Photoluminescence efficiencies of nanocrystalline versus bulk Y 2O3: Eu phosphor-revisited

Article abstract of DOI:10.1111/j.1551-2916.2010.04246.x

Highly efficient yttrium oxide doped with trivalent europium (Y ₂O₃:Eu) phosphor was prepared through precursors synthesized by hydrothermal method. Crystalline precursors, namely europium-doped yttrium carbonate (Y₂(CO

Full text of DOI:10.1111/j.1551-2916.2010.04246.x

J. Am. Ceram. Soc., 94 [5] 1627–1633 (2011)
DOI: 10.1111/j.1551-2916.2010.04246.x
r 2010 The American Ceramic Society
ournal
J
Photoluminescence Efficiencies of Nanocrystalline versus Bulk Y O :
2
3
Eu Phosphor—Revisited
z
z
y
z
Arunkumar Paulraj, Prabu Natarajan, Kottaisamy Munnisamy, Mujafar Kani Nagoor, Kamaraj
Parimana Nattar, Burkanudeen Abdulrazak, and Jeyakumar Duraisamy
z
z
w,z
z
Functional Materials Division, Central Electrochemical Research Institute, Council of Scientific and Industrial
Research, Karaikudi 630 006, Tamilnadu, India
yDepartment of Chemistry, Thiagarajar College of Engineering (Autonomous), Madurai - 625015, Tamilnadu, India
zDepartment of Chemistry, Jamal Mohamed College, Trichy 621 006, Tamilnadu, India
1
2
Highly efficient yttrium oxide doped with trivalent europium
(Y O :Eu) phosphor was prepared through precursors synthe-
sized by hydrothermal method. Crystalline precursors, namely
phor. Devaraju et al. have prepared nanorod- and spherical-
shaped Y :Eu by the solvothermal method and observed that
2
3
2 3
O
material with spherical morphology has higher PL efficiency than
the rod-shaped one. Combustion method has been successfully
.
europium-doped yttrium carbonate (Y (CO3)3 2H O:Eu) and
europium-doped yttrium hydroxy carbonate (Y(OH)CO :Eu),
2
2
13
3
2 3
used for the preparation of Y O :Eu nanophosphor by Vu et al.
14,15
and Kottaisamy et al.
16
Recent studies of Wang et al. on the
were prepared by varying the concentration of yttrium to
europium ions and urea in the reaction mixture. The precursor
materials on annealing at 7001C gave nanocrystalline Y O :Eu,
preparation of nano-Y O :Eu by the spray pyrolysis method has
2
shown that the emission efficiency has increased with the increase
in the crystallite size of the phosphor. The Y :Eu material pre-
pared at 14001C was found to have the crystallite size of 42.8 nm
and the PL efficiency was found to be 51.76% against the
theoretical quantum efficiency of 97%. The PL efficiency
of nanocrystalline phosphor versus bulk phosphor seems to be
ambiguous. In addition, in most of the papers, the diffused re-
flectance of phosphor materials was not taken into consideration.
Hence, it was felt that it would be desirable to study the param-
3
2
3
which was further processed at high temperatures in the absence
and presence of sintering aid to yield phosphor materials with
varying crystallite size and morphology. The precursors and
phosphor material were characterized using FTIR, TGA,
powder XRD, SEM, TEM, and photoluminescence (PL) spec-
tral studies. The emission efficiency was found to depend on the
crystallite size, morphology, and particle size of the phosphor
materials. It was observed that phosphor material with spherical
morphology and particle size of 0.5–1.0 lm with crystallite size
of 100 nm has the highest PL efficiency.
2 3
O
17
2 3
eters that affect the PL efficiency of Y O :Eu phosphor material.
We have been working in the preparation of phosphor
materials using combustion method, hydrothermal method,
Among these methods,
14,15
and solid-state metathesis reaction.
I. Introduction
the hydrothermal method has acquired potential importance in
material synthesis owing to the formation of highly crystalline
material and its industrial scalability. Hence, appropriate
protocols were evolved for the preparation of nanocrystalline
phosphor that was processed further by annealing at various
temperatures in the presence and absence of sintering aid to
obtain phosphor material with the desired crystallite size and
morphology. The PL efficiency of the processed material was
measured in comparison with the commercial standard phosphor
material and the data are reported.
TTRIUM OXIDE activated with europium (Y
2 3
O :Eu) is an
Y
unsurpassed red emitter extensively used in compact fluo-
1,2
3
rescent lamps, cathode ray tubes, currently used in high-
resolution and projection television systems and plasma display
devices. Recent advances in field emission display (FED) tech-
4
nology also points out that Y
2
O
3
:Eu is a potential candidate
5
as the red component. Recently, nanocrystalline Y O :Eu
2
3
phosphor has been anticipated to be superior to micrometer-
sized materials due to lower screen loading, higher screen
density, and high resolution in FED applications. However,
6
the reports on the photoluminescence (PL) efficiency of the
nanocrystalline phosphor compared with bulk are ambiguous.
Taxak et al., Wakefield et al., and Sharma et al. have adopted
II. Experimental Procedure
7
6
8
(
1) Materials
different methodologies for the preparation of nanocrystalline
All the reagents used in the present study were analytical-grade
materials. Yttrium oxide (99.99%, Hi-Media Laboratories Pvt.
Ltd., Mumbai, Maharashtra, India), europium (III) oxide
2 3
Y O :Eu and reported that the PL efficiency of the nanophos-
9
phor is better than the bulk material. Nelson et al. hav1e
explored the alkalide reduction method, Schmechel et al.
have adopted the chemical vapor synthesis (CVS) method, and
0
(
99.9%), lithium fluoride and sodium fluoride (Sigma-Aldrich
1
1
India (P) Ltd., Bangalore, Karnataka, India), and urea (99.5%,
Sisco Research Laboratories Pvt. Ltd., Mumbai, Maharashtra,
India) were used for the preparation of materials. Millipore wa-
ter was used for all the preparative work. Yttrium nitrate and
europium nitrate were prepared by dissolving the requisite quan-
tity of the oxides in AR nitric acid; the excess acid was removed
and used for the preparative work.
Hirai et al. have exploited the versatility of emulsion systems
for the synthesis of nanocrystalline Y :Eu material. These
2 3
O
groups have reported that the PL emission of phosphor mate-
rials decreased with the decrease in the particle size, indicating
that nanophosphors have lower efficiency than the bulk phos-
J. Ballato—contributing editor
(
2) Methods
Hydrothermal reactor (PARR Model 4836, PARR Instrument
company, Moline, IL) was used in this work. An autoclave was
Manuscript No. 27944. Received April 30, 2010; approved October 11, 2010.
Author to whom correspondence should be addressed. e-mail: djkr@rediffmail.com
w
1
627

1
628
Journal of the American Ceramic Society—Paulraj et al.
Vol. 94, No. 5
(2)
ꢁ
þ
2ꢁ
3
used as a control to study the effect of pressure on the mor-
phology of the synthesized materials. The synthesized materials
were characterized by powder X-ray diffractometer (PANalyt-
ical X’Per PRO Model X-ray diffractometer, PANalytical B.V.,
Almelo, the Netherlands) with CuKa radiation (a 5 1.5418 A),
the operation power is 2.2 KW. The 2y values for the measure-
ment of XRD were 10–901 in steps of 0.021 with a count time of
CNO þ 2H O ! NH þ CO
2
4
˚
Among these two steps, the first step was observed to be the
rate-limiting step and there is a sustained release of carbonate
species in the reaction medium. The pH of the solution when
urea alone was used in the PARR reactor was found to be
around 9.5, which can be effectively used for the precipitation of
0.2 s. Thermal analysis of the sample was carried out with TGA/
DTA instruments (TA Instruments, model SDT Q 600, New
Castle, DE). The samples were heated up to 10001C and the rate
of heating was 201C/min in a nitrogen atmosphere. FTIR spectral
measurements were carried out using a spectrometer (Thermo
Electron Corporation, Nexus 670 model, Centaurms 10 ꢀ micro-
scope, Waltham, MA). The particle size and morphology of the
sample were characterized using a scanning electron microscope
1
9
carbonates or basic carbonates of lanthanides. Herein, urea
hydrolysis through the hydrothermal method was carried out
with yttrium/europium nitrates to form yttrium/europium car-
bonate or hydroxy carbonate. The synthesized materials were
characterized using SEM, FTIR, and powder XRD studies.
3 3
The FTIR spectrum of the precursor prepared using Y(NO )
(Hitachi Model S-3000H, Hitachi High-Technologies Trading
and urea in 1:4 mol ratio using the PARR hydrothermal reactor
Corporation, Minato-Ku, Japan) with the resolution of 3.5 nm.
PL studies were carried out using a fluorescence spectrophotom-
eter (Cary AEclipse spectrometer, Varian Inc., Mulgrave, Austra-
at 1501C is presented in Fig. 1. It can be seen from the figure that
ꢁ
1
there are peaks at 758, 845, 1433, 1514, and 3480 cm . The first
two peaks arise due to the bending and the subsequent two
peaks due to the asymmetric stretching of carbonate functional
2 3
lia). Y O :Eu phosphor (Nichia Corporation, Tokyo, Japan) was
ꢁ1
used as the commercial standard. The solid sample holder was
supplied along with the equipment, has a quartz window at one
end, and a screw-thread arrangement at the other end. Three hun-
dred milligrams of the sample was weighed and placed in the
holder and tightened carefully to retain the close-packed material
in the holder. The PL emission spectra were measured with the
excitation and emission slit width of 5 nm for all the samples. All
the other parameters were kept the same for all the measurements,
and to ensure the reproducibility of data on PL emission efficiency,
the measurements were repeated independently. Transmission elec-
tron microscopy measurements were performed using a micro-
scope (Tecnai 20 G2, FEI company, Eindhoven, the Netherland).
group in the precursor. The broad band around 3480 cm sug-
gests the –OH stretching vibration. This is in good agreement
2
0
with the previous reports. The FTIR characteristics are similar
to the materials obtained using different compositions of Y:Eu
nitrates in the reactants. Further, there are no significant differ-
ences in the IR spectra of the products formed using the auto-
clave or the PARR high-pressure reactor. From the IR spectral
data alone, it would be difficult to conclude whether yttrium/
europium carbonate or yttrium/europium hydroxy carbonate
has formed during the course of the hydrothermal reaction.
The SEM pictures of the products formed by the hydrothermal
reaction and autoclave are presented in Fig. 2. Figure 2(b) depicts
that the material obtained from the autoclave is highly aggre-
gated, submicrometer-sized, and spherical-shaped particles. How-
ever, the morphology of product formed using high-pressure
PARR reactor was found to be platelet like and crystallized as
a flower, sized in the range of 5–10 mm. This difference may arise
due to the low temperature and pressure attainable in the auto-
clave, when compared with the PARR reactor. A maximum tem-
perature of 1201C and a pressure of 15 psi are realizable in an
autoclave while in the PARR reactor at a temperature of 1501C
resulted in the generation of an autogenic pressure of 100 psi,
resulting in the formation of a highly crystalline material. It is well
established that high pressure and temperature result in the for-
mation of highly crystalline materials. At this juncture, it is wor-
(
3) Preparation of Precursor Material
3 3
Y(NO ) (60 mM) and urea (240 mM) were dissolved in 100 mL
of water and the resulting solution was then charged into the
glass liner of the PARR high-pressure hydrothermal reactor. The
PARR reactor was heated to 1501C and maintained at this tem-
perature for 3 h. During the course of the reaction, the pressure
in the vessel reached a maximum of 100 psi. At the tenure of the
reaction, the reactor was allowed to cool to ambient temperature.
The glass liner was removed from the reactor. The white precip-
itate formed during the reaction was filtered, thoroughly washed
with water, and then dried in hot air oven at 701C overnight. This
material was used for further characterization.
thy to note the formation of dumbbell-like-shaped Y(OH)CO
using Y(NO and urea at 1501C for 22 h as reported by Zhang
et al. They have also observed that the size of the dumbbell
Y(OH)CO increased with the increase in the reaction time.
3
Europium doping was effected using a mixture of yttrium/
europium nitrates with the requisite mole ratio of europium in
the reaction mixture under the same experimental conditions.
Europium concentrations of 0.02, 0.03, 0.04, 0.05, 0.06, and
)
3 3
20
3
0
.1 mol ratio were used in the present study.
3 3
A control reaction was carried out using Y(NO ) and urea (60
and 240 mM, respectively) in an autoclave at 1201C with the au-
togenic pressure of 15 psi. The product was separated, thoroughly
washed with deionized water, dried, and used for further studies.
The nanocrystalline yttrium oxide was prepared by annealing
the precursor material at 7001C. This material was annealed at
1
0001, 11001, and 12001C for 6 h. Liquid-phase sintering of the
oxidic material was carried out using NaF/LiF as a sintering aid
at 10001, 11001, and 12001C for 6 h. The sintering aid was
washed thoroughly with hot water and the materials were dried
in an oven at 801C overnight. The materials thus obtained were
characterized using SEM and PL studies.
III. Results and Discussion
18
Hydrothermal reaction of urea is well established, resulting in
the generation of ammonium carbonate as the product in two
steps as depicted below.
Fig. 1. FTIR spectrum of the hydrothermal reaction product using
Y(NO ) and urea in the 1:4 mol ratio at 1501C.
3 3
þ
ꢁ
NH2CONH2 ! NH þ CNO
(1)
4

May 2011
PL Efficiencies of Y O :Eu
2
1629
3
3 3
Fig. 2. SEM image of products obtained from Y(NO ) and urea using (a) PARR hydrothermal reactor and (b) autoclave.
The powder XRD pattern of samples prepared using different
compositions of Y/Eu nitrates and reaction conditions were
measured and presented in Fig. 3. The diffraction pattern of the
precursor formed using autoclave is depicted in Fig. 3(a) and no
lines were observed suggesting the formation of amorphous
material. However, the material obtained using the PARR re-
actor shows sharp lines (Figs. 3(b)–(e)), indicating the crystalline
nature of the material. There are significant differences between
are o0.1, indicating that the product is highly crystalline. When
the urea-to-Y/Eu nitrate mole ratio was reduced to 1:1, then
the product formed was found to be Y/Eu(OH)CO
3
and was
characterized by XRD (Fig. 4). This is in accordance with the
2
0
report of Zhang et al., where they have shown that at 1:1 mol
ratio of Y(NO -to-urea, Y(OH)CO was the product, and at
)
3 3
3
higher mole ratio of urea, formation of yttrium carbonate was
observed. However, the role of europium on the formation of
hydroxy carbonate to carbonate was not explored by them.
The formation of hydroxy carbonate was observed either
at higher mol% of europium or lower mol% of urea was used in
the reaction mixture. It is fairly well established that lanthanide
ions due to their high charge and smaller ionic size tend to
the products obtained using Y(NO
europium when compared with Y(NO )
3
)
3
alone or with 3 mol%
with 5 or 10 mol%
3 3
europium in the reaction mixture. The observed lines were com-
pared with the standard XRD patterns of the carbonate and
hydroxy carbonate of yttrium. When yttrium nitrate alone or
with 3 mol% Eu were used in the reaction mixture with 4 mol
2
2
hydrate readily to form the aqua complexes. Rare-earth (RE)
aqua complex deprotonates to form a hydroxy aqua complex
owing to RE’s nature of Lewis acidity:
equivalents of urea, orthorhombic
x 5 0, 0.06) phase has formed exclusively, without any impuri-
ties and matched well with the ICDD No. 24-1419. This was in
Y2ꢁxEu (CO )
x 3 3
2
ꢂ 2H O
(
2
0
accordance with the earlier reports by Zhang et al. However,
Y1 Eu (OH)CO3 phase formation was observed for higher
LnðH2OÞnꢃ3þ ! ½LnðOHÞðH2OÞnꢁ1ꢃ2þ þ Hþ
½
(3)
ꢁx
x
mol% of europium (x 5 0.05, 0.1) in the reaction mixture.
The lines observed in the powder XRD pattern were in good
agreement with the ICDD No. 70-0278 and matched with earlier
This reaction may be favorable under hydrothermal condi-
tion with higher ratio of europium ions (ca. 5 and 10 mol%) in
the reaction mixture. This intermediate
2
1
reports. Further, the FWHM values of the diffraction peaks
2þ
þ CO 23 ꢁ ! LnðOHÞCO3
½
LnðOHÞðH2OÞnꢁ1
ꢃ
(4)
when reacted with the carbonate species generated during the
hydrolysis of urea resulted in the formation of the corresponding
hydroxy carbonate.
Fig. 3. XRD pattern of precursors formed by reacting Y(NO
urea in the 1:4 mol ratio using (a) autoclave and (b) PARR hydrother-
mal reactor, Y(NO with (c) 3 mol% Eu(NO , (d) 5 mol% Eu(NO
and (e) 10 mol% Eu(NO with urea where the Y/Eu(NO to urea
concentration is 1:4 mole ratio.
3 3
) and
)
3 3
)
3 3
3 3
) ,
)
3 3
)
3 3
Fig. 4. XRD pattern of precursors formed with Y(NO
1:1 mole ratio and (b) 1:4 mole ratio using PARR hydrothermal reactor.
3 3
) and urea (a)

1
630
Journal of the American Ceramic Society—Paulraj et al.
Vol. 94, No. 5
Similarly, at lower concentration of europium with higher
the Fig. 6. The diffraction pattern shows that the material
is crystalline and does not resemble the starting material. The
observed lines were compared with the cubic oxide phase
of yttria (ICDD No. 89-5592) and found to be in good agree-
ment. Further, the pattern shows line broadening, indicating
the reduction in crystallite size of the yttria than the starting
material. It is worthy to note that all precursor materials gave
rise to similar FWHM values on annealing. The crystallite size
of the material was calculated using the Scherrer’s formula:
mol% of urea in the reaction mixture resulted in the formation
of carbonate due to higher concentration of carbonate generated
in the reaction mixture, whereas at low urea concentration (1:1
mol ratio), hydroxy carbonate formation was found to occur.
The formation of carbonate or hydroxy carbonate has been
further confirmed by the TGA. The TG graphs of Y (CO ) ꢂ
2
3 3
2 3
2H O and Y0.95Eu0.05(OH)CO are presented in Fig. 5. The
thermal decomposition of these materials may be as depicted
below:
D ¼ Kl=b cos y
(8)
Y2ðCO Þ ꢂ 2H2O ! Y2O3 þ 3CO2 þ 2H2O
(5)
3
3
where D is the crystallite size, K is the dimension-less shape
factor (0.9), l is the X-ray wavelength, b is the FWHM, and y is
the Bragg’s angle. The crystallite size of the material was found
to be 27 nm. It is explicit from the data that the micrometer-
sized crystals of the precursors were disintegrated into nano-
crystalline oxidic material at 7001C. This arises due to the
2
Y0:95Eu0:05ðOHÞCO ! Y1:9O3
3
:
Eu0:1 þ 2CO2 þ H2O
(6)
It can be seen from the Eq. (5) that the carbonate precursor
would lose 3 mol of CO and 2 mol of H O that corresponds to
2
2
evolution of 2 mol equivalent of CO and 1 mol equivalent of
2
the weight loss of 42%. In the case of hydroxy carbonate pre-
cursor material, there will be loss of 1 mol of CO and half a
mole of H O per molecule of the precursor and this corresponds
to the weight loss of 31.0%. TG graphs of Y (CO ) ꢂ 2H O
H O molecule per 2 mol of hydroxy carbonate precursor and
2
2
similarly the evolution of 3 mol equivalent of CO
equivalent of H O molecule from the carbonate precursor.
The TEM picture along with the electron diffraction pattern
as inset) of the sample, Y0.95(OH)CO :Eu0.05 annealed at 7001C
and 2 mol
2
2
2
2
3 3
2
3
and Y0.9Eu0.1(OH)CO indicate weight loss of 44% and 31%,
(
3
respectively, and are in good agreement with the expected weight
loss. Further, it can be seen from the figure that initial weight
loss occurs around 2801C for Y (CO ) ꢂ 2H O. The initial
is presented in Fig. 7. It can be seen from the figure that
the material is nanosized and agglomerated with a platelet-
like morphology. The diffraction pattern clearly shows that
the oxide phase is crystalline. Annealing the precursor Y1.94
2
3 3
2
weight loss in the carbonate precursor may be due to the loss
of 2 mol of water and the subsequent weight loss due to the loss
of carbon dioxide. On the contrary, onset of weight loss for
(
CO
3
)
3
:Eu0.06 ꢂ 2H
2
O obtained from the PARR reactor also
showed similar characteristics in the TEM picture namely plate-
let-like morphology with the particle size in the range of 30 nm.
3
Y0.9Eu0.1(OH)CO was observed only after 4501C. The initial
weight loss seems to be more than the second one, indicating
that this process may be due to the loss of carbon dioxide.
However, the subsequent loss of water seems to be inseparable
3
This nanocrystalline Y1.9Eu0.1O was processed further by
annealing at 10001, 11001, and 12001C without or with sintering
aid to yield phosphor material with varying crystallite sizes and
morphologies. It is well established that usage of sintering
aid has improved the densification and defect-free surface of
the phosphor material. In the present work, we have used LiF
and NaF as the sintering aid in the 1:1 ratio and 4 wt% each of
the sintering aids was used for annealing the oxide at various
temperatures. After annealing at high temperature, the sintering
aid was removed by washing with hot water repeatedly and
the powder XRD patterns of the materials were recorded. The
selected XRD patterns of the processed materials are depicted
in the Fig. 8. The total removal of sintering aid was confirmed
by the absence of its XRD lines. The diffraction peak position
remains unchanged on annealing at high temperature and
corresponds to cubic oxide phase of yttria (ICDD No. 89-
5592). There was an increase in the line intensity of peaks and
reduction in FWHM values with the increase in the annealing
2 3
from the loss of CO . Y0.95Eu0.05(OH)CO was annealed
at 5501C for 3 h and the powder XRD pattern indicated the
disintegration of hydroxy carbonate phase completely and
the lines are broad. The line positions match with that of
2 2
oxidic phase. Hence, the loss of CO and H O appear to occur
simultaneously and the oxide phase may proceed through the
oxy hydroxide phase of yttrium. Yttria phase formation was
completed at 7001C as no further weight loss was observed:
Y0:95Eu0:05ðOHÞCO ! Y0:95Eu0:05OðOHÞ
3
!
Y1:9Eu0:1O3
(7)
The synthesized precursor material was annealed at 7001C for
h and its powder XRD pattern was measured and depicted in
3
Fig. 6. XRD pattern of the materials obtained by annealing the
Fig. 5. Thermogravimetric curves of (a) Y
0.95Eu0.05(OH)CO
(CO )
2 3 3
2
ꢂ 2H O and (b)
precursors at 7001C (a) Y
2
(CO
3
)
3
ꢂ 2H
2
O, (b) Y1.94Eu0.06(CO
)
3 3
2
ꢂ 2H O,
Y
3
.
(C) Y0.95Eu0.05(OH)CO , and (d) Y0.9Eu0.1(OH)CO .
3
3

May 2011
PL Efficiencies of Y O :Eu
2
1631
3
Fig. 8. XRD pattern of Y1.9Eu0.1
O
3
annealed at 7001, 10001, 11001, and
1
1001C with sintering aid.
the PL intensity of the sample annealed at 7001C due to the
formation of rigid oxidic phase. The PL spectrum of the
Y1.9O :Eu material sintered at 11001C with sintering aid along
3
0.1
Fig. 7. TEM image of Y1.9Eu0.1
O
3
obtained by annealing Y0.95Eu0.05
with the Nichia commercial standard phosphor material, which
is excited at wavelength of 254 nm, is presented in Fig. 10. It was
observed from the figure that the synthesized materials and
the commercial phosphor material have similar spectral charac-
teristics, with the predominant peak at 611 nm corresponding to
red emission. Y O crystallizes as bixbyite-type cubic structure
3
(OH)CO at 7001C with EDP in the inset.
temperature. These observations indicate the increase in the
crystallite size of the oxide material with an increase in the an-
nealing temperature. The crystallite size of these materials was
calculated using Scherrer’s formula and the values are tabulated
in Table I. Further, the crystallite size increases for a given an-
nealing temperature in the presence of sintering aid when com-
pared with its absence. It has been observed that the material
obtained by annealing at 11001C in the presence of sintering aid
has given the material with 98% of green density (4.93 g/mL).
The morphologies of these materials were studied using the
2
3
with yttrium occupying the center surrounded by six oxygen
atoms occupying the corner of the cube. Eu substitutes the
C2 and S6 sites of yttrium in oxide phase and their relative
2
3
abundance are in 3:1 ratio. The excitation spectra are due to
the Eu –O charge transfer band around 254 nm. The emis-
31
2ꢁ
5
7
sion lines are attributed to the D – F transitions (j 5 0, 1, 2, 3).
0
j
5
The emission at 580 nm is due to the D – F electronic tran-
7
0
0
5
sitions. The corresponding D – F electronic transition occurs
7
0
1
SEM. The SEM images of Y1.9Eu0.1
O
3
obtained at 11001 and
at 587, 592, and 599 nm from the C site, whereas the peaks at
2
1
1001C with sintering aid are presented in Fig. 9. The figure de-
582 and 594 nm are due to the S site. The intense peak at 611
6
nm can be assigned to the D – F , hypersensitive forced electric
0 2
2
dipole transitions from the C site. It has been established that
5
7
picts that the materials obtained at 11001C without sintering aid
have platelet morphology with the particle size of 1–4 mm, while
the material annealed at 11001C in the presence of sintering aid
showed spherical morphology with the particle size of 0.5–
there is a strong energy transfer from the europium occupying S6
site to C site resulting in the increase in the emission efficiency
2
1
.0 mm. The SEM image of the oxide prepared in an autoclave
at 611 nm. The efficiency of energy transfer increases with the
increase in the europium concentration in the yttria matrix.
showed porous structure when annealed at 10001C without
sintering aid. However, this material also showed spherical mor-
phology when annealed in the presence of sintering aid. Simi-
larly, the SEM picture (figure not shown) of the Nichia red
phosphor showed spherical morphology with the particle size in
the range of 1.0–4.0 mm. The EDS of Nichia and Y1.9Eu0.1O3
samples were measured and showed the values of 3.0 and 4.95
mol%, respectively.
The emission intensities of Y2 Eu O (x 5 0.04, 0.06, 0.08,
x
ꢁx
3
0.1, 0.12, and 0.2) at 611 nm are plotted against the mole ratio
of europium and are depicted in Fig. 11. It can be seen from the
figure that the emission intensity increases from x 5 0.06 to 0.1
and then decreased for x 5 0.2. The initial increase may be due
31
to the increase in the concentration of Eu in the material and
the enhanced energy transfer from the S site to C site. The
decrease in PL intensity for Y1.8Eu0.2O is due to concentration
6
2
Both
Y
1.94(CO
3
)
3
:Eu0.06 ꢂ 2H
2
O
and
Y
0.95Eu0.05(OH)CO
3
3
exhibit feeble emission intensity as seen under the UV lamp at
an excitation wavelength of 254 nm. This may be due to the
vibronic deactivation of the excited state, thereby yielding
low-emission intensity. There is a remarkable increase in
quenching. These observations are in agreement with the previ-
ous literature. As described already, Y1.9Eu0.1O material was
2
3
3
annealed at different temperatures and the PL spectra were
measured using Nichia phosphor material as the standard. The
Table I. Physical Characteristics and PL Efficiency of Y1.9Eu0.1O Phosphor
3
Diffused reflectance
at 254 nm (%)
Temperature (1C)
2y
FWHM (1)
Crystallite size (nm)
Shape of particle
Particle size
PL (%)w
7
1
1
1
1
00
000
100
29.19
29.20
29.08
29.11
29.10
29.28
0.3264
0.2448
0.1428
0.0816
0.0800
0.0984
25.6
33.7
57.2
44
36
28
22
22
22
Platelet
Platelet
Platelet
Spherical
Spherical
Spherical
30 nm
1–2 mm
1–4 mm
0.5–1 mm
3–5 mm
1–4 mm
17.0
56.0
114.0
165.0
120.0
100.0
100 with sintering aid
200 with sintering aid
100
4100
85.7
Nichia red phosphor
wComparison with Nichia standard phosphor material. PL, photoluminescence.

1
632
Journal of the American Ceramic Society—Paulraj et al.
Vol. 94, No. 5
Fig. 9. SEM image of (a) Y1.9Eu0.1
O
3
sintered at 11001C, (b) Y1.9Eu0.1
O
3
sintered at 11001C with sintering aid, and (c) Y1.9Eu0.1
O
3
sintered at 10001C.
relative PL intensities of the phosphor materials were measured
and are tabulated along with the crystallite size and diffused re-
flectance of the material at 254 nm (Table I). A perusal of the
table indicates the following. The nanocrystalline phosphor ma-
terial has the lowest PL efficiency, whereas the material annealed
at 11001C in the presence of sintering aid has the highest PL
intensity of 167% when compared with the commercial stan-
dard. These studies are in contrast to the reports made by Taxak
efficiency has dropped to 114%. It may be noted that the par-
ticle size of the materials was in the range of 1–4 mm. It is im-
perative that both the particle morphology and diffused
reflectance of the samples will play a role in determining the
PL efficiency of phosphor materials.
Nanocrystalline Y1.9Eu0.1O3 material was annealed in
the presence of sintering aid at 12001C. It was noted that the
diffused reflectance of the sample at 254 nm was found to be
same as the sample annealed at 11001C, ca. 22%. The PL
efficiency of the sample was found to be 120%, which is lower
than the sample annealed at 11001C with sintering aid. The
SEM picture (figure not shown) of the sample showed spherical
morphology with the particle size in the range of 3–5 mm. Hence,
the reason for such lowering of PL efficiency may be due to the
7
6
8
et al., Wakefield et al., and Sharma et al. and in accordance
with the reports by Hirai et al. and Nelson et al. It is fairly
well established that the available surface concentration of
1
1
9
2 3
europium centers in nanocrystalline Y O :Eu will be more
than the bulk material. Hence, it is anticipated that the PL
efficiency of nanocrystalline material should be more than the
bulk material. However, the scattering of light from nanomate-
rials will also be higher than the bulk material. We have
observed that the diffused reflectance of nanocrystalline mate-
rials is larger than that of the bulk material, ca. 44% vs. 22%.
Hence, the generation of excited state will be lower for nano-
crystalline material than the bulk. Further, the nanocrystalline
material may also have high surface defects than the material
obtained by annealing in the presence of sintering aid that may
deactivate the excited state through nonradiative processes. The
material annealed at 11001C with sintering aid has a spherical
morphology with the particle size in the range of 0.5–1.0 mm and
the density of the particle nearing the green density. Further, it
can be seen from the table that the material sintered at 11001C in
the absence of sintering aid showed the diffused reflectance of
31
larger size of the particle and surface defects. The Eu ions
buried inside the matrix may not contribute to the PL efficiency.
1
6
It is worthy to note the report of Wang et al., wherein they
have synthesized spherical phosphor material with the particle
size in the range of 0.5 mm and varying crystallite sizes by spray
pyrolysis method. The material synthesized at 14001C with the
crystallite size of 42.8 nm gave rise to the PL quantum efficiency
of 51.76%. It is understood that the spray pyrolysis method
generally produce materials with the porous structure due to the
evaporation of solvents from the droplets and shorter residency
time of pyrolysis. Hence, the diffused reflectance of the sample
may be higher, which may be the reason for the lower efficiency.
5
7
5
7
Further, the emission ratio of D – F to D – F is known as
0
2
0
1
1
6
16
the asymmetric ratio. Wang et al. have reported the prepa-
ration of Y :Eu by the spray pyrolysis method. They have
2
8% due to the platelet-like morphology and the relative PL
2 3
O
reported the crystallite size of 53.5 nm having a maximum asym-
metric ratio of 10.45, while in the present case, the asymmetric
ratio of 12.77 was observed for the sintering aid grown material
Fig. 10. Photoluminescence spectra of (a) Y1.9Eu0.1 annealed at
O
3
1
1001C with sintering aid and (b) Nichia commercial Y O :Eu red phos-
2 3
phor, (c) expanded view of emission in the wavelength of range 580–605
nm, and (d) excitation spectra of Y1.9Eu0.1 at lemi 5 611 nm.
2 3
Fig. 11. PL emission intensities of Y O :Eu with varying europium
concentrations in yttria matrix.
O
3

May 2011
PL Efficiencies of Y O :Eu
2
1633
3
2
C. R. Ronda, ‘‘Phosphors for Lamps and Displays: An Applicational View,’’
J. Alloys Compd., 225, 534–8 (1995).
at 11001C, and their relative emission intensity was found to be
167% compared with the Nichia commercial standard phosphor
3
T. Hasi, T. Kano, E. Nakazawa, and H. Yamamoto, ‘‘Phosphor Materials for
Cathode Ray-Tubes,’’ Adv. Electron. Electron Phys., 79, 271–373 (1990).
(with asymmetric ratio of 11.4). These observations are in
accordance with the literature.
4
C. Feldman, T. Justler, C. R. Ronda, and P. J. Schmidt, ‘‘Inorganic Lumines-
cent Materials: 100 Years of Research and Application,’’ Adv. Funct. Mater., 13,
The results clearly show that the nanocrystalline materials
have higher diffused reflectance at 254 nm that may reduce the
PL efficiency. On the contrary, the materials annealed at higher
temperature have lower PL efficiency that arises due to the
ineffective utility of the activator. Likewise, materials with
spherical morphology have better PL efficiency than the
one with platelet and porous morphology, which is in accor-
5
11–6 (2003).
V. A. Bolchouchine, E. T. Goldburt, B. N. Levonovitch, V. N. Litchmanova,
5
and N. P. Sochtine, ‘‘Designed, Highly-Efficient FED Phosphors and Screens,’’
J. Lumin., 87–89, 1277–9 (2000).
G. Wakefield, E. Holland, P. J. Dobson, and J. L. Hutchison, ‘‘Luminescence
6
Properties of Nanocrystalline Y
V. B. Taxak, S. P. Khatkar, S. D. Han, Rajeshkumar, and Mukeshkumar,
2
O
3
:Eu,’’ Adv. Mater., 13, 1557–60 (2001).
7
31
‘
Compd., 469, 224–8 (2009).
‘Tartaric Acid-Assisted Sol–Gel Synthesis of Y
2 3
O :Eu Nanoparticles,’’ J. Alloys
1
2
dance with the previous literature. An efficient phosphor
should have a spherical morphology due to its nature of min-
imizing the light scattering at its surfaces and thereby enhancing
the luminescent efficiency.
8
P. K. Sharma, R. Nass, and H. Schimidt, ‘‘Effect of Solvent, Host Precursor,
Dopant Concentration and Crystallite Size on the Fluorescence Properties of
Eu III Doped Yttria,’’ Opt. Mater., 10, 161–9 (1998).
9
2 3
J. A. Nelson, E. L. Brant, and M. J. Wagner, ‘‘Nanocrystalline Y O :Eu Phos-
phors Prepared by Alkalide Reduction,’’ Chem. Mater., 15, 688–93 (2003).
10
R. Schmechel, H. Winkler, L. Xaomao, M. Kennedy, M. Kolbe, A. Benker,
M. Winterer, R. A. Fischer, H. Hahn, and H. Von Seggern, ‘‘Photoluminescence
IV. Conclusions
31
Properties of Nanocrystalline Y
Mater., 44, 1213–7 (2001).
2
O
3
:Eu in Different Environments,’’ Scripta
Hydrothermal reaction has been successfully used for the prep-
aration of Y/Eu carbonate or hydroxycarbonate precursor ma-
terials, which on annealing at 7001C gave rise to nanocrystalline
11
31
2 3
T. Hirai, Y. Asada, and I. Komasawa, ‘‘Preparation of Y O :Eu Nanopar-
ticles in Reverse Micellar Systems and their Photoluminescence Properties,’’
J. Colloid Interface Sci., 276, 339–45 (2004).
1
2
Y O
2 3
:Eu. Phosphor materials with different crystallite size and
M. K. Devaraju, S. Yin, and T. Sato, ‘‘Solvothermal Synthesis, Controlled
31
morphology can be prepared by annealing the nanomaterials at
various temperatures in the presence and absence of sintering
aid. The study indicates that the nanocrystalline phosphor ma-
terial has lower PL efficiency compared with the sintering aid
grown material, due to higher diffused reflectance that arises due
to the smaller size and larger surface area to volume ratio.
Further, phosphor material with spherical morphology, opti-
mum particle size (0.5–1.0 mm), and higher crystallite size has the
highest PL efficiency. We may conclude that the optimal con-
centration of the activator, optimal size of the material with
spherical morphology, and proper densification of phosphor
material to have a smooth and defect-free surface, which lead
to lower diffused reflectance values, are all essential parameters
that decide the PL efficiency of phosphor material. Hence,
postannealing treatment will be essential to improve these char-
acteristics to have enhanced PL efficiency.
Morphology and Optical Properties of Y
Growth, 311, 580–4 (2009).
2 3
O :Eu
Nanocrystals,’’ J. Cryst.
13
N. Vu, T. K. Anh, G.-C. Yi, and W. Strek, ‘‘Photoluminescence and
Cathodoluminescence Properties of :Eu Nanophosphors Prepared by
Combustion Synthesis,’’ J. Lumin., 122–123, 776–9 (2007).
2 3
Y O
14
M. Kottaisamy, D. Jeyakumar, R. Jagannathan, and M. Mohan Rao,
‘Yttrium Oxide: Eu Red Phosphor by Self Propagating High Temperature
31
‘
Synthesis,’’ Mater. Res. Bull., 31 [8] 1013–20 (1996).
15
M. Kottaisamy, ‘‘New Synthetic Strategies for Phosphor Materials’’; Ph.D.
Thesis, Bharathidasan University, Tiruchirappalli, India, 1997.
16
W.-N. Wang, W. Widiyastuti, T. Ogi, I. W. Lenggoro, and K. Okuyama,
‘Correlations between Crystallite/Particle Size and Photoluminescence Properties
of Submicrometer Phosphors,’’ Chem. Mater., 19, 1723–30 (2007).
‘
17
T. X. Sun, ‘‘Combinatorial Search for Advanced Luminescence Materials,’’
Biotechnol. Bioeng., 61, 193–201 (1998).
18
D. Sordelet and M. Akinc, ‘‘Preparation of Spherical, Monosized Y
2 3
O
Precursor Particles,’’ J. Colloid Interface Sci., 122 [1] 47–59 (1988).
I. Haq and E. Matijevic, ‘‘Preparation and Properties of Uniform Coated
19
Inorganic Colloidal Particles—Titania on Copper Compounds,’’ Colloids Surf., A,
81, 153–9 (1993).
20
Y. Zhang, M. Gao, K. Han, Z. Fang, X. Yin, and Z. Xu, ‘‘Synthesis,
Characterization and Formation Mechanism of Dumbbell-Like YOHCO and
Rod-Like Y (CO .2.5H O,’’ J. Alloys Compd., 474, 598–604 (2009).
D. Zhao, Q. Yang, Z. Han, F. Sun, K. Tang, and F. Yu, ‘‘Rare Earth
3
2
)
3 3
2
Acknowledgments
21
The authors thank Director, CECRI, for his encouragement and permission to
publish this work. One of the authors, P. A., thanks UGC, India, for the award of
Rajiv Gandhi National Fellowship for pursuing higher studies.
Hydroxycarbonate Materials with Hierarchical Structures: Preparation and
Characterization and Catalytic Activity of Derived Oxides,’’ Solid State Sci., 10,
1028–36 (2008).
22
E. N. Rizkalla and G. R. Choppin, ‘‘Hydration of Lanthanides and Actinides
in Solutions,’’ J. Alloys Compd., 180, 325–36 (1992).
2
3
31
J. L. Ferrari, A. M. Pires, and M. R. Davolos, ‘‘The Effect of Eu Concen-
tration on the Y Host Lattice Obtained from Citrate Precursors,’’ Mater.
Chem. Phys., 113, 587–90 (2009).
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