be adsorbed onto nuclei (primary particles) and govern the growth rate of some specific faces of nuclei.21 Therefore, the surfaces of additives affect the morphology and polymorph of CaCO3. At present, the surface parameters of additives, such as surface functional groups, surface morphology, and surface porosity (micrometer scale), of self-assembled mercaptohexadecanoic acid on a glass cylinder, mica, and polycarbonate membranes, respectively, are thought to influence the polymorph of CaCO3.22 From this viewpoint, mesoporous silica is quite promising to have a distinctive effect to direct the crystallization of CaCO3 because mesoporous silica has unique surface properties, arising from high specific surface area, well-defined pore size, narrow pore size distribution, and selective adsorption ability. Among many kinds of mesoporous silica with different structures and porosities, KIT-6 was chosen as a typical example with relatively large pores. Here we report the polymorphism of CaCO3 in the presence of mesoporous silica, showing that the polymorph is effectively controlled under the presence of mesoporous silica.

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formation of vaterite (Figure 1a,b). When the precipitation time was increased from 2 to 6 h at 25 °C (entry 4), calcite polymorphs are formed in addition to vaterite (Figure 1c) On the other hand, aragonite and calcite polymorphs (Figure 1d) are formed in addition to vaterite when the precipitation time is increased from 2 to 6 h at 80 °C (entry 7). The supersaturation levels for polymorph are as follows: aragonite < vaterite < calcite.24 Therefore, aragonite is precipitated at relatively high temperature, 80 °C. The only calcite polymorph is shown at precipitation time in 24 h with KIT-6 additive at both 25 and 80 °C. This suggests that thermodynamically unstable vaterite is in the process of transformation to relatively stable aragonite or calcite polymorphs during treatment for 6 h, although direct crystallization of those phases from remaining calcium and carbonate ions dissolved in the aqueous phase should also be considered. On the other hand, the XRD pattern of the product obtained by precipitation without the additive at 25 °C for 2 h (entry 9) shows the peaks assigned to calcite and a very little fraction of vaterite (Figure 1e). The XRD pattern of the product obtained without the additive at 80 °C for 6 h (entry 10) exhibits only peaks due to calcite (Figure 1f). Therefore, it is strongly suggested that KIT-6 plays a crucial role in retaining the phase of vaterite, even under conditions for the formation of more thermodynamically stable calcite. The crystallite size of vaterite, estimated by the Scherrer equation for the unique peak indexed as (112), is around 19 nm for the vaterite samples (entries 1 and 2) (please note that the size of calcite formed as a very minor phase in entry 7 is around 50 nm (calculated for the lattice plane of (104)). Figure S2, Supporting Information shows a TEM image of a calcite nanoparticle about 50 nm in size. This result is consistent with that calculated by the Scherrer equation. The size does not vary much among the samples. The size is larger than the pore size of KIT-6, which means that the CaCO3 crystals form on the outer surfaces of KIT-6. This also means that the surface of mesoporous silica has a unique function to retain such a thermodynamically unstable phase, and this point will be discussed below. Table 1 shows the relative fractions of vaterite. The relative fraction of vaterite, f v, in precipitated CaCO3 can be estimated by Rao’s equation,25 where the I parameters indicate the intensities of XRD peaks due to various crystal planes of vaterite (subscript v) and calcite (subscript c).

EXPERIMENTAL SECTION

Sample Preparation. The preparative conditions for mesoporous silica KIT-6 are based on a previous report.23 The textural parameters of KIT-6 are 777 m2/g for specific surface area (BET), 1.2 cm3/g for pore volume, and 7.8 nm for pore size (BJH). Powder X-ray diffraction data, SEM, and TEM images are shown in the Supporting Information (Figure S1). In a typical reaction, calcium chloride (1 mmol, Wako Industries Ltd.) was dissolved in 10 mL of H2O, and then 0.1 g of mesoporous silica (KIT-6) as an additive was added into the solution. Into the above solution, an aqueous sodium carbonate solution (1 mmol Na2CO3 (Wako Industries Ltd.) in 10 mL of H2O) was added dropwise (drip rate 2 mL/min). Then, the solution was stirred constantly under several variable parameters, such as concentrations of CaCl2 and Na2CO3, reaction temperature, precipitation time, and presence or absence of the additive. Formed KIT-6−CaCO 3 composites were separated from turbid suspensions by centrifugation (4500 rpm, 10 min), washed with water and ethanol several times, and dried in vacuum. Table 1 lists the sample names with those preparative conditions. Characterizations. High-angle XRD measurement was performed on a Rigaku-Rint-Ultima III diffractometer with Cu Kα radiation at 40 kV and 40 mA. TEM images, SAED patterns, and EDX spectra were recorded by a JEOL JEM-2010 microscope using an accelerating voltage of 200 kV. Samples were suspended in ethanol and then dropped onto carbon grids. HRSEM images were recorded on a Hitachi S-5500 microscope at an accelerating voltage of 30 kV. Fourier transform infrared (FT-IR) spectra were obtained by the KBr disk technique using a Perkin-Elmer Spectrum One spectrometer. Nitrogen adsorption−desorption measurements of porous silicas were performed on an Autosorb 1 instrument (Quantachrome Instruments) at 77 K. Samples were preheated at 120 °C for 3 h under 1 × 10−2 Torr prior to the measurements. The pore size distributions were evaluated by the BJH method using adsorption branches. Specific surface areas were calculated by the BET method. The total pore volumes were evaluated from the amount adsorbed at a relative pressure (P/Po) of about 0.99.

fv =

I110v + I112v + I114v I110v + I112v + I114v + I104c

A large number of previous studies on the formation mechanisms of polymorphous CaCO3 suggest that heterogeneous nucleation is the first stage. The heterogeneous nucleation can be affected by several synthetic conditions, such as temperature on precipitation, precipitation time, concentration of calcium and carbonate ions in the starting solution, and the kind of additives. Table 1 shows that in general the presence of KIT-6 induces a preference for the formation of vaterite. However, longer time for the reaction results in decrease in the fraction of vaterite. In the presence of KIT-6, the highest fraction of vaterite was observed when the molar ratio of Ca2+/CO32− was 1 (entries 1 and 2). When the additive was not added, calcite is predominant (entries 9 and 10). Interestingly, when the molar ratio of Ca2+/CO32− was 0.5 (entries 3 and 5), the difference in the formation ratios of vaterite in the presence or

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RESULTS AND DISCUSSION The powder XRD patterns of KIT-6−CaCO3 composites (Table 1, entries 1 and 2), obtained by precipitation of the starting solutions at 25 and 80 °C for 2 h, exclusively show the 889



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Figure 2. FT-IR spectra of KIT-6−CaCO3 composites. The conditions are (a) 80 °C for 6 h without additive (entry 9), (b) 25 °C for 2 h with the additive (entry 1), and (c) 80 °C for 2 h with the additive (entry 2) (the asterisk indicates the KIT-6 additive).

crystals of vaterite are aggregated to form a spherical morphology (Figure 3). A lower magnification image shows many similar spherical particles on KIT-6 (Figure S4a, Supporting Information). The arrows indicate the presence of the KIT-6 mesoporous silica domain, which means that most vaterite particles are located on the KIT-6 domain. Figure 3c,d shows the magnified images where the KIT-6 domain is observed clearly.

absence of KIT-6 is not as large, the fraction of vaterite being 0.88 and 0.66, respectively. Notwithstanding such a comparatively small difference, the presence of KIT-6 results in a higher fraction of vaterite. When the molar ratio of Ca2+/CO32− was 2 (entries 6 and 8), the fraction of vaterite is relatively low even in the presence KIT-6. Though the variation in the ratio may influence the nucleation and crystallization behavior, the present data are not sufficient to discuss this point. It was reported that the presence of dissolved silica decreases the induction time for CaCO3 nucleation, but cannot control the polymorphism of CaCO3.19c However, the present study clarifies that mesoporous silica can control the CaCO3 polymorphism. Therefore, the influence of the slight dissolution of silica from the surface of KIT-6 should not be large. The pH value of 0.1 M CaCl2 aqueous solutions did not change even after the addition of KIT-6 mesoporous silica. Moreover, the pH value of the reaction solution is the same with and without the additive. Therefore, we can also claim that the effect of pH on the polymorph is minor in our system. The characteristic IR absorption bands of CaCO3 are frequencies of CO32− that show four kinds of vibration frequencies, ν1 (symmetric carbonate stretching), ν2 (carbonate out-of plane bending), ν3 (fundamental stretching), and ν4 (inplane bending mode).26 Two KIT-6−CaCO3 composites, obtained at 25 and 80 °C for 2 h (entries 1 and 2) (Figure 2b,c), show bands at 875 cm−1 (ν2 mode) and 745 cm−1 (ν4 mode), indicating the formation of vaterite, while the band at 712 cm−1 (ν4 mode) assignable to calcite was observed in the case of no additive (entry 9) (Figure 2a). With regard to the samples of entries 1 and 2, a large vibration around 1350−1600 cm−1 (ν3 mode) is observed with split bands at around 1410 cm−1 and 1490 cm−1, which can be assigned as a unique vibration mode of amorphous CaCO3 (ACC) or vaterite.26 A very broad ν3 band of CaCO3 for the sample prepared without the additive (Figure 2a) was observed around 1420 cm−1, which corresponds to the presence of calcite. The enlarged FT-IR spectra in the range of 1000−700 cm−1 are shown in Figure S3b,c, Supporting Information. The peak at 875 cm−1 is asymmetric. The peak at 864 cm−1, which is due to ν2 peak of amorphous calcium carbonate, may be overlapped with the peak at 875 cm−1. The SEM images of the KIT-6−CaCO3 composite (25 and 80 °C for 2 h, entries 1 and 2) demonstrate that nanoscale

Figure 3. SEM images of KIT-6−CaCO3 composites. The conditions are (a) 25 °C for 2 h (entry 1) and (b) 80 °C for 2 h (entry 2). Panels c and d show the magnified images of samples a and b, respectively, proving the presence of KIT-6. The arrow shows the presence of KIT6.

Smaller CaCO3 particles (25 °C, 2 h, entry 1) from 50 to 200 nm in size are also observed on the surface of KIT-6 mesoporous silica (Supporting Information, Figure S4b). The TEM analysis shows the presence of KIT-6 mesoporous silica and two kinds of CaCO3 particles (Figure 4). Each part of CaCO3 was characterized by SAED. The SAED pattern of Figure 4a,b indicates the crystalline phase of CaCO3. However, the SAED pattern of another part (Figure 4c,d) exhibits an amorphous phase of CaCO3 (ACC). The presence of Ca species in this part was confirmed by EDX analysis (Supporting Information, Figure S5). 890



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Figure 4. TEM image of KIT-6−CaCO3 composite (entry 1). (Inset, ED patterns of the selected areas, a,b for the left circle and c,d for the right.) In the ED patterns of parts b and d, these two particles are structurally different even though the morphologies look so similar. In fact, these two particles are too small (smaller than 100 nm), and such a size cannot directly be related to the crystal morphology. Under the size range from 70 to 120 nm of calcium carbonate, both phases of ACC and polycrystals can coexist.27.

Figure 5. XRD patterns of CaCO3 formed under the presence of Wakogel, silica nanoparticles 12 nm in particle size, and silica nanoparticles 30 nm in particle size.

To further investigate the influence of mesoporous silica on the CaCO3 polymorphism, a typical 2D-hexagonal mesoporous silica, SBA-15, was employed because it has surface area and pore size distribution similar to those of KIT-6 (3D-cubic). SBA-15 was prepared based on ref 28. When SBA-15-type mesoporous silica was used as an additive, the fraction of vaterite is similar to that found for the cases using the KIT-6 additive (Supporting Information, Figures S6 and S7). Therefore, the structural parameter of mesoporous silica does not affect the polymorphism of calcium carbonate. To prove the effect of pore size on the surfaces to direct the selective formation of the polymorph, CaCO3 was prepared under the presence of different types of silica including assembled silica nanoparticles without mesopores (Supporting Information, Figure S8). The nanoparticles were prepared based on ref 29. They possess void spaces as interparticle (interstitial) pores. The particle size of the assembled silica nanoparticles is 12 and 30 nm, as shown below. Commercially available silica gel (Wakogel, Wako Industries. Ltd.) was also used for comparison. The synthesis procedure of CaCO3 by using these silicas was the same as that for the cases using the KIT-6 additive (the conditions were same as those for entry 1 in Table 1). Wakogel silica and assembled silica nanoparticle additives (particle size 12 nm) mainly show the formation of vaterite polymorph. However, the additive of assembled silica nanoparticles (30 nm) exhibited a lower fraction of vaterite polymorph in addition to the major fraction of calcite, which is in clear contrast to the cases of silica nanoparticles (12 nm) and Wakogel silica (Figure 5). The textural properties of nanoparticles are shown in the Supporting Information (Table S1). The pore diameters, that are void space of interparticles of silica nanoparticles 12 and 30 nm in size, are calculated to be 4.39− 4.97 nm (tetrahedral and octahedral interstices) and 10.98− 12.42 nm, respectively, while the estimated pore size of Wakogel silica ranges from around 5 to 12 nm (Table S1, Supporting Information). This is very interesting to note the difference in these apparently similar silica materials. Layered silicate (without effective porosity), Ca-RUB-18, was used as an additive for precipitating CaCO3 in the same procedure as the case of KIT-6 additive. Calcite is a predominant polymorph by the precipitation time for 2 h (Figure S9, Supporting Information). Because there are several possible influential parameters of surface porosity, such as pore periodicity, pore size distribution, pore volume, and surface area, further studies are needed to fully understand the phenomena. In any case, it

should be noted that the shape of the pores of mesoporous silica is not involved in the selective polymorphism of calcium carbonate, because spherical (nonporous) silica nanoparticles having convex-type surfaces also exhibit a preference for a high fraction of vaterite. This study has clarified that mesoporous silica is an excellent additive to selectively produce vaterite. Previous reports on ACC have clarified that ACC is stabilized in a limited time of minutes.21,27,30 On the other hand, in this study, we can observe ACC for much longer time (longer than 2 h), as confirmed by TEM (please note that TEM used here is not cryo-TEM but conventional TEM), which is thought to be due to the stabilization on the surfaces of KIT-6 and SBA-15 (Figure 4 and Figure S5, Supporting Information). In this mineralization without additive, ACC cannot be observed after 1 h. Therefore, KIT-6 and SBA-15 can play a role of stabilizing ACC even under otherwise thermally inappropriate conditions. This finding is quite important for the control of mineralization of CaCO3. The crystallization process of CaCO3 occurs in the following two stages. First, particles of amorphous calcium carbonate (ACC) rapidly crystallize to form vaterite. Second, vaterite transforms to calcite, and the transformation rate is 10 times slower than that of the first stage.30 Therefore, the second step is the rate-determining step due to the slow reaction. However, we found that ACC can be stabilized in the presence of KIT-6 additive. It inferred that the rate of the first step is slower in the presence of KIT-6 additive. Therefore, the second step is not predominant to decide the rate of crystallization (from ACC to calcite) of calcium carbonate. It should be noted that mesoporous silica like KIT-6 may also inhibit the transformation from vaterite to calcite. With increasing the temperature, the rate of the second step is increased and is the rate-determining step.31 However, the fraction of vaterite mineralized in the presence of KIT-6 at 80 °C for 2 h is same as that at 25 °C for 2 h. The KIT-6 additive plays therefore a role to inhibit transformation from ACC to vaterite and finally to calcite. Moreover, we inferred that the second step is not ratedetermining in the presence of KIT-6. The pore size of mesoporous silica is critical to produce vaterite. The crystallite size of vaterite is larger than the pore size of mesoporous silica used in this study. Even if ACC cannot be present in the pores of mesoporous silica due to their large size, the stability of ACC may be affected by the surface of 891



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selective formation of polymorphs of CaCO3. The presence of relatively stabilized ACC on the KIT-6 additive clarified that the transformation of ACC to vaterite can be suppressed. This enables control of the polymorphism of CaCO3 due to the increase in the induction time in the primary stage. Furthermore, a sort of confinement effect on the surface of mesoporous silica contributes to stabilization of ACC. Because the rate of crystallization of CaCO3 is affected by the stabilization of ACC, the control of polymorphs of CaCO3 may be further controlled by adding appropriate inorganic additives.

mesoporous silica. When ACC is transformed to vaterite, dehydration and condensation of ACC must occur, but the morphological change is not so drastic.27 The crystalline and amorphous calcium carbonates are shown to be similar in size, which is proven by the data shown Figure 4 and Figure S4, Supporting Information. The pore size of mesoporous silica is an important factor for the selectivity of polymorph. ACC can be stabilized by the confinement alone.22b KIT-6 mesoporous silica has uniform pores on the surface. Even the surface of mesoporous silica can afford a unique concave shape exhibiting a confinement effect. Therefore, ACC confined on the surface of mesoporous silica would be stabilized. It should be noted that the confinement effect increases with decreasing pore diameter.32 On the other hand, according to the literature,30 the formation of calcite via vaterite means the dissolution of vaterite and the following precipitation of calcite. Therefore, the formation of calcite from vaterite should be a surface-controlled process. The rate of crystallization depends on the surface properties.35 Pores on surfaces may confine the nucleus along one or more directions.35 We speculate that the selective vaterite polymorph depends on the pore size of additive. ACC was observed on the surface of KIT-6 at precipitation time of 2 h. We expect that the rate of nucleation is slow, and then the crystallization of calcium carbonate is also slow in the KIT-6 additive. In addition, the nucleation rate is much more sensitive to pore size. Sear et al.35 reported that the smaller the pore the faster was pore filling but the slower was nucleation out of the pore. The roughness of the surface prevents the growth of the crystal due to trying to conform to the pit surface.36 In the initial stage of calcium carbonate formation, calcium carbonate clusters less than 4 nm are found by measurement of cryoTEM.27 We suppose that the nanoclusters of calcium carbonate generate in the pores, and the nucleation and crystallization procede on the surface (out of pores) of mesoporous silica. The striking difference between dissolved silica and mesoporous silica in the effect of silica species on the formation of calcium carbonate should be noted here because soluble silica species arising from mesoporous silica might affect the polymorph selectivity. However, the concentration of dissolved silica from mesoporous silica should be very low because the solubility is known to be ca. 100 ppm for amorphous silica.33 Even though mesoporosity might affect the solubility to some extent, the influence is expected to be not very high. The previous studies on the effects of dissolved silica on the polymorphs of CaCO3 used the concentration of Si in dissolved silica in the level of 2−15 mM. Dissolved silica (the concentration of Si is 2 mM) helps the formation of calcite.19c The induction time required for calcite nucleation decreases with increasing amount of dissolved silica,19c which is quite in contrast to the present study showing a reverse result. Another report showed that the rate of heterogeneous nucleation of ACC becomes slow and that ACC is thermodynamically stable in the presence of dissolved silica. But the Si concentration is ca. 1−7%,34 and this concentration is several hundred times higher than that of naturally dissolved silica. This finding cannot be applied to our present system. Therefore, the stabilization of ACC should be related to the nature of the surface, in particular, the solid surface of mesoporous silica having unique characteristics, though this point needs further and intensive studies. On the basis of these results described above, the surface properties of inorganic additives should play key roles for the

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CONCLUSION This study has depicted the effect of mesoporous silica on the stabilization of vaterite during the mineralization of CaCO3 under the conditions of the formation of calcite without the additive. Mesoporous silica additive plays two kinds of role during the mineralization. First, the surface of mesoporous silica increases the stability of initially formed ACC. The surface pores can also work as a medium for the confinement effect that can stabilize ACC. This novel mineralization at inorganic− inorganic interfaces should be applicable to other systems of the formation of scientifically and industrially important crystals.

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ASSOCIATED CONTENT

S Supporting Information *

Further characterization data (XRD patterns, SEM, TEM, EDX, and textural parameters). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81 3 5286 3199. Tel: +81 3 5286 3199.

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ACKNOWLEDGMENTS We very much appreciate Dr. A. Sugawara-Narutaki (University of Tokyo) and Professor Izumi Hirasawa (Waseda University) for their careful reading and constructive comments on the manuscript. The authors greatly appreciate the funding from Elements Science and Technology and Global COE program “Practical Chemical Wisdom” both from MEXT (Japan).

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