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Induced Growth of (0001)-Oriented Hydroxyapatite Nanorod Arrays on ZnO-Seeded Glass Substrate ZhenZhen Lu,† HaiYan Xu,*,‡ MuDi Xin,† KunWei Li,† and Hao Wang*,† The College of Materials Science and Engineering, Beijing UniVersity of Technology, Beijing 100124, P. R. China, and School of Materials & Chemical Engineering, Anhui UniVersity of Architecture, Hefei 230022, P. R. China ReceiVed: September 21, 2009; ReVised Manuscript ReceiVed: December 4, 2009
The oriented growth of the hydroxyapatite (HAp) nanorod array on the glass substrate with a ZnO seed layer was investigated. The samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. The HAp nanorods have a length of about 2 µm and a diameter of about 200 nm. The growth mechanism of the (0001)-oriented HAp nanorod array was attributed to the “mold effect” of the (0001)-oriented ZnO seed layer. The dissolution characteristics of the as-prepared HAp nanorod array have also been investigated. 1. Introduction As one of the most important constituents of human beings’ and animals’ teeth, hydroxyapatite (HAp, Ca10(PO4)6(OH)2) has attracted a great deal of attention. HAp crystals are packed into enamel prisms extending from the dentin-enamel junction to the tooth surface, with their crystallographic c-axis preferentially parallel to the prism axis. These structures coalesce to form the tough tissue of enamel, which can withstand high forces and resist damage by crack deflection.1-5 For obtaining oriented arranged HAp nanorod coatings, several coating methods, such as plasma spray,6,7 the sol-gel method,8,9 self-assembling monolayers,10,11 electrochemical deposition,12 and the biomimetic approach,13 are used to coat implant surfaces. The plasma spray process is a widely used commercial method for coating bioceramics on implants. However, the plasma spray process has several limitations, including microcracks, poor adhesion between the coating and substrate, phase changes due to high temperature exposure, nonuniformity in the coating density, and improper microstructural control, which could result in failure of the implanted system.14 Therefore, our strategy to control the shape and orientation of crystallites consists of growing thinfilm materials directly onto substrates from aqueous precursors in solution by monitoring the thermodynamics and kinetics of nucleation and growth of the materials by controlling experimentally their interfacial tension. This approach led to the development of a novel general concept; namely, oriented materials, which is dedicated to the design of materials with the appropriate structure, morphology, and orientation. To reach this aim, we adopt a ZnO-seeded layer as our template to obtain the oriented HAp films. Many groups have adopted a ZnO seed template to grow highly oriented ZnO nanorods.15-18 Such an approach has been successfully applied to demonstrate the ability to chemically grow, align, and orient nanoparticles onto substrates with an aqueous low-temperature coating process. However, on the basis of our current knowledge, no literature about HAp nanorods * Corresponding author. Phone: +86-10-67392733 (H.W.); +86-5533828152 (H.Y.X.). Fax: +86-10-67392445 (H.W.); +86-553-3828106 (H.Y.X.). E-mail:
[email protected] (H.W.);
[email protected] (H.Y.X.). † Beijing University of Technology. ‡ Anhui University of Architecture.
grown on a ZnO seed layer has been reported until now. The crystal lattice of ZnO is similar to that of HAp; both are hexagonal structures. In addition, the technology of growing a ZnO seed onto a substrate is mature and easy for us to put into practice. Moreover, ZnO is very popular in daily dental practice because it is the main substituent of some polycarboxylates,19 in temporary dental cement powders or in the formula of dental impression materials. Zinc content normally ranges between 0.012 and 0.025 wt % in human bone, which is relatively higher than the Zn content in adult tissues and plasma. There are some results showing that Zn has a stimulating effect on osteoblastic cell proliferation and bone formation.20 Herein, we adopt a ZnO seed template to grow HAp nanorods whose structure is like human teeth with nanorods piled up. We utilize the magnetron sputter method to obtain a (0001)oriented ZnO seed layer with high density. Then we adopt a hydrothermal method to grow HAp nanorods onto the ZnOseeded substrate. 2. Experimental Section 2.1. Preparation Process of HAp Nanorod Arrays on Glass Substrate. In this process, two steps were used for the preparation of HAp nanorod arrays. 2.1.1. First Step: ZnO Template Preparation. Glass substrates with a typical size of 10 × 10 × 1 mm3 were cut from microscope glass slides. They were ultrasonically cleaned in toluene, ethanol, and acetone, respectively, for 15 min to wash off organic pollutants, followed by hot air drying. The substrates were then loaded into the chamber of a magnetron sputterer. When the deposition chamber was evacuated down to ∼5 × 10-3 Pa, high purity argon with a pressure of 2.34 Pa was introduced into the chamber. Radio-frequency (13.56 MHz) power of 100 W was forwarded to the substrates to initialize plasma for surface cleaning. After that, direct-current power with a density of 0.4 W/cm2 was forwarded to a ZnO target (99.99%). Deposition was conducted for 20 min to establish a thin ZnO film on the glass substrate. 2.1.2. Second Step: Growth of HAp Nanorod Arrays on ZnO Template. Aqueous solutions for the growth of HAp nanorods were prepared using a 20 mL mixed solution of 0.2 mol/L CaNO3 · 4H2O and 0.2 mol/L ethylenediaminetetracetic acid disodium salt (EDTA) which was introduced into 20 mL
10.1021/jp9091078 2010 American Chemical Society Published on Web 12/28/2009
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Figure 1. SEM images of (a) plane image of an as-grown HAp nanorod array (scale bar, 5 µm), (b) profile image of a HAp nanorod array (scale bar, 2 µm), (c) extended area of a HAp nanorod plane image (scale bar, 20 µm), (d) TEM and electron diffraction image (inset) of a single HAp nanorod scraped from a glass substrate (scale bar, 100 nm), (e) HAp film deposited on bare glass (scale bar, 3 µm), and (f) ZnO seed film on bare glass (scale bar, 500 nm).
of 0.12 mol/L Na2HPO4 · 12H2O mixed solution. The pH of the solution was adjusted to 8 by adding NaOH solution, then the solution was stirred for 20 min. The glass substrates with a ZnO seed layer were suspended in the Teflon autoclave and kept at 140 °C for a certain period in an oven. After cooling to room temperature, the samples were washed with deionized water and dried in air at 70 °C for 6 h. 2.2. Characterization. X-ray Diffraction (XRD). Crystallographic phases and purity information of the prepared samples were investigated by X-ray diffraction at room temperature with a Bruker AXS D8 ADVANCE diffractometer using Cu KR1 radiation (λ ) 1.541 Å). The accelerating voltage, emission current, and scanning speed were 40 kV, 40 mA, and 0.2°/s, respectively. Infrared Spectroscopy (FTIR). The powders were scraped from the as-prepared HAp film and blended with KBr in a 1:20 molar ratio and pressed into a disk. The measurements were run in the wavenumber range from 400 to 4000 cm-1 at a 4 cm-1 resolution averaging 64 scans. The FTIR analysis was performed using a Vertex 70 FTIR machine. Scanning Electron Microscopy (SEM). To reveal the morphology of the as-prepared samples, they were stuck to electric adhesive tape and examined by Hitachi S4800 scanning electron microscopy.
Energy-DispersiWe X-ray (EDX). Element analysis was carried out by energy dispersion X-ray spectroscopy, which was directly connected to SEM. This data was used to analyze Ca/P, which is an important index for HAp. Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED). The single HAp nanorod was analyzed by TEM and SAED on a JEOL-JEM 2010F transmission electron microscope using an accelerating voltage of 200 kV. 2.3. The Vitro Experiment to Test the Dissolution Characteristic of the Prepared Samples. Three of the same prepared HAp films were set for average dissolution data. All samples were soaked in 40 mL of pH 7.4 distilled water (buffered using 0.05 M tris-hydroxymethylaminomethan) at 37 °C for times varying from several hours to 20 days. These solutions were not changed during the static immersion period. At the end of these time periods, all samples were taken out of the solution, gently washed with distilled water and with acetone, and then dried at 50 °C overnight. Then the calcium and phosphate concentrations of the trace elements dissolved in distilled water were measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, OPTIMA 4300, Perkin-Elmer).
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Figure 2. X-ray diffraction patterns of (a) HAp powders formed in the solution, (b) HAp film deposited on bare glass, (c) ZnO seed layer deposited by magnetron sputter for 10 min, (d) HAp film deposited on ZnO-seeded glass for 12 h, and (e) HAp film on ZnO-seeded glass for 24 h.
3. Results and Discussion Morphology and growth orientation of the highly oriented HAp nanorod arrays on a ZnO seed layer and flower-like HAp on bare glass were studied by SEM and TEM (Figure 1). The SEM images (Figure 1a-c) of HAp film deposited on a ZnO seed layer for 24 h show that uniform HAp nanorods with high distribution density were grown vertically on the ZnO layer. These hexagonal rods with an average diameter of 200 nm and length of 2 µm were piled up to a uniform array on a large area. A representative TEM image of a single HAp nanorod and an SAED pattern, collected along the [011j0] zone axis, are shown in Figure 1d. The [0001] crystallographic direction of the hexagonal HAp is parallel to the long axis of the nanorod, which is represented by the arrow. The SAED pattern indicates that the nanorods are single crystals. We also observed the morphology of HAp film deposited on bare glass substrate without a ZnO seed layer, and the corresponding SEM image is shown in Figure 1e. Flowerlike bundles exist (tied to one end of the pencil-like nanorods) with their one end tied up to the substrate and the other end fanned out to the surface. Each rod shows that the side surface is quite rough with one sharp end. Comparing the micrograph of HAp film deposited on a ZnO seed layer with film on bare glass, we get two points: First, the nucleation density of the HAp film on a ZnO seed layer is much higher than that on bare glass. Second, the HAp film on a ZnO seed layer shows preferential growth along the c-axis; thus, the nanorods grew vertically on the substrate. A top view of the ZnO seed layer that was deposited by magnetron sputter for 10 min is shown in Figure 1f. The layer is smooth and consists of many islandlike tiny grains. It seemed that the ZnO seed layer plays an important role in obtaining the highly oriented HAp array. The mechanism will be discussed later in detail. X-ray diffraction measurements have been applied to further confirm the crystallographic structure of the obtained samples and ZnO seed layer. Figure 2 shows the XRD patterns of HAp powders formed in the solution (Figure 2a), HAp film deposited on bare glass (Figure 2b), ZnO seed layer (Figure 2c), and HAp films on ZnO seed layer prepared for different times (Figure 2d and e), respectively. All samples were measured under the same instrumental parameters. It is obvious that the diffraction intensities of Figure 2c-e are many times stronger than those of Figure 2a and b. Therefore, the details of Figure 2a and b are enlarged in Figure 3. The diffraction patterns shown in
Lu et al.
Figure 3. X-ray diffraction patterns of (a) HAp powder formed in the solution and (b) HAp film deposited on bare glass. This figure is used to show the patterns of Figure 2a and b more clearly.
Figure 3a are consistent with the values of hydroxyapatite in the standard card (JCPDS No. 73-293), indicating that the powders formed in the solution are pure phase HAp. The HAp deposited on bare glass shows only a (211) diffraction peak (Figure 3b). However, it is noted that the intensity of the (211) peak of HAp powders is originally the highest one among all other peaks (Figure 3a). Moreover, the intensity of the (211) peak in Figure 3b is very low with a relatively high noisy background. Also seen from the SEM shown in Figure 1e, the HAp deposited on bare glass shows a morphology of flowerlike bundles, which clearly has no preferential orientation. Therefore, it is difficult to define it as the (211) orientation. Again, come back to Figure 2. From Figure 2c, we can see that the ZnO seed layer with good orientation was deposited by the seeding process. The XRD pattern of the seed layer displays only the (0002) diffraction peak of the wurtzite ZnO (2θ ) 34.28°), indicating the good orientation in the c-axis direction. The Scherrer line width analysis gives a seed diameter of around 40 nm. Figure 2d and e shows that only three distinct peaks exist in the XRD patterns of HAp films deposited on a ZnO layer, which are ascribed to the (0002) peak of ZnO (2θ ) 34.28°) and (0002) and (0004) peaks of HAp (2θ ) 25.85 and 53.22°), respectively. No other XRD peaks corresponding to HAp were observed in these HAp films, suggesting that the apatite crystallites were strongly oriented in the [0001] direction. From Figure 2d and e, we could also observe that the (0002) diffraction peak of ZnO is obviously stronger than the (0002) peak of HAp. However, we could observe from SEM images (Figure 1a-d) that the crystallinity of HAp nanorods is very good, and the ZnO seed layer was covered by the HAp film whose thickness is about 2 µm. Here, to compare the diffraction intensity of HAp with ZnO, we introduce a description of specific diffraction intensity, which was defined as I/Icor, where I is the intensity of the testing material and Icor is the intensity of corundum. From the JCPDS Card 73-293, we could find that the I/Icor of HAp is only about 1.06, whereas the I/Icor of ZnO is 5.43 according to the JCPDS Card 89-1397. It means that the X-ray sensitivity of ZnO is about 5 times higher than that of HAp. Therefore, the signal of ZnO could be directly observed by XRD in our samples. However, if we consider the difference of X-ray sensitivity between ZnO and HAp, we may deduce that the diffraction intensity of our HAp films is even higher than that of ZnO. Particularly, when we extended the deposition time from 12 h (Figure 2d) to 24 h (Figure 2e), the ratio of the intensity of the HAp (0002) peak to that of the ZnO (0002) peak could be significantly enhanced. In other words, if taking all factors into
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Figure 4. Schematic representation of the proposed nucleation and growth mechanism for the formation of HAp nanorod arrays on ZnO seed substrate.
consideration, including X-ray sensitivity and results both of XRD and SEM/TEM, we can conclude that the crystallinity of (0001)-oriented HAp films is at least as good as that of ZnO, even though the diffraction intensity of the HAp (0002) peak is less than that of the ZnO (0002) peak from XRD patterns. At the same time, as mentioned above, the diffraction intensities of the ZnO seed layer (Figure 2c) and seeded HAp film (Figure 2d and e) are many times stronger than the HAp powders (Figure 2a) and nonseeded HAp film (Figure 2b). There would be two reasons for this phenomenon. One is that the amount of HAp growing on the ZnO-seeded substrate is much more than that growing on bare glass, as seen in the SEM images shown in Figure 1. The fundamental cause of the increase lies in the fact that the seed layer greatly improved the surface distribution of HAp nanorods by providing an appropriate surface roughness and higher surface energy, which will be discussed later in detail. The other reason is the different orientation between the samples. According to refs 21 and 22, the integrated intensity of the individual (Bragg) peak depends on multiple factors that can be classified as follows: (1) Structure factors are determined by the crystal (atomic) structure of the material. (2) Specimen factors are related to preferred orientation, crystallite size, and the distribution as well as the internal strain of the sample. (3) Instrument factors include the type of focusing geometry, properties of radiation and detectors, etc. The integrated intensity of the individual diffraction peaks is given as
Ihkl ) KGθ PhklMhkl |Fhkl | 2
(1)
where K is the scale factor, Gθ is the grouped geometrical effects, Phkl is the preferred orientation factor, Mhkl is the multiplicity factor, and Fhkl is the structure factor. From Figure 2d and e, we could observe that the HAp films are highly oriented, which leads to the remarkable rise of Phkl (the preferred orientation factor). Therefore, under the inductive function of the ZnO seed layer, the as-obtained HAp X-ray diffractogram is intensified. To analyze the growth mechanism on the formation of HAp nanorod arrays on the ZnO seed layer, one has to first understand the structural characteristics of ZnO and HAp. HAp adopts a hexagonal structure in the P63/m space group and lattice parameters a ) 9.4225 Å and c ) 6.8850 Å.23 The structure can be viewed as consisting of unconnected PO43- tetrahedrons
with Ca2+ in the space between and a chain of OH- ions along the c-axis to balance the charge.24 The crystallite will be likely to grow according to the anisotropic structure of HAp; therefore, many kinds of elongated shapes along the c-axis could be obtained without difficulty, such as needle-, rod-, and wirelike morphologies, etc.25,26 Meanwhile, the thermodynamically stable crystal structure of ZnO is wurtzite (hexagonal crystal system) and occurs in nature as the mineral zincite (crystal class 6mm). This ionic and polar structure can be described as hexagonal close packing of oxygen and zinc atoms with zinc atoms in tetrahedral sites in space group P63mc and lattice parameters a ) 3.2495 Å and c ) 5.2069 Å (point group 3m). The occupancy of four of the eight tetrahedral sites of the hexagonal lattice controls the structure. The typical crystal habit exhibits a basal (pedion) polar oxygen plane (0001j), a top tetrahedron cornerexposed polar zinc (0001) face, and low-index faces (parallel to the c-axis) consisting of a nonpolar {11j00} family of crystal planes (and C6V symmetric ones). The “low-symmetry” nonpolar faces with 3-fold coordinated atoms are the most stable ones, and the polar ones are metastable. Additionally, there is no center of inversion in the wurtzite crystal structure, and therefore, an inherent asymmetry along the c-axis is present, which allows the anisotropic growth of the crystal along the [0001] direction. The velocities of crystal growth in different directions are reported to be [0001] > [1j011j] > [1j010] > [1j011] > [0001j].27 Accordingly, the theoretical and most stable crystal habit is a hexagon elongated along the c-axis. Therefore, we could obtain the (0001)-oriented ZnO seed template with ease. The wurtzite-structured ZnO crystal can be described schematically as a number of alternating planes composed of 4-fold, tetrahedral-coordinated O2- and Zn2- ions, stacked alternatively along the c-axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (0001j)-O polar surfaces. Wang’s group has reported that the Zn-terminated ZnO (0001) polar surface is chemically active and the oxygenterminated (0001j) polar surface is inert in the growth of nanocantilever arrays, which demonstrates that the polarity of the positively charged ZnO (0001) polar surface terminated with Zn plays an important role in determining the nanostructures grown on the surface.28,29 It is important to point out that, in our case, the ZnO seed layer exposed to the positively charged active (0001) surface would play a key role in inducing a controlled growth of HAp nanorod arrays during hydrothermal processing. The (0001)-oriented ZnO film, as shown by the SEM image in Figure 1f, yielded an array of the hexagonal tops with a high density. The majority of HAp nanorod arrays would have their [0001] axes perpendicular to the substrate surface, and most of them had their [0001] axes parallel to each other. On the basis of the above analysis and results, we suggest that the formation mechanism of (0001)-oriented HAp nanorod arrays with a high density on a ZnO seed layer can be expounded qualitatively from both crystal structure and thermodynamical angle in solution-solid phase transition processing. The nucleation of a HAp crystallite on the specific ZnO step identified here could be a result of a “mold effect”; that is, the boundary conditions imposed by ZnO surfaces on the evolution of the HAp phase, as suggested by Lifshitz et al.30 When the reacted solution reached saturation and the temperature of the reacted solution is the same as the glass, the HAp solute would emerge on the (0001)-oriented ZnO single crystal, which would influence the tropism of HAp. On the other hand, when HAp crystallizes from solution, an active substrate will provide the active point for nucleation with decreasing nucleation barrier, which is called heterogeneous
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Figure 5. Infrared spectrum of HAp nanorods grown on ZnO-seeded glass substrate.
nucleation. There are generally two favorable factors to decrease nucleation barrier for boosting the heterogeneous process: appropriate surface roughness and higher surface energy (lower contact angle). In this study, the fresh highly (0001)-oriented ZnO seed layer precisely met the need of the above two factors: it had a rough surface, seen from Figure 1f, and positively charged polar surface, which will lead to higher surface energy compared to bare glass. Particularly, the polar surface of the ZnO seed layer could not only decrease the contact angle between nuclei and substrate but also attract negative groups, such as PO43- and OH- to assemble them on its surface in order, which would induce a large amount of HAp nuclei to form onto the substrate due to the enhanced heterogeneous nucleation promoted by the ZnO seed layer, as shown in Figure 4. The phenomenon will cause two results: high nuclei density and rapid consumption of the reaction ions, which will be the precondition for obtaining HAp nanorod arrays with high surface distribution density in the following growth process. In addition, it is well-known that, if the fraction of ions consumed in the nucleation step is high, the growth velocity of the nuclei driven by supersaturation and the increase in rod size during growth processing immediately following the nucleation burst are drastically limited as a result of the closed finite system. Therefore, the growth habit of HAp is mainly affected by the interior structure under the relatively low supersaturation. The corresponding product will give an integrated hexagonal rod shape with a smooth surface, which is confirmed by the SEM and TEM results seen from Figure
Lu et al. 1a-d. However, in contrast, smooth-faced bare glass had neither a high surface energy nor roughness to decrease the nucleation barrier. Under the condition, the barrier is as high as that of homogeneous nucleation, which means high supersaturation is needed. Once a few nuclei are formed on the substrate, they will get an opportunity to grow quickly. There are two reasons for that: on one hand, it is quite difficult to form a new nucleus due to a high barrier; on the other hand, high supersaturation emerges with high growth velocity. Therefore, the produced HAp on bare glass is scattered flowerlike (binding with pencil-like rods) with the surface distribution density being much lower than the seeded sample (Figure 1e) due to the lower nucleation density. After nucleation, as shown by Goh and co-workers, hydroxyapatite crystallized on the adjacent seed islands and then continued to grow and approach each other, growing parallel to each other, thus preventing further lateral growth. This left crystallites that were growing mainly perpendicular to the substrate to continue growing, resulting in the c-axis preferential orientation.31 Close analysis of the flowerlike sample (Figure 1e) shows that each pencil-like rod with rough surface and quite sharp end is larger in size than the hexagonal nanrods in the seeded sample, which is convincing evidence to back the above interpretation. The powders scraped from the HAp film deposited on a ZnO seed layer were further characterized using the FTIR spectroscopic identification method; the FTIR spectrum is shown in Figure 5. The absorption bands at 472, 564, 603, 962, 1034, and 1099 cm-1 are the characteristic peaks of PO43-. The trace at 472 cm-1 is attributed to the ν2 bending vibration. The triply degenerated ν4 bending vibrations are reflected as traces at 564 and 603 cm-1. The band at 962 cm-1 corresponds to ν1, and the bands at 1034 and 1099 cm-1, to the ν3 vibrations of PO43- ions. The weak peak at 891 cm-1 (ν2) and the bimodal peaks at 1420 (ν3) and 1463 cm-1 (ν3) correspond to CO32-.32 The positions of these peaks indicate, according to ref 33, that the CO32- group substitutes for the PO43- group in the HAp (type B of the carbonate containing HAp). Carbonate ions are a common impurity in HAp. The broad, high-intensity band extending from 2500 to 3600 cm-1 derives from the ν3 and ν1 stretching modes of the hydrogen-bonded H2O molecules, and the band at 1642 cm-1 derives from the ν2 bending mode of the H2O molecules. The bands at 3573 and 639 cm-1 arise from the stretching and librational modes, respectively, of the OH- ions. To confirm the composition of nanorods shown in Figure 1b, they were subjected to EDX analysis. Figure 6 shows the EDX spectrum of the HAp nanorods deposited on a ZnO seed layer for
Figure 6. Energy-dispersive X-ray spectrum of HAp nanorods grown on ZnO-seeded glass substrate.
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J. Phys. Chem. C, Vol. 114, No. 2, 2010 825 about 5 µm and a diameter of about 200 nm. The growth mechanism in the initial stage was discussed. The results illuminate that the oriented ZnO seeds play crucial roles in effecting the nucleation density and growth orientation of the nanorod arrays. Arrays with better morphologies are expected by further modifying the seeding process. Such oriented HAp materials might be useful in dentistry and orthopedics. For instance, a surface with HAp c-face characteristics dominating could provide a long-lasting dental root prosthesis. Acknowledgment. This work was supported in part by the EducationDepartmentofAnhuiProvince(GrantNo.KJ2009B133), Anhui University of Architecture (Grant No. 20081204), and the Natural Science Foundation of Beijing (No. 4082008). References and Notes
Figure 7. Released Ca ion and P ion changes of HAp film deposited on ZnO-seeded glass via immersion duration in water: (a) the first 48 h and (b) the latter 18 days.
24 h. The EDX examination confirms the presence of Ca, P, and O in the sample. The relative amount of Ca and P revealed that the Ca/P ratio of the nanorod array was 1.55; this Ca/P ratio is close to that of stoichiometric hydroxyapatite (1.67). In the present study, the dissolution characteristics of the asprepared HAp films were evaluated in distilled water. Simulated body fluid (SBF) is a commonly used experimental medium for in vitro testing. The reason for using distilled water as an experimental medium instead of SBF is that it is easy to determine the dissolution behavior of HAp in distilled water rather than in SBF. Once HAp dissolves in SBF, apatite precipitates are formed on the HAp surface, meaning that it may be difficult to observe the dissolved surface. The amount of calcium and phosphate ions dissolving in immersed water is shown in Figure 7. The release of P ions took place, and the ion concentration continuously increased in the first 30 h as the immersion time increased (Figure 7a), while the release of Ca ions changed little. Then, the amount of released Ca and P kept steady in the latter 19 days (Figure 7b). As we concluded from the EDX result, the ratio of Ca/P is 1.55. Although in ideal Ca10(PO4)6(OH)2 the molar ratio is 1.67, the P is excessive in our sample. Therefore, the released amount of P ions is 4-5 times that of Ca ions. According to our results, we could conclude that the dissolution rate of the as-prepared sample is slow except for the beginning 30 h. The slow rate is attributed to the fewer defects of the well-crystallized HAp films prepared by the hydrothermal method. The nonstoichiometric apatites are of biological importance because of their similarity to the mineral part of bone.34 4. Conclusions The oriented growth of the vertical HAp nanorod arrays from a ZnO seed layer was realized. The HAp nanorods have a length of
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