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Self-Assembly of Hydroxyapatite Nanostructures by Microwave Irradiation
Article in Nanotechnology · December 2004
DOI: 10.1088/0957-4484/16/1/017
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INSTITUTE OF PHYSICS PUBLISHING
NANOTECHNOLOGY
Nanotechnology 16 (2005) 82–87
doi:10.1088/0957-4484/16/1/017
Self-assembly of hydroxyapatite
nanostructures by microwave irradiation
Jingbing Liu, Kunwei Li, Hao Wang1 , Mankang Zhu, Haiyan Xu
and Hui Yan
The College of Materials Science and Engineering, Beijing University of Technology,
Beijing 100022, People’s Republic of China
E-mail: haowang@bjut.edu.cn
Received 7 August 2004, in final form 4 October 2004
Published 2 December 2004
Online at stacks.iop.org/Nano/16/82
Abstract
Hydroxyapatite (HAp) nanorods, bowknot-like nanostructures and
flower-like architectures have been directly synthesized and assembled
under microwave irradiation without the help of any templates. The uniform
nanorods present an average diameter of about 40 nm and a length of up to
about 400 nm. The as-prepared bowknot-like nanostructures consist of
sword-like HAp nanorods with a typical width of 150 nm and lengths up to
1–2 µm. The flower-like architectures are composed of leaf-like flakes with
typical diameters of 150–200 nm and lengths up to 1–2 µm. The SAED and
HRTEM experiments imply that the sword-like HAp nanorods and leaf-like
flakes are single crystalline in nature and preferentially grow along the [001]
direction. It is found that the pH value and the complex reagent EDTA play
important roles in synthesis of the final HAp nanostructures. The possible
mechanism is discussed for the formation of the HAp nanostructures in the
presence of EDTA under microwave irradiation.
1. Introduction
The self-assembly of inorganic nanoparticles into superlattices
and nanostructures offers the potential to fabricate materials
with tunable physical and chemical properties [1]. These
properties of the superstructures are dependent on the design
and control of the shape, size and spatial organization of
the building blocks; thus, the control of the anisotropic
inorganic materials at the mesoscopic level is one of the most
challenging issues currently faced by synthetic chemists and
materials scientists. Recently, research on nanostructures
has expanded rapidly into the assembly of nanoparticles in
two-dimensional (2D) and three-dimensional (3D) ordered
superstructures [2, 3]. Much effort has been made in
the fabrication of patterns of well arranged nanocrystallites,
especially the arrangement of one-dimensional (1D) nanotubes
and nanorods, because of their interesting physical properties
and potential applications in many areas [4, 5].
A variety of nanofabrication techniques and crystal growth
methods have been used to achieve shape control, particularly
a template-assisted technique. Besides hard templates such
1 Author to whom any correspondence should be addressed.
0957-4484/05/010082+06$30.00 © 2005 IOP Publishing Ltd
as a carbon nanotube [6] and anodized alumina [7], soft
templates such as regular or inverse micelles [8, 9], liquid
crystal [10], polymer [11] and some biological assemblies [12]
were also employed to control the morphology of nanocrystals,
mainly via the chemical interaction between the reactants
and templates. However, the introduction of templates and
substrates introduces heterogeneous impurities and increases
the production cost, which may restrict the wide development
of research and applications. Thus how to develop facile, mild,
easily controlled, and template-free methods to control both
nanocrystalline morphology and the crystal size is of great
significance.
As one of the most important biomaterials, hydroxyapatite
(HAp), with compositions of stoichiometric Ca10 (PO4 )6 (OH)2
and Ca/P = 1.67 [13], has attracted much interest for many
years, due to its similarity with the mineral constituents of
human bones and teeth [14]. It is of importance in many
industrial applications and in the biomedical field, such as
catalysis, ion exchange, sensors and bioceramics [15, 16].
HAp is a polar hexagonal and highly anisotropic crystal, which
naturally grows into 1D nanostructures. HAp nanowhiskers
or nanowires can be synthesized by a variety of methods
such as precipitation hydrolysis [17], sol–gel [18], and
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Self-assembly of hydroxyapatite nanostructures by microwave irradiation
60
2θ (degree)
Figure 1. XRD pattern of the sample prepared in the solution with a
pH of 9 using EDTA as the complex reagent.
hydrothermal methods [19, 20]. The use of homogeneous
Ca/citrate/phosphate solutions in synthesis by the microwave
method has also been reported [21]. Only nanosized (30–
60 nm) needle-like HAp of poor crystallinity were obtained.
Compared with the conventional method, microwave
synthesis has the advantages of very short time, small particle
size, narrow particle size distribution, and high purity [22]. The
microwave heating is a quite fast, simple and efficient method
to prepare nano-sized inorganic materials. Although the exact
nature of the interaction between the microwaves and the
reactants during the synthesis of materials is somewhat unclear
and speculative, it is known that the interaction between
dielectric materials and the microwaves leads to dielectric
heating. Electric dipoles present in such materials respond to
the applied electric field. This constant reorientation creates
friction and collisions between molecules, which subsequently
generates heat.
In this paper, we report a general, controllable synthesis
and assembly of HAp nanostructures in one step under
microwave irradiation without any templates. Calcium nitrate
tetrahydrate (Ca(NO3 )2 ·4H2 O) and dibasic anhydrous sodium
phosphate (Na2 HPO4 ) have been utilized as starting materials,
using ethylenediaminetetracetic acid disodium salt (EDTA)
as complex reagent. The shape of the obtained nanocrystals
can be easily varied between the bowknot-like and flower-like
nanostructures as well as monorods by simply changing the
pH value of the solution.
2. Experimental procedure
All reagents were analytical grade and used without further
purification. In a typical procedure, 50 ml of a mixed solution
of Ca(NO3 )2 ·4H2 O (0.1 M) and EDTA (0.1 M) was introduced
into 50 ml of Na2 HPO4 (0.06 M) solution. The pH of initial
solution without adjusting was 4. The pH of solution was
adjusted to 9–13 by adding NaOH solution. After stirring for
several minutes, the ready-adjusted clear aqueous solutions
with a certain pH value were put into a household type
microwave oven of 700 W power with a refluxing system
and the reaction was performed under ambient air for 30 min.
Figure 2. TEM image of HAp powders synthesized from the
solution with a pH of 9. The inset shows the SAED pattern taken
from a single nanorod.
The microwave oven followed a working cycle of 6 s on and
10 s off (37% power). After cooling to room temperature, the
precipitate was centrifuged, washed with deionized water, and
dried in vacuum at 70 ◦ C for 2 h.
The x-ray diffraction (XRD) analysis was performed in θ –
2θ mode using a Bruker D8 x-ray diffractometer with graphite
monochromatized Cu Kα radiation (λ = 0.154 178 nm).
Scanning electron microscopy (SEM) images of samples
were obtained with a Hitachi model S-3500N scanning
electron microscope. Transmission electron microscopy
(TEM), selected area electron diffraction (SAED), and highresolution transmission electron microscopy (HRTEM) images
were taken on a JEOL-JEM 2010F transmission electron
microscope, using an accelerating voltage of 200 kV.
3. Results and discussion
XRD analysis was used to examine the crystal structure of
the products. Figure 1 shows the XRD pattern of the sample
prepared in the solution with a pH of 9 using EDTA as the
complex reagent. All the diffraction peaks in the pattern can be
indexed as the hexagonal HAp with lattice constants a = 9.430
and c = 6.882 Å, which are consistent with the values in the
standard card (JCPDS No. 73-293). Furthermore, it can be seen
that the diffraction peaks are high and narrow, implying that
the HAp crystallizes well, and for crystallographic orientations
in the direction parallel to the major axis sharper peaks will be
expected. As for the other samples prepared in solutions with
different pH values, the XRD patterns are almost identical to
figure 1.
Figure 2 shows the TEM micrograph of HAp powders
synthesized from the solution with a pH of 9. The uniform
nanorods with an average diameter of about 40 nm and a length
of up to about 400 nm had been obtained with this condition.
83
J B Liu et al
(c)
(a)
(b)
(d)
Figure 3. (a) Low magnification SEM image showing bowknot-like HAp nanostructures. (b) Higher magnification SEM image of a typical
bundle of the HAp nanostructures. (c) TEM image of a typical bowknot-like bundle of HAp. Inset: SAED pattern taken from an individual
HAp nanorod from the bowknot-like HAp nanostructures. (d) HRTEM image recorded from the tip of the individual sword-like HAp
nanorod.
The SAED pattern taken from a single nanorod shows that
the nanorod was a single crystal of hexagonal HAp (inset in
figure 2).
The SEM and TEM images of as-prepared HAp
nanostructures with a pH of 11 are shown in figure 3.
The overall morphology of the samples which is shown in
figure 3(a) indicates that there exist a great many uniform
bowknot-like bundles with their two ends fanning out while
the middle part is tied together. Typical bundles of the HAp
nanocrystals (figure 3(b)) indicate that the bundles consist of
HAp nanorods with a typical width of 150 nm and lengths up to
1–2 µm. After long-period ultrasonic treatment, the bowknotlike HAp nanostructures were not destroyed, indicating the
nanostructures were not due to aggregation. Figure 3(c) shows
the TEM image of a typical bowknot-like bundle of HAp. It
can be seen from this image that the end of the HAp nanorod
is a cusp like a sword. More detail about the structure of a
selected nanorod from the bowknot-like HAp was investigated
by the SAED pattern (inset in figure 3(c)). The highly arrayed
diffraction spots in the pattern indicate the single-crystalline
property of the HAp nanorod. Figure 3(d) shows the HRTEM
image recorded from the sword-like tip of the individual HAp
nanorod shown in figure 3(c). The regular spacing of the
observed lattice planes was about 0.82 nm, which is consistent
84
with the (100) lattice spacing of HAp, showing that the nanorod
was of uniform crystal structure. Further studies of both the
HRTEM image and SAED pattern demonstrate that the growth
axis is the [001] direction of HAp.
Figure 4 shows the SEM and TEM images of the typical
flower-like HAp nanostructures prepared in the solution of
pH 13. The morphology of the obtained HAp is in the
form of leaf-like flakes of 150–200 nm width and of 1–2 µm
length extending radially from the centre. The corresponding
SAED pattern (inset in figure 4(c)) taken from individual leaflike flake confirms that the flakes are well crystallized single
crystals. The typical HRTEM image (figure 4(d)), recorded
from an individual leaf-like flake, reveals a perfect crystal
structure, and unambiguously distinguishes the (100) and (002)
atomic planes of hexagonally structured HAp with interplanar
spacings of about 0.82 and 0.34 nm, respectively. The (002)
crystal planes are approximately vertical to the long axis of the
HAp flakes, which shows that the HAp flakes predominantly
grow along the [001] direction.
In order to reveal the growth mechanism and the effect
of EDTA, comparative experiments in the absence of EDTA
under the same synthesis conditions were studied. When
CaNO3 solution was added to Na2 HPO4 solution, a mass
of white precipitate of amorphous calcium phosphate (ACP)
Self-assembly of hydroxyapatite nanostructures by microwave irradiation
(c)
(a)
(b)
(d)
Figure 4. (a) Low-magnification SEM image showing flower-like HAp nanostructures. (b) Higher magnification SEM image of typical
flower-like HAp nanostructures. (c) TEM image of a typical flower-like HAp. Inset: SAED pattern taken from an individual HAp leaf-like
flake from the flower-like HAp nanostructures. (d) HRTEM image recorded from the edge of the individual leaf-like HAp flake.
immediately appeared at room temperature. An analogous
phenomenon was also reported in the literature [23]. After
30 min microwave irradiation, ACP can be transformed to the
HAp nanocrystals. Distinct different morphology could be
seen compared with the sample prepared in the presence of
EDTA. The TEM image of sample prepared from solution with
a pH of 11 in the absence of EDTA under microwave irradiation
for 30 min is shown in figure 5. The morphology is not regular;
it has a diameter of 5–50 nm and length of 100–200 nm. As for
the other samples prepared in solution with pH values of 9 and
13 in the absence of EDTA, only 1D HAp nanocrystals similar
to the sample prepared with a pH of 11 could be seen. The main
reason for the formation of smaller nanosized crystals without
EDTA is likely to be that the reaction rate of Ca2+ and PO3−
4
is too fast to form ACP, which served as the nucleus for HAp
crystals, under this reaction condition. If the fraction of ions
consumed in the nucleation step is high, the increase in particle
size due to the diffusion-controlled growth that immediately
follows the nucleation burst is drastically limited, and only
slightly larger particles than the nuclei can be obtained. It can
be confirmed that EDTA plays an important role in controlling
the nucleation and growth of the product.
Based on the above experiments and results, we suggest
the formation mechanism of the HAp in the presence of EDTA
as follows:
Figure 5. TEM image of sample prepared from solution with a pH
of 11 in the absence of EDTA under microwave irradiation for
30 min.
EDTA is a strong complex reagent with Ca2+ , which leads
to the formation of Ca–EDTA complexes. The formation of the
85
J B Liu et al
Figure 6. Molecular structure of Ca–EDTA complex.
1.0
Y
HY
H6Y
0.8
H 2Y
H 5Y
0.6
δ
0.4
H3Y
0.2
0.0
0
2
4
6
8
10
12
14
pH
Figure 7. Fraction of EDTA species as a function of pH value.
complex sharply decreased the free Ca2+ concentration in the
solution, and effectively prevented the formation of ACP upon
mixing sources of Ca and P in the presence of EDTA [24].
Figure 6 presents the molecular structure of the Ca–EDTA
complex. In the complex, EDTA acts as a hexadentate unit
by wrapping itself around the metal ion with four oxygen
atoms and two nitrogen atoms and forms several five-member
chelate rings. In solutions with different pH values, EDTA
(abbreviated to H4 Y, where Y is the EDTA residue) can present
in seven forms: H6 Y2+ , H5 Y+ , H4 Y, H3 Y− , H2 Y2− , HY3− ,
and Y4− . Figure 7 gives the fraction of each form of EDTA
as a function of pH value [25]. Since the anion Y4− is the
ligand species with the strongest complexability, the larger the
fraction of Y4− , the more stable is the complex. When the
pH value of solution is higher than 10, the species Y4− will
predominate. Therefore, with increasing pH value, the stability
of the complex improves.
On the other hand, the shape of a crystal is determined by
the relative specific surface energies associated with the facets
of this crystal. Before growth units are incorporated into the
crystal lattice, some facets of the cluster have a preference
to absorb OH− due to the different surface energy of the
crystallite facets [26]. Thus, the shielding effect of OH− ions
on the interface will control the growth rate of the OH-absorbed
facets. The quantity of the OH− and its hindrance effect are
varied on different facets. The larger the quantity of the OH−
present at the interface, the stronger is the hindrance effect
of OH− ions on the facet, which consequently reduces the
difference of growth rates in various crystal facets.
Usually, in the initial stage of the experiment, a threedimensional cluster of critical size would be formed, which
could act as a nucleus for HAp crystals and develop into
a crystallite under microwave irradiation.
The further
development of the crystallite will be controlled by the complex
stability of Ca–EDTA and the hindrance of OH− on the facets.
Figure 8. Schematic illustration of major steps involved in the microwave approach to the synthesis of HAp nanostructures.
86
Self-assembly of hydroxyapatite nanostructures by microwave irradiation
−
At lower pH value (lower OH concentration), the crystal
growth habit is mainly affected by the interior structure rather
than the exterior condition. In this condition, free Ca ions
can be easily released from Ca–EDTA complexes due to the
poor stability of Ca–EDTA. The free Ca ions will incorporate
into the crystal lattice site of the obtained crystallite and the
crystallite will grow according to the anisotropic structure of
HAp. Consequently, HAp nanorods are obtained at a pH
of 9. However, with increasing pH value, the concentration
of OH− and the stability of the complex increase. At a
pH of 13, each facet of the crystallite has almost the same
probability to generate active sites due to the strong absorption
of OH− ions. In this case, Ca will be incorporated in the
Ca–EDTA form, due to the higher stability of the complex,
into the active sites of the obtained crystallite. The distance
between two neighbouring Ca–EDTA causes the Ca atoms not
to arrange according to the crystal lattice of HAp. Thus,
a new nucleus is formed at the limited active sites with
absorption of Ca–EDTA. The absorbed Ca–EDTA complexes
undergo decomposition at an appropriate temperature under
microwave irradiation. Subsequently, HAp crystals can
present spontaneously preferential growth from the limited
active sites due to the anisotropic growth habit of HAp. So, the
morphology of particles prepared in solution with a pH of 13
gives the flower-like form. For the condition of pH equal to 11,
certain facets with higher free energies will preferentially form
the active sites and show higher growth rate; thus, the bowknotlike nanostructures are obtained. A schematic illustration of
the major steps involved in the microwave approach to the
synthesis of HAp nanostructures is shown in figure 8.
As is well known, the plane with the fastest growth rate
will disappear quickly. The (001) plane of HAp, the plane
with the most rapid growth rate, will disappear in the crystal
growth. Therefore, sword-like HAp nanorods (figure 3(c)) and
leaf-like HAp flakes (figure 4(c)) are achieved in our process.
4. Conclusion
In summary, uniform 1D and 3D (bowknot-like and flowerlike) HAp nanostructures have been successfully prepared
by microwave irradiation without any help of templates. A
mechanism was proposed to elucidate their formation. The
shape of the obtained crystals can be easily controlled by
changing the complex stability of Ca–EDTA and the hindrance
effect of OH− on the crystallite facets. These bowknot-like
and flower-like nanostructures, which have a highly specific
area on the surface of the particles, may bring some novel and
unexpected properties. We can foresee the scaling-up of the
process to form large quantities of this kind of nanomaterial.
Further work is under way to study the properties of these novel
3D nanostructures and the possibility of synthesizing other 3D
nanostructures.
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