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Controlled synthesis and thermal stability of hydroxyapatite hierarchical microstructures
Materials Research Bulletin 48 (2013) 1143–1147
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Controlled synthesis and thermal stability of hydroxyapatite hierarchical microstructures Ruixue Sun, Kezheng Chen *, Zhongmiao Liao, Nan Meng College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 March 2012 Received in revised form 5 December 2012 Accepted 5 December 2012 Available online 14 December 2012
Hydroxyapatite (HAp) hierarchical microstructures with novel 3D morphology were prepared through a template- and surfactant-free hydrothermal homogeneous precipitation method. Field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) were used to characterize the morphology and composition of the synthesized products. Interestingly, the obtained HAp with 3D structure is composed of one-dimensional (1D) nanorods or two-dimensional (2D) nanoribbons, and the length and morphology of these building blocks can be controlled through controlling the pH of the reaction. The building blocks are single crystalline and have different preferential orientation growth under different pH conditions. At low pH values, octacalcium phosphate (OCP) phase formed first and then transformed into HAp phase due to the increased pH value caused by the decomposition of urea. The investigation on the thermal stability reveals that the prepared HAp hierarchical microstructures are morphologically and structurally stable up to 800 8C. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics A. Nanostructures B. Crystal growth C. X-ray diffraction D. Crystal structure
1. Introduction Currently, there has been an increasing interest in the synthesis of materials with three-dimensional (3D) ordered superstructures because of their unique properties different from mono-morphological structures [1–3]. Nano/micro hierarchical structures often consist of nanometer-sized building blocks, like nanorods, nanoparticles, and nanosheets. Until now, many materials with hierarchical structures such as metal, metal oxide, and semiconducting materials have been synthesized by a variety of methods [4–8]. These materials with novel structures have enormous potential for applications in nanoelectronic devices, catalysis, and delivery vehicles because of their novel properties from the special morphologies. Methods commonly used to synthesize these novel structures usually involve a surfactant as the structure-directing agent or a complicated multistep synthesis process with a unique precursor, which are usually harmful to the environment and human health. Thus, developing facile, environment-friendly, and size-controlled methods to produce 3D hierarchical structures are strongly desirable. Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) is one of the most bioactive and biocompatible materials and has been widely investigated as bone substitutes and scaffolds for hard tissue
* Corresponding author. Tel.: +86 532 84023446; fax: +86 532 84022814. E-mail addresses: [email protected], [email protected] (K. Chen). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.12.013
engineering [9–11]. Recently, HAp has also been used as a reinforcement to improve the mechanical properties and reliability of biomedical ceramics (e.g. b-tricalcium phosphate and calcium phosphate cements) and polymers (e.g. polyetheretherketone, polyethylene and polymethylmethacrylate) [12–15]. The performance of HAp in this application depends on its crystallite size, composition, and morphologies. Up to now, considerable effort has been devoted to fabricating various HAp nanostructures, including spheres, rods, needles, and whiskers [16–18]. A class of sophisticated flower-like structures of HAp have also been synthesized recently. For example, uniform 3D structured carbonated apatite flowers have been successfully synthesized by Chang et al. [19]. Lin et al. prepared HAp microcrystals with flower-like morphologies using trisodium citrate as the structure-directing agent [20]. These 3D structures assembled from nanosheets may be an important alternative to whiskers and fibers (1D) as additives for the mechanical reinforcement of ceramics and polymers. To the best of our knowledge, there has been no report on the synthesis of HAp hierarchical 3D microstructures assembled from nanorods or nanoribbons in the absence of any surfactants. In this work, we reported the synthesis of HAp hierarchical microstructures assembled from nanorods and nanoribbons through a hydrothermal homogeneous precipitation method without adding any structure-directing agents and templates. Thermal stability of reinforcements at high temperatures directly affects the mechanical and biological properties of composites. So the thermal stability of the prepared HAp hierarchical microstructures was also studied.
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2. Experimental details 2.1. Materials Diammonium hydrogen phosphate [(NH4)2HPO4] and calcium nitrate tetrahydrate [Ca(NO3)24H2O] were supplied by Sinopharm Chemical Reagent Corporation (Shanghai, China). Urea was purchased from Gugangcheng Chemical Reagent Corporation (Tianjin, China). All chemicals used for the synthesis were of analytical grade and were used as received without any further purification. 2.2. Synthesis of HAp hierarchical structures In a typical synthesis, 42 mmol/L calcium and 25 mmol/L phosphate were prepared by dissolving Ca(NO3)24H2O and (NH4)2HPO4 in 0.05 mol/L HNO3 solution, respectively. 50 ml calcium and 50 ml phosphate prepared above were added into 20 ml 1 mol/L urea solution. The initial pH of the mixed solutions was adjusted to 3.0, 5.0, 7.0, 9.0, and 11.0 using 0.1 mol/L HNO3 or ammonium hydroxide. Then the obtained solution was transferred into stainless steel autoclaves and heated at 90 8C or 180 8C for 10 h, followed by cooling to room temperature naturally. The product was filtered off, washed with deionized water and anhydrous ethanol for three times, and finally dried in air at 80 8C. The thermal stability of the products was evaluated in a cabinet electric furnace at 600 8C and 800 8C for 2.5 h in air. 2.3. Characterization The morphology and selected area electron diffraction (SAED) of the as-synthesized HAp structures were characterized by field
emission scanning electron microscopy (FESEM, JEOL JSM-6700F, Japan) and high-resolution transmission electron microscopy (HRTEM, Tecnai 20U-TWIN, Philips). Powder X-ray diffraction was carried out with a Rigaku D/max-2500 XRD using Cu Ka radiation. 3. Results and discussion Fig. 1 shows the FESEM images of the as-synthesized powders after hydrothermal treatment at 180 8C for 10 h under various pH values. It is clear to see that the product prepared at pH = 3.0 (Fig. 1a) has a uniform 3D structured flower-like morphology and every individual flower possesses many radically out-extending 1D nanorods. The high magnification image inserted in Fig. 1a shows that the nanorods have different lengths with diameter about 60–150 nm. It is interesting to find that the building blocks of the products are changed from nanorods (1D) to nanoribbons (2D) with increasing the pH value of initial solution to 5.0 (Fig. 1b). Fig. 1b shows clearly that the nanoribbons of the hierarchical structures have lengths above 80 mm with widths about 500 nm and thickness about 100 nm. Upon a further increase of the pH value up to above 7.0, the microarchitectures disappear and are replaced by uniform nanorods with a diameter of about 20 nm. In addition, the morphology of the nanoparticles is changed from nanorods to nanospheres with increasing the pH value from 7.0 to 11.0. The composition and phase purity of the products prepared under various pH conditions were examined by XRD, which are shown in Fig. 2. The diffraction peaks of three samples can be indexed as a pure hexagonal phase, which coincides well with the standard data for HAp (JCPDS No. 09-0432). It is worth pointing out that there is a large difference from each other in the relative
Fig. 1. FESEM images of the as-obtained HAp powders prepared at different pH values: (a) pH = 3.0, (b) pH = 5.0, (c) pH = 7.0 and (d) pH = 11.0.
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Fig. 2. XRD patterns of the as-obtained HAp powders prepared at different pH values: (a) pH = 3.0; (b) pH = 5.0; and (c) pH = 9.0.
intensities based on (2 1 0), (2 1 1), and (3 0 0) peaks for three samples in the XRD patterns, indicating the possibility of different preferential orientation growth under different pH conditions. The shape of the strong diffraction peaks indicates that the obtained HAp powders are fairly well crystallized. Fig. 3 shows the XRD patterns of the products after hydrothermal treatment at 90 8C and 180 8C for different time. The product prepared by hydrothermal treatment at 90 8C was identified as the mixture of octacalcium phosphate (OCP) phase (JCPDS No. 79-0423) and HAP phase. With the increase of the hydrothermal temperature to 180 8C for 40 min (Fig. 3b), the product was all HAp phase and OCP phase disappeared completely. The crystallinity of the product increased greatly with further increasing the hydrothermal time to 1 h at 180 8C (Fig. 3c). OCP is thermodynamically less stable than HAp, and it is often found as an intermediate phase during the precipitation of HAp [21]. In this study, the pH of the initial solution is very low (about 3.0), and OCP will appear in acidic conditions. The transformation of OCP to HAp is initiated by raising the pH from acidic to alkaline. The uniform pH increase of the reaction is achieved by slow decomposition of urea above 80 8C [22]. Therefore, OCP phase was found in the product prepared by hydrothermal treatment at 90 8C.
Fig. 3. XRD patterns of the as-obtained HAp powders prepared at pH 3.0 after hydrothermal treatment at (a) 90 8C for 10 h, (b) 180 8C for 40 min and (c) 180 8C for 1 h.
As the temperature increases to 180 8C, OCP will transform to HAp completely due to the high pH value caused by the decomposition of urea. The crystalline structures of the HAp powders prepared at pH = 3.0 and 5.0 were further analyzed by HRTEM as shown in Fig. 4. It can be seen from Fig. 4a that the prepared HAp has hierarchical structures like flowers, which is consistent with the FESEM images, and the structure of the building units is further investigated as shown in Fig. 4b. Fig. 4b shows the clear onedimensional lattice fringes with interplanar spacing of 0.34 nm, which can be indexed to the (0 0 2) plane of the HAp crystal, indicating that the nanorods are growing along the [0 0 1] direction. The SAED pattern of the nanoribbons (Fig. 4c) confirms the growing direction of [0 0 1] and [1 0 0], indicating that the nanoribbons were obviously elongated along the c-axis and a-axis. Previous studies have indicated that the adsorption of OH ions onto the surface was necessary for the crystallization and growth of Ca5(PO4)3OH [20,23]. At high pH values (higher OH concentration), the adsorption probability of OH ions onto the growth surface of the Ca5(PO4)3OH nuclei was strong and each facet of the nuclei has almost the same probability, resulting in isotropic or weak-anisotropic growth [20]; that is, the crystallites grew into nanorods or nanospheres (pH = 7.0 or pH = 11.0), as shown in Fig. 1c and d, respectively. On the other hand, when the pH of the
Fig. 4. (a) TEM image of the as-obtained HAp prepared at pH = 3.0; (b) TEM and HRTEM image of the rodlike building unit and (c) HRTEM image and SAED pattern of a single nanoribbon of HAp prepared at pH = 5.0.
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Fig. 5. FESEM images of the as-obtained powders prepared at pH = 3.0 after hydrothermal treatment at 180 8C for (a) 40 min, (b) 1 h and (c) 3 h.
initial solution was decreased to 3.0 or 5.0, OCP was obtained firstly at the initial stages of reaction. The rise in pH due to the decomposition of urea at high temperature (above 80 8C) drives the OCP transformation to HAp [24]. To further investigate the formation mechanism of HAp with 3D structures, time-dependent experiments were performed to gain insight into the evolution process, and the products collected at different stages of the experiments are shown in Fig. 5. The 40 min reaction resulted in the rudiment of flower-like HAp which consisted of some incompact nanosheets (Fig. 5a), and it was noticed that the size and number of the nanosheets was small. In the initial stage of the
reaction, a three-dimensional cluster of OCP might be formed, which could act as nucleus for HAp crystals. After 1 h reaction, more and longer nanorods grew out and the shapes of the flowerlike HAp hierarchical structures were further developed (Fig. 5b). As shown in Fig. 5c, the product obtained after 3 h reaction is composed of well-shaped flower-like 3D structures with larger sizes which are assembled by numerous nanorods. With an extension of the reaction time, the morphology and size of HAp remained nearly unchanged, as shown in Fig. 1a. The formation mechanism of HAp hierarchical structures prepared at low pH values needs further studying.
Fig. 6. FESEM images of the as-obtained HAp powders prepared at pH = 3.0 after heat-treatment. (a) 600 8C, 2.5 h and (b) 800 8C, 2.5 h.
R. Sun et al. / Materials Research Bulletin 48 (2013) 1143–1147
β-TCP
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Mechanism was proposed to elucidate their formation. Moreover, the HAp with 3D structures had a high crystallinity and was structurally and morphologically stable at elevated temperatures below 800 8C. The HAp powders with these novel structures have many potential uses such as reinforced materials, catalysts, and adsorbents. Further work is under way to study the properties of these novel 3D microstructures. Acknowledgments
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Heat treatments are always involved in preparing HAp reinforced composites. The thermal stability of HAp reinforcements at high temperatures can directly affect the mechanical and biological properties of the composites. Fig. 6a and b shows the FESEM images of the HAp hierarchical structures after heattreated in air at 600 8C and 800 8C for 2.5 h, respectively. It can be seen clearly that the HAp flowers retained their morphology after heat-treated at 600 8C (Fig. 6a). After treatment at 800 8C, HAp still had a 3D morphology, the building units appeared to be sintered together (Fig. 6b) and had a smaller size compared with that treated at 600 8C. The phase change of the HAp hierarchical structures after heat treatment was analyzed by XRD. It can be seen from Fig. 7 that only a minor b-tricalcium phosphate (b-TCP) was found in the XRD pattern after heat-treated at 600 8C and 800 8C and the intensity of the HAp peaks had no significant decrease. 4. Conclusions This study demonstrates a facile synthesis of HAp hierarchical microstructures assembled with nanorods and nanoribbons without adding any structure-directing agents and templates. By altering the pH value of the reaction, the morphology and size of the building units of hierarchical structures can be controlled.
This work was financially supported by Promotive Research Fund for Excellent Young and Middle-Aged Scientists of the Shandong Province (Grant No. BS2010CL018). References [1] B.X. Li, Y.F. Wang, J. Phys. Chem. C 114 (2010) 890. [2] C.Q. Zhou, J. Han, R. Guo, Macromolecules 41 (2008) 6473. [3] P. Hu, X. Zhang, N. Han, W.C. Xiang, Y.B. Cao, F.L. Yuan, Cryst. Growth Des. 11 (2011) 1520. [4] G.J. Zhou, M.K. Lu, Z.S. Yang, F. Tian, Y.Y. Zhou, A.Y. Zhang, Cryst. Growth Des. 7 (2007) 187. [5] T. Liu, D.S. Li, D.R. Yang, M.H. Jiang, Langmuir 27 (2011) 6211. [6] D.S. Zheng, Z.L. Yin, W.M. Zhang, X.J. Tan, S.X. Sun, Cryst. Growth Des. 6 (2006) 1733. [7] Y. Masuda, T. Ohji, K. Kato, Cryst. Growth Des. 10 (2010) 913. [8] Q.J. He, Z.L. Huang, Y. Liu, W. Chen, T. Xu, Mater. Lett. 61 (2007) 141. [9] S.V. Dorozhkin, J. Mater. Sci. 44 (2009) 2343. [10] M. Uota, H. Arakawa, N. Kitamura, T. Yoshimura, J. Tanaka, T. Kijima, Langmuir 21 (2005) 4724. [11] M. Jevtic, M. Mitric, S. Skapin, B. Jancar, N. Ignjatovic, D. Uskokovic, Cryst. Growth Des. 8 (2008) 2217. [12] Y.M. Park, S.C. Ryu, S.Y. Yoon, R. Stevens, H.C. Park, Mater. Chem. Phys. 109 (2008) 440. [13] I.S. Neira, Y.V. Kolen’ko, K.P. Kommareddy, I. Manjubala, M. Yoshimura, F. Guitian, Appl. Mater. Interfaces 2 (2010) 3276. [14] G.L. Converse, W.M. Yue, R.K. Roeder, Biomaterials 28 (2007) 927. [15] M.S. Abu Bakar, M.H.W. Cheng, S.M. Tang, S.C. Yu, K. Liao, C.T. Tan, K.A. Khor, P. Cheang, Biomaterials 24 (2003) 2245. [16] H.Q. Cao, L. Zhang, H. Zheng, Z. Wang, J. Phys. Chem. C 114 (2010) 18352. [17] H.Q. Zhang, B.W. Darvell, Acta Biomater. 6 (2010) 3216. [18] V.S. Komlev, S.M. Barinov, E.V. Koplik, Biomaterials 23 (2002) 3449. [19] K.L. Lin, J. Chang, Y.J. Zhu, W. Wu, G.F. Cheng, Y. Zeng, M.L. Ruan, Cryst. Growth Des. 9 (2009) 177. [20] C.M. Zhang, J. Yang, Z.W. Quan, P.P. Yang, C.X. Li, Z.Y. Hou, J. Lin, Cryst. Growth Des. 9 (2009) 2752. [21] Y.H. Tseng, C.Y. Mou, J.C.C. Chan, J. Am. Chem. Soc. 128 (2006) 6909. [22] B. Jokic, M. Mitric, V. Radmilovic, S. Drmanic, R. Petrovic, D. Janackovic, Ceram. Int. 37 (2011) 167. [23] J.B. Liu, K.W. Li, H. Wang, M.K. Zhu, H. Yan, Chem. Phys. Lett. 396 (2004) 429. [24] J.H. Zhan, Y.H. Tseng, J.C.C. Chan, C.Y. Mou, Adv. Funct. Mater. 15 (2005) 2005.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Controlled synthesis and thermal stability of hydroxyapatite hierarchical microstructures Ruixue Sun, Kezheng Chen *, Zhongmiao Liao, Nan Meng College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 March 2012 Received in revised form 5 December 2012 Accepted 5 December 2012 Available online 14 December 2012
Hydroxyapatite (HAp) hierarchical microstructures with novel 3D morphology were prepared through a template- and surfactant-free hydrothermal homogeneous precipitation method. Field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) were used to characterize the morphology and composition of the synthesized products. Interestingly, the obtained HAp with 3D structure is composed of one-dimensional (1D) nanorods or two-dimensional (2D) nanoribbons, and the length and morphology of these building blocks can be controlled through controlling the pH of the reaction. The building blocks are single crystalline and have different preferential orientation growth under different pH conditions. At low pH values, octacalcium phosphate (OCP) phase formed first and then transformed into HAp phase due to the increased pH value caused by the decomposition of urea. The investigation on the thermal stability reveals that the prepared HAp hierarchical microstructures are morphologically and structurally stable up to 800 8C. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics A. Nanostructures B. Crystal growth C. X-ray diffraction D. Crystal structure
1. Introduction Currently, there has been an increasing interest in the synthesis of materials with three-dimensional (3D) ordered superstructures because of their unique properties different from mono-morphological structures [1–3]. Nano/micro hierarchical structures often consist of nanometer-sized building blocks, like nanorods, nanoparticles, and nanosheets. Until now, many materials with hierarchical structures such as metal, metal oxide, and semiconducting materials have been synthesized by a variety of methods [4–8]. These materials with novel structures have enormous potential for applications in nanoelectronic devices, catalysis, and delivery vehicles because of their novel properties from the special morphologies. Methods commonly used to synthesize these novel structures usually involve a surfactant as the structure-directing agent or a complicated multistep synthesis process with a unique precursor, which are usually harmful to the environment and human health. Thus, developing facile, environment-friendly, and size-controlled methods to produce 3D hierarchical structures are strongly desirable. Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) is one of the most bioactive and biocompatible materials and has been widely investigated as bone substitutes and scaffolds for hard tissue
* Corresponding author. Tel.: +86 532 84023446; fax: +86 532 84022814. E-mail addresses: [email protected], [email protected] (K. Chen). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.12.013
engineering [9–11]. Recently, HAp has also been used as a reinforcement to improve the mechanical properties and reliability of biomedical ceramics (e.g. b-tricalcium phosphate and calcium phosphate cements) and polymers (e.g. polyetheretherketone, polyethylene and polymethylmethacrylate) [12–15]. The performance of HAp in this application depends on its crystallite size, composition, and morphologies. Up to now, considerable effort has been devoted to fabricating various HAp nanostructures, including spheres, rods, needles, and whiskers [16–18]. A class of sophisticated flower-like structures of HAp have also been synthesized recently. For example, uniform 3D structured carbonated apatite flowers have been successfully synthesized by Chang et al. [19]. Lin et al. prepared HAp microcrystals with flower-like morphologies using trisodium citrate as the structure-directing agent [20]. These 3D structures assembled from nanosheets may be an important alternative to whiskers and fibers (1D) as additives for the mechanical reinforcement of ceramics and polymers. To the best of our knowledge, there has been no report on the synthesis of HAp hierarchical 3D microstructures assembled from nanorods or nanoribbons in the absence of any surfactants. In this work, we reported the synthesis of HAp hierarchical microstructures assembled from nanorods and nanoribbons through a hydrothermal homogeneous precipitation method without adding any structure-directing agents and templates. Thermal stability of reinforcements at high temperatures directly affects the mechanical and biological properties of composites. So the thermal stability of the prepared HAp hierarchical microstructures was also studied.
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2. Experimental details 2.1. Materials Diammonium hydrogen phosphate [(NH4)2HPO4] and calcium nitrate tetrahydrate [Ca(NO3)24H2O] were supplied by Sinopharm Chemical Reagent Corporation (Shanghai, China). Urea was purchased from Gugangcheng Chemical Reagent Corporation (Tianjin, China). All chemicals used for the synthesis were of analytical grade and were used as received without any further purification. 2.2. Synthesis of HAp hierarchical structures In a typical synthesis, 42 mmol/L calcium and 25 mmol/L phosphate were prepared by dissolving Ca(NO3)24H2O and (NH4)2HPO4 in 0.05 mol/L HNO3 solution, respectively. 50 ml calcium and 50 ml phosphate prepared above were added into 20 ml 1 mol/L urea solution. The initial pH of the mixed solutions was adjusted to 3.0, 5.0, 7.0, 9.0, and 11.0 using 0.1 mol/L HNO3 or ammonium hydroxide. Then the obtained solution was transferred into stainless steel autoclaves and heated at 90 8C or 180 8C for 10 h, followed by cooling to room temperature naturally. The product was filtered off, washed with deionized water and anhydrous ethanol for three times, and finally dried in air at 80 8C. The thermal stability of the products was evaluated in a cabinet electric furnace at 600 8C and 800 8C for 2.5 h in air. 2.3. Characterization The morphology and selected area electron diffraction (SAED) of the as-synthesized HAp structures were characterized by field
emission scanning electron microscopy (FESEM, JEOL JSM-6700F, Japan) and high-resolution transmission electron microscopy (HRTEM, Tecnai 20U-TWIN, Philips). Powder X-ray diffraction was carried out with a Rigaku D/max-2500 XRD using Cu Ka radiation. 3. Results and discussion Fig. 1 shows the FESEM images of the as-synthesized powders after hydrothermal treatment at 180 8C for 10 h under various pH values. It is clear to see that the product prepared at pH = 3.0 (Fig. 1a) has a uniform 3D structured flower-like morphology and every individual flower possesses many radically out-extending 1D nanorods. The high magnification image inserted in Fig. 1a shows that the nanorods have different lengths with diameter about 60–150 nm. It is interesting to find that the building blocks of the products are changed from nanorods (1D) to nanoribbons (2D) with increasing the pH value of initial solution to 5.0 (Fig. 1b). Fig. 1b shows clearly that the nanoribbons of the hierarchical structures have lengths above 80 mm with widths about 500 nm and thickness about 100 nm. Upon a further increase of the pH value up to above 7.0, the microarchitectures disappear and are replaced by uniform nanorods with a diameter of about 20 nm. In addition, the morphology of the nanoparticles is changed from nanorods to nanospheres with increasing the pH value from 7.0 to 11.0. The composition and phase purity of the products prepared under various pH conditions were examined by XRD, which are shown in Fig. 2. The diffraction peaks of three samples can be indexed as a pure hexagonal phase, which coincides well with the standard data for HAp (JCPDS No. 09-0432). It is worth pointing out that there is a large difference from each other in the relative
Fig. 1. FESEM images of the as-obtained HAp powders prepared at different pH values: (a) pH = 3.0, (b) pH = 5.0, (c) pH = 7.0 and (d) pH = 11.0.
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intensities based on (2 1 0), (2 1 1), and (3 0 0) peaks for three samples in the XRD patterns, indicating the possibility of different preferential orientation growth under different pH conditions. The shape of the strong diffraction peaks indicates that the obtained HAp powders are fairly well crystallized. Fig. 3 shows the XRD patterns of the products after hydrothermal treatment at 90 8C and 180 8C for different time. The product prepared by hydrothermal treatment at 90 8C was identified as the mixture of octacalcium phosphate (OCP) phase (JCPDS No. 79-0423) and HAP phase. With the increase of the hydrothermal temperature to 180 8C for 40 min (Fig. 3b), the product was all HAp phase and OCP phase disappeared completely. The crystallinity of the product increased greatly with further increasing the hydrothermal time to 1 h at 180 8C (Fig. 3c). OCP is thermodynamically less stable than HAp, and it is often found as an intermediate phase during the precipitation of HAp [21]. In this study, the pH of the initial solution is very low (about 3.0), and OCP will appear in acidic conditions. The transformation of OCP to HAp is initiated by raising the pH from acidic to alkaline. The uniform pH increase of the reaction is achieved by slow decomposition of urea above 80 8C [22]. Therefore, OCP phase was found in the product prepared by hydrothermal treatment at 90 8C.
Fig. 3. XRD patterns of the as-obtained HAp powders prepared at pH 3.0 after hydrothermal treatment at (a) 90 8C for 10 h, (b) 180 8C for 40 min and (c) 180 8C for 1 h.
As the temperature increases to 180 8C, OCP will transform to HAp completely due to the high pH value caused by the decomposition of urea. The crystalline structures of the HAp powders prepared at pH = 3.0 and 5.0 were further analyzed by HRTEM as shown in Fig. 4. It can be seen from Fig. 4a that the prepared HAp has hierarchical structures like flowers, which is consistent with the FESEM images, and the structure of the building units is further investigated as shown in Fig. 4b. Fig. 4b shows the clear onedimensional lattice fringes with interplanar spacing of 0.34 nm, which can be indexed to the (0 0 2) plane of the HAp crystal, indicating that the nanorods are growing along the [0 0 1] direction. The SAED pattern of the nanoribbons (Fig. 4c) confirms the growing direction of [0 0 1] and [1 0 0], indicating that the nanoribbons were obviously elongated along the c-axis and a-axis. Previous studies have indicated that the adsorption of OH ions onto the surface was necessary for the crystallization and growth of Ca5(PO4)3OH [20,23]. At high pH values (higher OH concentration), the adsorption probability of OH ions onto the growth surface of the Ca5(PO4)3OH nuclei was strong and each facet of the nuclei has almost the same probability, resulting in isotropic or weak-anisotropic growth [20]; that is, the crystallites grew into nanorods or nanospheres (pH = 7.0 or pH = 11.0), as shown in Fig. 1c and d, respectively. On the other hand, when the pH of the
Fig. 4. (a) TEM image of the as-obtained HAp prepared at pH = 3.0; (b) TEM and HRTEM image of the rodlike building unit and (c) HRTEM image and SAED pattern of a single nanoribbon of HAp prepared at pH = 5.0.
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Fig. 5. FESEM images of the as-obtained powders prepared at pH = 3.0 after hydrothermal treatment at 180 8C for (a) 40 min, (b) 1 h and (c) 3 h.
initial solution was decreased to 3.0 or 5.0, OCP was obtained firstly at the initial stages of reaction. The rise in pH due to the decomposition of urea at high temperature (above 80 8C) drives the OCP transformation to HAp [24]. To further investigate the formation mechanism of HAp with 3D structures, time-dependent experiments were performed to gain insight into the evolution process, and the products collected at different stages of the experiments are shown in Fig. 5. The 40 min reaction resulted in the rudiment of flower-like HAp which consisted of some incompact nanosheets (Fig. 5a), and it was noticed that the size and number of the nanosheets was small. In the initial stage of the
reaction, a three-dimensional cluster of OCP might be formed, which could act as nucleus for HAp crystals. After 1 h reaction, more and longer nanorods grew out and the shapes of the flowerlike HAp hierarchical structures were further developed (Fig. 5b). As shown in Fig. 5c, the product obtained after 3 h reaction is composed of well-shaped flower-like 3D structures with larger sizes which are assembled by numerous nanorods. With an extension of the reaction time, the morphology and size of HAp remained nearly unchanged, as shown in Fig. 1a. The formation mechanism of HAp hierarchical structures prepared at low pH values needs further studying.
Fig. 6. FESEM images of the as-obtained HAp powders prepared at pH = 3.0 after heat-treatment. (a) 600 8C, 2.5 h and (b) 800 8C, 2.5 h.
R. Sun et al. / Materials Research Bulletin 48 (2013) 1143–1147
β-TCP
Others: HAp
Intensity (a.u.)
(b)
Mechanism was proposed to elucidate their formation. Moreover, the HAp with 3D structures had a high crystallinity and was structurally and morphologically stable at elevated temperatures below 800 8C. The HAp powders with these novel structures have many potential uses such as reinforced materials, catalysts, and adsorbents. Further work is under way to study the properties of these novel 3D microstructures. Acknowledgments
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2θ (deg.) Fig. 7. XRD patterns of HAp powders prepared at pH = 3.0 after heat-treatment (a) 600 8C, 2.5 h and (b) 800 8C, 2.5 h.
Heat treatments are always involved in preparing HAp reinforced composites. The thermal stability of HAp reinforcements at high temperatures can directly affect the mechanical and biological properties of the composites. Fig. 6a and b shows the FESEM images of the HAp hierarchical structures after heattreated in air at 600 8C and 800 8C for 2.5 h, respectively. It can be seen clearly that the HAp flowers retained their morphology after heat-treated at 600 8C (Fig. 6a). After treatment at 800 8C, HAp still had a 3D morphology, the building units appeared to be sintered together (Fig. 6b) and had a smaller size compared with that treated at 600 8C. The phase change of the HAp hierarchical structures after heat treatment was analyzed by XRD. It can be seen from Fig. 7 that only a minor b-tricalcium phosphate (b-TCP) was found in the XRD pattern after heat-treated at 600 8C and 800 8C and the intensity of the HAp peaks had no significant decrease. 4. Conclusions This study demonstrates a facile synthesis of HAp hierarchical microstructures assembled with nanorods and nanoribbons without adding any structure-directing agents and templates. By altering the pH value of the reaction, the morphology and size of the building units of hierarchical structures can be controlled.
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