Materials Sciences and Applicatio ns, 2011, 2, 1194-1198
doi:10.4236/msa.2011.29161 Published Online September 2011 (http://www.SciRP.org/journal/msa)
Copyright © 2011 SciRes. MSA
Synthesis and Evaluation of Calcium-Deficient
Hydroxyapatite with SiO2
Atsushi Nakahira1,2, Kentaro Nakata1, Chiya Numako3, Hidenobu Murata4, Katsuyuki Matsunaga4
1Osaka Prefecture University, Sakai, Japan; 2Osaka Center, IMR, Tohoku University, Sendai, Japan; 3The University of Tokushima,
Tokushima, Japan; 4Nagoya University, Chikusa-ku, Japan.
Email: nakahira@mtr.osakafu-u.ac.jp
Received May 23rd, 2011; revised June 2nd, 2011; accepted June 9th, 2011.
ABSTRACT
Effect of SiO2 addition on the microstructures of calcium-deficient hydroxyapatite prepared by the heat-treatments was
examined. Obtained hydroxyapatites substituted with Si ion were characterized by XRD, FT-IR and XAFS measure-
ments. XANES results of Si K-edge and P K-edge in these modified hydroxyapatites indicated the shift of peaks of P
K-edge with the SiO2 contents, although no change of Si local structures. In this study, the effect of Si ion on micro- and
local structure of hydroxyapatite with SiO2 addition was mainly clarified.
Keywords: Hydroxyapatite, SiO2, XAFS, Local Structure
1. Introduction
It is well-known that hydroxyapatite (HAP) is a major
inorganic component of bone and teeth. Since HAP pos-
sesses high bioactivity, osteoconductivity and bio-com-
patibility, it is extremely useful as a biomaterial for arti-
ficial implant parts, bone fillers, and bone-cements etc.
[1-6].
On the contrary, HAP has also both the high ability of
ion exchange against cations and anions and adsorbent
for various amino acids and proteins among other inor-
ganic materials [5-10]. In bone, HAP is usually calcium-
deficient below Ca/P of 1.67, not stoichiometry
“(Ca10(PO4)6(OH)2) with Ca/P of 1.67”, for the achieve-
ment of doping metal ions and control of the solubility.
As a consequence, nonstoichiomeric HAP with Ca/P of
less than 1.67 possesses the high ion-exchange ability of
various cations. Therefore, calcium-deficient HAP is
expected to be applicable as an ion-exchange media for
solving environmental problems, such as the purification
for water and soil polluted with heavy metals as well as
biomedical applications [10,11]. Recently, the syntheses
of HAP doped with functional elements (Zn and Fe) are
also attempted for developing high performance bioce-
ramics for biomedical application and the clarification of
effects of functional elements on the properties and mi-
crostructures is carried out in order to develop the high
performance bioceramics.
The syntheses of HAP substituted with other metals
have been eagerly attempted by various synthetic pro-
cesses, such as hydrothermal treatments, ion-exchange
treatments, and normal heat-treatments at high tempera-
ture in order to enhance the ion exchange ability and
bioactivity of HAP [9-15]. For example, Fe substituted
HAP i.e. abbreviated by “HAP modified with Fe ions”,
synthesized by hydrothermal treatment indicated the
good exchange ability against As and Pb etc., compared
to monolithic HAP. However, in fact, monolithic HAP
doped with metal ions is significantly difficult to synthe-
size without any other phase.
Since Bonefield reported that HAP containing with
SiO2 indicated high bioactivity and osteoconductivity
[14], the synthesis of HAP with SiO2 and tricalcium
phosphate (TCP) with SiO2 have been performed by
many researchers. However, HAP with SiO2 addition
often results in the formation of reaction phase between
HAP and SiO2, such as calcium silicate, and phosphate
glass etc, and unreacted SiO2 residue. Thus, the SiO2
doped HAP without reaction phases and SiO2 residue is
difficult to synthesize by the conventional methods.
However, the successful synthesis of SiO2 doped HAP
will lead to understanding effect of SiO2 element on the
high bioactivity and osteoconductivity for SiO2 doped
HAP.
In this study, the main purpose is to synthesize SiO2
doped HAP. As a strategy, the incorporation of Si ions
Synthesis and Evaluation of Calcium-Deficient Hydroxyapatite with SiO1195
2
into HAP was especially carried out by using whisker-
like calcium-deficient HAP as a starting material, not
stoichiometric HAP. Whisker-like calcium-deficient HAP
powders were synthesized by the soft chemical process-
ing for the hydrolysis of α-TCP. In addition, calcium-
deficient HAP containing SiO2 was normally heat-treated
in air atmosphere and the effect of SiO2 addition on mi-
cro- and local structure of HAP was examined. Further-
more, Si and P local structures of these substituted HAP
were investigated by the XANES.
2. Experimental Procedures
2.1. Synthesis
HAP powder was synthesized by the soft chemical pro-
cessing for the hydrolysis of α-TCP [15]. 10 g of α-TCP
powder was stirred in 200 ml of 1-octanol at 70˚C for 48
h. The solution during reaction was kept at pH11 with
0.1 M NH4OH. The precipitation was filtered with the
membrane and washed with ethanol and ion-exchange
water sufficiently and then dried at 50˚C for 24 h. Com-
mercial SiO2 powder from Tokuyama Chemical (Toku-
shiru) was used as a source of Si ion. SiO2 (3, 10, and 30
wt%) was added into HAP powder and mixed in ethanol
with ball milling for 24 h. After mixing, the mixture was
filtered with membrane and subsequently dried at 323 K.
The dried powder was crashed with alumina mortal.
Mixed HAP/SiO2 powder was compacted with stainless
mold. Dimension of pellets was 2 mm in thickness and
20 mm in diameter. Pellets were heated at 600˚C to
1200˚C in air atmosphere. Heating rate was 5˚C/min.
After holding at the high temperature for 2 h, samples
were cooled in furnace. Heat-treated samples were poli-
shed with diamond paste.
2.2. Evaluations
The component of samples was identified with XRD
(Rint-2000: Rigaku, Tokyo, Japan). The microstructure
of samples was observed by SEM (S-450: Hitachi, Tokyo,
Japan) and TEM (FX-2010: JOEL, Tokyo, Japan). The
samples were evaluated with FT-IR equipment (Shimazu,
Tokyo, Japan). Si-K and P-K edge were measured at
UVSOR in Okazaki.
3. Results and Discussion
Figure 1 shows XRD and SEM results of HAP samples
synthesized by the soft chemical processing, i.e. the hy-
drolysis of α-TCP in 1-octanol at 70˚C for 48 h. XRD
results indicated that products were composed of mono-
lithic HAP and no other phase like calcium phosphate.
From SEM observation, HAP particles synthesized by
the hydrolysis of α-TCP were observed to be whisker-
like. From ICP measurements, Ca/P ratio of calcium-
3μm
3 μm
(a)
20 30 40 50
R.T.
900
1100
Intensity
2θ/°
No marked peaks: HAP
▼:β
-TCP
(b)
Figure 1. (a) XRD and (b) SEM image of whisker Ca-defi-
cient HAP synthesized by the hydrolysis of α-TCP in 1-
octanol at 70˚C for 48 h.
deficient HAP added with SiO2 (0 wt%, 3 wt%, 30 wt%)
were evaluated for samples synthesized by the soft che-
mical processing. ICP results indicated that this whisker-
like HAp had 1.580 of Ca/P ratio, suggesting that ob-
tained whisker-like HAP was calcium-deficient. On the
other hand, Ca/P ratio of calcium-deficient HAP added
with 3 wt% SiO2 and 30 wt% were 1.590 and 1.603, res-
pectively. It was obvious that the addition of SiO2 into
calcium-deficient HAP samples resulted in the decrease
of Ca/P ratio.
XRD results of calcium-deficient HAP samples con-
taining 3 to 30 wt% SiO2 heat-treated at 600˚C to 1200˚C
are shown in Figure 2. HAP samples without SiO2 heat-
treated at 600˚C were composed of HAP phase without
another calcium phosphate phase, although HAP samples
heat-treated at 1000˚C have the mixture of HAP and
Copyright © 2011 SciRes. MSA
Synthesis and Evaluation of Calcium-Deficient Hydroxyapatite with SiO
1196 2
30 wt %
3 wt %
0 wt %
10wt%
10 20 30 40 50 60
2θ/°
Intensity/a. u.
600
1000
1200
HAp
α-TCP
β-TCP
10 20 3040 5060
2θ/°
600
1000
1200
1020 30 4050 60
600
1000
1200
10 2030 40 50 60
2θ/°
600
1000
1200
HAp
α-TCP
β-TCP
HAp
α-TCP
β-TCP
HAp
α-TCP
β-TCP
Intensity/a. u.
Intensity/a.u.
Intensity/a.u.
2θ/°
30 wt %
3 wt %
0 wt %
10wt%
10 20 30 40 50 60
2θ/°
Intensity/a. u.
600
1000
1200
HAp
α-TCP
β-TCP
HAp
α-TCP
β-TCP
10 20 3040 5060
2θ/°
600
1000
1200
1020 30 4050 60
600
1000
1200
10 2030 40 50 60
2θ/°
600
1000
1200
HAp
α-TCP
β-TCP
HAp
α-TCP
β-TCP
HAp
α-TCP
β-TCP
HAp
α-TCP
β-TCP
HAp
α-TCP
β-TCP
HAp
α-TCP
β-TCP
Intensity/a. u.
Intensity/a.u.
Intensity/a.u.
2θ/°
Figure 2. XRD results of Ca-def HAp added with SiO2 (0
wt%, 3 wt%, 10 wt%, and 30 wt%) heat-treated at various
temperatures.
α-TCP. The component of samples heat-treated at 1200˚C
was the mixture of HAP and α-TCP. In case of HAP/
3%SiO2, samples heat-treated at 600˚C contained HAP
monophase. HAP/3%SiO2 samples heat-treated at 1200˚C
were composed of mainly α-TCP and in part HAP, al-
though the component of samples heat-treated at 1000˚C
were HAP and α-TCP. HAP/10%SiO2 and HAP/30%
SiO2 samples indicated the same dependence of heat-
treatment temperatures for XRD results. For HAP/10%
SiO2 and HAP/30%SiO2, samples heat-treated at 600˚C
were composed of monolithic HAP and ones at 1200˚C
were of α-TCP, respectively. On the other hand, both
HAP/10%SiO2 and HAP/30%SiO2 samples heat-treated
at 1000˚C were composed of HAP and α-TCP. Thus, it
was found that the addition of SiO2 into calcium-deficient
HAP enhanced the formation of α-TCP above 1000˚C.
Figure 3 shows the variation of FT-IR spectra of cal-
cium-deficient HAP/SiO2 samples heat-treated at 600˚C
with SiO2 content. HAP/SiO2 samples with higher SiO2
content indicated the stronger peak from 4
4
SiO
with
SiO2 content. However, as obviously shown in figure, the
peak form phosphate () for HAP/SiO2 samples de-
creased with increase of SiO2 content. In addition, the
peak from OH for HAP/SiO2 samples decreased with
high SiO2 content of 30%. In this study, XRD results
indicated no formation of calcium silicate for HAP/SiO2
samples heat-treated at 600˚C. These FT-IR results, in
conjunction with XRD and FT-IR results, indicate that
3
4
PO
Figure 3. FT-IR results of Ca-def HAp added with SiO2 (0
wt%, 3 wt%, 10 wt%, and 30 wt%) heat-treated at 600˚C.
SiO2 was incorporated into calcium-deficient HAP struc-
tures, although the excess addition of SiO2 (e.g. 30 wt%
addition) and heat treatment at high temperatures have
the possibility of formation of calcium silicate. Therefore,
these results indicate that , below the optimum
SiO2 contents, was substituted for site in calcium-
deficient HAP structure during the heat-treatment at
600˚C.
4
4
SiO
3
4
PO
The microstructures of these calcium-deficient HAP/3
~ 10 wt% SiO2 heat-treated at 600˚C were observed with
SEM. Figure 4 shows SEM images of calcium-deficient
HAP samples containing 0, 3 and 10 wt% SiO2 after the
heat-treatment at 600˚C. Although calcium-deficient HAP/
3 ~ 10 wt% SiO2 were whisker-like as well as calcium-
deficient HAP samples without SiO2, the length of
whisker-like products decreased with SiO2 content. Also,
no agglomerate and grain growth of HAP grains were
confirmed after heat-treatment at 600˚C. Thus, the addi-
tion of SiO2 into calcium-deficient HAP resulted in the
inhibition of HAP grain growth, which was caused by the
substitution of 4
4
SiO
for site in hydroxyapatite
structure during the heat-treatment.
3
4
PO
Figures 5 and 6 shows the spectra of XANES of P
K-edge and Si K-edge for calcium-deficient HAP/3 ~ 10
wt% SiO2 heat-treated at 600˚C, compared to monolithic
HAP and samples heat-treated at 600˚C and quartz as a
reference material. From measurement in Si K-edge,
Copyright © 2011 SciRes. MSA
Synthesis and Evaluation of Calcium-Deficient Hydroxyapatite with SiO1197
2
Figure 4. SEM images of calcium-deficient HAP samples
containing 0, 3 and 10 wt% SiO2 after the heat-treatment at
600˚C.
Figure 5. Results of Si K-edge XANES spectra of calcium-
deficient HAP/0 ~ 30wt%SiO2 heat treated at 600˚C.
Si-local structures of calcium-deficient HAP/3 ~ 10 wt%
SiO2 heat-treated at 600˚C was the same as the quartz
and had no peak shifts, as shown in Figures 5. However,
in case of P local structures, the peak of P K-edge was
shifted to higher energy with the SiO2 contents. This re-
sult indicated that local structure around P K-edge for
Figure 6. Results of P K-edge XANES spectra of calcium-
deficient HAP/0 ~ 30 wt%SiO2 heat-treated at 600˚C.
calcium-deficient HAP/3 wt% SiO2 heat-treated at 600˚C
was slightly different from monolithic HAP and HAP
with a large amount of SiO2. On the contrary, calcium-
deficient HAP/10 ~ 30 wt% SiO2 heat-treated at 600˚C
had the significantly different local structure around P
K-edge compared to monolithic HAP and HAP/3 wt%
SiO2. These XANES, XRD and FT-IR results also sug-
gest that 4
4
SiO
was substituted for in hydro-
xyapatite structure. The difference in XANES spectra
between HAP/3 wt% SiO2 samples and others are
thought to be caused by the electron state. According to
results reported by Harris et al. [16], the substitution of
3
4
PO
3
4
PO
site by carbonate (2 to 6 wt%) produced marked
change in EXAFS (extended X-ray absorption fine
structure), leading to the structural distortion in hy-
droxyapatite structure due to the incorporation of car-
bonate into hydroxyapatite. This phenomenon and fur-
thermore the theoretical defect energetics in hydroxyapa-
tite is under investigation. Thus, these results indicated
the noticeable structural changes accompany the substi-
tution of 4
4
SiO
into site in hydroxyapatite
structure.
3
4
PO
4. Conclusions
Effect of SiO2 addition on the microstructures of cal-
cium-deficient HAP prepared by the soft chemical proc-
essing was investigated. HAP substituted with Si ion was
synthesized by the normal heat-treatment at 600˚C to
1200˚C for 2 h in air atmosphere. The results of FT-IR
measurements for modified HAP indicated that the peak
from 3
4
PO
decreased with the SiO2 contents. Micro-
structural observations indicated that the incorporation of
SiO2 inhibited the growth of HAP grain. Evaluation of P
Copyright © 2011 SciRes. MSA
Synthesis and Evaluation of Calcium-Deficient Hydroxyapatite with SiO2
Copyright © 2011 SciRes. MSA
1198
K-edge indicated the XANES spectra of HAP/10 ~ 30
wt% SiO2 heat-treated at 600˚C were different from that
of monolithic HAP and HAP/3 wt% SiO2 samples. These
results of micro- and local structure of modified HAP
suggest that was substituted for
4
4
SiO3
4
PO
in hy-
droxyapatite structure.
5. Acknowledgements
We thank UVSOR for the support of XAFS measure-
ment at BL1A in UVSOR. This work was partly sup-
ported by Grant-in Aid for Scientific Research on Pri-
mary Area “Nano-materials Science for Atomic Scale
Modification (area No. 474)”.
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