Hydrothermal synthesis and characterization of ...

2017 14th International Conference on Electrical Engineering, Computing Science and Automatic Control (CCE), Mexico City, Mexico. September 20-22, 2017

Hydrothermal synthesis and characterization of hydroxyapatite microstructures S López Ortiz1, J Hernández- Avila1, M.P. Gutierrez2, Heberto Gómez-Pozos3, T.V.K. Karthik1 and V. Rodríguez Lugo1* 1

Academic Area Earth Science and Materials, Institute of Basic and Engineering, University of the State of Hidalgo, Pachuca-Tulancingo Km. 4.5, 42184 Highway Sciences. 2 Escuela superior de APAN, UAEH, Carretera APAN-Calpulalpan Km 8, APAN, Hidalgo, C.P. 43920 3 Academic Area of electrical engineering and computation, Institute of Basic and Engineering, University of the State of Hidalgo, Pachuca-Tulancingo Km. 4.5, 42184 Highway Sciences. * Corresponding author e-mail: [email protected], [email protected] Abstract - In the present work, hydroxyapatite (HAp, Ca10(PO4)6(OH)2) synthesis was carried out using hydrothermal method with calcium hydroxide (Ca(OH)2) and ammonium phosphate((NH4)2HPO4) as precursors. Two different HAp powders were obtaining by varying pH by adding nitric acid (pH =9) and ammonium hydroxide (pH = 10). All the synthesis was carried out in an autoclave at 200 ºC for 24 h. The obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques. The results obtained by XRD confirms that the diffraction peaks correspond to pure hydroxyapatite phase. The results of SEM show the formation of hydroxyapatite agglomerates, with an approximate size ranging from 5 to 40 μm, with a rough surface. EDS analysis show a Ca / P ratio ~1.95. powders obtained in this work, with the above characteristics can be utilized in gas sensors and thermoluminescence applications. Keywords: Hydroxyapatite, pH, hydrothermal, nitric acid, ammonium phosphate. I.

INTRODUCTION

Hydroxyapatite, most stable phase of calcium phosphate, is composed mainly of calcium and phosphate in a molar ratio of 1.67 and is a crystalline compound composed of three molecules of calcium phosphate and a molecule of calcium hydroxide, belongs to the apatite family with hexagonal structure [1]. Bones in the human body are constituted by 69% of hydroxyapatite, 9% of water content and 22% of other organic matter, which makes its use in synthetic form as a bone filler. It is also used as a coating material of the prosthesis or implants, due to its excellent biocompatibility and biological activity in addition to its great ion exchange power, which can be obtained by different methods and under different synthesis conditions [2].

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Many routes have been developed to synthesize hydroxyapatite by different methods such as mechano-chemical synthesis [3], conventional chemical precipitation [4], hydrolysis [5], emulsion [6], and hydrothermal method [7]. Hydrothermal synthesis is relatively simple, cost effective and adequate process to obtain different controllable morphologies because of usage of temperature and pressure in autoclave chamber [8]. One of the main characteristics of the hydrothermal method is that, the utilization of synthesis temperature (374.1 °C) and pressure (218.3 atm) makes the mother solution in the hydrothermal synthesis to obtain more homogenous and uniform powders. Different synthesis conditions such as reaction time, reaction temperature, concentrations and pH values [11] can be varied in a typical hydrothermal synthesis. Solution pH parameter is an important parameter to study because it directly affects the final morphology and structure of HAp. Various research groups have conducted studies to determine the behavior of the pH such as, Jingbing Liu et al. used COOH to reduce the pH from 9 to 6 and to increase the pH utilizing from 9 to 12 [12]. Mehdi Sadat-Shojai et al. summarized the different morphologies like particles, rods, laminated sheets and rod like structures at different pH values ranging from pH 4-14. the different structure and morphology depending the pH solutions [13]. Ravi Krishna Brindavanam et al. utilized ammonium hydroxide to adjust the pH of the solution to 9 to obtain HAp nanostructures [14]. From the above works it is clearly evident that the pH effects the morphological and structural properties of the resulted powders and an increased pH values ~10-12 results in spherical nanoparticles and lower pH values ~4-6 resulted in laminar and tubular nanostructures. Various precursors of calcium and phosphates such as calcium oxide, tricalcium beta phosphate, calcium nitrate [9] calcium nitrate tetrahydrate [10]and dibasic calcium phosphate are

utilized in hydrothermal synthesis depending on the final use of the HAp powders. Also the precursor selection depends upon the physical and chemical properties of final obtained HAp powders such as: geometry, size, density, pore size, mechanical strength, purity and chemical phase of the material. In this work hydroxyapatite was synthesized by polyol method and its structural and morphological properties were observed after adding nitric acid and ammonium hydroxide separately to change its pH value to 9 and 10, respectively. This work is a primary step of a series of experiments to obtain the optimized hydroxyapatite and to have more control of its physical characteristics. In future, these powders will be manually pressed for obtaining pellets and verified for its thermoluminescence and gas sensors properties.

4𝐻2 𝑂 + 10𝐻2 𝐶𝑎𝑂 + 6(𝑁𝐻4 )2 𝐻𝑃𝑂4 → 𝐶𝑎10 (𝑃𝑂4 )6(𝑂𝐻)2 + 6(𝑁𝐻4 )𝑂2 + 6𝐻2

B.

Characterization of hydroxyapatite powders

In resume, three HAp samples with pH 9.6, 10 and 9 were obtained and later the prepared HAp samples were characterized by INEL Equinox 2000 X-ray diffractometer (XRD) with a radiation of CuKα at 1.78 Å and 30 KV, for identifying the hydroxyapatite structural phases. Also, the morphology and particle sizes of the sample powders were observed by JEOL JSM-IT300 Scanning Electron Microscope (SEM) and an Oxford X-Max coupled EDS with a resolution of 5.9 KeV is used to obtain the composition of the samples. The samples were fixed with graphite tape to increase their conductivity. III.

II. A.

(1)

RESULTS AND DISCUSSIONS

EXPERIMENTAL PROCEDURE

Hydroxyapatite synthesis by the hydrothermal method

Aqueous 0.15 M calcium hydroxide and 0.09 M of ammonium phosphate were prepared separately and stirred gently for 30 min at 300 rpm (Fig. 1a). Ammonium phosphate solution was added dropwise to the previously prepared aqueous calcium hydroxide solution and left stirring at 150 rpm for 60 min. As prepared solution pH was found to be 9.6 (Fig. 1b). Later, in a part of prepared solution, ammonium hydroxide is added to raise the solution pH to 10 (Fig. 1c) and in the remaining part 2 M nitric acid was added to lower its pH to 9 (Fig. 1d). Finally, both the solutions were evacuated to an autoclave which was maintained at 200 °C for 24 h. Subsequently, the obtained solutions were removed from autoclave after its temperature reduces to 25 0C (Fig. 1e). Finally, the resultant precipitates were washed three times with deionized water (for each wash, precipitates are filtered under vacuum, Fig. 1f) and dried in an oven at 50 °C for 12 h (Fig.1g). Fig. 1h below shows the finally obtained HAp powders. In this work three HAp samples with pH 9.6, 10 and 9 were obtained.

A. SEM analysis The SEM micrographs of the three synthesized hydroxyapatite samples by the hydrothermal method are shown in Fig. 2.

Fig. 1. Step by step procedure of hydroxyapatite hydrothermal synthesis. The criteria for choosing the initial molarities of calcium hydroxide and ammonium phosphate is according the following equation (1) to obtain the pure and stoichiometric form of HAp.

Fig. 2. SEM micrographs of HAp samples with pH (a) 9.6, (c) 9 and (e) 10. (b), (d) and (f) are the higher magnifications of (a), (c) and (e) respectively.

Figs. 2a, 2c and 2e corresponds to samples with pH values 9.6, 9 and 10 respectively. Figs. 2b, 2d and 2f corresponds to the higher magnifications of Figs. 2a, 2c and 2e. From Figs. 2a, 2c and 2e we can clearly observe that the addition of nitric acid (Fig. 2c) and ammonium hydroxide (Fig. 2e) increased the agglomeration of the particles relatively which can be corroborated to the increased particle surface area due to the pH controller which subsequently leads to more agglomeration [15]. From Fig. 2b, a regular and porous surface is observed with agglomerates ~5 to 20 μm consisting of particles ranging from 5 to 40 nm. Unlikely, in Figs. 2d and 2f, we observe larger and very porous agglomerates ~ 5 to 40 μm. Table 1, the calculated values of the Ca / P ratio are reported by considering the EDS microanalysis. The obtained Ca/P ratio vary between 1.86 and 2.08 which is higher than a stochiometric Ca/P ratio of HAp (1.67). This result confirm that the obtained HAp powders are more carbonated. As the maximum temperature utilized in the present synthesis is 200 °C, may be further calcination treatment would reduce the amount of calcium in the obtained powders and modify the morphology of the powders which subsequently adjusts the Ca/P ration to 1.67 which is the ideal ratio for HAp structures. Table 1: Ca / P molar ratio of the synthesized samples Element/Sample

pH 9.6

pH 9

pH 10

O

49.4

50.16

47.38

Ca

27.8

27.92

30.58

P

14.9

13.42

15.92

Molar ratio Ca/P

1.86

2.08

1.92

B. XRD analysis Fig. 3 shows the X-ray diffractogram patterns of three obtained HAp samples. The results confirm the formation of the HAp phase and all the planes corresponds to hexagonal structure of HAp according to the JCPDS card 07-0028. Powders obtained without addition of ammonium hydroxide and nitric acid possess pH values 9.6 and show relatively high crystallinity compared to other powders. An additional plane (112) with very high intensity is observed for powders with pH 9.6. Therefore, addition of other acid or base compounds reduced the crystallinity of the HAp powders. A typical hydroxyapatite diffraction patterns with preferential orientation (211) and other planes are (002), (112) and (300), which appears in the range from 20º to 60º. Samples with pH 9 are more crystalline with defined peak intensities compared to the samples with pH 10. No additional phases were

found in the diffraction patterns apart from the peaks corresponding HAp.

Fig. 3. XRD patterns of all the synthesized HAp samples. XRD results are in good agreement with the SEM and EDS analysis, showing that further growth and crystallization is necessary to obtain the stoichiometric HAP powders. Further calcination of the powders around 800 °C will improve the crystallization and stoichiometry of the obtained HAp microstructures. In future, these powders will be calcinated and manually pressed to obtain HAp pellets and used as gas sensors. IV.

CONCLUSIONS

In this work, HAp microstructures are successfully synthesized using hydrothermal synthesis. Effect of pH on the structural and morphological properties were investigated in detail. SEM analysis evidences the formation porous agglomerates consisting of micro HAp structures. EDS analysis confirm that all the powders obtained are more carbonated with Ca/P ratio ranging from 1.86-2.08. XRD analysis reveals that the increase in the pH of the solution increased the crystallinity of the sample. Also, addition of nitric acid and ammonium hydroxide reduced the crystallinity of the samples. Relatively high crystalline hydroxyapatite with smooth and homogeneous surface consisting smaller particle size was for powders with a pH 9.6. Further calcination is required for primarily removing the carbonated content and for obtaining crystalline and stochiometric HAp nanostructures.

ACKNOWLEDGMENTS The authors are thankful to Dr. Francisco Raul Barrientos Hernández for their assistance in XRD analysis. Also, authors are thankful to Escuela superior de APAN, UAEH for SEM and EDS analysis. Additionally, authors are very thankful to Advanced Materials Group in Universidad Autónoma del Estado de Hidalgo(UAEH) Finally, authors are also grateful to Consejo Nacional de Ciencia y Tecnología (CONACYT) for its financial support to carry out this work. REFERENCES [1] N. E. V. H. M. G. V. José Luis Gómez, «Visualizacion cristalografica de la hidroxiapatita,» Facultad Ciencias Fisico Matematicas - UANL, Nuevo Leon, 2004. [2] M. v. G.-. Garduño y j. R. Garduño, «La Hidroxiapatita, su importancia en los tejidos mineralizados y su aplicacion biomedica,» Revista Especializada en Ciencias Quimico -Biologicas, 2006. [3] M.-T. K. E. D.-K. A. J. Mehdi Sadat-Shojai, «Synthesis methods for nanosized hydroxyapatite with diverse structures,» ELSERVIER, nº 9, pp. 7591-7621, 2012. [4] H. E. G. C. l. T. y. J. O. A.B. Martínez–Valencia, «Caracterización estructural y morfológica de hidroxiapatita nanoestructurada: estudio comparativo de diferentes métodos de síntesis,» Sociedad Mexicana de Ciencia y Tecnología de Superficies y Materiales, vol. 21, nº 4, pp. 18-21, 2008. [5] Y.-F. C. M.-C. W. M.-H. Wei-Jen Shih, «Crystal growth and morphology of the nano-sized hydroxyapatite powders synthesized from CaHPO4·2H2O and CaCO3 by hydrolysis method,» ELSERVIER, vol. 270, pp. 211-218, 2004. [6] W. T. V. B. Somnuk Jarudilokkul, «Synthesis of hydroxyapatite nanoparticles using an emulsion liquid membrane system,» ELSERVIER, vol. 294, pp. 149-153, 2007.

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