Enhanced luminescence of novel Y2Zr2O 7:Dy3+ phosphors by Li+ co-doping

Article abstract of DOI:10.1016/j.jallcom.2012.10.074

Novel blue-white Y₂Zr₂O₇:Dy³⁺, Li⁺ phosphors have been prepared by a facile and efficient sol-gelcombustion method at 800 °C, using ammonium nitrate as a fuel and citric acid as a complexing agent. No additional annealing was required, as crystalline Y₂Zr₂O₇ resulted directly from the combustion reaction. The structure and morphology were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The photoluminescence properties were measured using an Edinburgh FLS920 at room temperature. Stable blue-white emission from 425 nm to 625 nm has been observed from Y ₂Zr₂O₇:Dy³⁺ and Y₂Zr ₂O₇:Dy³⁺, Li⁺ phosphors. The optimal doping concentration of Dy3+ ions is 2 mol%. Experimental analysis and theoretical calculations reveal that the dipole-quadrupole (d-q) interaction should be the major concentration quenching mechanism of Dy³⁺ ions emission. Li⁺ co-doping improves luminescence intensity as well as crystallinity significantly. The incorporation of Li⁺ ions into Y₂Zr₂O₇ lattice could induce a remarkable increase of photoluminescence intensity by about 2 times. The mechanism of the enhanced emission by Li⁺ doping was discussed. Both the chromaticity coordinates of Y₂Zr₂O₇:Dy³⁺ and Y₂Zr₂O₇:Dy³⁺, Li⁺ samples were in the near cold-white region.

Full text of DOI:10.1016/j.jallcom.2012.10.074

Journal of Alloys and Compounds 552 (2013) 152–156
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Enhanced luminescence of novel Y2Zr2O7:Dy3+ phosphors by Li+ co-doping
b
c
a
Qingqing Du a, Guangjun Zhou a, , Juan Zhou , Xiao Jia , Haifeng Zhou
⇑
a State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
b Center for Disease Prevention and Control of Jinan Military Command, Jinan 250014, PR China
c College of Control Science and Engineering, Shandong University, Jinan 250100, PR China
a r t i c l e i n f o
a b s t r a c t
Novel blue–white Y2Zr2O7:Dy3+, Li+ phosphors have been prepared by a facile and efficient sol–gel-
combustion method at 800 °C, using ammonium nitrate as a fuel and citric acid as a complexing agent.
No additional annealing was required, as crystalline Y2Zr2O7 resulted directly from the combustion reac-
tion. The structure and morphology were characterized by X-ray diffraction (XRD) and scanning electron
microscope (SEM). The photoluminescence properties were measured using an Edinburgh FLS920 at
room temperature. Stable blue–white emission from 425 nm to 625 nm has been observed from
Y2Zr2O7:Dy3+ and Y2Zr2O7:Dy3+, Li+ phosphors. The optimal doping concentration of Dy3+ ions is
2 mol%. Experimental analysis and theoretical calculations reveal that the dipole–quadrupole (d–q)
interaction should be the major concentration quenching mechanism of Dy3+ ions emission. Li+ co-doping
improves luminescence intensity as well as crystallinity significantly. The incorporation of Li+ ions into
Y2Zr2O7 lattice could induce a remarkable increase of photoluminescence intensity by about 2 times.
The mechanism of the enhanced emission by Li+ doping was discussed. Both the chromaticity coordinates
of Y2Zr2O7:Dy3+ and Y2Zr2O7:Dy3+, Li+ samples were in the near cold-white region.
Article history:
Received 10 August 2012
Received in revised form 12 October 2012
Accepted 17 October 2012
Available online 25 October 2012
Keywords:
Y2Zr2O7:Dy3+, Li+
Phosphors
Luminescence
Sol–gel combustion
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Moreover, the sol–gel-combustion method is a promising method
to obtain nanocystalline materials with unique properties and bet-
Rare earth doped metal oxide crystalline powders are potential
phosphors with advantages such as heat resistance, chemical
stability and long working time. The rare-earth-metal zirconates
(Re2Zr2O7) exhibit a pyrochlore structure (space Fd3m) or in some
case a defect fluorite structure (space group Fm3m) [1]. These com-
plex oxides have attracted particular interest for many researchers
due to their important physical and chemical properties, such as
low thermal conductivity, high melting point, high thermal expan-
sion coefficient, high stability and ability to accommodate defects.
In view of these important properties, the complexes have been
widely applied for thermal barrier coatings and nuclear waste form
and good host materials for luminescent rare earth ions [2,3].
Re2Zr2O7 oxides have been synthesized by many methods,
including co-precipitation [4], sol–gel [5], hydrothermal [6], and
hydrazine methods [7]. As Re2Zr2O7 compounds have high melting
point, they are generally synthesized by solid-state reaction at very
high temperature. Compared with the conventional fabrication
methods, sol–gel combustion technique is more attractive owing
to the advantages of the simple equipment, time consumption,
low process temperature and easily controlled particle size [8,9].
ter chemical uniformity because the starting materials are homo-
geneously mixed in liquid phases [10,11].
Recently, Y2Zr2O7 have been reported to be good host materials
for rare earth phosphor emitting luminescence such as Y2Zr2O7:Eu,
Y2Zr2O7:Eu,Dy, Y2Zr2O7:Tb [12–14]. However, to the best of our
knowledge, the luminescence properties of Dy3+-activated
Y2Zr2O7 phosphor have not been reported yet. Dy3+ ions are well
known as activator dopants for many different inorganic lattices
because the fluorescence of Dy3+ ions originates from the 5d–4f
transition, and the 5d state is easily affected by the outer crystal
field. The intense blue and yellow transitions (4F9/2 ? 6H15/2 13/2
)
,
play an important role in synthesis of phosphor. On the other hand,
acting as a co-activator and/or a charge compensator, lithium ion
has been introduced into different phosphor host lattices. There
are also many reports on the significant enhancement in emission
intensity with Li+co-doping in different hosts [15–17]. To the best
of our knowledge, there is no report on improvement of blue–
white light with Li+ ion incorporation in Y2Zr2O7:Dy3+ phosphor.
In this study, Y2Zr2O7:Dy3+ phosphors were synthesized by sol–
gel-combustion method and their photoluminescence properties
were investigated. In addition, we reported the effect of Li+ ions
on structural and enhancement of photoluminescence properties
of Y2Zr2O7:Dy3+ phosphors.
⇑
Corresponding author. Tel./fax: +86 531 88361206.
E-mail address: gjunzhou@163.com (G. Zhou).
0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.10.074

Q. Du et al. / Journal of Alloys and Compounds 552 (2013) 152–156
2.2. Characterization
153
The crystalline structures were analyzed by powder X-ray diffraction patterns
(type Germany Bruker axs D8-Advance X-ray diffractometer with graphite mono-
chromatized Cu K irradiation (k = 1.542 Å)). Data were collected over the 2h range
of 20–70°, with a step width of 0.02° and count time of 0.4 s/step. Scanning electron
micrograph (SEM) images were taken with a JEOL JSM-6700f transmission electron
microscope. The composition measurement was operated by energy dispersive
spectrometric (EDS) analysis (Horba EMAX Energy, EX-350). The luminescent spec-
tra and absolute quantum efficiencies of the phosphor were measured on an Edin-
burgh FLS920 fluorescence spectrometer and its quantum yield measurement
system. All the measurements were performed at room temperature.
3. Results and discussion
3.1. Crystal structure and morphologies
The XRD patterns of pure Y2Zr2O7 calcined at different tempera-
ture, Y2Zr2O7:2%Dy and Y2Zr2O7:2%Dy, y%Li+ phosphors (y = 5, 10,
15, 20, 25) samples are shown in Fig. 1. Patterns of a–c suggest that
the increasing temperature results in a better crystallization. For
e–i patterns, a single phase of Y2Zr2O7:Dy, Li was also obtained
and all the four main peaks (222), (400), (440) and (622) are in
agreement with the formation of fluorite structured Y2Zr2O7 [18],
though the JCPDS data corresponding to Y2Zr2O7 phase are still
not available for comparison. In addition, the diffraction peaks
become much sharper after Li+ ions are introduced, which is attrib-
uted to the better crystallization. However, these characteristic
peaks can also be indexed on the basis of pyrochlore structure. Both
fluorite and pyrochlore have cubic symmetry with a space group of
Fm3m for the former and Fd3m for the latter [19,1]. Furthermore,
the pyochlore structure is characterized with ordered cations,
which results in more super lattice peaks in XRD compared with
the fluorite structure. Generally, there are two additional peaks
corresponding to (331) (2h = 36.31°) and (511) (2h = 43.61°) in
the pyrochlore structure. These two peaks are a key to distinguish
pyrochlore structure from fluorite structure [20,21]. The reflections
Fig. 1. XRD patterns (a–c) assigned to the pure Y2Zr2O7 calcined at 600 °C, 700 °C
and 800 °C, respectively; (d) Y2Zr2O7:Dy calcined at 800 °C; (e–i) Y2Zr2O7:2%Dy,
y%Li+ phosphors (y = 5, 10, 15, 20, 25) calcined at 800 °C, respectively.
2. Experimental
2.1. Synthesis of phosphors
The starting materials were ZrO(Ac)2, Y(NO3)3, Dy(NO3)3, Li(NO3), NH4NO3, and
citric acid. All the chemicals we used were analytic reagent. The aqueous rare earth
nitrate Y(NO3)3 and Dy(NO3)3 solutions were produced previously by dissolving the
corresponding rare earth oxides into diluted HNO3. For pure Y2Zr2O7, ZrO(Ac)2 and
Y(NO3)3 were first mixed with the fuel NH4NO3 in an appropriate proportion. After
30 min stirring, the requisite amount of citric acid was quickly added to the mixed
solution. The mixture was stirred for 1 h, and then the transparent sol became an
opal gel. The as-prepared gel was dried at 100 °C for 12 h to get the xerogel, and
then heated in a muffle furnace maintained at 800 °C for 30 min. The Dy3+-doped
and Dy3+, Li+ co-doped Y2Zr2O7 phosphors were synthesized just as mentioned
above except that the Dy(NO3)3 and LiNO3 was introduced. The doping content of
Dy3+ varied from 0.5 to 5 mol% and Li+ from 5 to 25 mol%.
Fig. 2. SEM micrographs of (a) pure Y2Zr2O7, (b) Y2Zr2O7:Dy (c) Y2Zr2O7:Dy, Li samples and (d) EDS spectra of the 2 mol% Dy3+-doped Y2Zr2O7 sample.

154
Q. Du et al. / Journal of Alloys and Compounds 552 (2013) 152–156
of (331) and (511) are inexistent in Fig. 1, which confirm the
absence of pyrochlore phases. Therefore, the Y2Zr2O7, Y2Zr2O7:Dy
and Y2Zr2O7:Dy, Li samples are of fluorite structure.
Fig. 2 shows the typical scanning electron microscope (SEM)
images of (a) pure Y2Zr2O7, (b) Y2Zr2O7:Dy and (c) Y2Zr2O7:Dy, Li
samples. The pure Y2Zr2O7 powders exhibit a uniform dispersion
of the particles. The Y2Zr2O7 and Y2Zr2O7:Dy particles in Fig. 2(a)
and (b) are irregular in shape and have pseudospherical morphol-
ogy, which are closely packed due to agglomeration. In addition, it
can be found that some pores and voids are in the as-prepared
samples. This phenomenon was probably induced due to the
escaping gases and a fast aggregation process, during the very ra-
pid combustion reaction. Evidence of a flux effect by Li+ is clearly
seen in Fig. 2(c), wherein the Dy3+ and Li+ co-doped powders shows
fibrous reticulated mesh like crystallites of up to ꢀ1
lm, compared
to <200 nm for the unmodified phosphor. In addition, the elemen-
tal analysis was carried out by using energy dispersive X-ray spec-
trometer (EDS). Because the lithium atomic number is too small, it
cannot be characterized by the EDS spectra. The EDS result of Y2-
Zr2O7:2%Dy3+ sample in Fig. 2(d) reveals that the sample contains
yttrium, zirconium, oxygen, and dysprosium elements, no other
impurities were detected. Furthermore, the atomic ratio measure-
ment of the given elements from the EDS spectrum presents Y/Zr/
O = 1.07:1:3.25, close to Y/Zr/O = 2:2:7, which demonstrates that
the sample probably is composed of Y2Zr2O7. To the 2 mol% Dy3+
-
doped Y2Zr2O7, the actual Dy3+ concentration estimated by
Fig. 2d is 1.32 mol%, indicating the presence of appropriate Dy3+
ions incorporated into the phosphor.
3.2. Luminescent properties
Photoluminescence excitation and emission spectra for Y2Zr2-
O7:Dy3+ phosphors are shown in Fig. 3. The excitation spectra
(Fig. 3(a)) in the range of 200–500 nm consist of several excitation
peaks ascribed to different transitions from ground state (6H15/2) to
various excited states of Dy3+ ions, which are centered at 297 nm
(6H15/2 ? 4D7/2), 325 nm (6H15/2 ? 6P3/2), 351 nm (6H15/2 ? 6P7/2),
364 nm (6H15/2 ? 4D5/2), 387 nm (6H15/2 ? 4M21/2), 425 nm (6H15/
2 ? 4G11/2), and 452 nm (6H15/2 ? 4J15/2), respectively [22–24].
The excitation maximum was located at 351 nm. Fig. 3(b) shows
emission spectra of Y2Zr2O7:x%Dy3+ (x = 0.5, 1, 2, 3, 4, 5) under
351 nm excitation. It shows no significant change in the emission
shape or position with increasing the Dy3+ doping concentration
except for the PL intensity. The 351 nm wavelength excites Dy3+
Fig. 3. The excitation spectrum (kem = 578 nm) and the emission spectra (kex = 351 -
nm) spectra of Y2Zr2O7:Dy. The inset of b is the evolution of the emission intensities
as a function of the Eu3+ doping concentration.
Dy3+ ions [28,29]. The concentration-quenching behavior is in-
duced mainly by the energy transfer between Dy3+ ions in the
phosphor [30]. According to the Dexter theory, the emission inten-
sity (I) per activator ion follows the equation:
6
ions resonantly to P7/2 state and then quickly relaxes non-radi-
4
4
I=x ¼ Kð1 þ bðxÞh=3Þꢁ1
ð1Þ
atively to F9/2 level. Radiative emission takes place from F9/2 to
6
6
the lower levels H15/2 (blue band at 485 nm) and H13/2 (yellow
band at 578 nm) [25,26]. It is well known that the 4F9/2 ? 6H15/2
transition is mainly magnetically allowed and hardly varies with
where x is the activator concentration, K and b are constants under
the same excitation condition for a given host crystal. Among the
concentration quenching caused by the multipolar interaction, the
dipole–dipole (d–d), dipole–quadrupole (d–q), or quadrupole–
quadrupole (q–q) interaction correspond to h = 6, 8, 10, respectively
[31]. Since the critical concentration of Dy3+ has been determined as
2 mol%, the phosphor with the doping Dy3+ concentration which is
not less than the critical concentration (2 mol%) is determined. The
curve of the selected Y2Zr2O7:Dy phosphor based on the emission
spectra and Eq. (1) is shown in Fig. 4. The electric multipolar char-
acter (h) can be obtained by the slope (ꢁh/3). It can be seen that the
plot is approximately linear and the slope is about ꢁ2.53. Thus, the
h value can be calculated as 7.6 (approximately equal to 8) accord-
ing to the linear fitting by using Eq. (1). The result indicates that the
dipole–quadrupole (d–q) interaction is the dominant mechanism
for the concentration quenching of Dy3+ ions emission in Y2Zr2O7:-
Dy phosphor.
the crystal field strength around the Dy3+ ion. However, the F9/
4
2 ? 6H13/2 transition is a forced electric dipole transition being pre-
dominant only at low symmetries with no inversion center [27]. In
the Dy3+-doped Y2Zr2O7 samples, Dy3+ ion is assumed to substitute
for the Y3+ ion, because of the same valence and similar ion radius
for Dy3+ (0.908 Å) and Y3+ (0.9 Å) ions. The emission intensity of the
blue emission is stronger than that of the yellow one in the inves-
tigated phosphors, which indicates that there is very little devia-
tion from inversion symmetry.
Doping concentration can affects the performance of lumines-
cent materials significantly. The inset of Fig. 3(b) describes the
variation of the PL intensity with the doping concentration. It can
be seen clearly that the optimal doping concentration of Dy3+ is
2 mol%. Lower doping concentration leads to weak emission inten-
sity because there are no sufficient luminescent centers. However,
if the doping concentration is higher than the critical value, the
intensity decreases due to the concentration quenching effect of
Fig. 5 displays the emission spectra of Y2Zr2O7:2%Dy3+, y%Li+
phosphors (y = 5, 10, 15, 20, 25). The co-doped samples also show

Q. Du et al. / Journal of Alloys and Compounds 552 (2013) 152–156
155
Fig. 4. The plot of log(xDy3+) vs log(I/xDy3+) in Y2Zr2O7:Dy phosphor.
Fig. 6. CIE chromaticity diagram for (a) Y2Zr2O7:Dy and (b) Y2Zr2O7:Dy, Li
phosphors.
Fig. 6 presents the chromaticity coordination (X,Y) of Y2Zr2-
O7:2%Dy and Y2Zr2O7:2%Dy, 20%Li in the CIE (Commission Interna-
tionale deI’ Eclairage) 1931 chromaticity diagram. Based on the
standard CIE coordinate-color graph, we can observe that both Y2-
Zr2O7:2%Dy and Y2Zr2O7:2%Dy, 20%Li can emit cold-white light
with CIE coordinates of (0.259, 0.296) and (0.267, 0.309), respec-
tively. The absolute quantum efficiencies of the phosphors are 4%
(Y2Zr2O7:2%Dy) and 10% (Y2Zr2O7:2%Dy, 20%Li). Hence, Y2Zr2O7:Dy
and Y2Zr2O7:Dy, Li are promising materials in application to LEDs.
4. Conclusions
Novel Y2Zr2O7:Dy and Y2Zr2O7:Dy, Li phosphors were prepared
by sol–gel-combustion method. The calcine temperature is much
lower than that of the traditional solid-state reaction method.
The photoluminescent properties have been investigated. Two
strong emission peaks are located at 485 nm and 578 nm under
UV excitation. Through theoretical calculation, we can infer that
the major mechanism for concentration quenching is the dipole–
quadrupole interaction. In addition, the emission intensity was
obviously enhanced after the doping of Li+ ions. The reason is that
Li+ ions may act as a self-promoter for better crystallization and the
compensator charge. Both the chromaticity coordinates of Y2Zr2-
O7:Dy and Y2Zr2O7:Dy, Li are located in the cold-white light region.
Fig. 5. The emission spectra of Y2Zr2O7:Dy, Li samples (kex = 351 nm). The inset is
the evolution of the emission intensities as
concentration.
a
function of the Li+ doping
the Dy3+ emission peaks at 485 nm (4F9/2 ? 6H15/2) and 578 nm
(4F9/2 ? 6H13/2). However, the emission intensity increases with
the Li+ concentration and attains its maximum at 20 mol% Li+.
The luminescence intensity enhanced by about 2 times with
20 mol% Li+ ion doping. It is proposed that the Li+ ions can act as
co-activators and/or charge compensators and may aid for the en-
ergy transfer from the host to the Eu3+ active center, resulting in a
higher quantum yield. On one hand, the enhanced luminescence
may be associated with the flux effect of Li+ ions [32]. Better crys-
tallinity has been confirmed by XRD results (Fig. 1) with the grow-
ing Li+ content, leading to less defects and stronger luminescence.
On the other hand, occupation of Y3+ sites by Li+ ions would give
rise to a substantial number of oxygen vacancies, which might
act as the sensitizer for the energy transfer to the rare earth ion
due to the strong mixing of charge transfer states resulting in the
highly enhanced luminescent intensity [33–34]. The inset of
Fig. 5 shows the evolution of the emission intensities as a function
of the Li+ doping concentration. The luminescence quenching oc-
curs at higher doping content of Li+ ions (>20 mol%) due to the ex-
cess oxygen vacancies of host, which destroy the crystallinity of
phosphor [35,36].
Acknowledgements
This work was supported by projects from National Science
Foundation of China (50972081; 51072098); The Chinese PLA Med-
ical Science and Technique Foundation (CWS11J243); Independent
Innovation Foundation of Shandong University, IIFSDU (2011JC024)
and The Excellent State Key Laboratory Research Program of the Na-
tional Natural Science Foundation of China (50823009).
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