Crystal Structure of Hydroxyapatite Nanorods Synthesized by

The XRD data of the sonochemically obtained HAp were used for the crystal-structure refinement (Figure 2). The structure of the HAp was refined in the...
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CRYSTAL GROWTH & DESIGN

Crystal Structure of Hydroxyapatite Nanorods Synthesized by Sonochemical Homogeneous Precipitation M. Jevtic´,*,† M. Mitric´,‡ S. Sˇkapin,§ B. Jancˇar,§,* N. Ignjatovic´,†,* and D. Uskokovic´†,* Institute of Technical Sciences of Serbian Academy of Sciences and Arts, Belgrade, Serbia, Institute of Nuclear Sciences Vinca, Belgrade, Serbia, and Jozef Stefan Institute, Ljubljana, SloVenia

2008 VOL. 8, NO. 7 2217–2222

ReceiVed August 3, 2007; ReVised Manuscript ReceiVed April 17, 2008

ABSTRACT: Using a homogeneous precipitation method in an ultrasound field, we synthesized nanosized, platelike hydroxyapatite (HAp). The internal structure of these platelike formations consists of specifically oriented and laterally connected nanorods. The synthesized HAp nanorods have a length of about 500 nm and a diameter of about 100 nm. A closer inspection of the microstructure of a single nanorod revealed a highly regular and defect-free lattice with unique crystallographic plane orientations. The obtained structure was related to the influence of the ultrasound on the growth mechanism. The samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Introduction Bone is a unique composite containing a collagenous hydrogel matrix. On a volumetric basis, it consists of about 33-43% apatite minerals, 32-44% organics, and 15-25% water. The bone mineral, a calcium phosphate complex dominated by hydroxyapatite, is deposited on the organic matrix, which is 90% collagen.1–3 One of the ways to solve problems related to the reparation and reconstruction of hard tissues is synthesizing materials that have properties similar to those of natural bone. Synthetic HAp is a bioceramic with excellent properties, like biocompatibility, bioactivity, osteoconductivity, and nontoxicity, as well as noninflammatory and nonimmunogenic behavior. These properties allow the extensive application of this material in different areas of medicine.4–7 Although many researchers have conducted research to classify the growth mechanism of apatite crystals,8–10 a consistent explanation has yet to be obtained.10 The application of HAp depends on the crystal shape, size, morphology, crystallinity, thermal stability, and solubility, all of which are strongly influenced by the fabrication route.11 Sonochemistry represents a novel synthesizing method with a wide range of applications in the fabrication of nanomaterials. The basic principle of this method is acoustic cavitation. Bubble collapse in liquids results in a large concentration of energy from the conversion of the kinetic energy of the liquid’s motion into the heating of the contents of the bubble. High local temperatures and pressures combined with extraordinarily rapid cooling provide a unique means for driving chemical reactions under extreme conditions.12 HAp has been synthesized previously with the application of ultrasound,13,14 but a detailed microstructural analysis of sonochemically synthesized HAp and the elucidation of the influence of ultrasonic effects on the microstructure and the crystal growth have not yet been reported. There have already been some results on the synthesis of monophase HAp by homogeneous precipitation, with urea as the precipitation agent.15 In those experiments, monophase HAp * Corresponding author. Telephone: 381 11 2636994. Fax: 381 11 2185 263. E-mail: [email protected]. † Institute of Technical Sciences of Serbian Academy of Sciences and Arts. ‡ Institute of Nuclear Sciences. § Jozef Stefan Institute.

was obtained after a prolonged aging time (48-120 h depending on the temperature), while shortening the time results in multiphase systems. The morphology of the HAp synthesized by the classical homogeneous method was built on large (50-150 µm in length) whiskerlike structures.15 After the homogeneous precipitation of HAp for 72 h at 90 °C, another group of authors obtained large fibrous single crystals (20-60 µm in length, 0.8-3.5 µm in width, and 100-300 nm in thickness).16 Some literature data indicate that the positive influence of ultrasound on the crystallization processes is shown by the dramatic reduction of the induction period, the supersaturation conditions and the metastable zone width. Sonocrystallization exhibits a number of features specific to ultrasonic wave that clearly distinguish it from ordinary crystallization. For most materials such features include (a) faster primary nucleation; (b) relatively easy nucleation in materials that are usually difficult to nucleate otherwise; (c) the initiation of secondary nucleation; and (d) the production of smaller, purer crystals that are more uniform in size.17–19 Because knowledge about the microstructural constitution of a material is strongly related to the understanding of its physical, mechanical, and chemical properties, in this work, we have analyzed the structure of HAp that was synthesized by a homogeneous precipitation process in a field of ultrasound. The predetermined orientation of the crystal growth is correlated with the influence of the ultrasound on the structure of the final products and the growth-orientation processes. Experimental Section Materials. Analytical-grade calcium-nitrate (Ca(NO3)2) (Zorka, Sˇabac, Serbia), ammonium-dihydrogenphosphate (NH4H2PO4) (Zorka, Sˇabac, Serbia), and urea (Kemika, Zagreb, Croatia) were used as the starting materials. Sonochemical HAp Synthesis. Ca(NO3)2 and NH4H2PO4 were dissolved in distilled water. The molar Ca/P ratio was fixed at 2 and the concentration of Ca2+ ions was 0.02 M. The solution was poured into the reaction vessel and heated up to 88 °C (as described in more detail in ref 20). When the solution reached an appropriate temperature, a 0.0 M aqueous solution of urea was added to regulate the pH value and to induce the homogeneous precipitation. The sonic horn was dipped into the solution in order to initiate the ultrasonic irradiation. This horn was a Sonics Vibra Sell high intensity ultrasonic processor (VCX 750 Newtown, CT).

10.1021/cg7007304 CCC: $40.75  2008 American Chemical Society Published on Web 06/17/2008

2218 Crystal Growth & Design, Vol. 8, No. 7, 2008

Jevtic´ et al. instrumental line broadening was removed with a deconvolution operation. The application of the Rietveld procedure to the LaB6 standard (using the same profile function) yields the separation of the Gaussian (βG,s) and Lorentzian (βL,s) integral breadth components of the instrumental profile. The so-obtained integral breadth values were then converted into the integral breadth values of the deconvoluted sample profile. Finally, these values allowed the calculation of the integral breadth of the total TCP-modified pseudo-Voigt profile (βv). The apparent sizes were calculated for each reflection using the ratio 〈Dhkl〉 ≈ 1/(βv). Scanning Electron Microscopy (SEM). The microscope analysis started with scanning electron microscopy, with the aim to reveal the shape and the size of the synthesized particles. The samples were dispersed in water by means of an ultrasound bath and examined on a JEOL JXA 840A scanning electron microscope. Transmission Electron Microscopy (TEM). A further structural analysis based on exploring the individual nanostructures was performed by TEM (JEM-2010F). The sample was prepared by dispersing the powder in acetone using an ultrasonic bath and subsequently dropping the suspension on a lacey carbon film supported by a 300-mesh copper grid. Images were obtained with the aim of identifying the crystallographic orientations and the crystallinity of the single-crystal nanorods.

Figure 1. Fourier transform infrared absorption spectrum of HAp synthesized in a field of ultrasound. Infrared Spectroscopy (FTIR). The FTIR spectrum was obtained using the KBr technique. The sample was blended with KBr in a 1:1 molar ratio and pressed into a disk. The measurements were run in the wavenumber range from 450 to 4000 cm-1 at a 4 cm-1 resolution averaging 64 scans. The FTIR analysis was performed using a Michelson interferometer with duplex mechanical bearings and a linear motor, resolution 32-0.5 cm-1, spectral range DTGS 7.8-400 cm-1, and an accuracy better than 0.01 cm-1. X-ray Diffraction (XRD). The XRD for the sonochemically synthesized sample was performed using a Philips PW-1050 diffractometer with Ni-filtered Cu KR radiation. The data were collected over the 2θ range 10-120° with a step size of 0.02° and a count time of 10 s. The obtained diffractograms were used for a qualitative phase analysis. The structural analysis was carried out using the Rietveld method21 and FullProof software.22 To make the microstructure analysis with the Rietveld method viable, the chosen diffraction profile function was the Thompson-Cox-Hastings (TCH) modified pseudo-Voigt.23 The

Results and Discussion The sonochemically synthesized sample was characterized using the FTIR spectroscopic identification method. The FTIR spectrum of the synthesized sample is presented in Figure 1. The characteristic bands for PO43- appear at 472, 583, 601, 961, 1032, and 1108 cm-1.24 The trace at 472 cm-1 is attributed to the ν2 bending vibration. The triply degenerated ν4 bending vibrations are reflected as traces at 583 and 601 cm-1. The band at 961 cm-1 corresponds to ν1 and the bands at 1032 and 1108 cm-1 to the ν3 vibrations of PO43- ions.25 The weak peak at 874 cm-1 (ν2) and the bimodal peaks at 1415 cm-1 (ν3) and 1455 cm-1 (ν3) correspond to CO32-. The positions of these peaks indicate, according to,26 that the CO32- group substitutes for the PO43- group in the HAp (type B of the carbonatecontaining HAp). HAp was synthesized from a Ca-sufficient precursor mixture (Ca/P ) 2), and it was to be expected that the absorption of the CO32--assigned peaks would be pro-

Figure 2. Rietveld method: red squares, observed; black line, calculated; blue line, difference X-ray diffraction data; and green line, Bragg positions.

HAp Nanorods Synthesized by Sonochemical Precipitation Table 1. Ion Positions and Refinement Parameters Ca2+ (4f) Ca2+ (6h) P5+ (6h) O2- (2) (6h) O2- (1) (6h) O2- (3) (12i) OH- (3m)

x

y

z

B (Å2)

1/3 0.2369(2) 0.3885(1) 0.3989(2) 0.6674(3) 0.3488(2) 0.00000 RB ) 8.2%

2/3 0.9840(3) 0.3671(1) 0.5097(3) 0.4896(2) 0.2791(2) 0.00000

0.0137(6) 1/4 1/4 1/4 1/4 0.0576(2) 0.1928 (8)

0.78(3) 0.78(3) 0.91(1) 2.78(8) 2.78(8) 2.78(8) 2.78(8)

Crystal Growth & Design, Vol. 8, No. 7, 2008 2219 Table 3. Microstructural Parameters Refined with the Rietveld Method

Table 2. Unit-Cell Parameters of the HAp in the Standards and in the Ultrasonically Synthesized Sample Refined with the Rietveld Method sample a

HAp-SRM HAp-SRMb HAp-SRMc HApd a

a (nm)

c (nm)

0.94238(9) 0.94244(2) 0.94174(2) 0.94330(4)

0.68854(6) 0.68854(2) 0.68854(2) 0.68765(3)

See ref.25 b See ref.30 c See ref.31 d This paper.

nounced.27 Carbonate ions are a common impurity in HAp.25 In this case, the presence of the carbonate peaks might be related to the organic precipitation agent, i.e., the hydrolysis of urea.27 The broad and 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 632 cm-1 arise from the stretching and librational modes, respectively, of the OH- ions.28–30 Figure 2 shows the XRD patterns of the sonicated system. The resulting diffractogram demonstrates that the sonochemically synthesized HAp is monophase (ICPDS File 240033), i.e., there were no other diffraction maxima except the maxima that correspond to the HAp structure. Under identical conditions, but without ultrasound, a multiphase system was obtained. An XRD analysis of the as-obtained powder has demonstrated that apart from the HAp phase the phases of calcium hydrogen phosphate hydrate (ICPDS File 790423), calcium phosphate (ICPDS File 898960), calcium phosphate hydroxide (ICPDS File 831886), monetite (ICPDS File 751520), and a complex of calcium phosphate and urea are observed. These results are in accordance with those obtained in previous studies,15 demonstrating that ordinary homogeneous precipitation yields merely multiphase phosphate systems, whereas HAp is synthesized with a prolonged aging time. The XRD data of the sonochemically obtained HAp were used for the crystal-structure refinement (Figure 2). The structure of the HAp was refined in the space group P63/m (No. 176) with the following ion positions: OH- is located at the crystallographic position 4e [00z] with the local symmetry 3m; P5+ is located at the crystallographic position 6h [xy1/4] with the local symmetry m; Ca2+ is located at two crystallographic positions, one at 6h [xy1/4] with the local symmetry m and the other at the position 4f [1/32/3z] with the local symmetry 3m; O2- is located at two different 6h positions, [xy1/4] with the local symmetry m, and at the general crystallographic position 12i [xyz] with the local symmetry 1. The atomic parameters, the fixed atomic positions and those obtained with the refinement are given in Table 1. The refined structural cell parameters are a ) 0.94330(4) nm, c ) 0.68765(3) nm, and the calculated cell volume is 0.5304(4) nm3. By comparing the cell parameters of the standards, 25,31,32 which are very pure materials (see Table 2), with the ultrasonically synthesized HAp, it is clear that there is a slight decrease in the c-axis length and an increase in the a-axis length. These

a

2θ (deg)

Dhkla (nm)

hkl

10.8172 16.8274 18.7922 21.7322 22.8325 25.3279 25.8682 28.8818 28.1117 35.4097 39.1571 41.9307

11.6 6.4 10.5 13.5 9.8 8.1 15.7 12.8 15.0 7.0 9.6 7.3

010 101 110 200 111 021 002 210 012 031 212 131

Apparent size of individual reflections.

changes result in an increase of the corresponding cell volume. On the other hand, the cell parameters of the HAp in the samples from different sources and synthesizing methods,33 in comparison with the sonochemically synthesized sample, show an increase in both parameters. The sonochemically obtained HAp was synthesized using a wet-chemistry method and so the lattice parameters might be influenced by water inclusion. Considering the fact that the SRMs were obtained by precipitation too, it can be assumed that atypical surroundings (like an ultrasonic field) might force the water inclusion and influence the detected differences in the lattice parameters. The microstructural analysis based on the parameters used in the input control file gave values of the apparent sizes for each individual reflection in order to take into account the eventual anisotropic broadening. The apparent size values (Dhkl) relate the size of the crystal domain with the orientation, and results presented in Table 3 are represents the isotropic crystallite sizes. The biggest apparent size value (D002 ) 15.7 nm) corresponds to the crystal direction along the c-axis and that perpendicular to the a-axis. The apparent size in the direction along the ab-axis is D110 ) 10.5 nm. The ratio of the apparent size in the direction along the c-axis (D002) and in the direction along the ab-axis and perpendicular to the c-axis (D110) is R (Dhkl) ) D002/D110 ) 1.5. This ratio is smaller than the standard. 25 But because the R (Dhkl) value is smaller than 2, it can be concluded that the primary sonochemically synthesized particles are of a cylindrical (rodlike) shape. This was also confirmed microscopically. The average apparent size (anisotropic crystallite size) and the standard deviation (anisotropy) were calculated too. Thus, the derived value for the average apparent size is about 10.2 nm, and 3.4 nm for the anisotropy. The next step in the structural analysis was to determine the morphology of the sonochemically synthesized monophase HAp particles. Scanning electron microscopy provided us with this information, and the corresponding SEM image is presented in Figure 3. This image shows the thin, irregular, platelike shape of the HAp particles with dimensions of about a micrometer. These monophase HAp crystals obtained during the sonochemical homogeneous precipitation process had significantly reduced dimensions and different shape compared to monophase HAp crystals obtained under ordinary homogeneous precipitation.15,16 The subsequent analyses were focused on exploring the morphology at the lower length-scale corresponding to the internal structure of these platelike formations. Transmission electron microscopy was performed with the aim of analyzing the size and morphology of the sonochemically synthesized platelike HAp crystals. A TEM image of the sonochemically synthesized HAp is represented in Figure 4 The figure indicates that the sample consists of mostly rodlike

2220 Crystal Growth & Design, Vol. 8, No. 7, 2008

Figure 3. SEM image of sonochemically synthesized platelike HAp.

Figure 4. TEM micrograph of sonochemically synthesized HAp crystals: (a) rodlike morphology and (b) magnified insert of the same image confirms the parallel orientation and the lateral connections of the nanorods.

nanoparticles, which confirms the previously mentioned XRD results. The synthesized HAp nanorod has a diameter of about 100 nm and a length of about 500 nm. These crystals are

Figure 5. TEM image and SAED pattern of a single nanorod of HAp.

Jevtic´ et al.

relatively homogeneous in terms of thickness and highly regular in shape. Also, most of these nanorods are oriented in the same direction, and are closely laterally connected, forming the platelike superstructures seen in the SEM image (Figure 3). Similar superstructures with a structure built upon perpendicular nanorods connected laterally have been reported for an another system.34 The crystallinity of the product was confirmed by the selected-area electron diffraction (SAED). A representative TEM image of a single HAp nanorod and an SAED pattern, collected along the [010] zone axis, are shown in Figure 5. The [001] 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. The crystallinity of the individual nanorods was gauged during the high-resolution transmission microscopy (HRTEM) investigations. Figure 6 shows a bright-field TEM image of HAp nanorod with the marked part that corresponds to the represented HRTEM micrographs. Many different parts along the nanorod were analyzed in this way and the results were the same, which implies good crystallinity and a defect-free lattice. The interplanar distances measured in segments of the HRTEM micrograph were 0.47 and 0.34 nm, corresponding to the interplanar spacing of the [200] and [002] planes of the hexagonal HAp, respectively. The preferred growth direction of the nanorods is appears to be parallel to the [001] direction of the hexagonal HAp. An illustration of the structure of these single-crystalline nanorods is schematically presented in Figure 7. The crystal structure of the sonochemically synthesized products can give more information about the synthesizing process. According to Suslick et al.,35 there are two locations of sonochemical activity: one is inside the collapsing bubbles, and the other is between the cavitation bubble and the surrounding bulk solution. If the reaction takes place inside the bubble, the products are amorphous, and if it takes place in the surrounding region, it is to be expected that the products are nanocrystalline. So in our case, because a highly crystalline and defect-free HAp powder was obtained, it is to be expected that the formation of the HAp probably occurs in the interfacial region. Consequently, we can assume that the obtained HAp particles have a platelike structure built upon nanorods. The key feature of their morphology is the specific orientation of the nanorods, which consisted of the orientation of the crystallographic planes

HAp Nanorods Synthesized by Sonochemical Precipitation

Crystal Growth & Design, Vol. 8, No. 7, 2008 2221

Figure 6. TEM and HRTEM micrographs of a sonochemically synthesized HAp nanorod.

Figure 7. Schematic illustration of HAp single-crystal nanorod structure. The gray spheres represent the Ca atoms, white represents the H atoms, red represents the O atoms, and pink represents the P atoms. The sample is viewed along the (100) direction and the nanorod grows along the (001) crystal face.

and the perfectly regular, defect-free lattice. This fact might be related to the mechanism of the HAp synthesis in the field of ultrasound. Some of the explanations related to the mechanism of the HAp synthesis suggested a ACP-OCP-HAp transformation.9,36,37 In our previous paper, we detected this intermediate and showed its transition to HAp in terms of the sonochemical synthesis conditions. Correlated with that, in this work, we have found the specific structure of synthesized HAp. In a field of ultrasound, particles have their natural, thermodynamically most stable growth direction, but are specifically oriented parallel and laterally connected. In view of the fact that the particles formed as the sonication products are pushed against each other as a result of the microjets created when the bubble collapses, and then the speeds at which they are projected can cause melting at the contact area when they collide,38 it can be assumed that in the field of ultrasound the HAp nanorods are

laterally “glued” during the sintering due to the effect of the microjets formed after the collapse of the bubble in the sonochemical process. The platelike structures are created as a result of their collisions. A similar effect with ultrasound has been detected in other materials.39 Conclusion In this work, nanosized hydroxyapatite (HAp), synthesized by the homogeneous precipitation method in a field of ultrasound, was structurally analyzed. The investigation revealed platelike formations of HAp crystals built upon parallel and laterally connected nanorods. Comparing these platelike structures with fiber- or whiskerlike structures, obtained with the classical homogeneous method, indicates different forms of crystal growths for the sonicated and nonsonicated systems.

2222 Crystal Growth & Design, Vol. 8, No. 7, 2008

These specifically oriented and laterally connected HAp nanorods, obtained in a field of ultrasound, have a length of about 500 nm and a diameter of about 100 nm each. The crystals were quite homogeneous in their structure. On the basis of these observations, it can be suggested that the individual nanorods are linked as a result of the sintering effect of the microjets formed after the collapse of the bubble in the sonochemical process. The HAp nanorods are created as a result of their collisions. A further investigation into the microstructure of a single nanorod revealed a unique crystallographic plane orientation along the whole rod, which crystallizes in a singlecrystalline orthorhombic structure. The identified segments clearly show good crystallinity, long-range order and a defectfree lattice. The highly ordered crystal structure indicates that the formation of HAp probably occurs in the interfacial region between the cavitation of the bubbles and the surrounding bulk solution. A comparison of sonochemical and ordinary homogeneous precipitation without ultrasound shows that sonochemical method yields monophase HAp with controlled morphology and crystal size. Besides the reduced sizes of the sonochemically obtained HAp crystals, another improvement with this method is the reduction of the time required for the preparation of the nanostructured, monophase HAp. Acknowledgment. The presented research was supported by The Ministry of Science of the Republic of Serbia, under Grant 142006.

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