The ZnWO4:Sm3+ compounds were prepared by hydrothermal method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering, absorption and photoluminescent (PL) techniques. This rare earth material presents high orange luminescence intensity under UV radiation. The excitation spectra of the compound presented broad bands arising from ligand-to-metal charge transfer (LMCT) (O®W and O®Sm3+) and narrow bands from 4f-intraconfigurational transitions. The emission spectra exhibited the 4G5/2®6HJ (J = 5/2, 7/2, 9/2 and 11/2) transition (direct excitation), for the system doped with Sm3+, while a broad band assigned to the LMCT (O®W) is observed when the excitation is monitored on the O®W LMCT state

An, Nguyen Manh

Hung, Nguyen Manh

Minh, Nguyen Van

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 22, No. 2 (2012), pp. 147-153
A STUDY OF THE OPTICAL PROPERTIES OF Sm-DOPED
ZnWO4 SYNTHESIZED BY HYDROTHERMAL METHOD
NGUYEN MANH HUNG
Center for Nano Science and Technology and Department of Physics,
Hanoi National University of Education
and
Hanoi University of Mining and Geology
NGUYEN MANH AN
Hong Duc University
NGUYEN VAN MINH
Center for Nano Science and Technology and Department of Physics,
Hanoi National University of Education
Abstract. The ZnWO4:Sm
3+ compounds were prepared by hydrothermal method and character-
ized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering, absorp-
tion and photoluminescent (PL) techniques. This rare earth material presents high orange lumi-
nescence intensity under UV radiation. The excitation spectra of the compound presented broad
bands arising from ligand-to-metal charge transfer (LMCT) (O→W and O→Sm3+) and narrow
bands from 4f-intraconfigurational transitions. The emission spectra exhibited the 4G5/2 →
6HJ (J
= 5/2, 7/2, 9/2 and 11/2) transition (direct excitation), for the system doped with Sm3+, while
the broad band assigned to the LMCT (O→W) is observed when the excitation is monitored on
the O→W LMCT state around 270 nm.
I. 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, including large-volume scintillators for high-energy physics
[5]. Besides that, the excited tungstate group may effectively transfer energy to rare earths.
In the past few years, there have been many papers concerned with the preparation and
photoluminescence properties of rare earth-doped tungstates crystals [6,7]. In these, the
trivalent samarium can generate red emissions in some tungstate crystals [7,8], that may
make the tungstates emit blue-green light and become a potential white-light phosphor.
However, Sm-doped tungstates in the forms of powder and film, especially Sm-doped
ZnWO4 material, have not been reported for the luminescence application.
ZnWO4 has been prepared by different routes such as the Czochralski method [9],
reaction in aqueous solution followed by heating of the precipitate [10], sol-gel reaction
148 OPTICAL PROPERTIES OF Sm-DOPED ZnWO4 SYNTHESIZED BY HYDROTHERMAL METHOD
[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 morphology.
One of the new routes to prepare ZnWO4 with nanometer size is hydrothermal method.
Although a few studies on the chemical synthesis of zinc tungstate by the hydrothermal
method have been reported, very few papers were concerned with effects of the size and
morphology and Sm dopant on the optical properties of ZnWO4 nanoparticles.
In this work, we report the synthesis of Sm doped ZnWO4 nanopowder by hy-
drothermal method at a low temperature of 180˚ C and investigate their structure, Raman
scattering, absorption and photoluminescence.
II. EXPERIMENT
Zinc tungstate (ZnWO4) nanoparticles were prepared by the hydrothermal reaction
of Zn(NO3)2.6H2O, Na2WO4.2H2O and Sm(NO3)3 at temperature of 180˚ C for 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) and Sm(NO3)3 in water (20 ml) with vigorous
stirring. H2O was added to make 40 ml of the solution, and pH of the solution was adjusted
to 6.68, respectively, with dilute of 30% NH3.H2O solution. The solution was then added
into a Teflon-lined stainless steel autoclave of 100 ml capacity. The autoclave was heated
to 180˚ C for 8 h without shaking or stirring. Afterwards, the autoclave was allowed to cool
to room temperature gradually. 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 measurements were performed in a back scat-
tering 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
absorption 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).
III. RESULT AND DISCUSSION
Fig. 1 shows the XRD patterns of Sm doped ZnWO4 powders heated for 8 h. 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), except the peak at 2θ = 270 for sample
with x ≥ 0.05. It was found that, the optimum temperature for the production of the
high-quality crystal was as high as 180˚ C [13].
The SEM micrographs of the ZnWO4 and Sm-doped ZnWO4 powders are shown in
Fig. 2, which indicates that the average particle size was about 200–500 nm for the ZnWO4
powders and about 150–400 nm for the Sm-doped powders. It is clear that the size was
decreased by the Sm substitution. This could be due to the inhibition in the crystalline
growth of the powders produced by the Sm substitution. It is generally believed that this
NGUYEN MANH HUNG, NGUYEN MANH AN, AND NGUYEN VAN MINH 149
20 30 40 50 60 70
x = 0.10
x = 0.05
x = 0.00In
te
n
si
ty
(a
rb
.
u
n
its
)
2-theta (degree)
(0
22
)
(2
02
)
(2
2
0)
(1
3
0)
(1
21
)
(2
0
0)(0
21
)
(0
20
)
(-
11
1)
(1
11
)
(1
10
)
(0
1
1)
Fig. 1. XRD patterns of Sm doped ZnWO4with x = 0.00 and 0.05
Fig. 2. SEM images of Sm doped ZnWO4 nanopowders
inhibition is associated with the crystal lattice distortion resulting from the larger ionic
radius of Sm3+ (1.04 A˚) compared with Zn2+ (0.74 A˚).
Fig. 3 shows a diffuse reflection spectrum of Sm doped 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 crystalline semicon-
ductor, the optical absorption near the band edge follows the equation: ahν = A(hν -
Eg)
1/n, where a, ν, Eg, and A are absorption coefficient, light frequency, band gap, and
a constant, respectively [14]. For the ZnWO4, n is determined to be 2. Thus, the band
gaps of the Sm doped ZnWO4 nanopowders were roughly estimated to be 3.74, 3.76 and
3.79, as shown in the inset of Fig. 3. Bonanni et al. [15] reported that the band gap of
ZnWO4 was 3.75 eV, which is in agreement to our experimental values. More further, in
the spectra of Sm doped powders appears an absorption peak around 405 nm. This would
originate from Sm dopant.
Fig. 4 illustrates the Raman spectra for Sm doped ZnWO4 nanopowdes. It is clear
that, the peak shifts to higher frequency as increasing the Sm content. Studies of the
optical properties and the Raman spectra of ZnWO4 at room temperature have been
150 OPTICAL PROPERTIES OF Sm-DOPED ZnWO4 SYNTHESIZED BY HYDROTHERMAL METHOD
300 350 400
x = 0.10
x= 0.05
x= 0.00
A
b
so
rp
tio
n
(a
rb
.
un
its
)
Wavelength (nm)
B
a
n
d
g
a
p
(e
V
)
Sm content
0.00 0.04 0.08
3.74
3.76
3.78
Fig. 3. Absorption spectra of Sm doped ZnWO4 nanopowders
reported in the literature [16]. ZnWO4 has the monoclinic wolframite structure 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, therefore, 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
Raman active (8Ag + 10Bg).
300 600 900
x=0.10
x=0.05
x=0.00
In
te
ns
ity
(a
rb
.u
ni
ts
)
Ramanshift (cm
-1
)
900
Fig. 4. Raman spectra of Sm doped ZnWO4 nanopowders. The right one is the
enlarged peak of 900 cm−1.
It is assigned to the Ag mode observed near 907 cm
−1 since, as in the case of the
regular octahedron, the symmetric 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
NGUYEN MANH HUNG, NGUYEN MANH AN, AND NGUYEN VAN MINH 151
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. [17] 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. [18] 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. For the Sm doped powders,
the peak shifts to higher wavenumber as increasing the Sm content and some new peaks
appear around 600 cm−1. This suggets the solubility of the Sm, although it can not be
abserved in XRD result.
Fig. 5a shows the representative PL spectra of the ZnWO4 crystallites synthesized
by the hydrothermal method for 8 h. With the exited wavelength at 324 nm, the cor-
responding emission peaks centered at ∼ 500 nm can be observed. This broad emission
band had a shoulder in the blue region, indicating it consisted of more than one emission
band. The PL spectra were fit to three peaks using a Gauss function as shown in Fig. 5a.
550 600 650 700 750
x =0.10
x =0.05
In
te
ns
ity
(a
rb
.u
ni
ts
)
Wavelength (nm)
(b)(a)
400 500 600 700
w−
In
te
n
si
ty
(a
rb
.u
n
its
)
Wavelength (nm)
Fig. 5. (a) PL spectra of the ZnWO4 synthesized by the hydro thermal method
for 8 h (the solid lines are fitting results by using Gauss distribution); (b) Emission
spectra of the Sm3+ doped ZnWO4 at room temperature
It is well established that theW 6−6 complex and a slight deviation from perfect order
in the crystal structure are responsible for the emission bands [19]. But there exist different
opinions concerning the origin of these bands. Lammers [20] and Grigorjeva [21] believed
152 OPTICAL PROPERTIES OF Sm-DOPED ZnWO4 SYNTHESIZED BY HYDROTHERMAL METHOD
that the blue and green emissions originated from the intrinsic W 6−6 complex with a
double emission from one and the same center (3T1u →
1A1g), whereas the yellow emission
is due to recombination of e–h pairs localized at oxygen atom deficient tungstate ions.
However, Ovechkin [22] ascribed the blue band to the self-trapped exciton in tungstenite
crystals with strong electron–phonon coupling, and the green and yellow bands to the
transitions of T2u →T2g and T1g →T2g in the W
6−
6 complex. Almost all investigations
in luminescent properties of ZnWO4 have been carried out for large crystals and films.
The nanosized ZnWO4 prepared via a hydrothermal route in the current work showed
luminescent properties similar to those of bulk ZnWO4. Further studies on PL property
of the as-prepared ZnWO4 crystallites are in progress.
The emission spectra of the Sm3+-doped compounds at room temperature, in the
range of 500-750 nm were shown in Fig. 5b. The samples doped with Sm showed an emis-
sion typical of samarium ions. These spectra show only the bands due to 4f-4f transitions
arising from the 4G5/2 emitting level. The relatively high intensity bands are those arising
from the 4G5/2.→
6HJ transitions (where J = 5/2, 7/2, 9/2 and 11/2), which are split in
the maximum number of (J+1/2) components, indicating that the Sm3+ ion occupies a
site with low symmetry. It is also observed that the forced electric dipole 4G5/2.→
6H9/2
transition presents the highest relative emission intensity at 643 nm.
IV. CONCLUSIONS
In conclusion, Sm doped ZnWO4 nano-particles were successfully synthesized at
180˚ C by a hydrothermal route. The morphology and dimension of the ZnWO4 crystallites
were affected by Sm doping.
The grain size of the powders was decreased with increasing Sm content. The
powders showed a broad excitation band and a broad deep blue-green emission band. The
Sm substitution resulted in emissions at 568, 603 and 642 nm.
ACKNOWLEDGMENTS
This work was supported by the National Foundation for Science and Technology
Development (NAFOSTED) of Vietnam and the Research Foundation – Flanders (FWO)
of Belgium (Code FWO.2011.23).
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Received 11 November 2011.


A Study of the Optical Properties of Sm-doped ZnWO4 Synthesized by Hydrothermal Method

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A Study of the Optical Properties of Sm-doped ZnWO4 Synthesized by Hydrothermal Method

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