Synthesis of Sr4Al14O25: Eu2+ green emitting luminescent nano- pigment by solution
combustion method
Somaye Karimkhani Zand1, Saeid Baghshahi1, Masoud Rajabi1*
1
Department of Materials Science and Engineering, Faculty of Technology and Engineering, Imam Khomeini International
University (IKIU), Qazvin, Iran, Phone :+982833901123, Fax :+982833901124 , P .O. Box :288
*Corresponding author: [email protected]
Abstract. Luminescent nano-pigments based on strontium aluminates doped with various amounts
of europium rare earth ion (Sr4Al14O25:Eu2+) were synthesized by solution combustion method
which involves the exothermic reaction of an oxidizer and an organic fuel. It is a suitable method
for producing chemically homogenous and pure powders with very fine particle size. Phases studies
by XRD indicated formation of pure phase Sr4Al14O25 with crystallite sizes about 52.46nm and peak
shift of about 2θ=0.26o degree due to Eu2+ doping. SEM micrographs also revealed that the particles
have irregular morphology and wide particle size distribution. The emission spectrum showed main
emission peak at about 480nm. Energy transfer mechanism between the two emission centers and
the effect of Eu2+ ion concentration on emission spectrum was studied and it was deduced that the
maximum emission intensity occurred at 4 at% of Eu2+.Absorption spectrum of the samples were
lower than 0.5%, especially in emission region of the doped pigments.
Keywords:
nano-pigments, Strontium aluminates, solution combustion synthesis, green luminescent
Nevertheless, these two methods have some
1. Introduction
Recently,
white
light
emitting
disadvantages including high color tolerance
diodes
and low color rendering index (CRI). To
(WLEDs) have attracted much attention, since
overcome these problems various phosphors
they have less negative effect on environment
doped with rare earth ions have been studied
and are more cost effective [1, 2]. Compared
to formulate new phosphors for this purpose
to conventional lighting devices, these diodes
[2]. Among the studied phosphors, alkaline
have some advantages, including low applied
earth aluminates doped with rare earth
voltage, high power efficiency and brightness
elements have proven to be useful, as they
and longer life [3-5]. Two methods have been
have strong luminescence in blue- green to
used to generate white light. One is by
red regions of visible spectrum [2, 6, 7]. They
utilizing primary tricolor phosphors excited
have high quantum yield, and are safe and
by near UV-LED chips and the other by using
chemically stable [7].
blue LED with yellow emitting phosphors.
1
In SrO-Al2O3 system, there are six known
as low calcinations temperature and very fine
phosphor hosts, namely Sr4Al14O25 [1],
particle size.
SrAl2O4 [2], Sr2Al6O11 [4, 7, 8], SrAl12O19 [1,
Crystallographic structure of Sr4Al14O25 were
4, 7, 8], Sr3Al2O6 [4, 8], and SrAl4O7 [8].
reported for the first time by Nadzhina et al
Recently, strontium aluminates, especially,
[11]. Sr4Al14O25 has orthorhombic structure
SrAl2O4 and Sr4Al14O25 doped with rare earth
with space group Pmma, and a = 24.785 Å, b
ions (Eu2+, Ce3+) have been investigated as
= 8.487 Å and c = 4.866Å. The structure
proper phosphors with high brightness and
consists of layers made up of AlO6 octahedra
broad and intense charge transfer bands or
separated by a double layer of AlO4 tetrahedra
host absorption bands in near UV regions.
Sr4Al14O25
demonstrates
strong
[6]. In Sr4Al14O25 structure, there are two
host
different strontium sites: the Sr1 site inside an
absorption band ranging from 260 nm to 340
oxygen-polyhedron composed of six O atoms
nm [7]. Moreover, Eu2+ is a lanthanide
and the Sr2 site inside a complicated oxygen-
element with f1 configuration [9] and is,
polyhedron composed of eight O atoms [6,
therefore, capable of creating luminescence
11].
from red to UV-region owing to strong crystal
In this work, synthesis of Sr4Al14O25:Eu2+
field dependence of 5d–4f transition energy
Sr4Al14O25:Eu2+
been
phase by solution combustion method and
investigated for its outstanding properties,
studying its luminescent properties have been
including high quantum efficiency and bright
investigated.
[8,
9].
also
has
blue- green LEDs fabricated by combining
NUV InGaN based LEDs with this phosphor
[6].
2. Experimental
Different processes such as solid state
A series of samples were prepared by solution
reaction
co-
combustion route. All of the starting materials
precipitation [10], microwave synthesis [1]
were of reagent grades. At first, stoichumetric
and solution combustion [9] are among the
amounts of Sr(NO3)2(99%), Al(NO3)3.9H2O,
methods
these
(99%), Eu(NO3)3, [prepared by adding HNO3
phosphors. Compared to the samples obtained
into proper amounts of Eu2O3 (99%) until a
by conventional solid state reaction method,
transparent solution was obtained] were
the phosphor materials synthesized by wet
mixed with double distilled water on a
chemical method have some advantages such
magnetic stirrer. Urea was added as fuel with
[1-3,
used
5,
for
7],
sol-gel
synthesis
[1],
of
2
oxidizer to fuel ratio of about 2.5. By adding
Where d is crystallite size (Å), λ is X-Ray
some ammonia, pH of the solution was fixed
wavelength (1.54056 Å), β is full width at
at 4. After that the solution was heated in hot
half maximum (FWHM) and θ is the angle
plate at about 80oC, until a gel was formed.
[1].
The viscous gel was inserted directly inside a
3. Results and discussion
furnace which was preheated to 600±2oC. In
3.1. Phase analysis
this way, the gel was rapidly dehydrated and
transformed into a foam. Then, it was self-
The XRD patterns of the sample after
ignited and a white fluffy mass was produced.
combustion and heat treatment at 1350oC are
After light crushing the product by a mortar
shown in Fig. 1. The patterns confirm the
and pestle, it was heat treated at 1350oC in a
partial and complete formation of Sr4Al14O25
tube furnace under reductive atmosphere of
(JCPDS
active carbon and N2/Ar gas.
combustion, respectively. The weak intensity
01-074-1810)
before
and
after
of the peaks before heat treatment is believed
The powder diffraction data was collected by
to be due to the short time of combustion
a X-Ray Diffractometer (Philips PW3040/60,
Cu
Kα:1.54056
Holland),
and
Å, 40KV
the
particle
and
process (about thirty seconds). Fig. 1 (A) also
30mA,
size
shows the presence of minor phases from the
and
SrO-Al2O3
morphology was determined by a scanning
sputtering
device.
SrAl2O4
026-097), and other unrecognizable phases,
Japan).The samples were coated with gold by
DC
including
(JCPDS 00-010-0061), SrAl12O19 (JCPDS 00-
electron microscope (SEM) (S4160, Hitachi,
a
system,
which almost disappeared after heat treatment
The
at 1350oC.
photoluminescence emission and excitation
spectra were carried out with a Perkin-Elmer
The crystallite sizes of the powders before
spectrophotometer LS50B equipped with
and after heat treatment were about 30.25nm
Xenon lamp as excitation source. The
and 52.46nm, respectively as determined by
absorption spectra were recorded on a UV-
using Scherer equation.
visible spectrophotometer (UV1800, Ray
Leigh, Japan).Crystallite sizes were calculated
by Scherer equation:
d= (0.9λ)/(βcosθ)
(1)
3
Intensity (arb.u)
SrAl2O4
Since Eu2+ radius (0.112nm) is smaller than
Sr4Al14O25
Sr2+ radius (0.114nm)[6], by doping Eu2+
SrAl12O19
activator ions into Sr4Al14O25 matrix, the host
structure contracts and this contraction affects
the lattice parameter and XRD pattern peak
B
positions, and causes a peak shift to higher
A
angles. This peak shift is shown in Fig. 3. The
value of this peak shift caused by doping
0
20
40
2θo
60
found to be about 2θ=0.26˚ compared to
80
undoped XRD pattern.
Fig1. XRD patterns of the samples containing
4%Eu2+ (A) after and (B) before heat
Intendity (arb.u)
treatment.
XRD patterns of the doped and undoped
samples are shown in Fig. 2. The patterns
2+
revealed that Eu
U
D
ion doping had no effects
on crystallization of the host material.
20
25
30
35
2θo
Intensity (arb.u)
Fig. 3. Peaks shift of 4% Eu2+ doped (D) and
undoped (U) samples after heat treatment
U
D
3-2. Spectroscopy
The luminescence spectrum of the 4% Eu2+
0
20
40
60
doped sample is shown in Fig. 4.The
80
spectrum shows a relatively broad excitation
2θ˚
Fig. 2.XRD patterns of 4% Eu
spectra with two maxima at about 250 and
2+
doped (D,
350nm. This broad excitation spectra could be
with red pattern) and undoped (U, with blue
related to electron transition from 4f7 ground
pattern) samples after heat treatment
state to the 4f65d1 excited state of Eu2+ [6].
4
Fig. 6 shows the effects of activator ion
Intensity(arb.u)
em:480nm
concentration on the emission spectra of
Sr4Al14O25:Eu2+ pigment. It can be seen that,
emission
intensity
first
increases
with
increasing activator ion concentration up to
0
200
400
wavelengh (nm)
600
4at%Eu2+. After which quenching effect in
host structure occurs which leads to decrease
of emission intensity.
Fig. 4 Excitation spectrum of Sr4Al14O25:4%
Eu2+
ex:350nm
When the excitation wavelength was 250nm,
the emission spectrum (Fig. 5) does not show
Intensity (arb.u)
x=1
very sharp emission spectra, but by excitation
with
350nm
wavelength,
the
emission
spectrum (Fig. 6) showed an intense green
x=4
x=8
x=12
emission with a maximum at 480nm, which
could be related to the 4f 65d1  4f7 transition
300
of Eu2+ ion [6,11].
400
500
600
700
wavelengh (nm)
Fig.6. Emission spectrum of
ex:250nm
Intensity (arb.u)
Sr4Al14O25:xat % Eu2+ (x= 1, 4, 8, 12)
pigment excited by 350nm radiation
x=4
x=1
As reported by many research groups [1, 2, 8,
10, 11], concentration quenching occurs when
0
200
400
600
wavelengh (nm)
activator concentration exceeds a proper
800
amount (usually about a few weight percent).
Fig. 5 Emission spectrum of
Sr4Al14O25:xat.%
2
Eu
(x=1,
Fig. 7 shows variation of peak emission
4)
pigment
intensity
excited by 250nm radiation
with
Eu2+
ion
concentration.
Previous research studies showed that in some
phosphors activated with rare earth ions, the
concentration quenching effect is very small
5
and in some phosphors all of replaceable ions
influenced by crystal field effects [8], as these
could be replaced with activator ions [12]. In
different excited states of Eu2+ lead to two
Sr4Al14O25:Eu2+, it is predicted that with
different crystal fields around them, and
further addition of activator ions (Eu2+) the
therefore two different emission peaks in
emission intensity does not show sensible
emission spectra.
variations (Fig. 7).
It was reported that although the number of
the both sites is equal, the 480nm emission is
more intense [6,8]. This might be due to
Intensity (arb.u)
energy transferring from Eu2+ ions with
emission at 400nm to Eu2+ ions emitting at
480nm
[8].
Another
reason
for
this
phenomenon could be overlapping 400nm
0
5
10
emission peak with excitation spectra [6]. It
15
can be seen from Fig. 7 that decrease of the
Eu2+ (at%)
emission intensity with increasing Eu2+ above
Fig.7. Peak emission intensity variation of
about 4% ion concentration is too high, but
Sr4Al14O25: Eu2+ pigment as function of
finally it levels off.
activator ion concentration (Eu2+)
Of course, when the concentration quenching
due to cross-relaxation (relaxation due to
In the spectrum shown in Fig. 6, there is also
resonant energy transfer between the same
another weak emission peak at about 400nm.
element atoms or ions) occurs on a particular
Existence of two emission band using only
level among several emitting levels, the
one activator ion could be related to the
emission color of phosphor changes with
presence of two different crystallographic
activator
sites for Sr2+ ions in the Sr4Al14O25 host
emission color which occurs by diminishing
structure as reported by Wu and Zhang et al
one of the emission wavelengths, is caused by
[6,11]. Since Eu2+ ion radius (0.112nm) is
cross relaxation between emission levels [12].
nearly similar to Sr2+ (0.114nm) [6] or
Hence, in relation to the Sr4Al14O25: Eu2+
(0.113nm) [8], Eu2+ ions could be easily
pigment, the energy transfer from 400nm
substituted for Sr2+ [6,8], leading to two types
emission center to 480nm could be the reason
of Eu2+ sites in the matrix structures [6,8,11].
for color changing with increasing Eu2+
The excited states of 5d in Eu2+ ion are
concentration greater than 1at%.
6
concentration.
The
change
in
Emission spectra at different excitation
wavelengths could be explained by the
wavelengths of 350, 390 and 400nm are
presence of different energy levels for holes
shown in Fig. 8.
inside pigment structure.
As could be seen from Fig. 5 and 8 with
emission
wavelength
did
not
Intensity (arb.u)
various excitation wavelengths, the main
change.
Excitation by shorter wavelengths (250nm)
led to the significant increase of the emission
ex=350nm
ex=390nm
ex=400nm
intensity, which is a result of high excitation
0
intensity at this wavelength. But under higher
500
1000
Wavelength (nm)
excitation wavelengths (350, 390 and 400nm)
the emission intensity decreases to half of it at
Fig.8.
390nm excitation. This could be explained by
4at%Eu2+
lower excitation intensity at 390nm in Fig. 4.
wavelengths
Emission
spectra
with
of
Sr4Al14O25:
different
excitation
But from another point of view, it can be seen
Fig. 9 shows SEM of the Sr4Al14O25:4at%
that the produced pigment shows emission
Eu2+ powders before and after heat treatment
peak at different excitation wavelengths, but
at 1350oC. It can be seen that the pigment has
main emission peak always occurs at a
a wide particle size distribution of 84nm-2μm
constant wavelength of 480nm. This shows
before heat treatment and 152nm-3.86μm
that emissions caused by relaxation of excited
electrons, have one exact level. But, in
after heat treatment.
excitation with shorter wavelengths, excited
electron after releasing part of its energy in
form of heat recombines to proper level,
causing emission. With the same reason,
broadening of the emission spectra could be
explained, that is emission caused by excited
electrons recombination could cause other
emission wavelengths in addition to main or
maximum emission wavelength, so emission
Fig.9. SEM images of Sr4Al14O25:4at%Eu2+
spectra become wider. The reason of emission
pigment, before (left images) and after (right
at the same wavelength produced by longer
images) heat treatment
7
Sr4Al14O25:4at%Eu2+
pigment(Fig.
10).As
observed, it shows less than 0.5% photon
absorption
Energy efficiency is one of the most
Sr4Al14O25:Eu2+
important factors in the field of luminescent
materials
application
at
at
synthesized
photoelectrical
emission
pigment.
pigments
peak
of
Therefore,
have
also
the
high
efficiency rate.
instruments. Parameters affecting energy
efficiency are given in the following equation:
η= (1-r) [hν]/(βEg(. ηt ηact ηesc
absorption (%)
(2)
Where, r, is backscatter coefficient, hν is the
mean photon energy of the photons emitted,
βEg is the energy needed to generate a
0.6
0.4
0.2
0
0
thermalized electron-hole pair (β being a pure
number and Eg being the band gap energy), ηt
is the transfer efficiency of electron-hole pairs
500
1000
Wavelength (nm)
1500
Fig.10. Absorption spectrum of Sr4Al14O25
to activators or sensitizers, ηact is the quantum
efficiency of the activator ions, and finally
4. Conclusions
ηesc is the ratio between photons leaving the
XRD phase analysis confirmed the formation
material and photons generated in the
of high purity Sr4Al14O25:Eu2+ phase with
material.
emission
crystallite sizes of about 52.46nm. It was
wavelength by matrix has a reverse relation
observed that by doping Eu2+, due to the
with (ηesc), that is, the less matrix phase
matrix phase structure contraction, peaks of
absorption, the more phase ηesc and energy
the X-Ray diffraction peaks shifted to higher
efficiency will be [13].
angles, but it did not affect phase formation.
The necessary requirement for phosphors
SEM analysis showed a relatively wide
applications for NUV-LED solid state light is
particle
Absorption
of
the
the high absorption rate in the 350-410nm [6],
size
distribution.
The
emission
2+
spectra of Sr4Al14O25: Eu pigments showed
2+
hence, Sr4Al14O25: Eu with wide excitation
two emission peaks, one with high intensity at
spectra in the 200-420nm (Fig. 4), would be a
480nm and the other one with lower intensity
suitable choice for application in LEDs which
at 400nm. Energy transfer between two
could be excited with violet photons. Photon
emission centers and also overlapping the
absorption at emission peak (about 480nm)
excitation spectra with emission peak at
determined
400nm were found to be major reasons for the
by
absorption
spectra
of
8
5. W.R. Liu, C.C. Lin, Y.C. Chiu, Y.T. Yeh,
S.M. Jang , R.S. Liu, “Luminescence
Properties of Green-emitting PhosphorsSr4Al14O25:Eu2+ for LED Applications”, The
13th Asia Pacific Confederation of Chemical
Engineering Congress, Taipei, 2010.
6. Z. Wu, J. Shi, J. Wang, M. Gong, Q. Su,
“Synthesis and luminescent properties of
Sr4Al14O25:Eu2+
blue–green
emitting
phosphor for white light-emitting diodes
(LEDs)”, J. Mater. Sci.: Mater. Electron. 19,
339–342, 2008.
intensity difference. The maximum emission
intensity was found at Eu2+ ion concentration
about 4at%. It was concluded that Sr4Al14O25:
Eu2+ pigment is a suitable choice for
optoelectronic device applications such as
NUV/VUV and LED/PDPs, because of its
wide excitation range and high emission
intensity and high luminescence efficiency
rate due to low matrix phase electromagnetic
7. H.N. Luitel, T. Watari, R. Chand, T.
Torikai, M. Yada, “Photoluminescence
properties of a novel orange red emitting
Sr4Al14O25:Sm3+
phosphor
and
PL
enhancement by Bi3+ co-doping”, Opt.
Mater., 34, 1375–1380, 2012.
absorption.
Acknowledgments the authors would like to thank
INSF of Iran for complete financial support
provided for this research work.
8. L. Qun, Z. Junwu, S. Feilong, “Energy
transfer mechanism of Sr4Al14O25:Eu2+
phosphor”, J. of Rare Earths, 28, p. 26-29,
2010.
5. References
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10