Synthesis, characterization and novel photoluminescence of SrWO4:Ln3+ nanocrystals

Article abstract of DOI:10.1166/jnn.2016.11848

SrWO₄:Ln³⁺ (Ln = Eu, Ce, and Tb) nanocrystals were successfully synthesized by a hydrothermal method, and were characterized by X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. The results indicated that the crystalline size of nanocrystals decreases with increasing Eu³⁺ concentrations and increases with increasing annealing temperature, gradually. The photoluminescence properties of SrWO₄:Ln³⁺ were investigated in detail. In the emission spectra of SrWO₄:Eu³⁺, the luminescence was dominated by ⁵D₀ → ⁷F₂ transition, indicating that Eu³⁺ occupied a site lacking inversion symmetry. The concentration quenching effect hardly occurs. In the excitation spectra of SrWO₄:Eu³⁺ nanocrystals monitored at 619 nm, the most intense peak is centered at 467 nm when the Eu³⁺ concentration was less than 10%, while the most intense peak is centered at 396 nm when the Eu³⁺ concentration was 15%. In the normalized emission spectra of SrWO₄:Ce³⁺/Tb³⁺ nanocrystals excited at 254 nm, the intensity ratio of the sharp emission peaks from Tb³⁺ ions to the broad emission band from Ce³⁺ ions increased with increasing Tb³⁺ concentration.

Full text of DOI:10.1166/jnn.2016.11848

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Synthesis, Characterization and Novel
Photoluminescence of SrWO4:Ln3+ Nanocrystals
Bingyu Xu, Mingxia Li, Kai Pan, Rong Li, Naiying Fan, and Guofeng Wang∗
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,
School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, China
SrWO4:Ln3+ (Ln = Eu, Ce, and Tb) nanocrystals were successfully synthesized by a hydrothermal
method, and were characterized by X-ray diffraction, transmission electron microscopy, and scan-
ning electron microscopy. The results indicated that the crystalline size of nanocrystals decreases
with increasing Eu3+ concentrations and increases with increasing annealing temperature, gradu-
ally. The photoluminescence properties of SrWO4:Ln3+ were investigated in detail. In the emission
7
spectra of SrWO4:Eu3+, the luminescence was dominated by 5D0 → F2 transition, indicating that
Eu3+ occupied a site lacking inversion symmetry. The concentration quenching effect hardly occurs.
In the excitation spectra of SrWO4:Eu3+ nanocrystals monitored at 619 nm, the most intense peak
is centered at 467 nm when the Eu3+ concentration was less than 10%, while the most intense
peak is centered at 396 nm when the Eu3+ concentration was 15%. In the normalized emission
spectra of SrWO4:Ce3+/Tb3+ nanocrystals excited at 254 nm, the intensity ratio of the sharp emis-
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sion peaks from Tb3+ ions to the broad emission band from Ce3+ ions increased with increasing
IP: 188.68.0.49 On: Mon, 13 Jun 2016 00:33:48
Tb3+ concentration.
Copyright: American Scientific Publishers
Keywords: SrWO4, Rare Earth, Nanocrystals, Luminescence.
1. INTRODUCTION
to date, several synthetic methods have been developed
to synthesize SrWO4 nanostructures, such as hydrothermal
method, the sonochemical method, moleten salt method,
microemulsion method, and solid state.18–25
In recent years, a great deal of research effort has
been devoted to the synthesis of inorganic compounds
doped with rare earth (RE) ions driven by the fact that
doped phosphors have potential applications in lasers,
optical telecommunication, optoelectronic devices, diag-
nostics, and biological labels.1–17 Especially, nano-sized
luminescent materials have been widely investigated for
their unique physical properties useful in developing novel
phosphors and building mini optoelectronic devices. It is
well known that host material is an important factor to
obtain high efficient fluorescent materials. So far, a range
of monodisperse RE ion doped oxide, fluoride, hydroxide,
vanadate, borate, phosphate, and thiooxide nanocrystals
have been successfully prepared.
In this paper, SrWO4:Ln3+ nanocrystals are synthe-
sized by CTAB-assisted hydrothermal method. The results
indicated that the crystalline size of nanocrystals grad-
ually decreases with increasing Eu3+ concentrations and
increases with increasing annealing temperature. The pho-
toluminescence properties of SrWO4:Ln3+ were investi-
gated in detail.
2. EXPERIMENTAL DETAILS
2.1. Synthesis
Analytical grade Ln(NO3ꢀ3 ·6H2O (Ln = Eu, Tb and Ce),
Na2WO4, Sr(NO3ꢀ2, hexadecyl trimethyl ammonium bro-
mide (CTAB), and ethylene glycol were obtained from
Beijing Chemical Reagents, China. All reagents and sol-
vents were used as received without further purification.
Deionized water was used throughout. In a typical synthe-
sis, 2.5 mmol of Sr(NO3ꢀ2 were separately dissolved in
15 ml ethylene glycol solution, and CTAB was added to
Metal tungstates (such as MgWO4, CaWO4, ZnWO4,
and CdWO4) represent a well-known class of inorganic
compounds which have been intensively investigated and
used in X-ray intensifying screens, fluorescent lamps,
microwave devices, optical fibers, and scintillators. Espe-
cially, SrWO4 is an important optoelectronic material. Up
∗Author to whom correspondence should be addressed.
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1533-4880/2016/16/3522/005
doi:10.1166/jnn.2016.11848

Xu et al.
Synthesis, Characterization and Novel Photoluminescence of SrWO4:Ln3+ Nanocrystals
Figure 3. (a) SEM and (b) TEM images of SrWO4:Eu3+ (5%) nano-
crystals without annealing.
Figure 1. (a) XRD pattern of SrWO4 nanocrystals without annealing.
(b–d) XRD patterns of SrWO4 nanocrystals after annealing at different
temperatures: (b) 400 ꢀC, (c) 600 ꢀC, and (d) 600 ꢀC.
scanning electron microscopy (SEM, Hitachi, S-4800)
and transmission electron microscopy (TEM, JEOL, JEM-
2010F). The luminescence spectra were recorded using
a Hitachi F-4600 fluorescence spectrophotometer. Raman
spectra were taken using Renishaw 1000 Micro-Raman
spectrometer. All experiments were performed at room
temperature.
the Sr(NO3ꢀ2 solution. The Na2WO4 solution was added
dropwise to the Sr(NO3ꢀ2 solution, followed by stirring
for 30 min. Subsequently, the resulting clear solution was
transferred to a 50 ml Teflon-lined autoclave, and heated
at 140 ꢀC for 12 h. The systems were then allowed to cool
to room temperate. The products were collected by cen-
trifugation, washed with distilled water and ethanol, and
3. RESULTS AND DISCUSSION
3.1. Crystal Structure and Morphology
Figure 1(a) shows the XRD pattern of pure SrWO4
nanocrystals without annealing. The position and rela-
tive intensity of all diffraction peaks are in good agree-
ꢀ
dried at 80 C in air. The as-prepared SrWO4 was then
annealed in air at 400–800 ꢀC for 2 h. SrWO4:Ln3+ nano-
crystals were prepared by the same procedure, except for
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addition of Ln(NO3ꢀ3 into the solution of Sr(NO3ꢀ2 at the
IP: 188.68.0.49 On: Mon, 13 Jun 2016 00:33:48
ment with the standard values for bulk tetragonal SrWO4
initial stage.
Copyright: American Scientific Publishers
(JCPDS 08-0490). No impurity is identified from the
XRD pattern. Figures 1(b)–(d) shows the XRD patterns
of pure SrWO4 nanocrystals after annealing at different
temperature. From the bottom to the top in Figure 1,
the XRD patterns become narrower and narrower, sug-
gesting that the crystalline size increases gradually with
increasing annealing temperature. Figure 2 shows the
XRD patterns of SrWO4:Eu3+ nanocrystals with differ-
ent Eu3+ concentrations after annealing at 600 ꢀC for
2 h. It was found that the XRD patterns become broader
with increasing Eu3+ concentrations, suggesting that the
2.2. Characterization
The crystal structure was determined by a Rigaku
(Japan) D/MAX-rA X-ray diffraction meter equipped
with graphite monochromatized Cu Kꢁ radiation (ꢂ =
1ꢃ541874 Å) keeping the operating voltage and cur-
rent at 40 kV and 40 mA respectively. The size and
morphology of the final products were investigated by
Figure 2. XRD patterns of SrWO4:Eu3+ nanocrystals with different
Eu3+ concentrations after annealing at 600 ꢀC: (a) 3%, (b) 5%, (c) 7%,
(d) 10%, and (e) 15%.
Figure 4. The emission spectra of SrWO4:Eu3+ (7%) nanocrystals after
annealing at 600 ꢀC excited at different wavelengths.
J. Nanosci. Nanotechnol. 16, 3522–3526, 2016
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Synthesis, Characterization and Novel Photoluminescence of SrWO4:Ln3+ Nanocrystals
Xu et al.
Figure 5. The excitation spectra of SrWO4:Eu3+ (7%) nanocrystals
Figure 7. The excitation spectra of SrWO4:Eu3+ nanocrystals with
different Eu3+ concentrations after annealing at 600 ꢀC monitored at
619 nm.
after annealing at 600 ꢀC monitored at different emission wavelengths.
crystalline size decreases with increasing Eu3+ concen-
trations. The corresponding SEM and TEM images of
SrWO4:Eu3+ (5%) nanocrystals without annealing are
shown in Figure 3. The high-resolution transmission elec-
tron microscopy (HRTEM) image of a single nanoparticle
is shown in the inset of Figure 3(b), which reveals the
single-crystals nature of the product.
7
5D0 → F3 transition exhibits a mixed magnetic dipole
5
7
and electric dipole character. The D0 → F4 is electric
dipole transition. It is well known that the intensity ratio
5
7
5
7
of D0 → F1 to D0 → F2 is strongly dependent on the
local symmetry of the Eu3+ ions. Therefore, Eu3+ ions
are often used as probes to detect local environments in a
3.2. Photoluminescence of SrWO4:Eu3+ Nanocrystals
5
7
matrix. In a site with inversion symmetry the D0 → F1
Figure 4 shows the emission spectra of SrWO4:Eu3+ (7%)
is dominating, while in a site without inversion symmetry
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ꢀ
5
7
nanocrystals after annealing at 600 C, excited at differ-
the D → F is the strongest. In the emission spectrum
IP: 188.68.0.49 On: Mon, 13 Jun 2016200:33:48
0
3+
5
7
Copyright: American Scientific Publishers
ent excitation wavelengths. It can be observed that the
emission intensities were the strongest when the excita-
of SrWO4:Eu (7%) nanocrystals, the D0 → F2 is much
stronger than the 5D0 → F1, indicating that Eu3+ occupies
7
5
7
tion was performed at 467 nm. The D0 → FJ (J = 1,
a site lacking inversion symmetry.
5
7
Figure 5 shows the excitation spectra of SrWO4:Eu3+
(7%) nanocrystals after annealing at 600 ꢀC monitored
at different emission wavelengths. The positions of these
excitation peaks are practically identical to the characteris-
tic absorption bands for f –f intra-configuration transitions
in trivalent europium. The most intense excitation peak is
centered at 467 nm.
2, 3, 4) transitions were observed. The D0 → F1 transi-
tion is magnetic-dipole-allowed and its intensity is almost
independent on the local environment around Eu3+ ions.26
5
7
The D0 → F2 transition is electric-dipole-allowed due to
an admixture of opposite parity 4f n−15d states by an odd
parity crystal-field component.27ꢄ28 Therefore, its intensity
is sensitive to the local structure around Eu3+ ions. The
Figure 6. The emission spectra of SrWO4:Eu3+ nanocrystals with dif-
Figure 8. The emission spectra of SrWO4:Eu3+ (7%) nanocrystals after
annealing at different temperatures excited at 397 nm.
ferent Eu3+ concentrations after annealing at 600 ꢀC excited at 397 nm.
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Xu et al.
Synthesis, Characterization and Novel Photoluminescence of SrWO4:Ln3+ Nanocrystals
Figure 9. Emission spectra of SrWO4:Ce3+ (10%)/Tb3+ (7%) nano-
crystals excited at different emission wavelengths.
Figure 11. The normalized emission spectra of SrWO4:Ce3+
(10%)/Tb3+ nanocrystals with different Tb3+ concentrations without
annealing excited at 254 nm.
Figure 6 shows the emission spectra of SrWO4:Eu3+
nanocrystals with different Eu3+ concentrations after
annealing at 600 ꢀC excited at 397 nm. The peak positions
and the shape of emissions were not influenced by Eu3+
concentration. The dependence of the intensity ratio of
3.3. Photoluminescence of SrWO4:Ce3+/Tb3+
Nanocrystals
Ce3+ doped ionic crystals have attracted increasing atten-
tion for their applications in scintillators and tunable lasers
in the ultraviolet and visible ranges. The luminescence
of Ce3+ ion is induced by the transitions from 5d to 4f
states. The 5d states strongly couple to the host mate-
rial because it is out of 5s and 5p shells, resulting in a
7
7
(5D0 → F2ꢀ to (5D0 → F1) on Eu3+ concentration indi-
cates that there are no any apparent change in the local
symmetry around Eu3+ ions with increasing Eu3+ con-
centration. The concentration quenching effect was not
observed.
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Figure 7 shows the excitation spectra of SrWO :Eu3+
broadening of the 5d state. Figure 9 shows the emission
IP: 188.68.0.494On: Mon, 13 Jun 2016 00:33:48
3+
spectra of SrWO4:Ce
(10%)/Tb3+ (7%) nanocrystals
nanocrystals with different Eu3+ concentrations after
Copyright: American Scientific Publishers
annealing at 600 C monitored at 619 nm. It is noted that
ꢀ
excited at different emission wavelengths. The broad emis-
sion bands are related to the f –d transitions of the Ce3+
ions, while the sharp peaks are related to the f –f transi-
tions of Tb3+ ions. Figure 10 shows the excitation spectra
of SrWO4:Ce3+ (10%)/Tb3+ (7%) nanocrystals monitored
at different emission wavelengths. Figure 11 shows the
normalized emission spectra of SrWO4:Ce3+ (10%)/Tb3+
nanocrystals with different Tb3+ concentrations without
annealing, excited at 254 nm. The results indicate that the
intensity ratio of the sharp emission peaks from Tb3+ ions
to the broad emission band from Ce3+ ions increases with
increasing Tb3+ concentration.
the most intense excitation peak is centered at 467 nm
when the Eu3+ concentrations were less than 10%, while
the most intense excitation peak is centered at 396 nm
when the Eu3+ concentrations was 15%. Figure 8 shows
the emission spectra of SrWO4:Eu3+ (7%) nanocrystals
after annealing at different temperatures. The lumines-
cence intensity increased with increasing annealing tem-
perature for experimental temperature range.
4. CONCLUSION
In summary, we successfully prepared the SrWO4 and
SrWO4:Ln3+ nanocrystals by using a CTAB-assisted
hydrothermal method. The results of X-ray diffrac-
tion investigations indicated that the crystalline size of
nanocrystals gradually decreases with increasing Eu3+
concentrations and increases with increasing anneal-
ing temperature. The photoluminescence properties of
SrWO4:Ln3+ were investigated in detail. The luminescence
5
7
was dominated by D0 → F2 transition in the emission
spectra of SrWO4:Eu3+, indicating that Eu3+ occupied a
site lacking inversion symmetry. The positions of emis-
sion peaks were independent of excitation wavelength, and
Figure 10. Excitation spectra of SrWO4:Ce3+ (10%)/Tb3+ (7%) nano-
crystals monitored at different emission wavelengths.
J. Nanosci. Nanotechnol. 16, 3522–3526, 2016
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Synthesis, Characterization and Novel Photoluminescence of SrWO4:Ln3+ Nanocrystals
Xu et al.
the concentration quenching effect was not detected. The
most intense excitation peak is centered at 467 nm in the
excitation spectra of SrWO4:Eu3+ nanocrystals monitored
at 619 nm when the Eu3+ concentration was less than
10%, while the most excitation intense peak is centered at
396 nm when the Eu3+ concentration was 15%. With the
increase of Tb3+ concentration, the intensity ratio of the
sharp emission peaks from Tb3+ ions to the broad emission
band from Ce3+ ions increased in the normalized emis-
sion spectra of SrWO4:Ce3+/Tb3+ nanocrystals excited at
254 nm.
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National Natural Science Foundation of China (21171052,
21471050, and 21473051), the Program for New Century
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for Innovative Research Team in University (IRT-1237),
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