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J. Mater. Sci. Technol., 2014, 30(4), 307e310
Synthesis and Characterization of Nano-hydroxyapatite Powder Using Wet
Chemical Precipitation Reaction
Syed Sibte Asghar Abidi1), Qasim Murtaza2)*
1) Centre of Excellence in Materials Science (Nanomaterials), Aligarh Muslim University, Aligarh, India
2) Mechanical and Production Engineering Department, Delhi Technology University (Formerly Delhi College of Engineering), Delhi
42, India
[Manuscript received January 19, 2012, in revised form December 20, 2012, Available online 17 October 2013]
Hydroxyapatite (HA) nano-powder was synthesized via wet chemical technique in a used precipitation reaction,
in which Ca(OH)2 and H3PO4 were used as precursors. Deionised water was used as a diluting media for the
reaction and ammonia was used to adjust the pH. The synthetic HA nano-powder has some medical
applications such as a coating material in orthopaedic implants and in dental. HA powder has been studied at
different temperatures from 100 to 800 C to achieve the stoichiometric Ca/P ratio 1.667. The optimum
temperature was found to be 600 C. Above this temperature, the HA powder decomposed to CaO. The
crystallite size of HA powder was found to be in the range of 8.47e24.47 nm. The crystallographic
properties were evaluated by X-ray diffraction, Fourier transform infrared spectroscopy, energy dispersive Xray spectroscopy and scanning electron microscopy. The results show that, high purity of nano-hydroxyapatite
powders could be obtained at low temperatures, and the crystallinity, crystallite size and Ca/P ratio of the
resulting nanoparticles were found to be dependent on the calcination temperature. When Ca/P ratio exceeded
1.75, formation of CaO phase was observed.
KEY WORDS: Hydroxyapatite; Nano-powder; Synthesis; Characterization
1. Introduction
Hydroxyapatite (HA) [Ca10(Po4)6(OH)2] is a naturally existing mineral in the inorganic component of human bone and tooth
enamel. The crystal size of HA in natural human bone is in nano
range. The constituent elements of HA are primarily calcium and
phosphorus, with a stoichiometric Ca/P ratio of 1.667[1]. HA is
composed primarily of calcium and phosphorous with hydroxide
ions that are eliminated at elevated temperatures. HA and other
related calcium phosphate minerals have been utilized extensively as implant materials for many years due to its excellent
biocompatibility and bone bonding ability and also due to its
structural and compositional similarity to that of the mineral
phase of hard tissue in human bones[1e3]. HA coatings have
good potential as they can exploit the biocompatible and bone
bonding properties of the ceramic, while utilizing the mechanical
properties of substrates such as Tie6Ale4V and other
*
Corresponding author. Ph.D; E-mail addresses: [email protected].
in, [email protected] (Q. Murtaza).
1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of
Journal of Materials Science & Technology. Published by Elsevier
Limited. All rights reserved.
http://dx.doi.org/10.1016/j.jmst.2013.10.011
biocompatible alloys. While the metallic materials have the
required mechanical properties, they benefit from the HA which
provides an osteoconductive surface for new bone growth,
anchoring the implant and transferring load to the skeleton,
helping to combat bone atrophy[1,4]. Their Ca/P ratio of
1.52 2.0 makes them an excellent choice for most dental and
orthopaedic applications in the form of bioceramic coatings. The
quality of a coating is closely dependent on the overall attributes
and characteristics of the synthesized powders. Such attributes
include phase composition, purity, crystallinity, particle size,
particle-size distribution, specific surface area, density and particle morphology. These important factors determine the resulting success of the HA coating deposited onto orthopaedic
implants through plasma thermal spraying due to poor mechanical properties of HA. The recent trend in bioceramic
research is focused on improving their mechanical and biological
properties using nanotechnology[5]. Common methods used to
produce synthetic nano-crystalline HA include precipitation[6],
hydrothermal[7], hydrolysis[8], mechanochemical[9] and solgel[10] method.
In the present work, nano-sized HA powder was synthesized
via wet chemical precipitation method using calcium hydroxide,
orthophosphoric acid and ammonia as precursors, and its crystallographic properties were characterized.
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S.S.A. Abidi and Q. Murtaza: J. Mater. Sci. Technol., 2014, 30(4), 307e310
2. Experimental
In the present work, calcium oxide (CaO) (S D Fine Chem
Limited, Mumbai, India), orthophosphoric acid (H3PO4) (Fisher
Scientific, Mumbai, INDIA), and ammonium hydroxide
(NH4OH) (Fisher Scientific, Mumbai, India) were used as
starting materials. Firstly, an analytical weighing scale was used
to accurately weigh CaO powder. 1.42 mol (79.55 g) CaO
powder was added to 500 ml of deionised water in a 1000 ml
beaker and vigorously stirred at 1000 r/min at 20 C for 24 h to
react and form a suspension of Ca(OH)2 in an excess of deionised water. The beaker was covered in order to avoid possible
contamination via contact with atmospheric conditions. The
temperature of the reaction (20 C) was maintained by a
thermostat-controlled water bath.
An analytical weighing scale was used to accurately weigh the
required quantity of orthophosphoric acid. 97.32 g of 85% H3PO4
was added to Ca(OH)2 solution at a rate of 1.5 ml/min. During the
course of the acid addition, the pH of the solution was monitored
via a handheld pH meter with an accuracy of 0.2. The reactants
were stirred for further 24 h to aid the maturation stage, under
continuous stirring conditions at 1000 r/min, held at the respective
reaction temperature of 20 C. 0.28 mol (9.94 g) NH4OH, was
added to the HA slurry after 24 h ripening period to stabilise the
pH of the super saturation solution to above 9. To maintain a Ca/P
ratio of 1.67 the solution was kept above 9[1,5]. A pH of 10 has
been considered to produce the correct stoichiometric condition by
which only pure single phase HA is formed[11].
Assay samples were taken for analysing the composition of
mixture in the barrel. A small crucible was filled with a sample
of the mixture in the mixing barrel and dried in a drying oven for
1 h at 100 C. The dried samples were then placed in a furnace
and sintered at 1200 C for 1 h. When the assays were cooled,
they were removed from the furnace and ground using a motor
and pestle. Scanning electron microscopy (SEM, HITACHI
model S-3700N at DTU) was used to observe the morphology
and the particle size of calcined HA powder. Elemental phase
composition of the HA powder was analysed using energy
dispersive X-ray spectroscopy (EDX) (HITACHI model S3700N at DTU). The X-ray diffraction (XRD) pattern of the final
HA nanoparticles was obtained with CuKa radiation
(l ¼ 0.15406 nm) on a diffractometer (RIGAKU MINIFLEX at
AMU). The XRD patterns were recorded in a 2q range of 20 e
60 with a step size of 0.02 and step duration of 0.5 s. The mean
crystallite size (D) of the particles was calculated from XRD line
broadening measurement using the Scherrer equation[12]:
D ¼
0:89l
bcos q
where l is the wavelength of CuKa, b the full width at half
maximum of the HA line and q the diffraction angle.
3. Results and Discussion
The following reactions were involved in the formation of HA
during the precipitation reaction:
CaO þ H2 O/CaðOHÞ2
10CaðOHÞ2 þ 6H3 PO4 /Ca10 ðPO4 Þ6 ðOHÞ2 þ 18H2 O
(1)
(2)
Fig. 1 XRD patterns of HA powders calcined at different temperatures.
Fig. 1 shows the XRD pattern of HA from 100 to 800 C. The
crystallite size was calculated by Scherrer equation with the most
intense plane that is 211 of eight calcined HA samples for each
temperature. The results are given in Table 1. It can be observed
that with increasing the calcination temperature the crystallite
size also increases. Similar phenomenon was observed previously[5,13]. Hydroxyapatite has two intrinsic properties that
enable it to form an amorphous phase. First, it is made up of
phosphates which are glass formers, and second, the composition
of hydroxyapatite is very close to the eutectic composition, thus
inducing a liquidus effect[14]. It has also been reported from
XRD spectra that HA calcined at higher temperatures exhibited
good crystallinity as the peaks become narrow and sharp moving
from 100 to 800 C. Also, it shows only a little or no activity
towards bioresorption which is important for the formation of
chemical bonding with surrounding hard tissues[5,15,16]. As it is
evident from the XRD spectra, the amorphous HA powders were
obtained at lower temperatures (100e200 C). Similar phenomena have been observed by others[5,13]. This indicates that
HA powder prepared in this study is a mixture of crystalline as
well as amorphous phases with the major content being the
amorphous phase of HA. It is expected to be metabolically more
active than the fully developed crystalline hydroxyapatite
structure which otherwise is insoluble in the physiological
environment[5,17].
The spectra in Fig. 1 are typically in agreement with those
published in literature; all XRD spectra obtained have characteristic peaks consistent with the International Centre for
Diffraction [JCPDS 2001] files for calcium phosphate. The
predominant HA phase was confirmed with JCPDS files number
09-432.
Table 1 Crystallite size (D) at different calcination temperatures
Sample No.
Temp. ( C)
D (nm)
1
2
3
4
5
6
7
8
100
200
300
400
500
600
700
800
8.4
12.1
12.4
12.6
14.5
14.5
14.6
24.4
S.S.A. Abidi and Q. Murtaza: J. Mater. Sci. Technol., 2014, 30(4), 307e310
Fig. 2 Plot of variation of Ca/P with calcination temperature.
This suggests that no foreign elements, such as sodium
(Na2þ), ammonium (NH4þ), potassium (Kþ), chloride (Cl) and
nitrate (NO3) ions, were involved in the synthesis reaction, as
there is strong evidence to suggest that these ions are easily
incorporated into the crystal lattice leading to the formation of
known stoichiometric HA. These elements are usually introduced into the precipitating systems with the reactants. The
absence of these elements can be attributed to the nature of the
raw materials used as precursors. In fact, it has been shown that
chloride ions enter into the crystal lattice substituting hydroxyl
groups while potassium ions are found to substitute calcium ions
into the HA crystal lattice forming locally non-stoichiometric
(i.e. impure) HA islands in the bulk crystal. Sodium ions also
show the evidence of substitutions. In order to avoid contamination of the products, use of calcium nitrate and phosphoric acid
instead of calcium chloride salts and potassium dihydrogen
phosphate, respectively is advantageous because the presence of
potassium ions is avoided and the nitrate ions are too large to
309
substitute hydroxyl groups in the crystal lattice of HA. These
substitutions were avoided when Ca(OH)2 and H3PO4 reactants
were used to prepare our samples. For all samples of the HA
studied no CaO was observed. This indicates that either small or
no carbonation of HA has occurred during the synthesis of HA
although no foreign elements were found.
The Ca/P stoichiometry of calcined HA at different temperatures (100e800 C) was analysed by EDX. Fig. 2 shows that HA
powder with a Ca/P ratio near 1.67 that is at 600 C and below,
indicates no CaO content and that with a Ca/P ratio near 1.75 and
above, and showed the formation of minor amounts of CaO for
calcined samples at 800 C. Other researchers have reported
similar formation of CaO in sol-gel processing of HA[5,18,19].
SEM observation was performed at Delhi Technical University (DTU). The HA sample is coated with gold and placed in a
HITACHI MODEL-S-3700N. The images were recorded at
different temperatures ranging from 100 to 800 C (Fig. 3). It
is known that spherical powders, in general, have better rheological properties than irregular powders and, thus, produce
better coatings for hip implants[1,11]. In order to produce dense,
high-quality HA powder for purpose like hip implant coating,
it is very important to predict or control granule spherical
morphology[20]. It has also been demonstrated that an increase in
synthesis temperature leads to an increase in HA precipitates
size.
Fourier transform infrared spectroscopy (FT-IR) was performed at Centre of Excellence in Materials Science (Nanomaterials) A.M.U. Aligarh. FT-IR machine (PARKIN ELMER)
with resolution of 4000e400 cm1 and reference material of
KBR were used. FT-IR patterns presented in Fig. 4 confirm the
formation of HA calcinated at different temperatures of 100e
700 C. The spectra possessed an (OH)1 group in the region of
3577 cm1 and (PO4)3 group comes out from the region of
1075 cm1. From this analysis the formation of HA is confirmed.
The peaks are quite sharp at intermediate temperatures (500e
Fig. 3 SEM images of HA powder at different temperatures.
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S.S.A. Abidi and Q. Murtaza: J. Mater. Sci. Technol., 2014, 30(4), 307e310
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Fig. 4 FT-IR spectrum of HA calcinated at different temperatures.
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4. Conclusion
Wet chemical precipitation method is used in the present work
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reported method, calcium hydroxide and orthophosphoric acid
are used as precursors. This process shows that high purity of
nano-hydroxyapatite powders could be obtained at low temperatures. The crystallinity, crystallite size and Ca/P ratio of the
resulting nanoparticles were found to be dependent on the
calcination temperature. When Ca/P ratio exceeded 1.75, the
formation of CaO phase was observed.
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