Comparative study of synthesis and characterization of monodispersed SiO2 @ Y2O3:Eu3+ and SiO2 @ Y2O3:Eu3+ @ SiO2 core-shell structure phosphor particles

Article abstract of DOI:10.1016/j.jallcom.2008.03.055

Monodispersed SiO₂ @ Y₂O₃:Eu³⁺ and SiO₂ @ Y₂O₃:Eu³⁺ @ SiO₂ nanospheres contain SiO₂ as core and Y₂O₃:Eu³⁺ as she

Full text of DOI:10.1016/j.jallcom.2008.03.055

Journal of Alloys and Compounds 471 (2009) 190–196
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Comparative study of synthesis and characterization of monodispersed SiO @
2
3+
3+
Y O :Eu and SiO @ Y O :Eu @ SiO core–shell structure phosphor particles
2
3
2
2
3
2
∗
Yuan-Xiang Fu, Yan-Hui Sun
School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China
a r t i c l e i n f o
a b s t r a c t
3
+
3+
Article history:
2 2 3 2 3 2
Monodispersed SiO @ Y O :Eu and SiO2 @ Y O :Eu @ SiO2 nanospheres contain SiO as core and
Received 24 January 2008
Received in revised form 9 March 2008
Accepted 12 March 2008
Available online 29 April 2008
3+
3+
Y2O3:Eu as shell or another SiO2 shell coated on the surface of SiO2 @ Y2O3:Eu particles were syn-
thesized, respectively. The synthesis of core–shell particles SiO2 @ Y2O3:Eu involved the SiO2 core
ultrasonically dispersed into a certain amount of polyethylene glycol solution, and the deposition of
Y2O3:Eu on the surface of SiO2 core through homogeneous precipitation of Y and Eu using urea
as precipitator. More over, the particles SiO2 @ Y2O3:Eu as core was ultrasonically dispersed into a cer-
tain amount of ethanol and followed by sol–gel processing of tetraethoxysilane to form another silica
shell. The morphology structures and sizes of the resulting particles were analyzed by SEM and TEM,
which indicated that the resulting particles dispersed regularly and the microstructures of the particles
are clearly core–shell or core–shell–shell structures. XRD patterns characterized the crystal structures of
3+
3
+
3+
3+
3
+
Keywords:
Coating materials
Rare earth alloys and compounds
Precipitation
Optical properties
Luminescence
3
+
◦
deposited Y2O3:Eu shell or SiO2 shell after heat treatment at 900 C. Photoluminescence determination
showed that both the phosphors can be excited by ultra violet at 254 nm which showed red-shift. Under
3
+
5
7
the excitation at 254 nm, the Eu ion mainly shows its characteristic red (612 nm, D0– F2) emission in
3
+
3+
the particles from Y2O3:Eu shell. The emission intensity of Eu was affected by the SiO2 core or shell in
some extent.
©
2008 Elsevier B.V. All rights reserved.
1
. Introduction
phosphor particles can be controlled by the size of the silica core or
the thickness of the shell and the number of coating cycles. Coating
particles with silica as a shell is a promising strategy which presents
several advantages especially in the field of biological and imaging
applications. Apart ensuring core protection and water-solubility,
the silica shell realizes two crucial functions. The first consists in
allowing functionalization by biological groups since silica can be
derived by organoalkoxysilanes containing reactive organic groups
(e.g. amine, thiol and isothiocyanate). The second is increasing the
optical response of luminescing cores used as optical tags [3].
It is well known that europium-doped Y O3 phosphor (Y O :
Recently, the synthesis of core–shell nanostructures has
emerged as an attractive research area in material chemistry since
the morphology and the size of the core–shell materials can be
tailored easily by changing the core materials’ shape, size or the
shell’s thickness [1]. More over, coating relative inexpensive cores
with the expensive shell materials or coating expensive shells on
the inexpensive cores materials can lower the cost of the functional
materials [2]. Especially, the shell material such as SiO2 with an
optical transparency can protect the surface of the functional mate-
rials such as the noble metals, magnetic compounds and phosphor
materials from contaminating by some impurity in surroundings
2
2
3
Eu3+) has been widely used in lighting and cathode ray tubes
as a red-emitting phosphor [4]. Recent researches have indicated
that it is also the promising candidate for field emission display
(FED) devices [5]. For the FED applications, it is highly desirable to
offer spherical phosphors with controllable diameters and narrow
size distributions [6]. For that purpose, core–shell particles have
attracted a great deal of interest because of their difference from
those of single-component materials [7–9]. The controlled coat-
ing of particles with homogeneous and organized layers without
aggregation remains a challenge for material scientists. Spherical
morphology of the phosphor is good for high brightness and high
resolution. Additionally, high packing densities and low scattering
of light can also be obtained by using spherical phosphors. So far,
[
3].
Silica is frequently used in core–shell structured materials, as
either a core or a shell because of their inexpensiveness, easiness
to get spherical particles with narrow size distribution, chemical
inertness, and optical transparency [3]. If the silica spheres are
coated with phosphor layers, a kind of core–shell phosphor materi-
als with spherical morphology will be obtained, and the size of the
∗ Corresponding author.
E-mail address: sunyanhui0102@163.com (Y.-H. Sun).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.03.055

Y.-X. Fu, Y.-H. Sun / Journal of Alloys and Compounds 471 (2009) 190–196
191
many synthetic routes have been developed to control the size and
distribution of phosphor particles, such as spray pyrolysis [10] and
urea homogeneous precipitation [11]. However, the obtained phos-
phor particles are still far from the ideally monodisperse spherical
morphology. On the other hand, the chemical properties of the
phosphor material usually are unstable, the surface of the phos-
phor material has many unsaturated bond and easily absorbed
some impurity which cut down the luminescence property of the
phosphors [3]. So coating some shells on the surface of phosphor
materials is essential in precondition of not affected the photolu-
minescence.
continuous stirring for 3 h. The resulting precursors were separated by centrifu-
gation, washed by distilled water and ethanol for several times, and then dried at
◦
◦
1
00 C for 4 h. At last, the samples were heated at 900 C in air for 3 h to produce the
3
+
final particles SiO2 @ Y2O3:Eu
.
.3. Synthesis core–shell–shell particles SiO2 @ Y2O3:Eu3+ @ SiO2
A certain quantity of SiO2 @ Y2O3:Eu3+ spheres obtained from the above step
2
were ultrasonically dispersed in 40 mL ethanol, and then 4 mL TEOS, 10 mL ammo-
nia and 8 mL distilled water were added into the reaction vessel in subsequently.
◦
After addition, the reaction lasted for 8 h at 40 C under vigorous stirring. Then the
resulting precursors were separated by centrifugation, washed by distilled water
◦
and ethanol for several times, and then dried at 100 C for 4 h. At last, the samples
◦
were heated at 900 C in air for 3 h to produce the final particles.
Up to now, there are many investigations of phosphor mate-
3
+
rials coated on the SiO2 core and resulting the SiO2 @ Y O :Eu
2
3
7–9], SiO @ YBO :Eu [12], SiO2 @ YVO :Dy , Sm3+ [13], SiO2
YAG:Ce , Tb [14] and so on, while there are few investiga-
3+
3+
2.4. Coating mechanism
[
2
3
3
4
+
3+
@
The silica submicrospheres prepared by the St o¨ ber method possess an inter-
esting substrates for the deposition of metals, oxides and functional polymers due
to the narrow size distribution, the easily modified surface with silanol composi-
tion and extent of hydrogen bonding, and the isotropic interactions in an aqueous
or organic suspension help to form ordered arrays on substrates [23]. The homoge-
neous precipitation method is a simple method for prepare monodispersed colloidal
particles and uniform coated particles. In this manuscript, a continuous europium-
doped yttrium oxide shell was coated on the surface of nanometer spherical silica
tions involves the coating of SiO shells on the surface of phosphors.
2
Much investigations for SiO2 as shells are focused on the metal or
magnetic materials such as Fe O3 @ SiO2 [15], Ag @ SiO2 [16], Au @
2
SiO2 [17] and so on.
Very recently, the synthesis and application of multilayered
nanostructures containing multiple shells also have begun to draw
attention in synthesis of noble metal core–shell structures such
as SiO2 @ Ag @ SiO2 [18] and SiO2 @ Au @ SiO2 [19]. While the
presence of multiple nanoparticles anchored on solid substrates
in close proximity could result in unique magnetic and optical cou-
pling [20], the encapsulation of metal or phosphor nanoparticles by
SiO2 as in such core–shell–shell nanostructures could also provide
an alternative way to increase the metal or phosphors nanoparti-
cles’ stability from aggregation or possible thermal coalescence at
higher temperatures [21].
3
+
by a homogeneous precipitation method, which leads to obtain SiO2 @ Y2O3:Eu
core–shell submicrospheres. Further more, the obtained particles SiO2 @ Y2O3:Eu3+
as core were ultrasonically dispersed into a certain amount of ethanol and followed
by sol–gel processing of tetraethoxysilane to form another silica shell to obtain the
core–shell–shell particles SiO2 @ Y O :Eu3+ @ SiO . The schematic representation
2
3
2
3
+
of the precipitation method for preparing SiO2 @ Y2O3:Eu core–shell submicro-
spheres and SiO2 @ Y2O3:Eu @ SiO2 particles are given in Schemes 1 and 2.
The surface Si–OH groups play an important role for bonding the metal ions
3
+
(
Y, Eu) and the –NH2 groups in urea from the coating precursor and forming the
(
Y1−xEux)2(OH)CO3 compounds on theSiO2 surfacesinthe following surface reaction
◦
process when heating the solution to 92 C in which the urea began to decom-
pose to NH4OH and H2CO3. Further more, in the following annealing process, the
Despite their unique properties and promising potential appli-
cations, to the best of our knowledge, there are few investigations
on the synthesis of core–shell–shell nanostructure have been
reported on the phosphor materials so far.
3
+
compounds (Y1−xEux)2(OH)CO3 decomposed into Y2O3:Eu and deposited on the
surface of SiO2. In this way, the core–shell structured SiO2 @ Y2O3:Eu3+ materials
have been obtained, and the whole process is shown in Scheme 1.
In the precursor solution, polyethyleneglycol (PEG, molecular, 10,000) is added
as across—linking agent, the two –OH groups in PEG can connect with the –NH2
In this paper, we reported a detail synthesis of core–shell–shell
3
+
nanoparticles SiO @ Y O :Eu @ SiO2 containing a silica core,
2
2
3
3
+
Y O :Eu nanoparticles shell and a second silica shell with a tun-
able size around the surface of Y O :Eu , and characterized their
2
3
3+
2
3
structure, morphology and photoluminescence property. As a com-
parison, the synthesis of core–shell phosphor particles of SiO
@
2
3
+
3+
Y O :Eu , the pure Y O :Eu powder phosphors are reported
2
3
2
3
too.
2. Experimental
The starting materials used in the experiments were tetraethoxysilane (TEOS,
8% SiO2, Tianjin No. 1 Chemicals Co., Ltd.), Y2O3 and Eu2O3 (99.99%, Shang-
2
hai Yuelong Nonferrous Metals Ltd.), NH4OH (25 wt.%, A.R., Guangzhou Donghong
Chemicals Co., Ltd.), HNO3 (A.R., Guangzhou Donghong Chemicals Co., Ltd.),
polyethylene glycol (PEG, molecular weight 10,000, A.R., Sanland-Chem. Inerna-
tional Incorp.) and ethanol (A.R., Guangzhou Donghong Chemicals Co., Ltd.).
2.1. Synthesis of silica cores
Monodispersed silica spheres were prepared with the procedure originally
described by St o¨ ber et al. [22] except that the reaction condition was changed in
some aspects, i.e., hydrolysis of TEOS in an ethanol solution containing water and
ammonia. In a typical experiment, 5 mL TEOS was added to 75 mL ethanol solution
of water and ammonia. The 80 mL mixture solution containing 0.279 mol/L TEOS,
◦
2
.55 mol/L H2O, and 1.15 mol/L ammonia was stirred at 29 C for 4 h. The resulting
silica spheres were centrifugally separated from the suspension and ultrasonically
washed with distilled water and ethanol.
2
.2. Coating of SiO2 cores with Y2O3:Eu3+ shells
A certain amount of Y2O3 and Eu2O3 powders were, respectively, dissolved in
excessive nitric acid to form Y(NO3)3 and Eu(NO3)3 solutions. A certain quantity of
silica spheres (0.3 g) was ultrasonically dispersed in 100 mL aqueous solution con-
−
2
−3
taining yttrium nitrate (2 × 10 mol/L), europium nitrate (1.2 × 10 mol/L), urea
◦
Scheme 1. Formation process of SiO2 @ Y O :Eu3+ core–shell particles.
(
2 mol/L) and certain amount of PEG. The dispersions were then aged at 92 C under
2
3

192
Y.-X. Fu, Y.-H. Sun / Journal of Alloys and Compounds 471 (2009) 190–196
◦ ◦ ◦
ꢁ = 29.2 , 48.5 and 38.4 , which indicates that Y O :Eu crystal
2 3
3+
2
3+
grows well. While the other peaks of Y O :Eu could not observed
2
3
3+
here may be enshroud by the background of SiO . SiO @ Y O :Eu
2
2
2
3
@
SiO2 sample in Fig. 1(c) illustrates the same patterns as that of
3+
SiO @ Y O :Eu except that the typical peaks were weak than that
of SiO2 @ Y O :Eu . This can be explained as that the silica shell
coated on the surface of the Y O :Eu
2
2
3
3+
2
3
3+
.
2
3
From the XRD patterns in Fig. 1(b) and (c), no other phase is
detected which suggesting that no reaction occurred between the
3
+
◦
SiO2 and the Y O :Eu shells even after annealing at 900 C. The
2
3
3
+
formation of Y O :Eu layer is due to the reaction of Y, Eu ions
2
3
on silica surface at high-temperature, as shown in Scheme 1. The
◦
literature showed that if annealed at 1000 C or higher a reaction
between the Y O3 shell and SiO2 core will occur, resulting in the
2
formation of impurity like Y SiO5 phase in the samples [13]. So we
2
◦
limit the annealing temperature at 900 C in this work.
In general, the nanocrystallite size can be estimated from the
Scherrer formula:Dhkl = Kꢀ/(ˇ cos ꢁ), where ꢀ is the X-ray wave-
length (0.15405 nm), ˇ is the full-width at half-maximum, ꢁ is the
diffraction angle, K is Scherrer constant (0.89), and Dhkl means the
Scheme 2. Formation process of SiO2 @ Y2O3:Eu3+ @ SiO2 core–shell–shell particles.
size along (h k l) direction. Here we take diffraction data along the
◦
(
2 2 2) plane at 2ꢁ = 29.2 to calculate the size of the nanocrystallites,
3+
and the estimated average crystallite sizes of Y O :Eu are 45 nm
for pure powders, 9.85 nm for crystallite sizes of Y O :Eu in SiO2
@ Y O :Eu core–shell particle and 9.42 nm for crystallite sizes of
Y O :Eu in SiO2 @ Y O :Eu @ SiO2 particle, which agrees well
2
3
groups in urea via hydration and making them more homogeneously distributed in
the solution. The PEG concentration will affect the viscosity of the precursor solution.
3+
2
3
3+
3
+
Low viscosity of the solution will result in easy deposition of a thin layer of Y2O3:Eu
on the SiO2 particles, and proper increasing of the solution viscosity can increase
2 3
3+
3+
2
3
2
3
3+
the thickness of Y2O3:Eu layer [13].
with the results in the SEM and TEM coating shell.
2.5. Characterization of the particles
3.2. SEM results
The X-ray diffraction (XRD) of the powder samples was examined on a Rigaku-
Dmax 2500 diffractometer using Cu K␣ radiation ꢀ = 0.15405 nm). The morphologies
of SiO2 @ Y2O3:Eu3 and SiO2 @ Y2O3:Eu @ SiO2 submicrospheres produced were
+
3+
Fig. 2 shows the SEM micrographs of bare silica (a), pure
3+
3+
3+
Y O :Eu (b), SiO2 @ Y O :Eu (c) and SiO2 @ Y O :Eu @ SiO2
observed using Quanta 400, Philips scanning electronic microscope (SEM) and trans-
mission electron microscope (JEOL JEM-2010 (TEM)). The excition and emission
spectra of samples were obtained using a Hitachi F-4500 FL Spectrophotometer
with a 150 W xenon lamp.
2
3
2
3
2
3
(d). From the SEM micrograph of Fig. 2(a), we can observed that the
formed SiO2 consists of spherical particles of average size 300 nm
and these particles are non-aggregated and dispersed uniformly. In
3+
contrast, the pure Y O :Eu powders in Fig. 2(b) contains irregu-
2
3
3. Results and discussion
lar morphology particles with a wide size distribution from about
0 nm to 250 nm and these particles are aggregated obviously. After
5
3
+
3.1. XRD results
functionalizing the silica particles as core with Y O :Eu coatings
2 3
3+
as shell, the resulting SiO @ Y O :Eu in Fig. 2(c) and SiO2
Y O :Eu particles as core with SiO2 as another shells resulting
SiO @ Y O :Eu @ SiO in Fig. 2(d) still keep the morphology as the
2 2 3 2
@
2
2
3
3+
X-ray diffraction patterns of bare silica spheres (a), SiO
@
2
2 3
3
+
3+
3+
Y O :Eu core–shell samples (b), and SiO2 @ Y O :Eu @ SiO2
2
3
2
3
samples (c) are shown in Fig. 1. The bare silica is amorphous, as evi-
dent from the presence of a broader hump in Fig. 1(a). In Fig. 1(b),
silica particle, i.e. these particles are still spherical with a smooth
surface and non-aggregated. The size became larger after the coat-
3
+
3+
SiO2 @ Y O :Eu core–shell sample presents three sharp peaks at
ing process, it is obvious that the size is 320 nm for SiO @ Y O :Eu
2
3
2 2 3
3+
and about 380 nm for SiO @ Y O :Eu @ SiO which agreeing well
2
2
3
2
with the XRD results. It should be mentioned that minor amount
3+
of the irregular fine particles like the pure Y O :Eu powders can
2
3
be observed in Fig. 2(c) and (d). Furthermore, the dispersivity of
the functionalizing particles were much uniform than that of pure
3+
Y O :Eu powders.
2
3
3.3. TEM results
The TEM photograph of the core–shell structure SiO
@
2
3+
Y O :Eu in Fig. 3(a) can be observed clearly due to the differ-
2
3
ent electron penetrability for cores and shells. The monodispersed
3
+
SiO @ Y O :Eu spheres about 320 nm in diameter contained SiO
2
2
3
2
3+
black core of 300 nm in diameter and Y O :Eu gray color shell
2
3
with an average thickness of about 10 nm can be observed from
Fig. 3(a) and the magnified image of a single sphere in Fig. 3(b).
Fig. 4(a) and (b) shows the uniform size distribution and clear
3
+
Fig. 1. X-ray diffraction patterns for SiO2 (a), SiO2 @ Y2O3:Eu3+ (b) and SiO2
Y2O3:Eu @ SiO2 (c).
@
core–shell–shell structure of SiO2 @ Y O :Eu @ SiO2 particles.
2
3
3+
The SiO2 cores are black spheres with an average size of 300 nm

Y.-X. Fu, Y.-H. Sun / Journal of Alloys and Compounds 471 (2009) 190–196
193
Fig. 2. SEM micrographs of SiO2 (a), Y2O3:Eu3+ (b), SiO2 @ Y2O3:Eu3+ (c) and SiO2 @ Y2O3:Eu3+ @ SiO2 (d) samples.
in diameter, and the shell have dark color with an average thick-
The Eu element was not detected clearly due to its low concentra-
tion below 5% (but it can be detected by the emission spectra, see
Section 3.4). This provides additional evidence for the formation of
3+
ness of 10 nm is Y O :Eu shell and the gray color with an average
2
3
thickness of 30 nm is SiO2 shell.
3+
In addition, there are some irregular powders can be seen in
Fig. 3(a) and Fig. 4(a) like that in the SEM photographs, which indi-
cated that the coating was not completely in this study.
Fig. 5 shows the energy dispersive X-ray spectrum performed on
coatings of crystalline SiO2 @ Y O :Eu @ SiO2 particles.
2
3
3.4. Photoluminescence properties
3+
3+
the particles SiO @ Y O :Eu @ SiO , and the composition analysis
The excitation and emission spectra of SiO @ Y O :Eu and
2 2 3
2
2
3
2
3+
of the particles suggests the existence of Si (from the cores and the
outside shell), O (from the cores and the shells), Y (from the inner
shell) as well as Cu and C (from the Cu gridding for measurement).
SiO @ Y O :Eu @ SiO samples are shown in Figs. 6 and 7, respec-
2 2 3 2
tively. As a comparison, the excitation and emission spectra of pure
3+
Y O :Eu was also showed in Figs. 6 and 7.
2
3
Fig. 3. TEM micrographs of core–shell nanospheres SiO2 @ Y2O3:Eu3+ (a) and the magnified image of a single sphere (b) for the samples.

194
Y.-X. Fu, Y.-H. Sun / Journal of Alloys and Compounds 471 (2009) 190–196
Fig. 4. TEM micrographs of core–shell–shell nanospheres SiO2 @ Y2O3:Eu3+ @ SiO2 (a) and the magnified image of a single sphere (b) for the samples.
3
+
Y O :Eu @ SiO2 under the complete identical conditions to the
2
3
3+
formers except that coating Y O :Eu on the surface of SiO2 or
embeding SiO2 @Y O :Eu into another SO2 shell.
2
3
3+
2
3
The excitation spectrum was performed by monitoring the
emission of Eu3 D – F transition at 612 nm. It can be seen clearly
+ 5
7
0
2
that the excitation spectrum consists of a broad band with maxi-
mum at 254 nm, which can be attributed to the charge-transfer
band (CTB) between O2 and Eu [9]. Here the excitation peak is
−
3+
3+
shifted toward the red from 250 nm in the pure Y O :Eu in this
2
3
3+
study and in the literature in core–shell SiO2 @ Y O :Eu [9] to
2
3
2
54 nm in this study. We know the excitation of Eu3+ belongs to the
charge-transfer state transition, which is related to the stability of
the electron of the surrounding O2 and the position of the exci-
−
Fig. 5. Energy dispersive X-ray spectrum for the sample SiO2 @ Y2O3:Eu3+ @ SiO2
particles.
2−
tation peak depends on the nature of surrounding O ions [24].
In a crystal lattice, an O2 ion is stabilized by surrounding positive
−
ions. However, as crystallite sizes decreasing from 45 nm for pure
Here, it should be mentioned that the three PL spectra were
determined in the same amount of rare earth oxide and the iden-
3+
3+
Y O :Eu to 9.85 nm and 9.42 nm for SiO2 @ Y O :Eu and SiO2
2
@
3
2
3
3
+
2−
Y O :Eu @ SiO , respectively, the surrounding of O ions in
2 3 2
3+
tical ratio of Y:Eu. That is, we used the same amount of Y and
Y2O3 were changed a lot. From XRD results in Fig. 1(b) and (c), the
3+
3+
Eu to prepare the pure Y O :Eu powders as that to coat the
2
3
3+
reflection peak intensity of Y O :Eu in the core–shell particles
2
3
SiO2 surfaces and to prepare the core–shell–shell particles SiO2
@
3+
(a), SiO2 @ Y2O3:Eu3+ (b) and SiO2
Fig. 7. Emission spectra of Y2O3:Eu3+ (a), SiO2 @ Y2O3:Eu3+ (b) and SiO2 @ Y2O3:Eu3+
@ SiO2 (c).
Fig. 6. Excitation spectra of Y2O3:Eu
Y2O3:Eu @ SiO2 (c).
@
3+

Y.-X. Fu, Y.-H. Sun / Journal of Alloys and Compounds 471 (2009) 190–196
195
became very weak and the width of half peak became more wide
On the other hand, the silica shell affecting the optical response
of luminescing cores was recently investigated for two different
mechanisms [28,29]. One is the influence of the coating was based
on the reduction of the luminescence quenching of the surface
hydroxil groups, and this enhancement of luminescence brought
by coating strongly depends on the nature of the core [28]. It can
be significant in the case of Mn-doped ZnS [28] or only slightly pro-
3+
compared with that of pure Y O :Eu (the XRD pattern of pure
2
3
3
+
Y O :Eu was not shown in this study). This indicated the param-
eter of the Y O :Eu crystal cell is more smaller. As crystallite size
2
3
3
+
2
3
decreases, the surface-to-volume ratio of atoms increases and the
degree of disorder of the nanostructured system increases. In that
2−
case, the O is less stable. As a result, it requires less energy to
remove an electron from an O2− ion; therefore, the charge-transfer
state band is shifted toward lower energy [25].
nounced in Eu doped YVO4 [30]. In some cases as pure Eu O3 [31]
2
or CdSe/ZnS [28], the luminescence is, on the contrary, found to be
slightly decreased or severely dropped just like that in this study
Upon excitation of the CTB at 254 nm, the obtained emission
5
7
3+
spectrum is composed of D0,1– FJ (J = 0, 1, 2, 3) emission lines of
the emission intensity of SiO2 @ Y O :Eu @ SiO2 is less than that
2
3
3
+
5
7
3+
Eu (Fig. 7), dominated by the hypersensitive red emission D – F
SiO2 @ Y O :Eu . Another mechanism is explained that the lumi-
0
2
2 3
3
+
transition of Eu at 612 nm. The other emission peaks located at
35 ( D – F ), 579 ( D – F ), 589, 595, 600 ( D – F ), and 653 nm
D – F ) are labeled in Fig. 7, which were very close to that in the
0 3
pure Y O :Eu and core–shell SiO @ Y O :Eu in literature [9].
nescence enhancement of the core–shell naoparticles is due to the
energy transfer between the two parts of a core–shell nanocom-
posite, the silica shell acting as an antenna which absorbs the light
and transfers it to the core then enhancing the optical response
[29]. But the latter mechanism was happened when the excitation
wavelength is shorter than 250 nm because the silica is found to
absorb significantly below 250 nm [32]. In this study, the energy
5
7
5
7
5
7
5
(
1 1 0 0 0 1
5
7
3
+
3+
2
3
2
2
3
3
+
3+
In cubic Y O :Eu , there are two crystallographic sites for Eu
:
2
3
one is with C2 symmetry and another with S symmetry. Eu3+ at
6
7
7
C2 site contributes D – F
transitions to the main part of visual
0
0,1,2
luminescence of Y O :Eu (580–640 nm). However, the 5D – F
3
+
7
transfer between SiO2 shell and the core SiO2 @ Y O :Eu com-
3+
2
3
0
1
2
3
transition lines are allowed for both C and S sites, are expected to
posites did not found because the excitation wavelength is 254 nm
which can not be absorbed well by SiO2 shell.
2
6
3
+
3+
arise from Eu (C ) and Eu (S ) sites simultaneously. The crystal
2
6
3
+ 5
7
field splitting of Eu
D – F transitions can be seen clearly, indi-
0 1,2
3
+
cating that the Y O :Eu layer is well crystallized on the surface
2
3
4. Conclusions
of SiO2 particles (agreeing well with the XRD and TEM results in
Sections 3.1 and 3.3).
Further more, it can be seen that the photoluminescent inten-
sities of the two samples (b) and (c) are affected by the addition of
In summary, we have demonstrated a simple approach for
3+
the synthesis of core–shell structure SiO2 @ Y O :Eu
and
2
3
3+
core–shell–shell particles like sandwich structure SiO @ Y O :Eu
2
2
3
SiO spheres both from Figs. 6 and 7. In Fig. 6, the relative excitation
2
@
SiO2 with uniform size distribution and spherical morphology.
3+
spectrum intensity is decreasing in the order of Y O :Eu > SiO @
2
3
2
Photoluminescence studies show that the existence of silica as core
or shell have some extent affect on the luminescence property of
3
+
3+
Y O :Eu > SiO2 @ Y O :Eu @ SiO2 at 254 nm. Similarly, the rel-
2
3
2
3
ative intensity of the emission spectrum at 612 nm is decreasing in
3+
3+
Y O :Eu shell. One can change the size of SiO2 core or Y O :Eu
2
3
2
3
3
+
3+
3+
order of Y O :Eu > SiO2 @ Y O :Eu > SiO2 @ Y O :Eu @ SiO ,
2
3
2
3
2
3
2
shell to obtain the best photoluminescence property.
3+
too. This may be related to the embeded structure of Y O :Eu in
2
3
the core–shell particles, and attributed to the fact that the emitting
3
+
3+
References
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2
3
2
3
3
+
3+
@
Y O :Eu > SiO @ Y O :Eu @ SiO [9,12,13]. This can be proved
by the fact that the PL intensity of coating particle SiO @ Y O :Eu
even coating 4 number is yet less than that of pure Y O :Eu
2
3
2
2
3
2
[
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[
While Lin and co-workers [9] give the result that the PL intensity
3
+
of one layer Y O :Eu coated on SiO2 particles increases with the
2
3
increase of SiO2 core particle size due to the decrease of surface
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[
[
[
3+
So the surface of Y O :Eu coated by SiO2 will decrease the
2
3
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[
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+
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2
2
3
2
3
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3+
Y O :Eu and SiO2 @ Y O :Eu material. This may be explained
2
3
2
3
3+
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2
3
107 (2003) 5475–5482.
3+
2
3
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2312–2313.
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2
3
[
3+
compared to that of SiO @ Y O :Eu material. This is investigated
2
2
3
in the next step.

196
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