Synthesis of mesoporous hydroxyapatite using a modified hard-templating route

Article abstract of DOI:10.1016/j.materresbull.2009.04.014

Mesoporous polycrystals of hydroxyapatite-calcium are synthesized via a modified hard-templating route. The structure properties of hydroxyapatite-calcium are characterized by means of X-ray diffraction, Fourier transform infrared spectroscopy, transmissi

Full text of DOI:10.1016/j.materresbull.2009.04.014

Materials Research Bulletin 44 (2009) 1626–1629
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Materials Research Bulletin
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Synthesis of mesoporous hydroxyapatite using a modified hard-templating route
*
Zhiguo Xia, Libing Liao , Senlin Zhao
School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Mesoporous polycrystals of hydroxyapatite-calcium are synthesized via a modified hard-templating
route. The structure properties of hydroxyapatite-calcium are characterized by means of X-ray
diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy and N2
adsorption–desorption isotherms. Wide-angle X-ray diffraction and Fourier transform infrared
spectroscopy measurements reveal that the crystalline grains consist of highly crystalline pure
hydroxyapatite phases. Transmission electron microscopy results show that rod-like hydroxyapatite-
calcium grains with an average diameter of about 100 nm long and about 20 nm wide are uniformly
distributed, which are also observed with an average pore size of 2–3 nm. Based on N2 adsorption–
desorption isotherms investigation, the pore size, surface area and pore volume of mesoporous
Received 10 March 2009
Received in revised form 14 April 2009
Accepted 30 April 2009
Available online 9 May 2009
Keywords:
A. Microporous materials
B. Chemical synthesis
D. Microstructure
Hydroxyapatite
hydroxyapatite-calcium are 2.73 nm, 42.43 m2 gÀ1 and 0.12 cm3 À1, respectively.
g
ß 2009 Elsevier Ltd. All rights reserved.
1. Introduction
and mesoporous [12] structure have been widely reported.
Among them, mesoporous HAP has attracted more attention,
thus, design and development on synthesis method becomes a
hot issue [12–14]. In this paper, we demonstrated a novel
modified ‘‘hard-templating’’ route used as the synthesis of
mesoporous HAP. We adopted the SBA-15 template and organic
carbon sources as the starting materials, and they can be used to
prepare ordered mesoporous carbon with two-dimensional
hexagonal structure, CMK-3, according to the reported techni-
que [15]. Then, mesoporous HAP was successfully synthesized
utilizing CMK-3 as hard template, which accorded with the
removal of carbon templates by combustion (600 8C/air, 8 h).
Recently, much attention has been attracted by mesoporous
materials due to their high surface areas, large and tunable pore
size, and large pore volumes, which are crucial for developing new
types of catalysts, adsorbents, drug delivery system, and so on
[1,2]. Therefore, many efforts have been placed on the synthesis of
mesoporous materials [3–5]. In 1992, Kresge et al. [3] successfully
developed a supermolecular templating technique, which could be
used to prepare silicon based mesoporous materials. After that,
Kim et al. [4] reported the first synthesis of a new type of
mesoporous carbon molecular sieve by carbonizing sucrose inside
the pores of the MCM-48 mesoporous silica molecular. The above
two routes demonstrated two kinds of typical strategies used as
the synthesis of mesoporous materials, which can be designed as
‘‘soft-templating’’ and ‘‘hard-templating’’ routes, respectively.
Hydroxyapatite-calcium, Ca10(PO4)6(OH)2 (denoted as HAP)
is a famous bioceramics materials due to its good biocompat-
ibility, excellent ability to form chemical bond with living bone
issue, and suitable osteoconductivity [6,7]. Moreover, HAP with
various morphologies and surface properties have also been
investigated as drug carriers for the delivery of a variety of
pharmaceutical molecules because of its nontoxic and nonin-
flammatory properties [8]. Considering the numerous applica-
tions of HAP, different morphologies HAP materials in the form
of ceramics body [9], nanostructure [10], uniform porous [11]
2. Experimental
2.1. Materials and synthesis procedure
All the reagents including CaCl2Á2H2O (A.R.), (NH4)2HPO4, NaOH
(A.R.), H2SO4 (A.R.) and sucrose (A.R.) were received without
further purification. High-quality SBA-15 was obtained from Zhao
et al. [16], which was prepared according to their previously
reported method. Fig. 1 shows the flow sheet of the procedure used
for the preparation of mesoporous HAP. Firstly, CMK-3 was
synthesized by SBA-15 as a silica template and sucrose as a carbon
precursor following the previously reported recipe [15], except for
the following modifications. The carbonization was completed by
pyrolysis with heating to typically 1173 K under N2 atmosphere.
The carbon–silica composite obtained after pyrolysis was washed
with 1 M NaOH solution (50 vol.% ethanol–50 vol.% H2O) twice at
373 K, to remove the silica template. The template-free carbon
* Corresponding author. Tel.: +86 10 82322104; fax: +86 10 82322974.
E-mail addresses: xiazg426@yahoo.com.cn (Z. Xia), lbliao@cugb.edu.cn (L. Liao).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.04.014

Z. Xia et al. / Materials Research Bulletin 44 (2009) 1626–1629
1627
Fig. 1. The formation procedure of mesoporous HAP in this work.
product thus obtained was filtered, washed with ethanol, and dried
at 393 K. Secondly, mesoporous HAP was prepared; by a modified
‘‘hard-templating’’ route, together with the refluxing wet chemical
method. In a typical process, stoichiometric amounts (Ca/P = 1.67)
of CaCl2 (2.9406 g CaCl2Á2H2O dissolved in 60 ml deionized water
[DW]) was added dropwise to a solution of (NH4)2HPO4 (1.5847 g)
and 0.36 g as-prepared CMK-3 dissolved in 120 ml DW; the pH was
adjusted to 10–11 using 2 M NaOH, yielding a milky suspension,
which was refluxed at 120 8C for 24 h. The final suspensions were
aged for 12 h at room temperature, filtered and thoroughly washed
with DW. The recovered gray precipitate was dried at 100 8C for
12 h in an oven, and then sintered at 600 8C for 8 h in air to remove
the carbon templates. Finally, a white and fluffy-like mesoporous
HAP was obtained.
evaluated from the desorption branch of isotherm based on
Barrett–Joyner–Halenda (BJH) model.
3. Results and discussions
3.1. XRD and FT-IR analysis
Fig. 2 shows the typical diffraction peaks of HAP, especially the
three strongest peaks of HAP at 2u = 25.88, 31.88 and 32.98, which
can be indexed as (0 0 2), (2 1 1) and (3 0 0), respectively,
according to the standard data [17]. The XRD pattern reveals
that the structure of as-synthesized HAP belongs to the hexagonal
˚
P63/m space group with lattice constant of a = 9.418 A and
˚
c = 6.884 A. It also testified that the present ‘‘hard-templating’’
route, together with the refluxing wet chemical method can obtain
pure HAP phase. Further, Fig. 3 shows the FT-IR spectrum of the
2.2. Characterization
calcinated HAP sample. Phosphate absorption bands [
occur at about 1031, 959, 606 and 564 cmÀ1, and a hydroxide
absorption band [
(OHÀ)] is observed at about 3436 cmÀ1, which
y
(PO43À)]
The wide-angle X-ray diffraction pattern was recorded by using
a SHIMADZU model XRD-6000 X-ray powder diffractometer (Cu
y
K
a
radiation, 40 kV, 30 mA and a scanning speed 2.08 (2
u
)/min).
are all characteristic for a typical HAP FT-IR spectrum [18]. In
addition, as shown by the enlarged view of the FT-IR spectrum in
the range of 500 and 700 cmÀ1 in the inset, we can obviously find
the fine bands at 606 and 564 cmÀ1, which are ascribed to the triply
degenerated y4 vibration of O–P–O bond.
Fourier transform infra-red (FT-IR) spectrum was collected on a
FTIT-AVATAR370 spectrophotometer over the range of wavenum-
ber 2000–400 cmÀ1, and the standard KBr pellet technique was
employed. Transmission electron microscope (TEM) images were
recorded on a JEOL-2010 Electron Microscope with an acceleration
voltage of 120 kV. N2 adsorption/desorption analysis was per-
formed at 77 K using a Quantachrome Quadrasorb SI apparatus.
The specific surface area was determined by the Bruaauer–
Emmett–Teller (BET) method using the data between 0.05 and 0.3.
The pore parameters (pore volume and pore diameter) were
3.2. TEM and NADI investigations
Fig. 4 presents the typical TEM images of as-synthesized
mesoporous HAP with different scale. It can be seen from
Fig. 4(a) that rod-like HAP grains with an average diameter of
Fig. 3. FT-IR spectrum of as-synthesized mesoporous HAP calcined at 600 8C, the
inset shows the enlarged FT-IR spectrum in the range of 500 and 700 cmÀ1
.
Fig. 2. XRD pattern of mesoporous HAP calcined at 600 8C.

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Z. Xia et al. / Materials Research Bulletin 44 (2009) 1626–1629
Fig. 4. Typical TEM images of mesoporous HAP calcined at 600 8C: (a) 100 nm scale and (b) 20 nm scale.
isotherms method. Fig. 5 shows a typical N2 adsorption–desorption
isotherms for mesoporous HAP calcined at 600 8C, which exhibit a
mesoporous materials type IV curve with a hysteresis loop. The
pore size distribution calculated from the adsorption branch of the
isotherms based on BJH model was shown in the inset of Fig. 5. As is
shown in the inset of Fig. 5, most of the pore size is distributed
around 2.73 nm. Besides, based on overall investigation on the N2
adsorption–desorption data, we can find that surface area and pore
volume of mesoporous HAP in this work is 42.43 m2 gÀ1 and
0.12 cm3 gÀ1, respectively. We can find that mesoporous HAP
prepared by the present ‘‘hard-templating’’ route has excellent
application properties with large surface areas and small pore size,
which can be promising as catalysts, absorbents, separation, and
host materials.
3.3. Discussion
Here, we will discuss the formation mechanism of mesoporous
HAP. In this work, we adopted a modified hard-templating route to
prepare mesoporous HAP materials. Fig. 6 gives the involved
experimental process and possible mechanism describing the
formation of mesoporous HAP. The structure of the CMK-3 carbon
is exactly an inverse replica of SBA-15, and we adopted the similar
method to inversely replicate CMK-3, and prepare mesoporous
HAP. As is reported in many references [15,19], the structure of
SBA-15 consists of the hexagonal arrangement of cylindrical
mesoporous tubes, and CMK-3 is the ordered arrangement of the
carbon nanorods. So we can expect that mesoporous HAP will form
the similar mesoporous tubes structure as SBA-15. But, irregular
quadrilateral shape pores are obtained in our experiments, and the
possible reasons can be ascribed to the complicated aqueous
system in the refluxing process.
Fig. 5. N2 adsorption–desorption isotherms for mesoporous HAP calcined at 600 8C,
inset: pore size distribution of mesoporous HAP based on nitrogen absorption
analysis.
about 100 nm long and about 20 nm wide are uniformly
distributed and the morphology of HAP samples are well-
defined. Moreover, there are many pores dispersed on the rod-
like HAP grains. Further, Fig. 4(b) gives the TEM images of
mesoporous HAP in 20 nm scale, which can be used to observe
the morphology of HAP samples in a facile way. It is found that
most pores are of irregular quadrilateral shape with an average
pore size of 2–3 nm.
The detailed parameters of the pores dispersed on the rod-like
HAP grains were determined by N2 adsorption–desorption
Fig. 6. The experimental process and possible mechanism describing the formation of mesoporous HAP in this work.

Z. Xia et al. / Materials Research Bulletin 44 (2009) 1626–1629
1629
4. Conclusions
College Student Research Innovation Program of China University
of Geosciences, Beijing.
Mesoporous polycrystals of hydroxyapatite-calcium are
synthesized via a modified hard-templating route in the present
work. Ordered mesoporous carbon CMK-3 was formed through
SBA-15 template and organic carbon sources. Mesoporous hydro-
xyapatite-calcium was successfully synthesized utilizing CMK-3 as
hard template, with the removal of carbon templates by
combustion (600 8C/air, 8 h). The structure properties of the
hydroxyapatite-calcium phase are characterized by powder X-ray
diffraction and Fourier transform infrared spectroscopy. Transmis-
sion electron microscopy results show that rod-like hydroxyapa-
tite-calcium grains about 100 nm long and about 20 nm wide are
uniformly distributed, with an average pore size of 2–3 nm. Based
on overall investigation on the N2 adsorption–desorption data, the
pore size, surface area and pore volume of this material was found
to be 2.73 nm, 42.43 m2 gÀ1 and 0.12 cm3 gÀ1, respectively.
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This present work was supported by the National Natural
Science Foundations of China (Grant No. 40272024), and the