Preparation and characteristics of core-shell structure Y 3Al5O12:Yb3+@SiO2 nanoparticles

Article abstract of DOI:10.1016/j.jallcom.2012.02.105

The club-shape (Y_0.98Yb_0.02)₃Al ₅O₁₂ nanoparticles (YAG:Yb³⁺), which were synthesized by the co-precipitation method, had been successfully coated with silica from hydrolysis and condensati

Full text of DOI:10.1016/j.jallcom.2012.02.105

Journal of Alloys and Compounds 528 (2012) 1–9
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
3
+
Preparation and characteristics of core–shell structure Y Al O :Yb @SiO
2
3
5
12
nanoparticles
Yan-Hui Suna,b, Zhong-Min Yang , Cheng-Ning Xie , Zhong-Hong Jiang
a,∗
a
a
a
Key Lab of Specially Functional Materials of Ministry of Education and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, PR
China
School of Environment and Chemistry, South China Normal University, Guangzhou 510006, PR China
b
a r t i c l e i n f o
a b s t r a c t
The club-shape (Y0.98Yb0.02) Al O nanoparticles (YAG:Yb3+), which were synthesized by the co-
Article history:
3
5
12
Received 14 October 2011
Received in revised form 17 February 2012
Accepted 17 February 2012
Available online 3 March 2012
precipitation method, had been successfully coated with silica from hydrolysis and condensation of
tetraethyl orthosilicate (TEOS). The thickness of SiO2 shell can be tuned by the amount of TEOS. The
influence of different modifying surfactants, polyvinylpyrrolidone (PVP), polyethylene glycol (PEG) and
cetyltrimethyl ammonium bromide (CTAB), on the final product had been investigated. The morphol-
ogy and structure of the nanoparticles were characterized by scanning electron microscopy (SEM) and
transmission electronic microscopy (TEM). Surface composition of the samples was studied by X-ray
photoelectron spectroscopy (XPS). The results show that PVP is much more beneficial to the dispersion
and formation of the core–shell structure than the others. The near-infrared photoluminescence emis-
sions of the nanoparticles were also investigated under the excitation of 940 nm laser diode. The results
indicate that SiO2 shell can increase the luminescence intensity and lifetime of YAG:Yb3+ in the near
infrared region emissions because the shell can reduce OH group content adsorbed on the surface of
the samples.
Keywords:
Coating materials
Rare earth alloys and compounds
Precipitaion
Sol–gel process
Optical properties
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
Many methods have been used to synthesize YAG nanopow-
ders, such as solid-state reaction [10], sol–gel method [9] or sol–gel
with microwave irradiation [11], mixed fuel solution combustion
method [12], hydrothermal method [13], liquid feed-flame spray
pyrolysis [14], microemulsion [15], and co-precipitation method
[8]. Compared with other methods, co-precipitation method is low-
cost and mass-production for preparing nano-sized powders. So
many detail studies on the precipitaion method have been reported
such as the effect of precipitant concentration on the formation pro-
cedure of YAG phase [8], microwave-assisted precipitation [16], a
novel mixed precipitation method [17] that alumina precursor is
first prepared and then two kinds of precursors are mixed through
the precipitation of yttrium ions in the suspension consisting of
yttrium ions solution and alumina precursor, and then YAG pow-
ders are obtained by calcining the mixed precursors in air. But it still
meets the defeats: aging phenomena [16,18], agglomeration and
irregular morphology of the products. Recently, core–shell struc-
ture materials have been investigated in detail because they can
modify the morphology of nano-particles and avoid the agglom-
eration [19–21,24,25,30]. Silica using as shell can manipulate the
interaction potential for dispersing colloids in a wide range of sol-
vents from very polar to apolar [19,20]. Moreover, SiO2 shell also
can protect the core from interaction with surrounding and avoid
agglomeration [21]. Furthermore, it is worth noting that moderate
SiO2 can prevent abnormal grain growth and completely removes
Yttrium aluminum garnet Y Al5O12 (YAG) is an attractive laser
3
host material because of its better thermal, chemical and mechan-
ical properties [1,2]. Ytterbium (Yb3 ) doped materials are of much
interest to the diode-pumped solid-state lasers due to their high
quantum efficiency and no concentration quenching [3–7]. Among
+
3
+
3+
them, Y Al5O12:Yb (YAG:Yb ) transparent ceramic lasers have
3
been intensely investigated [5–7]. However, the performance of
the laser ceramics, especially thermal shock resistance and trans-
parency still need to be improved [6,7]. Nanopowder technology
combined with vacuum sintering method is proved as an effective
process for the fabrication of transparent YAG ceramics [8]. The
laser damage of transparent ceramics is usually due to the existence
of second phase, i.e., pores and impurities. So the raw materials
with nano-sized, better-dispersed and regular morphology may be
beneficial to improve the performance of products [9]. In addition,
YAG:Yb nanoparticles coated with silica can be also used as labels
in biological assay and imaging instead of organic dyes and quan-
tum dots for overcoming the drawback such as high background
signals and the need for expensive pulse lasers.
3
+
∗ Corresponding author. Tel.: +86 20 22236303; fax: +86 20 87114204.
E-mail address: yangzm@scut.edu.cn (Z.-M. Yang).
0
925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2012.02.105

2
Y.-H. Sun et al. / Journal of Alloys and Compounds 528 (2012) 1–9
Chart 1. The formation mechanism and flow chart of YAG:Yb3+@SiO2.
the pores in YAG ceramics when used as sinter additives [22,23]. All
these advantages are beneficial to produce mono-dispersion YAG
powders.
Very recently, we have synthesized the submicron core–shell
3+
structure particles SiO @Y Al5O12:Er , which silica spherical par-
2
3
ticles was coated with an yttrium aluminum garnet (YAG) layer
3
+
doped with Er by the modified Pechini-type sol–gel method [24]
and co-precipitation process [25]. The results show that the lumi-
nescence spectra of the core–shell powders in the NIR region and
visible region are similar to those of the pure YAG:Er3 powders
except the relative lower intensity [24,25]. From the core–shell
+
structure a question will be brought forward that if SiO is used
2
as shell coating on the surface of YAG:Re3 (Re means rare earth
elements) how the luminescence emission of YAG:Re3 changes.
In this paper, the core–shell structure particles YAG:Yb3 @SiO2
will be prepared by controlling the hydrolysis of tetraethyl
orthosilicate (TEOS) in an ethanol/ammonia solution, and SiO2
would deposit on the surface of YAG particles synthesized by
co-precipitation method beforehand. An uniform silica layer will
successfully coat on the surface of YAG particles for the first time.
Different surfactants of polyvinylpyrrolidone (PVP), polyethylene
glycol (PEG) and cetyltrimethyl ammonium bromide (CTAB) are
used to modify the surface of YAG:Yb3 particles, and the effect and
mechanism are discussed in detail. The photoluminescence prop-
+
+
+
+
Fig. 1. TG–DSC measurements of pure SiO2 (A), the precursor of core–shell
3+
YAG:Yb3+@SiO particles using PVP as surfactant (B) and the precursor of pure
erties in near-infrared (NIR) region of YAG:Yb in the core–shell
2
3
+
3
+
YAG:Yb particles (C).
structure YAG:Yb @SiO are also investigated compared with that
2
3
+
of pure YAG:Yb particles.
was separated by centrifugation and washed with distilled water and ethanol fol-
lowed by drying at desiccator for 48 h. The dried precursors were crushed and then
calcined at 1000 C for 4 h to obtain YAG:Yb nanoparticles.
2
. Experimental
◦
3+
2
.1. Synthesis of YAG:Yb3+ nanoparticles
2
.2. Synthesis of core–shell structure YAG:Yb3+@SiO2
Y2O3 (wt. >99.99%) and Yb2O3 (wt. >99.99%) were produced by Guangzhou
Research Institute of Non-ferrous Metals. Nitric acid (A.R.) and Al(NO3)3·9H2O (A.R.)
TEOS (C.P., contain wt. 28% SiO2), PVP-360 (A.P.), CTAB (A.P.) and PEG-10000
were produced by Guangzhou Chemical Reagent Factory, and used as received with-
(A.P.) used in this study were obtained from Shanghai Bo’ao Biological Technol-
ogy Co. Ltd. Chart 1 shows the synthesis mechanism and procedure of core–shell
3
+
3+
out further purification. Y and Yb nitrate solutions were obtained by dissolving
3+
3+
Y2O3 and Yb2O3 in nitric acid while Al nitrate solution was obtained by dissolving
Al(NO3)3·9H2O in distilled water.
structure YAG:Yb @SiO2 powders with PVP as the functionalization surfactant. Two
steps are involved: functionalization with PVP surfactant and synthesis of the silica
shell. For PEG or CTAB as surfactant, the synthesis procedures were same as that
using PVP as surfactant. For comparison, the pure SiO2 was directly synthesized by
the Stöber method at the same condition.
The YAG:Yb3 nanoparticles were synthesized by a co-precipitation method
using NH4HCO3 as precipitant, the details were the same as previous study [26].
In order to obtain precursors with a higher homogeneity, the reverse-strike tech-
nique was used to make the precipitate steady at a moderate pH range. Briefly,
+
3+
3+
3+
certain amount of Y (Yb , Al ) nitrate solutions based on the stoichiometry of
Y0.98Yb0.02)3Al5O12 were mixed with ammonium sulfate, whose amount was the
2.2.1. Functionalization with surfactants
3
+
(
On the assumption that the surface of YAG:Yb colloids only adsorb one layer
surfactant, the amount of surfactant can be calculated according to the molecular
same as that of the total metal ions. The above solution was added into the NH4HCO3
solution (2 mol/L) at a speed of 2 mL/min. After aging 2 h, the obtained suspension
2
2
2
sectional area [27] 0.55 nm of PVP, 0.62 nm of PEG and 0.91 nm of CTAB and the

Y.-H. Sun et al. / Journal of Alloys and Compounds 528 (2012) 1–9
3
Fig. 2. The FT-IR spectra of the pure SiO2 (a), the core–shell YAG:Yb3+@SiO2 (b) and
Fig. 3. X-ray diffraction patterns for the core–shell YAG:Yb3+@SiO2 (a) and the pure
SiO2 (b) as well as the standard patterns of YAG at the bottom of the horizontal axis.
the pure YAG:Yb3+ powders (c).
surface area of YAG:Yb3+ [19]. The surfactants were dissolved in distil water and then
ultrasonically vibrated for 30 min to achieve complete dissolution. Subsequently, the
YAG:Yb3 powders were pre-dispersed in ethanol by ultrasonic and then mixed with
the surfactant solutions under stirring (750 rpm) for 8–10 h at room temperature.
Then the suspension was centrifugated to remove the supernate and obtain the
energy at 284.6 eV. The morphology and size of particles were observed by the field
emission scanning electron microscopy (FESEM, LEO 1530 VP) and transmission
electron microscopy (TEM, JEOL JEM-2010, 200 kV). The photoluminescence (PL)
spectra of the Yb ion were collected at room temperature using a Triax 320 flu-
orescence spectrometer (Jobin-Yvon Inc., France) with 940 nm laser diode as the
excitation source.
+
3
+
3+
surfactant-stabilized YAG:Yb particles.
2
.2.2. Synthesis of the silica shell
The surfactant-stabilized YAG:Yb3+ particles were dispersed in ethanol followed
3. Results and discussion
by adding certain amount of ammonia (the volume ratio of ethanol to ammonia is
2
0:1). Then TEOS solution (5 vol% in ethanol) was added under stirring (750 rpm)
◦
at 40 C for 12 h. The thickness of the silica shell was controlled by the amount of
TEOS. The resulting precursors were separated by centrifugation and washed with
distilled water and ethanol for three times respectively. After drying at 500 C in air
for 2 h to remove the surfactant, the final core–shell YAG:Yb @SiO2 particles have
been synthesized.
3.1. Formation and morphology of the core–shell particles
◦
3+
3.1.1. TG–DSC
Fig. 1 shows TG–DSC curves of pure SiO2 (A), the precursor of
3+
core–shell YAG:Yb @SiO2 particles (B) and the precursor of pure
YAG:Yb particles (C). As shown in Fig. 1(A), the weight loss of
2.3. Characterization
3+
pure SiO2 is about 10% of the total weight after calcination. It was
due to the removal of H2O, alcohol and the residual TEOS adhered
to the surface of SiO2 particles. The DSC curve presented an obvi-
The thermal history of the as-prepared particles was studied using TG–DSC ana-
◦
lyzer (NETZSCH STA 449C) from room temperature to 1100 C in air at a heating
◦
rate of 10 C/min. Fourier transmission infrared spectroscopy (FT-IR 17300, Perkin-
−
1
◦
Elmer, USA) was used to record the spectrum over range of 400–4000 cm . Phase
analysis was performed by X-ray diffraction (XRD) on a Philips Model PW1830
diffractometer using Cu K˛1 radiation. The surface binding structure of the sam-
ple was determined on an ESCA-LAB MKII spectrometer with monochromatic Mg
K˛ radiation (1253.6 eV) operated at 300 W, vacuum generator in an UHV cham-
ous wide exothermic peak at about 400 C, which was attributed to
the combustion of organic TEOS. In Fig. 1(B), the thermal decompos-
ing process of the core–shell YAG:Yb3+@SiO precursor was similar
2
to that of pure SiO , indicating that no reactions occur between
2
−6
3+
3+
ber (2.3 × 10 Pa). The energies of the spectra were calibrated using C (1s) binding
YAG:Yb core and SiO2 shell. The weight loss of YAG:Yb @SiO2 is
Fig. 4. XPS spectra for pure YAG:Yb3+ (A), core–shell YAG:Yb3+@SiO2 and the pure SiO2 (B).

4
Y.-H. Sun et al. / Journal of Alloys and Compounds 528 (2012) 1–9
3.1.2. FT-IR spectra
Fig. 2 shows FT-IR spectra of pure SiO2 powders (a), core–shell
YAG:Yb3+@SiO2 particles (b) and pure YAG:Yb particles (c). For
3+
−
1
pure SiO , the typical absorption bands of Si
O
Si (1120 cm
,
2
1
−
−1
8
05 cm ) and Si OH (475 cm ) are observed in Fig. 2(a), and the
unobvious absorption bands at 3550 cm assigned to the extend
−
1
−
1
vibration of hydroxyl OH and 1630 cm
assigned to the bend
vibration of hydroxyl OH from the adsorbed H O are observed
2
3
+
too. For pure YAG:Yb shown in Fig. 2(c), the obvious absorption
−
1
−1
bands at 3455 cm and 1630 cm are attributed to OH vibration
from the adsorbed water on the surface of YAG, while the absorp-
−
1
−1
−1
and 562 cm
tion bands at 805 cm , 736 cm
are the typical
vibrations of Al O and 562 cm 1 and 465 cm are assigned to Y
−
−1
O
vibration in YAG. The OH groups hanging on the surface of YAG
can form hydrogen bonds between the particles, resulting in the
formation of bridge-oxygen bonds and thus leading to the agglom-
eration of the powders. Moreover, the OH groups hanging on the
surface of YAG are harmful to SiO2 coating because the Si OH on
the surface of SiO repulses the OH on the YAG surface. In Fig. 2(b)
2
of YAG:Yb3+@SiO2 core–shell sample, the typical absorption peaks
of Y O and Al O bands of YAG core, as well as Si
O Si and Si OH
bands of the amorphous SiO2 layer are observed. Moreover, the
vibration of OH from the adsorbed water on the particle surface
is reduced a lot in core–shell structure compared to the pure YAG.
The results indicate that SiO2 has been successfully coating on the
YAG particles.
3.1.3. XRD patterns
Fig. 3 shows the X-ray diffraction patterns of core–shell
YAG:Yb3+@SiO2 (a), pure SiO2 (b) as well as the standard patterns
◦
of YAG at the bottom. For pure SiO , a broad peak around 2ꢀ = 22
2
is observed, which is characteristic pattern for the amorphous SiO2
(
0
Joint Committee on Powder Diffraction Standards (JCPDS) 0029-
085). For core–shell YAG:Yb3+@SiO , the distinct sharp peaks
2
belong to the yttrium aluminum garnet phase (JCPDS 0033-0044)
with cubic structure (space group: Ia3d), and the characteristic peak
of SiO2 was presented too. The XRD analysis revealed the presence
of amorphous SiO2 layer coating on the nanocrystalline YAG:Yb3+
core. The diffraction peak of SiO2 is very weak, indicating that the
thickness of the SiO layer is thin. No other phase has been detected,
2
which suggests that no reaction between the YAG core and SiO2
◦
shell occur even after calcination at 500 C.
3.1.4. XPS spectra
Fig. 4 shows the XPS spectra of pure YAG:Yb3+ (A), core–shell
YAG:Yb3+@SiO2 and pure SiO2 (B) in a wide energy range. The
carbon contamination is found in all spectra. In Fig. 4(A), besides
carbon, an excess of oxygen amount, in relation to the nominal
value, is found in the spectrum. This suggests a part of oxygen
exist as the contamination of hydroxyl group. The chemical com-
Fig. 5. The TEM morphology of core–shell YAG:Yb3+@SiO2 (a), SEM morphology of
pure YAG:Yb3 (b) and high resolution of YAG:Yb @SiO2 crystal (c).
+
3+
3+
position of YAG:Yb was calculated using the XPS spectrum, the
real value is Y2.94Yb0.06Al5O12.3, while the nominal composition is
Y2.94Yb0.06Al5O12. The yttrium and aluminum amount are in agree-
ment with the nominal one, while the oxygen amount shows excess
in relation to the nominal value. This proved the fact that the surface
about 18%. The additional 8% weight loss, which is corresponding
◦
to the exothermic peaks at 500 C, was due to the pyrolysis of PVP
adsorbed on the surface of YAG:Yb3 during the functionalization
+
3+
of YAG:Yb adsorb hydroxyl group and result the surface defects
process. From Fig. 1(C), 42% weight loss of YAG:Yb3 particles is
+
3+
of YAG:Yb crystals. Fig. 4(B) reveals that the surface structure of
observed, which was caused by the release of H O, CO , NH , NO
3+
2
2
3
2
YAG:Yb @SiO is the same as that of pure SiO , indicating that the
2
2
and SO gases from decomposition of the precursors of yttrium and
3+
2
surface of YAG:Yb is coated by SiO2 very well.
aluminum compounds. Accordingly, the DSC curve presents several
endothermal and exothermic peaks in the calcination process, and
.1.5. Morphology of core–shell YAG:Yb3+@SiO2 particles
3
◦
two exothermic peaks are found at 927 and 1035 C in NH HCO
4
3
Fig. 5(a) shows the TEM morphology of core–shell particles
precipitated precursor (Fig. 1(C)) corresponding to the crystalliza-
3+
YAG:Yb @SiO2 using PVP as modifier surfactant. As a reference,
tion of the intermediate phase YAlO (YAP) and the transformation
3+
3
the SEM image of pure YAG:Yb is shown in Fig. 5(b), in which
of YAP to YAG, which is confirmed by XRD analysis for different
calcination temperature in our previous study [26].
the particles are agglomerated to some extent and linked together
each other, and the club-shape particles have size of 120–150 nm

Y.-H. Sun et al. / Journal of Alloys and Compounds 528 (2012) 1–9
5
Fig. 6. SEM morphology of pure YAG:Yb3+ (a) and TEM morphologies of core–shell YAG:Yb3+@SiO2 synthesized using different surfactants PVP (b), PEG (c) and CTAB (d).
in length and 50 nm in diameter. Compared YAG:Yb3+@SiO2 with
pure YAG:Yb , the dark gray core with the club-shape represents
obtained using PVP as functional surfactant in Fig. 6(b). Most of
the YAG:Yb particles have been coated by SiO2 shell with uni-
form thickness about 25 nm. Compared with the SEM graph of pure
YAG:Yb3+, no obvious changes in size and shape but a better dis-
persion are observed thanks to the protection of silica layer from
agglomeration.
3
+
3+
3
+
3+
YAG:Yb and the pale gray layer coating on the YAG:Yb is SiO2
shell with thickness of 25 nm (Fig. 5(a)). Moreover, high resolu-
tion of one YAG:Yb3+@SiO2 crystal is illustrated in Fig. 5(c). Here
amorphous structure of SiO2 is observed outside and well-formed
ordered entities with the distance about 0.26 nm nanocrystallites
In nature, PVP is an amphiphilic polymer that can stabilize
3
+
3+
of YAG:Yb is observed inside. This further proved SiO2 is coated
on YAG:Yb surface successfully. As noted above, one can note
that the dispersion of YAG:Yb is good after coating and the size
of YAG:Yb @SiO2 is obviously larger than that of pure YAG:Yb
particles.
YAG:Yb particles in ethanol–water solvents; PEG is a non-ionic
3
+
surfactant that has hydrophile group (
bine the OH groups on the surface of YAG:Yb particles; CTAB is a
cationic surfactant that can adsorb on YAG:Yb due to the negative
O
CH2 CH2 )n to com-
3+
3+
3
+
3+
3+
3
+
surface potential. The functionalization of YAG:Yb surface can
prevent the particles from interaction and form homogeneously
dispersion, and link to SiO2 by the free hydrophile groups of sur-
factants. PVP just can stabilize colloidal particles in water and many
nonaqueous solvents, and this behavior arises from the presence of
a highly polar amide group within the pyrrolidone ring and a polar
methylene and methine groups in the ring and along its backbone.
It can adsorb onto many different surfaces due to its amphiphilic
characteristics. Thus YAG:Yb3+ can disperse homogeneously and
each particle is coated by PVP molecules, which can combine OH
existed on SiO2 surface and result the growth of silica shell on
each core particle. However, publications present that the adhe-
sion force between PEG and the surface of particles is weak and
the resultant PEG film is easy to be shelled off due to the thermal
movement of the particles [29]. For CTAB as modifier, the long chain
cetyltrimethyl ammonium cation can attract a lot of YAG:Yb3+ par-
ticles agglomerating together due to the negative zeta potential of
3.2. Influence of the modification surfactant
The surface electric behavior of particles is very important to
coating process. It is impossible to directly coat SiO2 on YAG with-
out surface functionalization because both the two surfaces are
electronegative [22,28], which has been confirmed by FT-IR in Sec-
tion 3.1.2. So the surfactant such as PVP, PEG and CTAB is necessary
to modify the YAG surface to be with stabilization and high-affinity
to the silica shell growth.
3+
Fig. 6 shows SEM morphology of pure YAG:Yb (a) and TEM
morphology of core–shell YAG:Yb3 @SiO2 particles functionalized
by different surfactant PVP (b), PEG (c) and CTAB (d). In Fig. 6(c) and
+
(
d), the coated and uncoated YAG:Yb3+ as well as the individual
SiO2 spheres co-exist, and the particles are seriously agglomer-
3+
ated. But more smooth morphology of YAG:Yb @SiO2 particle is

6
Y.-H. Sun et al. / Journal of Alloys and Compounds 528 (2012) 1–9
Fig. 7. The TEM morphology of YAG:Yb3+@SiO2 synthesized using different TEOS:YAG molar ratio of 5.6 (a), 14.7 (b), 28.1 (c) and the core particles YAG:Yb3+ (d).
YAG:Yb3+ surface, and little space was left for SiO shell growth and
SEM in Fig. 6. Here, the YAG crystalline is considered as a cylinder
2
thus some SiO2 spherical formed separately.
with diameter d and length l, thus
1
4
VYAG
=
ꢂd2l
(3)
3.3. Control of the thickness of SiO2 shell
and VSiO2 is the volume of SiO2 shell coated on one single YAG
crystalline.
In order to precisely control the thickness of SiO2 shell, a series
of simple formula to calculate the addition amount of TEOS are
proposed. This relation is based on the assumption that the total
ꢀ
ꢁ
2
d
1
VSiO2 = ꢂ
+ t (l + 2t) − 4 ꢂd2l
(4)
surface area of the core particles is fully covered by SiO . The equa-
2
2
tions are as follows:
here t is the thickness of the SiO shell.
2
Fig. 7 shows the morphology of YAG:Yb3+@SiO2 coated by dif-
ferent amounts of TEOS. All the three samples were prepared using
.1 g YAG:Yb as core and functionalized by equivalent amount
of PVP. The core particles are with diameter d = 50 nm and length
MTEOS
m
SiO2
mTEOS
=
(1)
(2)
M
SiO2
3+
0
and
3+
l = 100 nm and shown in Fig. 7(d). In these images, all YAG:Yb
particles are completely coated by SiO2. Originally, YAG:Yb3+@SiO2
ꢀ
ꢁ
mYAG
mSiO2 = ꢁSiO2
VSiO2
samples with SiO2 shell thickness t = 10 nm, 20 nm and 30 nm were
planned to prepare, and the molar ratio of added TEOS to YAG:Yb
ꢁYAGVYAG
3+
should be 5.6, 14.7 and 28.1 according to calculation. Based on
the practical addition amount of TEOS, the SiO2 shell thickness is
15 nm (Fig. 7(a)), 25 nm (Fig. 7(b)) and 40 nm (Fig. 7(c)), respec-
tively. The measured thickness is larger than the calculated one.
This might be due to the calculation is on the basis of the strictly
where mTEOS and mSiO2 are the mass of needed TEOS and SiO shell,
2
and mYAG is the mass of YAG:Yb3+ powders used as the core. MTEOS
and MSiO2 are the molecular weight of TEOS and SiO , and ꢁ
2
SiO2
3
3
and ꢁYAG are the density of SiO2 (1.9 g/cm ) and YAG (4.55 g/cm ),
respectively. VYAG is the volume of one single YAG:Yb3+ crystalline,
which can be calculated according to the micrograph of the TEM or
3+
assumption that YAG:Yb crystalline is a cylinder and perfectly
dispersed. Thus, well dispersion of YAG:Yb3+ particles, the proper

Y.-H. Sun et al. / Journal of Alloys and Compounds 528 (2012) 1–9
7
Fig. 8. The NIR emission spectra of pure YAG:Yb3+ (a), YAG:Yb3+@SiO2 with different
shell thickness, (b) for 15 nm, (c) for 25 nm and (d) for 40 nm, respectively. Inset:
The normalization spectra of the four samples (right up corner) and the magnified
one for 1024 nm emission (left up corner).
concentration and dropping rate of TEOS are the key points to
achieve the precise SiO2 shell thickness.
3.4. Photoluminescence characteristics
The near infrared (NIR) emission spectra of YAG:Yb3 @SiO with
+
2
different thickness SiO2 shell as well as pure YAG:Yb3+ are shown
in Fig. 8. With the excitation of 940 nm laser diode at room temper-
ature, all the samples show intense emissions within the region of
9
60–1150 nm. A strong peak at 1024 nm and three weak shoulders
at 975 nm, 1004 nm and 1044 nm are presented, which are assigned
to the f–f transitions of Yb3 from 2F5/2 to F7/2 [3]. Due to the ther-
+
2
mal vibration of crystal lattice at room temperature, the manifolds
3
+
2
2
3+
Fig. 9. Photoluminescence decay curves and lifetime of YAG:Yb powder (a) and
F5/2 and F of Yb ions, which are in D symmetry of the garnet
7/2
2
3+
the core–shell YAG:Yb @SiO2 with the shell thickness 15 nm (b), 25 nm (c) and
0 nm (d), respectively. (A) Original decay curves; (B) semi-logarithmic scale in y-
lattice, are expected to split. So a series of NIR emissions with differ-
4
ent peak values origin from the same transition of 2F5/2 → F are
2
7/2
axis decay curves.
observed. In order to compare the PL property of the same volume
3
+
YAG:Yb in different core–shell samples, the normalized spectra
are obtained using the original determined PL intensities to divide
the volume percent of YAG:Yb3 in core–shell samples, which are
shown in the inset spectra of Fig. 8 at the right up corner. Here, the
volume percent of YAG:Yb3 in core–shell samples was calculated
according to Section 3.3 based on the size of YAG:Yb core and the
thickness of SiO2 shell in Fig. 7. The volume percent of YAG:Yb3+
in core–shell structure is 44.7% for (b) with shell thickness 15 nm,
order of (c) > (b) > (a) > (d), as shown in the inset of Fig. 8. It is
interested that the influence of the SiO2 shell thickness on the
PL intensity does not exhibit the gradual change. The strongest PL
intensity occurs in particles with SiO2 shell thickness of 25 nm and
the weakest one in particles with SiO2 shell thickness of 40 nm.
+
+
3+
3+
The kinetic properties of the photoluminescence of Yb
2
2
3+
3+
( F → F7/2) in pure YAG:Yb and core–shell YAG:Yb @SiO2
5/2
3
1.4% for (c) with shell thickness 25 nm and 25.1% for (d) with shell
with different shell thickness were investigated by PL decay curves,
as shown in Fig. 9(A). These curves can be well fitted by a single-
exponential function as I(t) = I exp(−t/ꢃ) (I is the initial emission
thickness 40 nm. Moreover, for the sake of clarity of the spectra,
the normalized intensity at 1024 nm of the phosphors are enlarged
and inserted in Fig. 8 at the left up corner.
0
0
intensity at t = 0 and ꢃ is 1/e lifetime of the emission center), for
convenient, semi-logarithmic scale in y-axis is used to present the
The spectra of the four samples have same peak patterns indicat-
ing that the coating shell of SiO has no affect on the crystal lattice of
YAG:Yb . Compared with the three core–shell samples, the emis-
sion intensity at 1024 nm decreases with increasing of the thickness
decay curves in Fig. 9(B), that is ln I(t) = ln I − t/ꢃ, and the abscissa
2
0
3
+
is limited to lower than 5 ms because the intensity is constant after
5 ms for the four samples according to (a). The lifetime constant
3
+
(ꢃ) of 2F5/2 → F
2
3+
of SiO2 shell, which are all weaker than that of pure YAG:Yb
.
detected at 1024 nm of Yb is determined to
7/2
3+
This may be related to the embedded structure of YAG:Yb in the
core–shell particles, and attributed to the fact that the effective
emitting volume of YAG:Yb3 is decreased with increasing of the
thickness of SiO2 shell, just like that in literature [30], the effective
be 1.08 ms for (a), 1.33 ms for (b), 2.30 ms for (c) and 0.87 ms for
(d), respectively, which is in accordance with the lifetime 0.618 ms
and 1.229 ms of Yb doped in the YAG ceramic samples before
+
3+
and after annealing [31]. The lifetime of the core–shell samples
increases with increasing of SiO2 shell thickness to some extent,
but falls from 2.30 ms of YAG:Yb3+@SiO2 with 25 nm shell thick-
ness to only 0.87 ms of the core–shell particles with 40 nm shell
thickness.
3+
3+
emitting volume of Y O :Eu in core–shell SiO @Y O :Eu was
2
3
2
2
3
decreased with increasing of the SiO2 core diameter.
If the emission spectra are normalized to the same volume of
3
+
YAG:Yb , the emission intensity at 1024 nm is in the increasing

8
Y.-H. Sun et al. / Journal of Alloys and Compounds 528 (2012) 1–9
Fig. 10. The XPS spectra of (A) the survey of pure SiO2 and YAG:Yb3+@SiO2 with different shell thickness; deconvolved spectra for O1s line of pure YAG:Yb3+ (B), pure SiO2
C), core–shell YAG:Yb3 @SiO2 with shell thickness of 15 nm (D), 25 nm (E) and 40 nm (F).
+
(
In general, when the doped concentration of Yb3+ and the host
discrimination of size and shape between different core particles
and point defect of the core particles can be obviated because
YAG:Yb core of the samples are all the same batch and YAG:Yb
host has a good crystallinity. So the surface characteristic of the
sample and the local effective refractive index of nanocomposite
are the key factors to the photoluminescence property.
Fig. 10 shows the XPS survey spectra of pure SiO2 and core–shell
samples with different shell thickness for (A), deconvolved spectra
matrix are fixed, there are several factors affect the emission inten-
sity and the radiative lifetime of Yb3 of nanoparticle: (1) The
existence of OH groups on the surface of the nanoparticle; (2)
the point defect of the nanoparticle; (3) the size and shape of
luminescent nanoparticle and (4) the refractive index of their sur-
roundings [32]. In this study, the difference of the samples is just the
thickness of SiO2 shell and the size of the core–shell particles. The
+
3+
3+

Y.-H. Sun et al. / Journal of Alloys and Compounds 528 (2012) 1–9
9
for O1s line of pure YAG:Yb3+ (B), pure SiO2 (C), YAG:Yb3+@SiO2
Acknowledgments
with shell thickness of 15 nm (D), 25 nm (E) and 40 nm (F).
It can be seen that all the coated samples have same XPS spectra
as that of pure SiO2 in Fig. 10(A), which suggests the coat is perfect
and the shell thickness exceed X-ray penetrating depth of 20 A˚ , so
the YAG spectra are not appeared. The O1s lines of different samples
are deconvolved after a background calibration. Fig. 10(B) shows
the existence of three components with binding energy 531.3 eV,
This work is financially supported by the Key Technology Project
of Guangzhou (Grant No. 2006Z2-D0161), Applied Basic Research
Project of Guangzhou (Grant No. 2006J1-C0491) and National Natu-
ral Science Foundation of China (Grant No. 50602017) and the Open
Foundation of National Key Lab of Silicon Materials in Zhejiang
University (Grant No. SKL2008-9).
5
32.7 eV and 533.9 eV, which are corresponding to three crystal-
lographic position of the oxygen ion in the YAG garnet structure:
1) crystallized with two yttrium and aluminum ions in the octa-
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