Well-defined strontium tungstate hierarchical microspheres: Synthesis and photoluminescence properties

Article abstract of DOI:10.1166/jnn.2013.6856

Uniform and well-dispersed SrWO₄ microspheres have been successfully synthesized through a hydrothermal method by using trisodium citrate and SDS as surfactants. XRD and SEM results demonstrate that the as-synthesized SrWO₄ particles are high purity well crystallized and exhibit a relatively uniform spherical morphology. The as-obtained SrWO ₄:Ln³⁺ (Ln = Tb, Eu, Dy, and Sm) microspheres show intense light emissions with different colors coming from different Ln³⁺ ions under ultraviolet excitation, which might find potential applications in the fields such as light emitting phosphors, advanced flat panel displays, and light-emitting diodes (LEDs). Copyright

Full text of DOI:10.1166/jnn.2013.6856

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Journal of
Nanoscience and Nanotechnology
Vol. 13, 611–616, 2013
Well-Defined Strontium Tungstate
Hierarchical Microspheres: Synthesis and
Photoluminescence Properties
Guang Jia∗, Cuimiao Huang, Cuimiao Zhang, Shiwen Ding, and Liyong Wang
College of Chemistry and Environmental Science, Chemical Biology Key Laboratory of Hebei Province,
Hebei University, Baoding 071002, P. R. China
Uniform and well-dispersed SrWO4 microspheres have been successfully synthesized through a
hydrothermal method by using trisodium citrate and SDS as surfactants. XRD and SEM results
demonstrate that the as-synthesized SrWO4 particles are high purity well crystallized and exhibit a
relatively uniform spherical morphology. The as-obtained SrWO4:Ln3+ (Ln = Tb, Eu, Dy, and Sm)
microspheres show intense light emissions with different colors coming from different Ln3+ ions
under ultraviolet excitation, which might find potential applications in the fields such as light emitting
phosphors, advanced flat panel displays, and light-emitting diodes (LEDs).
Keywords: Strontium Tungstate, Microspheres, Hydrothermal Synthesis, Luminescence
Properties.
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1. INTRODUCTION
the dynamics of crystal growth and determining the final
morphology of the hierarchical nano-/microstructures dur-
ing the hydrothermal process.
During the past decades, self-assembled micrometer-sized
inorganic materials with special size, morphology, and
hierarchy were of special interest in the areas of materials
synthesis and device fabrication due to their hierarchi-
cal and repetitive structures, which show potential appli-
cations in wide fields of crystals, catalysis, diagnostics,
and pharmacology.1–4 In particular, three dimensional (3D)
hierarchical particles have attracted special attention due
to the practical importance related to some fractal growth
process. Because the physicochemical properties of crys-
tals depend not only on their chemical composition but
also on the morphology and size, controllable synthesis
of inorganic functional materials with high-ordered struc-
tures is very intriguing.5ꢀ6 Many synthesis routes have
been developed to fabricate the 3D inorganic hierarchical
particles, such as sol–gel processes, precipitation, com-
bustion, microemulsion, chemical vapor technique, and so
forth. In these fabrications, the hydrothermal method, as
a typical solution-based approach, has proved to be an
effective and convenient synthesis technique for prepar-
ing various inorganic materials with diverse morpholo-
gies and architectures. Moreover, the complexing agents
or surfactants always play an important role in controlling
Recently, the scheelite-type structured tungstates with
a general formula AWO4 (A = Ca, Sr, Ba, Pb) have
been extensively investigated due to their wide potential
industrial applications such as scintillators,7 phosphors,8–11
batteries,12 and solid-state lasers.13ꢀ14 Among these metal
tungstates, strontium tungstate (SrWO4ꢁ with typical
scheelite-type is widely used in optoelectronic industry
and solid state laser system due to its luminescence
behavior and stimulating Raman scattering property.15
In recent years, SrWO4 which has been widely applied
as eminent candidates for the matrix of lanthanide acti-
vator ions shows potential applications in optical displays
and light-emitting diodes (LEDs).16–19 To date, various
morphologies of SrWO4 have been prepared by differ-
ent synthesis routes, such as nanoparticles,20 nanobelts,21
nanopeanuts and nanorods,22 octahedron-like structure,23
and thin films.15ꢀ24 However, the synthesis of uniform and
well-dispersed SrWO4 microspheres has been rarely stud-
ied. On the other hand, it is well-established that the ideal
morphology of phosphor particles includes a perfect spher-
ical shape, narrow size distribution, and nonagglomera-
tion. The spherical morphology phosphors is good for high
brightness and high resolution because high packing den-
sities and low scattering of light can be obtained by using
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 1
1533-4880/2013/13/611/006
doi:10.1166/jnn.2013.6856
611

Well-Defined Strontium Tungstate Hierarchical Microspheres: Synthesis and Photoluminescence Properties
Jia et al.
such phosphors.25ꢀ26 So it is desirable to develop easily
controllable methods for the synthesis of spherical SrWO4
luminescent materials with promising novel luminescent
properties.
In the present work, uniform and well-dispersed SrWO4
hierarchical microspheres have been synthesized using a
facile hydrothermal process in the presence of trisodiun
citrate and sodium dodecyl benzenesulfonate (SDS). More-
over, the luminescent properties of lanthanide activator
ions Ln3+ (Ln = Tb, Eu, Dy, and Sm) doped SrWO4
samples, which may find potential applications in opti-
Fig. 1. XRD pattern of SrWO4 sample prepared with trisodium citrate
and SDS as surfactants. The standard data of tetragonal SrWO4 (JCPDS
No. 08-0490) is presented as a reference.
cal displays and light-emitting diodes, were investigated in
detail.
2.3. Characterization
2. EXPERIMENTAL DETAILS
The samples were characterized by powder X-ray diffrac-
tion (XRD) performed on a D8 Advance diffractome-
ter (Bruker). Fourier transform infrared spectroscopy
(FT–IR) spectra were measured with a Perkin–Elmer 580B
infrared spectrophotometer. The morphology and composi-
tion of the samples were inspected using JSM-7500F cold
field scanning electron microscope JEOL equipped with
an energy-dispersive X-ray spectrum. Photoluminescence
(PL) excitation and emission spectra were recorded with a
Hitachi F-4500 spectrophotometer. All measurements were
performed at room temperature.
2.1. Materials
Ln(NO3ꢁ3 (Ln = Eu, Dy, and Sm) and Tb(NO3ꢁ3 aqueous
solutions were obtained by dissolving Ln2O3 (99.99%) and
Tb4O7 (99.99%) in dilute HNO3 solution under heating
with agitation. All other chemicals were of analytical grade
and used directly without further purification.
2.2. Preparation
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In a typical synthesis, 2 mmol of Sr(Ac)2 · 0ꢂ5 H2O
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(0.429 g) was first dissolved in 20 mL of deionized
Copyright: American Scientific Publishers
water. Meanwhile 2 mmol (0.588 g) of trisodiun cit-
rate (C6H5Na3O7 · 2H2O) and 2 mmol (0.697 g) of
sodium dodecyl benzenesulfonate (SDS, C18H29NaO3S)
were added into the above solution under stirring. Subse-
quently, 4 mmol of Na2WO4 solution (1.320 g of Na2WO4 ·
2H2O dissolved in 15 mL of deionized water) was slowly
added to the above mixed solution. A well-controlled
amount of HAc solution was then added with magnetic
stirring to pH 5. After additional agitation for 10 min,
the precursor solution was transferred ꢀinto a Teflon-lined
autoclave, sealed, and heated to 200 C for 24 h. Then
the autoclave was cooled to room temperature naturally.
As the autoclave cooled to room temperature naturally, the
precipitate was separated by centrifugation, washed with
deionized water and ethanol in sequence, and then dried
in air to obtain the final sample.
3. RESULTS AND DISCUSSION
Figure 1 shows the XRD pattern of the SrWO4 sample pre-
ꢀ
pared at 200 C for 24 h with trisodium citrate and SDS
as surfactants. From Figure 1, one can see that the diffrac-
tion peaks agree well with the reported data of scheel-
ite structure SrWO4 with tetragonal phase [JCPDS Card
No. 08-0490, Space group: I41/a (88)]. No other addi-
tional peaks can be detected, indicating that the simple
hydrothermal method is a feasible route to prepare pure
phase of SrWO4. It should be mentioned that the diffrac-
tion peaks of SrWO4:Ln3+ (Ln = Eu, Tb, Dy, and Sm)
samples can also agree well with tetragonal scheelite-type
SrWO4, indicating that the Ln3+ ions have been effec-
tively doped into the SrWO4 host lattice. Moreover, the
XRD patterns of the samples prepared by solely adding
trisodium citrate or SDS also coincide with scheelite-type
SrWO4 (not shown). Moreover, it can also be seen that the
diffraction peaks of the SrWO4 sample are very strong and
sharp, so we can say this product has high crystallinity. It
is joyous, because when the phosphor has high crystallinity
it may has less traps and exhibit stronger luminescence.
The energy dispersive X-ray (EDX) spectrum was fur-
ther used to investigate the as-obtained SrWO4 sample
(Fig. 2). The EDX spectrum of the sample confirms the
presence of strontium (Sr), tungstate (W), and oxygen (O)
A similar process were employed to prepare Eu3+
,
Tb3+, Dy3+, and Sm3+ doped SrWO4 samples except for
adding a stoichiometric amount (5 mol%) of Tb(NO3ꢁ3,
Eu(NO3ꢁ3, Dy(NO3ꢁ3, and Sm(NO3ꢁ3 aqueous solution
instead of Sr(Ac)2 at the initial stage [0.1 mmol of
Ln(NO3ꢁ3 solution (0.1 M, 1 mL; Ln = Tb, Eu, Dy, or
Sm) was added into the Sr(Ac)2 solution which contained
1.9 mmol of Sr(Ac)2 ·0ꢂ5 H2O (0.408 g)]. For comparison,
the experiment was performed to prepare SrWO4 sample
via a similar process by solely adding trisodium citrate or
SDS as surfactant.
612
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Jia et al.
Well-Defined Strontium Tungstate Hierarchical Microspheres: Synthesis and Photoluminescence Properties
Fig. 2. EDX spectrum of the as-obtained SrWO4 sample.
elements. No other impurity peaks can be detected, which
can effectively support the XRD result of the sample.
Figure 3 shows the FT–IR spectrum of as-synthesized
SrWO4 sample. The absorption bands at 3423 cm−1 and
1622 cm−1 are assigned to O H stretching vibration and
H
O
H bending vibration,27ꢀ28 which are the charac-
teristic vibrations of water absorbed on the surface of
SrWO4 sample. A small band at 1406 cm−1 is charac-
teristic of the symmetrical vibrations of the carboxylate
groups, which proved the citric ions capping on SrWO4
sample. The absorption band centered at 2367 cm−1 is
attributed to the atmospheric absorbed CO on the sur-
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face of SrWO sample. The sharp band at 808 cm−1 is
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W O stretches of the WO4 tetrahedron,
4
due to the O
and the band at 442 cm−1 is assigned to the Au mode
of WO42–.20ꢀ28ꢀ29 The result agrees well with the XRD and
EDX analysis, which provides additional evidence for the
formation of the SrWO4 product.
The SEM and TEM images were used to characterize
the morphology of as-prepared SrWO4 samples. It should
be mentioned that doping a small amount of lanthanide
ions (Tb3+, Eu3+, Dy3+, and Sm3+ꢁ in the SrWO4 host
does not change the crystal phase and morphology of the
SrWO4 products in our present work. So here we only
Fig. 4. (a), (b) SEM and (c) TEM images of the SrWO4 sample pre-
pared with trisodium citrate and SDS as surfactants.
take pure SrWO4 sample as an example to illustrate the
morphology of the products. From the low-magnification
SEM images of the SrWO4 sample with trisodium citrate
and SDS as surfactants, it can be seen that the as-obtained
SrWO4 sample consists of a large scale of uniform and
well-dispersed microspheres (Fig. 4(a)). The diameters of
the microspheres are about 4 ꢃm. From the enlarged SEM
image, one can observe that the surface of the hierarchi-
cal microsphere is not smooth and an individual sphere
is not a single particle but the assembly of tiny packed
nanocrystallites (Fig. 4(b)). To provide further insight into
the SrWO4 composite particles, TEM investigation was
also performed (Fig. 4(c)). The TEM images of the SrWO4
sample exhibit the spherical morphology with good disper-
sion and the diameters of the microspheres are about 4 ꢃm,
which agrees well with the SEM images. The perfect
Fig. 3. FT–IR spectrum of SrWO4 sample.
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Well-Defined Strontium Tungstate Hierarchical Microspheres: Synthesis and Photoluminescence Properties
Jia et al.
Fig. 5. SEM images of the SrWO4 samples prepared with (a), (b)
trisodium citrate or (c), (d) SDS as surfactant.
spherical phosphor particles may have potential to be an
excellent host material for luminescent materials.25ꢀ26
In order to indentify the influence of the surfactants, the
SrWO4 samples were prepared by solely adding trisodium
citrate or SDS as surfactant during the hydrothermal pro-
cess. Figures 5(a), (b) shows the SEM images of SrWO4
sample prepared with trisodium citrate as surfactant. It can
be seen that the sample consist of microspheres and irreg-
Fig. 6. Photoluminescence (a) excitation and (b) emission spectra of
ular particles with broad size distribution. The SrWO
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4
SrWO4:Tb3+ sample.
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sample prepared with SDS as surfactant is mainly com-
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posed of microspheres with diameters of about 3.5 ꢃm,
5
5
5D4–7F6, D4–7F4, and D4–7F3 transitions of Tb3+ ions,
respectively (Fig. 6(b)). From the emission spectra, it can
be seen that no emission from the WO24– group is observed,
suggesting that the energy transfer from WO24– to Tb3+ is
very efficient.
but the sample becomes badly-dispersed in distribution
due to the aggregation of microspheres (Figs. 5(c), (d)).
On the basis of the results mentioned above, it is pre-
sumed that the trisodium citrate and SDS may exhibit a
synergistic effect in controlling the crystal growth and self-
assembly of SrWO4 sample during the hydrothermal pro-
cess, resulting in the uniform and well-dispersed SrWO4
microspheres.
The excitation spectrum of SrWO4:Eu3+ sample consists
of a broad band with a maximum at about 281 nm and
some sharp peaks between 350 and 500 nm (Fig. 7(a)).
The broad band at 281 nm can be assigned to the charge
transfer state from the excited 2p orbits of O2– to the
empty orbits of the central W6+ of the WO42– groups. The
dominant sharp peaks at 394 and 464 nm can be attributed
to the 7F0–5L6 and 7F0–5D2 transitions within the 4f 6 con-
figuration of Eu3+ ions, respectively.30 Upon excitation at
394 nm, the emission spectrum of SrWO4:Eu3+ sample is
composed of a group of lines at about 593, 617, 656, and
The photoluminescence (PL) properties of the as-
synthesized SrWO4:Ln3+ (Ln = Tb, Eu, Dy, and Sm) sam-
ples were characterized by the PL excitation and emission
spectra (Figs. 6, 7). The excitation spectra of SrWO4:Tb3+
sample consist of a broad band centered at 250 nm and
some weak peaks between 300 and 500 nm (Fig. 6(a)),
1
which are ascribed to the transition from the A1 ground-
1
state to the high vibration level of T2 within the WO42–
5
703 nm (Fig. 7(b)), which can be attributed to D0–7FJ
group and f –f transitions of Tb3+ ions.19 The absorption
intensity of the Tb3+ f –f transitions is very weak in com-
parison with that of the WO24– group, indicating that the
excitation of the Tb3+ ions is mainly through the energy
transfer from the WO42– group to Tb3+ ions. Upon exci-
tation into the WO24– group at 250 nm, one can see that
SrWO4:Tb3+ sample shows the most intense emission peak
(J = 1ꢀ2ꢀ3ꢀ4) transition lines of the Eu3+ ions, respec-
tively. The emission spectra are dominated by the 5D0–7F2
(617 nm) transition of Eu3+, which is an electric-dipole
allowed transition and hypersensitive to the environment.
The excitation spectra of SrWO4:Dy3+ and SrWO4:Sm3+
samples also consist of a broad band of WO24– groups
and the f –f transitions of Dy3+ and Sm3+ ions, which
are similar to that of SrWO4:Eu3+ sample (not shown).
The emission spectrum of SrWO4:Dy3+ sample consists of
5
corresponding to the green D4–7F5 hypersensitive tran-
sition of Tb3+ located at 544 nm. Three other peaks are
observed at 491, 589, and 624 nm, which correspond to the
614
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Jia et al.
Well-Defined Strontium Tungstate Hierarchical Microspheres: Synthesis and Photoluminescence Properties
Fig. 7. (a) Excitation and (b) emission spectra of SrWO4:Eu3+ sample and emission spectra of (c) SrWO4:Dy3+ and (d) SrWO4:Sm3+ samples.
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three emission lines at 487, 574, and 665 nm, which are
corresponds to the G5/2–6HJ (J = 5/2ꢀ7/2ꢀ9/2ꢀ11/2)
with the previous literatures.31ꢀ32
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assigned to the 4F9/2–6H15/2, 4F9/2–6H1C3/2opyright9:/A2 me1r1ic/2an Scientific Publishers
transition of Sm3+ ions (Fig. 7(d)). The result agrees well
, and 4F –6H
transition lines of Dy3+ ions (Fig. 7(c)), respectively. The
SrWO4:Sm3+ sample exhibits the characteristic emission
lines of Sm3+ at 564, 606, 647, and 706 nm, which
The SrWO4:Ln3+ (Ln = Tb, Eu, Dy, and Sm) samples
exhibit strong green, red, yellow, and orange–red emis-
sions under ultraviolet excitation, which can be confirmed
by the CIE (Commission Internationale de I’Eclairage
1931 chromaticity) coordinates for the emission spectra of
the SrWO4:Ln3+ samples (Fig. 8). The chromaticity coor-
dinates of SrWO4:Ln3+ (Ln = Tb, Eu, Dy, and Sm) sam-
ples are calculated to be x = 0ꢂ251, y = 0ꢂ602; x = 0ꢂ606,
y = 0ꢂ338; x = 0ꢂ429, y = 0ꢂ442; and x = 0ꢂ511, y = 0ꢂ356,
located in the green, red, yellow, and orange–red region,
respectively. On the basis of the above results, it can be
concluded that the PL emission colors of the uniform
SrWO4:Ln3+ microspheres can be tuned from green to yel-
low to red by doping different lanthanide activator ions.
4. CONCLUSIONS
In summary, a facile hydrothermal method has been devel-
oped to fabricate well-defined SrWO4 microspheres. The
crystal structure, morphology, element analysis, and lumi-
nescence properties were characterized by XRD, FT−IR,
EDX, SEM, and PL, respectively. Under ultraviolet light
excitation, the Tb3+-, Eu3+-, Dy3+-, and Sm3+-doped
SrWO4 phosphors show intense green, red, yellow, and
orange-red emissions, respectively. The as-obtained uni-
form and well-dispersed SrWO4:Ln3+ phosphors may be
Fig. 8. CIE chromaticity diagram of SrWO4:Ln3+ samples: (a) Tb3+
(b) Eu3+, (c) Dy3+, and (d) Sm3+
,
.
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Well-Defined Strontium Tungstate Hierarchical Microspheres: Synthesis and Photoluminescence Properties
Jia et al.
potentially applied in optical displays and light-emitting
diodes (LEDs). Furthermore, this general and facile
method may be of much significance in the synthesis of
other tungsten functional materials.
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Acknowledgments: This work is financially supported
by the Natural Science Foundation of Hebei Province
(Grant No. B2012201074), University Scientific Research
Project (Grant No. 2011171) and Outstanding Youth
Fund Project (Grant No. Y2012007) of Hebei Education
Department, and Research Plans of the Natural Science
Foundations (Grant Nos. 2010–200, 2012–234) and Labo-
ratory Open Foundations (Grant Nos. 2012015, 2012025)
of Hebei University.
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Received: 17 May 2012. Accepted: 29 September 2012.
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