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Journal of Alloys and Compounds 430 (2007) 330–333

Synthesis of nanocrystalline hydroxyapatite by using precipitation method

I. Mobasherpour a,∗, M. Soulati Heshajin b, A. Kazemzadeh a, M. Zakeri c

a Ceramics Department, Materials and Energy Research Center, P.O. Box 31787-316, Tehran, Iranb Biomaterials Engineering Department, Amir Kabir University of Tehran, Tehran, Iran

c Materials Engineering Department, Islamic Azad University of Saveh, Saveh, Iran

Received 12 February 2006; received in revised form 1 May 2006; accepted 4 May 2006Available online 23 June 2006

bstract

In this investigation, hydroxyapatite powder has been synthesized from the calcium nitrate hydrated and di-ammonium hydrogen phosphateolution by precipitation method and heat treatment of hydroxyapatite powders. In order to study the structural evolution, the Fourier transformnfrared spectroscopy (FTIR), the X-ray diffraction (XRD) and simultaneous thermal analysis (STA) were used. Transmission electron microscopyTEM) and scanning electron microscopy (SEM) were used to estimate the particle size of the powder and observe the morphology and agglomeration

tate of the powder. Results show that hydroxyapatite nanocrystalline can successfully be produced by precipitation technique from raw materials.ydroxyapatite grain gradually increased in size when temperature increased from 100 to 1200 ◦C, and the hydroxyapatite hexagonal-dipyramidalhase was not transformed to the other calcium phosphates phases up to 1200 ◦C.

2006 Elsevier B.V. All rights reserved.

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eywords: Hydroxyapatite; Precipitation; Nanocrystallization; X-ray diffractio

. Introduction

Hydroxyapatite (HAP: Ca10(PO4)6(OH)2) has attracteduch attention as a substitute material for damaged teeth or

ones over the past several decades, because of its crystallo-raphical and chemical similarity with various calcified tissuesf vertebrates [1,2].

Hydroxyapatite is the main mineral constituent of teeth andones. HAP ceramics does not exhibit any cytoxic effects. Ithows excellent biocompatibility with hard tissues and also withkin and muscle tissues. Moreover, HAP can directly bond tohe bone. Unfortunately, due to low reliability, especially in wetnvironments, the HAP cannot presently be used for heavy load-earing applications, like artificial teeth or bones [1].

Multiple techniques have been used for preparation of HAPowders, as reviewed in several works [3]. Two main ways forreparation of HAP powders are wet methods and solid state

eactions. In the case of HAP fabrication, the wet method can beivided into three groups: precipitation, hydrothermal technique,nd hydrolysis of other calcium phosphates [3,4]. Depending

∗ Corresponding author. Tel.: +98 261 6204131.E-mail address: I Mabasherpour@merc.ac.ir (I. Mobasherpour).

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925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2006.05.018

pon the technique, materials with various morphology, stoi-hiometry, and level of crystallinity can be obtained. Solid stateeactions usually give a stoichiometric and well-crystallizedroduct, but they require relatively high temperatures and longeat treatment times. Moreover, sinterability of such powders issually low. In the case of precipitation, where the temperatureoes not exceed 100 ◦C, nanometric-size crystals can be pre-ared. They have shapes of blades, needles, rods, or equiaxedarticles. Their crystallinity and Ca/P ratio depend strongly uponhe preparation conditions and are in many cases lower thanhose for well-crystallized stoichiometric hydroxyapatite. Theydrothermal technique usually gives HAP materials with a highegree of crystallinity and with a Ca/P close to the stoichiometricalue. Their crystal size is in the range of nanometers to millime-ers. Hydrolysis of tricalcium phosphate, monetite, brushite, orctacalcium phosphate requires low temperatures and results inAP needles or blades having the size of microns. However, inost cases, the hydrolysis product is highly nonstoichiometric

1].It is possible to improve the properties of HAP ceramic by

ontrolling important parameters of powder precursors such asarticle size and shape, particle distribution and agglomeration5]. Nanocrystalline HAP powders exhibit greater surface area6]. It can provide improved sinterability and enhanced densifi-

loys and Compounds 430 (2007) 330–333 331

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Itbvpawahexagonal-dipyramidal nanocrystals inside the powder particles.At room temperature the stable phase is hexagonal-dipyramidal.This hexagonal-phase-growth XRD data with increasing tem-perature is reported here for the first time, but the hexagonal-

I. Mobasherpour et al. / Journal of Al

ation to reduce sintering temperature [7]. Moreover, nanometerized HAP is also expected to have better bioactivity than coarserrystals [8,9], nanophase ceramics clearly represent a uniquend promising class of orthopedic/dental implant formulationsith improved osseointegrative properties [9,10]. Recently, these of precipitation processes for synthesis of HAP becomes anmportant research objective [11].

In this investigation, the precipitation method has beendapted to synthesize nanocrystalline HAP powder. Powderharacterization including phase composition, morphology andistribution of grain size has been performed. In this methodffers a molecular-level mixing of the calcium and phosphorusrecursors, which is capable of improving chemical homogene-ty and increasing phase transformation temperature to the otheralcium phosphates phase of resulting HAP in comparison withonventional method.

. Experimental

Hydroxyapatite compounds were prepared by solution-precipitation methodsing Ca(NO3)2·4H2O (Analar No. 10305) and (NH4)2HPO4 (Merck No. 1205)s starting materials and ammonia solution as agents for pH adjustment. A sus-ension of 0.24 M Ca(NO3)2·4H2O (23.61 g Ca(NO3)2·4H2O in 350 ml distilledater) was vigorously stirred and its temperature was maintained at 25 ◦C. A

olution of 0.29 M (NH4)2HPO4 (7.92 g (NH4)2HPO4 in 250 ml distilled water)as slowly added dropwise to the Ca(NO3)2·4H2O solution. In all experiments

he pH of Ca(NO3)2·4H2O solution by ammonia solution was 11. This can bexplained by the following reaction:

10Ca(NO3)2·4H2O + 6(NH4)2HPO4 + 8NH4OH

→ Ca10(PO4)6(OH)2 + 20NH4NO3 + 20H2O

n the next step the precipitated HAP has been removed from solution by cen-rifugation method at the rotation speed of 3000 rpm. The resulting powder wasried at 80 ◦C and then calcined at 100, 450, 900 and 1200 ◦C for 1 h.

The structure of the resulting powder after drying was evaluated by Fourierransform infrared (FT-IR) spectroscopy. The phase transformation during heatreatment and crystallite size evolutions were carried out by X-ray diffractionnalysis using a siemens diffractometer (30 kV and 25 mA) with Cu K� radiationλ = 1.5405 A). The scherrer method was used to evaluate the crystallite size.

DTA and TG were performed simultaneously in an STA (Polymer Labora-ories PL-1640) using alumina pans as sample holders. The sample used was inulk and weighed 8 g. The heating rate was 10 ◦C/min and air was used as theurging gas.

The morphology of the selected resulting powder was examined with a Cam-ridge scanning electron microscope (SEM) operating at 25 kV.

Finally, transmission electron microscopy (TEM) was used in characterizinghe particles. For this purpose, particles were deposited onto Cu grids, whichupport a “holey” carbon film. The particles were deposited onto the supportrids by deposition from a dilute suspension in acetone or ethanol. The particlehapes and sizes were characterized by diffraction (amplitude) contrast and, forrystalline materials, by high resolution (phase contrast) imaging.

. Results and discussion

The structure of the powder was analyzed using FT-IR spec-roscopy after dried at 80 ◦C, as shown in Fig. 1. In the FT-IR

nalysis, mainly the peaks for PO4

3− and OH− groups in theydroxyapatite can be identified in Fig. 1. Peaks at 560–610 and000–1100 cm−1 must be due to PO4

3−. For the OH− group theeak positions are 636 and 3572 cm−1. F

Fig. 1. FTIR spectroscopy analysis of the powder after drying at 80 ◦C.

The DTA and TGA (STA) curves for the hydroxyapatite pow-er after drying are illustrated in Fig. 2. The first endothermicegion range from 90 to 295 ◦C with a peak at about 250 ◦C,hich corresponds to the dehydration of the precipitating com-lex and the loss of physically adsorbed water molecules of theydroxyapatite powder. The weight loss in this region is 16%.ith increasing temperature from 295 to 1200 ◦C no peak has

een observed, except a weight loss of 6% is observed at theGA curve in the temperature range which is assumed to be

esulted gradual dehydroxyllin in hydroxyapatite powder. Thisan be explained by the following reaction [12]:

a10(PO4)6(OH)2 → Ca10(PO4)6(OH)2−2xOx�x + xH2O

n order to investigate the effect of heat treatment on nanocrys-allization and phase transformation in hydroxyapatite powdery precipitation method. Fig. 3 presents the XRD spectra atarious temperatures for hydroxyapatite calcined powders byrecipitation method. Particularly noteworthy, the hydroxyap-tite peaks gradually increased in intensity when the sampleas heated from 100, 450, 900, up to 1200 ◦C (1 h holding time

t each temperature), indicating further nucleation/growth of the

ig. 2. DTA and TGA traces of the hydroxyapatite powder after drying at 80 ◦C.

332 I. Mobasherpour et al. / Journal of Alloys and Compounds 430 (2007) 330–333

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ig. 3. XRD patterns of heat treatment hydroxyapatite powders for differentemperatures.

ipyramidal phase was not transformed to the other phases upo 1200 ◦C.

The patterns due to the as-prepared HAP at 100 ◦C bears witht the characteristic patterns of HAP but not with much resolutionnd intensity. It contains no other crystalline phase other thanAP. The broad patterns around at (2 1 1) and (0 0 2) indicate

hat the crystallites are very tiny in nature with much atomicscillations. The XRD patterns of the heat treatment HAP at50 and 900 ◦C show increase in intensity due to planes around2 1 1), (0 0 2), (3 0 1), (2 2 2) and (2 1 3). Again it rules out theormation of any new crystalline phase other than HAP. Againt the heat treatment temperature of 1200 ◦C, new crystallinehases are not seen. This precludes even amorphous phases, ashe patterns appear very well resolved. As the increase in inten-ity is noticed for every pattern with increase in heat treatmentemperature.

The decomposition of HAP partly into tricalcium phosphateas also reported by Raynaud et al. [13]. This decomposi-

ion into tricalcium phosphate was also not observed in theresent study when the HAP was heat treatment at 1200 ◦C.eported transformation of HAP into oxyapatite between 1200nd 1400 ◦C [14], but at 1200 ◦C no such transformation isbserved in the present study.

The apparent size of hydroxyapatite crystallites obtainedrom XRD profile analysis by Scherrer method [15–17] is shownn Table 1 and Fig. 4. In this method of broadening contributionue to the crystallites size are taken into account. A gradualncrease in crystallite size was observed for hydroxyapatite with

ncreasing in heat treatment temperature.

The microstructure of the powder prepared by the presentrocess after calcined at 1200 ◦C for 1 h, examined by SEM,

able 1alculated crystallite size from XRD pattern by Scherrer method

eat treatment temperature (◦C) Grain size (nm) R2HAP

100 7.75 0.96450 8.01 0.92900 27.79 0.99200 59.06 0.98 F

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ig. 4. The hydroxyapatite crystallites obtained from XRD profile analysis bycherrer method.

s shown in Fig. 5. As it can be seen from the morphologiesf particles, there is a distribution of small particles and largegglomerates. These agglomerates consist of very fine particleshat are cold welded together.

TEM micrographs of the hydroxyapatite powders before heatreatment and after heat treatment are seen in Figs. 6 and 7.he microstructure of the hydroxyapatite particles after dry-

ng is observed to be almost like spherical, with particle sizen the range 8–20 nm. When temperature increases to 1200 ◦C,he particle size of hydroxyapatite is found to be 40–50 nm.lthough the morphology of the particles is observed to be

lmost like irregular hexagonal. When the sample was heatedrom 100 to 1200 ◦C, indicating further nucleation/growth ofhe nanocrystals inside the powder. The exact particle nucleationnd growth mechanisms are not clear. Nucleation occurs proba-ly by either hydrolytic reactions or a salting-out phenomenon.rowth could be via diffusive molecular deposition or an aggre-ation of primary particles by increase temperature. This TEM

ig. 5. SEM micrograph of hydroxyapatite particles after calcined at 1200 ◦Cor 1 h.

I. Mobasherpour et al. / Journal of Alloys a

Fig. 6. TEM micrograph of the hydroxyapatite powder before heat treatment.

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ig. 7. TEM micrograph of the hydroxyapatite powder after heat treatment.

. Conclusions

Hydroxyapatite compound with nanoparticle can success-ully be produced by precipitation technique from Ca(NO3)2·H2O and (NH4)2HPO4 solution as starting materials for

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nd Compounds 430 (2007) 330–333 333

he first time. Heat treatment was carried out on hydrox-apatite samples with different temperature. The hydroxyap-tite grain gradually increased in size when the sample waseated from 100 to 1200 ◦C, and the hydroxyapatite hexagonal-ipyramidal phase was not transformed to the other calciumhosphates phases up to 1200 ◦C. In contrast, after heat treat-ent hydroxyapatite particles obtained from the homogeneous

recipitation are perfect monodispersed with sizes ranging fromanometers.

cknowledgments

The authors would like to acknowledge Mr. S. Razavi forelping in preparing of this paper and Mr. Kaviani for doingRD experiments.

eferences

[1] W. Suchanek, M. Yoshimura, J. Mater. Res. 13 (1) (1998).[2] L.L. Hench, J. Am. Ceram. Soc. 74 (1991) 1487.[3] H. Aoki, Science and Medical Applications of Hydroxyapatite, JAAS,

Tokyo, 1991.[4] R.Z. LeGeros, Calcium Phosphate in Oral Biology and Medicine, Karger

AG, 1991.[5] S. Best, W. Bonfield, J. Mater. Sci. Mater. Med. 5 (1994) 516.[6] R. Legeros, Clin. Mater. 14 (1993) 65.[7] K.C.B. Yeong, J. Wang, S.C. Ng, Mater. Lett. 38 (1999) 208.[8] S.I. Stupp, G.W. Ciegler, J. Biomed. Mater. Res. 26 (1992) 169.[9] T.J. Webster, C. Ergun, R.H. Doremus, R.W. Siegel, R. Bizios, Biomaterials

22 (2001) 1327.10] T.J. Webster, R.W. Siegel, R. Bizios, Biomaterials 20 (1999) 1221.11] Y. Han, S. Li, X. Wang, X. Chen, Mater. Res. Bull. 39 (2004) 25–

32.12] M.G.S. Murray, J. Wang, C.B. Pontoon, P.M. Marquis, J. Mater. Sci. 30

(1995) 3061.13] S. Raynaud, E. Champion, D. Bernache-Assollant, P. Thomas, Biomaterials

23 (2002) 1065.14] J. Zhou, J. Chen, X. Zhang, K. De Groot, J. Mater. Sci. Mater. Med. 4

(1993) 83.

Publishing, 1977.16] G.K. Williamson, W.H. Hall, Acta Metall. (January) (1953) 22–31.17] M. Zakeri, R. Yazdani-Rad, M.H. Enayati, M.R. Rahimipour, J. Alloys

Compd. 403 (2005) 258–291.