Spherical engineering and space-group dependent luminescence behavior of YBO3:Eu3+ red phosphors

Article abstract of DOI:10.1016/j.jallcom.2017.10.155

Spherical particles of (Y_1-xEu_x)BO₃ (x = 0–0.20, ～0.8–1.5 μm in diameter) have been converted from (Y_1-xEu_x)(B(OH)₄)CO₃ precursors synthesized via homogenous precipitation processing. Detailed characterizations of the samples were achieved by combined techniques of XRD, Raman, FE-SEM, HR-TEM, PLE/PL, and fluorescence decay analysis. Through homogenous precipitation processing in the absence of EG, orthorhombic (Y,Eu)(B(OH)₄)CO₃ cubic particles were obtained owing to their crystal habit. Proper EG addition leads to the formation of spherical particles, but nano-sized amorphous particles were resulted from the excessive EG content. Longing reaction time or elevating the reaction temperature induced the appearance of cubic ones. Calcining (Y,Eu)(B(OH)₄)CO₃ at 700 °C yielded hexagonal structured (Y,Eu)BO₃ phase with the space group of P6₃/m, but another hexagonal structured (Y,Eu)BO₃ phase (space group: P6₃/mmc) was obtained with the calcination temperatures of 800–1000 °C. The spherical shape and excellent dispersion of the original particles can be well retained up to 800 °C. Upon UV excitation, the (Y_1-xEu_x)BO₃ (x = 0–0.20) spheres exhibited a typical Eu³⁺ emission with the orange emission at 592 nm from the ⁵D₀-⁷F₁ magnetic dipole transitions of Eu³⁺ ions taking the dominant role. Space-group vibrations between in 700 °C-phase and 800–1000 °C phase resulted in the blue shift of CTB center, but they did not significantly affect the lifetimes. A higher Eu content (x = 0.01–0.10) leaded more Eu³⁺ ions occupying the non-centrosymmetric site instead of S₆ one, thus inducing enhanced emission of ⁵D₀→⁷F₂ transition, larger asymmetry factor, and red shift of the CIE chromaticity coordinates. However, the asymmetry factor remains nearly constant with the x value up to 0.20. The fluorescence lifetime is similar (5.3 ± 0.3 ms) up to 8 at% of Eu³⁺, followed by decreases at higher Eu³⁺ contents.

Full text of DOI:10.1016/j.jallcom.2017.10.155

Accepted Manuscript
Spherical engineering and space-group dependent luminescence behavior of
3+
YBO :Eu red phosphors
3
Qi Zhu, Shuo Wang, Ji-Guang Li, Xiaodong Li, Xudong Sun
PII:
S0925-8388(17)33591-0
10.1016/j.jallcom.2017.10.155
JALCOM 43552
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 8 September 2017
Revised Date: 17 October 2017
Accepted Date: 20 October 2017
Please cite this article as: Q. Zhu, S. Wang, J.-G. Li, X. Li, X. Sun, Spherical engineering and space-
3+
group dependent luminescence behavior of YBO :Eu red phosphors, Journal of Alloys and
3
Compounds (2017), doi: 10.1016/j.jallcom.2017.10.155.
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ACCEPTED MANUSCRIPT
Graphic abstract

ACCEPTED MANUSCRIPT
Spherical engineering and space-group dependent luminescence behavior
of YBO3:Eu3+ red phosphors
Qi Zhu1,2*, Shuo Wang1,2, Ji-Guang Li1,2,3*, Xiaodong Li1,2 and Xudong Sun1,2,4
1Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education),
Northeastern University, Shenyang, Liaoning 110819, China
2Institute of Ceramics and Powder Metallurgy, School of Materials Science and
Engineering, Northeastern University, Shenyang, Liaoning 110819, China
3Research Center for Functional Materials, National Institute for Materials Science,
Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
4School of Environmental and Chemical Engineering, Dalian University, Dalian,
Liaoning 116622, China
*Corresponding author
Dr. Qi Zhu
Tel: +86-24-8368-1680
E-mail: zhuq@smm.neu.edu.cn
Dr. Ji-Guang Li
Tel: +81-29-860-4394
E-mail: LI.Jiguang@nims.go.jp
1

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Abstract
Spherical particles of (Y1-xEux)BO3 (x=0-0.20, ~0.8-1.5 ꢀm in diameter) have been
converted from (Y1-xEux)(B(OH)4)CO3 precursors synthesized via homogenous
precipitation processing. Detailed characterizations of the samples were achieved by
combined techniques of XRD, Raman, FE-SEM, HR-TEM, PLE/PL, and fluorescence
decay analysis. Through homogenous precipitation processing in the absence of EG,
orthorhombic (Y,Eu)(B(OH)4)CO3 cubic particles were obtained owing to their crystal
habit. Proper EG addition leads to the formation of spherical particles, but nano-sized
amorphous particles were resulted from the excessive EG content. Longing reaction
time or elevating the reaction temperature induced the appearance of cubic ones.
o
Calcining (Y,Eu)(B(OH)4)CO3 at 700 C yielded hexagonal structured (Y,Eu)BO3
phase with the space group of P63/m, but another hexagonal structured (Y,Eu)BO3
phase (space group: P63/mmc) was obtained with the calcination temperatures of
o
800-1000 C. The spherical shape and excellent dispersion of the original particles
can be well retained up to 800 °C. Upon UV excitation, the (Y1-xEux)BO3 (x=0-0.20)
spheres exhibited a typical Eu3+ emission with the orange emission at 592 nm from
5
the D0-7F1 magnetic dipole transitions of Eu3+ ions taking the dominant role.
o
o
Space-group vibrations between in 700 C-phase and 800-1000 C phase resulted in
the blue shift of CTB centre, but they did not significantly affect the lifetimes. A
higher Eu content (x=0.01-0.10) leaded more Eu3+ ions occupying the
non-centrosymmetric site insdead of S6 one, thus inducing enhanced emission of
5D0→7F2 transition, larger asymmetry factor, and red shift of the CIE chromaticity
coordinates. However, the asymmetry factor remains nearly constant with the x value
up to 0.20. The fluorescence lifetime is similar (5.3±0.3 ms) up to 8 at% of Eu3+,
followed by decreases at higher Eu3+ contents.
Key words: Rare-earth orthoborates, Spherical particle, Homogenous precipitation,
Luminescence behavior, Asymmetry factor, Lifetime
2

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Graphic abstract
3

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Highlights
1. Boron hydroxyl carbonate spheres were produced as the precursors for YBO3:Eu3+
red phosphors.
2. Spherical engineering was achieved via controlled processing with ethylene glycol
addition.
3. Space-group dependent luminescence behavior was deciphered.
4. The effects of Eu3+ concentration and calcination on crystal structure and
luminescence were well addressed.
4

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1. Introduction
Rare-earth orthoborates of REBO3 (RE=lanthanide, yttrium, and scandium)
co-doped with activators, such as Eu3+ and Tb3+, have attracted much recent attention
because of their low toxicity, strong luminescence intensity, high chemical stability,
vacuum ultraviolet (VUV) transparency, and exceptional optical damage threshold
[1-4]. REBO3 usually crystallizes into several structures depending on the radius of
rare earth ions, including aragonite, calcite, and pseudo-vaterite types [5-8]. The
largest rare earth ion, La3+, forms an orthoborate with the orthorhombic aragonite
structure [9]. For the smaller rare earth ions, Y3+ and Gd3+, the orthoborates possess
the vaterite structure, which is consistent with a pseudohexagonal symmetry. For the
smallest rare earth ion, ScBO3 exhibits the rhombohedral calcite structure, having a
single site for the rare earth with S6 point symmetry [9]. Among the rare-earth
orthoborates, YBO3:Eu3+ is one of the best red phosphors to be widely used in PDPs
and a new generation of Hg-free fluorescent lamps, owing to its wide band gap
[10-15]. Although YBO3 exhibits relatively lower electron mobility than Y2O3, it
shows a higher host-to-activator energy transfer efficiency than that for Y2O3 at
moderate Eu3+ concentrations [16]. The low mobility is presumably due to the ionic
nature of the bonding in this material, and to the fact that the conduction band of
YBO3 is primarily composed of Y 4d states with little mixing of oxygen states [16].
From previous crystallographic studies, however, much debate and various space
groups have been proposed on the YBO3-type crystal structure. It is widely accept that
YBO3:Eu3+ possesses the hexagonal vaterite-type structure and the Eu3+ ion can
occupy the Y3+ site with the point symmetry S6 [17]. Therefore, the orange emission at
5
~591 nm from the transition of D0-7F1 takes the dominant role, thus resulting in an
orange-red rather than red chromaticity. Consequently, much effort is underway and
many approaches to improve the chromaticity have been reported during the past few
years. A “color effect” for YBO3: Eu3+ was observed by Dexpert-Ghys et al., and the
ratio of orange and red emission intensities was found to vary with the pressure in the
plasma setup, by consideration of the site selective excitation of Eu3+ in the
orthoborate matrix in the VUV range [18].
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It is generally understood that parameters such as size, shape, and crystal types of
YBO3:Eu3+, closely related to the synthetic routes, are fundamentally and
technologically related to luminescent properties of the materials [19-23]. So the
present tendency of development of such materials is not only oriented toward new
phases but also toward a better knowledge and a better control of microstructures of
existing materials. Because of high reaction temperature or iterative grind process, the
previously methods, such as solid-state reaction, sol-gel technology, and the
ammonium nitrate melt processing, always leads to a bad morphology of YBO3:Eu3+
[24-27]. Therefore several novel routes, such as electro-spinning processing,
hydro-/solvo- thermal methods, have been developed to control the shape, size and
distribution of orthoborate phosphors particles [28], and uniform YBO3:Eu3+ particles
were prepared in the previous literatures [29-36], including nanotubes and nanowires,
uniform drum-like microcrystals, doughnut-like assemblies and pancake-like
hierarchical architectures. However, phosphor particles of spherical shape, fine size,
and a narrow size distribution are desirable in high-definition displays to improve the
resolution by reducing the pixel size and improving the overall luminescent
performance [37-40]. Monodispersed spheres may also self-assemble into arrays or
domains, such as photonic crystals and close-packed films. This enables the formation
of functionalities not only from the spheres themselves but also from the long-range
periodic structures [37-40]. Therefore, some effort has been paid to the spherical
engineering of YBO3:Eu3+ particles, including template processing [41-43], but the
successful reports are rather limited. Recently, You et al. developed a hydrothermal
approach to form luminescent Eu3+-doped YBO3 phosphor onto the surface of
Y(OH)CO3:Eu3+
colloid
spheres
to
obtain
spherical
particles
of
Y(OH)CO3:Eu3+/YBO3:Eu3+ [44]. Then, the same research group synthesized
monodisperse YBO3:Eu/Tb microspheres by an ethylene glycol-mediated
solvothermal method [45]. This, though is a success in YBO3-based phosphors, may
not be widely applied in PDPs and other devices, since the low yield and lengthy
process of solvothermal synthesis. It is widely accept that many resultant phosphors,
such as oxides and sulfides, largely retained the original morphologies of their
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respective precursors, so much efforts have been paid to the synthesis and
morphology control of precursors. Urea-based homogeneous precipitation (UBHP)
technique is a typical method that can produce monodispersed basic carbonate colloid
spheres of lanthanides (including Y) by taking advantages of the slow decomposition
o
of urea at elevated temperatures (≥83 C), which are the ideal precursors for
luminescent oxides [46-49]. In this present work, spherical particles of
(Y,Eu)(B(OH)4)CO3 have been successfully prepared by UBHP method with a high
yielding, which converted into (Y,Eu)BO3 spheres by proper annealing. Formation
mechanism, calcination-dependent space group, and luminescence behavior of the
spheres were then investigated via detailed characterizations by combined techniques
of XRD, Raman, FE-SEM, HR-TEM, PLE/PL spectroscopy, and fluorescence decay
analysis. We believe that the outcomes of this work may enrich the applications of the
yttrium orthophosphate materials and may offer wide implications to other rare-earth
orthophosphate systems.
2. Experimental section
2.1 Synthesis
The starting rare-earth sources are Y2O3 and Eu2O3 all 99.99% pure products
purchased from Huizhou Ruier Rare-Chem. Hi-Tech. Co. Ltd (Huizhou, China). The
other reagents are of analytical grade and were purchased from Shenyang Chemical
Reagent Factory (Shenyang, China). The nitrate solution of RE3+ was prepared by
dissolving the corresponding oxide with a slightly excessive amount of nitric acid,
followed by evaporation at ~90 °C to dryness to remove the superfluous acid. In a
typical synthesis, 0.225 mol of H3BO3, 0.5 mol (30 g) of urea (CO(NH2)2), and 130
mL of ethylene glycol (EG) were dissolved in the mixed nitrate solution to make a
total volume of 1000 mL, which was then homogenized under magnetic stirring at
25 °C for 30 min with the pH value at ~ 4.5. In all the cases, the total concentration of
RE3+ was kept constant at 0.015 M. The mixed transparent solution was heated to
90 °C within 50 min in a water bath, followed by natural cooling to 50 °C after
reacting at ~90 °C for 2 h. The resultant colloidal particles were collected via
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centrifugation, washed with distilled water for three times to remove byproducts,
rinsed with absolute ethanol, and then dried in air at 60 °C for 24 h. (Y,Eu)BO3 was
obtained by calcining the precipitation products under flowing O2 gas (200 mL/min)
at selected temperatures for 2 h with a heating rate of 10 oC/min at the ramp stage.
2.2 Characterization techniques
Phase identification was performed by X-ray diffractometry (XRD, Model
SmartLab, Rigaku, Tokyo, Japan) operating at 40 kV/40mA using nickel filtered Cu
Kα radiation and a scanning speed of 6.0° 2θ/min. Thermogravimetry (TG, Model
SETSYS Evolution-16, SETARAM, France) of the dried precursor was made in
flowing oxygen gas with a heating rate of 10 °C/min. The Raman scattering
measurements were carried out using the Raman microscope (Model R-XploRA Plus,
Horiba, Kyoto) with a 785 nm He-Ne laser as the excitation source. The laser power
was 49.1 mW at the sample. Morphologies of the products were observed via field
emission scanning electron microscopy (FE-SEMꢀModel JSM-7001F, JEOL, Tokyo)
and transmission electron microscopy (TEM, Model JEM-2000FX, JEOL, Tokyo).
Photoluminescence and fluorescence decay of the phosphors were analyzed with an
FP-8600 fluorospectrophotometer (Jasco, Tokyo).
8

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(e)
Y(B(OH)4)CO3
, JCPDS no.78-2062
(f)
YBO
3
,
JCPDS no.74-1929
10
20
30
40
50
60
70
2θ (degree)
Figure 1. FE-SEM images and XRD patterns of the (Y0.95Eu0.05)(B(OH)4)CO3 (a,e)
and (Y0.95Eu0.05)BO3 (b,f) spheres. (c,d) are TEM and SAED patterns of the
(Y0.95Eu0.05)BO3 spheres.
3. Results and discussion
3.1 Phase structure and morphology of the precursors and orthoborates spheres
The systematic studies performed in this work indicate that spherical particles of
(Y,Eu)BO3 can be converted from the precursors (Y,Eu)(B(OH)4)CO3 by a proper
annealing. In a typical synthesis, (Y0.95Eu0.05)(B(OH)4)CO3 spheres, with a diameter
of 0.8-1.5 ꢀm, were synthesized by UBHP processing in the presence of ethylene
glycol (EG) (Figure 1a). XRD patterns indicated that the obtained spheres were
identified along with the orthorhombic (Y0.95Eu0.05)(B(OH)4)CO3 (Y(B(OH)4)CO3,
JCPDS No. 78-2062) (Figure 1e). Calculation from the diffractions data yielded the
cell constants being a=0.9089 nm, b=1.2247 nm, c=0.8930 nm, larger than that of
orthorhombic Y(B(OH)4)CO3 from JCPDS No. 78-2062 (a=0.9089 nm, b=1.2244 nm,
c=0.8926 nm), mainly due to the larger ionic size of Eu3+ (for 8-fold coordination,
=0.1019 nm and
=0.1066 nm) [50]. Calcining the (Y0.95Eu0.05)(B(OH)4)CO3
rEu3+
r
Y 3+
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at 800 °C for 2 h, the resultant products have displayed all the characteristic
diffractions corresponding to the hexagonal (Y0.95Eu0.05)BO3 (YBO3, JCPDS No.
74-1929) (Figure 1f). Also owing to the larger ionic size of Eu3+, larger cell constants
than that of JCPDS No. 74-1929 were observed, which would be discussed in the later
text. It is seen that the spherical shape of the original particles can be well retained to
the (Y0.95Eu0.05)BO3 (Figure 1b,c). Mainly due to the decomposition of hydroxyl and
carbonate, smaller particles were observed. The HR-TEM and SAED analysis further
confirmed that the spheres belong to hexagonal crystal structure (Figure 1c,d). The
well resolved lattice fringes suggest their excellent crystallinity, while the spacings of
~0.323 nm correspond well to the (100) plain of hexagonal structured YBO3
(d(100)=~0.32 nm, JCPDS No. 74-1929) (Figure 1c). The SAED results indicated that
the obtained spheres are polycrystalline. It should be mentioned that successful
synthesis of nano/micro-materials with ideal morphologies in a solution-based system
not only depends on the intrinsic structure of the target compounds but also requires
more fastidious control of the growth parameters such as temperatures, organic
additives, pH values and so forth. Therefore, spherical engineering of the precursor
particles will be illustrated in the following section.
3.2 Spherical engineering of the precursor particles
Figure 2a shows the FE-SEM morphology of the particles synthesized via UBHP
processing in the absence of EG, from which cubic particles are clearly discernible
with the majority of particles having lateral size of 1-1.5 ꢀm. However, spherical
particles were observed rather than the cubic ones via adding 130 mL EG in the
reaction system (Figure 2d). Samples in Figure 2b and Figure 2c were mixtures of
cubic and spherical particles, indicating that they were in the process of morphology
transformation from cubic ones to spheres at the mild EG addtion of 100-110 mL.
Further adding EG up to 150-200 mL yields nano-sized amorphous particles, which
aggregated into larger irregular ones as shown in Figure 2e,f. Figure 3 shows the
corresponding XRD patterns of the samples indicated in Figure 2. Obviously, the
obtained cubic particles and spheres were both identified along with the orthorhombic
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(Y0.95Eu0.05)(B(OH)4)CO3 (Y(B(OH)4)CO3, JCPDS No. 78-2062), indicating that the
phase structure is independent on the EG addition until the EG content up to 150 mL.
The well-developed cubic morphology (Figure 2a) should be derived from the
preferred crystallographic growth habit of the orthorhombic symmetry. However,
gradual rounding corners instead of original sharp ones in cubic particles were
observed through EG addition, thus leading to the final spherical morphology by
increasing the EG from 0 to 130 mL, indicating that the particles can be spherified by
EG. Because the sharp corners are high-energy ones, the EG will preferentially be
adsorbed on them and then inducing rounded corners by inhibiting growth. Stronger
effect of the inhibiting growth on the particles were observed by increasing the EG
addition, but nano-sized amorphous particles were resulted from the excessive EG
content above 150 mL (Figure 2e,f and Figure 3e,f).
Figure 2. FE-SEM morphologies of the precursors synthesized via homogeneous
precipitation at 90 oC for 2h with various amounts of EG in the reaction system: (a) 0
mL, (b) 100mL, (c) 110mL, (d) 130mL, (e) 150mL, (f) 200mL.
11

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(f)
(e)
(d)
(c)
(b)
(a)
Y(
B(OH)
4
)
CO , JCPDS no.78-2062
3
10
20
30
40
50
60
70
2
θ (degree)
Figure 3. XRD patterns of the precursors synthesized via homogeneous precipitation
o
at 90 C for 2h with various amounts of EG in the reaction system: (a) 0 mL, (b)
100mL, (c) 110mL, (d) 130mL, (e) 150mL, (f) 200mL.
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Figure 4. FE-SEM morphologies of the precursors synthesized via homogeneous
precipitation at 90 oC (130 mL EG in reaction system) for (a) 1h, (b) 2h, (c) 3h, (d) 4h,
(e) 5h.
Besides for the EG content, the effect of reaction time and temperature on the
particle shape and crystal structure was systematically studied in this work. Figure 4
shows the FE-SEM morphology of the particles synthesized via UBHP processing at
90 oC with the EG content at 130 mL for different reaction time. At the reaction time
of 1h, the resultant products are nano-sized amorphous particles of ~50 nm in the
absence of spherical shape (Figures 4a and S1). Longing the reaction time to 2h,
spherical particles of orthorhombic (Y0.95Eu0.05)(B(OH)4)CO3 crystallized (Figures 4b
and S1). Reaction for another hour (up to 3h, Figures 4c), the sphere shape was
maintained. However, the trace of cubic shape was discernible at 4h with the majority
of particles having the spherical shape without changing the crystal structure (Figure
4d). More and more cubic ones were obtained by longing the reaction time up to 5 h
(Figure 4e). However, the crystal structure is independent on the cubic and spherical
shapes (Figure S1). Another interesting finding is that the particles shape was
significantly affected by the reaction temperature. The orthorhombic
o
(Y0.95Eu0.05)(B(OH)4)CO3 crystallizes into ellipsoids of ~1.0 ꢀm in length at 80 C
(Figures 5a and S2), and they tend to spherical ones at a higher temperature of 85 oC
with the mixture of some ellipsoids (Figures 5b and S2). Almost all spheres were
observed in the field of view at 90 oC (Figures 5c and S2). Increasing the temperature
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o
to 98 C yields cubic particles (Figures 5e and S2), which is similar to that in Figure
4e. The mixture of spheres and cubic ones was observed in Figure 5d, indicating that
they were in the process of morphology transformation from spheres to cubic particles
o
at the temperature of 95 C. Since the orthorhombic (Y0.95Eu0.05)(B(OH)4)CO3 tends
to crystallizing into cubic particles owing to their crystal habit, longing reaction time
or elevating the reaction temperature induced the appearance of cubic ones, which is
mainly due to their better crystallization (Figures S1 and S2). Therefore, spherical
particles can be obtained by optimizing the crystallization processing, at a proper
reaction temperature of 90 oC and the reaction time from 2h to 3h.
Figure 5. FE-SEM morphologies of the precursors synthesized via homogeneous
precipitation (130 mL EG in reaction system) at various temperature for 2h: (a) 80oC,
(b) 85oC, (c) 90oC, (d) 95oC, (e) 98oC.
14

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3.3 Crystal structure, space group and morphology of the derived orthoborates
spheres
Thermal decomposition process of the orthorhombic (Y0.95Eu0.05)(B(OH)4)CO3
were investigated via TG analysis in Figure S3, indicating that this compound
o
decomposes to (Y0.95Eu0.05)BO3 up to the temperature of ~700 C. With a holding
period of 2 h, the precursors calcined at 700 °C have displayed all the characteristic
diffractions corresponding to the hexagonal structured (Y0.95Eu0.05)BO3 phase (space
group: P63/m, JCPDS No. 16-0277) with the (102)/(100) ratio of ~1.1 ((102)/(100)
ratio of ~1.1 (above 1) in JCPDS No. 16-0277, Figure 6). It is interesting to note that
the (Y0.95Eu0.05)BO3 phosphor powders calcined from this spheres precursor show the
o
higher (102)/(100) ratio of ~0.9 (below 1) at the elevated temperatures of 800 C
(Figure 6). The (102)/(100) ratio almost kept at a constant of ~0.7 (below 1) at the
o
calcination temperature of 900-1000 C. Since the (102)/(100) ratio, with the value
below 1, is close to that in JCPDS No. 74-1929, the products may be assigned to
another hexagonal structured (Y0.95Eu0.05)BO3 phase (space group: P63/mmc, JCPDS
No. 74-1929). Until now, seven main crystallographic structures of the vaterite-type
orthoborate YBO3 at room temperature have been determined. However, two possible
structure models, a disordered hexagonal structure and an ordered hexagonal structure,
are accepted in orthoborate YBO3. In order to gain more insight into the effects of
calcination temperature on the crystal structure, Raman spectra was employed and the
results are shown in Figure 7. The Raman spectrum for (Y0.95Eu0.05)BO3 calcined at
o
700 C exhibits vibrational bands typical for B3O9 rings [51]. The B3O9 ring has D3h
symmetry with twelve atoms, which leads to 20 vibrational modes. The B-O-B
bending of the BO4 units includes Raman bands at the range from 250 to 500 cm-1
[51]. B3O9 borate ring deformation includes Raman bands from 600 to 850 cm-1 [51].
The bands from 1000 to 1400 cm-1 can be assigned to the stretching of tetrahedral
BO4 groups, which share vertices, thus forming three-membered ring borate B3O9
units [51]. Raman modes below 250 cm-1 are attributed to the translation of the Y3+
cations and the B3O9 groups [51]. All these Raman peaks provide evidence for the
existence of B3O9 rings in the hexagonal phase [51]. Elevating the calcination
15

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temperature from 700 to 1000 oC, new bands at 877 and 1306 cm-1 appeared with the
o
temperature increasing to 800-1000 C. The above results indicated that small
o
variation existed in lattice vibration, and two camps for 700 C-phase (space group:
o
P63/m, JCPDS No. 16-0277) and 800-1000 C phase (space group: P63/mmc, JCPDS
No. 74-1929) were obviously observed, although the basic structures are the
hexagonal ones, which are in good agreement with the XRD results. Figure 8
demonstrates XRD patterns of the (Y1-xEux)BO3 (x=0-0.20) obtained by calcination at
800 °C for 2h, which was identified along with the hexagonal structured orthoborates
(YBO3, JCPDS No. 74-1929). The XRD peaks steadily shift toward the low angle
side along with increasing Eu addition, indicating a gradually expanded unit cell of
the crystal structure owing to the incorporation of larger Eu3+ ions. Lattice constant of
the (Y1-xEux)BO3 (x=0-0.20) oxide follows the Vegard’s law and linearly increases
along with the x value increasing from 0 to 0.20. This indicates the already formation
of solid solutions at this annealing temperature, due to the atomic level homogeneous
mixing of Y3+ and Eu3+. Because no obvious changes in the morphology with various
Eu3+ content were observed, the data was not shown here. However, the particle
morphology was significantly affected by the calcination temperatures.
YBO
3
,
JCPDS no.74-1929
(d)
(c)
(b)
(a)
YBO
3
,
JCPDS no.16-0277
10
20
30
40
50
60
70
2θ
(degree)
Figure 6. XRD patterns of the spherical particles of precursor compounds calcined at
the various temperatures for 2h: (a) 700 oC, (b) 800 oC, (c) 900 oC, (d) 1000 oC.
16

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λex = 785 nm
877
1000 ºC
1306
900 ºC
800 ºC
Translation of Y3+and B
3O9
B3O9 rings
700 ºC
200
400
600
800
1000
1200
1400
Raman shift
(
cm-1
)
Figure 7. Raman spectra of (Y0.95Eu0.05)BO3 calcined at various temperatures: (a) 700
oC, (b) 800 oC, (c) 900 oC, (d) 1000 oC.
x=0.20
(b)
(a)
x=0.20
x=0.15
x=0.13
x=0.15
x=0.13
x=0.10
x=0.08
x=0.05
x=0.10
x=0.08
x=0.05
x=0.03
x=0.01
x=0.03
x=0.01
x=0
x=0
YBO
3
, JCPDS no.74-1929
YBO
3
, JCPDS no.74-1929
25
26
27
28
29
30
10
20
30
40
50
60
70
2
θ
(degree)
2θ
(degree)
(d)
3.796
3.794
3.792
3.790
3.788
3.786
3.784
3.782
3.780
3.778
3.776
(c)
8.84
8.83
8.82
8.81
8.80
0.00
0.05
0.10
0.15
0.20
0.00
0.05
0.10
0.15
0.20
The x value
The x value
Figure 8. (a,b) XRD patterns of the (Y1-xEux)BO3 phosphors solid solutions calcined
at 800 °C for 2h, as a function of the Eu content. (c) and (d) are the lattice constants.
17

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Figure 9. FE-SEM morphologies of the products calcined at various temperatures for
o
2h from the typical sphere precursors (Figure 2d): (a) 700 oC, (b) 800 C, (c) 900 oC,
(d) 1000 oC.
Figure 9 shows FE-SEM morphologies of the orthoborates powders at various
temperatures, taking (Y0.95Eu0.05)BO3 as an example. It is seen that the spherical shape
and excellent dispersion of the original particles can be well retained to the
(Y0.95Eu0.05)BO3 up to 800 °C (Figure 9a,b). Shrinking of particle diameter was
observed, owing to the removal of adsorbed water and the densification process of
fine crystals. At a higher temperature of 900 °C, the resultant particles still exhibit
excellent dispersion, though the spherical shape has been instead of nutshell one
(Figure 9c). The particles lost their spherical shape and underwent significant
aggregation at 1000 °C (Figure 9d).
3.4 Luminescence properties of the derived orthoborates spheres
The photoluminescence properties of (Y0.95Eu0.05)BO3 calcined at various
temperatures were investigated in Figure 10. As shown in Figure 10a, the excitation
18

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spectrum of the samples consists of a wide absorption band centered at 209-214 nm
and several weak lines at 323, 395 and 464 nm, which can be ascribed to the
charge-transfer band (CTB) between the O2- and Eu3+ ions and the f-f transitions
within the 4f6 configuration of the Eu3+ ions, respectively [52]. It is interesting to find
that CTB center shifting from 214 nm to 209 nm by elevating the temperature from
o
o
700 to 800 C, and it keeps at 209 nm at higher temperature of 900-1000 C. In the
charge transfer excitation, the O2- 2p electrons are excited into the 4f levels of Eu3+,
and the position of the CTB is determined by the energy difference between the O2- 2p
valence band and the 4f levels of Eu3+ [53,54]. The 4f orbital is not involved in the
bonding between rare earth ions and the ligands. The interaction between the Eu3+ 5d
and O2- 2p orbitals, however, leads to bonding (2p) and anti-bonding (5d) molecular
orbitals [53,54]. One could, therefore, expect that the energy difference between the
Eu3+ 4f and O2- 2p electrons, significantly related to the CTB position, is affected by
o
the lattice vibration. Lattice vibration between in 700 C-phase (space group: P63/m,
o
JCPDS No. 16-0277) and 800-1000 C phase (space group: P63/mmc, JCPDS No.
74-1929), resulted in the blue shift of CTB centre as shown in Figure 10a. Upon
excitation with 214 nm wavelength light, the emission spectrum of (Y0.95Eu0.05)BO3
shows a typical Eu3+ emission. These emission peaks are centered at 592, 610 (627),
650 (673), and 693 (709) nm, which can be described as the well-known 5D0-7FJ (J=0,
1, 2, 3, 4) radiative transition of the Eu3+ ions, as labeled in Figure 10b, respectively
[52]. Obviously, the strongest emission peak is located at 592 nm, and the next are
5
located at 610 and 627 nm. The former is due to the magnetic dipole D0-7F1 of the
Eu3+ ions, whereas the latter is due to the typical forced electric dipole transition
5D0-7F2 of the Eu3+ ions [55-58]. As well-known to us, the intensities of different
5D0-7FJ transitions depend on the local symmetry of the crystal field of the Eu3+ ions.
If the Eu3+ ions occupied an inversion symmetry center in the crystal lattice, the
5
allowed orange D0-7F1 (around 592 nm) magnetic dipole transition would be the
dominant transition; by contrast, the 5D0-7F2 electric dipole transition (around 610-630
nm) would become dominant [55-58]. The Eu3+ ions in the hexagonal vaterite-type
YBO3:Eu3+ structure can occupy the Y3+ site with the point symmetry S6. Therefore,
19

ACCEPTED MANUSCRIPT
5
the orange emission at 592 nm from the D0-7F1 magnetic dipole transitions of the
Eu3+ ions takes the dominant role. Increasing the calcination temperature does not
alter appreciably the positions of the PLE and PL bands, but significantly improves
the intensities of the two. A 100 °C increase in the temperature from 700 to 800 °C
yielded a 31% increase in the intensity of the 592 nm emission (Figure 10b). Another
100 °C temperature increase from 800 to 900 °C improves the emission intensity by
43 %, and that from 900 to 1000 °C only yields a 1% improvement. These
luminescence enhancements are mainly due to the crystal growth and crystal
perfection at higher temperatures. However, the spherical-shape loss and aggregation
contributed to the weaken luminescence, thus inducing the littlie enhanced intensity
from 900 to 1000 oC. Overall considering the results of luminescence and morphology,
an optimized calcination temperature of 800 °C was suggested. The (Y1-xEux)BO3
(x=0-0.20) spheres calcined at 800 °C were investigated at room temperature in
Figure 11. It is clearly seen that the intensity of the 592 nm emission rapidly increases
from x=0.01 to x=0.10, and then it remains nearly constant for x=0.10-0.20. Another
5
interesting finding is that the D0-7F2 electric dipole transition (around 610-630 nm)
increased faster than the 5D0-7F1 (around 592 nm) one with increasing the Eu content,
5
and the D0-7F2 became dominant at the x value up to 0.05. Therefore, the CIE
chromaticity coordinates moved from orange area to red area with the heavy Eu3+
5
doping, as shown in Figure 11b. As the D0→7F2 transition is hyper-sensitive to the
local Eu3+ environment while 5D0→7F1 not, the intensity ratio I(5D0→7F2)/I(5D0→7F1),
called asymmetry factor, has been effectively used to probe the local coordination
environment of Eu3+ [49]. We analyzed the asymmetry factor for the whole series of
orthoborates solid solutions and the results are plotted in Figure 12 as a function of
the Eu content. It is clearly seen that the factor rapidly increases from ~1.34 at x=0.01
to ~2.20 at x=0.10, and then it remains nearly constant at ~2.20 up to x=0.20.
Although Eu3+ ions in the hexagonal vaterite-type YBO3:Eu3+ structure (ordered
hexagonal one) occupied the Y3+ site with the point symmetry S6, a few Eu3+ ions
occupied the Y3+ in non-centrosymmetric site arising from the disordered hexagonal
structure co-existing in orthoborate YBO3. The Eu3+ ions have a priority to occupy the
20

ACCEPTED MANUSCRIPT
S6 instead of the non-centrosymmetric site. A higher Eu content (x=0.01-0.10.) would
lead more Eu3+ ions occupying the latter one because of the lattice disoration, thus
5
inducing enhanced emission of D0→7F2 transition, larger asymmetry factor, and red
shift of the CIE chromaticity coordinates. However, the ratio of S6 site to the
non-centrosymmetric one is a constant, so the asymmetry factor remains nearly
constant at ~2.20 up to x=0.20.
(a)
700 ºC
800 ºC
900 ºC
CT B
λ
=627 nm
em
209 nm
214 nm
1000 ºC
O2--Eu3+
200
250
300
350
400
450
500
Wavelength (nm)
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592
(b)
700 ºC
800 ºC
900 ºC
1000 ºC
627
λ
=214 nm
ex
500
550
600
650
700
750
800
Wavelength (nm)
Figure 10. PLE (a) and PL (b) spectra of (Y0.95Eu0.05)BO3 calcined at various
temperatures for 2h.
x=0.01
x=0.03
627
(A)
592
x=0.05
x=0.08
x=0.10
x=0.13
x=0.15
x=0.20
0.00
0.05
0.10
0.15
0.20
The x value
λ
=214 nm
ex
550
600
650
700
750
800
Wavelength (nm)
22

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Figure 11. (A) PL spectra and (B) the CIE chromaticity coordinates of the
(Y1-xEux)BO3 phosphors solid solutions calcined at 800 °C for 2h.
2.4
2.2
2.0
1.8
1.6
1.4
1.2
0.00
0.05
0.10
0.15
0.20
The x value
Figure 12. I(5D0→7F2)/I(5D0→7F1) ratio (luminescence asymmetry factor) for the
(Y1-xEux)BO3 phosphors calcined at 800 oC for 2h, as a function of the Eu content.
23

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6.0
5.5
5.0
4.5
4.0
3.5
3.0
0.00
0.05
0.10
0.15
0.20
The x value
Figure 13. Lifetime of the 627 nm emission for the (Y1-xEux)BO3 phosphors calcined
at 800oC for 2h, as a function of Eu content.
Fluorescence decay kinetics of the 627-nm emission are presented in Figure S4 for
the (Y1-xEux)BO3 (x=0-0.20) calcined at 800 °C for 2 h. The decay curves can be fitted
to single exponentials according to Eq. 1,
(1)
I = Aexp(-t/τR) + B
where τR is the fluorescence lifetime, t the delay time, I the relative intensity, and A
and B are constants. The relationship between fluorescence lifetime and Eu3+ content
has been studied in Figure 13. It is seen that the lifetime is similar (5.3±0.3 ms) for the
samples up to 8 at% of Eu3+ but then turns shorter. When Eu3+ concentration is below
8 at%, the luminescent center can be thought of as isolated. In such a case the energy
transfer from the interior to surface sites is ineffective due to the longer distance
among the Eu3+ ions, and thus a relatively longer lifetime is observed. A higher Eu3+
concentration would lead to the formation of a resonant energy transfer net, which
acts as an additional channel to the nonradiative centers on the surface and therefore a
shorter lifetime is resulted. Figure S5 presented the fluorescence decay kinetics of the
(Y0.95Eu0.05)BO3 sample (calcined at 700 °C, space group P63/m), and the fitted
24

ACCEPTED MANUSCRIPT
lifetime 5.13±0.06 ms is similar to that from (Y0.95Eu0.05)BO3 calcined at 800 °C
(5.14±0.05 ms, space group P63/mmc, Figure S4). This indicated the independent
inference of space group on the lifetimes.
4. Conclusions
In the present work, (Y1-xEux)BO3 (x=0-0.20) spheres (~0.8-1.5 ꢀm in diameter)
have been successfully synthesized via homogenous precipitation processing,
followed by calcination. Detailed characterizations of the products by combined
techniques of XRD, Raman, FE-SEM, HR-TEM, PLE/PL, and fluorescence decay
analysis have yielded the following main conclusions:
(1) Orthorhombic (Y,Eu)(B(OH)4)CO3 were synthesized by homogenous precipitation
treatment, which tends to crystallizing into cubic particles owing to their crystal habit.
Proper EG addition leads to the formation of spherical particles, but nano-sized
amorphous particles were resulted from the excessive EG content. Longing reaction
time or elevating the reaction temperature induced the appearance of cubic ones.
Spherical particles can be obtained by optimizing the crystallization processing, at a
proper reaction temperature of 90 oC and the reaction time from 2h to 3h.
o
(2) Calcining (Y,Eu)(B(OH)4)CO3 at 700 C yielded hexagonal structured (Y,Eu)BO3
phase with the space group P63/m, but another hexagonal structured (Y,Eu)BO3 phase
o
(space group: P63/mmc) were obtained with the temperature of 800-1000 C. The
spherical shape and excellent dispersion of the original particles can be well retained
up to 800 °C. At a higher temperature of 900 °C, the resultant particles still exhibit
excellent dispersion, though the spherical shape has been instead of nutshell one. The
particles lost their spherical shape and underwent significant aggregation at 1000 °C.
(3) Upon UV excitation, the (Y1-xEux)BO3 (x=0-0.20) spheres exhibited a typical Eu3+
5
emission with the orange emission at 592 nm from the D0-7F1 magnetic dipole
transitions of Eu3+ ions taking the dominant role. Space-group vibrations between in
700 oC-phase and 800-1000 oC phase resulted in the blue shift of CTB centre, but they
did not significantly affect the lifetimes. A higher Eu content (x=0.01-0.10.) leaded
25

ACCEPTED MANUSCRIPT
more Eu3+ ions occupying the non-centrosymmetric site, thus inducing enhanced
5
emission of D0→7F2 transition, larger asymmetry factor, and red shift of the CIE
chromaticity coordinates. However, the asymmetry factor remains nearly constant
with the x value up to 0.20. The fluorescence lifetime is similar (5.3±0.3 ms) up to 8
at% of Eu3+, followed by decreases at higher Eu3+ contents.
Acknowledgments
This work was supported in part by the Fundamental Research Funds for the Central
Universities (Grants N160204008), and the National Natural Science Foundation of
China (Grant 51672039).
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