Al2O3:Eu3+ spherical red-emitting phosphor: Synthesis by hydrothermal technique and luminescence characteristics

Article abstract of DOI:10.1016/j.jallcom.2017.05.250

An experimental assessment of europium doped Al₂O₃ spherical phosphors has been conducted, in conjunction with a controllable morphology via one-pot hydrothermal synthesis route and typical red light emission. The most important synthesis step of spheres is utilizing of glucose as structuring additive, followed by a subsequent calcination process in a range of 600–1300 °C. It was found that the final products Al₂O₃:Eu³⁺ exhibited a micro-spherical structure. SEM images indicate that the spheres consist of cross-linked sheets at lower calcination temperatures or coral-like subunits at higher temperatures. XRD patterns show that the adequate calcination temperature can guarantee a well-crystallized Al₂O₃ phase formation. Furthermore, under excitation of a xenon lamp (λ_ex = 392 nm), the Al₂O₃:Eu³⁺ phosphors exhibit a bright red emission. All results of this research can also contribute to a simple synthesis route of metal oxide based phosphors with spherical morphology and lower temperature conditions.

Full text of DOI:10.1016/j.jallcom.2017.05.250

Accepted Manuscript
3+
Al O :Eu spherical red-emitting phosphor: Synthesis by hydrothermal technique
2 3
and luminescence characteristics
Yue Hu, Qian Liu, Guanghui Liu, Zhenzhen Zhou
PII:
S0925-8388(17)31867-4
10.1016/j.jallcom.2017.05.250
JALCOM 41981
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 7 February 2017
Revised Date: 21 May 2017
Accepted Date: 24 May 2017
3+
Please cite this article as: Y. Hu, Q. Liu, G. Liu, Z. Zhou, Al O :Eu spherical red-emitting phosphor:
2 3
Synthesis by hydrothermal technique and luminescence characteristics, Journal of Alloys and
Compounds (2017), doi: 10.1016/j.jallcom.2017.05.250.
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ACCEPTED MANUSCRIPT
Al2O3:Eu3+ spherical red-emitting phosphor: Synthesis by
hydrothermal technique and luminescence characteristics
Yue Hu†, ‡, §, Qian Liu*, †, ‡, Guanghui Liu†, ‡, Zhenzhen Zhou†, ‡
†State Key Laboratory of High Performance Ceramics and Superfine Microstructure,
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road,
Shanghai, 200050, China.
‡Shanghai Institute of Materials Genome, 99 Shangda Road, Shanghai 200444, China
§University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049,
China
*Correspondence author: qianliu@mail.sic.ac.cn
Abstract
An experimental assessment of europium doped Al2O3 spherical phosphors has
been conducted, in conjunction with a controllable morphology via one-pot
hydrothermal synthesis route and typical red light emission. The most important
synthesis step of spheres is utilizing of glucose as structuring additive, followed by a
subsequent calcination process in a range of 600-1300 ꢀ. It was found that the final
products Al2O3:Eu3+ exhibited a micro-spherical structure. SEM images indicate that the
spheres consist of cross-linked sheets at lower calcination temperatures or coral-like
subunits at higher temperatures. XRD patterns show that the adequate calcination
temperature can guarantee a well-crystallized Al2O3 phase formation. Furthermore,
under excitation of a xenon lamp (λex= 392 nm), the Al2O3:Eu3+ phosphors exhibit a

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bright red emission. All results of this research can also contribute to a simple synthesis
route of metal oxide based phosphors with spherical morphology and lower temperature
conditions.
Keywords: Hydrothermal, Al2O3:Eu3+ phosphor, Spherical morphology, Red emission,
Luminescent property
1. Introduction
Rare earth (RE) doped luminescent materials have attracted considerable attention
due to their interesting fundamental properties and tremendous potential applications in
optoelectronic fields, such as, phosphors, laser materials, optical amplifiers, and
photocatalysts [1-4]. Among the RE ions, trivalent europium ion (Eu3+) is a potential
candidate to be used in electroluminescent devices, emitting phosphors light, optical
amplifiers, high-density optical storage, and lasers [5, 6], because of its exceptional
spectroscopic properties [7-10]. The emission spectrum of Eu3+ shows a wide spectral
5
range from visible to the near infrared, assigned to D0→7FJ (J=0, 1, 2, 3, 4, 5, 6)
5
transitions. Especially, the D0→7F2 transition (known as hypersensitive transition) can
determine the symmetry site of ions in the crystal structure (unit cell) of host materials.
The selection of host materials for RE ions is fundamental for the development of
luminescent materials [11]. The choice of a suitable host and rare earth ions include the
proper combination of solubility, large optical band gap, and low phonon energies to
achieve luminescent efficiency and color chromaticity. Alumina is an attractive host
because of its high optical transparency from ultraviolet to near-infrared and excellent
mechanical properties with high thermal and chemical stability [12]. Although
alumina-based phosphors have been extensively studied since the 1960sꢁin recent yearsꢁ
alumina powders doped with different rare earth ions (such as Tb3+, Dy3+, Er3+, Yb3+ ,

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Nd3+, Tm3+ ions) still arouse great research interests in its low-temperature synthesis
methods, tailoring of morphology, spectral property controlling, energy-transfer
processes, possibility as up-conversion luminescent materials, and so on [13-18].
Turning to the Eu3+ doped alumina phosphor (Al2O3:Eu3+), it has been prepared by
sol-gel techniques, combustion, solvent evaporation, co-precipitation technique, and
beam implantation method as well [19-23]. However, the preparation of Al2O3:Eu3+ with
well-defined morphologies is still a great challenge.
Based on previous researches, the ideal morphology of phosphor particles includes
a perfect spherical shape, narrow size distribution, and non-agglomeration, mainly for
two reasons: (1) the size of phosphor particles decides its luminescence, and thus a
narrow size distribution allows better luminescence uniformity, that is important for
display screens, and (2) the spherical phosphors are easy to be packed densely and
meanwhile can reduce the scattering of light to improve luminescence [24]. Of the
various methods, the hydrothermal synthesis is advantageous in producing high-quality
and well-dispersed crystals in one step, accompanied with a homogeneous dopant
distribution. A series of phosphor spheres such as Y2O3:(Er3+-Yb3+, Eu3+), Gb2O3:Tb3+,
Lu2O2S:Eu3+, PbWO4:Ln3+ (Ln=Tb, Eu, Dy, and Sm) and so on have been synthesized
by hydrothermal method [25-29]. From another angle, glucose has been widely used as
a hard template for the universal synthesis of metal oxide spheres by hydrothermal
technique [30]. Naskar et al. have exploited the hydrothermal synthesis of rod shaped
alumina in the presence of glucose [31]. Zhou et al. have shown the use of glucose as
templating agent in the synthesis of rattle-type carbon−alumina core−shell spheres by
hydrothermal method [32]. However, as far as our best knowledge, the synthesis and
mass production of Al2O3:Eu3+ powders with uniform spheres and sharp size
distribution via a simple synthesis method hasn’t been reported.

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In this manuscript, we present an effective synthesis route of spherical Al2O3:Eu3+
red phosphors via a facile one-pot hydrothermal process that utilizes glucose as
structuring guide and additive. The luminescent properties of the as-prepared spherical
Al2O3:Eu3+ phosphors are also reported.
2. Experimental procedures
2.1. Synthesis of Al2O3:Eu3+ spheres
Different concentrations of Eu3+ ions activated Al2O3 (Al2-xO3:xEu3+, simply
named as Al2O3:xEu3+) spheres were prepared with the mole ratio of Al:Eu=(2-x):x by a
one-step hydrothermal method, in which x=0.2, 0.5, 1.0, 1.2, 1.5 mol %. The utilized
aluminum and europium sources are Al(NO3)3·9H2O (≥ 99.0 %; A. R., Sinopharm
Chemical Reagent Co., Ltd., Shanghai, China) and Eu(NO3)3·6H2O (≥ 99.0 %; A. R.,
Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), respectively. To begin the
process, the stoichiometric amounts of Al(NO3)3·9H2O and Eu(NO3)3·6H2O were
dissolved into 60 ml de-ionized water under stirring, followed by the addition of 7.5 g
glucose (A. R., Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) at room
temperature to form a clear solution. Typically, the concentration of [Al3+ + Eu3+] is 0.5
M. The mixed solution was placed in a 80 ml Teflon-lined stainless steel autoclave,
followed by heat-treatment to 180 ꢀ for 20 h. The brown solid precursors formed
were separated by centrifugation, washed with de-ionized water and alcohol for three
cycles, and then dried at 80 ꢀ for 12 h. Finally, the precursors were calcined at
600-1300 ꢀ for 5 h. Eu3+-undoped Al2O3 spheres were also obtained by the same
preparation methods as a comparison.
2.2. Characterization
Powder X-ray diffraction patterns of the powder samples were characterized on a

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D/MAXX-ray diffractometer (Rigaku, Tokyo, Japan). The particle size and morphology
of the samples were observed on a S-4800 Field Emission Scanning Electron
Microscope (Hitachi, Tokyo, Japan). Photoluminescence measurements (PL and PLE)
were performed on a Hitachi F-4600 spectrometer equipped with a 150 W xenon lamp
as the excitation source at room temperature (RT, 25 ꢀ) conditions.
3. Results and discussion
All the Al2O3:xEu3+ powders thus prepared by one-step hydrothermal route
calcined in the temperature range of 600-1300 ꢀ are spherical in shape, including the
fabricated hydrothermal precursor. The typical SEM morphology images of
Al2O3:0.01Eu3+ (Fig. 1) after calcination at 600 ꢀ and 1300 ꢀ indicate the
individual spherical morphology of phosphor samples. Otherwise, our SEM
observations on Al2O3:xEu3+ prove that both calcination temperature and doping
concentration do not cause any significant changes in spherical shape. The rough
surfaces of the spheres imply that these spheres may be comprised of smaller subunits.
The surface of sample calcined at lower 600 ꢀ is composed of many cross-linked
sheets with the thickness about 15 nm. However, after calcined at higher 1300 ꢀ for 5
h, the smaller subunits of cross-linked sheets turn into a coral-like morphology whose
size is about 40 nm. Moreover, the spherical morphologies can be maintained after
calcination perhaps due to the higher activation energies needed for collapse or change
of the spherical powders structures [33]. Based on the hydrothermal processing and
SEM micrographs, the formation mechanism of Al2O3:xEu3+ spheres can be briefly
deducted as following (illustrated in Scheme 1): firstly, Al3+ and Eu3+ ions are produced
after Al(NO3)3·9H2O and Eu(NO3)3·6H2O dissolving in deionized water. Then Al3+ and

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Eu3+ cations can react with the added glucose to form lamellar organic-inorganic hybrid
structures under hydrothermal [34] (Scheme 1- ꢂ). Secondly, upon subsequent
dehydration of organic-inorganic hybrid and carbonization of glucose, small particles
are formed [35] (Scheme 1-ꢃ). Afterwards, the particles aggregate via self-assembly
forming large sheets (Scheme 1-ꢄ). Subsequent loss of water by self-assembly leads to
further coalescence of sheets to form spherical structures [36] (Scheme 1-ꢅ). Finally,
during the process of calcinations, the organic component and water vapor are removed,
resulting in the formation of Al2O3:xEu3+ spheres with a porous structure (Scheme 1-
ꢆ).
Fig.2 shows XRD patterns of the as-prepared Al2O3:xEu3+ (x=0.2, 0.5, 1.0, 1.2, 1.5
mol %) calcined at the highest 1300 ꢀ, with the standard peak position of α-Al2O3
phase (JCPDS NO.10-0173) inserted at the bottom as reference. All of samples are well
crystallized and possess sharp XRD patterns. However, the undoped Al2O3 and
low-doped Al2O3:0.002Eu3+ are both indicated to be pure α-Al2O3 phase. While, with
regard to the samples having a concentration of Eu3+ ions larger than 0.5 mol %, their
XRD patterns show a two-phase mixture of α-Al2O3 and EuAlO3. The EuAlO3 is
considered as an impurity phase and can be produced via the reaction 0.5Al2O3
+0.5Eu2O3ꢇEuAlO3, limited by the solubility of Eu3+ in Al2O3 [37].
Fig.3a and Fig.3b show the excitation spectra (PLE) of Al2O3:xEu3+ (x=0, 0.2, 0.5,
1.0, 1.2, and 1.5 mol %) calcined at 800 ꢀ and 1300 ꢀ, respectively, by monitoring
5
Eu3+ emission of D0ꢇ7F2 transition at 613 nm, as representative. It indicates that the
excitation spectra consist of two regions: one is a strong broadband centered at about
255 nm for the sample calcined at 800 ꢀ but at about 255-320 nm for the sample
calcined at 1300 ꢀ, another one is a series of sharp lines (from 360 nm to 540 nm) for

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all samples calcined at both 800 ꢀ and 1300 ꢀ. The broad band corresponds to the
Eu3+→O2- charge transfer (CTB), which is the electronic transition from the 2p orbital
of O2- to the 4f orbital of Eu3+, while the sharp lines are ascribed to the transitions
within the Eu3+ 4f6 electronic configurations: 7F0→5D4 (~360 nm), 7F0→5L7 (~380 nm),
7F0→5L6 (~392 nm), 7F0→5D3 (~412 nm), 7F0→5D2 (~464 nm), and 7F0→5D1 (~532 nm)
transitions. A weak absorption peak embedded in the CTB can be found in the PLE
7
spectrum which can be attributed to the F0→5H3 transitions (~317 nm). The samples
calcined at other temperatures (600, 700, 900 and 1150 ꢀ) exhibit a similar spectral
profile (seen in Fig.S1 in the Supporting Information). However, the band maxima of
the CTB shifts towards a lower energy side or a longer wavelength side (Fig.3c) with
increasing of samples calcination temperatures, especially for the samples calcined at
higher 1300 ꢀ. Normally, the CTB position of Eu-doped oxide phosphors depends on
the Eu-O bond length and the coordination environment around the Eu3+ ions [38]. The
shorter the Eu-O bond is, the shorter the wavelength of the CT band position would be
[39-41]. On the basis of the variations of CTB peak position with calcined temperatures
shown in Fig.3c, it can be supposed that the length of Eu-O bond in phosphor samples
becomes longer with increasing of calcination temperatures. Nevertheless, in the
Al2O3:xEu3+ phosphors, the lengthened Eu-O bond may be related to the phase
transformation of γ (cubic) → θ (monoclinic) → α-alumina (rhombohedral) from low
calcination temperature of 600 ꢀ to high 1300 ꢀ, although the location of Eu3+ in
Al2O3 matrix structure is still unclear.
Fig.4a and Fig. 4b show the PL spectra of Al2O3:xEu3+ (x=0, 0.2, 0.5, 1.0, 1.2, and
1.5 mol %), calcined at 800 ꢀ and 1300 ꢀ, measured through intra-4f6 direct
5
excitation at 392 nm, as representative. The PL spectra all display a typical D0→7FJ
5
(J=0-4) transitions of Eu3+. The spectra illustrate the band transitions: D0→7F0 (~578

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nm), 5D0→7F1 (591 and 598 nm), 5D0→7F2 (613 nm), 5D0→7F3 (654 nm), and 5D0→7F5
(687 and 702 nm). No significant shift of peak position is observed for samples with
various Eu3+ ion doping concentrations. As a supplement, the Al2O3:xEu3+ samples
calcined at 600, 700, 900 ꢀ, and 1150 ꢀalso exhibit a similar spectral profile, seen in
Fig.S2a-c in the Supporting Information. It has to be emphasized that the emission
spectra of the samples (whose x=0 and 0.2 mol % calcined at 1150 ꢀ as well as all the
doped samples calcined at 1300 ꢀ) present a detectable intense and narrow peak at
around 694 nm assigned to the Cr3+ emission (R line), which overlaps with the
5
transition D0→7F5 of Eu3+. The emission R line of Cr3+ ions observed in Al2O3:xEu3+
(calcined at 1150 and 1300 ꢀ) may serve as an additional argument for identifying the
formation of crystalline phase α-Al2O3 [42]. As well-known, the Cr3+ is often co-existed
with dominant Al3+ component as an impurity difficult to remove.
Optical spectra of Eu3+ are well known to be strongly dependent on its site
symmetries located in the host crystal. The Judd-Ofelt parity law predicts that the
electric dipole (5D0→7F2, 613 nm) allowed transition can be dominant when Eu3+
occupies the lattice site of a non-centrosymmetric environment in the host matrix, while
the magnetic dipole (5D0→7F1, 591 and 598 nm) transition does depend on the local
environment. The intensity ratio of transitions (5D0→7F2)/(5D0→7F1), so-called
asymmetry ratio, can be used as an optical probe to determine the symmetry site where
the Eu3+ is inserted. The asymmetry ratio is presented by A21, where subscripts ‘1’ and
‘2’ refer to transitions of (5D0 to 7Fj), j=1 and 2, respectively. The A21 is represented by
the equation as follows:
ꢃꢄꢅ Iꢀ dλ
ꢂ
ꢃꢆꢅ
Aꢀꢁ
=
ꢃꢆꢅ Iꢁ dλ
ꢂ
ꢇꢈꢀ

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The larger the A21 is, the smaller the degree of the symmetry of the crystalline
region is, where the Eu3+ is surrounding. The calculated A21 values for Al2O3:0.01Eu3+
calcined at different temperatures are shown in Fig.4c. Based on the A21 calculation
results, the Eu3+ ions in Al2O3:xEu3+ systems are understood to be in the
non-centrosymmetric lattice site, and A21 values indicate a decreasing trend with
calcination temperature increasing due to the lattice perfection change in the final phase
of phosphors.
4. Conclusions
In summary, Al2O3:xEu3+ spherical phosphors have been successfully synthesized
via a one–step hydrothermal method followed by a subsequent heat treatment, utilizing
commercially available starting materials, especially using glucose as effective
structuring guide for spheres formation. The synthesis route is practical and
easily-controlled. The adequate addition of Eu3+ ions and calcination temperature
greatly influence phase formation and phase composition of matrix Al2O3. EuAlO3
impurity phase appears due to the reaction between Al2O3 and heavy-doping Eu2O3. The
emission spectra of as-prepared samples show an intense red light emission from
5D0→7F2 transition of Eu3+ peaked at 613 nm. The developed synthesis process is
relatively simple and cost-effectively, which is a suitable method for preparing
Al2O3:RE or even other metal oxide based spherical phosphors.
Acknowledgements
Many thanks for the financial supports from the National Key Research and
Development Program of China (Grant No. 2016YFB0700204 and 2016YFB0701004)
and the Research Program of Shanghai Sciences and Technology Commission

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Foundation (Grant No.14DZ2261203).
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Figure captions
Fig.1 SEM images of as-prepared Al2O3:0.01Eu3+ calcined at (a) 600 ℃ and (b)
1300 ℃, insets are high magnification SEM images of powder surfaces.
Fig.2 XRD patterns of Al2O3:xEu3+ with different doping concentrations calcined at
1300 ℃. (♦: EuAlO3)
Fig.3 Excitation spectra (λem=613 nm) of Al2O3:xEu3+ with different doping
concentrations calcined at (a) 800 ℃, (b) 1300 ℃ and (c) Variations of peak
position of CTB with calcined temperatures for Al2O3:0.01Eu3+.
.
Fig.4 Emission spectra (λex=392 nm) of Al2O3:xEu3+ with different doping
concentrations calcined at (a) 800 ℃, (b) 1300 ℃ and (c) Asymmetric ratio vs
calcination temperatures for Al2O3:0.01Eu3+.
Scheme1. Schematic illustration of formation evolution of Al2O3:xEu3+ spheres.

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Supporting Information captions
Fig. S1 Excitation spectra of Al2O3:xEu3+ with different doping concentrations
calcined at (a) 600 ℃; (b) 700 ℃; (c) 900 ℃; (d) 1150 ℃. (λem=613 nm)
Fig. S2 Emission spectra of Al2O3:xEu3+ with different doping concentrations calcined
at (a) 600 ℃; (b) 700 ℃; (c) 900 ℃; (d) 1150 ℃. (λex=392nm)

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Highlights
ꢀ Al2O3:xEu3+ phosphors were prepared by a simple hydrothermal method.
ꢀ Glucose was used as structuring additive during synthesis.
ꢀ Al2O3:xEu3+ phosphors present a spherical morphology.
ꢀ Emission spectra of Al2O3:xEu3+ phosphors show a red emission peaked at 613
nm.