Controllable synthesis of hierarchical nanostructures of CaWO4 and SrWO4 via a facile low-temperature route

Article abstract of DOI:10.1016/j.materresbull.2008.04.008

CaWO₄ and SrWO₄ nanostructures have been synthesized via a simple microemulsion-mediated route. With careful control of the fundamental experimental parameters including the concentration of reactants, the reaction time and the tempe

Full text of DOI:10.1016/j.materresbull.2008.04.008

Materials Research Bulletin 44 (2009) 45–50
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Controllable synthesis of hierarchical nanostructures of CaWO4 and
SrWO4 via a facile low-temperature route
*
Z. Chen, Q. Gong, J. Zhu, Y.P. Yuan, L.W. Qian, X.F. Qian
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
CaWO4 and SrWO4 nanostructures have been synthesized via a simple microemulsion-mediated route.
With careful control of the fundamental experimental parameters including the concentration of
reactants, the reaction time and the temperature, the products with different morphologies of dumbbell,
coral, rod and dendrite have been obtained, respectively. The possible formation mechanism of these
unique morphologies has been proposed based on surfactant self-assembly under different experimental
conditions. The as-synthesized CaWO4 samples with various morphologies exhibit different photo-
luminescence properties. X-ray powder diffraction, transmission electron microscopy, field-emission
scanning electron microscopy, and luminescence spectroscopy were used to characterize these products.
Crown Copyright ß 2008 Published by Elsevier Ltd. All rights reserved.
Received 9 January 2008
Received in revised form 3 April 2008
Accepted 9 April 2008
Available online 22 April 2008
Keywords:
A. Nanostructures
B. Chemical synthesis
1. Introduction
In this paper, we described the controllable synthesis of CaWO4
and SrWO4 nanomaterials through the reverse micelle/micro-
Inorganic nanomaterials of uniform size and shape are of
special interest from both theoretical and practical perspectives
[1–3]. To systematically manipulate the shapes of inorganic
compounds would greatly benefit various fields, including optics,
electronics, catalysis, and medicine [4–6]. The existing methods for
size and shape control generally employ capping agents, e.g.
surfactants, ligands, polymers or dendrimers, to confine the crystal
growth in the nanometer regime [7–9].
In recent years, calcium and strontium tungstates from the
scheelite family remain centre of attraction for crystal growers,
radiologists, material scientists and physicists due to their
luminescence, thermoluminescence and stimulated Raman scat-
tering behavior [10], and have potential application in the field of
photonics and optoelectronics. For example, calcium tungstate is
one of the most widely used phosphors in industrial radiology and
medical diagnosis [11], and it can be employed for a variety of
applications, e.g. tunable fluorescence and sensor for dark matter
search [12,13]. Although nanosized CaWO4 and SrWO4 with
various morphologies have been prepared, such as nanoparticles
[14], nanofilms [15], nanopeanuts [16], and bipolar petal [17], to
the best of our knowledge, few works has been reported on the
synthesis of CaWO4 with dumbbell, sandglass or coral shape until
now.
emulsion system, in which the morphologies and the organized
particles sizes can be readily tuned by adjusting experimental
parameters. The photoluminescence performance of the self-
organized crystals was also studied.
2. Experimental
Analytical-grade chemicals cetyltrimethylammonium bromide
(CTAB), Na2WO4Á2H2O, CaCl2 and SrCl2Á6H2O were purchased from
Shanghai Chemical Industrial Corporation and used without
further purification. The reaction was carried out in a 50 mL glass
jar at a designated temperature.
The synthesis of CaWO4 or SrWO4 nanocrystals was simply
achieved via
a microemulsion-mediated route. The detailed
reaction conditions and the obtained products are given in
Table 1. In a typical procedure, two microemulsion solutions
were prepared by adding 2 mL of 0.2 M CaCl2 and 2 mL of 0.2 M
Na2WO4 aqueous solutions to n-octane/CTAB/n-butanol systems
(the molar ratio of n-butanol/CTAB is 4.15 M, the molar ratio of
H2O/CTAB is 33 M, and [CTAB] is 0.2 M), respectively, then they
were stirred for 30 min until they became transparent. After that,
the two obtained solutions were mixed rapidly and then were set
aside at 10 8C for certain time. White products were separated by
centrifugation, washed with deionized water and absolute ethanol
several times, and then dried at 60 8C for 6 h.
The as-prepared products were characterized by powder X-ray
* Corresponding author. Tel.: +86 21 54743262; fax: +86 21 54741297.
E-mail address: xfqian@sjtu.edu.cn (X.F. Qian).
diffraction (XRD, Shimadzu XRD-6000 with Cu K
a radiation,
0025-5408/$ – see front matter. Crown Copyright ß 2008 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2008.04.008

46
Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–50
Table 1
Detailed reaction conditions and the obtained products
Sample
Formulation detected by XRD
Ca (mol/L)
T (8C)
Time
Morphologies
1
2
3
4
5
6
CaWO4
CaWO4
CaWO4
CaWO4
SrWO4
SrWO4
0.2
0.2
0.5
0.2
0.2
0.2
10
10
10
25
10
25
5 min
6 h
Nanoparticles
Dumbbell-like nanocrystals
Coral-like nanocrystals
Nanorods
6 h
6 h
1 h
Dendritic nanocrystals
Dendritic nanocrystals
1 h
a
2À
C is the initial concentration of Ca2+ and WO4 ions in the microemulsions.
˚
l
= 1.5406 A), field-emission scanning electron microscopy
that all the diffraction peaks can be indexed as a pure tetragonal
˚
(FESEM, FEI SIRION 200), transmission electron microscopy
(TEM, JEOL 2010), and luminescence spectroscopy (PerkinElmer
LS50B luminescence spectrometer).
scheelite structure with cell parameters of a = 5.24 A and
˚
c = 11.25 A (JCPDS Card No. 77-2233), which confirms that the
as-prepared product is CaWO4.
Other samples prepared under different conditions have similar
XRD patterns. The peaks of pattern 1 are obviously wider than
those of the other three patterns implies the smaller size of the
obtained nanoparticles (sample 1).
3. Results and discussion
3.1. Detailed structural characterization of the CaWO4 superstructure
with different morphologies
The typical morphology and structure of CaWO4 were further
characterized by transmission electron microscopy (TEM) and
scanning electron microscopy (FESEM). Fig. 2b–d shows the
obtained CaWO4 nanocrystals were in dumbbell-like 3D archi-
tectures with highly self-assembled hierarchical and repetitive
superstructures, rather than the disordered arrangements
reported by the previous studies [14,18]. We can also find that
only nanoparticles with mean diameters of 20–30 nm were
obtained when the reaction was carried for 5 min at 10 8C in
Fig. 2a. Though the HRTEM image (Fig. 2e) is not very clear, careful
observation still reveals that the interplanar spacing of nano-
particles is 0.312 nm, corresponding to the (1 1 2) lattice spacing
of CaWO4. And the SAED image (inset of e) also confirms the
single-crystalline nature. As the reaction time was prolonged to
6 h, dumbbell-like superstructures of CaWO4 crystals were
obtained. A high-magnification FESEM image clearly reveals that
CaWO4 nanocrystals were obtained by the reaction between
Ca2+ and WO4 ions solubilized in a cationic surfactant-CTAB-
2À
microemulsion system. In Fig. 1a, the XRD pattern clearly shows
the dumbbell superstructure of CaWO4 is about 1.5–2
length, and the top of the dumbbell is like a cauliflower with about
m in diameter (Fig. 2d). These dumbbells are constructed from
mm in
1
m
the nanoparticles, in which these highly oriented nanoparticles
connect with each other to form architectures with recognizable
boundaries. The spacing between adjacent lattice fringes along
orientation direction is 0.285 nm, corresponding to the (0 0 4)
lattice spacing of CaWO4 (Fig. 2f). The adjacent particles have the
same lattice fringe (d = 0.285 nm) along the same direction,
confirming that they have the same orientation. This structure is
similar to the mesocrystal structure.
Reverse micelles or microemulsion systems have been widely
used as ideal media to prepare nanoparticles [19–21]. Water-in-oil
(w/o) microemulsion is a transparent and isotropic liquid medium
with nanosized water pools dispersed in a continuous phase and
stabilized by surfactant and co-surfactant molecules at the water/
oil interface. These water pools offer ideal microreactors for the
formation of nanoparticles. Compared to other synthetic methods,
the microemulsion approach has distinct advantages in effective
control over the size and shape of the particles, such as relatively
low reaction temperature, less time consumption, high homo-
geneity, and well-crystallized products with definite composition.
A schematic diagram of the proposed growth process is shown in
Fig. 3. When the microemulsion solutions containing Ca2+ and
WO42À, respectively, were mixed, CaWO4 nucleation and micellar
fusion may be concomitant. And nanoparticles with surfactant
coatings are formed in
a very short time. The obtained
nanoparticles can be regarded as ‘‘spherical core–shell nanopar-
ticles’’ with an inorganic core and an organic surfactant shell. Then
the dumbbell superstructures of CaWO4 were formed via a self-
Fig. 1. (a) XRD patterns of samples 1–4 (as described in Table 1) and (b) XRD
patterns of SrWO4 (sample 5).

Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–50
47
Fig. 2. Images of CaWO4 nanocrystals formed at 10 8C, with [Ca2+] = [WO42À] = 0.2 M. (a) TEM images of the product formed within 5 min; (b–d) TEM and FESEM images of the
product formed for 6 h; (e) HRTEM image of the product formed within 5 min; inset shows the corresponding ED patterns; (f) HRTEM image of the product formed for 6 h.
assembly process through the surfactant interactions between the
building blocks. At last, prolonging the time leads the products to
experience an Ostwald ripening process.
dumbbell CaWO4 crystals (Fig. 4a). The ‘‘arms’’ of the coral-like 3D
superstructures is in radial structure with about 200–300 nm in
length. The size of the nanoparticle building blocks was about 25–
40 nm. The large amount of ‘‘holes’’ appeared on the TEM and
FESEM images revealed that the obtained architectures are built by
the original particles. This result also indicated that a higher
For comparison, the effect of the initial concentration of Ca2+
2À
and WO4 ions in the microemulsions was examined. When the
2À
concentration of Ca2+ and WO4 ions was increased from 0.2 to
concentration of Ca2+ and WO4
favored the formation of
2À
0.5 M, coral-like CaWO4 aggregation was obtained instead of the

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Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–50
nanocrystals with higher aspect ratios, and the initial concentra-
tion of Ca2+ and WO42À ions in microemulsions played a key role in
the final morphology of products.
Further studies indicated that reaction temperature is also an
important influence factor for the shape and size of CaWO4
products. When the reaction was carried out at room temperature,
the main morphology of products is in rod shape with lengths in
the range of 2–3
mm (Fig. 4b). The holes on the surfaces of products
suggest that the obtained rods were assembled by the original
particles. In addition, these obtained rods tend to aggregate
towards flowerlike, organized by two, four or more rods. When the
reaction temperature increased, the reaction rate and the reactant
diffusion increased considerably between the microemulsion
droplets, which led to a large size of nanoparticles and unclosed
deposit.
3.2. Optical properties of CaWO4 superstructure with different
morphologies
The photoluminescence performance of the as-synthesized
CaWO4 nanostructures was also studied because of its well-known
photoluminescent properties. Fig. 5 shows the luminescence
spectra of the obtained CaWO4 nanostructure with different
morphologies, which are recorded from 300 to 460 nm wave-
length. The curves 1–4 were obtained from samples 1–4,
respectively (as shown in Table 1). It is well known that the
intrinsic luminescence of CaWO4 arises from the radioactive
2À
recombination of electrons and holes at WO4 sites [22]. When
excited at 246 nm, the emission took place at 416 nm for these
samples.
However, careful observation revealed that the photolumines-
cence performances of the obtained CaWO4 nanostructure are
depended on their morphologies and sizes [23,24]. Compared
sample 2 (dumbbell-like shape) with sample 3 (coral-like shape)
and sample 4 (nanorods), there is a remarkable change of the
absolute luminescence intensity. This result might be attributed to
their different morphologies and crystalline nature. It is well
known that higher disposal temperature is benefit to the crystal-
linity of products. On the other hand, the reaction rate also has
important effect on the crystallinity, and lower reaction rate is
favorable for the crystal growth. As described above, dumbbell-like
crystals (sample 2) were produced at a lower concentration and
reaction temperature, which would be benefit for its crystallinity
than others. Moreover, sample 1 has the most absolute lumines-
cence intensity. The shapes of the spectrum of the CaWO4
superstructures are very similar to that of the CaWO4 nanopar-
ticles, but the fluorescence intensity of CaWO4 superstructures is
weaker than that of the nanoparticles. The reason is that the
CaWO4 superstructures are formed by the aggregation of the
nanoparticles. The different morphologies of CaWO4 have similar
spectra because that they are all consisted of the primary
nanoparticles. On the other hand, the primary nanoparticles of
the superstructures can grow via oriented attachment, the
fluorescence intensity of the superstructures decrease with the
particles size increasing. The results are well agreed with the
proposed growth mechanism of dumbbell-like superstructures of
CaWO4 crystals [25]. As to the exact photoluminescence property
of CaWO4 crystals need further to study.
3.3. Detailed structural characterization of the SrWO4 superstructure
Similarly, SrWO4 superstructures are also successfully obtained
by the reaction between Sr2+ and WO42À in CTAB microemulsions.
The morphology of the sample 5 is in dendrite structure. The XRD
pattern confirms that the products are pure SrWO4 crystals in a
Fig. 3. Schematic illustration of the formation of hierarchical CaWO4 at 10 8C, with
[Ca2+] = [WO42À] = 0.2 M.

Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–50
49
Fig. 4. (a and b) TEM and SEM images of the sample 3 obtained at 10 8C, with [Ca2+] = [WO42À] = 0.5 M, (c and d) TEM and FESEM images of the sample 4 obtained at room
temperature, with [Ca2+] = [WO42À] = 0.2 M.
˚
tetragonal structure with cell parameters of a = 5.41 A and
˚
c = 11.95 A, which is according with the JCPDS Card No. 8-490
(Fig. 1b). From Fig. 6a and b, the TEM and FESEM images clearly
shows that exclusive dendrite architecture is about 2–3
mm in
length, and the building block is about 40–50 nm. If the reaction
was carried out at room temperature for 1 h, dendrite like SrWO4
superstructure can also be obtained (Fig. 6c and d).
4. Conclusions
In summary, CaWO4 and SrWO4 nanostructures have been
synthesized via a simple microemulsion-mediated route. With
careful control of the fundamental experimental parameters
including the concentration of reactants, the reaction time and the
reaction temperature, CaWO4 with various morphologies of
dumbbell, coral, rods, and dendrite has been efficiently obtained,
respectively. The possible formation mechanism of these unique
morphologies also has been proposed based on surfactant self-
assembly under different experimental conditions. The as-
synthesized CaWO4 samples with various morphologies exhibited
different photoluminescence properties. This approach is
expected to provide a new general route for the controlled
synthesis of tungstate luminescence materials in restricted
dimensions.
Fig. 5. PL spectra of the CaWO4 nanostructures. Curves 1–4 were obtained from
samples 1–4 (as shown in Table 1).

50
Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–50
Fig. 6. (a and b) TEM and SEM images of the sample 5 obtained at 10 8C, with [Sr2+] = [WO42À] = 0.2 M, (c and d) TEM and FESEM images of the sample 4 obtained at room
temperature, with [Sr2+] = [WO42À] = 0.2 M.
[12] M.V. Nazarov, D.Y. Jeon, J.H. Kang, M.V. Zamoryanskaya, B.S. Tsukerblat, Solid
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Science Foundation of China (No. 20671061) and the Program for
New Century Excellent Talents of Education Ministry of China.
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