Enhanced upconversion emission in ZrO2-Al2O3 composite oxide

Article abstract of DOI:10.1039/c5dt04551g

Aqueous solutions of zirconium oxychloride and aluminum nitrate were coprecipitated and crystallized to form a ZrO₂-Al₂O₃ solid solution. The upconversion (UC) emission from different Er³⁺-doped samples was studied. An enhancement of the green UC emission by as much as 22 times was achieved by co-doping with Yb³⁺ and Mo⁶⁺ ions due to an energy transfer at a higher excited-state energy, which partly avoided the non-radiative decay processes at the lower energy levels of Er³⁺. The UC emission of the ZrO₂-Al₂O₃ composite system series doped with different agents was enhanced. Excess oxygen vacancies are generated by forming ZrO₂-Al₂O₃ solid solutions, which have an energy level close to the ⁴F_7/2 level of the Er³⁺ ions. The defect state promoted the energy transfer process resulting in an eight-fold increased green UC emission in ZrO₂-Al₂O₃ solid solutions. The solid solutions have a superior color chromaticity of x = 0.25 and y = 0.71 due to the evident enhancement in the green to red emission ratio in the 8ZrO₂-2Al₂O₃ sample.

Full text of DOI:10.1039/c5dt04551g

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Page 1 of 24
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Enhanced Upconversion Emission in ZrO2ꢀAl2O3 Composite Oxide
Yan Cong, Bin Dong*, Naisen Yu, Yangyang He, Ying Zhao, Yang Yang
School of Physics&Materials Engineering, Dalian Nationalities University, Dalian
116600, China
Aqueous solutions of zirconium oxychloride and aluminum nitrate were
coprecipitated and crystallized to form a ZrO2ꢀAl2O3 solid solution. The upconversion
(UC) emission from different Er3+ꢀdoped samples was studied. Enhancement of the
green UC emission by as much as 22 times was achieved by coꢀdoping with Yb3+ and
Mo6+ ions due to an energy transfer at a higher excitedꢀstate energy, which partly
avoided the nonꢀradiative decay processes at the lower energy levels of Er3+. The UC
emission of the ZrO2ꢀAl2O3 composite system series doped with different agents were
both enhanced. Excess oxygen vacancies are generated by forming ZrO2ꢀAl2O3 solid
4
solutions, which have an energy level close to the F7/2 level of the Er3+ ions. The
defect state promoted the energy trasfer process resulting in an eightꢀfold increased
green UC emission in ZrO2ꢀAl2O3 solid solutions. The solid solutions has a superior
color chromaticity of x=0.25 and y=0.71 due to the evident enhancement in the green
to red emission ratio in the 8ZrO2ꢀ2Al2O3 sample.
* Correspondence author. Tel:+86ꢀ411ꢀ87924857; fax: +86ꢀ411ꢀ87556959.
Eꢀmail: congyan@dlnu.edu.cn; dong@dlnu.edu.cn

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1. Introduction
Upconversion (UC) luminescent materials with antiꢀstokes optical properties
have garnered considerable attention because of their broad potential applications,
including color display, solidꢀstate lasers, sensor technology, solar cells and biological
labeling.1ꢀ5 Several rareꢀearth (RE) ions have been reported for their efficient UC
luminescence. Among them, Er3+ ion which can efficiently emit photons in the green,
red and nearꢀinfrared (NIR) regions is the most frequently used activator. Very often,
Yb3+ is used as a sensitizer. Actually, as Yb3+ has a much larger absorption cross
section than Er3+ near 980 nm, the NIR photon energy is efficiently transferred from
Yb3+ to Er3+ resulting in an efficient energy transfer (ET).3,4 Lately, much attention
has been focused on the transition metal (TM) as sensitizer for RE ions. The
intraꢀconfigurational dꢀd transitions in the TM resulting in low electronꢀphonon
coupling and low multiphonon relaxation rate make the TM a potential sensitizer in
UC luminescent materials.5ꢀ7 In our study on the ErꢀYbꢀMo and TmꢀYbꢀMo systems
in oxidic matrix materials, we observed the intense UC luminescence in both.5,8
Moreover, the luminescence efficiency also depends on the performance of the host
material. To obtain a higher UC efficiency, the host should have low lattice phonon
energy which can greatly reduce the nonradiative decay rate of multiphonon
relaxations. Fluorides are efficient hosts for the UC luminescence of RE ions due to
their appropriate energy phonons which produce strong UC emission. Compared with
fluorides, oxides have high chemical durability, thermal stability and mechanical
strength.5,9,10 However their corresponding high phonon energies failed to suppress
the nonradiative loss, resulting in weak UC emissions, which greatly hampers their
applicability. To solve these problems above, the designing of new oxides materials
capable of converting low phonon energy has been carried out.
ZrO2ꢀAl2O3 a composite oxide which can improve the current properties and
create many new functions has attracted our attention. In the last two decades, ZrO2
has been recognized as a new oxide host due to its low phonon energy (470 cmꢀ1),
high chemical stability and broad optical transparence from visible to NIR

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wavelength.10ꢀ12 Alumina is another important variety of oxide host for UC
luminescence.13,14 A ZrO2ꢀAl2O3 solid solution, formed by mixing Al2O3 and ZrO2, is
widely used as a potential catalyst, oxygen sensor, and hard ceramic material,15,16 and
most of the research has been focused on finding new synthesis methods to extend the
Al2O3 in ZrO2. A solution of this ZrO2ꢀAl2O3 composite oxide has received little
attention as potential UC host material. Thus, it is very necessary to study the
luminescent properties of this system in order to make the best use of it as a
luminescent host.
Here, to improve the UC emissions in the oxide host, we prepared various
ZrO2ꢀAl2O3 composites doped with different doping agents and studied the UC
luminescent properties in detail. In the composite system, ZrO2ꢀAl2O3 composite
samples which had been singleꢀdoped with Er3+ or triꢀdoped with ErꢀYbꢀMo both
show a great enhancement of the UC luminescence. Characterization of the
ZrO2ꢀAl2O3 composites series was performed by Xꢀray diffraction (XRD), Fourier
transform infrared (FTIR) spectroscopy, UVꢀvis absorption spectroscopy and
upconversion photoluminescence (UPL) emission spectroscopy. In addition, the
enhancement mechanism of the UC emission in the ZrO2ꢀAl2O3 composites was
investigated.
2. Experimental
2.1. Chemicals and materials.
ZrOCl2·8H2O, Al(NO3)3·9H2O and (NH4)6Mo7O24·4H2O were purchased from
Sinopharm Chemical Reagent Co., Ltd. Er(NO3)3·5H2O and Yb(NO3)3·5H2O were
purchased from Aldrich. All chemicals were of analytical grade reagents and were
used directly without further purification.
2.2. Sample Preparation.
ZrO2ꢀAl2O3 solid solutions doped with different doping agents were prepared by
the coꢀprecipitation method. The synthesis procedure was as follows: 1 mmol
ZrOCl2·8H2O (0.3223g) was dissolved under vigorous stirring in an aqueous solution

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(50 mL). Al(NO3)3·9H2O was added into the above solution according to the molar
ratios of ZrO2/Al2O3=2:8, 4:6, 5:5, 6:4, and 8:2. Er(NO3)3·5H2O, Yb(NO3)3·5H2O,
and (NH4)6Mo7O24·4H2O with corresponding mole ratios (for 1 mol% Er3+, 10 mol%
Yb3+ and 10 mol% Mo6+) were added into the above solution as doping agents. The
polyethylene glycol (PEG, molecular weight=20000, A.R.) was added as a dispersant
(0.02g/mL). A solution of 0.5 mol/L NH3·H2O was slowly added into the solution as a
precipitating aid agent until the pH was 7~8. The precipitate was suctionꢀfiltered,
washed 4 or 5 times with deꢀionized water, and dried at 100 °C to obtain a white
powder. Finally, the asꢀprepared powder samples were annealed at 1100 °C for 3 h to
produce the final samples. We denote the series of ZrO2ꢀAl2O3 samples as follows:
0.2ZrO2ꢀ0.8Al2O3 (2Z-8A), 0.4ZrO2ꢀ0.6Al2O3 (4Z-6A), 0.5ZrO2ꢀ0.5Al2O3 (5Z-5A),
0.6ZrO2ꢀ0.4Al2O3 (6Z-4A), and 0.8ZrO2ꢀ0.2Al2O3 (8Z-2A). In addition, the pure
ZrO2 doped with different doping agents were prepared by the same procedure above
for comparison.
2.3. Characterization.
The structural characterization was performed by Xꢀray diffraction (XRD;
Rigaku D/maxꢀIIB) spectra with Cu Kα radiation (λ=0.15405 nm). The FTIR
absorption spectra were measured by a Nicolet Avatar 370 FTIR spectrometer
(Thermo Fisher Scientific, Waltham, MA, USA). UVꢀvis diffuse reflectance spectra
were collected on a Lambda 750 UV/Vis/NIR spectrophotometer (Perkin Elmer,
Waltham, MA, USA). UCL emission spectra were obtained on a Hitachi Fꢀ4600
fluorescence spectrometer, with an external 980 nm diode laser with a maximum
power of 1 W and a focused area of 0.25 mm2 as the excitation source, in place of the
xenon lamp in the spectrometer. The spectrometer operated with excitation slit at 0
nm, and emission slit at 2.5 and 5 nm, respectively. The element analysis was
performed on an inductively coupled plasma mass spectrometer (Optima 2000 DV,
Perkin Elmer).
3. Results and Discussion

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The XRD patterns of the ZrO2 doped with Er, ErꢀYb, or ErꢀYbꢀMo sintered at
1100 °C are displayed in Fig. 1(1). ZrO2 has three crystallographic phases, namely the
monoclinic (m), tetragonal (t), and cubic (c) phases. Bulk mꢀzirconia changes to tꢀ and
cꢀZrO2 at 1170 and 2370 °C, respectively. When doped with 1 mol% Er3+ ,the sample
exhibits a monoclinic form with less amount of tetragonal form from the (0,1,1)t and
(1,1,2)t reflections peaks at 30.2° and 50.2°, respectively. However, the tetragonal
phase was stabilized when the sample was doped with 10 mol% Yb3+. Oxygen ion
vacancies can be generated via doping trivalent cations within the ZrO2 lattice.
Actually, the stability of the tetragonal phase in ZrO2 has been attributed to the
presence of oxygen vacancies produced by doping an appropriate amount of oversized
17
trivalent cations.
When coꢀdoped with 10 mol% Yb3+ and 10 mol% Mo6+, the
dominant crystalline structure turned to be a strong tetragonal phase containing traces
of the monoclinic phase.
The XRD patterns of the ErꢀYbꢀMo triꢀdoped ZrO2ꢀAl2O3 solid solutions are
summarized in Fig.1(2). The mixture of Al2O3 in ZrO2 forms a solid solution of
composition Zr1ꢀxAlxO2ꢀx/2. During this phase transformation, the Al2O3 incorporates
into the ZrO2 lattice according to the following reaction equation:
ZrO2
Al2O3 → 2AlZ' r +VO•• + 3OOX
(1)
Nonequivalent substitution of Al2O3 for ZrO2 produced Al′Zr neagtiv defects and
••
X
oxygen vacancies VO and OO . However, Al3+ (0.57Å) is apparently too small to
substitute extensively for Zr4+ (0.79Å), resulting in a rather limited equilibrium
solubility. The sample 8Z-2A shows a dominant tetragonal ZrO2 form contain traces
of αꢀAl2O3. The αꢀAl2O3 reflection peaks in other samples (samples 6Z-4A to 2Z-8A)
increased gradually. In addition, the ZrO2ꢀrich portion of the ZrO2ꢀAl2O3 system is a
tꢀ(Zr,Al)O2 solid solution.18 Actually, in the ZrO2ꢀAl2O3 system all the diffraction
peaks corresponding to tetragonal ZrO2 appear but display a systematic redshift. A
close inspection of the XRD peak at 30.2° (0,1,1) in Fig. 1(2b) reveals that when the
content of Al2O3 reached to 20 mol% in sample 8Z-2A, the diffraction peak redshift

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was obvious, and no gradual shift in diffraction peaks occurred upon increasing the
Al2O3 content. Accordingly, the redshift provides evidence of Al2O3 incorporation into
the ZrO2 lattice, and that no additional shift occured with the increase of the Al2O3
content, which is further evidence that the solid solution limit is about 20 mol% Al2O3,
at which point no more Al2O3 could be incorporated into the ZrO2 lattice.
Fig. 2 shows the UC emission spectra of pure ZrO2 host with different doping
agents upon laser excitation at the wavelength of 980 nm (1W/mm2). By choosing
suitable concentrations for the sensitizer and emitter molecules, an intense green UC
emission was obtained with 1 mol% Er3+, 10 mol% Yb3+, 10 mol% Mo6+ triꢀdoped
ZrO2. The ZrO2 singleꢀdoped with 1 mol% Er3+ radiated a green band at 550 nm
(4S3/2ꢀ4I15/2 transitions), while a strong red band at 654/678 nm (4F9/2ꢀ4I15/2 transitions)
was obtained after further doping 10 mol% Yb3+. The strong red UC emission with
the quenching of the green emission is due to the sensitization of Yb3+ to Er3+ which is
well known and has been previously described.3,4 ZrO2 triꢀdoped with ErꢀYbꢀMo
exhibits a great luminescence enhancement at 550 nm, corresponding to the 4S3/2ꢀ4I15/2
transitions (22 times higher than the singleꢀdoped ZrO2:Er3+). Notably, the 525 nm
green emission from the 2H11/2ꢀ4I15/2 transitions, which are too weak to be observed in
singleꢀdoped ZrO2:Er3+, markedly increased about 220 times in the ErꢀYbꢀMo
triꢀdoped ZrO2. Due to the extending emission region and the efficiently increasing
emission intensity, a novel intense yellowꢀgreen UC emission was obtained in the
ErꢀYbꢀMo triꢀdoped ZrO2 phosphor. A similar enhancement was also observed in
other ErꢀYbꢀMo coꢀdoped oxidic matrix materials, such as Al2O3, TiO2, Gd2O3, and
CeO2. We explain this intense UC emission as a special high excited state ET process,
in which the ET occurs at a higher excited state energy (2F7/2,3T2> state of
Yb3+ꢀMoO42ꢀ dimer) thus avoiding the nonradiative decay processes that occurs at the
lower energy levels of the Er3+ ions.5,8,19
2ꢀ
Besides examining the effective sensitization of the Yb3+ꢀMoO4 dimer, our study
also focused on the evaluation of efficient host materials. Al2O3 mixed in a ZrO2
matrix formed ZrO2ꢀAl2O3 solid solutions, and the UC luminescence properties of

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the ZrO2ꢀAl2O3 composite system series doped with different doping agents were
investigated in detail. It is remarkable to see significant enhancement of the UC
emission intensity in ZrO2ꢀAl2O3 solid solutions samples compared to the pristine
oxide samples−ZrO2 and Al2O3. It reveals that the composite oxide structure has a
profound effect on the UC efficiency. The comparison of UC emission spectra for
the series of ZrO2ꢀAl2O3 samples singleꢀdoped with 1 mol% Er3+ are shown in Fig. 3.
The green emission is predominant one among all the Er3+ ꢀdoped composite
4
samples. Additionally, the S3/2ꢀ4I15/2 transition at 550 nm dominantly contributes to
the green emission, as shown in Fig. 3. Actually, an increment in intensity is
observed after introducing Al2O3 into ZrO2 to produce the ZrO2ꢀAl2O3 composite
oxide. The green emission at 550 nm increased in the ZrO2ꢀAl2O3 composite
samples, and the strongest green emission is produced by sample 8Z-2A, which
exhibits about a 40–fold increment compared with the pure ZrO2 sample. The
significant increase of UC emission in sample 8Z-2A may be due to the better
miscibility of Er3+ in Al2O3ꢀcoꢀdoped zirconia, which prevents the creation of large
clusters and circumvents the concentration quenching process for the same
concentration of Er3+.20,21
The UC luminescence mechanism in Er3+ ꢀdoped systems, which has been well
established in the literature,3,4,20,21 is depicted in Fig. 4. First, the Er3+ ion is excited
4
4
from the ground state I15/2 to the excited state I11/2. Subsequently, nonradiative
4
4
relaxations of I11/2→4I13/2 also populate the I13/2 level. Following firstꢀlevel
4
4
excitation, the excited atoms are excited from the I11/2 to the F7/2 levels or from the
4
4
4I13/2 to the F9/2 states. The populated F7/2 may mostly nonradiatively relax to the
4
2H11/2 and S3/2 levels, which produce two green UC emissions. In addition, the red
4
emission was obtained due to the F9/2→4I15/2 transitions. The mechanism of
cooperative energy transfer process between two nearby Er3+ ions also contributes to
the enhancement of green emission at the higher doping concentration. Two excited
4
4
Er3+ at I11/2 level interact with each other and one Er3+ is deꢀexcited to I15/2 level
4
while the other is excite to F7/2 level. The cooperative energy transfer becomes

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dominant in the sample with high Er3+ concentrations due to shortening the average
distance between dopant ions and enhancing the interionic interaction.
The results of the element content analysis by ICP are listed in Table 1. The Er3+
ions content is 0.085 mol% for the 8Z-2A sample, 0.024 mol% for the 6Z-4A sample,
0.052 mol% for the 5Z-5A sample, 0.044 mol% for the 4Z-6A sample, and 0.026
mol% for the 2Z-8A sample. This indicates that the 8ZrO2ꢀ2Al2O3 composite system
benefits the efficient doping of Er3+.
The absorption spectra of the series of ZrO2ꢀAl2O3 samples singleꢀdoped with 1
mol% Er3+ are shown in Fig. 5. The absorption band centered at 284 nm in the ZrO2
sample corresponds to the host absorption. This band shows a blueshift in ZrO2ꢀAl2O3
composite samples, which is produced by an increase of the band gap caused by the
stabilization of the tetragonal phase at 1100 °C. The main absorption peaks for Er3+
and the corresponding transitions are: 1524 nm (4I13/2), 962 nm (4I11/2), 652 nm (4F9/2),
525 nm (2H11/2), 490 (4F7/2), and 378 nm (4G11/2). The introduction of Al2O3 enhances
the absorption properties of the Er3+ ions in these ZrO2ꢀAl2O3 composite samples. In
particular, for the sample 8Z-2A the absorption peak intensity exhibits a sixꢀfold
increment in the visible as well as the 4I11/2 excited states near 980 nm compared with
the sample without Al2O3. The enhancement of absorption by the 8ZrO2ꢀ2Al2O3
composite system can be attributed to higher doping concentration of Er3+ as
mentioned above in the ICP results. The Er3+ ions in sample 8Z-2A, which has the
solid solution limitation of Al2O3, are localized at either the zirconium or aluminum
lattice site that prevents aggregation. The bypassing of the nonradiative process
(2H11/2→4I9/2)/(4I15/2→4I13/2) due to cross relaxation, which leads to the decay of the
4
4S3/2/2H11/2 levels, mostly radiatively to the I15/2 level, and a remarkable 40ꢀfold
enhancement of the intensity.
The phonon energy of pure ZrO2 is around 470 cmꢀ1, which makes it a good
candidate to insert rare earth ions for UC photoluminescence applications. FTIR was
performed to ascertain how the introduction of Al2O3 affects the phonon energy
distribution in these hosts of solid solutions. For the pure ZrO2 sample, the principal

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transmittance band is centered at 470 cmꢀ1, as shown in Fig. 6. This principal band
shifts to longer wave numbers at 510 cmꢀ1 in the ZrO2ꢀAl2O3 composite samples,
which allows higher phonon energy to be produced. Additionally, the FTIR study
reveals that the ZrO2ꢀAl2O3 composite has a lower hydroxyl group content. A broad
band centered at 3435 cmꢀ1 is due to the OH stretching vibrations of water, and the
corresponding bending vibration is at 1640 cmꢀ1. Both of the peaks decrease in the
ZrO2ꢀAl2O3 composite, and are mostly absent in the 8ZrO2ꢀ2Al2O3 composite sample.
Surface hydroxyl groups can influence the nonradiative relaxation rate,22 thus the
hydroxyl content is another reason for the increase of the UC emission intensity
observed in sample 8Z-2A.
The ZrO2ꢀAl2O3 solid solutions also induced an increase of the UC emitted in the
ErꢀYbꢀMo triꢀdoped samples. A high efficiency, as much as 22 times, in the green UC
emission was achieved by triꢀdoping with Er3+ꢀYb3+ꢀMo6+ ions in pure ZrO2 (Fig. 2).
The high efficiency of the UC emission was further enhanced 8ꢀfold in the
8ZrO2ꢀ2Al2O3 solid solutions. The UC emission spectra of the ErꢀYbꢀMo triꢀdoped
series of the ZrO2ꢀAl2O3 composite system are shown in Fig. 7. Interestingly, the UC
emission intensity in the ZrO2ꢀAl2O3 composite samples was enhanced compared with
pure ZrO2 and Al2O3 samples. The 8Z-2A sample shows the strongest emission of all
the samples studied and the relative emission intensity is about 7ꢀand 8ꢀfold increased
compared with pure ZrO2 and Al2O3 samples, respectively.
A simplified diagram illustrating the possible mechanism of UC emission and the
energy transfer process is depicted in Fig. 8. Our previous work suggest that Mo
2ꢀ
substitutes for the triꢀvalent Yb3+ ions to form Yb3+ꢀMoO4 dimmer. Accurate
firstꢀprinciples calculations show that configuration with Mo occupying Yb sites have
2ꢀ
a total energy lower than other configurations.5 Firstly, the Yb3+ꢀMoO4 dimer
1
absorbs a 980 nm photon and is excited from the ground state 2F7/2, A1> to the
1
intermediate excited state2F5/2, A1> level (GSA). Next, the excited ion absorbs a
second photon in an excited state absorption (ESA) process and is promoted to the
1
1
relevant higher level of 2F7/2, T1>. The excited state of 2F7/2, T1> is much higher

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in the ErꢀYbꢀMo system than in the ErꢀYbꢀdoped ZrO2. The ET takes place at a higher
excited state energy and partly avoids the nonradiative decay processes that occur at
the lower energy levels of Er3+, leading to the enhancement of the efficiency
enhancement of the green UC emission in the ErꢀYbꢀMo triꢀdoped samples.5,8,19 In the
host of the ZrO2ꢀAl2O3 solid solutions, the oxygen ion vacancies (VO••) can be
generated as a result of charge balance by incorporating an appropriate amount of
Al2O3, as shown in equation (1). The oxygen vacancy serves as the F center and
greatly affects the luminescent properties in ZrO2. The blue emission centered at
470ꢀ490 nm from ZrO2 is attributed to the VO defects.23ꢀ25 Moreover, the F7/2 level
of Er3+ ions (490 nm) is very close to the defect state. When the above twoꢀphoton
••
4
••
process occurs, the excited ions are easy to be captured by the VO defects near the
1
4
••
2F7/2, T1> state, and then transfer to F7/2 level of Er3+. The presence of the VO
defects in the ZrO2ꢀAl2O3 system favors the promotion of the ET process, in which
1
the Yb3+ꢀMoO42ꢀ dimer sensitizes the Er3+ ions at the 2F7/2, T1> excited state, which
nonradiatively relaxes to 2H11/2 and 4S3/2 levels and subsequently results in two strong
4
green emissions. The red emission caused by populated F9/2 level in the ZrO2ꢀAl2O3
system is one order of magnitude weaker than the green emission, as shown in Fig. 7,
and provides further evidence of the high excited state energy transfer UC process.
The dependence of the green UC intensity (empty bars) and red UC intensity
(shaded bars) on different ZrO2ꢀAl2O3 composite samples is depicted in Fig. 9. The
intensity of the green emission relative to the intensity of the red emission is 8.12 and
3.06 for the sample 8Z-2A and pure ZrO2, respectively. A predominantly green
emission was obtained, and as the ratio was 8:1, the emission was almost exclusively
green in the sample 8Z-2A. The evident enhancement in the green to red emission
ratio in comparison with the ZrO2 sample makes the ZrO2ꢀAl2O3 composite sample an
efficient green UC phosphor with high color purity. The Commission Internationale
del′ Eclairage (CIE) chromaticity coordinates of sample 8Z-2A, calculated from the
corresponding upconverted emission spectrum, is (0.25, 0.71), while the index of
ZrO2 is (0.32, 0.41). In addition, the corresponding hue of the samples are green and

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yellowꢀgreen, respectively, as indicated in the chromaticity diagram inset in Fig. 9. It
is well known that the green and red UC intensities are proportional to the population
4
4
of I11/2 and I13/2 levels, which are strongly governed by the nonradiative relaxation
from the two levels. Thus, decreasing the nonradiative relaxation rate can increase the
4
4
population ratio of the I11/2 to I13/2 levels, so as to enhance the green UC emission
and suppress the red emission, simultaneously.
4. Conclusion
A series of ZrO2ꢀAl2O3 composites containing different zirconium compositions
(20 to 100 mol%) were prepared by the coꢀprecipitation method, and the optimum
composition was determined to be 8ZrO2ꢀ2Al2O3. The visible green UC emission was
significantly enhanced by doping with different dopant ions in the 8ZrO2ꢀ2Al2O3
composites. The enhancement of the green UC emission from the 4S3/2ꢀ4I15/2 transition
in Er3+ singleꢀdoped 8ZrO2ꢀ2Al2O3 is ascribed by reducing the nonradiative relaxation
due to the better miscibility of Er3+. The UC emissions were further increased by
triꢀdoping Er3+ꢀYb3+ꢀMo6+ ions through a high ET process which occurred at higher
excited state energy and avoided the nonradiative decay processes in the Er3+ ions.
Furthermore, ZrO2ꢀAl2O3 phosphor exhibits high color saturation with the appropriate
tuning of the concentration of the Al2O3 solid solution.
Acknowledgment
This work is supported by the National Natural Science Foundation of China
(Grant Nos.11274057, 11474046, and 11474045), Program for New Century
Excellent Talents in University (Grant No. NCETꢀ13ꢀ0702), Science and Technology
Project of Liaoning Province (Grant No. 2012222009), Fundamental Research Funds
for the Central Universities (Grant No. DC201502080302, DC201502080203, and
DC201502080406), Program for Liaoning Excellent Talents in University (LNET)
(Grant No. LR2015016), and Science and Technique Foundation of Dalian (Grant No.
2014J11JH134, 2015J12JH201).

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Figure captions
Fig. 1 (1) XRD patterns of ZrO2 doped separately with Er, ErꢀYb, ErꢀYbꢀMo. (2)XRD
patterns of the series of the ErꢀYbꢀMo: ZrO2ꢀAl2O3 solid solution samples (a), and
partial enlarge detail of (0,1,1)t peak (b).
Fig. 2 UC emission spectra of the Er:ZrO2, ErꢀYb:ZrO2, and ErꢀYbꢀMo:ZrO2 samples
(Emission slit=5 nm).
Fig. 3 UC emission spectra of the Er3+ doped series of the ZrO2ꢀAl2O3 composite
system (Emission slit=5 nm).
Fig. 4 UC luminescence mechanism of Er3+.
Fig. 5 Absorption spectra of the series of ZrO2ꢀAl2O3 solid solutions samples
singleꢀdoped with 1 mol% Er3+.
Fig. 6 FTIR spectra of the ZrO2ꢀAl2O3 solid solutions samples.
Fig. 7 UC emission spectra of 1 mol% Er3+, 10 mol% Yb3+, 10 mol% Mo6+ doped
series of ZrO2ꢀAl2O3 composite system (Emission slit=2.5 nm).
Fig. 8 Schematic energy level diagram of the ErꢀYbꢀMo triꢀdoped ZrO2ꢀAl2O3
composite system.
Fig. 9 Dependence of the green UC intensity (empty bars) and the red UC intensity
(shaded bars) on different ZrO2ꢀAl2O3 composite samples. Inset shows the CIE
chromaticity diagram for the ZrO2ꢀAl2O3 composite and ZrO2 samples.
TABLE 1: ICP Results of Different Samples

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Y. Cong et al. Fig. 1

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Y. Cong et al. Fig. 2

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Y. Cong et al. Fig. 3

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Y. Cong et al. Fig. 4

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Y. Cong et al. Fig. 5

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Y. Cong et al. Fig. 8

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Y. Cong et al. Fig. 9

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Y. Cong et al. table 1
Samples
8Z-2A
6Z-4A
5Z-5A
4Z-6A
2Z-8A
Er3+ content
0.85 mol%
0.1206 wt%
0.24 mol%
0.0355 wt%
0.52 mol%
0.0768 wt%
0.44 mol%
0.0671 wt%
0.26 mol%
0.0411 wt%