A new sol-gel route to synthesize YPO4:Tb as a green-emitting phosphor for the plasma display panels

Article abstract of DOI:10.1246/cl.2005.566

This work adopts a novel and low-cost sol-gel route to synthesize Tb ³⁺-doped YPO₄ as a green-emitting phosphor for the plasma display panel (PDP). The phosphor obtained by this route shows improved luminescence efficiency in vacuum ultraviolet (VUV) excitation, compared with that obtained by a solid-state reaction. Copyright

Full text of DOI:10.1246/cl.2005.566

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Chemistry Letters Vol.34, No.4 (2005)
A New Sol–Gel Route to Synthesize YPO :Tb as a Green-emitting Phosphor
4
for the Plasma Display Panels
ꢀ
Weihua Di, Xiaojun Wang, Baojiu Chen, and Xiaoxia Zhao
Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and physics,
Chinese Academy of Sciences, Changchun, 130033, P. R. China
(Received January 6, 2005; CL-050022)
3
þ
This work adopts a novel and low-cost sol–gel route to syn-
of N2/H2 to prevent the oxidation of Tb
.
thesize Tb3 -doped YPO4 as a green-emitting phosphor for the
plasma display panel (PDP). The phosphor obtained by this route
shows improved luminescence efficiency in vacuum ultraviolet
þ
All the measurements were performed at room temperature.
XRD studies were conducted on a Rigaku D/max-2000 X-ray
powder diffractometer using Cu Kꢀ radiation. SEM images were
taken on a KYKY-1000 scanning electron microscopy. The ex-
citation spectra were measured by using a vacuum ultraviolet
monochromator and a deuterium (D2) lamp and were normalized
to that of sodium salicylate. The emission spectra were obtained
under 147-nm excitation.
(
VUV) excitation, compared with that obtained by a solid-state
reaction.
Recently, the research in the vacuum ultraviolet (VUV) re-
gion has become a challenge in the field of luminescence of rare
earth ions because of the development of phosphors used in the
Figure 1 presents XRD patterns of YPO4:Tb from sol–gel
ꢃ
route annealed at 1000 C. It is seen that all the diffraction peaks
1
,2
plasma display panel (PDP). The host for PDP phosphors is re-
3
are well indexed to tetragonal xenotime structure of YPO4
(JCPDS File No. 84-0335), indicating the formation of YPO4
phase. The XRD data (not shown here) of the sample obtained
quired to have band-gap absorption in VUV region. The phos-
phates are suggested to be a suitable host for PDP phosphors ow-
ing to high VUV absorption. Tb3 -activated YPO4 and LaPO4
are typical examples that are used as green-emitting PDP phos-
phors.4 The luminescent performance of PDP phosphors de-
pends strongly on the synthetic route and procedure. In the case
of synthesis of YPO4:Tb, a solid-state reaction is usually used.
However, this method has several disadvantages such as high-
temperature process, long-time calcination and repeated milling
and grinding. Accordingly, the phosphor particles by the solid-
state reaction possess a large size, irregular shape, and coarse
surface. In contrast, the wet chemical route may overcome these
þ
ꢃ
from a solid-state reaction and calcinated at 1200 C shows all
the diffraction lines with almost the same intensity as observed
in the sample from sol–gel route. This indicates the crystalliza-
,5
ꢃ
tion temperature for sol–gel route is 200 C lower than that for a
solid-state reaction. This is due to the fact that the precursor
powders obtained from sol–gel route have very small size, and
thus large surface area. This low-temperature synthesis not only
prevents the introduction of the impurity, but controls the growth
of phosphor particles effectively.
6
,7
disadvantages mentioned above. Furthermore, the phosphor
particles by the wet chemical route commonly have high compo-
sition homogeneity and phase purity. However, few report has
been made for the synthesis of YPO4 by sol–gel method, the only
(200)
8
one even involved the use of metal alkoxides, which are very
(112)
(
312)
expensive and pollutive. This work aims at reporting a novel
and low-cost sol–gel route to synthesize Tb-activated YPO4.
The luminescent characteristic of YPO4:Tb from different syn-
thetic routes under VUV excitation is compared.
(
220) (301)
(321)
(400)
(
211)
(202) (103)
20
30
40
50
60
2
θ
Appropriate amount of Y2O3 (4N) and 1% mol Tb4O7 (4N)
were dissolved in concentrated nitric acid to form Y(NO3)3 and
Tb(NO3)3 solutions. These solutions were then passed through
Figure 1. X-ray diffraction pattern of YPO :Tb obtained from
4
sol–gel route and annealed at 1000 C.
ꢃ
ꢁ
an OH exchange column with Dowex 1 ꢂ 4 (50–100 mesh) res-
in at room temperature. The flow of the solution was controlled
to maintain the pH of the collected solution to be 11.0. Thus, the
clear sols of Y(OH)3 and Tb(OH)3 were obtained. Trimethyl
phosphate and ethanol were mixed in 1:10 ratio to get trimethyl
phosphate stock solution. The stiochiometric amount of Y(OH)3,
Tb(OH)3, and trimethyl phosphate solution, together with a
small amount of boric acid as a flux were transferred to a round
Figure 2 shows the morphology of YPO4:Tb particle syn-
thesized by sol–gel route and solid-state reactions, respectively.
The YPO4:Tb particle from sol–gle route possesses almost
spherical shape, small size and smooth surface. In contrast, the
irregular shape, large size, and poor surface quality are observed
for the YPO4:Tb particle from solid-state reactions. This mor-
phological difference originates from different synthetic route.
As is known, all of the starting materials are mixed at the molec-
ular level in sol–gel route. In addition, a lower processing tem-
perature is required and no milling and grinding processes are in-
troduced. Thus the superior morphology of phosphor particles is
observed, as shown in Figure 2a.
ꢃ
bottom flask, and peptized at 80 C for 5 h in a stirrer. After the
pepitization, sol/gels were left in a container for 24 to 48 h until
ꢃ
they became a thick gel, which was dried at 80 C in an oven.
After drying, the gel powder underwent two-step heat treatments
ꢃ
at 300 and 1000 C, respectively for 2 h in a reducing atmosphere
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.4 (2005)
567
sol-gel route
solid state reaction
4
50
500
550
600
650
Wavelength / nm
Figure 3. The emission spectra of YPO4:Tb from two different
synthetic routes in 147-nm excitation.
(
a)
_
a) and solid-state reaction (b).
2 µm
(b)
_
6 µm
Figure 2. SEM images of YPO4:Tb obtained by sol–gel route
(
It is well known that the luminescent performance of PDP
sol-gel route
phosphors depends strongly on the shape, size, surface, and dop-
ant dispersity.9 Figure 3 presents the emission spectra of
YPO4:Tb from sol–gel route and solid-state reaction in 147-
nm excitation. The spectrum consists of lines ranging from
solid state reaction
,10
480 to 630 nm, which originate from the transitions between dif-
ferent f-electron states of Tb , i.e., between the excited D4 to
1
50
200
250
3þ
5
Wavelength / nm
7
the Fj (j ¼ 6, 5, 4, 3) ground states. The most intense emission
Figure 4. VUV-UV excitation spectrum of YPO4:Tb in 140–
5
5
7
is the D4 to F5 transition peaking around 543 nm, correspond-
ing to the green emission. It is worth noting that the emission in-
tensity of YPO4:Tb from sol–gel route is about 1.5 times higher
than that from solid-state reactions. The luminescent efficiency
of a phosphor is closely related to the host matrix purity and dop-
ant homogeneity. Since the molecular-level reaction is achieved
in sol–gel route, the doping of activators through solutions is
straightforward, efficient, and highly homogeneous. Therefore,
the homogeneous dispersion of dopant ions lowers the probabil-
ity of ion interactions, thus enhancing the luminescent efficien-
cy. Furthermore, as we have known, under VUV excitation,
the energy is absorbed first by the host, and then transferred to
the activators.11 Therefore, the luminescence efficiency under
VUV excitation depends strongly on the host absorption efficien-
cy. However, VUV radiation only penetrates into a thin surface
layer of phosphor particle within several tens nanometers, thus,
the luminance of phosphors in VUV excitation largely comes
7
2
80-nm range by monitoring the D4 to F5 transition.
the host absorption to VUV light. Therefore, the luminescent
efficiency in VUV excitation is improved by sol–gel route.
In conclusion, a novel and low-cost sol–gel route is used
to synthesize Tb -doped YPO4. High phase purity and dopant
homogeneity, and superior particle morphology can be achieved
by this route, thus improving the luminescent efficiency of
YPO4:Tb as a PDP phosphor.
3þ
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 50172046, 50102006) and
the State 863 Project (Grant No. 2002AA311160).
References
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3
,11
from the surface contribution.
Therefore, the surface quality
2
of phosphor particles is a key factor to determine the lumines-
cent efficiency. As observed in Figure 2a, the YPO4:Tb particle
synthesized by sol–gel route have smooth surface and almost
spherical shape. Such morphology may effectively decrease
the scattering of incident light, thus enhancing the absorption
to VUV radiation. In order to confirm this, the excitation spectra
are shown in Figure 4. The spectrum consists of a few broad
bands ranging from 140 to 280 nm. The band peaking around
3
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150 nm should be ascribed to the host absorption. Other bands
in the excitation spectrum are ascribed to the transitions from
8
the lower energy level of the 4f configuration to the energy lev-
7
3þ
els of the 4f 5d configuration of the Tb . It is noted that the in-
tensity of host absorption band of YPO4 from sol–gel route is
stronger than that from a solid-state reaction. This indicates that
the efficiency of host absorption to VUV light is related to the
synthetic route. The smooth surface and almost spherical shape
of phosphor particles synthesized by sol–gel route contribute to
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Published on the web (Advance View) March 12, 2005; DOI 10.1246/cl.2005.566