Materials Sciences and Applicatio n, 2011, 2, 988-992
doi:10.4236/msa.2011.28133 Published Online August 2011 (
Copyright © 2011 SciRes. MSA
A Study of the Optical Properties in ZnWO4
Nanorods Synthesized by Hydrothermal Method#
Nguyen Van Minh1*, Nguyen Manh Hung1,2
1Center for Nano Science and Technology, and Department of Physics, Hanoi National University of Education, Hanoi, Vietnam; 1,2
Hanoi University of Mining and Geology, Dong Ngac, Tu Liem, Hanoi, Vietnam.
Email: *;
Received May 17th, 2011; revised April 20th, 2011; accepted June 7th, 2011.
We investigate the effect of synthesized time on the structure, as well as optical properties in ZnWO4 nano rod prepared
by hydrothermal method. The prepared rods were characterized by X-ray diffraction (XRD), scanning electron micros-
copy (SEM), Raman scattering, absorption and photoluminescent (PL) spectra techniques. The size and morphology of
ZnWO4 nano-rod can be controlled by adjusting the reaction time. The resultant sample is a pure phase of ZnWO4
without any impurities. The results showed that the optical property of ZnWO4 nanoparticles obviously relied on their
rod sizes.
Keywords: ZnWO4 Nano Rod, Raman Spectroscopy, Absorption, Photoluminescence
1. Introduction
Zinc tungstate (ZnWO4) with a wolframite structure has
been of practical interest for a long time because of its
attractive luminescence [1]. ZnWO4 has been applied as
a possible new material for microwave amplification by
stimulated emission of radiation [2], scintillator [3] and
optical hole burning lattice material [4], etc. Recently,
new applications for this material have emerged, includ-
ing large-volume scintillators for high-energy physics [5]
In particular, ZnWO4, also known by its mineral name
sanmartinite, is a wide-gap semiconductor, with band gap
energy close to 4 eV [6], and is a promising material for
the new generation of radiation detectors.
ZnWO4 has been prepared by different routes such as
the Czochralski method [7], sintering of WO3 and ZnO
or ZnCO4 powders [8], reaction in aqueous solution fol-
lowed by heating of the precipitate [9], heating of ZnO
thin films with WO3 vapor [10], sol-gel reaction [11],
and hydrothermal reaction over an extensive period [12].
However, ZnWO4 particles prepared by these routes are
relatively large in particle size and irregular in morphol-
ogy. Furthermore, higher calcining temperature is still
needed. It is very significant whether in fundamental or
applied field to explore new routes to ZnWO4, especially
for ZnWO4 crystallites with nanometer size, which would
have unique properties compared to traditional products
[13,14]. To obtain nanosized powders, the solid state
methods have several problems, because the WO3 has a
tendency to vaporize at high temperatures [15], nonho-
mogeneous compounds might be easily formed during
the solid-state and melting processing and the tempera-
ture for the solid state reaction is relatively high [16].
These problems could be solved by applying the hydro-
thermal method. However, a few studies on the chemical
synthesis of zinc tungstate by the hydrothermal method
have been reported. Furthermore, very few papers were
concerned with effects of the size and morphology on the
optical properties of ZnWO4 nanoparticles.
In this work, we report the synthesis of ZnWO4 nano
rod by hydrothermal method at a low temperature of
180˚C and investigate their structure, Raman scattering,
absorption and photoluminescence.
2. Experiment
Zinc tungstate (ZnWO4) nanoparticles were prepared by
the hydrothermal reaction of Zn(NO3)2·6H2O and
Na2WO4·2H2O at temperature of 180˚C, and various re-
action times (2, 4, 6 and 8 h). In a typical procedure for
the preparation of sample, Zn(NO3)2·6H2O (1 mmol) in
water (10 ml) was added Na2WO4·2H2O (1 mmol) in
water (20 ml) with vigorous stirring. H2O was added to
make 40 ml of the solution, and pH of the solution was
#This work has been supported by The Vietnam’s National Foundation
for Science and Technology Development (NAFOSTED).
A Study of the Optical Properties in ZnWONanorods Synthesized by Hydrothermal Method989
adjusted to 6.68, respectively, with dilute of 30%
NH3·H2O solution. The solution was then added into a
Teon-lined stainless steel autoclave of 100 ml capacity.
The autoclave was heated to 180˚C for 2, 4, 6 and 8 h,
respectively, without shaking or stirring. Afterwards, the
autoclave was allowed to cool to room temperature gradu-
ally. The white precipitate collected was washed with
distilled water four times. The solid was then heated at
80˚C and dried under vacuum for 2.5 h.
Structural characterization was performed by means of
X-ray diffraction using a D5005 diffractometer with Cu
Kα radiation. The FE-SEM observation was carried out
by using a S4800 (Hitachi) microscope. Raman meas-
urements were performed in a back scattering geometry
using Jobin Yvon T 64000 triple spectrometer equipped
with a cryogenic charge-coupled device (CCD) array
detector, and the 514.5 nm line of Ar ion laser. The ab-
sorption spectra were recorded by using Jasco 670 UV-vis
spectrometer and the room temperature luminescent
spectra were recorded on a spectrofluorometer (PL,
Fluorolog-3, Jobin Yvon Inc, USA).
3. Result and Discussion
3.1. Structure
Figure 1 shows the XRD patterns of ZnWO4 powders
heated for 2, 4, 6 and 8 h as a function of the reaction
time. It is noted that the ZnWO4 single phase could be
observed in all XRD patterns. All diffraction peaks of
ZnWO4 crystal appeared when the sample was prepared
at 180˚C, which could be easily indexed as a pure,
monoclinic wolframite tungstate structure according to
the standard card (JCPDS Card number: 73-0554). It was
found that, the optimum temperature for the production
Particle size (nm)
Time (h)
y( )
20 30 40 50 60 70
2-theta (degree)
Figure 1. XRD patterns of nanosamples with various reac-
tion times. The inset shows the particle size vs. reaction
of the high-quality crystal was as high as 180˚C [17].
The morphologies and microstructures of the samples
were then investigated with SEM. Figure 2 shows that
the morphologies and dimensions of the samples were
strongly dependent on the reaction time. SEM micro-
graph for the sample synthesized for 2 h was basically
irregular (Figure 2a). With the increase of the reaction
time to 4 h, the rod-shaped crystals can be seen (Figure
2b), and the rod size was in the range from several na-
nometers to several tens of nanometers. For 6 h, the
crystal was basically rod-like (Figure 2c). When the re-
action time was raised to 8 h, the rod-shaped crystals
grew larger and longer and a majority of the crystals
have exceeded 50 nm in length, the sizes of the rods be-
came homogeneous (Figure 2d). However, it was shown
that, for longer reactive times, such as 48 h, the crystal
phase instead became inhomogeneous, which could be
due to the breakage of the large crystal under such condi-
tions, suggesting the worse crystallinity [12]. The inset of
Figure 1 shows the average crystallite sizes for the
heat-treated powders calculated by XRD line broadening
method [18]. The calculated average crystallite sizes
were 22.02, 25.00, 26.05 and 26.00 nm for the heat-treat-
ed powders at 2, 4, 6 and 8 h, respectively. These are
corresponding to the SEM observation in Figure 2
showing an ordinary tendency to increase with the reac-
tion time from 2 h to 8 h. However, Zhao et al. [19] has
investigated the calcinations time and concluded that the
calcination time plays little effect on the crystal phase of
ZnWO4. This comment was contrary to our result.
3.2. Absorption Spectroscopy
Figure 3 shows a diffuse reection spectrum of ZnWO4
nanopowder. Steep shape of the spectra indicated that the
UV light absorption was due to the band-gap transition
instead of the transition from the impurity level. For a
Intensity (arb.units)
Figure 2. SEM images of the nanosamples synthesized in
different time: for 2 h (a), 4 h (b), 6 h.
Copyright © 2011 SciRes. MSA
A Study of the Optical Properties in ZnWONanorods Synthesized by Hydrothermal Method
990 4
8 h
2 h
Ab so rp tio n (a rb . u n its)
Band gap (eV)
Time (h)
250 300 350 400 450
Wavelength (nm)
Figure 3. Diffuse reection spectra of ZnWO4 nanopowders.
The inset shows the optic al band gap vs. reaction time.
crystalline semiconductor, the optical absorption near the
band edge follows the equation: ah
= A(h
where a,
, Eg and A are absorption coefficient, light
frequency, band gap, and a constant, respectively [20].
For the ZnWO4, n is determined to be 2. Thus, the band
gaps of the ZnWO4 nanopowders were roughly estimated
to be 3.78, 3.76, 3.73 and 3.72 eV, as shown in the inset
of Figure 3. Bonanni et al. [21] reported that the band
gap of ZnWO4 was 3.75 eV, which is in agreement to our
experimental values. However, the band gap becomes
narrower in the sample with longer reaction time. There-
fore, the crystallization degree may contribute to the ab-
sorption edge shift.
3.3. Raman Spectroscopy
Figure 4 illustrates the Raman spectra for ZnWO4
nanopowdes. It is clear that, the peak shifts to higher
frequency as increasing the reaction time. Studies of the
optical properties and the Raman spectra of ZnWO4 at
room temperature have been reported in the literature
[22]. ZnWO4 has the monoclinic wolframite structure
880 900 920
200 400 600 8001000
8 h
6 h
4 h
2 h
Intensity (Arb. units)
Raman shift (cm
Figure 4. Raman spectra of ZnWO4 nanopowders.
with C2h point group symmetry and P2/c space group. It
has two formula units per unit cell. The W-O interatomic
distance is substantially smaller than that of Zn-O, there-
fore, to a first order approximation, the lattices can be
separated into internal vibrations of the octahedra and the
external vibrations in which an octahedron vibrates as a
unit. A group theoretical calculation of the ZnWO4
structure yields 36 lattices modes, of which 18 are Ra-
man active (8Ag + 10Bg).
its) Absorption (arb.un
It is assigned to the Ag mode observed near 907 cm–1
since, as is the case of the regular octahedron, the sym-
metric stretch is expected to have the highest frequency
of all the internal modes. The Eg mode (asymmetric
stretch) of the regular octahedron splits into Ag + Bg by
the crystal field. Again, these modes are expected to have
frequencies that are higher than those of the bending
mode (T2g) of the regular octahedron. The obvious
choices are the Bg and Ag modes observed near 786 and
709 cm–1, respectively. The remaining modes 2Ag + Bg
with frequencies of 407, 342 and 190 cm–1 are assigned
to the T2g mode of the regular octahedron.
In a first attempt to identify the six internal stretching
modes of the W-O atoms in the distorted WO6 octahedra
of ZnWO4, Liu et al. [23] assigned them to the modes at
906, 787 and 407 cm–1 on the basis of the bond lengths
and Raman frequencies in the WO6 group. Afterwards,
Wang et al. [24] assigned the internal stretching modes
to the phonons observed near 906, 787, 709, 407, 342,
and 190 cm–1 on the basis of the temperature dependence
of the Raman frequencies. However, this assignment is in
contradiction with the fact that the frequencies of the
internal modes are expected to be higher than those of
the external modes. These authors argue in favor of their
assignment that the oxygen sharing between WO6 and
ZnO6 octahedra may cause a considerable overlap in the
frequency range for the two types of vibrations.
3.4. Photoluminescence
Figure 5 shows the representative PL spectra of the
ZnWO4 crystallites synthesized by the hydrothermal
method for 2, 4, 6 and 8 h. With the exited wavelength at
320 nm, the corresponding emission peaks centered at
~500 nm can be observed. Obviously, the ZnWO4 nano-
rods, prepared at the same calcining temperature (180˚C)
with a shorter holding time, exhibit lower emission in-
tensity than that from longer time. It implies PL proper-
ties of the ZnWO4 nano-rods are strongly affected by
their long scale. This broad emission band had a shoulder
in the blue region, indicating it consisted of more than
one emission band. The PL spectra were t to three peaks
using a Gauss function as shown in Figure 6.
The PL spectrum for the ZnWO4 lm also consisted of
an emission band at 2.50 eV (495 nm) and two emission
Copyright © 2011 SciRes. MSA
A Study of the Optical Properties in ZnWONanorods Synthesized by Hydrothermal Method991
400 500 600
8 h
6 h
4 h
2 h
PL intensity (arb. units)
Wavelength (nm)
Figure 5. PL spectra ZnWO4 nanopowders.
400 500 600 700
PL Intensity (Arb. units)
W avelength (nm)
Figure 6. The PL spectra were t to three peaks using a
Gauss function. To clarify, we shows only fitting for PL of
the sample with reaction time of 8 h.
bands at 2.80 eV (448 nm) and 2.28 eV (545 nm). To
clarify, we show only the fitting figure of the sample
with reaction time for 8 h.
It is well established that the complex and a
slight deviation from perfect order in the crystal structure
are responsible for the emission bands [25]. But there
exist different opinions concerning the origin of these
bands. Lammers [26] and Grigorjeva [27] believed that
the blue and green emissions originated from the intrinsic
complex with a double emission from one and the
same center (3T1u1A1g), whereas the yellow emission is
due to recombination of e-h pairs localized at oxygen
atom decient tungstate ions. However, Ovechkin [28]
ascribed the blue band to the self-trapped exciton in
tungstenite crystals with strong electron–phonon cou-
pling, and the green and yellow bands to the transitions
of T2uT2g and T1gT2g in the complex. Almost
all investigations in luminescent properties of ZnWO4
have been carried out for large crystals and lms. The
nanosized ZnWO4 prepared via a hydrothermal route in
the current work showed luminescent properties similar
to those of bulk ZnWO4. The luminescence intensities
varied from sample synthesized for 2 h to sample synthe-
sized for 8 h. Nanorods with a longer synthesized time
exhibited a strong luminescence, and nanorods with a
shorter synthesized time gave weak luminescence. These
results suggest that morphologies and sizes of nanoparti-
cles may aect their luminescence characteristics. Fur-
ther studies on PL property of the as-prepared ZnWO4
crystallites are in progress.
4. Conclusions
In conclusion, ZnWO4 nano-particles were successfully
synthesized at 180˚C by a hydrothermal route. The mor-
phology and dimension of the ZnWO4 crystallites were
affected by synthesized time. The optical band gap be-
comes narrower as increasing the reaction time. The im-
proved PL properties of the ZnWO4 crystallites can be
obtained with the increase of the rod scale. Because the
crystallite size of ZnWO4 for the nano rod is far large,
therefore, the absorption edge shift cannot result from
quantum size effect but prolonged rod.
5. Acknowledgements
This work has been supported by The Vietnam’s Na-
tional Foundation for Science and Technology Develop-
ment (NAFOSTED).
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