Low-Temperature Synthesis of Eu-Doped Cubic Phase BaAl2S4 ...

Low-Temperature Synthesis of Eu-Doped Cubic Phase BaAl 2S 4
Blue Phosphor Using Liquid-Phase Reaction
Yang Hwi Cho, a Do Hyung Park, b and Byung Tae Ahn a, * ,z
a
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,
Yuseong-gu, Daejeon 305-701, Korea
b
Samsung SDI, Yongin, Gyeonggi, 446-577, Korea
A Eu-doped BaAl 2S 4 phosphor was synthesized by a liquid-phase reaction using BaS, EuS, Al, and S, where Al acts as a liquid
source during the reaction, instead of Al 2S 3. The synthetic temperature of the cubic BaAl 2S 4 phase could be reduced to as low as
660°C, considerably lower than that required for a phosphor synthesized via a solid-state reaction using BaS, EuS, and Al 2S 3. The
cubic BaAl 2S 4:Eu phosphor synthesized using the liquid phase showed larger grains and higher crystallinity. The photoluminescence
of the cubic BaAl 2S 4:Eu phosphor showed a blue emission centered at 470 nm with a CIE color coordinate at x = 0.12,
y = 0.11. In addition to well-defined blue color, the PL intensity was doubled relative to the solid-state reaction case.
© 2007 The Electrochemical Society. DOI: 10.1149/1.2804380 All rights reserved.
Manuscript submitted July 11, 2007; revised manuscript received October 8, 2007. Available electronically November 26, 2007.
The ternary compound semiconductor BaAl 2S 4 has an optical
bandgap of 3.99 eV, which is suitable for application to optoelectronic
devices operating at the violet-blue wavelength. 1-3 In particular,
Eu-doped BaAl 2S 4 BaAl 2S 4:Eu is known to a blue emitting
phosphor centered at 467 nm. 1,4 BaAl 2S 4:Eu phosphors have received
attention for application in inorganic electroluminescence devices,
because BaAl 2S 4:Eu thin films in EL devices show a blue
emission at 470 nm with high brightness. 5-8
BaAl 2S 4:Eu phosphors are commonly synthesized by a solidstate
reaction of Eu-doped BaS and Al 2S 3. 1,2,4,9 However, solid-state
synthesis of the cubic BaAl 2S 4 phase requires temperatures above
900°C. Secondary phases that reduce the color purity of BaAl 2S 4
exist when the reaction temperature is below 900°C. In addition to
high-temperature synthesis, handling starting material, such as
Al 2S 3, is difficult because it reacts easily with water vapor or oxygen.
The selection of Al and S instead of Al 2S 3 can reduce this
difficulty. 10
Because the melting point of Al is lower than the reaction temperature
of the cubic BaAl 2S 4, the existence of liquid Al can enhance
the reactivity of the reactants. The liquid in the reactant acts
as a medium that allows fast transportation of the starting materials
through the liquid. 11
In this study, BaAl 2S 4:Eu blue phosphor is synthesized utilizing
Al melt instead of Al 2S 3 and the structure and microstructural
changes of the phosphor are investigated.
Experimental
A BaAl2S4:Eu phosphor was synthesized from BaS Kojundo
99.9%, EuS Kojundo 99.9%, AlKojundo 99.99% up, and S
Aldrich 99.99%. The ratio of BaS, EuS, Al, and S was
0.97:0.03:2:3 for 3 mol % Eu doping. To obtain pure BaAl2S4 phase, excess sulfur was added to the starting materials. The starting
materials were mixed in a mortar in air. The mixture was put into an
alumina crucible, which was then placed into a quartz ampule. The
quartz ampule was sealed under 10−3 torr vacuum. The mixture was
reacted in a temperature range of 600–850°C for 12 h.
For comparison, BaAl2S4:Eu was also synthesized form BaS,
EuS, and Al2S3 Kojundo 98%. The ratio of BaS, EuS, and Al2S3 was 0.97:0.03:1. In this case, mixing was performed in Ar, because
Al2S3 is easily hydrolyzed in moist air. The mixture was reacted in
a temperature range of 600–850°C for 12 h.
The phase was characterized by X-ray diffraction XRD operated
at 40 kV and 45 mA using Cu K = 1.5405 Å radiation.
The scanning range 2 was from 15 to 40°. The microstructure was
* Electrochemical Society Active Member.
z E-mail: btahn@kaist.ac.kr
Journal of The Electrochemical Society, 155 1 J41-J44 2008
0013-4651/2007/1551/J41/4/$23.00 © The Electrochemical Society
analyzed via scanning electron microscopy SEM. The photoluminescent
PL properties were measured under an excitation wavelength
of 350 nm.
Results and Discussion
Figure 1 shows the XRD patterns of synthesized powders with
various Al to S ratios in the reactant composition on the phase formation
at 850°C. When the Al to S ratio is 2:3, which corresponds
to the stoichiometric BaAl2S4 composition, a cubic BaAl2S4 and a
secondary Ba2Al2S5 phase were formed. The major reaction can be
expressed as
BaS+2Al+3S=BaAl2S4 The minor reaction can be expressed as 12
BaS+Al+ 3
2 S=1
2 Ba 2Al 2S 5
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It is considered that S will not be fully incorporated due to a lack of
S supply when the Al to S ratio corresponds with the stoichiometric
composition. When the Al to S ratio is 2:6, only cubic BaAl 2S 4
phase is formed. Ba 2Al 2S 5 does not exist, suggesting that S is fully
supplied. When the Al to S ratio is 2:9, BaS 3, sulfur-rich phase,
appears in addition to BaAl 2S 4.
Figure 2a shows the XRD patterns of the phosphors reacted with
Al 2S 3 as a source material at various temperatures. At 600°C, the
XRD peaks of the starting materials, BaS and Al 2S 3, are detected. At
700°C, BaS and Al 2S 3 phases are reduced by reaction. The XRD
peaks of the synthesized powders have low intensities and are not
Figure 1. XRD patterns of synthesized powders with various Al to S ratios
after heat-treatment at 850°C.

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Figure 2. XRD patterns of synthesized powders with various temperatures
using a Al 2S 3 and b Al + S Al:S=2:6.
clearly distinguished from the background. At 800°C, cubic
BaAl 2S 4 phase and BaAl 4S 7 phase are formed. The BaAl 4S 7 phase
has a relatively low synthetic temperature and is not stable at higher
temperature. 9 At 850°C, BaAl 4S 7 phase disappears and only a cubic
BaAl 2S 4 phase exists.
Figure 2b shows the XRD patterns of phosphors with an Al to S
ratio of 2:6 as a source material, reacted at various temperatures.
Orthorhombic BaAl 2S 4 phase and BaS 3 phase are formed at 600°C,
while no reaction takes place at this temperature when Al 2S 3 is used
as a source material. At 650°C, BaS 3 phase disappears and only
orthorhombic BaAl 2S 4 phase exists. Note that all starting materials
are reacted and changed to BaAl 2S 4 phase at 650°C, while no
BaAl 2S 4 phase is formed when the starting material contains Al 2S 3.
At 700°C, the synthesized BaAl 2S 4 powder has a cubic phase. The
temperature at which cubic BaAl 2S 4 can be obtained when using
Al + S is lower than that in the case of using Al 2S 3. Even though it
is a secondary phase, BaS 3 exists at 700°C and can be removed by
controlling the Al to S ratio.
Figure 3 shows XRD patterns of phosphor with an Al to S ratio
of 2:6, reacted at 650, 660, and 670°C. At 650°C, BaAl 2S 4 phase is
orthorhombic. BaAl 2S 4 phase is changed from orthorhombic to cubic
when the reaction temperature increases above 660°C, which is
near the aluminum melting point. It appears that the liquid-phase
reaction by aluminum melting lowers the phase formation temperature
from orthorhombic to cubic. As a result, the synthetic temperature
is lowered by 140°C.
To understand the reaction mechanism more precisely, the phases
in the specimen with partial reaction are analyzed. Figure 4 shows a
XRD pattern of the powder with an Al to S ratio of 2: 6, heated from
Figure 3. XRD patterns of synthesized powders using Al + S Al:S=2:6
at a 650°C, b 660°C, and c 670°C.
room temperature to 700°C for 70 min and then cooled down immediately.
If Al and S are mixed, they form Al2S3 at 700°C. But if
BaS, Al, and S are mixed the reaction is quite different. At 700°C,
BaS and S react to form BaS3 first, as seen in the XRD pattern. The
XRD pattern shows the BaS3, Al, and BaAl2S4 phase. Therefore, it
is considered that BaAl2S4 is formed by the following reaction
mechanism
BaS3 +2Al+S=BaAl2S4 The Al2S3 phase is not observed, as shown in Fig. 4. When solid
Al2S3 powder was used, cubic phase BaAl2S4 cannot be formed
below 800°C. 9
Figure 5 shows microstructures of synthesized BaAl2S4:Eu powder
at 850°C using a Al2S3 and b Al + S. The synthesized pow-
Figure 4. A XRD pattern of the powder, heated from room temperature to
700°C for 70 min and then cooled down immediately, using Al + S Al:S
=2:6.

Figure 5. SEM images of synthesized BaAl 2S 4 powders at 850°C using a
Al 2S 3 and b Al + S Al:S=2:6.
der using Al 2S 3 Fig. 5a has a conglomerate structure comprised of
many round grains. The grain size is generally below 1 m diam.
The synthesized powder using Al + S Fig. 5b has faceted grains.
This indicates that the liquid-phase reaction occurs. The synthesized
powder using Al + S has relatively large grains ranging from
3to15m diam. From the microstructure, it appears that the crystallinity
crystal quality of cubic BaAl 2S 4 is improved by the liquidphase
reaction.
To investigate the difference in the crystallinities according to the
synthesis method, the full widths at half-maximum fwhms of the
311 X-ray diffraction peaks are compared in Fig. 6. The fwhm of
the synthesized powder using Al + S is 0.141, while that using
Al 2S 3 is 0.196. These results indicate that the synthesized powder
using Al + S has better crystallinity than that using Al 2S 3.
Figure 7 shows the PL spectra of powders synthesized at 850°C
with various Al to S ratios. When the Al to S ratio is changed from
2:3 to 2:6, the peak of the synthesized powders shifts from 483 nm
fwhm: 51 nm to 470 nm fwhm: 43 nm. This is caused by the
disappearance of Ba 2Al 2S 5 phase emitting at 487 nm. When the Al
to S ratio is 2:6, the powder has a blue emission centered at 470 nm
with the CIE color coordinate at x = 0.12 and y = 0.11 due to luminescence
from Eu-doped cubic BaAl 2S 4 phase only. When the Al to
S ratio is changed from 2:6 to 2:9, the powder shows a small redshift
to 473.5 nm.
The emission of BaAl 4S 7:Eu is also slightly redshifted compared
to BaAl 2S 4:Eu and the emission peak is broadened. 4 However, no
BaAl 4S 7:Eu was detected in Fig. 1 and 2b. The emission at Al:S
= 2:9 had slightly smaller fwhm compared to the emission at
Al:S = 2:6 fwhm of 2:6, 43 nm; fwhm of 2:9, 42 nm. Thus, it is
Journal of The Electrochemical Society, 155 1 J41-J44 2008 J43
Figure 6. 311 XRD patterns of synthesized BaAl 2S 4 powders at 850°C
using Al 2S 3 and Al + S Al:S=2:6.
considered that BaAl 4S 7 does not affect the redshift and the existence
of BaS 3 affects the crystal field of BaAl 2S 4:Eu. BaS 3:Eu, reacted
at 850°C for 12 h, did not show emission.
Figure 8 shows PL spectra of synthesized powders reacted at
various temperatures with an Al to S ratio of 2:6. The powder is
composed of orthorhombic BaAl 2S 4:Eu below 650°C and shows a
peak at 476 nm. At 700°C, the powder contains BaS 3 second
Figure 7. PL spectra of synthesized powders with various Al to S ratios after
heat-treatment at 850°C.
Figure 8. PL spectra of synthesized powders with various temperature at
Al:S=2:6.

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Figure 9. PL spectra of synthesized BaAl 2S 4:Eu powders at 850°C using
Al 2S 3 and Al + S Al:S=2:6.
phase in cubic BaAl 2S 4:Eu and shows a peak at 473.5 nm. At
850°C, the powder is composed of only cubic BaAl 2S 4:Eu and
shows a peak at 470 nm.
Figure 9 shows PL spectra of BaAl 2S 4:Eu powders synthesized
using Al 2S 3 and Al + S. The peak intensity of BaAl 2S 4:Eu powder
using Al + S is twice that of BaAl 2S 4:Eu powder using Al 2S 3. This
is attributed to the larger grain size Fig. 5 and higher crystallinity
Fig. 6 of the powder synthesized using a liquid-phase reaction. It
has been reported that the quality of grains strongly affects phosphor
emission intensity. 13,14
Conclusion
In this work, a BaAl2S4:Eu phosphor was synthesized by a
liquid-phase reaction using BaS, EuS, Al, and S. Al and S were used
instead of Al2S3, as they are easy to treat in an air atmosphere and as
they act as a liquid source during the reaction. The Al to S ratio
affected the phase formation in the synthesized powder. When the Al
to S ratio was 2:6 at 850°C, only cubic BaAl 2S 4 phase appeared.
The synthetic temperature of cubic BaAl 2S 4 phase could be reduced
to as low as 660°C compared to the case of a solid-state reaction
800°C using BaS, EuS, and Al 2S 3. A cubic BaAl 2S 4:Eu phosphor
synthesized using the liquid phase showed larger grains and higher
crystallinity. The photoluminescence of the cubic BaAl 2S 4:Eu phosphor
showed blue emission centered at 470 nm with a CIE color
coordinate at x = 0.12, y = 0.11. In addition to well-defined blue
color, the PL intensity of the phosphor using the liquid-phase reaction
was twice that of a phosphor synthesized via a solid-state reaction,
due to the larger grains and higher crystallinity of the former.
Korea Advanced Institute of Science and Technology assisted in meeting
the publication costs of this article.
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