Controls of Tricalcium Phosphate Single-Crystal Formation from Its

Controls of Tricalcium Phosphate Single-Crystal Formation from Its Amorphous Precursor by Interfacial Energy Jinhui Tao,† Haihua Pan,† Halei Zhai,† Jieru Wang,‡ Li Li,† Jia Wu,† Wenge Jiang,† Xurong Xu,† and Ruikang Tang*

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 3154–3160

Department of Chemistry and Center for Biomaterials and Biopathways and Centers of Analysis and Measurement, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed October 10, 2008; ReVised Manuscript ReceiVed February 3, 2009

ABSTRACT: Different from the conventional solution precipitation, amorphous precursor involves widely in biomineralizations. It is believed that the development of crystalline structures with a well-defined shape in biological systems is essentially facilitated by the occurrence of these transient amorphous phases. However, the previous studies have not elucidated the physicochemical factors influencing the transformation from the transient phase into the stable phase. In this study, the evolutions from the amorphous calcium phosphate to the different-shaped (hexagon and octahedron; octahedron is an unexpected morphology of the crystal with space group of R3jc) single crystals of β-tricalcium phosphate (β-TCP) were examined. The hexagonal β-TCP crystals were formed via the phase transformation of amorphous precursor in CaCl2-Na2HPO4-ethylene glycol solution; however, the octahedral β-TCP crystals were formed in Ca(OH)2-(NH4)2HPO4-ethylene glycol solution. Because the interfacial energies between amorphous phase and crystals were much smaller than those between solutions and crystals, the crystallization of the β-TCP phase occurred directly in the amorphous substrate rather than from the solution. It was interesting that the final morphology of product was also determined by the interfacial energy between the transformed crystal and solution. The current work demonstrated that the amorphous precursor epitaxial nucleation process and morphology selection of crystals in the amorphous phase could also be understood by an interfacial energy control. This result might provide an in-depth understanding of the biomimetic synthesis of crystals via a pathway of amorphous precursors. Introduction The ability to synthetically tune sizes, structures, and morphologies of inorganic crystals is an important objective in current materials science and device fabrications. The same crystal may have different applications as its properties change with size or shape.1 For example, the catalytic property of platinum has been found to be highly dependent on which facets terminate the surface.2 In our previous study, the mineralization and demineralization behaviors of biomaterials such as β-tricalcium phosphate (β-TCP, Ca3(PO4)2) and hydroxyapatite (HAP) are highly dependent on their exposed surfaces to biological milieus,3 which are also related to the protein adsorptions and cell attachments.4 Size- and shape-controlled synthesis of many inorganic compounds such as noble metals,5 semiconductors,6 and magnetites7 have been achieved to modulate their electrical, optical, magnetic, and catalytic properties. In contrast, the challenge of controlling crystal shape of biominerals has been met with a limited success. But crystal polymorph is an important feature of natural biominerals. Different from the conventional solution precipitation, it has been observed that amorphous precursor involves widely in biological crystallizations. Living organisms usually use amorphous phases as the building materials, stabilizing them over their lifetime, or depositing them as transient phases that transform in a controlled manner into the specific crystalline structure and morphology. For example, during the formation of calcitic sea urchin spine and larval spicules, the amorphous calcium carbonate is first formed before the final crystal generation.8,9 Amorphous materials are also identified during the formations of mollusk and skeletal minerals.9-11 It is * Corresponding author. Tel/Fax: 86-571-87953736. E-mail: rtang@ zju.edu.cn. † Department of Chemistry and Center for Biomaterials and Biopathways, Zhejiang University. ‡ Centers of Analysis and Measurement, Zhejiang University.

believed that the development of crystalline structures with a well-defined shape in biological systems is essentially facilitated by the occurrence of these transient amorphous phases.8-11 However, the previous studies have not elucidated the physicochemical factors influencing the transformation from the transient phase into the stable phase. Biological control over the selection of mineral form and morphology indicates complex interactions between the organism and the amorphous precursor, which are not fully discovered. In this study, we examine the evolution from the amorphous precursor to the different-shaped (hexagon and octahedron, octahedron is an unexpected morphology of the crystal with space group R3jc) single crystals. It is revealed experimentally that crystal nucleated directly from the amorphous precursor. The epitaxial nucleation process and shape selection of crystals in the amorphous phase can be addressed by an interfacial energetic control. This result provides an in-depth understanding of the biomimetic crystallizations via a pathway of amorphous precursors. Calcium phosphates have excellent biocompatible properties since they are main component of biological bone and tooth.12 In particular, β-TCP, an important resorbable calcium phosphate biomaterials, is an intermediate phase of calcium phosphate. β-TCP has been used as an ideal candidate for bone substitute,13 inorganic filling of biodegradable composites,14 substrate for evaluation of cell seeding efficacy, proliferation, osteogenic differentiation,15 and carrier for bone growth factors to stimulate bone healing and formation, because of its excellent osteoconductive and biodegenerative characteristics.16 Besides, it can also find other applications of this compound, which involve drug carrier, luminescence materials, and catalyst.17 It has been reported that the protein adsorption property of β-TCP is dependent upon its size and terminal facets.4 The synthesis method with size and shape control ability may provide an effective way for the biological modulation. There are several synthesis methods to produce β-TCP but none of them can form

10.1021/cg801130w CCC: $40.75  2009 American Chemical Society Published on Web 05/12/2009

Calcium Phosphate Phase Transformation

the uniform and faceted crystals. These conventional methods include solid-state reactions between CaHPO4 and CaCO3, (NH4)2HPO4 and CaCO3, NH4H2PO4 and CaCO3, Ca2P2O7 and CaCO3,18 or wet-chemical methods.19 The solid-state reactions usually take place at high temperatures (∼1000 °C) and the formed products are usually agglomerated without any defined shapes. The wet-chemical route results in calcium deficient apatite (CDHA), which is then transformed into β-TCP by calcination at 700-800 °C. Although some other methods have been tried to fabricate nano β-TCP, the shape and structureproperty relationship for this material can hardly be controlled.19e All these methods cannot be used to understand the biological formations of calcium phosphate. Herein, we propose a bioinspired pathway for a large scale synthesis of β-TCP using amorphous calcium phosphate (ACP) as the precursor. Hexagon and octahedron of the well-crystallized β-TCP can be achieved from the identical ACP precursor under the different solvent conditions. Experimental Section Amorphous Precursor. One-tenth of a gram of CaCl2 · 2H2O was added into 50 mL of ethylene glycol (EG), and the mixture was heated to 150 °C under vigorous stirring. Next, 1.36 mL of 0.3 M Na2HPO4 (aqueous solution) and 120 µL of 1.3 M NaOH (aqueous solution) were mixed with 20 mL of EG at a temperature of 105 °C. The phosphatecontaining EG solution was poured into the calcium-containing ethylene glycol solution within 10 s. The precipitation sustained for 5 s and the slurry was then poured into a vial immersed in ice-acetone bath (-16 °C) to quench reaction. The solids were collected by centrifugation (10 000 g) and -4 °C. They were washed with ethanol for 3 times. Synthesis of β-TCP via Amorphous Precursor. For hexagons, 20 mg of amorphous precursor was dispersed in 70 mL of EG containing CaCl2 (7.6 mM) and Na2HPO4 (3.7 mM), and the slurry was heated to 150 °C. For octahedron, 20 mg of precursor was dispersed in 70 mL of ethylene glycol containing Ca(OH)2 (7.6 mM) and (NH4)2HPO4 (3.7 mM), and the slurry was heated to 150 °C. In the size-controlled synthesis, the precursor amounts were altered accordingly. For the study of evolution process, the samples were withdrawn from the reaction milieu periodically using a glass pipet. The extractions were injected into vials immersed in ice-acetone bath (-16 °C) to quench the reaction. All the above solids were harvested by centrifuging at 4000 g and -4 °C. The tablets were washed with ethanol and water repeatedly 3 times to remove the residual solvent and other impurities. The crystals were dried under a vacuum condition at room temperature. Interfacial Energy Determinations. Solid samples were dispersed in chloroform-ethanol mixed solvent (v:v ) 2:1) with a weight ratio of 0.6%. A 100 µL of this dispersion was carefully dipped onto the silicon substrates. The solvent was evaporated in air at room temperature and the films could be formed on the substrates. The growth solutions of hexagon and octahedron were filtered through membrane with pore diameter of 220 nm before use. The surface tensions of these solutions were measured by pendant method at 20 °C and relative humidity was 70%. To measure the interfacial energy of solid films in air, we used four probing liquids: water, EG, n-octane, and DMSO. The contact angles were measured by sessile drop and thin layer wicking at 20 °C and relative humidity of 70%. At least five independent values were measured for each solid film and liquid. Characterizations. Transmission electron microscopy (TEM) observations were performed by using JEM-200CX TEM (JEOL, Japan) at an acceleration voltage of 160 kV and JEM-2010HR HRTEM (JEOL, Japan) at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) characterization was performed on S-4800 fieldemission scanning electron microscope (HITACHI, Japan) at an acceleration voltage of 5 kV. The phase of the solids was examined by X-ray diffraction (XRD, D/max-2550pc Rigaku, Japan) with monochromatized Cu KR radiation. The FT-IR spectra were collected form 4000 to 400 cm-1 in transmission mode by a Nexus-670 spectrometer (Nicolet, USA). The contact angle data were measured on an OCA15+ optical contact-measuring device (Data Physics Instruments GmbH, Germany).

Crystal Growth & Design, Vol. 9, No. 7, 2009 3155 Simulations. Computer simulations were performed using the morphology modules of Material Studio 3.1 packages. The initial configuration of β-TCP crystal was taken from the X-ray crystal structure. Initial face list was generated by Bravais-Friedel DonnayHarker (BFDH) method, which used the crystal lattice and symmetry to generate a list of possible growth faces. The minimum d-spacing was set to be 1.3 Å. The maximum of indices along a, b, c was chosen to be 5, 5, 10 respectively. Finally, A face list consist of 1942 unique crystal facets was generated. This face list was used as input for further calculation of attachment energy. In the part of energy calculation consistent-valence force field (CVFF) was used. Ewald summation method was adopted for treatment of electrostatic terms with accuracy of 0.001 kcal/mol. An atom-based summation method was applied for van der Waals terms with the cutoff distance of 1.25 nm.

Results and Discussion ACP is the least stable of the calcium phosphate phases and it is identified at the early stage of the biological formations of apatite.11 Amorphous mineral is moldable; this characteristic results in the diverse crystal structures of bioinorganic crystals. In the current study, the precursor ACP is synthesized and stabilized in the laboratory by mixing of CaCl2 and Na2HPO4 in EG. TEM and SEM images of the resulting ACP precipitates are shown in Figure 1. Energy-dispersive spectroscopy (EDS) and chemical analysis (atomic adsorption for calcium and UV for phosphate) shows the solids mainly contained calcium and phosphorus and their molar ratio is 1.47 ( 0.05. The chemical composition of the resulted ACP is similar to Ca3(PO4)2. The selected area electron diffraction (SAED) pattern is weak and dispersive, indicating the poor crystallinity of the phase (insert of Figure 1A). FT-IR result shows the broad and featureless phosphate absorption bands (Figure 1D). The triply degenerated asymmetric stretching (1087, 1046, and 1032 cm-1) and bending vibrations of PO43- (602, 574, and 561 cm-1) in crystallized solids are not detected.20 These results confirm that the precipitate in EG is the amorphous phase. The peaks of CO32(1419 and 874 cm-1) in FT-IR implies that some carbonate ions incorporated into the ACP.20 The incorporation of carbonate is a common phenomenon during the formation of biological calcium phosphate. The presence of HPO42- in the amorphous precursor may also contributes to the absorption at 874 cm-1.20 It is previously revealed that the short-range order is always present in the bulk of amorphous phase including ACP.21 The similar result is also observed in our samples. The highresolution TEM (HRTEM) study shows a few of nano ordered domains in the amorphous phase for their different contrasts compared with the surrounding disordered regions (Figure 1B). Such an order-related contrast has also been reported in some amorphous binary alloys.22 However, this short-range order cannot be detected by conventional XRD and the amorphous nature of the precipitates is clarified by the featureless humps in its pattern (Figure 1E). The formed ACP solids can be stabilized up to several months under vacuum conditions at room temperature. We study the phase transformation at temperature of 150 °C in EG in the presence of calcium and phosphate ions (these ions are used to prevent the dissolution of ACP in the solvent). ACP solids are redispersed in a CaCl2-Na2HPO4-EG solution. The hexagon can be eventually formed from ACP (Figure 2A, 2B). The typical diameter of the hexagonal face can be tuned from 550 nm to 1 µm by changing precursor concentration from 10 mg/ 70 mL to 40 mg/70 mL (powder to solution). The thickness of the hexagon, ∼220-250 nm, keeps almost unchanged under the different experimental conditions (Figure 3). The XRD pattern collected on the hexagons can be indexed to β-TCP (R3jc, a ) b ) 10.42 Å, c ) 37.38 Å; R ) β ) 90°, γ ) 120°, JCPDS

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Figure 1. (A) TEM image and the corresponding SAED pattern of ACP, the precursor, extracted from the reaction at 15 s. (B) HRTEM image of the amorphous precursor in and no crystal lattice fringe is observed. (C) SEM image of the amorphous precursor. (D, E) FT-IR spectrum and XRD pattern of the precursor.

Figure 2. SEM and HRTEM images of final β-TCP hexagon and octahedron. (A) SEM of a typical hexagon. (B) TEM of β-TCP hexagon recorded along [001] zone axis. (C, D) HRTEM images of the right and left side marked in B. (E) SEM of a typical octahedron. (F) TEM image of β-TCP octahedron recorded along the [010] direction; the angle between the adjacent surfaces is 76°. (G) HRTEM image recorded from the left side surface marked in F; the lattice fringe parallel with the outer surface is corresponding to (006) lattice plane. (H) HRTEM image recorded from the right side surface marked in F. The lattice fringe parallel with the outer surface is corresponding to (101j) lattice plane.

09-0169). Ca, P, and O elements are detected by EDS, and the measured calcium to phosphate molar ratio is 1.51 ( 0.02.3a HRTEM and SAED show that the hexagon is terminated by {100} and {001} planes (Figures 2C, 2D, and 3). The size of product increases in proportional to precursor concentration in the range from 10 mg/70 mL to 40 mg/70 mL (Figure 3). Further increase in precursor concentration (>40 mg/70 mL) has no obvious influence on the product size any more. However, it is also noted that the different concentrations of calcium (5-8 mM) and phosphate ions (2.5-4 mM) cannot result in a significant change of crystal sizes. This result implies the essential role of ACP precursor in the formation of β-TCP. The reaction temperature can influence the phase and shape of the products. β-TCP hexagons with rough {100} planes are formed at a temperature of 115 °C. At 100 °C, the hexagons with smooth apex, surface cracks, and holes (see Figure S1 in the

Supporting Information) can be resulted and they become roundlike. Thus, the crystal perfection and crystallinity are temperature-dependent. The morphology of transformed β-TCP is dependent on the solution conditions. It is abnormal but interesting that octahedrons with dimensions of 300∼400 nm can be observed in Ca(OH)2-(NH4)2HPO4-EG solution by using ACP as the starting material (Figure 2E). Even though β-TCP indexed in the space group R3jc is not expected to grow with this exceptional morphology.23 XRD experiments of samples confirm that the resulted material is β-TCP (see Figure S2 in the Supporting Information). It is found that the surface of octahedron is not atomic flat under HRTEM and SEM (Figure 2E-H). Some atomic steps can be observed on the surfaces. The lattice planes parallel to the surfaces are uniquely indexed as (006) and (101j) according to the lattice spacings, 0.622 and

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Figure 3. TEM and SEM images and SAED pattern of samples synthesized at 150 °C using different precursor amounts. The total volume of EG is 70 mL (CaCl2 and Na2HPO4 concentrations of 7.6 and 3.7 mM, respectively). (A, C) 10 mg precursor. (B) SAED pattern of the hexagon marked in A. (D, E) 15 mg precursor. (F, G) 26 mg precursor. (H, I) 40 mg precursor. This study shows that the size of hexagonal plates can be adjustable by the amorphous precursor amounts.

Figure 4. Hydroxyapatite nanorods formed after phase transformation by amorphous precursor in water. (A) TEM image of the sample. (B) Corresponding SAED pattern of this sample in A clarified the phase is hydroxyapatite.

0.877 nm, respectively. As a confirmation of these indices, the angle between the planes is 77° by calculation, which is consistent with the measured value, 76°. It is important to mention that biominerals usually have shapes that defy the strict geometric restrictions of 230 classical space groups. The symmetry breaking during the phase transformation from isotropic amorphous to anisotropic crystal is an interesting phenomenon. Its reason is unclear and further efforts will be paid for an explanation. It is well-reported that β-TCP cannot be formed in aqueous solution as the involvement of proton and hydroxyl ions.19 If the same amorphous phase is dispersed in water or calcium phosphate aqueous solution, the resulted products are the rodlike hydroxyapatite nanocrystals (Figure 4). However, the pure β-TCP phase can be synthesized in nonaqueous solvent such as EG or methanol.3a,19e EG is a solvent with relatively high

dielectric constant that can dissolve many salts. Another property of EG is its high boiling points (∼196 °C), which is suitable for synthesis of highly crystallized materials.5d Xia and coworkers have successfully controlled the shape of noble metals nanocrystals using EG.5a,c,d It is found that EG has a strong effect on mechanism of ion solvation and dissociation.3a The molar conductivity of ions in aqueous environment is approximately one order larger than that in EG, which indicates that greater activities of ions in water than in EG. It is suggested that the relatively low driving force for precipitation in EG may be beneficial to the formations of uniform crystals.6c,24 To some extent, our results have phenomenological similarities to the “gel-sol” mechanism proposed by Sugimoto.25 This mechanism is first proposed on the basis of a metal hydroxide gel to be transformed into uniform metal oxide sol through a dissolution-recrystallization process. During this process, a highly viscous metal hydroxide gel network is used as a matrix for holding the nuclei and growing particles to protect them from aggregation even in strong ionic strength conditions, and also as a reservoir of metal ions or hydroxide ions to compensate a drastically reduced supersaturation during the growth of crystal. The dissolution-recrystallization model is frequently discussed in the phase transformation of calcium minerals. However, our system shows a different pathway that the crystal nucleates directly at the precursor and its shape can be controlled just by changing the growth environment. The precursor need not to dissolve to provide nutrient for crystallization, and it can be understood by an energetic controls of the interfaces. To investigate the evolution process of the ACP precursor in EG, we withdrew the samples from the same reaction system periodically (Figure 5). The extracted mixture is quickly

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so-called dissolution-recrystallization25 can explain the revealed evolution process indicated by TEM observation as well as in the viewpoint of interfacial energy control. Figure 6A shows the initial state of the phase transformation in quite a short time (within 1 min). An extended reaction time (∼1-2 min) leads to a significant decrease in precursor amount and increase in the crystal sizes (Figure 6B). When the reaction time is prolonged to 3 min, the amorphous precursor disappears completely and only the crystals with smooth outer surfaces can be observed (Figure 6C). The sharp edge forms within 7 min. Further extension of reaction time shows no obvious improvement in crystallinity and size. According to Ostwald’s phase rule,26 the first formed phase in polymorphism is normally the one that is closest in free energy to the mother solution; that is, the least stable phase, followed by phases with increasing stability. Amorphous precursor mediated crystallization is a specific example of Ostwald’s rule that has attracted great attention. Experimentally, this mechanism is observed during the crystal growth of proteins and colloids.27 As revealed by the previous literature,28 the nucleation rate, Γ, can be represented as eq 1 Figure 5. HRTEM images of the phase transformation within 1 min. (A) At 10s, different contrasts indicate that the clusters generated among the amorphous matrix. (B) Spherical aggregates are formed in the precursor with a low contrast buffer layer at about 20 s. (C) Spherical aggregate remolded into hexagonal crystallite at about 32 s. The inset SEM image indicates that the crystallite stems from the amorphous precursor as the continuous connection between the precursor and the crystallite. (D) Sample extracted at 50 s. The hexagon grows at the expense of precursor. The inset SEM image indicates that the crystallite has an improved shape.

quenched to -16 °C to terminate the reaction, and the product is collected by centrifugation (4000 g) at -4 °C. After transformation for 10s, the amorphous solids are still continuous without significant change. However, the clusters with ∼2-5 nm among the amorphous precursor are detected inside the ACP. These initially crystallized clusters are considered to provide the nucleation and growth sites for the crystal phase. The lattice structure in these clusters can be detected and the interplanar spacing, 0.208 nm, is consistent with the crystallographic data of the (00,18) plane of β-TCP (Figure 5A). The clusters are randomly distributed in the amorphous phase as the ringlike diffraction patterns are obtained. The density and size of these clusters increase with the reaction process. At a time of 20 s, the spherical aggregates begin to form within the amorphous phase. A covering film of lower contrast on their surfaces acts as a buffer between original precursor and aggregates (Figure 5B). This buffer layer is an indication of nucleation inside the amorphous precursor. Furthermore, the spherical aggregate is remolded into crystallites with hexagonal shapes at 32 s (Figure 5C). The creation of well-faceted crystallites distorts the precursor film for the generation of stress between these two phases (inset SEM image in Figure 5C). It also shows that the crystallites and the precursors are actually integrated without any obvious boundary. Another important experimental phenomenon is that the increasing the crystallized phase is proportional to the decreasing of the amorphous one. It is also noted that in the current phase transformation system, the solid precursor, ACP, shares a similar chemical composition (Ca/P ) 1.47 ( 0.05) with β-TCP (Ca/P ) 1.51 ( 0.02) crystallites and no additional ions are required during the evolution. Thus, we conclude that the crystallite may directly nucleated by solid-solid phase transformation from the precursor (Figure 5D). The amorphous precursor epitaxial nucleation rather than

Γ ) νexp(-∆G∗ /kBT)

(1)

where ∆G* is the height of the free energy barrier separating the metastable phase from the crystal phase. The kinetic factor, ν, is a measure of the rate at which critical nuclei, once formed, transform into larger crystallites. The variation of the nucleation rate is dominated by the variation in the barrier height. The form of ∆G* can be given by the classical nucleation theory

∆G∗ ) 16πγ3 /(3F2∆µ2)

(2)

where γ is the interfacial energy per unit area of the phase interface, F is the number density of the solid phase, and ∆µ is the difference in chemical potential between the metastable phase and the crystal phase. After the addition of amorphous precursor in our system, the new equilibrium between the amorphous precursor and solution is reached. The free energy barrier, ∆G*, is directly determined by the interfacial energy, γ. The surface tension components of amorphous precursor, hexagon and octahedron are determined by wicking techniques with probing liquids of water, n-octane, ethylene glycol, and DMSO. The solid surface tension components, Lifshitz-van der Waals (γLW) and Lewis acid-base (γAB ) 2(γ+γ-)1/2)29 are obtained when Young eq 3 is solved

(1 + cos θ)γL ) 2 √γSLWγLLW + √γS+γL- + √γS-γL+

(

) (3)

where the subscripts S and L represent the solid surface and test liquids, respectively. γ+ is the Lewis acid (electron-acceptor) and γ- is the Lewis base (electron-donor) parameters. θ is the contact angle between the test liquid and solid surface. The observed contact angles of the various liquids on the ACP, hexagon, and octahedron β-TCP are listed in Table 1. The standard parameters of liquids and the calculated results of amorphous precursor, hexagon, and octahedron are given in Table 2. The total interfacial tension between two different condensed phases can be estimated from eq 4

γij )

(√γ

LW i

- √γjLW + 2 √γi+γi- + √γj+γj- - √γi+γj- 2

)

(

√γi-γj+)

(4)

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Figure 6. SEM images of the sample evolutions. (A) Initial hexagon sample in the amorphous phase at 30 s, the crystallites are indicated by the white arrows. (B) Samples at 1.2 min. The crystallites increase in both density and size; at this stage, the precursor coexists with the crystals. (C) Samples at 2.3 min. The precursor has completely disappeared and the hexagons result. (D) The 3D atomic model of β-TCP hexagon. (E) Initial octahedrons at 30 s; their morphology is spherelike. (F) Octahedron sample at 2.5 min, The white arrow indicates the crystallites, but they are not fully developed; octahedral shape can be observed at this stage but the amorphous precursor still exists. (G) Sample at 90 min; the uniform octahedral crystals formed. The inset is a high-magnification image of the β-TCP octahedron. (H) The 3D atomic model of β-TCP octahedron. Table 1. Contact Angles of Probing Liquids on Amorphous Precursor, β-TCP Hexagon, and Octahedron Surfaces at 20 °C and Relative Humidity of 70% ACP DMSO EG n-octane water CaCl2-Na2HPO4-EG Ca(OH)2-(NH4)2HPO4-EG

hexagon

21.9 ( 3.0 22.6 ( 2.8 0 21.6 ( 3.6

19.4 16.3 0 ≈0 19.3 18.7

( 4.1 ( 0.7 ( 0.9 ( 0.6

octahedron 19.0 22.8 0 18.8 25.5 22.4

( 0.5 ( 2.0 ( 2.7 ( 0.6 ( 3.5

Table 2. Surface Tension Components of Different Solvents and Parameters of Amorphous Precursor, β-TCP Hexagon, and β-TCP Octahedron Determined by Wicking Method at 20 °C (mJ/m2) DMSO EG n-octane water CaCl2-Na2HPO4-EG Ca(OH)2-(NH4)2HPO4-EG ACP hexagon octahedron

γ

γLW

γAB

γ+

γ-

44.00 48.00 21.62 72.80 48.45 48.12 45.63 47.38 45.98

36.00 29.00 21.62 21.80

8.00 19.00 0 51.00

0.50 1.92 0 25.50

32.00 47.00 0 25.50

21.86 21.79 22.03

23.77 25.59 23.95

2.14 2.21 2.12

65.87 74.11 67.62

The interfacial energies between the amorphous precursor and β-TCP hexagon (γAm-Hex), amorphous precursor and β-TCP octahedron (γAm-Oct) are 0.026 and 0.006 mJ/m2, respectively, which are calculated from eq 4 using the data in Table 2. In contrast, the interfacial energy between β-TCP hexagon and CaCl2-Na2HPO-EG solution (γSH-Hex), β-TCP octahedron and Ca(OH)2-(NH4)2HPO4-EG solution (γSO-Oct) are 1.65 and 1.50 mJ/m2, respectively, by using eq 5 and data in Tables 1 and 2

γsolid-solution ) γsolid - γsolutioncos θ

according to eq 2. Thus, the nucleation of β-TCP in the amorphous precursor is thermodynamically preferred as shown in eq 1. The amorphous precursor epitaxial nucleation process occurs during the formation of both hexagon and octahedron. Interestingly, the life spans of amorphous precursor in these two solutions are quite different. The amorphous precursor disappeared at 2.3 min in the formation of hexagon (Figure 6C). In the case of octahedron formation, the precursor still exists widely at the reaction time of 2.5 min (Figure 6F). Besides, the interfacial energies between the growth solutions and crystals with different shapes are quite different. The interfacial energies between octahedron and CaCl2-Na2HPO4-EG solution (γSH-Oct), hexagon, and Ca(OH)2-(NH4)2HPO4-EG solution (γSO-Hex) are 2.24 and 1.80 mJ/m2, respectively. It is mentioned that γSO-Hex is larger than γSH-Hex and γSH-Oct is larger than γSO-Oct. Therefore, both formations of octahedron in CaCl2-Na2HPO4-EG solution and hexagon in Ca(OH)2-(NH4)2HPO4-EG solution are thermodynamically unfavorable for their relatively large interfacial energies. This also indicates that the final morphology of crystal is determined by the crystal-solution interfacial energies, because the amorphous precursor disappears eventually. Only the crystal-solution interfaces are present at the end of phase transformation. Therefore, the amorphous precursor alters the kinetic evolution pathway instead of changes the thermodynamically stable shape of product. Furthermore, our attachment energy calculation of β-TCP without any additives have selected out the lattice planes with the lowest attachment energy, that is, the planes with the lowest growth rate that determine the final morphology.30 The lattice planes that enclose the hexagon and octahedron have the lowest attachment energies in the plane list of β-TCP phase, as shown in Table 3.

(5)

The interfacial energy between crystal and amorphous precursor (γAm-Hex or γAm-Oct) is lower compared to that between crystal and solution (γSH-Hex or γSO-Oct) by about two magnitudes. Hence, the free energy barrier between amorphous precursor and β-TCP is far lower than that between β-TCP crystals and solutions

Conclusion In summary, β-TCP crystals with different morphologies and sizes are synthesized in an organic solvent using ACP as the starting material. In this method, the resulted nano octahedrons can be even against the classical crystal symmetry of β-TCP. It

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Table 3. Lattice Planes with the Lowest Attachment Energy, Where Eatt is the Attachment Energy; These Planes Were Used to Construct the Surface of Crystals Together with the HRTEM Lattice Images crystal face index (010), (01j0) (11j0), (1j10) (100), (1j00) (110), (1j1j0) (12j0), (1j20) (21j0), (2j10) (001), (001j) (101j), (1j01) (11j1), (1j11j) (011), (01j1j)

Eatt (kcal/mol)

45.19 57.97 60.96 100.27 100.29 100.31

is found that the crystallization of crystalline phases can occur and develop directly within the ACP phases because of the lower interfacial energies between the solids. However, the final shape of crystals is controlled by alteration of crystal-solution interfacial energy. The crystallized phase can also be controlled by intervention the ACP precursor epitaxial crystallization by different temperatures and precursor concentrations, etc. This study suggests that a combination of amorphous precursors and energetic controls can provide a novel strategy of material manufacture and its mechanism may be applied in the studies of biomineralization. Acknowledgment. We thank Dr. Dexi Zhu and Prof. Hui Ye for their assistance in the examinations. This work was supported by National Natural Science Foundation of China (20571064 and 20601023) and Cheung Kong Scholars Program (RT). Supporting Information Available: Samples synthesized at different temperatures (Figure S1), XRD patterns of hexagon and octahedron β-TCP crystals (Figure S2) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (a) Burda, C.; Chen, X.; Narayanan, R.; EI-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (b) Sugimoto, T. Chem. Eng. Technol. 2003, 26, 313. (2) (a) Ren, J.; Tilley, R. D. J. Am. Chem. Soc. 2007, 129, 3287. (b) Tian, N.; Zhou, Z.; Sun, S.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (3) (a) Tao, J.; Jiang, W.; Zhai, H.; Pan, H.; Xu, X.; Tang, R. Cryst. Growth Des. 2008, 8, 2227. (b) Pan, H.; Tao, J.; Yu, X.; Fu, L.; Zhang, J.; Zeng, X.; Xu, G.; Tang, R. J. Phys. Chem. B 2008, 112, 7162. (4) (a) Dos Santos, E. A.; Farina, M.; Soares, G. A.; Anselme, K. J. Mater. Sci: Mater. Med. 2008, 19, 2307. (b) Yin, X.; Stott, M. J. J. Chem. Phys. 2006, 124, 124701. (5) (a) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (b) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. Nat. Mater. 2007, 6, 692. (c) Xiong, Y.; Cai, H.; Wiley, B. J.; Wang, J.; Kim, M. J.; Xia, Y. J. Am. Chem. Soc. 2007, 129, 3665. (d) Wiley, B. J.; Sun, Y.; Mayers, B.; Xia, Y. Chem.sEur. J. 2005, 11, 454. (e) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (6) (a) Peng, X. AdV. Mater. 2003, 15, 459. (b) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (c) Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664. (7) (a) Hinotsu, T.; Jeyadevan, B.; Chinnasamy, C. N.; Shinoda, K.; Tohji, K. J. Appl. Phys. 2004, 95, 7477. (b) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (c) Chen, M.; Kim, J.; Liu, J. P.; Fan, H.; Sun, S. J. Am. Chem. Soc. 2006, 128, 7132. (8) (a) Beniash, E.; Aizenberg, J.; Addadi, L.; Weiner, S. Proc. R. Soc. London, Ser. B 1997, 264, 461. (b) Politi, Y.; Arad, T.; Klein, E.; Weiner, S.; Addadi, L. Science 2004, 306, 1161. (9) Addadi, L.; Raz, S.; Weiner, S. AdV. Mater. 2003, 15, 959.

Tao et al. (10) Weiss, I. M.; Tuross, N.; Addadi, L.; Weiner, S. J. Exp. Zool. 2002, 293, 478. (11) (a) Lowenstam, H. A.; Weiner, S. Science 1985, 227, 51. (b) Mahamid, J.; Sharir, A.; Addadi, L.; Weiner, S. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 12748. (c) Tao, J.; Pan, H.; Wang, J.; Wu, J.; Wang, B.; Xu, X.; Tang, R. J. Phys. Chem. C 2008, 112, 14929. (d) Le´ve´que, I.; Cusack, M.; Davis, S. A.; Mann, S. Angew. Chem., Int. Ed. 2004, 43, 885. (12) Dorozhkin, S. V.; Epple, M. Angew. Chem., Int. Ed. 2002, 41, 3130. (13) (a) van Haaren, E. H.; Smit, T. H.; Phipps, K.; Wuisman, P. I. J. M.; Blunn, G.; Heyligers, I. C. J. Bone Joint Surg. 2005, 87-B, 267. (b) Ogose, A.; Hotta, T.; Kawashima, H.; Kondo, N.; Gu, W.; Kamura, T.; Endo, N. J. Biomed. Mater. Res. 2004, 72B, 94. (c) Fujita, R.; Yokoyama, A.; Nodasaka, Y.; Kohgo, T.; Kawasaki, T. Tissue Cell 2003, 35, 427. (d) Guo, X.; Wang, C.; Zhang, Y.; Xia, R.; Hu, M.; Duan, C.; Zhao, Q.; Dong, L.; Lu, J.; Song, Y. Tissue Eng. 2004, 10, 1818. (14) Lee, Y. M.; Park, Y. J.; Lee, S. J.; Ku, Y.; Han, S. B.; Choi, S. M.; Klokkevold, P. R.; Chung, C. P. J. Periodontol 2000, 71, 410. (15) (a) Sous, M.; Bareille, R.; Rouais, F.; Cle´ment, D.; Ame´de´e, J.; Dupuy, B.; Baquey, C. Biomaterials 1998, 19, 2147. (b) Takahashi, Y.; Yamamoto, M.; Tabata, Y. Biomaterials 2005, 26, 3587. (16) Laffargue, P.; Fialdes, P.; Frayssinet, P.; Rtaimate, M.; Hildebrand, H. F.; Marchandise, X. J. Biomed. Mater. Res. 2000, 49, 415. (17) (a) Gineba, M. P.; Traykova, T.; Planell, J. A. J. Controlled Release 2006, 113, 102. (b) Paul, W.; Sharma, C. P. J. Biomater. Appl. 2003, 17, 253. (c) Donker, H.; Smit, W. M. A.; Blasse, G. J. Electrochem. Soc. 1989, 136, 3130. (d) Legrouri, A.; Lenzi, J.; Lenzi, M. React. Kinet. Catal. Lett. 1998, 65, 227. (18) (a) Yashima, M.; Sakai, A.; Kamiyama, T.; Hoshikawa, A. J. Solid State Chem. 2003, 175, 272. (b) Bigi, A.; Foresti, E.; Gandolfi, M.; Gazzano, M.; Roveri, N. J. Inorg. Biochem. 1997, 66, 259. (c) Pan, Y.; Huang, J.; Shao, C. Y. J. Mater. Sci. 2003, 38, 1049. (d) Wei, X.; Akinc, M. J. Am. Ceram. Soc. 2007, 90, 2709. (e) Belik, A. A.; Izumi, F.; Stefanovich, S. Y.; Malakho, A. P.; Lazoryak, B. I.; Leonidov, I. A.; Leonidova, O. N.; Davydov, S. A. Chem. Mater. 2002, 14, 3197. ¨ zgu¨r Engin, N.; Cu¨neyt Tas, A. J. Am. Ceram. Soc. 2000, 83, (19) (a) O 1581. (b) Gibson, I. R.; Rehman, I.; Best, S. M.; Bonfield, W. J. Mater. Sci.:Mater. Med. 2000, 11, 533. (c) Kannan, S.; Ventura, J. M.; Ferreira, J. M. F. Ceram. Int. 2007, 33, 637. (d) Kwon, S.; Jun, Y.; Hong, S.; Kim, H. J. Eu. Ceram. Soc. 2003, 23, 1039. (e) Bow, J.; Liou, S.; Chen, S. Biomaterials 2004, 25, 3155. (20) Koutsopoulos, S. J. Biomed. Mater. Res., A 2002, 62, 600. (21) (a) Posner, A. S.; Betts, F. Acc. Chem. Res. 1975, 8, 273. (b) Betts, F.; Blumenthal, N. C.; Posner, A. S.; Becker, G. L.; Lehninger, A. L. Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 2008. (c) Levi-Kalisman, Y.; Raz, S.; Weiner, S.; Addadi, L.; Sagi, I. AdV. Funct. Mater. 2002, 12, 43. (d) Levi-Kalisman, Y.; Raz, S.; Weiner, S.; Addadi, L.; Sagi, I. J. Chem. Soc., Dalton Trans. 2000, 3977. (e) Politi, Y.; Levi-Kalisman, Y.; Raz, S.; Wilt, F.; Addadi, L.; Weiner, S.; Sagi, I. AdV. Funct. Mater. 2006, 16, 1289. (22) Saida, J.; Matsushita, M.; Inoue, A. J. Appl. Phys. 2001, 90, 4717. (23) Grassmann, O.; Neder, R. B.; Putnis, A.; Lo¨bmann, P. Am. Mineral. 2003, 88, 647. (24) Jiang, X.; Herricks, T.; Xia, Y. AdV. Mater. 2003, 15, 1205. (25) (a) Sugimoto, T.; Sakata, K. J. Colloid Interface Sci. 1992, 152, 587. (b) Sugimoto, T.; Sakata, K.; Muramatsu, A. J. Colloid Interface Sci. 1993, 159, 372. (c) Sugimoto, T.; Muramatsu, A. J. Colloid Interface Sci. 1996, 184, 626. (d) Sugimoto, T.; Wang, Y. J. Colloid Interface Sci. 1998, 207, 137. (e) Sugimoto, T. J. Colloid Interface Sci. 2007, 309, 106. (26) Ostwald, W. Z. Phys. Chem. 1897, 22, 289. (27) (a) Kuznetsov, Y. G.; Malkin, A. J.; McPherson, A. J. Cryst. Growth 2001, 232, 30. (b) Vekilov, P. G. Cryst. Growth Des. 2004, 4, 671. (c) Chen, X.; Samia, A. C. S.; Lou, Y.; Burda, C. J. Am. Chem. Soc. 2005, 127, 4372. (d) Lutsko, J. F.; Nicolis, G. Phys. ReV. Lett. 2006, 96, 046102. (e) Zhang, T. H.; Liu, X. Y. J. Am. Chem. Soc. 2007, 129, 13520. (28) Ten Wolde, P. R.; Frenkel, D. Science 1997, 277, 1975. (29) (a) Wu, W.; Giese, R. F., Jr.; van Oss, C. J. Langmuir 1995, 11, 379. (b) Wu, W.; Nancollas, G. H. AdV. Colloid Interface Sci. 1999, 79, 229. (30) Berkovitch-Yellin, Z. J. Am. Chem. Soc. 1985, 107, 8239.

CG801130W