- Email: [email protected]
Hydroxyapatite formation from calcium carbonate single crystal under hydrothermal condition: Effects of processing temperature
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 1886–1890 www.elsevier.com/locate/ceramint
Hydroxyapatite formation from calcium carbonate single crystal under hydrothermal condition: Effects of processing temperature Ill Yong Kimn, Chikara Ohtsuki Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, B2-3(611) Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 20 August 2015; received in revised form 29 September 2015; accepted 29 September 2015 Available online 9 October 2015
Abstract Hydrothermal processing has been used for synthesis of hydroxyapatite (HAp) crystals with unique morphologies. Our previous studies reported that the morphology of HAp was affected by dissolution behavior of starting materials under a hydrothermal condition. In this study we evaluated the effects of processing temperature on formation and morphology of HAp crystals under a hydrothermal condition. Calcite (CaCO3) single crystal as a starting material was hydrothermally treated in a phosphate solution at various temperatures. After hydrothermal processing for 24 h, needle-shaped HAp crystals formed on the surfaces of all the samples. The size of HAp crystals increased with increasing the processing temperature. Plate-shaped HAp was observed on the cross-section of the starting material treated at 200 1C. The structure of HAp plates were generated by arrangement of needle-shaped HAp crystals during their crystal growth. The difference in the processing temperature affects the morphology and structure of HAp formed on the starting material under the hydrothermal condition. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Microstructure-final; Hydroxyapatite; Hydrothermal processing; Crystal growth
1. Introduction Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) has attracted great attention in biomedical applications such as bone and tooth repairing. Since HAp shows bonding ability to living bone and unique protein adsorption property [1,2]. The morphology of HAp crystals affects the performance in their biological applications. Therefore, the morphology control of HAp crystals has been remained as one of important assignments. From the previous studies, it is well known that hydrothermal processing is an attractive method to control morphology of well-crystallized HAp [3–7]. Under the hydrothermal processing, morphology of HAp crystals significantly depends on not only hydrothermal conditions such as processing temperature and period, and additives but also starting materials. It was reported that hydrothermal processing allows HAp synthesis from calcium carbonate in a phosphate solution by a dissolution–precipitation reaction [8]. Starting materials have n
Corresponding author. Tel.: þ81 52 789 3183; fax: þ 81 52 789 3182. E-mail address: [email protected] (I.Y. Kim).
http://dx.doi.org/10.1016/j.ceramint.2015.09.156 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
to be dissolved to supply calcium ions for HAp formation when a calcium compound is used as a starting material. So, HAp formation and dissolution of calcium carbonates would occur on their surface simultaneously. We reported that HAp formation was also generated on single crystals of calcium compounds [9,10]. Single crystals of calcium compounds as starting materials are suitable calcium sources on HAp synthesis under hydrothermal conditions because the effects of specific surface area and particle distribution on dissolution behavior of starting materials, can be ignored. Besides, each surface of a single crystal would show uniform dissolution behavior under hydrothermal conditions. Therefore, the effects of starting materials under hydrothermal conditions can be evaluated fundamentally on HAp formation using single crystals of calcium compounds. Crystal faces of a calcium carbonate (calcite) single crystal also affected not only growth rate but also growth direction of HAp crystals [9]. The influence of crystal faces on HAp formation would be governed by the difference in dissolution behavior of each crystal face. In addition, morphology of the resultant HAp crystals depend on solubility of a starting
I.Y. Kim, C. Ohtsuki / Ceramics International 42 (2016) 1886–1890 HAp Cal-200
Cal -100 Intensity / a. u.
Intensity / a. u.
Cal-160
Before HP
20
Cal- 200
20
30
30 2 / degree
40
1887
microscopy (TEM, H-800, Hitachi Ltd., Tokyo, Japan) with selected area electron diffraction (SAED) was used to evaluate the morphology and structure of the reaction products on the samples. The products on the samples were applied to measure Ca/ P ratio and carbonate contents by energy-dispersive X-ray analyser (EDX) and thermogravimetry-differential thermal analysis (TGDTA, DTG-60AH, Shimadzu. Co. Kyoto, Japan). 3. Results
40
50
2 / degree
Fig. 1. XRD patterns of the calcium carbonate single crystals before and after hydrothermal processing (HP) at various temperatures for 24 h.
material [11]. Using calcium tartrate, calcium carbonate and calcium fluoride single crystals, nano-sized, needle-shaped and plate-shaped HAp crystals were formed on their surfaces, respectively [10]. This means that the morphology of resultant HAp crystals can be controlled by the dissolution rate of starting materials under a hydrothermal condition. Dissolution behavior of a starting material is affected by processing temperature. However, the effects of the processing temperature on HAp formation on starting materials are not cleared yet. In the present study, we investigated the effects of processing temperature on HAp formation in a phosphate solution under a hydrothermal condition. Calcite single crystal was used as a starting material for HAp formation by the dissolution–precipitation reaction. After the hydrothermal processing, we evaluated morphology of the HAp crystals formed on the surfaces of the starting material. 2. Materials and methods Natural calcite single crystal (CaCO3, Mexico) with {104} faces was used as a starting material to supply calcium ions for HAp formation. The single crystal sample with 10 10 1 mm3 in size was placed in a Teflons-lined autoclave with 30 cm3 of 1 mol dm 3 diammonium hydrogen phosphate ((NH4)2HPO4, Nacalai tesque, Inc., Kyoto, Japan) solution. The pH of the phosphate solution was adjusted to 9.8 by 25% ammonia solution (NH4OH, Nacalai tesque, Inc., Kyoto, Japan). The autoclave was sealed tightly and then heated at 100, 160 or 200 1C for 24 h. After heating, the autoclave was cooled rapidly with a fan. The sample was removed from the phosphate solution, rinsed with ultra-pure water and acetone to terminate any further reaction, and then kept at 40 1C to dry for 1 d. Notation of specimens is decided as Cal-temperature. For example, Cal-200 is the hydrothermally treated specimen at 200 1C. The processing products were characterized using a field emission scanning electron microscope (SEM, JSM-5600, JEOL Ltd., Tokyo, Japan) to observe the morphological change of the surfaces and cross-sections of the samples. X-ray diffraction (XRD, RINT PC 2100, Rigaku Co., Tokyo, Japan) was used to confirm HAp formation by the processing. Transmission electron
The external shape and size of the samples were maintained, and the transparent samples became opaque after the hydrothermal treatment. Fig. 1 shows XRD patterns of the surfaces of calcite samples before and after the hydrothermal processing at various temperatures for 24 h. The strong (104) peak of calcite at about 291 was detected before the hydrothermal processing. The peak of starting material disappeared in case of Cal-160 and Cal-200. After the hydrothermal processing, the peaks assigned to HAp were detected although the peak intensity of HAp is lower than that of calcite. The extended XRD pattern of Cal-200 shows clearly formation of HAp crystals with high crystallinity on the specimen. The peak intensity of HAp increased with the processing temperature. Fig. 2 shows SEM photos of the surfaces of the calcite samples before and after the hydrothermal processing at various temperatures for 24 h. While the samples had smooth surface before the hydrothermal processing, needle-shaped HAp crystals on their surfaces were formed randomly by the hydrothermal processing. The width and length of the crystals on each sample increased with increasing the processing temperature. In case of Cal-200, the length and width of the needle-shaped HAp crystals were about 4 and 0.4 μm, respectively. Fig. 3 shows SEM photos of the cross-section of the calcite samples after the hydrothermal processing at various temperatures for 24 h. The thickness of HAp layer formed on the calcites increased with increasing the processing temperature. On Cal-100, HAp crystals were grown randomly while HAp crystals were oriented vertically to the Cal-160 [9] and Cal-200. Fig. 4 shows SEM and TEM photos of the cross-section of Cal-200 after hydrothermal processing for 24 h. The photo (a) shows random HAp growth around the surface of the sample. The photo (b) shows plate-shaped HAp crystals like nacre structure. The Fig. 4(c) shows the cross-section after polishing the (b) part. From SEM photo of the polished surface, it was confirmed that HAp crystals were oriented. TEM photo shows orientation of HAp crystals and electron diffraction pattern shows that each HAp was a single crystal. Fig. 5 shows SEM photo of the oblique-sections and its illustration of Cal-200 after the hydrothermal treatment. The plate-shaped HAp structure was composed of needle-shaped HAp crystals. It was confirmed that HAp crystals were oriented and stacked like showing the illustration. Table 1 shows Ca/P ratio and carbonate content of the HAp prepared on the samples. The Ca/P atomic ration of the HAp crystals were slightly increased from 1.62 to 1.63 with increasing the processing temperature. And the carbonate
1888
I.Y. Kim, C. Ohtsuki / Ceramics International 42 (2016) 1886–1890
Fig. 2. SEM photos of the surfaces of the starting materials before and after hydrothermal processing (HP) at various temperatures for 24 h.
Fig. 3. SEM photos of the cross-section of the starting materials after hydrothermal processing at various temperatures for 24 h.
contents of the HAp crystals were slightly increased with increasing the processing temperature. 4. Discussion HAp formation on calcium carbonate in a phosphate solution was triggered by its dissolution. Calcium ions released from calcium carbonate react with phosphate ions to form calcium phosphate as followed. CaCO3 þ Phosphate solution-Ca10(PO4)6(OH)2 þ H2O þ CO2 HAp has the lowest solubility in calcium phosphates at neutral and basic conditions. So, HAp formation was occurred at the basic condition in this study. This reaction would generate rapidly on the surface of a calcium compound in a phosphate solution because calcium ions are released from its surface [8]. This reaction for HAp formation from calcium salts was used in the other studies [12–17]. To supply calcium ions to form HAp crystals, corals, natural gypsum, calcium carbonate, eggshell and calcium phosphates were used as
starting materials. HAp crystals with various morphologies could be synthesized through a hydrothermal processing. Rodshaped HAp crystals were obtained from α-tricalcium phosphate (Ca3(PO4)2) as a starting material [18]. HAp whiskers were obtained from β-tricalcium phosphate (Ca3(PO4)2) with addition of citric acid [19]. Plate-shaped HAp crystals had also been synthesized from octacalcium phosphate (Ca8(HPO4)2(PO4)4 5H2O) plates [20]. Our previous study also showed that the oriented structure of needle-shaped HAp crystals was prepared from a calcium carbonate single crystal under a hydrothermal processing [9]. The oriented structure of HAp was generated by uniform dissolution of the calcium carbonate single crystal. In this study, to control the dissolution rate of a starting material, the processing temperature was changed. It was reported that the solubility of calcite increased with increasing the processing temperature under a hydrothermal condition [21]. On the contrary, the solubility of HAp decreases with increasing temperature [22]. Resultantly, the reaction rate of calcium ions with phosphate ions for HAp formation might increase with increasing the processing temperature. The
I.Y. Kim, C. Ohtsuki / Ceramics International 42 (2016) 1886–1890
1889
Fig. 4. SEM and TEM photos of the cross-section of Cal-200 after hydrothermal processing for 24 h.
Fig. 5. SEM photos of the oblique-section of the Cal-200 after hydrothermal processing for 24 h.
thickness of HAp layer well agreed with the dissolution rate of starting materials [10]. The thickness of HAp layer on Cal-200 was greater than those on Cal-100 and Cal-160. The difference in the Ca/P ratio and carbonate content can be affected by the processing temperature. In this study, the Ca/P ratio and carbonate contents of the HAp crystals were slightly increased with increasing the processing temperature. The carbonate content affects to Ca/P ratio of HAp crystals. Using a calcium carbonate as a starting materials for HAp preparation under a hydrothermal condition, carbonate is generated by its dissolution. Therefore carbonate amount in a vessel during hydrothermal processing would be increased with increasing the formed HAp amount resulted in increasing the carbonate content in HAp crystals. The results in this study were evidenced clearly the effects of processing temperature on morphology of HAp crystals under the hydrothermal condition. Previously, it was reported that the morphology of HAp crystals depended on dissolution behavior of starting materials [11]. According to the reports, plate-shaped HAp crystals would be prepared easily on starting
Table 1 Ca/P ratio and carbonate contents of the HAp prepared on the samples. Sample
Ca/P atomic ratio
Carbonate content (mass %)
Cal-100 Cal-160 Cal-200
1.62 1.62 1.63
7.8% 7.9% 8.1%
materials with higher dissolution rate than that with lower dissolution rate. However, in this study, plate-shaped HAp crystal was obtained on the sample treated at 200 1C although the dissolution rate of calcite increased with increasing the processing temperature. From observation of the obliquesection for Cal-200 (See Fig. 5), it was confirmed that the stacked structure of plate-shaped HAp crystals like a nacre was composed of needle-shaped HAp crystals. The plate-shaped HAp was fabricated by arrangement of needle-shaped HAp like the illustration (Fig. 5). Previously we found that HAp
1890
I.Y. Kim, C. Ohtsuki / Ceramics International 42 (2016) 1886–1890
crystals are grown up to inside of a calcium compound single crystal [9]. The HAp crystal growth was governed by supplying calcium ions from the surface of starting materials resulted in the oriented structure of HAp crystals. The structure of HAp might be originated by rapid dissolution of calcite single crystal as a starting material. Consequently, the results obtained in this study suggested that dissolution rate of a starting material affects not only morphology of the formed HAp crystals but also their arrangement under a hydrothermal condition. 5. Conclusions The effects of processing temperature on HAp formation were investigated using calcite single crystals under a hydrothermal condition. After hydrothermal processing, needleshaped HAp crystals were formed on all the samples. The size of HAp crystal increased with increasing the processing temperature. From the observation of cross-section of the samples, the thickness of HAp layer increased with increasing the processing temperature. These results were caused by increase of dissolution rate of the starting materials. Moreover, plate-shaped HAp structure was observed on the cross-section of Cal-200. This stacked structure of plate-shaped HAp was composed of needle-like HAp crystals. Dissolution rate of a starting materials affects morphology and structure of the resultantly formed HAp. Acknowledgments This work was supported by Grant-in-Aid for Scientific Research on the Innovative Areas: “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (No. 2206) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). References [1] J.B. Park, Bioceramics: Properties, Characterizations and Applications, Springer, New York, 1998. [2] G. Bernardi, Chromatography of nucleic acids on hydroxyapatite, Nature 20 (1965) 779–783. [3] H. Zhang, K. Zhou, Z. Li, S. Huang, Plate-like hydroxyapatite nanoparticles synthesized by the hydrothermal method, J. Phys. Chem. Solids 70 (2009) 243–248. [4] T. Goto, I.Y. Kim, K. Kikuta, C. Ohtsuki, Hydroxyapatite formation by solvothermal treatment of α-tricalcium phosphate with water–ethanol solution, Ceram. Intern. 38 (2012) 1003–1010.
[5] C.M. Zhang, J. Yang, Z.W. Quan, P.P. Yang, C.X. Li, Z.Y. Hou, J. Lin, Hydroxyapatite nano- and microcrystals with multiform morphologies: controllable synthesis and luminescence properties, Cryst. Growth Des. 9 (2009) 2725–2733. [6] I.S. Neira, Y.V. Kolen’ko, O.I. Lebedev, G. Van Tendeloo, H.S. Gupta, F. Guitián, M. Yoshimura, An effective morphology control of hydroxyapatite crystals via hydrothermal synthesis, Cryst. Growth Des. 9 (2009) 466–474. [7] M. Yoshimura, H. Suda, K. Okamoto, K. Ioku, Hydrothermal synthesis of biocompatible whiskers, J. Mater. Sci. 29 (1994) 3399–3402. [8] K. Ishikawa, S. Matsuya, X. Lin, Z. Lei, T. Yuasa, Y. Miyamoto, Fabrication of low crystalline B-type carbonate apatite block from low crystalline calcite block, J. Ceram. Soc. Japan 118 (2010) 341–344. [9] I.Y. Kim, K. Kikuta, C. Ohtsuki, Formation of oriented hydroxyapatite rods by hydrothermal treatment of calcite single crystal, Korean, J. Mater. Res. 22 (2012) 397–402. [10] I.Y. Kim, K. Koichi, C. Ohtsuki, Hydroxyapatite formation through dissolution-precipitation reaction: effects of solubility of starting materials, Ceram. Intern. 40 (2014) 14385–14390. [11] T. Goto, I.Y. Kim, K. Kikuta, C. Ohtsuki, Comparative study of hydroxyapatite formation from α- and β-tricalcium phosphates under hydrothermal conditions, J. Ceram. Soc. Japan 120 (2012) 131–137. [12] K.P. Sanosh, M.C. Chu, A. Balakrishnan, T.N. Kim, S.J. Cho, Utilization of biowaste eggshells to synthesize nanocrystalline hydroxyapatite powders, Mater. Lett. 63 (2009) 2100–2102. [13] T. Watanabe, G. Kawachi, M. Kamitakahara, K. Kikuta, C. Ohtsuki, Formation of needle-like hydroxyapatite by hydrothermal treatment of CaHPO4 2H2O combined with β-Ca3(PO4)2, J. Ceram. Soc. Japan 117 (2009) 759–764. [14] G.B. Ramírez-Rodríguez, J.M. Delgado-López, J. Gómez-Morales, Evolution of calcium phosphate precipitation in hanging drop vapor diffusion by in situ Raman microspectroscopy, CrystEngComm 15 (2013) 2206–2212. [15] M. Sivakumar, T.S. Sampath Kumar., K.L. Shantha, K. Panduranga Rao, Development of hydroxyapatite derived from Indian coral, Biomaterials 17 (1996) 1709–1714. [16] E. Pujiyanto, P.W. Laksono, J. Triyono, Synthesis and characterization of hydroxyapatite powder from natural gypsum rock, Adv. Mater. Res. 898 (2014) 56–59. [17] S. Jinawath, D. Pongkao, M. Yoshimura, Hydrothermal synthesis of hydroxyapatite from natural source, J. Mater. Sci.: Mater. Med. 13 (2002) 491–494. [18] K. Ioku, G. Kawachi, S. Sasaki, H. Fujimori, S. Goto, Hydrothermal preparation of tailored hydroxyapatite, J. Mater. Sci. 41 (2006) 1341–1344. [19] M. Yoshimura, H. Suda, K. Okamaoto, K. Ioku, Hydrothermal synthesis of biocompatible whiskers, J. Mater. Sci. 29 (1994) 3399–3402. [20] M. Kamitakahara, N. Ito, S. Murakami, N. Watanabe, K. Ioku, Hydrothermal synthesis of hydroxyapatite from octacalcium phosphate: effect of hydrothermal temperature, J. Ceram. Soc. Japan 117 (2009) 385–387. [21] E.R. Segnit, H.D. Holland, C.J. Biscardi, The solubility of calcite in aqueous solutions-1. The solubility of calcite in water between 751 and 2001 at CO2 pressures up to 60 atm, Geochim. Cosmochim. Acta 26 (1962) 1301–1331. [22] J.B. Liu, X.Y. Ye, H. Wang, M.K. Zhu, B. Wang, H. Yan, The influence of pH and temperature on the morphology of hydroxyapatite synthesized by hydrothermal method, Ceram. Intern. 29 (2003) 629–633.
CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 1886–1890 www.elsevier.com/locate/ceramint
Hydroxyapatite formation from calcium carbonate single crystal under hydrothermal condition: Effects of processing temperature Ill Yong Kimn, Chikara Ohtsuki Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, B2-3(611) Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 20 August 2015; received in revised form 29 September 2015; accepted 29 September 2015 Available online 9 October 2015
Abstract Hydrothermal processing has been used for synthesis of hydroxyapatite (HAp) crystals with unique morphologies. Our previous studies reported that the morphology of HAp was affected by dissolution behavior of starting materials under a hydrothermal condition. In this study we evaluated the effects of processing temperature on formation and morphology of HAp crystals under a hydrothermal condition. Calcite (CaCO3) single crystal as a starting material was hydrothermally treated in a phosphate solution at various temperatures. After hydrothermal processing for 24 h, needle-shaped HAp crystals formed on the surfaces of all the samples. The size of HAp crystals increased with increasing the processing temperature. Plate-shaped HAp was observed on the cross-section of the starting material treated at 200 1C. The structure of HAp plates were generated by arrangement of needle-shaped HAp crystals during their crystal growth. The difference in the processing temperature affects the morphology and structure of HAp formed on the starting material under the hydrothermal condition. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Microstructure-final; Hydroxyapatite; Hydrothermal processing; Crystal growth
1. Introduction Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) has attracted great attention in biomedical applications such as bone and tooth repairing. Since HAp shows bonding ability to living bone and unique protein adsorption property [1,2]. The morphology of HAp crystals affects the performance in their biological applications. Therefore, the morphology control of HAp crystals has been remained as one of important assignments. From the previous studies, it is well known that hydrothermal processing is an attractive method to control morphology of well-crystallized HAp [3–7]. Under the hydrothermal processing, morphology of HAp crystals significantly depends on not only hydrothermal conditions such as processing temperature and period, and additives but also starting materials. It was reported that hydrothermal processing allows HAp synthesis from calcium carbonate in a phosphate solution by a dissolution–precipitation reaction [8]. Starting materials have n
Corresponding author. Tel.: þ81 52 789 3183; fax: þ 81 52 789 3182. E-mail address: [email protected] (I.Y. Kim).
http://dx.doi.org/10.1016/j.ceramint.2015.09.156 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
to be dissolved to supply calcium ions for HAp formation when a calcium compound is used as a starting material. So, HAp formation and dissolution of calcium carbonates would occur on their surface simultaneously. We reported that HAp formation was also generated on single crystals of calcium compounds [9,10]. Single crystals of calcium compounds as starting materials are suitable calcium sources on HAp synthesis under hydrothermal conditions because the effects of specific surface area and particle distribution on dissolution behavior of starting materials, can be ignored. Besides, each surface of a single crystal would show uniform dissolution behavior under hydrothermal conditions. Therefore, the effects of starting materials under hydrothermal conditions can be evaluated fundamentally on HAp formation using single crystals of calcium compounds. Crystal faces of a calcium carbonate (calcite) single crystal also affected not only growth rate but also growth direction of HAp crystals [9]. The influence of crystal faces on HAp formation would be governed by the difference in dissolution behavior of each crystal face. In addition, morphology of the resultant HAp crystals depend on solubility of a starting
I.Y. Kim, C. Ohtsuki / Ceramics International 42 (2016) 1886–1890 HAp Cal-200
Cal -100 Intensity / a. u.
Intensity / a. u.
Cal-160
Before HP
20
Cal- 200
20
30
30 2 / degree
40
1887
microscopy (TEM, H-800, Hitachi Ltd., Tokyo, Japan) with selected area electron diffraction (SAED) was used to evaluate the morphology and structure of the reaction products on the samples. The products on the samples were applied to measure Ca/ P ratio and carbonate contents by energy-dispersive X-ray analyser (EDX) and thermogravimetry-differential thermal analysis (TGDTA, DTG-60AH, Shimadzu. Co. Kyoto, Japan). 3. Results
40
50
2 / degree
Fig. 1. XRD patterns of the calcium carbonate single crystals before and after hydrothermal processing (HP) at various temperatures for 24 h.
material [11]. Using calcium tartrate, calcium carbonate and calcium fluoride single crystals, nano-sized, needle-shaped and plate-shaped HAp crystals were formed on their surfaces, respectively [10]. This means that the morphology of resultant HAp crystals can be controlled by the dissolution rate of starting materials under a hydrothermal condition. Dissolution behavior of a starting material is affected by processing temperature. However, the effects of the processing temperature on HAp formation on starting materials are not cleared yet. In the present study, we investigated the effects of processing temperature on HAp formation in a phosphate solution under a hydrothermal condition. Calcite single crystal was used as a starting material for HAp formation by the dissolution–precipitation reaction. After the hydrothermal processing, we evaluated morphology of the HAp crystals formed on the surfaces of the starting material. 2. Materials and methods Natural calcite single crystal (CaCO3, Mexico) with {104} faces was used as a starting material to supply calcium ions for HAp formation. The single crystal sample with 10 10 1 mm3 in size was placed in a Teflons-lined autoclave with 30 cm3 of 1 mol dm 3 diammonium hydrogen phosphate ((NH4)2HPO4, Nacalai tesque, Inc., Kyoto, Japan) solution. The pH of the phosphate solution was adjusted to 9.8 by 25% ammonia solution (NH4OH, Nacalai tesque, Inc., Kyoto, Japan). The autoclave was sealed tightly and then heated at 100, 160 or 200 1C for 24 h. After heating, the autoclave was cooled rapidly with a fan. The sample was removed from the phosphate solution, rinsed with ultra-pure water and acetone to terminate any further reaction, and then kept at 40 1C to dry for 1 d. Notation of specimens is decided as Cal-temperature. For example, Cal-200 is the hydrothermally treated specimen at 200 1C. The processing products were characterized using a field emission scanning electron microscope (SEM, JSM-5600, JEOL Ltd., Tokyo, Japan) to observe the morphological change of the surfaces and cross-sections of the samples. X-ray diffraction (XRD, RINT PC 2100, Rigaku Co., Tokyo, Japan) was used to confirm HAp formation by the processing. Transmission electron
The external shape and size of the samples were maintained, and the transparent samples became opaque after the hydrothermal treatment. Fig. 1 shows XRD patterns of the surfaces of calcite samples before and after the hydrothermal processing at various temperatures for 24 h. The strong (104) peak of calcite at about 291 was detected before the hydrothermal processing. The peak of starting material disappeared in case of Cal-160 and Cal-200. After the hydrothermal processing, the peaks assigned to HAp were detected although the peak intensity of HAp is lower than that of calcite. The extended XRD pattern of Cal-200 shows clearly formation of HAp crystals with high crystallinity on the specimen. The peak intensity of HAp increased with the processing temperature. Fig. 2 shows SEM photos of the surfaces of the calcite samples before and after the hydrothermal processing at various temperatures for 24 h. While the samples had smooth surface before the hydrothermal processing, needle-shaped HAp crystals on their surfaces were formed randomly by the hydrothermal processing. The width and length of the crystals on each sample increased with increasing the processing temperature. In case of Cal-200, the length and width of the needle-shaped HAp crystals were about 4 and 0.4 μm, respectively. Fig. 3 shows SEM photos of the cross-section of the calcite samples after the hydrothermal processing at various temperatures for 24 h. The thickness of HAp layer formed on the calcites increased with increasing the processing temperature. On Cal-100, HAp crystals were grown randomly while HAp crystals were oriented vertically to the Cal-160 [9] and Cal-200. Fig. 4 shows SEM and TEM photos of the cross-section of Cal-200 after hydrothermal processing for 24 h. The photo (a) shows random HAp growth around the surface of the sample. The photo (b) shows plate-shaped HAp crystals like nacre structure. The Fig. 4(c) shows the cross-section after polishing the (b) part. From SEM photo of the polished surface, it was confirmed that HAp crystals were oriented. TEM photo shows orientation of HAp crystals and electron diffraction pattern shows that each HAp was a single crystal. Fig. 5 shows SEM photo of the oblique-sections and its illustration of Cal-200 after the hydrothermal treatment. The plate-shaped HAp structure was composed of needle-shaped HAp crystals. It was confirmed that HAp crystals were oriented and stacked like showing the illustration. Table 1 shows Ca/P ratio and carbonate content of the HAp prepared on the samples. The Ca/P atomic ration of the HAp crystals were slightly increased from 1.62 to 1.63 with increasing the processing temperature. And the carbonate
1888
I.Y. Kim, C. Ohtsuki / Ceramics International 42 (2016) 1886–1890
Fig. 2. SEM photos of the surfaces of the starting materials before and after hydrothermal processing (HP) at various temperatures for 24 h.
Fig. 3. SEM photos of the cross-section of the starting materials after hydrothermal processing at various temperatures for 24 h.
contents of the HAp crystals were slightly increased with increasing the processing temperature. 4. Discussion HAp formation on calcium carbonate in a phosphate solution was triggered by its dissolution. Calcium ions released from calcium carbonate react with phosphate ions to form calcium phosphate as followed. CaCO3 þ Phosphate solution-Ca10(PO4)6(OH)2 þ H2O þ CO2 HAp has the lowest solubility in calcium phosphates at neutral and basic conditions. So, HAp formation was occurred at the basic condition in this study. This reaction would generate rapidly on the surface of a calcium compound in a phosphate solution because calcium ions are released from its surface [8]. This reaction for HAp formation from calcium salts was used in the other studies [12–17]. To supply calcium ions to form HAp crystals, corals, natural gypsum, calcium carbonate, eggshell and calcium phosphates were used as
starting materials. HAp crystals with various morphologies could be synthesized through a hydrothermal processing. Rodshaped HAp crystals were obtained from α-tricalcium phosphate (Ca3(PO4)2) as a starting material [18]. HAp whiskers were obtained from β-tricalcium phosphate (Ca3(PO4)2) with addition of citric acid [19]. Plate-shaped HAp crystals had also been synthesized from octacalcium phosphate (Ca8(HPO4)2(PO4)4 5H2O) plates [20]. Our previous study also showed that the oriented structure of needle-shaped HAp crystals was prepared from a calcium carbonate single crystal under a hydrothermal processing [9]. The oriented structure of HAp was generated by uniform dissolution of the calcium carbonate single crystal. In this study, to control the dissolution rate of a starting material, the processing temperature was changed. It was reported that the solubility of calcite increased with increasing the processing temperature under a hydrothermal condition [21]. On the contrary, the solubility of HAp decreases with increasing temperature [22]. Resultantly, the reaction rate of calcium ions with phosphate ions for HAp formation might increase with increasing the processing temperature. The
I.Y. Kim, C. Ohtsuki / Ceramics International 42 (2016) 1886–1890
1889
Fig. 4. SEM and TEM photos of the cross-section of Cal-200 after hydrothermal processing for 24 h.
Fig. 5. SEM photos of the oblique-section of the Cal-200 after hydrothermal processing for 24 h.
thickness of HAp layer well agreed with the dissolution rate of starting materials [10]. The thickness of HAp layer on Cal-200 was greater than those on Cal-100 and Cal-160. The difference in the Ca/P ratio and carbonate content can be affected by the processing temperature. In this study, the Ca/P ratio and carbonate contents of the HAp crystals were slightly increased with increasing the processing temperature. The carbonate content affects to Ca/P ratio of HAp crystals. Using a calcium carbonate as a starting materials for HAp preparation under a hydrothermal condition, carbonate is generated by its dissolution. Therefore carbonate amount in a vessel during hydrothermal processing would be increased with increasing the formed HAp amount resulted in increasing the carbonate content in HAp crystals. The results in this study were evidenced clearly the effects of processing temperature on morphology of HAp crystals under the hydrothermal condition. Previously, it was reported that the morphology of HAp crystals depended on dissolution behavior of starting materials [11]. According to the reports, plate-shaped HAp crystals would be prepared easily on starting
Table 1 Ca/P ratio and carbonate contents of the HAp prepared on the samples. Sample
Ca/P atomic ratio
Carbonate content (mass %)
Cal-100 Cal-160 Cal-200
1.62 1.62 1.63
7.8% 7.9% 8.1%
materials with higher dissolution rate than that with lower dissolution rate. However, in this study, plate-shaped HAp crystal was obtained on the sample treated at 200 1C although the dissolution rate of calcite increased with increasing the processing temperature. From observation of the obliquesection for Cal-200 (See Fig. 5), it was confirmed that the stacked structure of plate-shaped HAp crystals like a nacre was composed of needle-shaped HAp crystals. The plate-shaped HAp was fabricated by arrangement of needle-shaped HAp like the illustration (Fig. 5). Previously we found that HAp
1890
I.Y. Kim, C. Ohtsuki / Ceramics International 42 (2016) 1886–1890
crystals are grown up to inside of a calcium compound single crystal [9]. The HAp crystal growth was governed by supplying calcium ions from the surface of starting materials resulted in the oriented structure of HAp crystals. The structure of HAp might be originated by rapid dissolution of calcite single crystal as a starting material. Consequently, the results obtained in this study suggested that dissolution rate of a starting material affects not only morphology of the formed HAp crystals but also their arrangement under a hydrothermal condition. 5. Conclusions The effects of processing temperature on HAp formation were investigated using calcite single crystals under a hydrothermal condition. After hydrothermal processing, needleshaped HAp crystals were formed on all the samples. The size of HAp crystal increased with increasing the processing temperature. From the observation of cross-section of the samples, the thickness of HAp layer increased with increasing the processing temperature. These results were caused by increase of dissolution rate of the starting materials. Moreover, plate-shaped HAp structure was observed on the cross-section of Cal-200. This stacked structure of plate-shaped HAp was composed of needle-like HAp crystals. Dissolution rate of a starting materials affects morphology and structure of the resultantly formed HAp. Acknowledgments This work was supported by Grant-in-Aid for Scientific Research on the Innovative Areas: “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (No. 2206) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). References [1] J.B. Park, Bioceramics: Properties, Characterizations and Applications, Springer, New York, 1998. [2] G. Bernardi, Chromatography of nucleic acids on hydroxyapatite, Nature 20 (1965) 779–783. [3] H. Zhang, K. Zhou, Z. Li, S. Huang, Plate-like hydroxyapatite nanoparticles synthesized by the hydrothermal method, J. Phys. Chem. Solids 70 (2009) 243–248. [4] T. Goto, I.Y. Kim, K. Kikuta, C. Ohtsuki, Hydroxyapatite formation by solvothermal treatment of α-tricalcium phosphate with water–ethanol solution, Ceram. Intern. 38 (2012) 1003–1010.
[5] C.M. Zhang, J. Yang, Z.W. Quan, P.P. Yang, C.X. Li, Z.Y. Hou, J. Lin, Hydroxyapatite nano- and microcrystals with multiform morphologies: controllable synthesis and luminescence properties, Cryst. Growth Des. 9 (2009) 2725–2733. [6] I.S. Neira, Y.V. Kolen’ko, O.I. Lebedev, G. Van Tendeloo, H.S. Gupta, F. Guitián, M. Yoshimura, An effective morphology control of hydroxyapatite crystals via hydrothermal synthesis, Cryst. Growth Des. 9 (2009) 466–474. [7] M. Yoshimura, H. Suda, K. Okamoto, K. Ioku, Hydrothermal synthesis of biocompatible whiskers, J. Mater. Sci. 29 (1994) 3399–3402. [8] K. Ishikawa, S. Matsuya, X. Lin, Z. Lei, T. Yuasa, Y. Miyamoto, Fabrication of low crystalline B-type carbonate apatite block from low crystalline calcite block, J. Ceram. Soc. Japan 118 (2010) 341–344. [9] I.Y. Kim, K. Kikuta, C. Ohtsuki, Formation of oriented hydroxyapatite rods by hydrothermal treatment of calcite single crystal, Korean, J. Mater. Res. 22 (2012) 397–402. [10] I.Y. Kim, K. Koichi, C. Ohtsuki, Hydroxyapatite formation through dissolution-precipitation reaction: effects of solubility of starting materials, Ceram. Intern. 40 (2014) 14385–14390. [11] T. Goto, I.Y. Kim, K. Kikuta, C. Ohtsuki, Comparative study of hydroxyapatite formation from α- and β-tricalcium phosphates under hydrothermal conditions, J. Ceram. Soc. Japan 120 (2012) 131–137. [12] K.P. Sanosh, M.C. Chu, A. Balakrishnan, T.N. Kim, S.J. Cho, Utilization of biowaste eggshells to synthesize nanocrystalline hydroxyapatite powders, Mater. Lett. 63 (2009) 2100–2102. [13] T. Watanabe, G. Kawachi, M. Kamitakahara, K. Kikuta, C. Ohtsuki, Formation of needle-like hydroxyapatite by hydrothermal treatment of CaHPO4 2H2O combined with β-Ca3(PO4)2, J. Ceram. Soc. Japan 117 (2009) 759–764. [14] G.B. Ramírez-Rodríguez, J.M. Delgado-López, J. Gómez-Morales, Evolution of calcium phosphate precipitation in hanging drop vapor diffusion by in situ Raman microspectroscopy, CrystEngComm 15 (2013) 2206–2212. [15] M. Sivakumar, T.S. Sampath Kumar., K.L. Shantha, K. Panduranga Rao, Development of hydroxyapatite derived from Indian coral, Biomaterials 17 (1996) 1709–1714. [16] E. Pujiyanto, P.W. Laksono, J. Triyono, Synthesis and characterization of hydroxyapatite powder from natural gypsum rock, Adv. Mater. Res. 898 (2014) 56–59. [17] S. Jinawath, D. Pongkao, M. Yoshimura, Hydrothermal synthesis of hydroxyapatite from natural source, J. Mater. Sci.: Mater. Med. 13 (2002) 491–494. [18] K. Ioku, G. Kawachi, S. Sasaki, H. Fujimori, S. Goto, Hydrothermal preparation of tailored hydroxyapatite, J. Mater. Sci. 41 (2006) 1341–1344. [19] M. Yoshimura, H. Suda, K. Okamaoto, K. Ioku, Hydrothermal synthesis of biocompatible whiskers, J. Mater. Sci. 29 (1994) 3399–3402. [20] M. Kamitakahara, N. Ito, S. Murakami, N. Watanabe, K. Ioku, Hydrothermal synthesis of hydroxyapatite from octacalcium phosphate: effect of hydrothermal temperature, J. Ceram. Soc. Japan 117 (2009) 385–387. [21] E.R. Segnit, H.D. Holland, C.J. Biscardi, The solubility of calcite in aqueous solutions-1. The solubility of calcite in water between 751 and 2001 at CO2 pressures up to 60 atm, Geochim. Cosmochim. Acta 26 (1962) 1301–1331. [22] J.B. Liu, X.Y. Ye, H. Wang, M.K. Zhu, B. Wang, H. Yan, The influence of pH and temperature on the morphology of hydroxyapatite synthesized by hydrothermal method, Ceram. Intern. 29 (2003) 629–633.