Sol-gel synthesis and characterization of Si O2 @NaGd (W O4) 2: Eu3+ core-shell-structured spherical phosphor particles

Article abstract of DOI:10.1149/1.2392601

Monodisperse, core-shell-structured Si O2 @NaGd (W O4) 2: Eu3+ particles were prepared by the sol-gel method. The samples were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, photoluminescence (PL), and low-voltage cathodoluminescence (CL) as well as time-resolved PL spectra and lifetimes. PL and CL study revealed that the core-shell-structured Si O2 @NaGd (W O4) 2: Eu3+ particles show strong red emission dominated by the D05 - F27 transition of Eu3+ at 614 nm with a lifetime of 0.74 ms. The PL and CL emission intensity can be tuned by the coating number of NaGd (W O4) 2: Eu3+ phosphor layers on Si O2 and by accelerating voltage and the filament current, respectively.

Full text of DOI:10.1149/1.2392601

Journal of The Electrochemical Society, 154 ͑1͒ J39-J43 ͑2007͒
J39
0013-4651/2006/154͑1͒/J39/5/$20.00 © The Electrochemical Society
Sol-Gel Synthesis and Characterization
of SiO2@NaGd„WO4…2:Eu3+ Core-Shell-Structured
Spherical Phosphor Particles
Peiyun Jia,a,b Xiaoming Liu,a,b Yan Luo,a,b Min Yu,a and Jun Lina,z
aKey Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Changchun 130022, P. R. China
bGraduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China
Monodisperse, core-shell-structured SiO2@NaGd͑WO4͒2:Eu3+ particles were prepared by the sol-gel method. The samples were
characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, photolumines-
cence ͑PL͒, and low-voltage cathodoluminescence ͑CL͒ as well as time-resolved PL spectra and lifetimes. PL and CL study
revealed that the core-shell-structured SiO2@NaGd͑WO4͒2:Eu3+ particles show strong red emission dominated by the 5D0–7F2
transition of Eu3+ at 614 nm with a lifetime of 0.74 ms. The PL and CL emission intensity can be tuned by the coating number of
NaGd͑WO4͒2:Eu3+ phosphor layers on SiO2 and by accelerating voltage and the filament current, respectively.
© 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2392601͔ All rights reserved.
Manuscript submitted July 27, 2006; revised manuscript received August 23, 2006. Available electronically November 22, 2006.
Recently considerable efforts have been devoted to the design
and preparation of composite particles consisting of a core covered
by shells of different chemical composition. The interests in such
core-shell materials stem from the fact that the properties ͑mechani-
cal, optical, electrical, magnetic, and catalytic, etc.͒ of cores or
shells can be tailored by their size, morphology, component, and
structure of the shell or core.1-6 Core-shell materials can be used to
protect the medicine or other materials from decomposition or hy-
drolysis and strengthen the polymeric materials.1,2 Some magnetic
materials have also been designed as core-shell materials to protect
them from reaction with various species in the environment and
increase their solubility in some solvent.3,4 A great number of pro-
cesses have been developed to prepare core-shell materials, such as
intramolecular polymerization,7 template-directed self-assembly,8
precipitation and surface reaction,9 controlled deposition of pre-
formed inorganic colloids,5 layer-by-layer technique,10 sonochemi-
cal method, etc.11
The current demand for high resolution, high brightness, and
high efficiency in phosphors for cathode ray tubes, field emissive
displays, and plasma display panels has promoted the development
of phosphors. Particularly, phosphors with nonagglomerated, mono-
disperse, spherical ͑Ͻ2 ␮m͒ morphology are of great interest, be-
cause they offer higher packing density, lower scattering of light,
brighter luminescent performance, higher definition, and more im-
proved screen packing.12-14 Today, many synthetic routes have been
developed to control the size, morphology, and distribution of phos-
phor particles, such as spray pyrolysis15,16 and flux precipitation,17
but it is very difficult to obtain highly monodispere spherical phos-
phor particles from these methods.
It is well known that monodispersed spherical silica particles
from nano- to submicrometer range can be prepared by hydrolysis
and condensation of tetraethoxysilane ͑TEOS͒ catalyzed by
ammonia.18 If the silica spheres are coated with layers of phosphors,
a kind of core-shell phosphor materials with spherical morphology
will be obtained, and the size of the phosphor particles can be con-
trolled by the silica cores. Furthermore, because silica is cheaper
than most of the phosphor materials ͑which often employ the expen-
sive rare-earth elements as the activators and/or host components͒,
the core-shell phosphor materials are cheaper than the pure phos-
phor materials in unit mass. Scheelite structure materials have been
widely used as a phosphor host. With the exception of CaWO4, there
is still a class of materials with scheelite structure, namely,
AB͑MO4͒2, where A = Li+, Na+, K+, Rb+, Cs+, and Ag+, B = rare
earth ions, and M = Mo and W.19 Accordingly, in the present work
we prepared monodispersed spherical silica microspheres coated
with NaGd͑WO4͒2:Eu3+ layers via the sol-gel process to obtain the
core-shell-structured SiO2@NaGd͑WO4͒2:Eu3+ phosphor particles.
Detailed characterizations for the structure, morphology, and lumi-
nescent properties of the samples were performed.
Experimental
Amorphous submicrometer spheres of silica with a size of about
400 nm were synthesized by the well-known Stöber process, i.e., the
hydrolysis and condensation of TEOS in an ethanol solution con-
taining water and ammonia.18 This method yielded the colloidal so-
lution of silica particles with a narrow size distribution in the sub-
micrometer range, and the particle size of silica depends on the
relative concentration of the reactants. In a typical experiment,
10 mL TEOS ͓99 wt %, analytical reagent ͑AR͒ Beijing Beihua
Chemicals Co., Ltd.͔, 20 mL deionized H2O, and 100 mL of
NH4OH ͑25–28 wt %, AR Beijing Beihua Chemicals Co., Ltd.͒
were added into 80 mL absolute ethanol and stirred at room tem-
perature for 4 h, resulting in the formation of white silica colloidal
suspension. The silica particles were centrifugally separated from
the suspension and then dried at 100°C for 12 h. In order to com-
pare with the core-shell particles, some of the as-prepared SiO2 were
annealed at 800°C for 2 h.
SiO2@NaGd͑WO4͒2:Eu3+ core-shell phosphors were prepared
by a Pechini sol-gel process. 0.1590 g Na2CO3, 0.5159 g Gd2O3
͑99.99%͒, and 0.0264 g Eu2O3 ͑99.99%͒ were dissolved in dilute
HNO3 ͑AR͒ under vigorous stirring, and superfluous HNO3 was
driven off until the pH value of the solution was between two and
three. The as-prepared mixture was mixed with 1.6500 g ammonium
͑meta͒ tungstate hydrate ͑H26N6O41W12·18H2O, Fluka͒, 30 mL
water-ethanol ͑V/V = 25:5͒ solution containing citric acid ͑AR͒ as a
chelating agent for the metal ions. The molar ratio of metal ions to
citric acid was 1:2. As a cross-linking agent, polyethylene glycol
͑PEG, molecular weight = 10000, AR͒ was added with a final con-
centration of 0.20 g/mL. The solution was stirred for 1 h to form a
sol, and then the silica particles were added under stirring. The
suspension was further stirred for another 4 h, and then the silica
particles were separated by centrifugation. The samples were dried
at 100°C for 1 h immediately. Then the dried samples were an-
nealed to 500°C with a heating rate of 1°C/min and held there for
2 h in air. The above process was repeated several times to increase
the thickness of the NaGd͑WO4͒2:Eu3+ shells. After these processes,
the sample was annealed to 800°C with a heating rate of 5°C/min
and held there for 2 h in air. In this way, the core-shell-structured
SiO2@NaGd0.95Eu0.05͑WO4͒2 materials have been obtained. For the
purpose of comparison, the coating sol was evaporated in an 80°C
z E-mail address: jlin@ns.ciac.jl.cn
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Journal of The Electrochemical Society, 154 ͑1͒ J39-J43 ͑2007͒
Figure 1. XRD patterns of ͑a͒ SiO2, ͑b͒ SiO2@NaGd0.95Eu0.05͑WO4͒2, and
͑c͒ NaGd0.95Eu0.05͑WO4͒ particles annealed at 800°C, as well as the stan-
2
dard data for NaGd͑WO4͒ ͑JCPDS No.25-0829͒ as a reference.
2
water bath to form gel, which was annealed in a similar process to
produce the pure NaGd0.95Eu0.05͑WO4͒2 powder phosphor.
The X-ray diffraction ͑XRD͒ of as-prepared samples were exam-
ined on a Rigaku-Dmax 2500 diffractometer using Cu K␣ radiation
͑␭ = 0.15405 nm͒. The morphology of SiO2 and coated SiO2 were
inspected using a field emission scanning election microscope
͑FESEM, XL30, Philips͒ and transmission electron microscope
͑TEM, JEOL-2010, 200 KV͒. The photoluminescence ͑PL͒ excita-
tion and emission spectra were taken on a Hitachi F-4500 spectro-
fluorimeter equipped with a 150 W xenon lamp as the excitation
source. Time-resolved spectra were obtained using an Acton
spectrapro-2758 spectrograph with a Hama Matsu R928 PMT, and
the luminescence decay curves were measured on a LeCroy Wave
Runner 6100 Digital Oscilloscope ͑1 GHz͒ using 249 nm laser
͑pulse width = 4 ns, 10 Hz͒ as the excitation source ͑Continuum
Sunlite OPO͒. The cathodeluminescence ͑CL͒ was measured on an
instrument fabricated by our group under 10−6 Pa vacuum, filament
current 102.5 mA, grid voltage 20 V, and anode voltage from
1000 to 6000 V. All the measurements were performed at room
temperature.
Figure 2. FESEM micrographs of ͑a͒ amorphous SiO2 and ͑b͒ core-shell-
structured SiO2@NaGd0.95Eu0.05͑WO4͒ particles annealed at 800°C.
2
culate the size of the nanocrystalline, and the estimated average
crystallite size is 31 nm.
Figure 2 shows the FESEM micrographs of the SiO2 and
SiO2@NaGd͑WO4͒2:Eu3+ ͑four times coated͒ particles annealed at
800°C, respectively. From the FESEM micrograph of Fig. 2a, we
can observe that the SiO2 sample consists of nonagglomerated,
monodisperse, spherical particles with an average size of about
400 nm. After the silica particles were functionalized
Results and Discussion
by
NaGd͑WO4͒2:Eu3+
coatings,
the
resulting
SiO2@
NaGd͑WO4͒2:Eu3+ particles still keep the morphology of the silica
particles, i.e., these particles are still nonaggregated, monodispersed,
and spherical, with an average size of about 480 nm, slightly larger
than the pure silica particles due to the additional layers of
NaGd͑WO4͒2:Eu3+ on them, as shown in Fig. 3b. Moreover, no
separate and irregular fine particles of NaGd͑WO4͒2:Eu3+ can be
Figure 1 shows the XRD patterns of 800°C annealed bare
SiO2 microspheres, NaGd0.95Eu0.05͑WO4͒2, and SiO2@NaGd0.95
Eu0.05͑WO4͒2 particles, as well as the standard data for
NaGd͑WO4͒2 ͑JCPDS no. 25-0829͒ as a reference, respectively. For
SiO2 microspheres annealed at 800°C ͑Fig. 1a͒, no diffraction peaks
are observed except for a broad-band centered at 2␪ = 22.0°, which
is the characteristic peak for amorphous SiO2 ͑JCPDS 29-0085͒. For
the SiO2@NaGd0.95Eu0.05͑WO4͒2 core-shell sample fired at 800°C
͑Fig. 1b͒, besides the broad-band at 2␪ = 22.0° from amorphous
SiO2, all the diffraction peaks belonging to crystalline NaGd͑WO4͒2
are present, suggesting that the coatings of NaGd0.95Eu0.05͑WO4͒2
have crystallized well on the surfaces of amorphous silica micro-
spheres. This coincides well with the XRD pattern of pure
NaGd0.95Eu0.05͑WO4͒2 particles annealed at 800°C ͑Fig. 1c͒ and the
standard data for NaGd͑WO4͒2 ͑JCPDS no. 25-0829͒. No other
phase has been detected, indicating that no reaction occurred be-
tween the core ͑SiO2͒ and the shell ͓NaGd0.95Eu0.05͑WO4͒2͔ com-
ponents. In general, the nanocrystallite size can be estimated from
the Scherrer formula, Dhkl = K␭/͑␤ cos ␪͒, where ␭ is the X-ray
wavelength ͑0.15405 nm͒, ␤ is the full-width-at-half-maximum, ␪ is
the diffraction angle, K is a constant ͑0.89͒, and Dhkl means the size
along ͑hkl͒ direction.23 Here we take diffraction data along the
͑101͒, ͑112͒, ͑200͒, ͑204͒, ͑312͒, ͑224͒, and ͑208͒ direction to cal-
Figure 3. TEM micrographs of ͑a͒ amorphous SiO2 and ͑b͒ core-shell-
structured SiO2@NaGd0.95Eu0.05͑WO4͒ particles annealed at 800°C.
2
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Journal of The Electrochemical Society, 154 ͑1͒ J39-J43 ͑2007͒
J41
Figure 5. Decay curves for the luminescence of Eu3+ in
SiO2@NaGd0.95Eu0.05͑WO4͒2.
Figure
4.
͑a͒
Excitation
and
͑b͒
emission
spectra
of
SiO2@NaGd0.95Eu0.05͑WO4͒2.
The PL decay curve for Eu3+ ͑614, 5D0–7F2͒ in
SiO2@NaGd͑WO4͒2:Eu3+ is shown in Fig. 5. This curve can be
fitted into a single exponential function as I = I0 exp͑−t/␶͒ ͑I0 is the
initial intensity at t = 0, ␶ is the 1/e lifetime͒ and the lifetime can be
determined as 0.74 ms.
observed in Fig. 3b. This indicates that almost all of the
NaGd͑WO4͒2:Eu3+ materials have been coated on the surfaces of
silica particles by our experimental process.
TEM was performed to verify the core-shell structure of
In order to further understand the energy-transfer process
from WO24− to Eu3+ in SiO2@NaGd͑WO4͒2:Eu3+, time-resolved
spectra have been performed, as shown in Fig. 6. For
SiO2@NaGd͑WO4͒2:Eu3+, the spectra collected at different delay
times exhibit two types of emission, namely, the band emission from
WO24− and characteristic emission of Eu3+. The emission band of
WO24− first increased with the delay time, reaching a maximum at
t = 2 ␮s, then began to decrease and disappeared completely at t
= 50 ␮s. The characteristic emission of Eu3+ also increases with
delay time, reaching a maximum at t = 50 ␮s, then decreases with
time. The above emission characteristics of WO24− and Eu3+
reflect the energy transfer process from WO24− to Eu3+ in
SiO2@NaGd͑WO4͒2:Eu3+ sample. For the transition within WO24−
͑always in microsecond range͒, it is allowed; yet, for the transition
within Eu3+ 4f6 configuration, it is forbidden. The lifetime of WO24−
is much shorter than that of Eu3+, so at the beginning ͑very short
delay time͒ the excitation energy of WO24− has not been transferred
to the Eu3+. As a result, the emission of WO24− is dominated and the
emission of Eu3+ is very weak. With the increase of time, the exci-
tation energy began to transfer to Eu3+. When t = 50 ␮s, most of the
excitation energy of WO24− nonradiatively transferred to Eu3+, lead-
SiO2@NaGd͑WO4͒2:Eu3+ particles directly. Figure
3 shows
the TEM micrographs of bare SiO2 ͑Fig. 3a͒ and
SiO2@NaGd͑WO4͒2:Eu3+ samples ͑Fig. 3b͒. Only black spheres
͑400 nm in diameter͒ were observed for bare SiO2 particles. In con-
trast, the core-shell structure for the SiO2@NaGd͑WO4͒2:Eu3+ par-
ticles can be seen clearly due to the different electron penetrability
for the cores and shells. The cores are black spheres with an average
size of 400 nm ͑similar to the pure SiO2 particles in Fig. 3a͒, and the
shells have a gray color with an average thickness of 40 nm. The
electron diffraction measurement performed in the interface region
of the core and shell of a particle ͑inset of Fig. 3b͒ has shown
electron diffraction rings with some disorder, demonstrating the co-
existence of crystalline phase ͓NaGd͑WO4͒2:Eu3+͔ and amorphous
phase ͑SiO2͒ in the interface region of the core-shell particle.
Under ultraviolet radiation, SiO2@NaGd͑WO4͒2:Eu3+ phosphor
particles show red emission. Figure 4a shows the excitation spec-
trum of SiO2@NaGd͑WO4͒2:Eu3+ monitored by 614 nm emission
of Eu3+ ͑5D0–7F2͒. The excitation spectrum contains a broad-band
with a maximum at 249 nm due to the charge-transfer transition in
WO24− groups in which an oxygen ͑O͒ 2p electron goes into empty
tungsten ͑W͒ 5d orbital. This indicates that there is energy transfer
21
from WO24− to Eu3+
.
The f-f transition lines at 397 ͑7F0–5L6͒ and
468 ͑7F0–5D2͒ of Eu3+ in the longer wavelength have also been
detected.24,25 Excitation into CTB of WO24− at 249 nm yields the
characteristic emission of Eu3+ ͑Fig. 4b͒, which not only contains
the characteristic emission from the lowest excited 5D0 but also
those from higher levels ͑5D1,5D2͒ of Eu3+ with a very weak inten-
sity. The presence of emission from higher excited states of Eu3+
͑5D1,5D2͒ is attributed to the low vibration energy of WO24− groups.
The multiphonon relaxation by WO24− is not able to bridge the gaps
5
between the higher energy levels ͑5D1,5D2͒ and D0 level of Eu3+
completely, resulting in the weak emissions from these levels. Ob-
5
viously, the emission is dominated by the red D0–7F2 hypersensi-
tive transition of Eu3+, which is due to the crystalline structure of
¯
NaGd͑WO4͒2 belonging to space group I41 a with distorted
scheelite structure and there is no inversion center at the side of
21
Gd3+
.
In the short wavelength region, we can observe a broad
emission band from 350 to 550 nm, even though its intensity is very
weak, which reveals that the excitation band at 249 nm originates
from charge-transfer transition of WO24−, and the energy transfer
from WO24− group to Eu3+ is not complete.
Figure 6. Time-resolved PL spectra of SiO2@NaGd0.95Eu0.05͑WO4͒ ͑␭
2
ex
= 249 nm, ␭ = 614 nm͒.
em
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J42
Journal of The Electrochemical Society, 154 ͑1͒ J39-J43 ͑2007͒
Figure 7. Luminescent intensity of SiO2@NaGd0.95Eu0.05͑WO4͒ as a func-
2
tion of the coating number.
ing to the quenching of the emission of WO24− accompanied by that
of Eu3+ reaching a maximum value. Later on ͑t Ͼ 50 ␮s͒, the emis-
sion of Eu3+ began to decrease due to the depopulation of the ex-
cited state.
The coating number ͓or the thickness of NaGd͑WO4͒2͔ has more
effect on the PL intensity. In order to investigate the effects of the
coating number on the PL intensity, SiO2@NaGd͑WO4͒2:Eu3+
phosphor particles with a different coating number have been pre-
pared, and Fig. 7 provides the PL intensity as a function of the
coating number. As shown in the figure, the PL intensity increases
with the coating numbers. For the samples coated three and four
times, the PL intensity has little difference, which means that the
four times of coating can reach an ideal thickness of coatings. Figure
Figure 9. CL emission intensity of SiO2@NaGd0.95Eu0.05͑WO4͒ as a func-
2
tion of ͑a͒ accelerating voltage and ͑b͒ anode current.
8
gives the PL spectra of NaGd͑WO4͒2:Eu3+ ͑Fig. 8a͒,
the PL emission spectra ͑Fig. 8b͒. The CL intensity of
SiO2@NaGd͑WO4͒2:Eu3+ has been investigated as a function of
accelerating voltage and anode current and the results are provided
in Fig. 9. It can be seen clearly from the figure that the CL intensity
increases with the increase of accelerating voltage from 1 to 6 kV
͑Fig. 9a͒ and anode current from 14 to 18 mA ͑Fig. 9b͒. With the
increase of accelerating voltage or anode, more plasmas are pro-
duced by the incident electrons, resulting in more WO24− and Eu3+
ions being excited and thus higher CL intensity. Due to the strong
low-voltage CL intensity and excellent dispersing properties of
SiO2@NaGd͑WO4͒2:Eu3+ core-shell phosphors, they are potentially
applied in field emission display ͑FED͒ devices as red phosphor.
SiO2@NaGd͑WO4͒2:Eu3+ ͑Fig. 8b͒, and the CL spectrum of
SiO2@NaGd͑WO4͒2:Eu3+ ͑Fig. 8c͒. From Fig. 8, we can find that
the PL intensity of core-shell-structured SiO2@NaGd͑WO4͒2:Eu3+
͑Fig. 8b͒ can be compared with that of NaGd͑WO4͒2:Eu3+ ͑Fig. 8a͒
phosphor powders.
Similar to the emission under UV light illumination, the
SiO2@NaGd͑WO4͒2:Eu3+ core-shell particles also exhibit strong
red emission under the excitation of electron beam. The typical
emission spectra under the excitation of electron beam
͑6 kV, 18 mA͒ are shown in Fig. 8c, which basically agrees
Conclusions
A simple and effective sol-gel process has been developed to
coat NaGd͑WO4͒2:Eu3+ phosphor layers on monodispersed spheri-
cal
SiO2
particles. The
obtained
core-shell-structured
SiO2@NaGd͑WO4͒2:Eu3+ phosphors maintain spherical morphol-
ogy, submicrometer size, and narrow size distribution. Under
UV light and low-voltage electron-beam excitation, the
SiO2@NaGd͑WO4͒2:Eu3+ phosphors show the characteristic red
emission of Eu3+ dominated by 614 nm. The coating number also
plays an important role in the luminescent intensity, namely, with
the increase of the coating number, the intensity of emission also
increases, which provides an effective means to tune the intensity of
emission by changing this parameter. The obtained core-shell phos-
phors have potential applications in FED.
Acknowledgment
This project is financially supported by the foundation of Bairen
Jihua of the Chinese Academy of Sciences, the National Natural
Science Foundation of China ͑50225205, 50572103, 20431030,
00310530, 0061022͒, and the MOST of China ͑No.
Figure 8. PL emission spectra of ͑a͒ NaGd0.95Eu0.05͑WO4͒2, ͑b͒
SiO2@NaGd0.95Eu0.05͑WO4͒2, and ͑c͒ CL emission spectrum of
SiO2@NaGd0.95Eu0.05͑WO4͒ ͓accelerating voltage = 6 kV, anode current
2
= 18 mA͔.
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Journal of The Electrochemical Society, 154 ͑1͒ J39-J43 ͑2007͒
J43
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