Chiang Mai J. Sci. 2010; 37(2) 243

Chiang Mai J. Sci. 2010; 37(2) : 243-251www.science.cmu.ac.th/journal-science/josci.htmlContributed Paper

Nanocrystalline Hydroxyapatite Powders by aPolymerized Complex MethodJutharatana Klinkaewnarong and Santi Maensiri*Small and Strong Materials Group (SSMG), Department of Physics, Faculty of Science,Khon Kaen University, Khon Kaen 40002, Thailand.*Author for correspondence; e-mail: sanmae@kku.ac.th; santimaensiri@gmail.com

Received: 15 December 2009Accepted: 19 January 2010

ABSTRACTNanocrystalline hydroxyapatite (HAp) powders were successfully synthesized by

a polymerized complex method and calcination at 750, 800, 900 and 1,000oC for 12 h.The specific surface areas of the calcined powders obtained by BET adsorption techniquewere 1.83 - 42.82 m2/g, depending on the calcination temperature and soaking time. Thephase composition of the calcined powders was studied by X – ray diffraction (XRD) technique.The XRD results confirmed the formation of HAp phase with a small trace of β–TCP phase.With increasing calcination temperature, the crystallity of the HAp increased, showing thehexagonal structure of HAp with the lattice parameter a in a range of 0.9351 – 0.9446 nm andc of 0.6843 - 0.6906 nm The particle sizes of the powder were found to be 16 – 125 nm andas evaluated by the XRD line broadening technique and were 44 – 1,044 nm as obtained fromthe BET surface area data. The chemical compositions of the calcined powders werecharacterized by Raman and FTIR spectroscopies. The Raman spectra showed the phosphatevibration mode (ν1(PO4)) at 963 cm-1 for all the calcined powders. The peaks of the phosphatecarbonate and hydroxyl vibration modes were observed in the FTIR spectra for all the calcinedpowders. The morphology of the powders was spherical of size less than 200 nm, as revealedby TEM. Increasing the calcination temperature resulted in the transition from polycrystallineto single crystalline phase of the HAp, as clearly confirmed by the analysis of TEM diffractionpatterns.

Keywords: hydroxyapatite; nanocrystalline powders; bioceramics; polymerized complexmethod; characterization.

1. INTRODUCTIONCalcium phosphate – based materials,

especially bioactive hydroxyapatite (HAp,Ca10(PO4)6(OH)2), is the main component ofbone mineral [1]. HAp has been widelyconsidered as one of the most importantbioceramics for medical and dental applicationdue to its excellent biocompatibility [2]. It has

been also applied in non-medical fields, suchas gas sensors, catalysis and host material forlasers [3]. Unfortunately, due to its lowreliability, especially in wet environments, HApcannot presently be used for heavy load -bearing applications, like artificial teeth orbones [4]. Properties of HAp, including

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bioactivity, biocompatibility, solubility, sin-terability, castability, fracture toughness andabsorption can be tailored over wide rangesby controlling the particle composition, sizeand morphology [5]. Therefore, many signifi-cant synthesis methods have been exploredto prepare HAp with controllable properties[6]. Especially for nanoscale HAp, there area lot of especial behaviors. For example,nanocrystalline HAp powder could enhancedensification and improve the fracturetoughness of HAp ceramics [7]. Manymethods have been developed to prepareHAp nanopowders including sol–gel method[8-10], hydrothermal reaction [11-13],precipitation method [14,15], mechano-chemical [16], emulsion technique [17-19], andwet-chemical method [20]. However, mostof these methods are costly and require astrict pH control, vigorous agitation and longtime for hydrolysis. In this study, polymerizedcomplex method is chosen because this routeis relative simplicity (not strict pH control) andusefulness for obtaining a homogeneous andfine powder precursor.

The polymeric precursor solution wasprepared using the polymerized complex (PC)method which has been used to synthesizepolycation oxide powders [21]. It is basedon metallic citrate polymerization with theuse of ethylene glycol. A hydrocarboxylic acidsuch as citric acid was used to cheated cationsin aqueous solution. The addition of a glycol,such as ethylene glycol, leads to organic esterformation. Polymerization promoted byheating the mixture results in a homogeneousresin in which metal ions are uniformlydistributed throughout the organic matrix.This synthesis method has been previouslyused for synthesis of the nanoparticles ofFe-doped La0.5Sr0.5 TiO3-δ [22], Co–doped(La,Sr)TiO3-δ [23], and Co–doped ZnO [24].

In this article, we demonstrate thesynthesis of nanocrystalline HAp powders by

a polymerized complex method (PC). Thesynthesized HAp powders were characterizedby thermogravimetric differential thermalanalysis (TG/DTA), X-ray diffraction (XRD),Fourier transform infrared spectroscopy(FTIR) and transmission electron microscopy(TEM).

2. EXPERIMENTAL PROCEDUREIn this study, citric acid, C6H8O.H2O,

(99.7% purity, BDH) was dissolved in ethyleneglycol, CH2OHCH2OH (99.5% purity, CarloErba Reagenti) and constant temperature(80oC) with vigorous stirring. Subsequently,calcium nitrate, Ca(NO3)2

.4H2O (99.9%purity, Kanto Chemical) and diammoniumhydrogen phosphate, (NH4)2HPO4 (99%purity, BDH) were slowly added to thissolution. The molar ration of Ca/P was keptat 1.67 as HAp. Citric acid and ethylene glycolwere mixed in the respective proportion of4 and 60 moles for each mole of metal cations.The mixed solution was continuously stirredat 200oC until homogeneous highly viscouspolymeric was obtained. The black resinprecursor was dried at 350oC. In order todetermine the temperature of possibledecomposition and crystallization of thenanoparticles, the dried precursor wassubjected to thermogravimetric differentialthermal analysis (TG/DTA) (Pyris DiamondTG/DTA, PerkinElmer Instrument).The crystallization seemed to occur attemperature above 700oC (Figure 1) and thencalcined at 750, 800, 900 and 1,000oC for12 h in air. Phase analysis of the calcined HAppowders were conducted using X-ray Dif-fraction (XRD) (PW3710, The Netherlands)with CuKα radiation (λ = 0.15406 nm).Specific surface area measurement was madeusing the BET (S. Brunauer, P. H. Emmett,and E. Teller) method [25] utilizing adsorptionof N2 gas at 120oC (Coulter SA-3100, SurfaceArea and Pore Size Measurement Analyzer).

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The products were also characterized byFourier transform infrared spectroscopy(FTIR, PerkinElmer instruments, spectrumone) in the range of 4,000-450 cm-1 and Ramanspectroscopy were recorded at room tem-perature by using a triple spectrometer (JobinYvon/Atago-Bussan T-64000, France) witha liquid nitrogen cooled CCD detector for800 s, in micro-mode. The Ar+ laser beamwith the excitation λ = 514.5 nm was focusedunder 90x microscope objective and the laserspot size was between 1 and 2 μm. Ramanspectra were recorded in the 1,200–200 cm-1

range with the spectral resolution of 1 cm-1.The particle sizes of the calcined powderswere obtained by BET method. Thesemethods for determination of the surfacearea (Surface Area and Pore Size MeasurementAnalyzer, Coulter SA–3100) of powder arebased on the phenomenon of gas adsorption.The morphology of the calcined powderswas characterized by transmission electronmicroscopy (TEM, JEOL JEM2010, 200kV).

3. RESULTS AND DISCUSSIONThe thermogravimetric analysis of dried

precursor was carried out between 30oC and1,000oC in air at heating rate of 5oC/min.The simultaneous TG/DTA curves are shownin Figure 1. There is a minute weight loss(<5%) around 300oC. It is assigned to weaklyentrapped water in the dried precursor. Itis followed by major weight loss (60%)between 300 and 500oC. It is assigned to thedehydration of nitrate in raw materials.Almost no weight loss could be observed atabove 700oC, suggesting the formation ofcrystalline HAp as a decomposed product.The first strong exothermic peak is seen inthe DTA at about 500oC results from thecombustion of organic materials.

The XRD patterns of the precursor andcalcined HAp powders are shown in Figure 2.The XRD results that the precursor (350oC)and the 750oC calcined sample are calciumhydrogen phosphate (monotite, CaHPO4)phase, whereas the HAp samples calcined at

Figure 1. TG/DTA curves of thermal decomposition of nanocrystallineHAp powder precursor at a heating rate of 5oC/min in static air.

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800, 900 and 1,000oC have a main phase ofHAp (PDF Card No.9-432 of hexagonalHAp phase) with a small trace of monotiteand β–TCP phases, which have a Ca/P ratiodeviated from 1.67 [26]. It is suggested thatthe HAp phase does not form in the precursorand the 750oC calcined samples but it fullyformed after calcination at above 800oC. Thisresults demonstrate that the calcinationstemperature play an important role in theformation of HAp. As the calcinationstemperature is increased from 350 to 1,000oCseveral of HAp lines become more distinctat higher temperature, and also the widths ofthe lines become narrower, which suggest anincrease in the crystalline degree.

The line broadening of the (002), (211),(300), (202), (310), (222) and (213) reflectionswas used to evaluate the mean crystal size, Lof the prepared HAp powders, which iscalculated from the Scherrer equation [27]:

where λ is the wavelength of the X – rayradiation, k is a constant generally taken to be1.0 [27], θ is the diffraction angle, and βr isthe full width at half maximum (FWHM) andis given by βr

2 = βo2 - βi

2 , where βo and βi arethe width from the observed X-ray peak andthe width due to instrumental effects,respectively. The estimated crystal sizes were16, 43, 125 and 125 nm for the HAp samplescalcined at 750, 800, 900 and 1,000oC for 12 h,respectively. The values of lattice parameter acalculated from the XRD spectra were 0.9351

0.0110, 0.9446 0.0031, 0.9446 0.0031and 0.9428 0.0000 nm and those of latticeparameter c were 0.6906 0.0029, 0.6843 0.0137, 0.6894 0.0083, and 0.6880 0.0016 nm for the HAp powders calcined at750, 800, 900 and 1,000oC for 12 h,respectively. These data are tabulated inTable 1.

BET measurements show the differentspecific surface area of 42.82, 5.83, 2.18,and 1.83 m2/g for the HAp powders calcined

Figure 2. XRD patterns of HAp precursor and powders calcinedin air at 750, 800, 900 and 1,000oC for 12 h.

Inte

nsi

ty (

a.u

.)

Diffraction Angle (2θθθθθ)

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at 750, 800, 900, and 1,000oC for 12 h,respectively. The average particle size canbe obtained from the surface area data.Assuming that the powder is unagglomeratedand that the particles are spherical and dense,the particle size, D can be estimated from theequation [25]:

where Sw is the specific surface area (m2/g)and ρs is the density of the solid sample (3.16g/cm3). The estimated particle sizes were 44,327, 678, and 1,041 nm for the HAp samplescalcined at 750, 800, 900 and 1,000oC for12 h, respectively, as also tabulated in Table 1.

FTIR spectra of the precursor and calcinedsamples are shown in Figure 3. There is abroad envelope between 3,800 and 2,600 cm-1

due to the O - H stretch of water and HAp.The O - H groups are hydrogen bounded [10].The sharp peak at 3,575 cm-1 was assigned tofree O - H stretch which may be presented atthe surface of the crystalline of the HAppowders calcined at 800, 900 and 1,000oCfor 12 h. The intense broad peaks between1,100 and 900 cm-1 are assigned to non-separated ν1, ν3 and ν4 vibration modes ofPO4

3- groups. The stretching and the bendingmodes of PO4

3- appear at ~607 and 567 cm-1

as intense sharp peak. Absorption bands inthe range of 1,400 to 1,570 cm-1 can be

Calcination Particle size (nm) Hexagonal lattice Hexagonal latticetemperature (oC) From XRD From BET parameter a (nm) parameter c (nm)

750 oC/12h 16 44 0.9351 0.0110 0.6906 0.0029800 oC/12h 43 327 0.9446 0.0031 0.6843 0.0137900 oC/12h 125 678 0.9446 0.0031 0.6894 0.00831,000 oC/12h 125 1,041 0.9428 0.0000 0.6880 0.0016

Table 1. Particle sizes from XRD line broadening, hexagonal lattice parameters a and ccalculated from XRD patterns of the HAp samples calcined in air for 12 h at 750, 800, 900and 1,000oC.

Figure 3. Raman spectra of HAp precursor and powders calcined in air at 750,800, 900 and 1,000oC for 12 h.

4,000 3,500 3,000 2,500 2,000 1,500 1,000 500

Wavenumber (cm-1)

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attributed to C-O and C=O stretchingvibrations of organic residuals [28,29].

The Raman spectra of the HAp powdersare shown in Figure 4. The Raman spectra ofHAp have been reported previously [30-33].The phosphate modes at ~1,072, 1,046 and1,029 cm-1 are due to (ν3) P-O stretchingsymmetric. The mode at ~963 cm-1 has veryhigh intensity, which is attributed to the mode(ν1) P-O-P totally stretching with freetetrahedral phosphate ion associating HAp.

Modes at ~593 and 581 cm-1 are attributedto the (ν4) O-P-O stretching and symmetricand that at ~431 cm-1 is due to the (ν2) O-P-Owith stronger bands. The internal Ramanvibrations of phosphate carbonate and hy-droxyl ions in apatite appear above 400 cm-1.It is seen from Figure 4 that all the calcinedpowders have high intensity Raman peak at~963 cm-1 indicating a strong characteristicof HAp.

The TEM images of the HAp samples

Figure 4. FTIR spectra of HAp precursor and powders calcined inair at 750, 800, 900 and 1,000oC for 12 h.

calcined at 750, 800, 900 and 1,000oC for 12h, respectively are shown in Figure 5. All theHAp samples show spherical morphology ofsize less than 200 nm with agglomeration. Anincrease in calcinations temperature gives thesystem adequate kinetics to permit furthergrowth of the HAp grains, as observed inFigure 5d. It suggest that increasing thecalcination temperature resulted in thetransition from polycrystalline to singlecrystalline phase of the HAp, as clearlyconfirmed by the analysis of TEM diffraction

patterns. The selected area electron diffraction(SAED) patterns shown in Figure 5 (a)-(b),show spotty ring patterns indicating thecrystalline phase of HAp with the randomorientation. The HAp sample calcined at 900and 1,000oC for 12 h (Figure 5c-d) show theSAED spots patterns with the clear diffractionpatterns indicating strong development ofHAp single crystal. These SAED analysesconfirmed the formation of hexagonalstructure of HAp, and are in agreement withthe XRD results.

Inte

nsi

ty (

a.u

.)

400 600 800 1,000 1,200

Wavenumber (cm-1)

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4. CONCLUSIONSNanocrystalline HAp powders were

successfully synthesized by a polymerizedcomplex method. Structural and morpholo-gical properties and chemical composition ofthe synthesized powders were characterizedby XRD, FTIR, Raman, and TEM. The XRDresults showed that the synthesized nano-crystalline HAp powders have hexagonalstructure and were fully formed aftercalcination in air at 1,000oC for 12 h.The synthesized nanocrystalline HAp powdersare spherical nanoparticles having the particlesize less than 200 nm. The crystal size of HApsamples increased with increasing calcinationtemperature.

ACKNOWLEDGEMENTSThe authors would like to thank the

Department of Chemistry, Khon KaenUniversity for providing TG/DTA, FTIRspectroscopy facilities, the Science andTechnology Service Center (Chiang Mai

Figure 5. TEM images with corresponding selected area electron diffraction (SAED) patternsof HAp powders calcined in air for 12 h at (a) 750oC, (b) 800oC, (c) 900oC, and (d) 1,000oC.

University) for providing TEM facilities.This work is supported by The IntegratedNanotechnology Research Center (INRC),Khon Kaen University, Thailand.

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