Synthesis and Evaluation of Calcium-Deficient Hydroxyapatite with SiO<sub>2</sub>

doi:10.4236/msa.2011.29161

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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)

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: [email protected]

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

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

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.

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.

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

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

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.

SiO

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

SiO



for site in hydroxyapatite

structure during the heat-treatment.

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,

Synthesis and Evaluation of Calcium-Deficient Hydroxyapatite with SiO1197

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

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

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

SiO



into site in hydroxyapatite

structure.

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



decreased with the SiO2 contents. Micro-

structural observations indicated that the incorporation of

SiO2 inhibited the growth of HAP grain. Evaluation of P

Synthesis and Evaluation of Calcium-Deficient Hydroxyapatite with SiO2

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

SiO3



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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