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Morphology control of brushite prepared by aqueous solution synthesis
Journal of Asian Ceramic Societies 2 (2014) 52–56
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Journal of Asian Ceramic Societies journal homepage: www.elsevier.com/locate/jascer
Morphology control of brushite prepared by aqueous solution synthesis T. Toshima a,∗ , R. Hamai a,b , M. Tafu a , Y. Takemura a , S. Fujita a , T. Chohji a , S. Tanda c,d , S. Li e , G.W. Qin e a
Toyama National College of Technology, 13 Hongo-machi, Toyama 939-8630, Japan Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Fukuoka 808-0196, Japan c Department of Applied Physics, Hokkaido University, Hokkaido 060-8628, Japan d Center of Education and Research for Topological Science and Technology, Hokkaido University, Hokkaido 060-8628, Japan e Key Laboratory for Anisotropy and Texture of Materials (MOE), Northeastern University, Shenyang 110819, China b
a r t i c l e
i n f o
Article history: Received 30 August 2013 Received in revised form 7 January 2014 Accepted 7 January 2014 Available online 4 February 2014 Keywords: Morphology Crystal growth Brushite Calcium phosphate
a b s t r a c t Dicalcium phosphate dihydrate (DCPD, CaHPO4 ·2H2 O), also known as brushite, is one of the important bioceramics due to not only diseases factors such as kidney stone and plaque formation but also purpose as fluoride insolubilization material. It is used medicinally to supply calcium, and is of interest for its unique properties in biological and pathological mineralization. It is important to control the crystal morphology of brushite since its chemical reactivity depends strongly on its surface properties; thus, its morphology is a key issue for its applications as a functional material or precursor for other bioceramics. Here, we report the effects of the initial pH and the Ca and phosphate ion concentrations on the morphology of DCPD particles during aqueous solution synthesis. Crystal morphologies were analyzed by scanning electron microscopy and X-ray diffraction. The morphology phase diagram of DCPD crystallization revealed that increasing the initial pH and/or ion concentration transformed DCPD morphology from petal-like into plate-like structures. © 2014 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction Calcium phosphate bioceramics are the main constituents of the bones and teeth of vertebrates, including most human hard tissues [1,2]. They have been the subjects of intensive research in a range of fields, including biomineralization, medicine [3–6], and the environment [7]. In particular, dicalcium phosphate dihydrate (DCPD, CaHPO4 ·2H2 O), also known as brushite, is one of the key components in calcium phosphate systems, because DCPD can not only be deposited as a coating for orthopedic implants [8], but is also recognized as a starting material in the preparation of other calcium phosphate bioceramics [9–11]. So far, DCPD has been applied as many functional materials, such as the abrasive constituent of toothpaste, a feed additive, and water treatment compounds [7,12].
∗ Corresponding author. Tel.: +81 76 493 5476; fax: +81 76 493 5476. E-mail addresses: [email protected], [email protected] (T. Toshima). Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.
2187-0764 © 2014 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jascer.2014.01.004
There has been substantial research into the morphology control of calcium phosphate bioceramics. In general, the performance of a functional material significantly depends on its shape and surface properties [13–15]. Extensive investigations have been undertaken to establish effective parameters controlling the shapes of these bioceramics because of the complexity of the crystallization system [16,17]. Research on DCPD crystallization has relevance for the preparation of other calcium phosphate bioceramics. For example, crystalline octacalcium phosphate (OCP, Ca8 (H2 PO4 )2 (PO4 )4 ·5H2 O) is well known as a bone component precursor [18] and is attractive to biochemists because of its controllable morphology [19,20]. The parametric factors in the biomineralization processes of these materials have attracted the interest of biochemists, with a key issue being the case of hydroxyapatite (HAp, Ca10 (PO4 )6 (OH)2 ) formation [21,22]. Many reports concern the transformation of DCPD to HAp [23–28]. Information on DCPD morphology is very useful for predicting in vivo bone and/or tooth bioactivity and for accelerating bone regeneration. Such detour methods may provide important clues for controlling the shape of the final product. Therefore, elucidating the effects of DCPD crystallization parameters is very important. In recent studies of calcium phosphate bioceramics, various crystallization conditions (e.g., calcium and phosphorous concentrations, temperature, ionic strengths, and solution pH) were investigated [29–36]. We previously reported the effects of both acetate anion and pH on the morphology of DCPD crystallization [37]. The addition of acetate ion changed the morphology of the DCPD crystals and their yields.
T. Toshima et al. / Journal of Asian Ceramic Societies 2 (2014) 52–56
The objective of this work was to demonstrate the effects of the initial pH and ion concentrations on the morphology of DCPD crystallization during aqueous solution synthesis. It is well known that pH has a great influence on both the nucleation rate and crystal growth rate. The stability and solubility of DCPD crystals are also strongly dependent on the solution pH. Both the pH and the ion concentration are known to affect the supersaturation ratio of DCPD crystals. Therefore, pH and ion concentration were examined as key parameters for controlling the morphology of DCPD crystals in this study. The product structures were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). 2. Materials and methods DCPD crystals were prepared according to the flow diagram shown in Fig. 1. Aqueous solutions of ammonium dihydrogen phosphate (NH4 H2 PO4 , Nacalai, 99%) and calcium nitrate (Ca(NO3 )2 , Wako, 98.5%) were prepared in 100 mL glass beakers at room temperature. The initial pH of the solutions was adjusted to the desired value by the addition of 0.01 and 0.1 M nitric acid (Kanto Chemical)
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and ammonia solution (Wako, 25%). The initial pH values, from 4.0 to 7.0, were tested with a pH meter (SevenMulti, Mettler-Toledo). The initial molar concentrations were adjusted to 0.02, 0.10, and 0.20 mol/L, respectively. The solutions (100 mL each) of Ca(NO3 )2 and NH4 H2 PO4 with the same initial pH values and concentrations were rapidly mixed in a 200 mL beaker and stirred magnetically at 200 rpm. The key point is mixing two solutions at once, without taking long time for mixing. The pH values of the mixtures were monitored over the first hour after mixing. If a solid was not observed in the solution at this stage, then the solution was stirred up to 24 h. After 24 h without evidence of crystallization, the reaction was stopped and recorded as “no product.” After the reaction, all solutions were filtered through a 0.45 m membrane filter. The solid products were dried at 40 ◦ C for 24 h. The products from each set of crystal growth conditions were examined by SEM (JSM-6390AX, JEOL, accelerating voltage of 3–10 kV without coating) and XRD (Mini Flex, Rigaku, 35 kV, 15 mA, 2◦ /min). 3. Results and discussion Table 1 lists the results at 1 h and 24 h for the various crystal growth conditions. At concentrations below 0.01 mol/L, no solid products were observed with an initial pH of 5.5 or lower, even after 24 h. A solid product was observed at an initial pH of 6.0 after 24 h, however, the amount of solid formed after 1 h was not sufficient for testing. At an initial pH of 7.0, solids were observed within 1 h. At other concentrations, the crystallizations exhibited the same behaviors. As the concentration increased, crystallization occurred more readily at lower initial pH, at which the products were never obtained under lower concentration conditions. Fig. 2 illustrates the pH changes after 1 h with respect to the initial pH and concentration. The pH values of the solutions decreased under all conditions, whether solid products were obtained or not. The pH difference increased when the initial pH Table 1 Initial pH value and concentration dependence of solid product formation. Initial pH
0.01 mol/L
0.05 mol/L
0.10 mol/L
4 5 6 7
× ×
×
×
×: no product after 24 h. : no product after 1 h, product after 24 h. : product after 1 h.
Fig. 1. Experimental flow design. Starting solutions with the same initial pH and concentration values were initially mixed. (Example: Ca(NO3 )2 (0.02 mol/L, pH 4) + NH4 H2 O4 (0.02 mol/L, pH 4) → mixture (0.01 mol/L, pH 4).)
Fig. 2. pH change as a function of initial pH and ion concentration.
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and/or concentrations were high. When such solutions were combined, the mixtures became milky due to the rapid formation of the solid product. Therefore, each parameter contributed to the driving force for crystallization.
The dependence of DCPD morphology on the initial preparation conditions was further examined. As shown in Fig. 3, plate-like and petal-like particles were observed by SEM. The plate-like particles mostly formed in parallelogram shapes that were stacked
Fig. 3. SEM images of DCPD crystals grown under each set of conditions. Images to the left show macroscopic views, while those to the right show expanded views of typical particles.
T. Toshima et al. / Journal of Asian Ceramic Societies 2 (2014) 52–56
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Fig. 4. (a) SEM image of cross-section of petal-like particle. (b) Schematic image of particle in (a). Nested structure was observed at the center of each petal-like particle.
in multiple layers. Their dimensions ranged from a few micrometers to <100 m in one direction and to several micrometers in the other direction. However, some particles exhibited different aspect ratios, such as more than 100 m to <1 mm. The thickness of the plate-like particles was no larger than about 1–2 m. The petallike particles appeared to be aggregates of the plate-like particles. The total length of the polycrystal diameter was <100 m; therefore, each plate-like crystal was on the order of 10 m. The centers of the petal-like particles were of nested structures (Fig. 4); these results were similar to the case of galenite [38]. Fig. 5 shows the XRD spectra of DCPD synthesized at 0.10 mol/L and different initial pH values. All observed diffraction peaks were
Fig. 5. XRD patterns of DCPD powders at different initial pH values and 0.1 mol/L precursor molar concentration. The products at other precursor concentrations showed the same behavior.
related to DCPD and no impurity phases were found. The XRD results could be categorized into two types. The first displayed a very strong (0 2 0) peak with other weaker diffraction peaks at higher initial pH (5.0–6.0). The products with such XRD patterns always exhibited a plate-like structure. Therefore, this shape is based on an a–c plate. DCPD crystals usually form such plate-like structures, analogous to these observations. The other type also had a strong (0 2 0) peak, and other diffraction peaks such as (1¯ 2 1), (1¯ 1 2), and (1¯ 4 1) were of comparable strength. In the case of the latter species, the product morphology displayed the petal-like structure. These results show the same behavior as in our previous report [37]. In summary, the results of the SEM and XRD analyses revealed that the DCPD particles with petal-like structures were composed of smaller plate-like particles. This result shows that there is no difference during crystal growing process between the conditions we tried. Key point of formation mechanism of nested structure is not only reducing concentration during crystallization but also aggregate crystal nuclei at the early timing. Therefore controlling the early crystallization process, particularly the nucleation rate, is the most important factor influencing the shape of the DCPD crystals. Morphology phase diagrams were thus drawn for the DCPD crystallization showing the dependence on initial pH, the supersaturation, and pH change (Fig. 6). We added other experimental results (0.02 mol/L and 0.2 mol/L) to draw the diagram more clearly. The desired product is formed according to the following chemical reactions: Ca(NO3 )2 → Ca2+ + 2NO3 −
(1)
NH4 H2 PO4 → NH4 + + H2 PO4 −
(2)
Fig. 6. Morphology phase diagrams for DCPD crystallization under various crystal growth conditions. (Left) Dependence on initial pH value and supersaturation. (Right) Dependence on initial pH value and pH change. Products in light gray colored area is petal-shaped, gray colored is mixture and dark gray colored is plate-shaped, respectively.
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H2 PO4 − ↔ H+ + HPO4 2− 2+
Ca
+ HPO4
2−
+ H2 O ↔ CaHPO4 ·2H2 O
(3)
References
(4)
[1] J.C. Elliott, “Structure and Chemistry of the Apatites and Other Calcium Orthophosphates”, Elsevier, Amsterdam (1994). ´ ´ Serbian Dent. J., 59, (1) 7–21 (2012). [2] V. Babic-Ivanˇ cic´ and M.D. Sikiric, [3] A.M. Young, P.Y.J. Ng, U. Gbureck, S.N. Nazhat, J.E. Barralet and M.P. Hofmann, Acta Biomater., 4, 1081–1088 (2008). [4] M.-P. Ginebra, C. Canal, M. Espanol, D. Pastorino and E.B. Montufar, Adv. Drug Deliv. Rev., 64, 1090–1110 (2012). [5] C.-Y. Wen, L. Qin, K.-M. Lee and K.-M. Chan, J. Biomed. Mater. Res. Part B, 89, (2) 466–474 (2009). [6] A. Kawasaki, K. Ishikawa, T. Suge, H. Shimizu, K. Suzuki, T. Matsuo and S. Ebisu, J. Oral Rehabil., 28, 439–449 (2001). [7] T. Yang, C. Kim, J. Jho and I.W. Kim, Colloids Surf. A: Physicochem. Eng. Aspects, 401, 126–136 (2012). [8] M. Kumar, H. Dasarathy and C. Riley, J. Biomed. Mater. Res., 45, (4) 302–310 (1999). [9] H.E.L. Madsen, J. Cryst. Growth, 310, 2602–2612 (2008). [10] K. Furutaka, H. Monma, T. Okura and S. Takahashi, J. Eur. Ceram. Soc., 26, 543–547 (2006). [11] C. Oliveira, P. Georgieva, F. Rocha, A. Ferreira and S.F. de Azevedo, J. Cryst. Growth, 305, 201–210 (2007). [12] M. Tafu and T. Chohji, J. Eur. Ceram. Soc., 26, 767–770 (2006). [13] T. Lee and Y.C. Lin, Cryst. Growth Des., 11, 2973–2992 (2011). [14] O. Borkiewicz, J. Rakovan and C.L. Cahill, Am. Mineral., 95, 1224–1236 (2010). [15] H. Ito, Y. Oaki and H. Imai, Cryst. Growth Des., 8, 1055–1059 (2008). [16] F. Abbona, F. Christensson, M.F. Angela and E.L. Madsen, J. Cryst. Growth, 131, 331–346 (1993). [17] T. Yokoi, M. Kawashita and C. Ohtsuki, J. Asian Ceram. Soc., 1, 155–162 (2013). [18] W.E. Brown, N. Eidelman and B. Tomazic, Adv. Dent. Res., l, (2) 306–313 (1987). [19] M. Kitamura, C. Ohtsuki, S. Ogata, M. Kamitakahara, M. Tanihara and T. Miyazaki, J. Am. Ceram. Soc., 88, 822–826 (2005). [20] X. Yang, X. Gao, Y. Gan, C. Gao, X. Zhang, K. Ting, B.M. Wu and Z. Gou, J. Phys. Chem. C, 114, 6265–6271 (2010). [21] J.V. Rau, M. Fosca, V.S. Komlev, I.V. Fadeeva, V.R. Albertini and S.M. Barinov, Cryst. Growth Des., 10, 3824–3834 (2010). [22] M.S.-A. Johnsson and G.H. Nancollas, Crit. Rev. Oral Biol. Med., 3, (1/2) 61–82 (1992). [23] X.-D. Sun, Y.-L. Zhou, J.-Y. Ren, F.-Z. Cui and H.-D. Li, Curr. Appl. Phys., 7S1, e75–e79 (2007). [24] H. Monma and T. Kamiya, J. Mater. Sci., 22, 4247–4250 (1987). [25] A.C. Tas and S.B. Bhaduri, J. Am. Ceram. Soc., 87, 2195–2220 (2004). [26] J.A. Juhasz, S.M. Best, A.D. Auffret and W. Bonfield, J. Mater. Sci. Mater. Med., 19, 1823–1829 (2008). [27] X. Chu, W. Jiang, Z. Zhang, Y. Yan, H. Pan, X. Xu and R. Tang, J. Phys. Chem. B, 115, 1151–1157 (2011). [28] M. Aslanidou, T. Vaimakis, A. Mitsionis and C. Trapalis, Ceram. Int., 39, 539–546 (2013). [29] G.B. Ramírez-Rodríguez, J.M. Delgado-López and J. Gómez-Morales, CrystEngComm, 15, 2206–2212 (2013). [30] N.S. Al-Qasas and S. Rohani, Sep. Sci. Technol., 40, (15) 3187–3224 (2007). [31] N. Eliaz and T.M. Sridhar, Cryst. Growth Des., 8, 3965–3977 (2008). [32] L. Pach, S. Duncan, R. Roy and S. Komarneni, J. Mater. Sci., 31, 6565–6569 (1996). [33] O. Mekmene, S. Quillard, T. Rouillon, J.M. Bouler, M. Piot and F. Gaucheron, Dairy Sci. Technol., 89, 301–316 (2009). [34] S. Mandel and A.C. Tas, Mater. Sci. Eng. C, 30, (2) 245–254 (2010). [35] M.A. Miller, M.R. Kendall, M.K. Jain, P.R. Larson, A.S. Madden and A.C. Tas, J. Am. Ceram. Soc, 95, (7) 2178–2188 (2012). [36] F.Z. Cui, L.F. Zhou, H. Cui, C.L. Ma, H.B. Lu and H.D. Li, J. Cryst. Growth, 169, 557–562 (1996). [37] R. Hamai, T. Toshima, M. Tafu, T. Masutani and T. Chohji, Key Eng. Mater., 529–530, 55–60 (2012). [38] Y. Aoki, Mem. Sci., Kyushu Univ., Ser. D, 24, 75–108 (1979).
We calculated the supersaturation ratio S from following formulas: S(DCPD) =
IP(DCPD) KSP (DCPD)
IP(DCPD) = [Ca2+ ][HPO2− 4 ]
(5) (6)
where IP is the ion activity product and KSP , the solubility product constant, is 2.59 × 10−7 (mol/L)2 for DCPD. An increase in the supersaturation ratio and/or initial pH value shifted the morphology from the liquid phase (i.e., no product for at least 24 h) to the petal-like structure, and then to the plate-like structure. Occasionally, borderline conditions resulted in a mixture of plate-like and petal-like structures. When DCPD crystals form, both the pH value and the ion concentrations decrease. Under such circumstances, the crystal growth conditions may alternate between the two states. The morphology phase diagram comprising the initial pH values and pH changes showed very similar distributions to the preceding results. For solution conditions with high precursor concentrations and/or initial pH values, the pH change increased and the resulting crystalline form was of the plate-like structure. Initial crystal growth condition rules early nucleation condition; that is equal to say that mixing process of two solutions is a key for controlling the DCPD morphology. If one takes a long time for mixing solutions, morphology phase diagram is shifted and expand the area of petal structure. 4. Conclusion This study focused on controlling the shape of DCPD crystals by changing the initial pH of the solution and initial concentrations of HPO4 2− and Ca2+ ions at room temperature. The shapes of DCPD particles were strongly dependent on both these parameters: an increase in either the initial pH or the initial ion concentration led to the transformation of the petal-like to the plate-like structure. Morphology phase diagrams of DCPD crystallization were constructed from these results. To control the shapes of other calcium phosphate bioceramics, it would be of considerable interest to use the uniquely shaped DCPD crystals as precursors. Acknowledgements This study was supported by a Grant-in-Aid for Scientific Research (B), from the Japan Society for the Promotion of Science (JSPS) (FY 2011-2013).
Contents lists available at ScienceDirect
Journal of Asian Ceramic Societies journal homepage: www.elsevier.com/locate/jascer
Morphology control of brushite prepared by aqueous solution synthesis T. Toshima a,∗ , R. Hamai a,b , M. Tafu a , Y. Takemura a , S. Fujita a , T. Chohji a , S. Tanda c,d , S. Li e , G.W. Qin e a
Toyama National College of Technology, 13 Hongo-machi, Toyama 939-8630, Japan Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Fukuoka 808-0196, Japan c Department of Applied Physics, Hokkaido University, Hokkaido 060-8628, Japan d Center of Education and Research for Topological Science and Technology, Hokkaido University, Hokkaido 060-8628, Japan e Key Laboratory for Anisotropy and Texture of Materials (MOE), Northeastern University, Shenyang 110819, China b
a r t i c l e
i n f o
Article history: Received 30 August 2013 Received in revised form 7 January 2014 Accepted 7 January 2014 Available online 4 February 2014 Keywords: Morphology Crystal growth Brushite Calcium phosphate
a b s t r a c t Dicalcium phosphate dihydrate (DCPD, CaHPO4 ·2H2 O), also known as brushite, is one of the important bioceramics due to not only diseases factors such as kidney stone and plaque formation but also purpose as fluoride insolubilization material. It is used medicinally to supply calcium, and is of interest for its unique properties in biological and pathological mineralization. It is important to control the crystal morphology of brushite since its chemical reactivity depends strongly on its surface properties; thus, its morphology is a key issue for its applications as a functional material or precursor for other bioceramics. Here, we report the effects of the initial pH and the Ca and phosphate ion concentrations on the morphology of DCPD particles during aqueous solution synthesis. Crystal morphologies were analyzed by scanning electron microscopy and X-ray diffraction. The morphology phase diagram of DCPD crystallization revealed that increasing the initial pH and/or ion concentration transformed DCPD morphology from petal-like into plate-like structures. © 2014 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction Calcium phosphate bioceramics are the main constituents of the bones and teeth of vertebrates, including most human hard tissues [1,2]. They have been the subjects of intensive research in a range of fields, including biomineralization, medicine [3–6], and the environment [7]. In particular, dicalcium phosphate dihydrate (DCPD, CaHPO4 ·2H2 O), also known as brushite, is one of the key components in calcium phosphate systems, because DCPD can not only be deposited as a coating for orthopedic implants [8], but is also recognized as a starting material in the preparation of other calcium phosphate bioceramics [9–11]. So far, DCPD has been applied as many functional materials, such as the abrasive constituent of toothpaste, a feed additive, and water treatment compounds [7,12].
∗ Corresponding author. Tel.: +81 76 493 5476; fax: +81 76 493 5476. E-mail addresses: [email protected], [email protected] (T. Toshima). Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.
2187-0764 © 2014 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jascer.2014.01.004
There has been substantial research into the morphology control of calcium phosphate bioceramics. In general, the performance of a functional material significantly depends on its shape and surface properties [13–15]. Extensive investigations have been undertaken to establish effective parameters controlling the shapes of these bioceramics because of the complexity of the crystallization system [16,17]. Research on DCPD crystallization has relevance for the preparation of other calcium phosphate bioceramics. For example, crystalline octacalcium phosphate (OCP, Ca8 (H2 PO4 )2 (PO4 )4 ·5H2 O) is well known as a bone component precursor [18] and is attractive to biochemists because of its controllable morphology [19,20]. The parametric factors in the biomineralization processes of these materials have attracted the interest of biochemists, with a key issue being the case of hydroxyapatite (HAp, Ca10 (PO4 )6 (OH)2 ) formation [21,22]. Many reports concern the transformation of DCPD to HAp [23–28]. Information on DCPD morphology is very useful for predicting in vivo bone and/or tooth bioactivity and for accelerating bone regeneration. Such detour methods may provide important clues for controlling the shape of the final product. Therefore, elucidating the effects of DCPD crystallization parameters is very important. In recent studies of calcium phosphate bioceramics, various crystallization conditions (e.g., calcium and phosphorous concentrations, temperature, ionic strengths, and solution pH) were investigated [29–36]. We previously reported the effects of both acetate anion and pH on the morphology of DCPD crystallization [37]. The addition of acetate ion changed the morphology of the DCPD crystals and their yields.
T. Toshima et al. / Journal of Asian Ceramic Societies 2 (2014) 52–56
The objective of this work was to demonstrate the effects of the initial pH and ion concentrations on the morphology of DCPD crystallization during aqueous solution synthesis. It is well known that pH has a great influence on both the nucleation rate and crystal growth rate. The stability and solubility of DCPD crystals are also strongly dependent on the solution pH. Both the pH and the ion concentration are known to affect the supersaturation ratio of DCPD crystals. Therefore, pH and ion concentration were examined as key parameters for controlling the morphology of DCPD crystals in this study. The product structures were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). 2. Materials and methods DCPD crystals were prepared according to the flow diagram shown in Fig. 1. Aqueous solutions of ammonium dihydrogen phosphate (NH4 H2 PO4 , Nacalai, 99%) and calcium nitrate (Ca(NO3 )2 , Wako, 98.5%) were prepared in 100 mL glass beakers at room temperature. The initial pH of the solutions was adjusted to the desired value by the addition of 0.01 and 0.1 M nitric acid (Kanto Chemical)
53
and ammonia solution (Wako, 25%). The initial pH values, from 4.0 to 7.0, were tested with a pH meter (SevenMulti, Mettler-Toledo). The initial molar concentrations were adjusted to 0.02, 0.10, and 0.20 mol/L, respectively. The solutions (100 mL each) of Ca(NO3 )2 and NH4 H2 PO4 with the same initial pH values and concentrations were rapidly mixed in a 200 mL beaker and stirred magnetically at 200 rpm. The key point is mixing two solutions at once, without taking long time for mixing. The pH values of the mixtures were monitored over the first hour after mixing. If a solid was not observed in the solution at this stage, then the solution was stirred up to 24 h. After 24 h without evidence of crystallization, the reaction was stopped and recorded as “no product.” After the reaction, all solutions were filtered through a 0.45 m membrane filter. The solid products were dried at 40 ◦ C for 24 h. The products from each set of crystal growth conditions were examined by SEM (JSM-6390AX, JEOL, accelerating voltage of 3–10 kV without coating) and XRD (Mini Flex, Rigaku, 35 kV, 15 mA, 2◦ /min). 3. Results and discussion Table 1 lists the results at 1 h and 24 h for the various crystal growth conditions. At concentrations below 0.01 mol/L, no solid products were observed with an initial pH of 5.5 or lower, even after 24 h. A solid product was observed at an initial pH of 6.0 after 24 h, however, the amount of solid formed after 1 h was not sufficient for testing. At an initial pH of 7.0, solids were observed within 1 h. At other concentrations, the crystallizations exhibited the same behaviors. As the concentration increased, crystallization occurred more readily at lower initial pH, at which the products were never obtained under lower concentration conditions. Fig. 2 illustrates the pH changes after 1 h with respect to the initial pH and concentration. The pH values of the solutions decreased under all conditions, whether solid products were obtained or not. The pH difference increased when the initial pH Table 1 Initial pH value and concentration dependence of solid product formation. Initial pH
0.01 mol/L
0.05 mol/L
0.10 mol/L
4 5 6 7
× ×
×
×
×: no product after 24 h. : no product after 1 h, product after 24 h. : product after 1 h.
Fig. 1. Experimental flow design. Starting solutions with the same initial pH and concentration values were initially mixed. (Example: Ca(NO3 )2 (0.02 mol/L, pH 4) + NH4 H2 O4 (0.02 mol/L, pH 4) → mixture (0.01 mol/L, pH 4).)
Fig. 2. pH change as a function of initial pH and ion concentration.
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T. Toshima et al. / Journal of Asian Ceramic Societies 2 (2014) 52–56
and/or concentrations were high. When such solutions were combined, the mixtures became milky due to the rapid formation of the solid product. Therefore, each parameter contributed to the driving force for crystallization.
The dependence of DCPD morphology on the initial preparation conditions was further examined. As shown in Fig. 3, plate-like and petal-like particles were observed by SEM. The plate-like particles mostly formed in parallelogram shapes that were stacked
Fig. 3. SEM images of DCPD crystals grown under each set of conditions. Images to the left show macroscopic views, while those to the right show expanded views of typical particles.
T. Toshima et al. / Journal of Asian Ceramic Societies 2 (2014) 52–56
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Fig. 4. (a) SEM image of cross-section of petal-like particle. (b) Schematic image of particle in (a). Nested structure was observed at the center of each petal-like particle.
in multiple layers. Their dimensions ranged from a few micrometers to <100 m in one direction and to several micrometers in the other direction. However, some particles exhibited different aspect ratios, such as more than 100 m to <1 mm. The thickness of the plate-like particles was no larger than about 1–2 m. The petallike particles appeared to be aggregates of the plate-like particles. The total length of the polycrystal diameter was <100 m; therefore, each plate-like crystal was on the order of 10 m. The centers of the petal-like particles were of nested structures (Fig. 4); these results were similar to the case of galenite [38]. Fig. 5 shows the XRD spectra of DCPD synthesized at 0.10 mol/L and different initial pH values. All observed diffraction peaks were
Fig. 5. XRD patterns of DCPD powders at different initial pH values and 0.1 mol/L precursor molar concentration. The products at other precursor concentrations showed the same behavior.
related to DCPD and no impurity phases were found. The XRD results could be categorized into two types. The first displayed a very strong (0 2 0) peak with other weaker diffraction peaks at higher initial pH (5.0–6.0). The products with such XRD patterns always exhibited a plate-like structure. Therefore, this shape is based on an a–c plate. DCPD crystals usually form such plate-like structures, analogous to these observations. The other type also had a strong (0 2 0) peak, and other diffraction peaks such as (1¯ 2 1), (1¯ 1 2), and (1¯ 4 1) were of comparable strength. In the case of the latter species, the product morphology displayed the petal-like structure. These results show the same behavior as in our previous report [37]. In summary, the results of the SEM and XRD analyses revealed that the DCPD particles with petal-like structures were composed of smaller plate-like particles. This result shows that there is no difference during crystal growing process between the conditions we tried. Key point of formation mechanism of nested structure is not only reducing concentration during crystallization but also aggregate crystal nuclei at the early timing. Therefore controlling the early crystallization process, particularly the nucleation rate, is the most important factor influencing the shape of the DCPD crystals. Morphology phase diagrams were thus drawn for the DCPD crystallization showing the dependence on initial pH, the supersaturation, and pH change (Fig. 6). We added other experimental results (0.02 mol/L and 0.2 mol/L) to draw the diagram more clearly. The desired product is formed according to the following chemical reactions: Ca(NO3 )2 → Ca2+ + 2NO3 −
(1)
NH4 H2 PO4 → NH4 + + H2 PO4 −
(2)
Fig. 6. Morphology phase diagrams for DCPD crystallization under various crystal growth conditions. (Left) Dependence on initial pH value and supersaturation. (Right) Dependence on initial pH value and pH change. Products in light gray colored area is petal-shaped, gray colored is mixture and dark gray colored is plate-shaped, respectively.
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T. Toshima et al. / Journal of Asian Ceramic Societies 2 (2014) 52–56
H2 PO4 − ↔ H+ + HPO4 2− 2+
Ca
+ HPO4
2−
+ H2 O ↔ CaHPO4 ·2H2 O
(3)
References
(4)
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We calculated the supersaturation ratio S from following formulas: S(DCPD) =
IP(DCPD) KSP (DCPD)
IP(DCPD) = [Ca2+ ][HPO2− 4 ]
(5) (6)
where IP is the ion activity product and KSP , the solubility product constant, is 2.59 × 10−7 (mol/L)2 for DCPD. An increase in the supersaturation ratio and/or initial pH value shifted the morphology from the liquid phase (i.e., no product for at least 24 h) to the petal-like structure, and then to the plate-like structure. Occasionally, borderline conditions resulted in a mixture of plate-like and petal-like structures. When DCPD crystals form, both the pH value and the ion concentrations decrease. Under such circumstances, the crystal growth conditions may alternate between the two states. The morphology phase diagram comprising the initial pH values and pH changes showed very similar distributions to the preceding results. For solution conditions with high precursor concentrations and/or initial pH values, the pH change increased and the resulting crystalline form was of the plate-like structure. Initial crystal growth condition rules early nucleation condition; that is equal to say that mixing process of two solutions is a key for controlling the DCPD morphology. If one takes a long time for mixing solutions, morphology phase diagram is shifted and expand the area of petal structure. 4. Conclusion This study focused on controlling the shape of DCPD crystals by changing the initial pH of the solution and initial concentrations of HPO4 2− and Ca2+ ions at room temperature. The shapes of DCPD particles were strongly dependent on both these parameters: an increase in either the initial pH or the initial ion concentration led to the transformation of the petal-like to the plate-like structure. Morphology phase diagrams of DCPD crystallization were constructed from these results. To control the shapes of other calcium phosphate bioceramics, it would be of considerable interest to use the uniquely shaped DCPD crystals as precursors. Acknowledgements This study was supported by a Grant-in-Aid for Scientific Research (B), from the Japan Society for the Promotion of Science (JSPS) (FY 2011-2013).