Sol-gel synthesis and photoluminescence of RExLu2-x O3:Eu3+ (RE = Y, Gd) nanophosphors

Article abstract of DOI:10.1007/s10789-005-0179-9

RE_xLu_2-x O₃:Eu³⁺ (RE = Y, Gd) nanophosphors with x = 0.4, 1.0, and 1.6 were synthesized using rare-earth salicylate coordination polymers as precursors for the luminescent species RE_xLu_2-x O₃:Eu³⁺ (RE = Y, Gd) composed of polyethylene glycol both as a dispersing medium and fuel. Their particle sizes were around 100 nm as determined by scanning electron microscopy and x-ray diffraction.

Full text of DOI:10.1007/s10789-005-0179-9

Inorganic Materials, Vol. 41, No. 6, 2005, pp. 613–617. From Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 706–710.
Original English Text Copyright © 2005 by Zhou, Yan.
Sol–Gel Synthesis and Photoluminescence
3+
1
of RE Lu O :Eu (RE = Y, Gd) Nanophosphors
x
2 – x
3
L. Zhou and B. Yan
Department of Chemistry, Tongji University, Shanghai 200092, People’s Republic of China
e-mail: byan@tongji.edu.cn
Received August 24, 2004
3
+
Abstract—RE Lu O :Eu (RE = Y, Gd) nanophosphors with x = 0.4, 1.0, and 1.6 were synthesized using
x
2 – x
3
3
+
rare-earth salicylate coordination polymers as precursors for the luminescent species RE Lu O : Eu (RE =
x
2 – x
3
Y, Gd) composed of polyethylene glycol both as a dispersing medium and fuel. Their particle sizes were around
1
00 nm as determined by scanning electron microscopy and x-ray diffraction.
1
INTRODUCTION
Nanotechnology, revealed at the end of the last cen-
irradiation [9]. Many researchers tried to improve this
red-emitting material with respect to size [8], concen-
tration, method [10], new substrate, and more seldom
through doping with Lu.
tury, has a great impact on the whole science and tech-
nology fields. Various researches have focused on
nanomaterials because they have shown some special
optical, electronic, or structural properties compared
with traditional materials, and more reliable informa-
tion could be acquired from nanocrystals. The bulk
properties of nanocrystals depend on the high percent-
age of grain boundaries in the total material, although
the local atomic structure at the interfaces is not com-
pletely understood [1].
In our experiments, we try to introduce lutetium into
3+
Y O :Eu
(or Gd O :Eu). The synthesis of
2
3
2 3
3
+
RE Lu O :Eu (RE = Y, Gd) by the sol–gel method
x
2 − x
3
was described in [11]. Sol–gel chemistry, where all
chemicals are mixed in matrices on a molecular scale,
combined with the spin-coating or dip-coating tech-
nique, is an efficient method for producing rare-earth-
doped planar waveguides [12, 13]. The advantages of
the sol–gel method are obvious: a homogeneous phase
3
+
Due to their favorable luminescent characteristics,
stability in high vacuum, and absence of corrosive gas
emission under electron bombardment, many works
were carried out on rare-earth oxide phosphors [2]. The
most typical ones are europium-dopedY O and related
can easily be formed, and Eu can be well dispersed.
The thermolysis temperature was 600°C for removing
the organic component in rare-earth coordination poly-
meric systems.
2
3
materials, which are common phosphors in optical dis-
play and lighting applications [3, 4]. Due to its chemi-
cal stability, Y O is an excellent substrate for Eu. Over
EXPERIMENTAL
The starting materials were rare-earth oxides (99.9
2
3
the past decades, a number of new chemical techniques, or 99.99%). The other chemicals used were of pure
such as thin-film chemical vapor deposition [5], com- grade. Rare-earth nitrates were prepared from by treat-
bustion synthesis [6] and colloidal reactions [7], have ing oxides in concentrated nitric acid.
3
+
3
+
been developed to synthesize nanosized Y O :Eu .
2
3
The synthesis of RE Lu O :Eu (RE = Y, Gd) by
x 2 – x 3
Recently, some reports on the enhancement of photolu- the sol–gel method was adopted as the similar proce-
minescence in nanocrystals of Y O :Eu [8] were pub- dure according to [11] except for the start mixed solu-
lished. A group in Virginia described the optical spec- tion of Lu(NO ) and RE(NO ) (RE = Y, Gd) (total
2
3
3
3
3 3
3
+
troscopy of nanocrystalline Eu O and Y O :Eu [1]. 1.3 mmol) with different molar ratios and thermal decom-
2
3
2
3
3
+
3+
Due to its quantum efficiency, nanosized Y O :Eu is position at 600°C. The synthesis of RE Lu O :Eu
potential material for a variety of applications and may (RE =Y, Gd) by rare-earth salicylate coordination poly-
become a new type of luminescent material. Other mers—PEG precursors was described in the following.
authors adopted different hosts. Bartram synthesized a Excess salicylic acid (5.0 mmol) was dissolved in 95%
sample of Eu -activated lutetium sesquioxide trans-
parent ceramic, which was investigated by scintillation
and thermoluminescence excited by prolonged gamma
2
3
x
2 – x
3
3
+
ethanol, and pH value of 10 ml solution was adjusted to
about 7.0 with aqueous ammonia. Next, the solutions of
rare earth nitrates (Lu(NO ) , RE(NO ) (RE = Y, Gd)
and Eu(NO ) ) with different molar ratios were homo-
geneously mixed. The solutions of urea (10 ml) and
3 3
3 3
3
3
1
This article was submitted by the authors in English.
0
020-1685/05/4106-0613 © 2005 Pleiades Publishing, Inc.

6
14
ZHOU, YAN
0
20
40
60
80
2
θ, deg
Fig. 1. XRD pattern of nanocrystalline Gd1.6Lu0.4O :Eu3+.
3
PEG (10 ml) were added to the above solutions and
In order to observe the configuration of nanophase
heated under 60°C by dipping a 1% ammonia solution samples, we used SEM (Fig. 2). All the nanophosphors
until the pH value became alkaline. After that, the heat- show dimensional uniformity. It can be approximately
ing temperature was increased to 100°C to evaporate predicted that the crystal particle size is less than
the solvent until the solution became viscous, and the 100 nm, which is in good agreement with the XRD
precursors were finally obtained. To remove the organic results. There also exist some conglomeration phenom-
component in rare earth coordination polymeric sys- ena which may take place during thermal decomposi-
tems, the precursors were subjected to thermolysis at tion. Rare-earth coordination polymers with salicylic
6
00°C for 2 h. The scheme for the synthesis can be acid have infinite chain polymeric structures and are
shown as below:
used as precursors to prepare luminescent species [16].
Organic polymers are not only good dispersing media
but also polymeric hybrid precursor templates with
nanometer size. So the nanometer size can be con-
trolled and determined by hybrid precursors. Therefore,
this preparation technology connects the assembly of
hybrid material with synthesis of nanomaterials and
may be used for the synthesis of other luminescent
materials based on rare-earth oxides.
xLu(NO ) + (2 – x)RE(NO ) + 0.02Eu(NO )
3 3
3
3
3 3
+
yHSAl + zNH · H O
[RE(Sal)3]n
3
2
coordination polymer + mPEG + nNH –CO–NH +
z'NH · H O
NH ↑ + NO ↑ (RE = Y, Gd).
The nanoparticles were characterized by powder
x-ray diffraction (XRD) analysis (40 kV, 20 mA, CuKα,
Philips PW1710) and scanning electronic microscopy
2
2
3
+
Lu RE O :Eu + CO ↑ + H O↑ +
3
2
x
2 – x
3
2
2
3
2
Figure 3 shows the excitation spectra of the
3
+
3+
(
SEM) on a Philips XL-30. Excitation and emission nanophosphors Lu O :Eu , Y2 – xLu O :Eu , and
2 3 x 3
3
+
spectra for powders of these phosphors were measured Gd2 − xLu O :Eu , respectively (λ = 612 nm). There
x
3
em
with Perkin-Elmer LS-55 model fluorophotometer
excitation wavelength = 612 nm, scan rate =
is an asymmetric band with three peaks at about 212, 227,
(
3+
and 243 nm for Lu O :Eu , three peaks at about 228,
2
3
1
5
000 nm/s, excitation and emission slit widths of
.0 nm).
3+
2
43, and 254 nm for Y2 – xLu O :Eu , and three peaks at
x 3
3+
about 220, 244, and 256 nm for Gd2 – xLu O : Eu ,
x
3
respectively. These peaks are attributed to the transition
RESULTS AND DISCUSSION
toward the charge transfer state due to Eu–O interaction
7
–1
3+
(4f 2p charge transfer band) lying in the band gap
region of the host matrix [17]. This indicates superim-
position of the additional excitation component on the
excitation band. Figures 4 and 5 show compare the
Figure 1 shows the XRD pattern of a Gd Lu O :Eu
1.6
0.4
3
nanophosphor. The full width at half maximum of the
diffraction peak was used to estimate the average parti-
cle size by the Scherrer equation [14, 15]. The particle
size was found to range from 18 to 106 nm. Similar
results were reported earlier [9].
3
+
emission spectra of nanocrystalline Gd Lu O :Eu
2 − x
x
3
3
+
and Y2 – xLu O :Eu , respectively, both of which could
x
3
INORGANIC MATERIALS Vol. 41 No. 6 2005

3
+
SOL–GEL SYNTHESIS AND PHOTOLUMINESCENCE OF RE Lu O :Eu
615
x
2 – x
3
3
+
Lu O :Eu
2
3
3
+
Y2–xLu O :Eu
x
3
3
+
Gd2–xLu O :Eu
x
3
Acc.V Spot Magn
Det WD Exp
2 µm
(
a)
3
0.0 kV 2.7 30000x SE 7.2
0
2
00
250 Wavelength, nm
Fig. 3. Excitation spectra of nanocrystalline Lu O :Eu3+,
2
3
Y2 Lu O :Eu3+, and Gd2 – xLu O :Eu (λem = 612 nm).
3+
− x
x
3
x 3
3
+
GdLuO :Eu
3
Acc.V Spot Magn
Det WD Exp
2 µm
(
b) 3
0.0 kV 2.7
30000x SE 6.8
0
3
+
Gd0.4Lu1.6O :Eu
3
Gd1.6Lu0.4O :Eu3
+
3
6
00
650
700
Wavelength, nm
Fig.
4.
Emission
spectra
of
nanocrystalline
3
+
Gd2 Lu O :Eu with x = 1.6, 1.0, and 0.4 (λex = 245 nm).
− x
x 3
Acc.V Spot Magn
Det WD Exp
2 µm
(
c)
The two strongest emission peaks are located at
11 nm, corresponding to the typical hypersensitive
3
0.0 kV 2.7 20000x SE 9.8
0
6
5
7
3+
transition D
F for the Eu ion. This is a parity-
2
0
Fig. 2. SEM images of nanocrystalline (a) Lu O :Eu3+,
2
3
forbidden f–f intraconfigurational transition. In the
(
b) Gd1.6Lu0.4O :Eu3+, and (c) Y0.4Lu1.6O :Eu3+.
3
3
body-centered cubic type Y O and Gd O , there are
2
3
2
3
3
+
two types of cationic sites occupied by RE having C2
and S6 local symmetries. The former site lacking inver-
sion center is quite favorable for observing the electric
dipole transition as a forced transition due to the admix-
5
7
be described by the well-known D
, 3, 4) line emission of the F configuration of the Eu
FJ (J = 0, 1,
0
4
3+
2
6
ion. Some of the emission was eliminated by the selec- ture of the odd parity states, whereas the latter is unfa-
tion rule, and some was too weak to be observed [18]. vorable for the electric dipole transition due to the pres-
INORGANIC MATERIALS Vol. 41 No. 6 2005

6
16
ZHOU, YAN
obtain enough information on the possible multiple
3
+
3+
sites occupied by Eu . Compared with bulkY O :Eu
Y0.4Lu1.6O :Eu3
+
2
3
3
at the emission of 618 nm, the nanophosphors exhibit a
blue shift of 7 nm.
3
+
YLuO :Eu3
3
+
In conclusion, the size of Lu RE O :Eu (RE =
x 2 – x 3
Y, Gd) nanophosphors synthesized by rare-earth
nitrate—PEG precursors was 10–20 nm. It may be
explained by its low thermal decomposition tempera-
ture of 600°C. These nanomaterials show an especially
strong emission at 611 nm. In comparison with bulk
materials the nanophosphors have a blue shift of 7 nm.
Y1.6Lu0.4O :Eu3
3
+
ACKNOWLEDGMENTS
6
00
650
700
The authors are supported by the Science Founda-
tion of Shanghai Universities for Excellent Youth Sci-
entists.
Wavelength, nm
Fig.
5.
Emission
spectra
of
nanocrystalline
3
+
Y2 Lu O :Eu with x = 1.6, 1.0, and 0.4 (λex = 247 nm).
− x
x
3
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