# Effects of Particle Size and Type of Alumina on  the Morphology and Photoluminescence Properties ofSr4Al14O25:Eu2+/Dy3+ Phosphor

Authors: Hom Nath Luitel; Takanori Watari; Toshio Torikai; Mitsunori Yada
Journal: Research Letters in Materials Science
 (2009)

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## Abstract

Sr4Al14O25:Eu2+/Dy3+ phosphor with high luminescence intensity and long afterglow duration was synthesized using 1.0 μm (α), 0.1 μm (α), and 0.05 μm (γ) particle sizes of Al2O3. SEM observation results showed that spike-like thin particles were formed when 0.05 μm γ-Al2O3 was used as raw material. Hexagonal thick particles were observed when 0.1 μm α-Al2O3 was used. But irregular, thin particles were observed while using 1 μm α-Al2O3. Photoluminescence measurements showed that both the initial intensity and the long persistency were much higher for the phosphor prepared using γ-Al2O3 of 0.05 μm particle size.


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## Body

## 1. Introduction

Of  the  phosphors, the alkali earth aluminates containing rare
earth ions are functional inorganic materials with strong luminescence in blue
to red regions [1–3]. These
materials are widely used in various fields; to highlight a few, one may include
emergency signs, low level lightening escape systems, military applications,
textile fibers, lightening apparatus, exit signboards, and many more [4]. These materials, due to better safe, chemical
stability, excellent photo resistance, very high brightness, and long-lasting
afterglow with no radio active radiations [5], form the important materials in
various ceramics industries [6].In recent years,SrAl2O4 and Sr4Al14O25 doped with Eu2+ and Dy3+ have been regarded as an excellent phosphor and
attracted the researcher's interest. For the improvement of phosphorescence, many researches have been focused on regarding additives, molar
ratio of constituents, and the preparation methods [6–8]. It was found
that the shape and size of phosphor particles play important role for the phosphorescence
properties. When the particle size reaches the nanoscale, new properties are
appeared like the blue shift of emission intensity [9]. If the phosphor particles are regular and flat plate-like, they are
expected to give a better light absorption and form a dense compact by their
orientation, resulting in higher phosphorescence intensity. The shape and size
of the phosphor particles may depend on the crystal type and particle size of the
starting materials as well as the method of preparation. This letter reports
the effect of raw Al2O3 type and size on the formation of
regular particles for the higher luminescence intensity of Sr4Al14O25:Eu2+, Dy3+ phosphor.


## 2. Experimental

Strontium aluminates doped withEu2+ and Dy3+ (Sr4Al14O25:Eu2+/Dy3+) were prepared by the reaction between strontium
carbonate (SrCO3; Sigma-Aldrich, Inc., Mo, USA), aluminum
oxide (Al2O3 Sigma-Aldrich, Inc., Mo, USA), europium oxide (Eu2O3; Sigma-Aldrich, Inc., Mo, USA, 99.9+ %), and  dysprosium oxide (Dy2O3;
Sigma-Aldrich, Inc., Mo, USA, 99.9+ %). Boric acid (H3BO3; Katayama Chemicals, Japan) was
used as a flux. The powders were mixed according to the nominal composition of
4SrCO3+7Al2O3+0.4B (as H3BO3)  +  4 at% Eu and 8 at% Dy. In this work, different particle size alumina of γ and
α types (0.05 μm
γ-type, 0.1 μm, and 1.0 μm, α-type) were used to reveal the effect of raw alumina
powder. The mixing was performed thoroughly by mortal and pestle with the help
of ethanol. Pellets were prepared and preheated in air atmosphere at 1000°C
for 4 hours, pulverized and then heated at 1300°C for 5 hours in a reducing
atmosphere of H2/N2=1/9.Phase identification was carried out using a Shimadzu
XRD-6300 instrument with CuKα radiation at room temperature. Scanning electron microscopy
(SEM) observations were carried out using a JEOLJSM-5510LV instrument.
Photoluminescence (PL) spectra were measured using USB 4000 UV-VIS miniature
fiber optic spectrometer (Ocean Optics, Fla, USA). The decay curves
were obtained at room temperature using a brightness meter (Konica Minolta LS-100). Before  decay curves measurement, the samples were exposed
to standard 60 W xenon lamps for 25 minutes. All measurements were carried out
at room temperature.


## 3. Results and Discussion

The  X-ray diffraction patterns of the specimens prepared
from the different particle sizes
and crystal types
ofAl2O3 were almost the same as shown
in Figure 1. The same Sr4Al14O25 phase was formed (except very small impurities of SrAl4O7 phase
in all three samples as in Figure 1) which was confirmed when compared with
standard JCPDS card data (52–1876). This
result indicates that the reaction conditions
under this work were enough for the formation of Sr4Al14O25 phase. Also the change of particle size and crystal type of starting Al2O3 did not affect the phase formation.X-ray diffraction
pattern ofSr4Al14O25:Eu2+/Dy3+ phosphor prepared using (a) 0.05 μm;
γ-Al2O3, (b) 0.1 μm; α-Al2O3, and (c) 1.0 μm; α-Al2O3.
(a)
0.05μm Al2O3(b)
0.1μm Al2O3(c)
1.0μm Al2O3Figure2 shows the SEM microstructure of the three samples
prepared using Al2O3of different particle sizes: (a) 0.05 μm (γ), (b) 0.1 μm (α), and (c) 1.0 μm (α). Although the same phase was observed as in XRD observations, their
microstructures were quite different. When γ-Al2O3 of
smaller particle size was used as precursor powder, a ball-like microstructure
composed of spikes or elongated plates was observed. The average size of the
plate particle was roughly 1 μm long, 0.2 μm wide, and 20 nm thick. Due to the formation of regular and smaller particles, these
small plates overlapped with each other, forming compact ball-like structure. The size of the ball was about 3 μm in diameter. When 0.1 μm α-Al2O3 was used, irregular porous microstructure
composed of bigger and thicker hexagonal plates with 1 μm long, 0.75 μm wide, and 0.5 μm thick was formed as in Figure
2(b). These plates connected with each other and form walls, making the spongy
structure. While, using α-Al2O3 of 1.0 μm particle size,
much bigger but thinner, irregular flat plates
of 3 μm long 2 μm wide, and 30 nm thick were observed as in Figure 2(c). Due to
the high aspect ratio of the irregular plates, porous microstructure was formed.  The porous microstructure was further supported by the
density of the prepared pellets that decreased on increasing the particle size
of raw alumina powder and also the phosphor particles (0.05μ–1.909g/cm3>0.1μ–1.87g/cm3>1.0μ–1.803g/cm3).
The particle shape dependency according to the starting alumina powder
may be explained on the basis of dissolution and precipitation mechanisms [10]. The
detail mechanism for the explanation of different shapes according to the starting material is under
progress.SEM microstructure ofSr4Al14O25:Eu2+/Dy3+ phosphor
prepared using (a) 0.05 μm; γ-Al2O3, (b) 0.1 μm; α-Al2O3,
and (c) 1.0 μm; α-Al2O3.
(a)(b)(c)Figure3 shows the excitation and emission spectra of Sr4Al14O25:Eu2+/Dy3+ phosphor prepared using 0.05 μm, 0.1 μm, and 1.0 μm Al2O3 as starting materials. All samples prepared from
these three types of Al2O3 were
excitated by the light of wavelength ranging from 220 to 440 nm. These samples gave the similar emission
spectra with peaks at ~500 nm showing blue-green color. However, there was a small blue shift in the case of phosphor prepared
using 0.05μAl2O3. It is obvious that the
decrease in particle size increases the surface energy, which results in the
distortion of atomic structure around the Eu2+, and consequently, the
blue shift in the emission peak is observed [9]. The emission intensity
of these phosphors, however, varied drastically according to the used raw
alumina powder as shown in the inserted graph in Figure 4. The initial emission
intensities of the
products from 0.05 μm, 0.1 μm, and 1.0 μm Al2O3 were 2316, 137,9 and 1025mcd⋅m−2,
respectively. The emission intensity increased with the decreasing of Al2O3 particle size and became the highest for the phosphor prepared using 0.05 μm Al2O3. The decrease in phosphorescence intensity might be due to the formation
of bigger particles with much porous microstructure on
increasing the Al2O3 particle size as explained in the
SEM microstructure observations. Obviously, as the
packing density decreases, the volume content of the
phosphor decreases leading to
the decrease of the emission intensity of the phosphor.Figure 3
Excitation (normalized) and emission
spectra ofSr4Al14O25:Eu2+/Dy3+ phosphor using (a) 0.05 μm; γ-Al2O3, (b) 0.1 μm; α-Al2O3,
and (c) 1.0 μm; α-Al2O3.Figure 4
Effect ofAl2O3 particle size on the photoluminescence properties of Sr4Al14O25:Eu2+/Dy3+ phosphor. Inserted figure shows the variation
of emission intensity with particle size of Al2O3.Figure4 shows  the  change in  the afterglow emission intensity
of the phosphor products with time. The afterglow intensity was decreased by
>1/100th of the initial intensity within 1 hour. The afterglow emission
intensity and duration were higher for the phosphor prepared using 0.05 μm γ-Al2O3,
and its afterglow duration over the value of 5 mcd⋅m−2 was more than 20 hours.
The longer afterglow duration of the smaller particles phosphor may be
explained as, upon exposure to light source, the direct excitation of Eu2+ due to 4f to 4f5d transition occurs (Figure 5),
and a great numbers of holes are generated near the valence band. Some of these
free holes are released thermally to the valence band, migrate through the valence band, and
captured by the Dy3+-borate complex [6, 11]. When the excitation
source is removed, the trapped holes are released thermally to the valence
band, migrate to
the excited Eu2+, and consequently, the
recombination takes
place, which leads to the long afterglow. So, the long afterglow duration depends on
the number of captured holes. The numbers of captured holes, in turn, depend on
the concentration of Dy-borate complexes and the trap
depth of it. With the decrease of particle size,
the blue shift of the trap depth occurs [9] that leads to increase in the
afterglow duration. The detailed explanation of the mechanism with other
supporting information is under progress.Figure 5
The mechanism of long afterglow ofSr4Al14O25:Eu2+/Dy3+ phosphor.


## 4. Conclusions

Long-afterglowSr4Al14O25:Eu2+/Dy3+ phosphor was synthesized by solid phase reaction method using α and γ types Al2O3 of different particle sizes. Though the same Sr4Al14O25 phase was formed on changing the alumina precursors, quite different structures
were observed. When 0.05μmγ-Al2O3 was used, spike-like
phosphor particles were
observed with minimum particle size, which agglomerated
to form a ball-like structure. When 0.1μmα-Al2O3 was
used, bigger particles with the shape of hexagon were observed, while using 1.0μmα-Al2O3 much bigger but thinner irregular particles were
observed, that is, the shape and size of phosphor particles can be controlled
by changing the alumina precursor under the experimental conditions. The better
phosphorescence intensity and persistency were observed for the phosphor prepared using 0.05μmγ-Al2O3 that might be due to the smaller and regular
particles formation.



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*Source: 475074-2009-03-04.xml*