Structural Components and Anisotropic Dissolution Behaviors in One

Publication Date (Web): June 5, 2008 ... dissolution behaviors of these two parts in one single crystal resulted in the formation of porous, gearlike,...
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Structural Components and Anisotropic Dissolution Behaviors in One Hexagonal Single Crystal of β-Tricalcium Phosphate Jinhui Tao, Wenge Jiang, Halei Zhai, Haihua Pan, Xurong Xu, and Ruikang Tang* Department of Chemistry and Center for Biomaterials and Biopathways, Zhejiang UniVersity, Hangzhou, 310027, P. R. China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2227–2234

ReceiVed August 27, 2007; ReVised Manuscript ReceiVed NoVember 20, 2007

ABSTRACT: Large-scale β-tricalcium phosphate (β-TCP) hexagonal single crystals were synthesized at a relatively low temperature (150 °C) by using a solution-phase method. The solvent, ethylene glycol, played an important role during the formation of the homogeneous submicron-sized crystals. Unlike the conventional understanding of a single crystal, the wall of the formed β-TCP hexagonal was well crystallized, showing different physicochemical properties from the bulk part. The dissolution spots were anisotropically distributed throughout the single crystal. The bulk part dissolved readily from the top and bottom planes in the undersaturated solutions, but the thin hexagonal wall could be stable against any dissolution even in pure water. These differences between the wall and the bulk part were attributed to the different crystallinities and defect densities in their structures. It was suggested that the low defect number might stem from the solvent-interface exchange that was allowed the edge surfaces in contact with the solution. And the rapid growth of the particles resulted in the randomly distributed defects in the bulk part, which induced a selective dissolution along the c-axis of β-TCP. Furthermore, the stability of wall could be explained by a size effect during the nanodemineralization. It was interesting that both the wall and the bulk part shared the exact same lattice fringes under the transmission electron microscope. This phenomenon implied that both components were crystallographically identical so that they were constructed into an integral single crystal of β-TCP. The distinct dissolution behaviors of these two parts in one single crystal resulted in the formation of porous, gearlike, and ringlike single crystals at different demineralization stages, which demonstrated an easy control of crystal morphology patterns by using the anisotropic dissolution behavior. Introduction Because of the dependence of physical and chemical properties on the size, morphology and microstructure of materials,1–3 controllable synthesis of nanocrystals with various shapes and structural complexities with high precision presents a great challenge in nanosized materials synthesis.4–7 The morphology control of single crystals of natural minerals such as calcium carbonates and calcium phosphates is also an essential characteristic of biomineralization.8–11 The precise control of crystals is intensively investigated in biominerals.12–16 Many organisms shows exceptional control over the gross morphology, physical properties, and nanoscale organization of biomaterials, creating shapes that defy strict geometrical restrictions.8,10,13–16 A remarkable category of biominerals is the single crystal with complex form although they have the complicated structures.8,14,15 Inspired by biomineralization, various approaches have been developed to the large-scale control of structures and morphologies of nanoparticles, mainly by altering additives or solvents,17–19 template-aided synthesis,20–24 and self-assembly.25 These methods usually include relatively complicated operations, low yields, or poor controllability in uniformities and shapes. Besides biomineralization, it is also noted that biodemineralization is another useful strategy in the control of single crystals in living systems. Here, we demonstrate that polymorph control of β-tricalcium phosphate (β-TCP) can be conveniently achieved by an anisotropic dissolution behavior of the hexagonal single crystals. A series of the derivative morphologies including porous, gearlike, and ringlike are achieved at different time scales of demineralization. β-TCP is an important biomineral since it has potential applications in bone grafting, calcium phosphate cements and * Corresponding author: Department of Chemistry, Zhejiang University, Hangzhou, 310027, China, Tel/fax: +86-571-87953736. E-mail: rtang@ zju.edu.cn.

surgical implants.26 In the present work, we report that the hexagonal single crystals of β-TCP are first synthesized by using a solution method under a relatively low temperature. Unlike the conventional understanding of a single crystal, the crystallinities of six edges and the bulk part in as-prepared β-TCP are different although their chemical compositions, phases, and crystallographic structures are exactly identical. The improved crystallinity and thin thickness of the edge wall can protect this part against dissolution reaction in water even though the bulk part is completely etched. It shows that the anisotropic dissolution of the structural complex in one single crystal can result in an easy but effective control of morphologies of the single crystal. Experimental Section The hexagonal β-TCP plates were synthesized by a solution-phase method. Ethylene glycol was used as solvent and CaCl2 and Na2HPO4 were used as calcium and phosphate sources for the precipitation, respectively. 0.10 g CaCl2 · 2H2O was mixed with 50 mL of ethylene glycol and the slurry was heated to 150 °C under vigorous magnetic stirring. A mixed aqueous solution of 1.36 mL of 0.3 M Na2HPO4 and 120 µL of 1.3 M NaOH was added to 20 mL of ethylene glycol at a temperature of 95 °C. The phosphate-containing ethylene glycol solution was added dropwise into the calcium containing ethylene glycol solution at a rate of 20 mL/min. The mixture was bathed at 150 °C for 90 min and then was cooled in air. The solids were separated by centrifugation at 2000g and were washed using ethanol and water alternatively 3 times to remove the residual solvent or other impurities. The products were dried under a vacuum condition at 30 °C. The chemical compositions and structures of the solids were characterized by chemical analysis (atomic adsorption for calcium and UV for phosphate). The molar conductivities of CaCl2 and Na2HPO4 in water and in ethylene glycol were also examined to discuss the roles of solvent in the formation of β-TCP. In the demineralization experiments, 1.5 mg of solids was dispersed into 50 mL of water (pH ) 7.0) under a stirring condition. One milliliter slurry samples were withdrawn at different experimental periods. The

10.1021/cg700808h CCC: $40.75  2008 American Chemical Society Published on Web 06/05/2008

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Figure 1. SEM micrographs of samples extracted at different time scales. (a) SEM of the synthesized hexagonal plates of β-TCP, the side view of plates could give thickness information (white circle and inset); the other inset is the magnified image of the plate indicated by the white arrow, which shows the pits on the surface (arrows). (b) Samples after demineralization for 21 h. The density and size of the pits increased obviously; some of them even passed throughout the plate to form the holes. The inset image is the magnification of the plate denoted by the white arrow. (c) Samples after demineralization for 12 days. Only the rings survived, and they had the same dimensions as the solid plates. The magnified graph of the single ring indicated by the white arrow is shown as the inset. (d) XRD pattern of hexagonal solids, all the peaks could be assigned to β-TCP. The XRD pattern of hollow rings was exactly the same (Figure S4). solids were separated by centrifugation (10000g). In order to investigate the effects of undersaturation on the dissolution of β-TCP, a parallel experiment was performed by using a low content (0.015 mg) of seeds to increase the final undersaturation level. Some synthesized β-TCP crystallites were also heated to 500 °C in the presence of flowed air to examine the influence of calcination and organic residuals on the dissolution kinetics. All the solids were examined by using a JEM-200CX (JEOL, Japan) transmission electron microscope (TEM) and a JEM-2010HR (JEOL, Japan) high resolution TEM (HRTEM). Scanning electron microscopy (SEM) was performed using a S-4800 field-emission scanning electron microscope (HITACHI, Japan). The samples were also measured by a Nanoscope IVa atomic force microscope (AFM, Veeco). The phase of the solids was examined by a D/max-2550pc XRD (Rigaku, Japan) with monochromatized Cu KR radiation at the working voltage of 40 kV, and the scanning step was 0.02°.

Results and Discussion The phase of the obtained solid was examined by X-ray diffraction (XRD, Figure 1). All the peaks could be well indexed by using the standard card of β-TCP (JCPDS: 09-0169, a ) b ) 10.42 Å, c ) 37.38 Å; R ) β ) 90°, γ ) 120°; space group of R3jc (167), Figure S4, Supporting Information). The result of chemical analysis showed that the atomic molar ratio of calcium to phosphate of the solids was 1.51 ( 0.02, which was consistent with the stoichemical value of ideal β-TCP, 1.50. These results confirmed that we obtained β-TCP crystals by using a feasible, large-scale, and controllable synthesis method in the laboratory. β-TCP was widely used as the calcium phosphate bone cement in biomedical areas. The other important applications of this compound included fertilizers, polishing, dental powders, porcelains, pottery, and animal food supple-

ments. In the previous literature,26 it was widely accepted that β-TCP could only be obtained by calcination of calcium deficient hydroxyapatite at temperature above 800 °C. The previously synthesized β-TCP crystallites had the irregular morphologies and nonuniform sizes.27 However, our preparation was performed at a much lower temperature (150 °C) in ethylene glycol. The formed β-TCP crystals were hexagonal plates, and their sizes could be well controlled. This new method provided a convenient but effective pathway to prepare β-TCP crystallites. It was believed that the solvent, ethylene glycol, played a key role in the crystallization. The molar conductivity of CaCl2 and Na2HPO4 in water and in ethylene glycol was measured (Figure S1, Supporting Information). The curves indicated that the amounts of free calcium and phosphate ions in the aqueous solution were significantly greater than those in ethylene glycol. Besides, the influence of electrolyte concentration on its molar conductivity in ethylene glycol was negligible since the molar conductivities of CaCl2 and Na2HPO4 were almost unchanged in Figure S1. This result indicated that ethylene glycol provided a medium for the controlled release of free calcium and phosphate ions from their electrolyte solids. Thus, a low but stable driving force was maintained during the precipitation of β-TCP in the ethylene glycol solvent, which promoted the formation of the well-crystallized crystals. The obtained β-TCP were examined by SEM, TEM, and AFM. A typical SEM of the as-prepared samples is shown in Figure 1a. It can be seen that the hexagonal plates had the size distribution of 750-800 nm. The thickness of the plates, 200-250 nm, was measured by their side view (Figures 1 and S2, Supporting Information). The result of selected area electron

Anisotropic Dissolution of β-TCP

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Figure 2. HRTEM studies of hexagonal β-TCP plates. (a) A single hexagonal plate and its corresponding SAED recorded along the [001] zone axis. Three different sites (circles) were used for the measurement of the lattice structures. (b) Magnified TEM image of site 1. (c) Magnified TEM image of site 2. (d) Magnified TEM image of site 3. The lattice fringes of {110} planes (d ) 0.52 nm) and {300} planes (d ) 0.30 nm) can be seen.

diffraction (SAED) indicated that the top/bottom surface of the hexagonal plate was identical to (001) facets of β-TCP. The diffraction dots and their 6-fold symmetry showed that the whole plate was a single crystal (Figure 2a), which was also supported by the direct measurements of their lattice structures (Figures 2b, 2c, and 2d). Different from the nature of the perfect single crystals, the structure of the β-TCP hexagonal plate was not consistent, for each plate of β-TCP, the six thin sides acted as a wall to wrap the inside part, the bulk. This structural complex was well displayed by a demineralization reaction of the solids (Figure 1), which showed the distinct behaviors of two components of the crystal. The dissolution phenomena clearly implied that the edge wall and the bulk part might have different physicochemical properties although they were in one single crystal. The differences in contrast under bright-field TEM image (Figure 2a) indicated that the internal texture of the bulk part was actually not uniform, which might be caused by the different crystallinity or thickness. By using SEM, it was noted that the surface of the bulk part was not perfect too and some pits were present (black arrows, inset of Figure 1a). As the previous understanding,28,29 these pits could provide the active sites to initiate crystal dissolution. Thus, the spontaneous demineralization of the plate surface occurred spontaneously when an undersaturated medium, e.g. water, was introduced. When the particles were immersed into water for 21 h (free drift dissolution), the pits extended to contribute to the dissolution reaction. However, the kinetic rates of these pit developments

were anisotropic. It seemed that their dissolution directions were more preferred along the c-axis to penetrate the plates. As a result, the dissolution holes were formed (Figure 1b). Actually, a similar selective dissolution process had been reported and explained in a demineralization model of dental enamel,28b in which the etched enamel surfaces only developed along the c-axes of hydroxyapatite. Furthermore, the resulting pits and holes on the β-TCP were almost irregular, e.g. the density, morphology, and size of the pits and holes, resulting in various porous structures (Figures 1b, 3a, and 3d). This phenomenon also implied the random and heterogeneous internal texture of the bulk part of the β-TCP hexagonal plates. During the dissolution process, the layered structure of the bulk during the dissolution was also revealed (Figure 3b). It could be found that each layer had the same crystallographic lattice structure and orientations. The layers packed along the c-axis. This ordered texture was another proof to confirm that the single crystal structure was formed in the bulk part. Although the dissolution spots on the plates were random, it was interesting to note that no dissolution occurred on the six edges. Figure 3d clearly showed that the wall structure was maintained well in the partially dissolved plates. In contrast, the conventional crystal dissolution model described that the edges should be more readily dissolved since they provided more natural dislocation sources. When the dissolution reaction was extended to 62 h, the porouslike β-TCP crystallites evolved into the gearlike rings (Figure 4a). At this stage, most of the bulk part disappeared

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Figure 3. HRTEM images of the β-TCP samples with dissolution period of 21 h. (a) Morphology and SAED pattern (along [001]) of a hexagonal plate with partial dissolution. (b and c) The enlarged TEM image of the sites denoted by 1 and 2 in (a), respectively. The layered structure of the bulk part was shown in (b). The detailed structure with defects of the bulk was detected on a remaining thin layer. (d) Partially dissolved hexagonal plates; the dissolution period extended to 2 days in this case.

Figure 4. SEM image of the β-TCP samples with dissolution of 62 h in water. (a) Most materials were etched but the sites at the six vertexes of the hexagon were still present against the dissolution. The gearlike morphology of β-TCP single crystal was formed. (b) The curves of F(θ) against θ for the concave corner (green) and the flat plane (blue).

and the hexagonal crystals became hollow. Again, it was emphasized that the six edges and the wall structure remained without any dissolution. Another interesting phenomenon was that the β-TCP compounds in all six concave corners of the hexagon were also not dissolved, implying that the demineralization was somehow retarded at these sites. Actually, Figure 3d showed that the six corners were also against dissolution reaction in the intermediate state. It could be understood by using a thermodynamical model of the growth/dissolution on

different crystal substrates. Analogous to crystallization, the energy barrier, ∆g*, of dissolving a crystal unit could be given by eq 1,28c

∆g/ )

16πγSL3 3∆gv2

F(θ)

(1)

where ∆gv was the change of free energy per unit volume before and after dissolution, γSL, the nucleus-liquid interface energy,

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Figure 5. HRTEM images of the hollow rings at the end of dissolution (12 days). (a) The remaining rings. (b) The SAED pattern of the rings along the [001] zone axis, showing that the plate had a top/bottom (001) surface and outer (100) surface. (c) The detailed structure of the boundary of wall and bulk (white circle in a). (d) The lattice fringes of the edge wall (dark circles in a).

and F(θ), a function of shape of crystal face and contact angle of the unit and substrate. For the dissolution cases, F1(θ) on the flat crystal face could be described by eq 2.

1 F1(θ) ) - (2 - 3 cos θ + cos3 θ) 4

(2)

At the concave corners (the angle was set as 120°), F2(θ) was much more complicated as a description by Trivedi and Sholl,30,31

{

(

)

√3 1 2sin2 θcosθcos-1 cotθ + 4π 3 √3 2√3 2 1 cos θ sin2 θ - cos2 θ - 4cosθcos-1 cotθ + 3 3 3 1 (3) cos-1 2sinθ

F2(θ) ) -

√

(

) ( )}

therefore, a difference of the energy barrier at the concave corner, ∆g2/ to that on the flat surface, ∆g1/could be represented by

∆g/2 - ∆g/1 )

16πγSL3 3∆gv2

{F2(θ) - F1(θ)}

(4)

and a curve of F(θ) vs θ was also illustrated in Figure 4b. It was noted that F1(θ) was always less than F2(θ) within a range of all contact angle zone. The curves implied that, under the same experimental condition, the dissolution barrier at the concave corner was always greater than that in the bulk or on

the edge. Besides the wall itself, the sites around the hexagonal corners of the wall were more difficult to be dissolved. Thus, the formation of the gearlike structure could be understood. Unlike the wall, which was really stable against the dissolution, the remained β-TCP at the corner sites could be dissolved eventually with the reaction time. At the end of dissolution (12 days), the hexagonal dentations almost disappeared and only the six edges survived, forming the hexagonal ring (Figures 1c and 5). Most of the rings could keep their hexagonal structures without any deformation. No obvious dissolution was detected even that the resulting rings were redispersed in pure water. The sizes of the hollow rings were 750-800 nm, and the heights were 200-250 nm (Figure S3, Supporting Information), which were in good agreement with the dimensions of the original solid hexagons of β-TCP. The chemical composition and phase of the remaining rings were also checked by using XRD (Figure S4) and SAED (Figure 5b). The results confirmed that the remaining walls were still pure β-TCP and there was no detectable phase transformation during the reaction. Thus, it was surprising that the wall and the bulk have different dissolution properties even though they are identical in the crystal. By increasing the undersaturation level in the demineralization solution, similar dissolution results could be observed (Figure S5, Supporting Information). However, a promoted dissolution rate of the bulk part was detected since the hollow hexagonal rings could be obtained within only 5 days. This experiment indicated that the anisotropic dissolution behaviors could not be affected by the change of undersaturation.

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Figure 6. Dark-field TEM images of the β-TCP plate along the different zone axes: (a) side view, (b) top view. The insets show their corresponding diffraction pattern, “o” indicates the transmitted beam, and the white arrows indicate the diffracted beam of the (110) face, which was used for the dark-field imaging.

In order to reveal the structural difference of the different parts in the β-TCP single crystal, the solid hexagonal plates, hollow rings, and their intermediate states were studied by HRTEM. The lattice parameters at the different sites on the top/bottom surfaces were examined. However, they had the same crystallographic structure and orientation as shown in Figure 2b. The interplanar distance (d-spacing), 0.52 nm, was attributed to the (110) face of β-TCP. Together with the SAED pattern, the orientation of the single crystal could be confirmed. The d-spacings of two different edges of the hexagon (sites 2 and 3 in Figure 2a) clearly showed that all six side faces of the walls were assigned to the {100} crystal face group. Since β-TCP has the space group of R3jc, the marked faces, (100) and (11j0), were actually equivalent. Besides, the planes of (21j0) and (1j20) belonged to the {110} group too, and (300) was identical to (33j0). The typical included angles of the hexagonal structure, 120°, could be obtained by using these lattice directions (Figures 2c and 2d). It could be found that the two neighboring edges shared an integral and continuous lattice structure as their lattice fringes could match with each other well. The study of the other sides reached the same conclusion. Thus, the whole hexagonal wall was constructed by six equivalent {100} thin crystal planes of β-TCP, and it could be treated as a complete hollowed hexagonal single crystal. This suggested model was also confirmed by the SAED result of the rings (Figure 5b). The 6-fold-symmetry of the diffraction patterns of the wall showed a typical pattern of the hexagonal single crystal of β-TCP. The lattice structure of the bulk part (Figure 2b) coincided in that of the edges (Figures 2c and 2d) too. Both the wall and the bulk part shared the identical crystallographic structure and orientation in a hexagonal plate, e.g. the in situ measured (110) faces in the bulk part (Figures 2b) and that in the wall (Figure 2d) were exactly the same, which agreed with the features for a single crystal of β-TCP. This conclusion was also confirmed by the HRTEM image recorded from the inner edge (Figure 5c). The coexistence of wall (dark area) and the remaining part (light area) provided an opportunity to study their interface in detail. Although the boundary of the wall and the bulk was obvious, their lattice structure (d-spacing) could be attributed to (110) and (12j0) in one single crystal, respectively. The lattice structures of the wall and bulk under HRTEM clearly showed that the complex of them was an integral single crystal. In some cases, the distinct dissolution behaviors were due to the different crystallographic orientation of the crystals. However, this explanation could not be applied in the present case of β-TCP

dissolution as the wall and bulk part had the same crystallographic structure. It had been mentioned that the internal texture of the bulk part was not uniform, which implied that the bulk part was not perfect. During the dissolution, the detailed structure of the center part could be studied by their remaining thin layer. A high density of defects of the bulk was demonstrated in the lattice fringe image of these thin layers (Figure 3c). The dislocation lines and the lattice-disordered regions were marked by the lines and the arrows. In some domains, there was no lattice fringe and it was an indication of the uncontinuous crystal structures. However, such defects were rarely detected in the wall structure. Figure 5c showed the consistence of the wall structure and some remaining bulk fragments. The domains with the discontinued lattice structure were separated by the dotted lines. All the marked lines were in the bulk part (light region). In contrast, the lattice structure of the wall (dark region) was almost perfect. Moreover, the continuous and complete lattice fringes at the other sites of the wall were demonstrated clearly in Figure 5d, which confirmed the perfection of the wall structure. In order to observe the overall dislocation distributions in the whole hexagonal plates, the dark field TEM images along the [001] and the [100] zone axes were recorded (Figure 6), and the diffracted beam of (110) indicated by the white arrows in SAED patterns was used for the imaging. A perfect single crystal should be shown by a uniformly bright image due to its consistent lattice structure. However, dark lines or dark regions appeared if the crystal contained dislocations for the bending of lattice planes in the strain field, which caused the local changes in the Bragg conditions. It was noted that such dislocations were frequently observed in the bulk part and on the border between the bulk part and wall (indicated by the arrows). The distribution of these dislocations was also random in the bulk part. This feature could explain why the dissolution process initiated randomly on the face of bulk (Figure 1b). The relative uniformity in brightness in the wall structure suggested the low density of the dislocations. It was also interesting to find that the width of this bright region, 30-40 nm, was similar to the thickness of the resulting rings after the demineralization. The difference in the crystallinities of the wall and the bulk part might be caused by the fast formation of hexagonal β-TCP during the preparation. The nuclei of the hexagonal plates were formed within only two minutes (Figure S6, Supporting Information). During such a rapid process, the internal structure

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Figure 7. AFM height image of hexagonal plates. (a) AFM image shows the smooth edge (wall surface) of a single hexagonal plate. (b) The rough top (001) surface of the bulk contains many domains in the size of 20-60 nm.

of the plate could not be well organized and the defects resulted. However, as the outer surfaces contacted with the reaction medium, the precipitated ions on the surfaces had the opportunity to exchange with the reaction solution at the solid-liquid interfaces. The lattice structure could be reorganized during an aging period so that the crystallinity of the wall could be improved. Figure S6 shows that the smooth edges of the plates evolved within five minutes. However, this reorganization effect only occurred at the interface and it could not penetrate into the bulk. Thus, the formed defects were proposed to be “kinetically trapped” within the bulk part. This rapid growth induced defect formation phenomenon had been previously observed in other crystal system such as KDP.32,33 The dark field TEM image recorded by the diffraction of (110) faces in Figure 6b indicated that the six side surfaces were different from the central part. The brightness of the side surfaces was much stronger and more uniform than the top/bottom surfaces, indicating the well-crystallized structure of the edge wall. The curves and holelike lines in the bulk part demonstrated the distortions of crystal faces, which were caused by the existed dislocations and defects. The difference in face flatness between side faces and top/bottom faces was also confirmed in the bright field TEM image of side view of the hexagonal plates (Figures S2 and S3). That the side faces had different crystallinities from the top/bottom faces could be understood by the intrinsic structural features of the β-TCP.34 Only three calcium ions were distributed in the different ways over the six sites lining from bottom to top along the [001] direction. The incomplete distributions of calcium ions over these sites could inevitably generate calcium vacancies, which led to the local residual charge or the dangling bond along the [001] direction. The top/ bottom (001) facets were the polar ones of β-TCP. The surface energies calculation also confirmed that the surface stability of the {100} side faces was greater than the {001}.35 The polar surface (001) was usually considered as an energetically unfavorable one in the solution where the dislocations were more readily generated on it than on the six equivalent nonpolar surfaces {100}. A similar effect was also observed in the case of ZnO dissolution.6 Furthermore, the strain field of these dislocations in the bulk could induce the formation of etch pits much more readily than the defect-free wall.29 These differences of dislocation distribution between the bulk part and the wall,

the side faces and the top/bottom faces, might result in the anisotropic dissolutions in one single crystal. Besides, the size effect was the most important factor for the abnormal stability of the wall. It had been suggested, and confirmed by experiment, that demineralization of sparingly soluble salts such as calcium phosphate was generally initiated and accompanied by the formation and development of pits on the crystal surfaces and that the dissolution rates were also determined by the pit densities and spreading velocities.28 However, only the large pits (greater than a critical size) could provide the active dissolution sites, contributing to the reaction. The anisotropic behavior of the hexagonal β-TCP dissolution had already been described. It implied that the dissolution along [001] was initiated by the large pits on the top/bottom surface of the plate, or the (001) crystal facet as shown by SEM (Figure 1), TEM (Figures 2 and S2) and AFM surface height profiles (Figure 7). The wall had a relatively defect-free structure, and the initiation of dissolution was more difficult than that of the bulk. In order to dissolve the wall, the active pits on the (001) narrow surface of the wall were required. The dimension (width) of this facet was less than 40 nm. However, the critical size for the active pit for β-TCP dissolution was of tens of nanometers.27a,28c Thus, the active pit was extremely difficult to be produced on the limited dimensions. As the nanodissolution model proposed,28a the thin edge wall could be dynamically self-presevered by the size effect. A similar size effect was also found in biodemineralizaiton of tooth enamel.28a,b However, they were not single crystals but polycrystallites. The identical chemical and crystal properties of apatite in cores and on walls were observed in the rods. Analogous to the present work, the demineralization of the enamel cores around the rod c-axis was privileged as the core was always emptied while the wall remained. However, the dissolution inhibition of the wall of the enamel rod may be explained by the presence of some organic residuals in the frame. In order to examine the possible effect of the remaining organic solvent on the abnormal dissolution, the hexagonal plates were calcinated at 500 °C for 2 h to remove the organic compounds. TEM characterizations showed that the size, morphology, and the structure of the plates were almost unaffected after the calcination. Furthermore, they underwent the same demineralization to form the hexagonal rings (Figure S7, Supporting Information) eventually. Therefore, the interest-

2234 Crystal Growth & Design, Vol. 8, No. 7, 2008 Scheme 1. Schematic Representation of a Single Hexagonal Plate of β-TCPa

Tao et al. Supporting Information Available: Supporting figures: conductivity measurement of CaCl2 and Na2HPO4 in water and in ethylene glycol (Figure S1), the side views of the solid (Figure S2) and hollow (Figure S3) β-TCP single crystals, XRD of the hollow hexagonal crystals (Figure S4), dissolution of β-TCP at a higher undersaturation level (Figure S5), fast formation of hexagonal single crystals (Figure S6), and hexagonal plates calcinated at 500 °C and their dissolution results (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

References

a The thin edge wall (blue) had well-crystallized structure; but the bulk part (green) contained lots of defects. The blue part could be stabilized by size effect against dissolution, and the green part could be dissolved readily in water.

ing dissolution behavior of these hexagonal β-TCP had no direct relationship with the involvement of organic additives, which should be eliminated by calcination. However, it could be contributed to the unique structural complex of the single crystal as indicated by HRTEM and dark-field TEM images. Actually, the size effect of bulk β-TCP particles had already been revealed in our previous constant composition dissolution study.27a Based on the collected structural information, a scheme of β-TCP nanoplate was suggested as Scheme 1: the two parts, the wall and the bulk part (displayed by blue and green, respectively), had different dissolution features despite their being integrated in one single crystal. The dark circles represented the defects in the bulk. The schematic structure was also supported by the surface morphology information, obtained by AFM (Figure 7). The thin wall had a relatively smooth facet; on the surfaces of the bulk, many tiny domains in the size of 20-60 nm were separated by the block boundaries, irregularly shaped holes, which represented a higher density of the defects. Conclusion By using ethylene glycol as the solvent, we have succeeded in the synthesis of a uniform hexagonal submicron single crystal of β-TCP phase at relatively low temperature. However, this single crystal has a complex structure, a well-crystallized wall and a poorly crystallized bulk part. These two components have different physicochemical properties, resulting in anisotropic dissolution behaviors. This abnormal but interesting feature can be used to produce various structures, porous, gearlike, and hexagonal rings of β-TCP single crystals by controlled demineralization reaction. The technique presented here might be regarded as an effective and feasible approach to synthesize complicated structures of functional materials without the involvement of template and complicated operations. Acknowledgment. We thank Profs. Jianguo Hu, Ying Chen (Fudan University) and Dr. Yaowu Zeng for their help in HRTEM and Drs. Youwen Wang and Jieru Wang for their help in TEM and SEM. This work is supported by National Natural Science Foundation of China (20571064 and 20601023) and Changjiang Scholar Program (RT).

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