Facile preparation of strontium tungstate and tungsten trioxide hollow spheres

Article abstract of DOI:10.1111/j.1551-2916.2006.01140.x

A simple and low-cost technique was developed to produce unique SrWO ₄ and WO₃ hollow spheres. The SrWO₄ hollow spheres were prepared via a precipitation reaction between SrCl₂ and Na₂WO₄ in the presence of polymethacrylic acid, and the synthesis of WO₃ spheres used the as-prepared SrWO₄ hollow spheres as both precursors and templates. After soaking in HNO₃ and calcinations, the SrWO₄ were completely transformed into WO ₃ while the hollow structures were perfectly retained. The polymer-cation template model was proposed to describe the formation of the SrWO₄ hollow spheres. This technique is feasible in fabricating other inorganic hollow-structured materials.

Full text of DOI:10.1111/j.1551-2916.2006.01140.x

J. Am. Ceram. Soc., 89 [9] 2960–2963 (2006)
DOI: 10.1111/j.1551-2916.2006.01140.x
r 2006 The American Ceramic Society
ournal
J
Facile Preparation of Strontium Tungstate and Tungsten Trioxide
Hollow Spheres
Xiufeng Zhao, Teresa L. Y. Cheung, Xitian Zhang, and Dickon H. L. Ngw
Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
Jiaguo Yu
State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of
Technology, Wuhan, China
A simple and low-cost technique was developed to produce
unique SrWO and WO hollow spheres. The SrWO hollow
acrylonitrile, or acrylic acid with a certain type of cations (M).
This idea had inspired us to develop an alternative route for
preparing inorganic materials with hollow structures. If the M
on the surface of the in situ formed cross-links were able to react
with free anion species (X) in the reaction solution, it was
possible that these cross-links could be used as the so-called
‘‘polymer-cation templates’’ for the growth of inorganic MX
hollow structures.
4
3
4
spheres were prepared via a precipitation reaction between SrCl2
and Na WO in the presence of polymethacrylic acid, and the
synthesis of WO spheres used the as-prepared SrWO hollow
2
4
3
4
spheres as both precursors and templates. After soaking in
HNO and calcinations, the SrWO were completely trans-
formed into WO while the hollow structures were perfectly
3
4
3
retained. The ‘‘polymer-cation template’’ model was proposed to
describe the formation of the SrWO4 hollow spheres. This
technique is feasible in fabricating other inorganic hollow-
structured materials.
Inexpensive commercially available PMAA is highly soluble
in water, because its helical chain buries the hydrophobic back-
bone and shows only the hydrophilic –COOH ions on the outer
1
3
rim of its chain. It is therefore possible for the PMAA
macromolecules to form cross-links with the co-existing M in
the solution. In this work, PMAA was chosen to generate the
2
1
‘
‘polymer-cation template’’ with the divalent Sr , and the
I. Introduction
polymer-cation templating technique was demonstrated by
synthesizing hollow-structured SrWO4.
ICROMETER-SIZED and nanometer-sized inorganic hollow
spheres, with unique microstructures and large specific
M
The potential applications of SrWO
3
WO ) have attracted our attention. Scheelite-type SrWO
4
and tungsten trioxide
has
areas, have a wide range of applications in medicine release,
catalysis, sensing, filtering, chromatograph separation, optical,
(
4
been widely used in the optoelectronic industry and the solid-
state laser system due to its luminescence behavior and stimulat-
ing Raman scattering property. There is no report on the
1
–5
and electronic devices.
Techniques for preparing inorganic
hollow spheres are becoming subjects of intense research. Nu-
merous physical and chemical methods based on templating
processes had been developed to fabricate various inorganic
hollow spheres in glass, ceramic, semi-conducting and magnetic
14
4
fabrication of SrWO hollow spheres so far, although the
structural properties and optical characteristics of SrWO crys-
4
15–18
tals have been extensively investigated.
3
WO is an electro-
6
–10
materials, and biominerals.
Among these, a technique using
mixed polymer-surfactant to produce soft micelles templates
chromic, optochromic, and gaschromic material, which is widely
used and found in electrochromic windows, infrared switching
devices, photo-catalysis, writing–reading erasing optical devices,
9
,10
was particularly remarkable.
Both inorganic CaCO and Ag
3
hollow spheres were successfully prepared. In this approach, a
so-called double hydrophilic block copolymer polyethylene
oxide-block-poly methacrylic acid (PEO-b-PMAA) and a sur-
and gas sensors for detecting NO
2
, H S, NH
2
3 2 3
, H , O , and
19,20
21
Li et al. had synthesized WO
H
2
O.
3
nanometer-sized
hollow spheres via the hydrolysis of WCl using pre-synthesized
6
factant sodium dodecylsulfate were used. More recently, CaCO3
carbon micro-spheres as templates. However, considering the
hollow spheres were also synthesized by using another mixed
polymer-surfactant system, which was composed of polystyrene-
alt-maleic acid (PSMA) and cetyltrimethyl ammonium bro-
instability and toxicity of WCl
extensive studies in exploring new routes for preparing WO
hollow spheres remain a challenge. In this article, we report the
preparation of micrometer-sized SrWO hollow spheres based
6
and the high production cost,
3
1
1
mide. However, the relatively high costs of PEO-b-PMAA
and PSMA, and the needs of compatible surfactants, had
limited this technique to producing hollow structures on a small
scale. Thus, there is still a pressing need for exploring other
facile, general, and low-cost routes.
4
on the ‘‘polymer-cation’’ templating technique. In addition, a
novel route for the preparation of WO hollow spheres using the
3
as-prepared SrWO hollow spheres as precursors and templates
4
is also presented.
1
2
We had studied an early report on ionic elastomers. It was
suggested that ionic cross-links could be formed by the neutra-
lization of polymeric chains in the copolymer of butadiene,
II. Experimental Procedure
P. Guoma—contributing editor
Analytical-grade SrCl ꢀ 6H O (Beijing Chemical Industrial
2
2
Company, Beijing, China), Na WO
2
4
ꢀ 2H
2
O (International
Laboratory, Las Vegas, NV), and PMAA (Sigma Aldrich,
St. Louis, MO, 30% aqueous solution, M 6500) were used in
this work. In a typical procedure for preparing SrWO hollow
Manuscript No. 21579. Received March 13, 2006; approved April 18, 2006.
This work was supported by the United College–Lee Hysan Foundation, Endowment
Fund Research Grant Scheme (Project code: CA11066), and the RGC Earmarked Research
Grant (Project code: 2150421/4233-04E).
w
4
2
spheres, 10 mL SrCl solution (0.1 mol/L), 0.5 mL PMAA
wAuthor to whom correspondence should be addressed. e-mail: dng@phy.cuhk.edu.hk
solution (10 g/L), and 29.5 mL distilled water were mixed in a
2
960

September 2006
Communications of the American Ceramic Society
2961
beaker under magnetic stirring for 5 min. Then, 10 mL Na WO
2
4
solution (0.1 mol/L) was slowly added under vigorous stirring,
giving final concentrations of 0.02 mol/L for both Sr and
21
2
4
ꢁ
WO
, and 0.1 g/L for PMMA. The starting pH value of the
mixture was adjusted to 12 using 2 mol/L NaOH and HCl
solutions. After continuous vigorous stirring for 5 min, the
mixture was sealed and incubated for 12 h at room temperature.
Thereafter, the resulting white precipitate was collected, washed
with distilled water and ethanol, and dried in air. To prepare
WO
HNO
3
hollow spheres, the as-synthesized SrWO
4
was soaked in
3
(8 mol/L) for 12 h at room temperature, followed by
calcination at 5001C for 2 h, resulting in a yellow powder.
X-ray diffractometry (XRD) was performed using a Rigaku
RU-300 X-ray diffractometer (Spring, TX) with monochromatic
˚
CuKa radiation (l 5 1.5406 A). Scanning electron microscopy
(
SEM) and energy-dispersive X-ray spectrometry (EDS) were
performed with a LEO1450 VP scanning electron microscope
equipped with an EDS spectrometer. The Raman spectra of the
samples were obtained by a Renishaw RM 1000B spectrometer
with a radiation of 514.5 nm from an argon ion laser.
Fig. 2. Energy-dispersive X-ray spectrometry spectra of the as-pre-
4 3
pared (a) SrWO , and (b) WO .
III. Results and Discussion
The SrWO
4
samples were produced via a precipitation reaction
between SrCl (0.02 mol/L) and Na WO (0.02 mol/L) solutions
close spheres of SrWO spheres were also hollow. As an alter-
4
native to show that the as-prepared SrWO spheres were hollow,
4
2
2
4
in the presence of PMAA (0.1 g/L) at room temperature. Figure
(a) shows an XRD pattern of the SrWO product. All the
peaks were indexed, and it was confirmed that a pure tetragonal
we had produced WO hollow spheres by using these SrWO
3
4
1
4
spheres as precursors (to be shown later). This could provide
extra evidence that the SrWO product was hollow.
4
scheelite-type structured SrWO was produced. (JCPDS card
4
We propose the ‘‘polymer-cation template’’ model to describe
the mechanism in the formation of these SrWO hollow spheres.
˚
˚
no. 08-0490, a 5 5.4168 A, and c 5 11.951 A). EDS was used to
determine the composition of the prepared SrWO product. The
4
4
In this model, the PMAA macromolecules were cross-linked via
the electrostatic interaction between the ionized carboxylic acid
EDS result shown in Fig. 2(a) indicates that the product was
composed of Sr, W, and O with an atomic ratio of 1:1.10:3.88
for Sr, W, and O, which was close to the stiochiometric ratio of
ꢁ
21
groups (COO ) and the Sr cations in the reaction solution,
21
resulting in the formation of spherical ‘‘PMAA–Sr ’’ cross-link
micron-sized lumps. On the surface of these lumps, the Sr ions
readily reacted with the free-floating WO ions in the reaction
solution and formed SrWO crystals. As a result, nucleation and
21
SrWO
4
.
2
4
ꢁ
The SEM micrograph in Fig. 3(a) shows that the product
consisted of well-defined SrWO spheres, with size ranging from
4
4
2
an open SrWO sphere, revealing its hollow nature. The rugged
to 3 mm. A high-magnification SEM image in Fig. 3(b) shows
growth of SrWO crystallites occurred on the surface of the
4
4
spherical templates, and eventually, SrWO engulfed the entire
4
surface suggested that the hollow spheres were composed of
smaller crystallites. This observation was in agreement with the
grain size calculation based on the width of the (112) reflection
peak from the XRD pattern (Fig. 1(a)), which determined that
the size of the crystallites was about 15 nm. In Fig. 3(a), only a
cross-link lump and formed a closed shell. This proposal was
supported by the observed fact that the PMAA solution became
turbid immediately after the addition of SrCl , implying a strong
2
21
interaction between Sr and PMAA. In contrast, white pre-
cipitates could only be observed approximately 10 min after the
addition of Na WO solution. It has to be mentioned that the
few open SrWO
tempts had been made to produce more SrWO
4
spheres (about 0.5%) can be observed. At-
open spheres by
2
4
4
rest of the free-floating PMAA in the reaction solution acted as
an inhibitor. It functioned to halt the further growth of the
SrWO4 once they were formed, and prevented them from
aggregation. As can be seen in the SEM images of these samples,
sonication of the as-prepared precipitate, but it was not success-
ful. Although the hollow nature of the open spheres had been
observed by SEM, there was no solid evidence to verify that the
Fig. 3. Scanning electron microscopy images of (a) the typical as-
prepared SrWO product, (b) magnified image of a hollow sphere;
4
products obtained from controlled experiments, (c) when poly-
methacrylic acid (PMAA) was not present, and (d) when the pH value
was 4 in the presence of PMAA.
Fig. 1. X-ray diffraction patterns of the as-prepared (a) SrWO
WO samples.
3
4
, and (b)

2
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Communications of the American Ceramic Society
Vol. 89, No. 9
the range of the size of the spheres was relatively narrow, and no
aggregation was observed in the SrWO precipitate.
4
Some controlled experiments were performed to verify
whether both PMAA and a suitable pH value were vital to
the formation of the SrWO hollow spheres. When PMAA was
4
4
not present, the SrWO product did not appear spherical as
shown in Fig. 3(c). It was because no ‘‘polymer-cation tem-
plates’’ could be formed. Moreover, when the pH value was
adjusted from 12 to 4, no SrWO hollow spheres were formed as
4
shown in Fig. 3(d). It was evident that the formation of those
‘
‘polymer-cation templates’’ required a high degree of ionization
–
i.e., high concentration of COO ). Reducing the pH value of the
(
system would reduce the degree of ionization in PMAA. Thus,
was
21
the electrostatic interaction between PMAA and Sr
weakened, and the formation of the ‘‘polymer-cation templates’’
became difficult.
In the fabrication of the WO
SrWO spheres were used as the precursors. The SrWO
was bathed in HNO3 (8 mol/L), followed by calcination at
001C. Figure 1(b) shows the XRD pattern of the product. The
reflection peaks were indexed to a pure monoclinic WO
3
hollow spheres, the as-prepared
4
4
sample
5
Fig. 5. Raman spectra of (a) SrWO
4 3
, and (b) WO hollow spheres.
3
˚
˚
(
JCPDS card no. 83-0951, a 5 7.301 A, b 5 7.539 A, c 5 7.689
˚
A, and b 5 90.8931). The EDS spectrum shown in Fig. 2(b)
further confirmed that the product was WO . The quantitative
analysis indicated that the elemental ratio of W to O was 1:2.9.
Thus, SrWO had been successfully transformed to WO after
the nitric acid treatment and calcination.
The SEM image in Fig. 4(a) shows the as-prepared WO
hollow spheres with size ranging from 2 to 3 mm, which is similar
to that of the SrWO precursors. A closeup view of the outer
WO hollow spheres. As the WO spheres were proven to be
3
3
3
hollow, the hollow nature of the SrWO
However, it was possible that some PMAA cross-links would
have been enclosed inside the closed SrWO spheres. After the
HNO treatment and calcinations, all the polymer cross-links
3
inside the spheres should have been removed.
Raman spectroscopy was conducted on the as-prepared
4
spheres was confirmed.
4
3
4
3
SrWO and the WO hollow spheres. The scheelite-type struc-
4 3
4
surface of a WO hollow sphere is shown in Fig. 4(b). It reveals
3
tured tungstates such as CaWO , SrWO , PbWO , SrMoO ,
4
4
4
4
5
that each shell was composed of a large number of flake-like
nano-crystallites with a thickness of about 45 nm. From another
micrograph shown in Fig. 4(c), the thickness of the shell was
determined to be about 500 nm.
and CaMoO
4
are known to have strongly bonded XO
4
groups
(X5 W or Mo), but these groups were loosely bonded to the
cations. Thus, many of the internal vibration modes in these
tungstates were rather similar. The Raman spectrum of the
2
2
The SrWO
4
was completely transformed into WO . During
3
SrWO
4
hollow spheres is shown in Fig. 5(a). The bands located
at 920.9, 834.8, 797.9, 369.7, and 335.2 cm corresponded to
21
ꢁ1
the nitric acid treatment, the Sr was removed from SrWO
4
,
while the original hollow structures were retained. At the same
time, the calcination process removed the water content and
the vibration modes n (A ), n (B ), n (E ), n (B ), and n (A ) of
1
g
3
g
3
g
4
g
2
g
2ꢁ
the WO
were close to those of an early report in bulk samples.
Notably, the almost identical intensities of bands at 834.8 and
4
groups in the SrWO
4
crystal lattice, and these values
2
2
3
promoted the crystallization of WO to form stable shells. The
reactions are decribed as follows:
ꢁ
1
797.9 cm suggested that identical growth had occurred along
the a and b axes of the nano-crystallites in the SrWO hollow
þ
SrWO4 ðhollow sphereÞ þ 2H þ ðn ꢁ 1Þ H O
!
4
2
23
spheres. Figure 5(b) shows the Raman spectrum of the WO
3
2þ
ꢁ1
WO3 ꢀ nH2O ðhollow sphereÞ þ Sr
(1)
(2)
hollow spheres. The bands located at 805.3 and 709.3 cm were
related to the O–W–O stretching modes of the bridging oxygens,
ꢁ
1
and the bands at 325.4 and 268.8 cm corresponded to the O–
WO3 ꢀ nH2O ðhollow sphereÞ
Calcination
24
W–O bending modes. The weak band at 135.9 cm was
ꢁ1
25
attributed to the O–O deformation. The terminal W 5 O
stretching mode at around 950 cm was not observed, and
ꢁ1
ꢁ
!
WO3 ðhollow sphereÞ þ nH2O
2
4
this mode was associated with WO
3
ꢀ H
2
O. This result was
consistent with that of the XRD (Fig. 1(b)) in which no
WO ꢀ H O phase was detected.
The as-synthesized SrWO hollow spheres had not only acted
4
as precursors but also as ‘‘hard templates’’ for the formation of
3
2
IV. Conclusions
A foremost fabrication method to produce micrometer-sized
SrWO and WO hollow spheres has been developed. SrWO
hollow spheres were formed via a simple precipitation reaction
between SrCl and Na WO solutions in the presence of PMAA.
A ‘‘polymer-cation template’’ model was established to describe
the formation mechanisms. WO hollow spheres with flake-like
nano-crystallites growing perpendicularly to its surface were
produced by subsequent HNO treatment and calcination.
4
3
4
2
2
4
3
3
This study is of practical importance due to the commercial
availability and relatively low cost of PMAA, and the simplicity
of the method wherein no surfactant is required. It is expected
that this approach can be applied as a general and facile method
for large-scale production of other inorganic hollow-structured
materials. Furthermore, unusual WO hollow spheres have been
3
Fig. 4. Scanning electron microscopy images of the WO
spheres (a, b) before, and (c) after sonication.
3
hollow
facilely prepared by using pre-synthesized SrWO4 hollow
spheres as precursors and templates. This approach is novel

September 2006
Communications of the American Ceramic Society
2963
13
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14
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4
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