Synthesis of SiO2/Y2O3:Eu core-shell materials and hollow spheres

Article abstract of DOI:10.1016/j.jssc.2005.03.010

The SiO₂/Y₂O₃:Eu core-shell materials and hollow spheres were first synthesized by a template-mediated method. X-ray diffraction patterns indicated that the broadened diffraction peaks result from nanocrystals of Y₂O₃:Eu shells and hollow spheres. X-ray photoelectron spectra showed that the Y₂O₃:Eu shells are linked with silica cores by Si-O-Y chemical bond. SEM and TEM observations showed that the size of SiO₂/Y₂O₃:Eu core-shell structure is in the range of 140-180 nm, and the thickness of Y ₂O₃:Eu hollow spherical shell is about 20-40 nm. The photoluminescence spectra of SiO₂/Y₂O₃:Eu core-shell materials and Y₂O₃:Eu hollow spheres have better red luminescent properties, and the broadened emission bands came from the size effects of nanocrystals composed of Y₂O₃:Eu shell.

Full text of DOI:10.1016/j.jssc.2005.03.010

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 1647–1651
www.elsevier.com/locate/jssc
Synthesis of SiO /Y O :Eu core-shell materials and hollow spheres
2 2 3
Ã
Guixia Liu, Guangyan Hong
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, PR China
Received 22 January 2005; received in revised form 26 February 2005; accepted 2 March 2005
Abstract
The SiO /Y O :Eu core-shell materials and hollow spheres were first synthesized by a template-mediated method. X-ray
2
2
3
diffraction patterns indicated that the broadened diffraction peaks result from nanocrystals of Y
X-ray photoelectron spectra showed that the Y :Eu shells are linked with silica cores by Si–O–Ychemical bond. SEM and TEM
observations showed that the size of SiO /Y O :Eu core-shell structure is in the range of 140–180 nm, and the thickness of Y O :Eu
2 3
O :Eu shells and hollow spheres.
2
O
3
2
2
3
2
3
2 2 3 2 3
hollow spherical shell is about 20–40 nm. The photoluminescence spectra of SiO /Y O :Eu core-shell materials and Y O :Eu hollow
spheres have better red luminescent properties, and the broadened emission bands came from the size effects of nanocrystals
composed of Y O :Eu shell.
r 2005 Published by Elsevier Inc.
2
3
Keywords: Y :Eu; Core-shell; Hollow spheres; Luminescence properties
2 3
O
1
. Introduction
Nanometer- and micrometer-sized core-shell structure
desired materials on the template cores to form core-
shell spheres and then removing the template by either
calcinating in air or selective etching with a solvent. The
common template materials are polymer colloids and
inorganic colloids such as polystyrene colloids and
silica, Ag or Au nanoparticles.
particles and hollow spheres with special physical and
chemical properties have attracted great attention due to
their potential applications in photonic crystals, cata-
lyst, diagnostics, pharmacology, etc. [1–3]. In particular,
inorganic hollow nanospheres with well-defined archi-
tectures are of great increasing interest due to their low
density, high surface area, stability, and surface perme-
ability, they may find a wide range of applications as
catalysis, shape-selective adsorbents, chemical sensors
and capsules for controlled release of therapeutic agents
For example, Yang et al. [14] used sulfonated
polystyrene core-shell gel particles as templates to
prepare hollow titania spheres. Xu et al. [15] used silica
particles as template to fabricate polymer hollow
spheres by dispersion polymerization around monodis-
persed silica particles, and then removed the silica cores
by HF etching. Recently, we have synthesized SiO2/
Gd O :Eu core-shell luminescent materials by coating
[
4–8]. A number of methods have been employed to
2
3
synthesize the core-shell materials and hollow spheres,
for example, the interface assembly strategies [9,10],
layer-by-layer self-assembly processes [11], hydrother-
mal and solvothermal precipitation method [12,13]. The
most common route for preparing hollow spheres is
sacrificial template method; the process is to coat the
Gd O :Eu on silica nanoparticles with homogeneous
2 3
precipitation method [16]. It is well known that Y O :Eu
2
3
is a kind of widely used red phosphor and display
material, it has also been used as luminescent probe for
3
+
high sensitivity of Eu
Y O :Eu with different morphology such as spherical
ions to their surroundings.
2
3
nanocrystal and nanotubes has been prepared [17–20],
but the core-shell structure Y O :Eu and hollow spheres
have not been prepared. The core-shell structure and
Ã
Corresponding author. Fax: +86 431 5698 041.
E-mail address: gyhang@ns.ciac.jl.cn (G. Hong).
2
3
0022-4596/$ - see front matter r 2005 Published by Elsevier Inc.
doi:10.1016/j.jssc.2005.03.010

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G. Liu, G. Hong / Journal of Solid State Chemistry 178 (2005) 1647–1651
1648
hollow spheres of Y O :Eu red phosphors will achieve a
2
the silica template to obtain hollow spheres. The
detailed procedure was as follows: First, yttrium oxide
(99.99%) and europium oxide (99.99%) were dissolved
in a minimum amount of diluted nitric acid (1:1),
evaporated to dryness, cooled to room temperature,
then dissolved in distilled water to form yttrium nitrate
and europium nitrate solutions. Secondly, a certain
amount of prepared SiO2 cores were ultrasonically
3
reduction in the amount of expensive rare earth,
additionally, due to the low density of hollow spherical
materials, when coating screen in display applications,
the phosphors can be dispersed well, enhance the
uniformity and give high packing densities of the
coating. In this paper, we prepared SiO /Y O :Eu
2
2
3
core-shell structure materials by using silica spheres as
template, and dissolved silica cores with sodium hydro-
xide to obtain Y O :Eu hollow spheres. The core-shell
ꢀ
1
dispersed in the stock solutions containing 0.02 mol L
ꢀ
1
Y(NO ) , 0.001 mol L Eu(NO ) (Eu/Y ¼ 5 at%) and
2
3
3 3
ꢀ
3 3
1
2 mol L (NH ) CO. The dispersion was then aged at
structure and hollow spherical Y O :Eu still has
2
3
2 2
retained red luminescent properties.
85–90 1C in tightly capped beaker under continuous
stirring for 5 h. Then the resulting product was filtered
and washed with distilled water for three times. The
samples were heated at 800 1C in air for 2 h after being
dried at 80 1C for 2 h and then the core-shell materials
were obtained. At last, the core-shell materials were
dispersed in 1 mol L NaOH solutions for 12 h, and
washed with distilled water until the pH value is equal to
7, dried for 12 h and obtained the hollow spheres.
The sample structure was detected by a RIGAKU D/
max-IIB X-ray diffractometer. Particles’ surface element
binding energy and chemical composition were char-
acterized by a VGESCALAB MK II X-ray photoelec-
tron spectrometer. The morphologies of samples were
observed using a PHILIPS XL30 ESEM FEG scanning
electron microscopy (SEM) and a JEOL JEM-2010
transmission electron microscopy (TEM). The excita-
tion and emission spectra of the samples were measured
by a HITACHI F-4500 fluorescence spectrophotometer
using an Xe lamp as the excitation source.
2
. Experimental section
The SiO /Y O :Eu core-shell materials and hollow
2
2
3
ꢀ
1
spheres were synthesized by silica spheres as template.
The template silica was prepared according to Ref. [16].
The homogeneous precipitation method was used to
coat europium-doped yttria on silica, and then dissolved
c
b
3. Results and discussion
a
3.1. XRD patterns
0
10
20
30
40
50
60
70
80
90
The X-ray diffraction (XRD) patterns of silica cores,
the Y O :Eu-coated silica calcinated at 800 1C and the
2
theta(˚)
2
3
calcinated samples etched by NaOH are shown in
Figs. 1a–c. The silica cores are amorphous as evident
Fig. 1. XRD patterns of SiO
2
cores (a), SiO
2
2 3
/Y O :Eu calcinated at
800 1C (b), the calcinated sample etched by NaOH (c).
5
5
4
4
3
3
2
2
1
1
500
000
500
000
500
000
500
000
500
000
1
57.9
Si2p
103.9
Y3d5/2
O1s
533.8
31.8
uncoated
uncoated
1
01.6
5
coated
coated
1
45
150
155
160
165
170 90
95
100
105
110 525
530
535
540
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
Fig. 2. XPS spectra of Y3d5/2, Si2p and O1s of bare silica cores and Y
2
O :Eu-coated silica core-shell structure materials.
3

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G. Liu, G. Hong / Journal of Solid State Chemistry 178 (2005) 1647–1651
1649
from the presence of a broader hump in Fig. 1(a). The
curve (b) is the pattern of the Y O :Eu-coated silica,
2
3
there are two broadened diffraction peaks which
coincide with the peaks of cubic yttria assigned as
(
indicates that crystalline Y O has been formed on the
222) and (440) reflection lines, respectively, which
2
3
surface of silica, the broadened diffraction peaks are
probably due to the nanometer grain sizes and the
effects of the amorphous silica cores. In curve (c), after
the samples etched by sodium hydroxide, the peak of
silica disappears and the peaks of cubic yttria are not
changed, this indicates that the hollow shells are
crystalline Y O nanoparticles.
2
3
3
.2. XPS spectra
In order to study the combination of silica cores and
Y O :Eu shells of core-shell structure materials, X-ray
2
3
photoelectron spectra (XPS) of samples are measured.
The XPS spectra of Y3d5/2, Si2p and O1s of bare silica
cores and Y O :Eu-coated silica core-shell structure
2
3
materials are shown in Fig. 2. From Fig. 2a, the peak of
Y3d5/2 (157.9 eV) can be seen on Y O :Eu-coated silica
2
3
core-shell structure materials, which comes from the
Y O :Eu coatings. From Fig. 2b and c, the binding
energy of Si2p and O1s of Y O :Eu-coated silica core-
2
3
2
3
shell particles is lowered to that of the bare silica
particles by 2.3 and 2.0 eV, respectively. It is inferred
that Y O :Eu is linked to the surface of silica by a
2
3
Si–O–Ychemical bond. Because the electronegativity of
Yis lower than that of Si, which strengthens the
electronic density on the surface of Si, then the shielding
effect of the surface is strengthened, and the binding
energy of Si2p is lowered. For O , the forming Si–O–Y
1s
bond on the surface of the particles makes the electronic
density of O combined with Si increase, so the binding
energy of O1s for core-shell particle has a chemical shift
of ꢀ2.0 eV.
3
.3. Electron microscopic studies
The image of silica cores is shown in Fig. 3(a). From
the photograph, we can see that the core particles of
silica are all in spheres and the size of particles is
uniform in the range of 120–140 nm. After coating with
the europium-doped yttria, the microscopy is shown in
Fig. 3(b), noted that it is still spherical, but the surface of
core-shell structure particles is rougher than that of bare
silica cores, and the particle size is larger than that of the
bare silica cores and increased to 140–180 nm, which
indicates that the thickness of coating is about
Fig. 3. SEM micrographs of the silica cores (a), Y
(
2
O
3
:Eu-coated silica
b) and the calcinated samples etched by sodium hydroxide (c).
arrows and the inset magnified image, which conforms
that the hollow spheres are obtained by etching of
sodium hydroxide.
Fig. 4 shows the TEM images of samples etched by
sodium hydroxide. Note that the samples are partly in
core-shell structures and partly in hollow spheres, the
thickness of shells is about 20 nm, which is conformed
by the magnified image of Fig. 4(b). The difference in
contrast between the particles of TEM photographs is
2
0–40 nm. From Fig. 3(c), the SEM microscopy of
calcinated samples etched by sodium hydroxide, the
particles are still in spheres, and they are partly
agglomerated as a result of the heat treatment. In
addition, there are some void in the particles directed by

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G. Liu, G. Hong / Journal of Solid State Chemistry 178 (2005) 1647–1651
1650
Fig. 4. TEM micrographs of calcinated sample etched by sodium hydroxide: (a) the entire image; (b) the magnified image.
6
00
00
248
1000
247
5
8
00
00
00
00
0
204
λem=611nm
400
6
4
2
3
00
00
203
2
100
0
200
250
300
350
200
250
300
350
(a)
wavelength(nm)
(a)
Wavelength(nm)
6
00
00
6
11
1000
5
611
8
00
00
00
00
0
λex=254nm
400
00
200
6
4
2
3
586
5
86
575
5
76
100
0
−
100
500
550
600
650
700
5
00
550
600
650
700
(b)
Wavelength(nm)
(b)
wavelength(nm)
Fig. 5. The excitation spectrum (a) and emission spectrum (b) of SiO
2
/
Fig. 6. The excitation spectrum (a) and emission spectrum (b) of
:Eu hollow spheres.
2 3
Y O :Eu core-shell structure materials.
2 3
Y O
due to incomplete etching of silica cores by sodium
hydroxide for some core-shell particles. In addition, it is
interesting to note that the hollow sphere shell is
composed of some small nanoparticles, which makes
the surface of shells rougher.
3.4. Photoluminescence properties
The excitation spectra and emission spectra of SiO2/
Y O :Eu core-shell structure particles and Y O :Eu
hollow spheres are shown in Figs. 5 and 6, respectively.
2
3
2
3

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G. Liu, G. Hong / Journal of Solid State Chemistry 178 (2005) 1647–1651
1651
The monitoring wavelength of the excitation spectra
is 611 nm and the excitation wavelength of emissi-
on spectra is 254 nm. Note that the luminescent spectra
of SiO /Y O :Eu core-shell structure particles and
Y O :Eu core-shell materials and Y O :Eu hollow
2 3 2 3
spheres have better red luminescent properties.
2
2
3
References
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2
3
spectra, the band near 250 nm (248 and 247 nm) is
known to be a charge transfer band (CTB) which is an
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11 nm is the red characteristic emission of Y O :Eu,
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7
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0
2
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3
transition of Eu , and there are other two peaks
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5
7
5
7
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0
0
0
1
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3
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(
[
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[
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2
3
3
+
higher disorder of the Eu
3
crystalline environment, a
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(
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larger portion of the Eu
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[
[
[
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4
. Conclusion
The SiO /Y O :Eu core-shell structure and hollow
2
2
3
spherical luminescent materials were synthesized using
amorphous SiO as a template. The Y O :Eu shells are
[
[
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2
2
3
linked with silica cores by a Si–O–Ychemical bond. The
size of SiO /Y O :Eu core-shell structure is in the range
of 140–180 nm, and the thickness of Y O :Eu hollow
2
2
3
[
2
3
spheres is about 20–40 nm. The PL spectra of SiO2/