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P ratio and ultrasonic power on the crystallinity and morphology of hydroxyapatite nanoparticles prepared with a novel ultrasonic precipitation method
Materials Letters 59 (2005) 1902 – 1906 www.elsevier.com/locate/matlet
Influence of temperature, [Ca2+], Ca/P ratio and ultrasonic power on the crystallinity and morphology of hydroxyapatite nanoparticles prepared with a novel ultrasonic precipitation method Cao Li-yuna,T, Zhang Chuan-bob, Huang Jian-fengc a
College of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xianyang, Shaanxi, P.R. China, 712081 b College of Resource and Environment, Shaanxi University of Science and Technology, Xianyang, Shaanxi, P.R. China, 712081 c School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xianyang, Shaanxi, P.R. China, 712081 Received 28 June 2004; received in revised form 5 February 2005; accepted 10 February 2005 Available online 10 March 2005
Abstract Nanocrystalline hydroxyapatite was prepared by a precipitation method with the aid of ultrasonic irradiation using Ca(NO3)2 and NH4H2PO4 as source material and carbamide (NH2CONH2) as precipitator. The influence of Ca/P molar ratio, precipitation temperature, concentration of Ca2+ ([Ca2+]) and ultrasonic power on the crystallinity of the nanopowder were systematically investigated by XRD analysis. The size of the as-prepared particles was analyzed using TEM and XRD methods. The results revealed that the monophase hydroxyapatite could be obtained at the following technological conditions: [Ca2+] = 0.01–0.1 mol/L, ultrasonic power = 300 W, Ca/P (mol) = 1.2–2.5 and T = 313–353 K. In addition, the acicular and spherical particles could be prepared at different ultrasonic powers of 300 and 200 W, respectively. D 2005 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Ultrasound; Precipitation; Nanopowder
1. Introduction Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) is one of the most important bioceramics for medical and dental applications such as dental implants, alveolar bridge augmentation, orthopaedics, maxillofacial surgery and drug delivery systems due to its biocompatibility and chemical and biological affinity with bone tissue [1–3]. To produce high quality HAp bioceramics for artificial bone substitution, ultrafine HAp powder was usually employed [4]. NanoHAp powder results in easy handling, casting and sintering, leading to an excellent sintered body in the bioceramics preparing process.
T Corresponding author. Tel.: +86 910 3570704; fax: +86 910 3579723. E-mail addresses: [email protected] (C. Li-yun)8 [email protected], [email protected] (H. Jian-feng). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.007
To date, many methods have been developed to prepare HAp powders. The techniques include sol–gel [5], homogeneous precipitation [6], hydrothermal [7], mechano-chemical [8], RF plasma spray [9], spray dry [10], combustion synthesis [11], supersonic rectangular jet impingement [12], ultrasonic spray freeze-drying [13], sonochmical synthesis [14] methods, etc. In these methods, wet chemical process was usually used to prepare HAp powders because it is easy to operate and does not require expensive equipment. However, it needs highly qualified and controlled parameters such as the nature and composition of the starting materials, pH and temperature of the solutions prepared to obtain HAp monophase. Moreover, the nanoparticles produced from wet chemical method usually present an acicular or plate-like morphology. On the other hand, ultrasonic spray freeze-drying could obtain the spherical particles. Spherical powders, in general, have better rheological properties than irregular powders and, thus,
C. Li-yun et al. / Materials Letters 59 (2005) 1902–1906
produce better products such as bulk bioceramics and coatings for bone substitution and implants [4,15]. In the present work, nanocrystalline HAp was prepared by a precipitation method with the aid of ultrasonic irradiation using Ca(NO3)2 and NH4H2PO4 as source material. The influence of Ca/P mole ratio, precipitation temperature, concentration of Ca2+([Ca2+]) and ultrasonic frequency on the crystallinity of the nanopowder were primarily investigated. The acicular and spherical particles were also synthesized in different conditions.
2. Experiment Analytical grade Ca(NO3)2, NH2CONH2 and NH4H2PO4 were selected for use in this work. A tool for irradiating ultrasound was an available commercial device (SC-III, ultrasonic irradiator, Jiu Zhou, China) with a variable power of 100–300 W. The sonic horn made of Ti (dip diameter length = A10 70 mm) was driven by a PZT transducer. The diagram of the experimental device is shown in Fig. 1. Firstly, the Ca(NO3)2 and NH4H2PO4 were weighed in molar ratio of Ca/P from 1.2 to 2.5 and dissolved in distilled water to make a homogenous solution. The starting concentration of Ca2+([Ca2+]) was also designed to vary from 0.01–0.1 mol/L to investigate the influence of Ca/P ratio and [Ca2+] on the crystallinity of the powders. Then, the solution was heated with a water bath that provides an invariable reactive temperature. In the experiment, the reactive temperature was set from 293 to 353 K. In the meantime, the sonic horn was dipped into the solution around 10 mm in distance from the bottom of the flask. In the preparation process, the sonicating power and amplitude
NH2CONH2 solution
steam cooling device
ultrasonic irradiation head
water bath
Fig. 1. Sketch diagram of the experimental device for preparing hydroxyapatite powder.
1903
were adjusted to provide different power outputs of the generator. The periods of irradiation time (reactive time) was constant at 2 h. Next, the 12 wt.% NH2CONH2 solution was added to the solution to fix the solution pH in the vicinity of 7.4. After 2 h reaction, the precipitates were separated from the mother liquor by filtration and washed with de-ionized water 4 times. The filtrates were dried in a vacuum dryer for 5 h at 353 K. The as-prepared powders were characterized by X-ray diffractometry (XRD) using a high-resolution Seifert-FPM diffractometer operating with CuKa radiation at 40 kV and 40 mA, divergence slit of 18, receiving slit width of 0.1 mm and a scan rate of 28/min. Peak position and line width variation were controlled with the measurement of XRD lines of a standard silicon sample. The Debye–Scherrer equation, r = Kk / B cosh, was used in order to determine the crystallite sizes along (002) directions: r is the crystallite size in the chosen direction, K is a constant, k is the wavelength of the CuKa radiation and B is the line width of the XRD peak. The crystallite morphology of the samples was analyzed in a transmission electron microscope (TEM) operated at 200 kV. TEM specimens were prepared by depositing a few drops of HAp dispersed in acetone on a carbon-coated copper grid.
3. Results and discussion Fig. 2 shows the XRD patterns of the powders prepared in different [Ca2+]. It can be observed that the sample crytallinity was variable. At the range of 0.01–0.1 mol/L of [Ca2+], the HAp crystallites were obtained, while the phosphate crystallites with the mixture phases of HAp and CaHPO4 were acquired when the [Ca2+] increased to 0.2 mol/L. Fig. 2 also displays the improvement in crystallization with the increase of [Ca2+]. The HAp peaks are generally broad as is typical of HAp synthesized from the sol–gel and chemical precipitation process [5,6] without irradiating ultrasound. This is implying that the ultrasonic irradiation applied in the preparation procedure is helpful to synthesize a finer particle and this deduction is verified by the XRD analysis of particle size shown in Fig. 3. Compared with the powder size of 2.47 Am [6] and 38–67 nm [16] prepared by chemical precipitation technique, the present method produced an 8.9–38 nm particle. In addition, Fig. 3 reveals an increase in particle size with the increase of [Ca2+], indicating an important influence of [Ca2+] on the sample crystallization. The reason needs more research in the future. The XRD patterns of the powders prepared in different ultrasonic powers are given in Fig. 4. Lower than 300 W, the monophase HAp could not be achieved. Some other peaks of phosphate crystallites such as Ca3(PO4)2 and Ca2P2O7 appear in the XRD patterns. This indicates that ultrasonic power is crucial to the synthesis of HAp and the HAp monophase can be obtained only when the ultrasonic power
C. Li-yun et al. / Materials Letters 59 (2005) 1902–1906
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Fig. 2. XRD patterns of the powders prepared in different [Ca ] (Ca/ P= 2.0, 300 W, 353 K).
exceeds a critical value. It has been well recognized that the ultrasonic irradiation caused cavitation in an aqueous medium where the formation, growth and collapse of microbubbles occurred. This can result in extreme conditions of temperature (N2000 K) and pressure (N500 bar) on a microsecond timescale leading to the formation of reactive intermediates such as radicals. This can stimulate the reactivity of chemical species involved, resulting in the acceleration of the heterogeneous reactions between liquid and solid reactants effectively [17]. Therefore, the increase of ultrasonic power could cause a rapid reaction that leads to the formation of monophase of HAp. The weakening of the other phosphate peaks with the strengthening of ultrasonic power shown in Fig. 4 also verifies it. The particle size of the HAp phases in different ultrasonic powers analyzed by XRD method is shown in Fig. 5. The particle size of the HAp crystallites decreases linearly with the increase of ultrasonic power, which infers
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∆ ∆ ♦∆ ♦ ∆ ♦♦ ♦ ♦ ∆ ∆ ∆
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Fig. 4. XRD patterns of the powders prepared in different ultrasonic powers ([Ca2+] = 0.02, Ca/P= 2.0, 353 K).
that the irradiation of ultrasound on the reaction solution during the precipitation process is helpful to make finer particles. Fig. 6 shows the TEM images of the HAp powders prepared in different ultrasonic powers. It reveals the formation of acicular (Fig. 6a) and spherical (Fig. 6b) particles in different ultrasonic powers of 200 and 300 W, respectively. This is quite different from the particle morphologies (acicular or plate-like) reported by other wet chemical methods [5–8,16]. From the TEM images, the sizes of the acicular and spherical particles were determined to be 20 100 and 10 nm, respectively. By XRD calculation according to the Scherrer formula, the acicular crystallite size in the perpendicular direction of the (002) Miller plane was calculated to be around 20 nm. This is much smaller when compared to the TEM images (almost 100 nm). However, the spherical crystallite size as determined from the XRD calculations was comparable to the TEM micrographs.
35 40 30
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35
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30 25 20
25 20 15
15 10
10 5 0.00
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5 100
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ultrasonic power/W Fig. 5. Particle size of HAp powder as a function of ultrasonic power.
C. Li-yun et al. / Materials Letters 59 (2005) 1902–1906
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2θ/degree Fig. 8. XRD patterns of the powders prepared in different reactive temperatures (Ca/P= 2.0, [Ca2+] = 0.02 mol/L, 300 W).
Fig. 6. TEM images of the HAp powders prepared at 353 K, [Ca2+] = 0.02 mol/L, Ca/P= 2.0 and different ultrasonic powers: (a) 200 W and (b) 300 W.
Fig. 7 reveals the XRD patterns of the powders prepared in different Ca/P ratios. Monophase of HAp could be gained in a wide range of Ca/P ratio from 1.2 to 2.5. The formation range of Ca/P for HAp is wider than the research results (1.5–1.667) of Raynaud et al. [18]. When the Ca/P ratio is 1.67, the (002) peak intensity of the crystallites is the strongest, implying a preferential growth along the (002) direction. This is agreement with the other work [19]. The preferential growth along the (002) direction also obviously
appears in different reactive temperatures. Fig. 8 displays the influence of reactive temperature on the cystallinity of the HAp powder. It is clear that the (002) peak intensity decreases with the reactive temperature increase, which infers that the acicular or plate-like HAp crystallites could be formed at lower temperature. In addition, in the experiment, a little powder was acquired when the reactive temperature was controlled at 293 K although the reactive time was prolonged to 3 h. It is different from the results reported by Pang and Bao[20] that the HAp powders could be synthesized at 288 K and the peak intensity and the particle size increase with the temperature increase. In contrast to it, in our research, the HAp diffraction peak became broader with the increase of temperature (see Fig. 8) and the particle size of the HAp crystallites decreases with the temperature increase (see Fig. 9a). The reason could be attributed to the irradiation of ultrasound in our research though the convincing reason needs more research in the future.
♦: HAp
16
Ca/P=2.5 15
particle size/nm
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Ca/P=2.0
Ca/P=1.67
Ca/P=1.2
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0
10
20
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2θ/degree Fig. 7. XRD patterns of the powders prepared in different Ca/P ratios ([Ca2+] = 0.01 mol/L, 300 W, 353 K).
9
310
320
330
340
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temperature/K Fig. 9. Particle size of HAp powder as a function of reactive temperature.
1906
C. Li-yun et al. / Materials Letters 59 (2005) 1902–1906
4. Conclusion The chemical precipitation with the aid of ultrasonic irradiation on aqueous solutions provides a simple and economic route for synthesis of monophase hydroxyapatite nanoparticles. The crystallinity and morphology of the resulting nanoparticles are dependent on the Ca/P ratio, concentration of Ca2+, ultrasonic power and synthetic temperature. When the concentration of Ca2+ exceeded 0.2 mol/L and the ultrasonic power was lower than 300 W, the monophase of HAp powder could not be obtained. When the reactive temperature and Ca/P increase from 313 to 353 K and 1.67 to 2.5, respectively, the as-prepared crystallites exhibit a preferential growth along the (002) direction and the nanoparticles show an acicular morphology. On the other hand, spherical nanoparticles could be obtained at high synthetic temperature (353 K), ultrasonic power (300 W) and high Ca/P (2.0–2.5). In addition, the crystallite size of the HAp nanoparticles decreases with the decrease of [Ca2+] and the increase of synthetic temperature and ultrasonic power.
References [1] K.J.L. Burg, S. Porter, J.F. Kellam, Biomaterial development for bone tissue engineering, Biomaterials 21 (2000) 2347 – 2359. [2] A.K. Dash, G.C. Cudworth, Therapeutic applications of implantable drug delivery systems, J. Pharmacol. Toxicol. Methods 40 (1998) 1 – 12. [3] W. Suchanek, M. Yoshimura, Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants, J. Mater. Res. 13 (1998) 94 – 117. [4] D.M. Liu, Preparation and characterization of porous hydroxyapatite bioceramics via a slip-casting route, Ceram. Int. 24 (1998) 441 – 446. [5] G. Bezzi, G. Celotti, E. Landi, T.M.G. La Torretta, I. Sopyan, A. Tampieri, A novel sol–gel technique for hydroxyapatite preparation, Mater. Chem. Phys. 78 (2003) 816 – 824.
[6] H.Q. Zhang, S.P. Li, Y.H. Yan, Dissolution behavior of hydroxyapatite in hydrothermal solution, Ceram. Int. 27 (2001) 451 – 454. [7] H.S. Liu, T.S. Chin, L.S. Lai, S.Y. Chiu, K.H. Chuang, C.S. Chang, M.T. Lui, Hydroxyapatite synthesized by a simplified hydrothermal method, Ceram. Int. 23 (1997) 19 – 25. [8] M. Toriyama, A. Ravaglioli, A. Krajewski, G. Celotti, A. Piancastelli, Synthesis of hydroxyapatite-based powders by mechanochemical method and their sintering, J. Eur. Ceram. Soc. 16 (1996) 429 – 436. [9] R. Kumar, P. Cheang, K.A. Khor, RF plasma processing of ultra-fine hydroxyapatite powders, J. Mater. Process. Technol. 113 (2001) 456 – 462. [10] P. Luo, T.G. Nieh, Synthesis of ultrafine hydroxyapatite particles by a spray dry method, Mater. Sci. Eng., C 3 (1995) 75 – 78. [11] A. Cuneyt Tas, Combustion synthesis of calcium phosphate bioceramic powders, J. Eur. Ceram. Soc. 20 (2000) 2389 – 2394. [12] V. Shukla, G.S. Elliott, B.H. Kear, Hyperkinetic deposition of nanopowders by supersonic rectangular jet impingement, Scr. Mater. 44 (2001) 2179 – 2182. [13] K. Itatani, K. Iwafune, F. Scott Howell, M. Aizawa, Preparation of various calcium-phosphate powders by ultrasonic spray freeze-drying technique, Mater. Res. Bull. 35 (2000) 575 – 585. [14] W. Kim, F. Saito, Sonochemical synthesis of hydroxyapatite from H3PO4 solution with Ca(OH)2, Ultrason. Sonochem. 8 (2001) 85 – 88. [15] S.W.K. Kweha, K.A. Khora, P. Cheang, An in vitro investigation of plasma sprayed hydroxyapatite (HA) coatings produced with flamespheroidized feedstock, Biomaterials 23 (2002) 775 – 785. [16] E. Mavropoulos, A.M. Rossi, N.C.C. Rocha, G.A. Soares, J.C. Moreira, G.T. Moure, Dissolution of calcium-deficient hydroxyapatite synthesized at different conditions, Mater. Charact. 5578 (2003) 1 – 5. [17] G.J. Price, Recent developments in sonochemical polymerization, Ultrason. Sonochem. 10 (2003) 277 – 283. [18] S. Raynaud, E. Champion, D. Bernache-Assollant, P. Thomas, Calcium phosphate apatites with variable Ca/P atomic ratio: I. Synthesis, characterization and thermal stability of powders, Biomaterials 23 (2002) 1065 – 1072. [19] J.C. Knowles, S. Callcut, G. Georgiou, Characterisation of the rheological properties and zeta potential of a range of hydroxyapatite powders, Biomaterials 21 (2000) 1387 – 1392. [20] Y.X. Pang, X. Bao, Influence of temperature, ripening time and calcination on the morphology and crystallinity of hydroxyapatite nanoparticles, J. Eur. Ceram. Soc. 23 (2003) 1697 – 1704.
Influence of temperature, [Ca2+], Ca/P ratio and ultrasonic power on the crystallinity and morphology of hydroxyapatite nanoparticles prepared with a novel ultrasonic precipitation method Cao Li-yuna,T, Zhang Chuan-bob, Huang Jian-fengc a
College of Chemistry and Chemical Engineering, Shaanxi University of Science and Technology, Xianyang, Shaanxi, P.R. China, 712081 b College of Resource and Environment, Shaanxi University of Science and Technology, Xianyang, Shaanxi, P.R. China, 712081 c School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xianyang, Shaanxi, P.R. China, 712081 Received 28 June 2004; received in revised form 5 February 2005; accepted 10 February 2005 Available online 10 March 2005
Abstract Nanocrystalline hydroxyapatite was prepared by a precipitation method with the aid of ultrasonic irradiation using Ca(NO3)2 and NH4H2PO4 as source material and carbamide (NH2CONH2) as precipitator. The influence of Ca/P molar ratio, precipitation temperature, concentration of Ca2+ ([Ca2+]) and ultrasonic power on the crystallinity of the nanopowder were systematically investigated by XRD analysis. The size of the as-prepared particles was analyzed using TEM and XRD methods. The results revealed that the monophase hydroxyapatite could be obtained at the following technological conditions: [Ca2+] = 0.01–0.1 mol/L, ultrasonic power = 300 W, Ca/P (mol) = 1.2–2.5 and T = 313–353 K. In addition, the acicular and spherical particles could be prepared at different ultrasonic powers of 300 and 200 W, respectively. D 2005 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Ultrasound; Precipitation; Nanopowder
1. Introduction Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) is one of the most important bioceramics for medical and dental applications such as dental implants, alveolar bridge augmentation, orthopaedics, maxillofacial surgery and drug delivery systems due to its biocompatibility and chemical and biological affinity with bone tissue [1–3]. To produce high quality HAp bioceramics for artificial bone substitution, ultrafine HAp powder was usually employed [4]. NanoHAp powder results in easy handling, casting and sintering, leading to an excellent sintered body in the bioceramics preparing process.
T Corresponding author. Tel.: +86 910 3570704; fax: +86 910 3579723. E-mail addresses: [email protected] (C. Li-yun)8 [email protected], [email protected] (H. Jian-feng). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.007
To date, many methods have been developed to prepare HAp powders. The techniques include sol–gel [5], homogeneous precipitation [6], hydrothermal [7], mechano-chemical [8], RF plasma spray [9], spray dry [10], combustion synthesis [11], supersonic rectangular jet impingement [12], ultrasonic spray freeze-drying [13], sonochmical synthesis [14] methods, etc. In these methods, wet chemical process was usually used to prepare HAp powders because it is easy to operate and does not require expensive equipment. However, it needs highly qualified and controlled parameters such as the nature and composition of the starting materials, pH and temperature of the solutions prepared to obtain HAp monophase. Moreover, the nanoparticles produced from wet chemical method usually present an acicular or plate-like morphology. On the other hand, ultrasonic spray freeze-drying could obtain the spherical particles. Spherical powders, in general, have better rheological properties than irregular powders and, thus,
C. Li-yun et al. / Materials Letters 59 (2005) 1902–1906
produce better products such as bulk bioceramics and coatings for bone substitution and implants [4,15]. In the present work, nanocrystalline HAp was prepared by a precipitation method with the aid of ultrasonic irradiation using Ca(NO3)2 and NH4H2PO4 as source material. The influence of Ca/P mole ratio, precipitation temperature, concentration of Ca2+([Ca2+]) and ultrasonic frequency on the crystallinity of the nanopowder were primarily investigated. The acicular and spherical particles were also synthesized in different conditions.
2. Experiment Analytical grade Ca(NO3)2, NH2CONH2 and NH4H2PO4 were selected for use in this work. A tool for irradiating ultrasound was an available commercial device (SC-III, ultrasonic irradiator, Jiu Zhou, China) with a variable power of 100–300 W. The sonic horn made of Ti (dip diameter length = A10 70 mm) was driven by a PZT transducer. The diagram of the experimental device is shown in Fig. 1. Firstly, the Ca(NO3)2 and NH4H2PO4 were weighed in molar ratio of Ca/P from 1.2 to 2.5 and dissolved in distilled water to make a homogenous solution. The starting concentration of Ca2+([Ca2+]) was also designed to vary from 0.01–0.1 mol/L to investigate the influence of Ca/P ratio and [Ca2+] on the crystallinity of the powders. Then, the solution was heated with a water bath that provides an invariable reactive temperature. In the experiment, the reactive temperature was set from 293 to 353 K. In the meantime, the sonic horn was dipped into the solution around 10 mm in distance from the bottom of the flask. In the preparation process, the sonicating power and amplitude
NH2CONH2 solution
steam cooling device
ultrasonic irradiation head
water bath
Fig. 1. Sketch diagram of the experimental device for preparing hydroxyapatite powder.
1903
were adjusted to provide different power outputs of the generator. The periods of irradiation time (reactive time) was constant at 2 h. Next, the 12 wt.% NH2CONH2 solution was added to the solution to fix the solution pH in the vicinity of 7.4. After 2 h reaction, the precipitates were separated from the mother liquor by filtration and washed with de-ionized water 4 times. The filtrates were dried in a vacuum dryer for 5 h at 353 K. The as-prepared powders were characterized by X-ray diffractometry (XRD) using a high-resolution Seifert-FPM diffractometer operating with CuKa radiation at 40 kV and 40 mA, divergence slit of 18, receiving slit width of 0.1 mm and a scan rate of 28/min. Peak position and line width variation were controlled with the measurement of XRD lines of a standard silicon sample. The Debye–Scherrer equation, r = Kk / B cosh, was used in order to determine the crystallite sizes along (002) directions: r is the crystallite size in the chosen direction, K is a constant, k is the wavelength of the CuKa radiation and B is the line width of the XRD peak. The crystallite morphology of the samples was analyzed in a transmission electron microscope (TEM) operated at 200 kV. TEM specimens were prepared by depositing a few drops of HAp dispersed in acetone on a carbon-coated copper grid.
3. Results and discussion Fig. 2 shows the XRD patterns of the powders prepared in different [Ca2+]. It can be observed that the sample crytallinity was variable. At the range of 0.01–0.1 mol/L of [Ca2+], the HAp crystallites were obtained, while the phosphate crystallites with the mixture phases of HAp and CaHPO4 were acquired when the [Ca2+] increased to 0.2 mol/L. Fig. 2 also displays the improvement in crystallization with the increase of [Ca2+]. The HAp peaks are generally broad as is typical of HAp synthesized from the sol–gel and chemical precipitation process [5,6] without irradiating ultrasound. This is implying that the ultrasonic irradiation applied in the preparation procedure is helpful to synthesize a finer particle and this deduction is verified by the XRD analysis of particle size shown in Fig. 3. Compared with the powder size of 2.47 Am [6] and 38–67 nm [16] prepared by chemical precipitation technique, the present method produced an 8.9–38 nm particle. In addition, Fig. 3 reveals an increase in particle size with the increase of [Ca2+], indicating an important influence of [Ca2+] on the sample crystallization. The reason needs more research in the future. The XRD patterns of the powders prepared in different ultrasonic powers are given in Fig. 4. Lower than 300 W, the monophase HAp could not be achieved. Some other peaks of phosphate crystallites such as Ca3(PO4)2 and Ca2P2O7 appear in the XRD patterns. This indicates that ultrasonic power is crucial to the synthesis of HAp and the HAp monophase can be obtained only when the ultrasonic power
C. Li-yun et al. / Materials Letters 59 (2005) 1902–1906
♦♦ ♦
♦ ♦♦ ♦
∆ ∆∆ ♦
Intensity
Intensity
♦
♦
♦
♦♦
♦ ♦♦ ♦
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200W
♦ ∆∆ ∆
∆ ♦
♦ ∆ ∆♦♦
∆
∆
[Ca ]=0.1mol/L ♦
♦
♦ ♦♦
∆ ∆ ∆♦ ∆ ∆ ♦♦ ∆♦ ∆ ∆ ∆∆ ∆ ∆ ∆ ∆∆ ∆ ∆ ∆ ∆
∆
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♦
♦ ♦♦♦ ♦
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♦
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60
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40
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Fig. 2. XRD patterns of the powders prepared in different [Ca ] (Ca/ P= 2.0, 300 W, 353 K).
exceeds a critical value. It has been well recognized that the ultrasonic irradiation caused cavitation in an aqueous medium where the formation, growth and collapse of microbubbles occurred. This can result in extreme conditions of temperature (N2000 K) and pressure (N500 bar) on a microsecond timescale leading to the formation of reactive intermediates such as radicals. This can stimulate the reactivity of chemical species involved, resulting in the acceleration of the heterogeneous reactions between liquid and solid reactants effectively [17]. Therefore, the increase of ultrasonic power could cause a rapid reaction that leads to the formation of monophase of HAp. The weakening of the other phosphate peaks with the strengthening of ultrasonic power shown in Fig. 4 also verifies it. The particle size of the HAp phases in different ultrasonic powers analyzed by XRD method is shown in Fig. 5. The particle size of the HAp crystallites decreases linearly with the increase of ultrasonic power, which infers
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50
60
70
2θ/degree
70
2θ/degree
∆
50
∆
40
∆
30
∆
20
∆
10
0
♦♦ ♦♦
∆
[Ca ]=0.01mol/L
♦
∆
2+
10
(002) ♦
∆ ∆ ♦∆ ♦ ∆ ♦♦ ♦ ♦ ∆ ∆ ∆
2+
[Ca ]=0.02mol/L
Fig. 4. XRD patterns of the powders prepared in different ultrasonic powers ([Ca2+] = 0.02, Ca/P= 2.0, 353 K).
that the irradiation of ultrasound on the reaction solution during the precipitation process is helpful to make finer particles. Fig. 6 shows the TEM images of the HAp powders prepared in different ultrasonic powers. It reveals the formation of acicular (Fig. 6a) and spherical (Fig. 6b) particles in different ultrasonic powers of 200 and 300 W, respectively. This is quite different from the particle morphologies (acicular or plate-like) reported by other wet chemical methods [5–8,16]. From the TEM images, the sizes of the acicular and spherical particles were determined to be 20 100 and 10 nm, respectively. By XRD calculation according to the Scherrer formula, the acicular crystallite size in the perpendicular direction of the (002) Miller plane was calculated to be around 20 nm. This is much smaller when compared to the TEM images (almost 100 nm). However, the spherical crystallite size as determined from the XRD calculations was comparable to the TEM micrographs.
35 40 30
particle size/nm
35
particle size/nm
30 25 20
25 20 15
15 10
10 5 0.00
0.05
0.10
0.15
0.20
[Ca2+]/mol/L Fig. 3. Particle size of HAp powder as a function of [Ca2+].
5 100
150
200
250
300
ultrasonic power/W Fig. 5. Particle size of HAp powder as a function of ultrasonic power.
C. Li-yun et al. / Materials Letters 59 (2005) 1902–1906
1905
♦:HAp
(211)
Intensity
353K
(210) (002)
333K
♦♦ ♦♦ ♦
♦
10
♦♦
♦
313K
20
30
♦ ♦
♦
40
♦ ♦
50
♦
♦♦
60
70
2θ/degree Fig. 8. XRD patterns of the powders prepared in different reactive temperatures (Ca/P= 2.0, [Ca2+] = 0.02 mol/L, 300 W).
Fig. 6. TEM images of the HAp powders prepared at 353 K, [Ca2+] = 0.02 mol/L, Ca/P= 2.0 and different ultrasonic powers: (a) 200 W and (b) 300 W.
Fig. 7 reveals the XRD patterns of the powders prepared in different Ca/P ratios. Monophase of HAp could be gained in a wide range of Ca/P ratio from 1.2 to 2.5. The formation range of Ca/P for HAp is wider than the research results (1.5–1.667) of Raynaud et al. [18]. When the Ca/P ratio is 1.67, the (002) peak intensity of the crystallites is the strongest, implying a preferential growth along the (002) direction. This is agreement with the other work [19]. The preferential growth along the (002) direction also obviously
appears in different reactive temperatures. Fig. 8 displays the influence of reactive temperature on the cystallinity of the HAp powder. It is clear that the (002) peak intensity decreases with the reactive temperature increase, which infers that the acicular or plate-like HAp crystallites could be formed at lower temperature. In addition, in the experiment, a little powder was acquired when the reactive temperature was controlled at 293 K although the reactive time was prolonged to 3 h. It is different from the results reported by Pang and Bao[20] that the HAp powders could be synthesized at 288 K and the peak intensity and the particle size increase with the temperature increase. In contrast to it, in our research, the HAp diffraction peak became broader with the increase of temperature (see Fig. 8) and the particle size of the HAp crystallites decreases with the temperature increase (see Fig. 9a). The reason could be attributed to the irradiation of ultrasound in our research though the convincing reason needs more research in the future.
♦: HAp
16
Ca/P=2.5 15
particle size/nm
Intensity
Ca/P=2.0
Ca/P=1.67
Ca/P=1.2
(002) ♦
♦♦
0
10
20
♦
(211) (112) ♦(300) ♦
30
♦ ♦
(310) ♦♦ ♦
14 13 12 11
(213) (222)
40
♦ ♦ ♦ ♦♦
50
♦♦
60
10 70
2θ/degree Fig. 7. XRD patterns of the powders prepared in different Ca/P ratios ([Ca2+] = 0.01 mol/L, 300 W, 353 K).
9
310
320
330
340
350
360
temperature/K Fig. 9. Particle size of HAp powder as a function of reactive temperature.
1906
C. Li-yun et al. / Materials Letters 59 (2005) 1902–1906
4. Conclusion The chemical precipitation with the aid of ultrasonic irradiation on aqueous solutions provides a simple and economic route for synthesis of monophase hydroxyapatite nanoparticles. The crystallinity and morphology of the resulting nanoparticles are dependent on the Ca/P ratio, concentration of Ca2+, ultrasonic power and synthetic temperature. When the concentration of Ca2+ exceeded 0.2 mol/L and the ultrasonic power was lower than 300 W, the monophase of HAp powder could not be obtained. When the reactive temperature and Ca/P increase from 313 to 353 K and 1.67 to 2.5, respectively, the as-prepared crystallites exhibit a preferential growth along the (002) direction and the nanoparticles show an acicular morphology. On the other hand, spherical nanoparticles could be obtained at high synthetic temperature (353 K), ultrasonic power (300 W) and high Ca/P (2.0–2.5). In addition, the crystallite size of the HAp nanoparticles decreases with the decrease of [Ca2+] and the increase of synthetic temperature and ultrasonic power.
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