Growth process and optical properties of SrWO 4 microcrystal prepared by a microwave-assisted method

Article abstract of DOI:10.1080/15533174.2011.652283

A convenient microwave-assisted process has been applied to prepare Scheelite-type SrWO₄ microcrystals. The phase structures and morphologies of the products obtained at different time have been characterized by X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscopy. The results show that SrWO₄ with various morphologies including nanoparticles, nanorod, dumbbell, and sphere shapes were synthesized. A possible mechanism to illustrate the formation of the SrWO₄ structures was discussed. The examination of photoluminescence (PL) property revealed that their PL emission properties of the as-prepared SrWO₄ microcrystals can be dependent on the morphologies, degree of crystal, and sizes of SrWO₄.

Full text of DOI:10.1080/15533174.2011.652283

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Growth Process and Optical Properties of SrWO4
Microcrystal Prepared by a Microwave-Assisted Method
a b
a b
a b
Guizhen Wang
a
, Shiwei Lin
& Gengping Wan
Key Laboratory of Ministry of Education for Application Technology of Chemical Materials
in Hainan Superior Resources, The College of Materials and Chemical Engineering, Hainan
University , Haikou , P. R. China
b
Key Laboratory of Chinese Education Ministry for Tropical Biological Resources, Hainan
University , Haikou , P. R. China
Accepted author version posted online: 18 Apr 2012.Published online: 11 Jul 2012.
To cite this article: Guizhen Wang , Shiwei Lin & Gengping Wan (2012) Growth Process and Optical Properties of SrWO4
Microcrystal Prepared by a Microwave-Assisted Method, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal
Chemistry, 42:6, 888-891
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 42:888–891, 2012
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.652283
Growth Process and Optical Properties of SrWO4
Microcrystal Prepared by a Microwave-Assisted Method
1
,2
1,2
1,2
Guizhen Wang, Shiwei Lin, and Gengping Wan
1
Key Laboratory of Ministry of Education for Application Technology of Chemical Materials
in Hainan Superior Resources, The College of Materials and Chemical Engineering, Hainan University,
Haikou, P. R. China
2
Key Laboratory of Chinese Education Ministry for Tropical Biological Resources, Hainan University,
Haikou, P. R. China
And it was also verified to be an excellent Raman medium for
A convenient microwave-assisted process has been applied to the frequency conversion of the high-energy nanosecond laser
prepare Scheelite-type SrWO
microcrystals. The phase struc- pulses.[ Recently, many works have been focused on the syn-
6,7]
4
tures and morphologies of the products obtained at different time
have been characterized by X-ray diffraction, transmission elec-
tron microscopy, and field-emission scanning electron microscopy.
thesis of SrWO4 with different morphologies to enhance its
physical properties and applications.
coworker reported the morphology evolution of SrWO4 crystals
[8–11]
For instance, Huh and
4
The results show that SrWO with various morphologies including
nanoparticles, nanorod, dumbbell, and sphere shapes were syn- obtained using a simple precipitation method without any sur-
thesized. A possible mechanism to illustrate the formation of the factants or templates.[ Further, strontium tungstate nanorods
12]
SrWO
nescence (PL) property revealed that their PL emission properties
of the as-prepared SrWO microcrystals can be dependent on the
morphologies, degree of crystal, and sizes of SrWO
4
structures was discussed. The examination of photolumi-
with uniform morphology were controllably synthesized via a
facile liquid–solid–solution process through oil/water interface
by Wu et al.[ Nevertheless, it is still very important to de-
velop some other fast methods for the preparation of SrWO4
with controlled morphologies and properties in mild reaction
conditions.
4
13]
4
.
Keywords microcrystal, microwave radiation, optical properties,
SrWO
4
Microwave radiation has become an important tool in chem-
istry for its applications in the synthesis and modification of both
organic and inorganic materials. When microwave radiation is
supplied to chemical solutions, one or more of the components
dissolving in the solutions is capable of coupling with the ra-
diation. It can lead to higher heating rate than that achieved by
conventional method. Microwave radiation can solve the prob-
lems of temperature and concentration gradients.[ As a result,
this has provided the possibility to synthesize tungstate materials
in a short time.[
For the present research, we describe the controllable synthe-
sis of SrWO4 microcrystals through a facile and fast microwave-
assisted route and proposed a possible mechanism to illustrate
the formation of the SrWO4 structures. The as-prepared samples
exhibit some novel optical properties.
INTRODUCTION
Scheelite-type metal tungstates have received great atten-
tion because of their structural properties and wide applications
in photoluminescence (PL), solid state optical masers, opti-
cal fibers, scintillating materials, humidity sensors, catalysts,
14]
and so on.[
1–5]
In particular, as compared to other well-known
Scheelite-type tungstate materials, SrWO4 was predicted and
confirmed to be perspective for Raman converters, lasers, and
amplifiers by the study of spontaneous Raman spectroscopy.
15,16]
Received 24 August 2011; accepted 1 November 2011.
This work was supported by Program for New Century Excellent
Talents in University (NCET-09–0110), the Key Project of Chinese
Ministry of Education (210171), Analysis and Testing Program from
Collaborative and Shared Network of Hainan Large Instrument, and
Young Teacher Fund of the College of Materials and Chemical Engi-
neering in Hainan University.
Address correspondence to Guizhen Wang, Key Laboratory of Min-
istry of Education for Application Technology of Chemical Materials
in Hainan Superior Resources, The College of Materials and Chemical
Engineering, Hainan University, Haikou 570228, P. R. China. E-mail:
wangguizhen0@hotmail.com
EXPERIMENTAL
Sr(NO3)2, Na2WO4·2H2O, NaOH, and cetyltrimethylam-
monium bromide (CTAB) were of analytical grade and used
as received without any further purification. In a typical
experimental procedure, solution A was prepared by dissolving
0.5 mmol of CTAB in 100 mL of 5 mM Sr(NO3)2 aqueous
solution under magnetic stirring. Solution B was prepared
888

GROWTH PROCESS AND OPTICAL PROPERTIES OF SRWO4
889
by dissolving 0.5 mmol of CTAB in 100 mL of 5 mM
Na2WO4·2H2O aqueous solution under magnetic stirring. Two
solutions were mixed quickly under magnetic stirring at room
temperature. The mixture was placed in the center of an im-
proved household microwave oven (Galanz WP750, 2450 MHz,
750 W, China) with a refluxing system outside. 60% (450 W) of
the microwave-heated power was used to irradiate the mixture
for 6 min. Then, the microwave heating was terminated, and
the solution was cooled to room temperature. The products was
separated by centrifugation, washed with distilled water and
◦
absolute ethanol three times, and dried at 45 C in vacuum.
X-ray powder diffraction (XRD) analysis was performed on
a Bruker AXS D8 Advance diffractometer (Germany) with Cu
Kα radiation (λ = 1.54178 Å) using a 40 kV operation voltage
and 40 mA current. Scanning electron microscopy (SEM) was
recorded on a Jeol JSM-6700F field-emission scanning electron
microscope (FE-SEM, Japan). The transmission electron
microscopy (TEM) images were collected on a JEM2000EX
transmission electron microscope (JEOL, Japan) with an accel-
erating voltage of 160 kV. Fourier transform infrared (FTIR)
spectra were determined on a TENSOR 27 spectrophotometer
FIG. 1. XRD patterns of the products prepared at different times.
increased to 6 min. Large numbers of dumbbells with diameters
of 3–4 µm and lengths of 5–7 µm can be observed in Figure 2.
Ultimately, the SrWO4 sphere-like structures with diameters of
4
–9 µm were formed at 10 min shown in Figure 2. In addition, as
(Bruker, Germany) using KBr pressed disk. The Raman spectra
SEM images are shown in the inset to Figure 2, these dumbbells
and microspheres were constructed from the nanoparticles, in
which these highly oriented nanoparticles connect with each
other to form the present architectures.
On the basis of the previous experimental results and analy-
sis, we propose a possible mechanism to illustrate the formation
of the SrWO4 structures. The resultant SrWO4 microspheres can
be formed by templating the CTAB aggregates induced by the
microwave irradiation. The as formed micellar aggregates are
metastable and characterized by active ligands on the surface,
of the products were obtained by Raman spectrometer (Horiaba
Jobin Yvon T64000, France) with a radiation of 514.5 nm from
an argon ion laser. The PL was studied on a Hitachi F-7000
fluorescence spectrophotometer (Japan) with Xe lamp at room
temperature. The resulting products were dispersed in water
and measured in a standard quartz cuvette.
RESULTS AND DISCUSSION
The XRD patterns of SrWO4 microcrystal obtained at dif-
ferent reaction time are shown in Figure 1, and all diffraction
peaks of the products can be indexed to pure Scheelite-type
tetragonal SrWO4 phase with space group I41/a, in agreement
with the respective JCPDS (Joint Committee on Powder Diffrac-
tion Standards) card No. 08-0490. No peaks of other impurity
phases are detected from these patterns. The only difference is
that the diffraction intensity of the product obtained in longer
time is much higher, indicating that the better crystalline product
can be obtained with increasing aging time. This result shows
that the microwave irradiation method can accelerate the forma-
tion of crystalline SrWO4 microcrystal with low heat treatment
temperature and reduced processing time than the conventional
methods.[
5,12]
The typical morphology and structure of SrWO4 were further
characterized by TEM and SEM. It can be found that only
SrWO4 nanoparticles with mean diameters of 20–30 nm were
obtained when the reaction was carried for 0.5 min, as shown
in Figure 2. When the reaction time was prolonged to 2 min, it
can be seen from Figure 2 that the sample was dominated by
large amounts of nanorods with diameters of 500–600 nm and
lengths of 2–2.5 µm. Dumbbell-like superstructures of SrWO4
microcrystals were obtained as the reaction time was further
FIG. 2. SEM images of the products prepared at different times.

890
G. WANG ET AL.
SCH. 1. Schematic illustration for the possible formation mechanism of SrWO4 microspheres.
which act as temporary templates during the formation of the about 460 nm.[22–24] Here, SrWO products obtained at longer
4
sphere-like structure. A schematic illustration of the possible time (2 min, 6 min, 10 min) exhibit similar emission peaks at
formation mechanism is shown in Scheme 1. When the reac- about 457 nm. The emission peaks are considered to be from the
tion system containing Sr2+ and WO4 was mixed, the SrWO4
2−
1
1
2−
[25–27]
T → A transition of electrons within [WO ] anions.
2
1
4
nucleation was formed, and nanoparticles with CTAB coatings The shoulders are from some defects and/or impurities, and in-
were formed in a very short time. With the reaction proceed- terpreted as extrinsic transitions. But the SrWO prepared at
4
ing, the formed SrWO4 nanoparticles underwent mineralization
to form a relatively compact SrWO4 rod on the surface of the
CTAB aggregates. Then the dumbbell superstructures of SrWO4
were formed via a self-assembly process through the surfactant
interactions between the rod units. At last, prolonging the time
led the products to experience an Ostwald ripening process, and
then dumbbells grew up to microspheres.
Figure 3 shows FTIR spectra of SrWO4 synthesized in this
work. For Td symmetry, ν3(F2) and ν4(F2) are IR active and
correspond to stretching and bending modes, respectively.[
The spectra of SrWO4 obtained at different times shows a com-
17,18]
−1
mon characteristic band at 808 cm corresponding to W-O
stretching vibration. It is one of the internal modes specified as
[
17]
ν3(F2) antisymmetric stretching vibration.
(
The broad band
3600–3000 cm ) centered at 3454 cm can be assigned to
hydrogen bonded O-H stretching vibration arising from crystal
−1
−1
−1
water. The absorption band at 1635 cm on spectra refers to
the vibration of remainder H2O in present samples.
FIG. 3. FTIR spectra of the products prepared at different times.
The Raman spectra of the SrWO4 products are shown in Fig-
ure 4. Raman spectra of these products show five bands in the
−1
−1
range of 100–1200 cm . The Raman peak at 920.43 cm is
assigned to the mode of vibration ν1(Ag) from the symmetric
stretching of the WO2 group in the SrWO4 crystal lattice. The
–
4
−1
peaks at 836.53 and 796.90 cm correspond to the modes of vi-
bration, ν3(Bg) and ν3(Eg), from antisymmetric stretching of the
2
−
−1
WO group. The peaks at 370.87 and 334.71 cm correspond
4
to the modes vibration, ν4(Bg), and ν2(Ag), from antisymmetric
and symmetric bending, respectively, of the WO2 group.
−
[19,20]
4
Each vibration mode is in accord with Raman vibrations an-
[
12,20]
alyzed by other researchers
and no peaks in the Raman
spectra from other impurities were detected. Raman spectra
[
9,21,22]
provide the evidence of Scheelite-type structure.
The room-temperature PL spectra of the SrWO4 samples
with different time are shown in Figure 5, which were measured
from 400 to 600 nm using a 270 nm exciting wavelength. For
most emission spectra of SrWO4, their blue emissions were at
FIG. 4. Raman spectra of the products prepared at different times.

GROWTH PROCESS AND OPTICAL PROPERTIES OF SRWO4
891
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ferent time. The Fourier transform infrared spectra show W-O
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stretching vibration in [WO4]2 tetrahedrons at 808 cm for
SrWO4. Raman spectra provide the evidence of Scheelite-type
structure of SrWO4. SrWO4 obtained at longer time (2 min,
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1
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can play important roles in their emission properties.