Synthesis process and luminescence properties of Tm3+ in AWO4 (A = Ca, Sr, Ba) blue phosphors

Article abstract of DOI:10.1016/j.jallcom.2009.08.068

AWO₄:Tm³⁺ (A = Ca, Sr and Ba) blue phosphors were prepared via a mild and facile hydrothermal route. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curve were

Full text of DOI:10.1016/j.jallcom.2009.08.068

Journal of Alloys and Compounds 487 (2009) 758–762
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis process and luminescence properties of Tm3+
in AWO4 (A = Ca, Sr, Ba) blue phosphors
Jinsheng Liao∗, Bao Qiu, Herui Wen, Jinglin Chen, Weixiong You, Liangbin Liu
School of Material and Chemistry Engineering, Jiangxi University of Science and Technology, Ganzhou, Jiangxi 341000, China
a r t i c l e i n f o
a b s t r a c t
AWO4:Tm3+ (A = Ca, Sr and Ba) blue phosphors were prepared via a mild and facile hydrothermal route. X-
ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission
spectra and decay curve were used to characterize the AWO4:Tm3+ phosphors. By analyzing the excita-
tion and emission spectra of AWO4:Tm and undoped AWO4 samples, the result indicates that there exists
Article history:
Received 4 May 2009
Received in revised form 14 August 2009
Accepted 14 August 2009
Available online 22 August 2009
the energy transfer from the WO4 group to the Tm3+ ion only in CaWO4:Tm phosphor. The lumines-
2−
cence decay curves from the 1D2 state of Tm3+ in AWO4 phosphors show bi-exponential characteristics
when excited directly to the 1D2 state, implying that there exist two luminescence centers in AWO4:Tm
phosphors. Because AWO4:Tm3+ phosphors exhibit intensive blue emission and high color purity under
UV excitation, it is considered to be a new promising blue phosphor for field emission display application.
© 2009 Elsevier B.V. All rights reserved.
Keywords:
Optical material
Chemical synthesis
Luminescence
3
3
1D2 → F4 and 1G4 → H6 transitions [24–29]. Their luminescence
spectra consist of several very narrow bands, which make them
effective luminescent centers. These investigations of Tm3+-doped
materials mainly focused on host matrix, such as fluorides [25],
phosphates [27], and vanandates [28]. However, to the best of our
knowledge, there is no study that has been devoted to the lumines-
cence properties of AWO4:Tm phosphors.
1. Introduction
Recently, rare-earth activated inorganic material has been
widely employed in some important applications, such as fluores-
cent lamps [1–4], solid-state lasers [5–8], scintillators [9,10], and
field emission display (FED) [11–14]. It is very important to develop
novel phosphors for the important field of FED application [11].
However, there are not many examples of new phosphors which
have been designed for FED. It has been difficult to find a suitable
blue phosphor [12]. At present, the most efficient blue phosphors
contain sulfur in the host material composition [15,16], for exam-
ple, ZnS, ZnSO4 and Y2O2S. Although they have good luminescence
performances, the lifetime of the display is limited by the degrada-
tion of these sulfide phosphors during FED operation [11,12]. With a
motivation to develop a new blue phosphor, tungstate-based phos-
phor may emerge as an alternate choice for blue phosphor, due to
their superior chemical and thermal stability [17].
In this paper, AWO4:Tm blue phosphors were synthesized by the
mild hydrothermal method. The energy transfer from the WO4
2−
group to the Tm3+ ions is investigated in the AWO4:Tm phosphors
based on the luminescence properties of AWO4:Tm and undoped
AWO4 samples. The 1D2 excited-state dynamics of Tm3+ in AWO4
phosphors were analyzed by utilizing the double-exponential func-
tion. The blue emission in the visible range were observed in
AWO4:Tm phosphors. The high color purity of the luminescence
in AWO4:Tm phosphors was suggested as a blue phosphor for FED
application.
The alkaline-earth metal tungstates AWO4 (A = Ca2+, Sr2+, Ba2+
)
have been extensively investigated as a self-activating phosphor
emitting blue or green light under ultraviolet or X-ray excitation
[18,19]. The full-width at half-maximum of blue emission is broad
in AWO4 phosphor (about 100 nm) [18], which is disadvantage
to improve the color purity. At present, AWO4 were reported to
be efficient luminescent hosts for rare earth [20–23]. Tm3+ ion
is widely used as activator of blue emission corresponding to its
2. Experimental
2.1. Preparation
Analytical reagent (AR) chemicals of A(NO3)2 (A = Ca, Sr, and Ba), Na2WO4·2H2O,
citric acid, HNO3 and NaOH were purchased from Shanghai Chemical Industrial Co.,
and used as the starting materials without any further purification. A procedure for
the sample synthesis is typically described as follows: 0.25 mmol Tm(NO3)3·6H2O
with purity of 99.99% was first dissolved in deionized water under heating to obtain
solution. Meanwhile 4.5 mmol A(NO3)2 and 2.5 mmol citric acid as chelating agent
for the metal ions were added into the Tm(NO3)3 solution while stirring. Second,
5 mmol Na2WO4·2H2O was slowly added to the mixed solution until little white
precipitate formed (when the pH value of solution is about 3, white precipitate
∗
Corresponding author. Fax: +86 797 8312204.
E-mail address: jsliao1209@126.com (J. Liao).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.08.068

J. Liao et al. / Journal of Alloys and Compounds 487 (2009) 758–762
759
is polytungstate species). A well-controlled amount of NaOH solution was then
added with magnetic stirring to pH 9 to form a little white precipitate (white pre-
cipitate is monotungstate species) [30,31]. The precipitate was transferred into a
Teflon-lined stainless-steel autoclave, and heated to 180 ◦C for 12 h. Then the auto-
clave was cooled to room temperature naturally. The white powders obtained were
washed three times by dispersing in deionized water and ethanol and followed
by the centrifugation. The white powder products were dried in air at 100 ◦C for
2−
4 h. To investigate the energy transfer from the WO4 group to the Tm3+ ions in
AWO4:Tm phosphors, undoped AWO4 samples were prepared by the same proce-
dure.
2.2. Characterization
The crystal structural characterization and phase identification of the as-
prepared samples were carried out using
a PANalytical X’Pert Pro X-ray
diffractometer (XRD) with Cu K␣1 radiation (ꢀ = 0.154187 nm) for 2ꢁ rang-
ing from 10◦ to 80◦. The particle size and morphology of the samples
were inspected using scanning electronic microscope (SEM Philips XL30).
The photoluminescence spectra in the visible region and the luminescence
decay were recorded on an Edinburgh Instruments FLS920 spectrofluo-
rimeter equipped with both continuous (450 W) xenon and pulsed xenon
(nanosecond) lamps. All spectra were corrected for the intensities as well
as the line positions. All measurements were performed at room tempera-
ture.
Fig. 1. XRD spectra for AWO4:Tm (A = Ca, Sr and Ba) sample prepared by hydrother-
mal method.
Fig. 2. SEM image (left) and particle size distributions (right) of the AWO4:Tm (A = Ca, Sr and Ba) sample. (a) CaWO4:Tm, (b) SrWO4:Tm and (c) BaWO4:Tm.

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J. Liao et al. / Journal of Alloys and Compounds 487 (2009) 758–762
Fig. 3. Excitation spectra of AWO4:Tm (A = Ca, Sr and Ba) sample, monitoring the
Tm3+ emission at 453 nm.
Fig. 5. Emission spectra of AWO4:Tm (A = Ca, Sr and Ba) samples under direct exci-
tation of 358 nm at room temperature.
3. Results and discussion
for BaWO4:Tm phosphor become broader than those of CaWO4:Tm
and SrWO4:Tm phosphors (Fig. 1), it is indicated that the crys-
tal size of BaWO4:Tm phosphor is the smallest among AWO4:Tm
phoshors. SEM image (the left of Fig. 2) shows that the three sam-
ples composed of a number of monodispersed microparticles with
spherical shape. The particle size distribution (the right of Fig. 2)
was counted from SEM images. The distributions are very close
to the normal curves. They are in ranges of 1.75–5.22, 1.95–5.36,
and 1.48–3.97 ␮m with the average of 3.30 1.73, 3.48 1.70,
and 2.52 1.24 ␮m in diameter for CaWO4:Tm, SrWO4:Tm and
BaWO4:Tm phosphors, respectively. The average size of BaWO4:Tm
phosphors is smaller than that of CaWO4:Tm and SrWO4:Tm phos-
phors. The results of SEM agree with those of XRD.
Thecrystallinityand phasepurity oftheas-preparedsamples are
examined with XRD. Fig. 1 presents XRD patterns of the AWO4:Tm
(A = Ca, Sr and Ba) samples. From Fig. 1, it can be seen that all
diffraction peaks matched well the standard data of scheelite phase
AWO4 (A = Ca, Sr and Ba) (Joint Committee for Power Diffrac-
tions Standards, JCPDS card NO. 41-1431, 85-0587 and 43-0646).
Their strongest intensity peaks diffracted from the same plane
assigned as (1 1 2). They have scheelite structure with tetragonal
crystal system and I41/a space group. No additional diffractions
that could be attributed to impurities are observed. Therefore, all
as-prepared samples are pure phase. The peaks of the XRD patterns
Fig. 3 shows the excitation spectra of AWO4:Tm phosphors by
monitoring the room temperature emission at 453 nm due to the
3
1D2 → F4 transition. It can be clearly seen that the excitation spec-
tra of CaWO4:Tm phsophor consist of a broad band from 210 to
290 nm with a maximum at about 255 nm, which electrons are
excited from the oxygen 2p states to tungsten 5d states through
2−
the absorption of photons within the WO4
group [32]. In the
excitation spectra of SrWO4:Tm and BaWO4:Tm phosphors, the
relative intensities of charge-transfer band (W6+–O2−) are much
weaker than that of CaWO4:Tm phosphor. The results indicate that
CaWO4:Tm phosphor is the most effective for the energy transfer
group to the Tm3+ ions in AWO4:Tm phosphors
2−
from the WO4
(discussed in next paragraph). In the longer wavelength region, the
f–f transitions within the Tm3+ 4f12 configuration can be detected
with stronger intensity with respect to charge-transfer transition,
which is assigned as transition from the 3H6 ground state to the
1D2 excited state of Tm3+ located from 340 to 370 nm with several
sharp lines.
2−
To investigate energy transfer from the WO4
group to the
Tm3+ ions in AWO4:Tm phosphors, the excitation and emission
spectra for undoped AWO4 samples are shown in Fig. 4. It can be
clearly seen that CaWO4 samples have strong and broad excitation
peak at about 255 nm (39,215 cm−1). Since the energy gap between
the WO42− group and the excited state 3P2 (∼38,315 cm−1) of Tm3+
[33] is about 900 cm−1 and the maximum phonon energy of AWO4
crystal is about 900 cm−1 [18], the probability of the energy trans-
fer from the WO4 group to the excited state 3P2 of Tm3+ is great
2−
via one phonon-assisted process in the CaWO4:Tm phosphors. For
SrWO4 sample, the energy gap between its weak excitation peak
Fig. 4. Excitation (a) and emission (b) spectra of AWO4 (A = Ca, Sr and Ba) sample (for
excitation spectra, monitoring the WO4 group 440 nm; for emission spectra, the
excitation wavelengths are 255, 310, and 358 nm corresponding to CaWO4, SrWO4,
and BaWO4, respectively).
at 310 nm (32,258 cm−1) and the excited state 1D2 (27,933 cm−1
)
2−
of Tm3+ [33] is 4325 cm−1, the energy transfer from the WO4
2−
group to the excited state 1D2 of Tm3+ is nearly impossible occurred

J. Liao et al. / Journal of Alloys and Compounds 487 (2009) 758–762
761
Table 1
The double-exponential fitting parameters of AWO4:Tm phosphors.
AWO4:Tm
A1
ꢂ1 (␮s)
A2
ꢂ2 (␮s)
ꢂav (␮s)
CaWO4:Tm
SrWO4:Tm
BaWO4:Tm
342
120
214
0.122
0.118
0.111
215
139
216
1.69
2.34
2.52
1.53
2.25
2.42
0.065) corresponding to CaWO4:Tm, SrWO4:Tm, and BaWO4:Tm,
respectively.
The luminescence lifetime of the 1D2 state was measured
under the excitation at 358 nm, and the decay curve deviated
significantly from single exponential. It is suggested that the
non-exponential dynamics are contributed by two different lumi-
nescent Tm3+ centers. The process has also been observed in Yb3+
,
Er3+-codoped PbWO4 powder [34], which is the same scheelite
3
structure as AWO4. The decay curve for 1D2 → F4 (453 nm) of
Tm3+ ions can be well fitted into a double-exponential function as
I = A1 exp(−t/ꢂ1) + A2 exp(−t/ꢂ2) (ꢂ1 and ꢂ2 are fast and slow com-
ponents of the luminescence lifetimes, respectively. A1 and A2 are
the fitting parameters), and fitting curves are shown in Fig. 6 in
semilog scale. The fitting parameters are listed in Table 1. It can
be seen from Table 1 that the short lifetimes (ꢂ1) of AWO4:Tm are
nearly same (about 0.12 ␮s), while the long lifetimes (ꢂ2) are dif-
ferent. The short lifetimes are expected for the surface site because
the surface defects or impurity ligands such as OH− in samples
might act as the channels of nonradiative relaxation. The differ-
ence of the long lifetimes is expected for the lattice site because
Tm3+ ions locate different lattice environment. The average life-
times for 1D2 energy level of Tm3+ ions are about 1.53, 2.25, and
2.42 ␮s for CaWO4:Tm, SrWO4:Tm, and BaWO4:Tm, respectively,
as determined by the formula as ꢂav = (A1ꢂ12 + A2ꢂ22)/(A1ꢂ1 + A2ꢂ2)
[35].
For further identifying the two Tm3+ luminescence centers in
AWO4:Tm, the site-selective spectroscopy at low temperature, may
also be taken into consideration, for which additional measure-
ments would be required. These will be the subjects of our future
study.
4. Conclusions
In summary, we have prepared Tm3+-activated AWO4 (A = Ca,
Sr and Ba) blue phosphors by the mild hydrothermal method. The
SEM photograph demonstrated that the crystalline particles are
nearly monodisperse and spherical shape. By analyzing excitation
and emission spectra of AWO4:Tm and undoped AWO4 samples,
Fig. 6. Luminescence decay of 5D0 state of AWO4:Tm (A = Ca, Sr and Ba) sample
(excitation wavelength 358 nm, monitoring the Tm3+ emission at 453 nm).
the result indicates that there exists the energy transfer from the
group to the Tm3+ ion only in CaWO4:Tm phosphor. The
2−
via five phonon-assisted process in the SrWO4:Tm phosphors. For
BaWO4 sample, although the energy of its excitation peak at 358 nm
matches with the excited state 1D2 of Tm3+, the absorption peak of
WO4
blue emissions for the Tm3+ ions decay bi-exponentially, suggest-
ing that there exists two luminescence centers in the AWO4:Tm
phosphor, i.e. the surface and lattice sites. The characteristic of
Tm3+ ions of f–f transitions is observed upon UV excitation. Among
2−
BaWO4 sample is too weak to transfer the energy from the WO4
group to the excited state 1D2 of Tm3+. Therefore, the energy trans-
fer from the WO4 group to the Tm3+ ions is observed only in the
2−
3
these transitions, it is found that the 1D2 → F4 transition with
CaWO4:Tm phosphors.
blue emission is strong, while the other transitions are too weak
to affect the intensity and chromaticity of blue emission. There-
fore, the high color purity of luminescence and easy preparation
of these systems make them potential candidates for FED applica-
tion.
Fig. 5 shows the emission spectra of 5 at.% AWO4:Tm phosphors
under the excitation of continuous Xe-lamp irradiation (358 nm)
at room temperature. The strong blue emission band centered at
453 nm is observed from all samples. It corresponds to the emis-
sion from 1D2 state to the 3F4 state. The other very weak band is
3
also present at 650 nm assigned to 1G4 → F4 transition, while the
3
transition of 1G4 → H6 at about 470 nm is too weak to detect in
Acknowledgements
AWO4:Tm phosphors. Therefore, the strongest emission of Tm3+ in
3
AWO4 phosphors is 1D2 → F4 transition in blue region, showing
This work has been financially supported by Ganzhou City Public
Services Platform for Technical Innovation of Nonferrous Metal and
the National Natural Science Foundation of China.
that high color purity of the luminescence whose CIE chromaticity
color coordinates were (0.158, 0.064), (0.159, 0.063), and (0.157,

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J. Liao et al. / Journal of Alloys and Compounds 487 (2009) 758–762
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