Sol-gel synthesis and characterization of Zn2SiO 4:Mn@SiO2 spherical core-shell particles

Article abstract of DOI:10.1149/1.1990612

The synthesis and characterization of Zn₂SiO₄:Mn phosphor layers on spherical silica spheres, i.e., core-shell particles of Zn₂SiO₄:Mn@SiO₂ are described in this paper. First, monodisperse silica spheres with an average size around 750 nm have been obtained via the Stoeber method by the hydrolysis and condensation of tetraethoxysilane Si(OC₂H₅)₄ under base condition (using NH₄OH as the catalyst). Second, the silica spheres are coated with Zn₂SiO₄:Mn phosphor layers by a sol-gel process. The resulting core-shell particles are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscope, energy-dispersive X-ray spectroscopy, transmission electron microscopy, photoluminescence, low-voltage cathodoluminescence, as well as kinetic decay. The results confirm that the 1000°C-annealed sample consists of crystalline Zn₂SiO₄:Mn shells and amorphous SiO ₂ cores with spherical morphology and narrow size distribution. The Zn₂SiO₄:Mn@SiO₂ particles show the green emission at 521 nm corresponding to ⁴T₁( ⁴G)-⁶A₁(⁶S) transition of Mn ²⁺ under the excitation of UV (250 nm), vacuum UV (172 nm), and electron-beams (1-6 kV). The luminescence intensity has been studied as a function of coating number, accelerating voltage, and filament current, respectively.

Full text of DOI:10.1149/1.1990612

Sol-gel Synthesis and Characterization of Zn2SiO4?:?Mn@SiO2
Spherical Core-Shell Particles
D. Y. Kong, M. Yu, C. K. Lin, X. M. Liu, J. Lin and J. Fang
J. Electrochem. Soc. 2005, Volume 152, Issue 9, Pages H146-H151.
doi: 10.1149/1.1990612
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2005 ECS - The Electrochemical Society
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H146
Journal of The Electrochemical Society, 152 ͑9͒ H146-H151 ͑2005͒
0
013-4651/2005/152͑9͒/H146/6/$7.00 © The Electrochemical Society, Inc.
Sol-gel Synthesis and Characterization of Zn SiO :Mn@SiO
2
2
4
Spherical Core-Shell Particles
a
a,b
a
a
a,z
c
D. Y. Kong, M. Yu, C. K. Lin, X. M. Liu, J. Lin, and J. Fang
a
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Changchun 130022, and Graduate School of the Chinese Academy of Sciences,
Beijing 100049, China
b
Department of Chemistry, Northeast Normal University, Changchun 130024, China
c
Department of Chemistry and Advanced Materials Research Institute, University of New Orleans,
New Orleans, Louisiana 70148, USA
The synthesis and characterization of Zn SiO :Mn phosphor layers on spherical silica spheres, i.e., core-shell particles of
2
4
Zn SiO :Mn@SiO are described in this paper. First, monodisperse silica spheres with an average size around 750 nm have been
2
4
2
obtained via the Stöber method by the hydrolysis and condensation of tetraethoxysilane Si͑OC H ͒ under base condition ͑using
2
5 4
NH OH as the catalyst͒. Second, the silica spheres are coated with Zn SiO :Mn phosphor layers by a sol-gel process. The
4
2
4
resulting core-shell particles are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, field emission scan-
ning electron microscope, energy-dispersive X-ray spectroscopy, transmission electron microscopy, photoluminescence, low-
voltage cathodoluminescence, as well as kinetic decay. The results confirm that the 1000°C-annealed sample consists of crystalline
Zn SiO :Mn shells and amorphous SiO cores with spherical morphology and narrow size distribution. The Zn SiO :Mn@SiO
2
4
2
2
4
2
4
4
6
6
2+
particles show the green emission at 521 nm corresponding to T ͑ G͒- A ͑ S͒ transition of Mn under the excitation of UV
1
1
͑
250 nm͒, vacuum UV ͑172 nm͒, and electron-beams ͑1–6 kV͒. The luminescence intensity has been studied as a function of
coating number, accelerating voltage, and filament current, respectively.
2005 The Electrochemical Society. ͓DOI: 10.1149/1.1990612͔ All rights reserved.
©
Manuscript submitted January 24, 2005; revised manuscript received April 13, 2005. Available electronically July 28, 2005.
1
8,19
The core-shell composite particles are of extensive scientific and
technological interest due to the ability to fine-tune their
sol-gel process.
It would be of great interest and importance to
check if the core-shell phosphor materials can be prepared in a simi-
lar process.
1
-5
properties. Core-shell materials consist of a core structural do-
main covered by a shell domain. The core and shell domains may be
composed of a variety of materials including polymers, inorganic
solids, and metals. The structure, size, and composition of these
particles can be easily altered in a controllable way to tailor their
magnetic, optical, mechanical, thermal, electrical, electro-optical,
2
+
It is well known that Mn -doped zinc orthosilicate ͑␣-Zn SiO ,
2
4
willemite͒ has been widely used as a green component in plasma
display panels ͑PDPs͒, CRTs, and electroluminescent devices due to
its highly saturated color, strong luminescence, and long
Recently, much attention has been paid to the synthesis
and performance of nanostructural Zn2SiO4:Mn phosphors by sol-
gel, hydrothermal, spray pyrolysis, and combustion methods,
Interestingly, ZnO nanowires sheathed with Zn SiO were
reported by Wang’s group. Herein, we select Zn SiO :Mn as the
phosphor shells and silica spheres as the cores, respectively, to ob-
tain the core-shell structured Zn SiO :Mn@SiO materials via the
sol-gel process and characterize the structure, morphology, and pho-
toluminescent and cathodoluminescent properties of the resulting
samples.
2
0,21
lifespan.
1-8
and catalytic properties. Until now, many routes have been devel-
oped to fabricate such core-shell materials, such as the sol-gel
22-26
etc..
1
9
2 4
process,
layer-by-layer
technique,
template
directed
27
1
0
2
4
self-assembly, 1and encapsulation of silica nanoparticles by in situ
1
polymerization.
2
4
2
The demand for high resolution and increased efficiency in phos-
phors for cathode ray tubes ͑CRTs͒ and field emissive displays has
promoted the development of phosphors that perform at low
1
2
voltages. In particular, phosphors made up of small, ideally spheri-
cal particles are of interest because they offer the possibility of
brighter cathodoluminescent performance, high definition, and much
Experimental
13
The starting materials for the preparation of Zn SiO :Mn@SiO
2
core-shell phosphors were tetraethoxysilane ͑TEOS͒ Si͑OC H ͒
analytical reagent, AR͒, ammonium hydroxide NH OH ͑AR͒, zinc
acetate dihydrate Zn͑OOCCH ͒ ·2H O ͑99.99%͒, manganese ac-
etate tetrahydrate Mn͑OOCCH ͒ ·4H O ͑99.99%͒, citric acid
monohydrate ͑AR͒, polyethylene glycol ͑PEG, molecular weight
2
4
improved screen packing. The ideal morphology of phosphor par-
ticles includes a perfect spherical shape, narrow size distribution
2
5 4
͑
4
͑
0.5–3 ␮m͒, and nonagglomeration. Spherical morphology of the
3
2
2
phosphors is good for high brightness and high resolution. Addition-
ally, high packing densities and low scattering of light can also be
obtained by using spherical phosphors. Nowadays, many synthetic
routes have been developed to control the size and distribution
of phosphor particles, such as spray pyrolysis
precipitation.
Silica can be easily made controllably in spherical morphology
3
2
2
1
0,000; AR͒, nitric acid HNO ͑AR͒, ethanol C H OH ͑AR͒, and
deionized water.
3 2 5
1
4,15
and fluxes
1
6
Preparation of monodisperse SiO2 spheres.—Monodisperse
silica spheres were prepared by the Stöber method. Typically,
5.7 mL TEOS was hydrolyzed in 171 mL ethanol in the presence
of 196 mL ammonium hydroxide and 7.0 mL deionized water.
The obtained silica spheres were centrifugally separated from the
suspension and dried in an oven at 100°C for 24 h.
17
17
2
from nano- to micrometer size. If the silica spheres are coated with
layers of phosphors, a kind of core-shell phosphor materials with
spherical morphology is obtained, and the size of the phosphor par-
ticles can be controlled by the silica cores. Furthermore, because
silica is cheaper than most of the phosphor materials, the core-shell
phosphor materials are cheaper than the pure phosphor materials in
unit mass. Our group has been preparing various kinds of phosphor
coatings on bulk silica glass and silicon wafer substrates via the
Surface coating of Zn SiO :Mn layer on SiO spheres.—
The doping concentration of Mn is 2 mol %, substituting for Zn
in Zn SiO layer, which has been optimized previously.
2
4
2
2+
2+
2
0-25
2
4
Stoichiometric
weights
of
Zn͑CH COO͒ ·2H O
and
3
2
2
Mn͑CH COO͒ ·4H O were dissolved in 50 mL ethanol-water ͑8:1
3
2
2
in volume͒ solution in the presence of 1 mL nitric acid under stir-
ring. Citric acid monohydrate as chelating agent ͑citric acid/metal
z
E-mail: jlin@ns.ciac.jl.cn
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Journal of The Electrochemical Society, 152 ͑9͒ H146-H151 ͑2005͒
ion = 2:1͒ and PEG as cross-linking agent were dissolved in the
H147
above solution, then silica spheres were added into the solution and
the resulting mixture was further stirred for 3 h. The precipitates
were filtered and dried at 100°C for 2 h, then preheated at 500°C
for 3 h in a furnace with a heating rate of 60°C/h. The preheated
samples were annealed to 1000°C at 100°C/h and held there for
3
h. In this way, the core-shell structured Zn SiO :Mn@SiO mate-
2 4 2
rials have been obtained, and the whole process is shown in Scheme
Figure 1. XRD patterns for ͑a͒ the 1000°C-annealed SiO2, ͑b͒
Zn SiO :Mn@SiO core-shell particles, ͑c͒ pure Zn SiO :Mn powder
2
4
2
2
4
sample, and ͑d͒ the JCPDS card ͑no. 37-1485͒ for Zn SiO . Miller indices of
2
4
diffracting lattice planes are also shown.
before the silica spheres were added. The other processes were the
same. The above process was repeated several times to increase the
thickness of Zn SiO :Mn shells.
2
4
Characterizations.—The X-ray diffraction ͑XRD͒ of the powder
samples was examined on a Rigaku-Dmax-IIB using Cu K␣ radia-
tion ͑␭ = 0.15405 nm͒. Fourier transform infrared ͑FTIR͒ spectra
were measured with Perkin-Elmer 580B infrared spectrophotometer
with the KBr pellet technique. The morphology and composition of
the samples were inspected using field emission scanning electron
microscope ͑FE-SEM, XL30, Philips͒, transmission electron micro-
scope ͑TEM, JEOL-2010, 200 kV͒, and energy-dispersive X-ray
spectroscopy ͑EDS, JEOL JXA-840͒. The photoluminescence ͑PL͒
and the cathodoluminescence ͑CL͒ spectra were taken on a Hitachi
F-4500 spectrofluorimeter equipped with a 150 W xenon lamp,
1
72 nm vacuum-uv ͑VUV͒ lamp ͑self-made͒ and 1–6 kV electron
gun ͑self-made͒ as the excitation sources, respectively. The lumines-
cence decay curves were obtained from a Lecroy Wave Runner 6100
digital oscilloscope ͑1 GHz͒ using 250 nm laser ͑pulse width
=
4 ns, gate = 50 ns͒ as the excitation source ͑Continuum Sunlite
OPO͒. All the measurements were performed at room temperature.
Results and Discussion
Formation and morphology of Zn SiO :Mn@SiO core-shell
2
4
2
particles.—XRD.—Figure 1 shows the XRD patterns for ͑a͒ the
annealed SiO , ͑b͒ Zn SiO :Mn@SiO , and ͑c͒ pure Zn SiO :Mn
2
2
4
2
2
4
powder samples, as well as ͑d͒ the JCPDS card ͑no. 37-1485͒ for
Zn SiO as a reference. For SiO particles annealed at 1000°C ͑Fig.
2
4
2
1
a͒, no diffraction peak is observed except for a broad band centered
at 2␪ = 22°, which is the characteristic peak for amorphous SiO2
JCPDS 29-0085͒. For the Zn SiO :Mn@SiO composite phosphors
͑
2
4
2
fired at 1000°C ͑Fig. 1b͒, besides the broad band at 2␪ = 22° from
the amorphous SiO , all the diffraction peaks belonging to crystal-
2
line willemite ͑␣-Zn SiO , JCPDS card 37-1485͒ ͑Fig. 1d͒ are
2
4
Scheme 1. Formation process of Zn SiO :Mn@SiO core-shell particles.
2
4
2
present, indicating that the coating layer of Zn SiO :Mn has crys-
2
4
tallized well on the surface of silica spheres. This agrees well with
the situation for the pure Zn SiO :Mn powder annealed at 1000°C
2
4
1
. For multiple coating, the precursor solution is a little different
͑
Fig. 1c, in which well-crystalline Zn SiO is observed͒. No other
2 4
from the above one because the surface of silica spheres were cov-
ered by Zn SiO :Mn layers which could not continue to react with
Zn and Mn . Instead of citric acid and PEG, the stoichiometric
weights of TEOS were added as silicon source in the mixing solu-
tion of Zn and Mn ; and then the solution was stirred for 2 days
phase can be detected. Formation of the Zn SiO :Mn layer is due to
2
4
2
4
2+
2+
the reaction of Zn , Mn ions ͑on silica surfaces͒ with SiO cores
2
+
2+
2
at high temperature ͑1000°C͒, as shown in Scheme 1. In the Pechini
2+
process, the citric acid first formed chelate complexes with Zn and
2
+
2+
2+
Mn , then the left carboxylic acid groups in the citric acid reacted
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H148
Journal of The Electrochemical Society, 152 ͑9͒ H146-H151 ͑2005͒
Figure 2. FTIR spectra of ͑a͒ the as-formed SiO , ͑b͒ 1000°C-annealed
2
Zn SiO :Mn@SiO core-shell sample, and ͑c͒ pure Zn SiO :Mn powder.
2
4
2
2
4
2
+
with PEG to form polyester with a suitable viscosity. The Zn and
2
+
Mn , stabilized by the chelating and polymerizing process, were
28
homogeneously distributed in the solution. The Stöber process-
derived silica particles contained a large amount of free hydroxyl
groups ͑-OH͒ and silanol groups ͑Si–OH͒ on their surface ͑evi-
denced by FTIR spectra, see next section͒. By stirring silica particles
in the solution, a lot of Zn2+ and Mn were absorbed onto the silica
particles by physical and chemical interactions. After the drying and
2+
annealing process, the Zn2+ and Mn reacted with surface SiO to
2+
2
form a Zn SiO :Mn layer on the silica cores.
2
4
FTIR.—The FTIR spectra of the as-formed SiO , 1000°C fired
2
Zn SiO :Mn@SiO core-shell sample, and pure Zn SiO :Mn pow-
2
4
2
2
4
der are shown in Fig. 2a-c, respectively. In Fig. 2a for the as-formed
SiO particles, the absorption bands and their corresponding vibra-
2
−
1
−1
tional models are 3432 cm ͑␯ , O–H͒, 1630 cm ͑O–H in H O͒,
102 cm
␯1, Si–O–Si͒, 474 cm ͑␯ , Si–O͒, where ␯ is symmetric stretch-
ing, ␯ is asymmetric stretching, and ␯ is asymmetric deformation
1
2
−
1
−1
−1
1
͑
͑␯ , Si–O–Si͒, 952 cm
͑␯ , Si–OH͒, 802 cm
3
1
−
1
4 1
3
4
2
9-32
vibrations, respectively.
This indicates that the as-formed SiO2
particles contain a large amount of OH groups and H O on their
2
surfaces. The surface Si–OH groups play an important role for bond-
ing the metal ions from the coating sol and forming the Zn SiO :Mn
2
4
layer on the SiO surfaces in the following annealing process, as
2
shown in Scheme 1. In Fig. 2b for the Zn SiO :Mn@SiO core-
2
4
2
shell sample, apart from the absorption peaks from silica cores
−
1
͑
͑
1125, 474 cm ͒, the characteristic absorption peaks due to SiO
4
−
1
−1
␯ , 995, 894 cm ͒ and ZnO ͑␯ , 577 cm from crystallized
Zn SiO are present.
1
4
3
33,34
These absorption peaks are basically simi-
2
4
lar to those for the pure Zn SiO :Mn sample, as shown in Fig. 2c.
Figure 3. FE-SEM micrographs of ͑a͒ the as-formed SiO particles, ͑b͒ the
1000°C-annealed core-shell Zn2SiO4:Mn@SiO2 particles, and ͑c͒ EDS of
2
4
2
This suggests that crystalline phase ͑Zn SiO ͒ has formed after an-
2
4
Zn SiO :Mn@SiO sample.
nealing at 1000°C, agreeing well with the results of XRD and fur-
ther demonstrating the formation of crystalline Zn SiO :Mn coating
2
4
2
2
4
layers on the silica surfaces via the sol-gel process and annealing
treatment. Note that weak OH vibration can be found in
Zn SiO :Mn@SiO and Zn SiO :Mn samples due to the absorption
The core-shell Zn SiO :Mn@SiO particles ͑Fig. 3b͒ are also uni-
form spheres with a little aggregation, whose average size ͑diam-
2
4
2
2
4
2
2
4
of traces of water in the course of measurement.
eter͒ seems larger than the pure SiO particles due to the extra layers
2
FESEM, EDS, TEM.—Figure 3 shows the FE-SEM micrographs of
of Zn SiO :Mn on the SiO spheres. However, it should be men-
2
4
2
͑
a͒ the as-formed SiO particles, ͑b͒ the single Zn SiO :Mn layer
tioned that the FE-SEM micrographs can only provide the basic
2
2
4
coated SiO core-shell ͑Zn SiO :Mn@SiO ͒ particles annealed at
information on the morphology of Zn SiO :Mn@SiO particles in
2
2
4
2
2
4
2
1
000°C, and ͑c͒ the EDS of Zn SiO :Mn@SiO sample. From the
large scale, and the core-shell structure of Zn SiO :Mn@SiO par-
2
4
2
2 4 2
FESEM micrograph of Fig. 3a we can observe that the as-formed
ticles cannot be resolved from the FE-SEM micrographs. To further
confirm the existence of Zn SiO :Mn layers on the surfaces of silica
SiO particles are monodisperse spheres with smooth surfaces and
2
2
4
nonaggregation, whose average size ͑in diameter͒ is about 750 nm.
particles, EDS was performed on the sample in Fig. 3b, as shown in
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Journal of The Electrochemical Society, 152 ͑9͒ H146-H151 ͑2005͒
H149
Figure 4. TEM micrographs for 1000°C-annealed ͑a͒ single and ͑b͒ double
layer of Zn SiO :Mn-coated SiO particles and ͑d͒ the bare SiO particles as
well as ͑c͒ the electron-diffraction pattern for ͑a͒.
2
4
2
2
Figure 5. ͑a͒ Excitation and ͑b, c͒ emission spectra of ͑single-layer͒
Zn SiO :Mn@SiO core-shell particles.
2
4
2
Fig. 3c. Si, Zn, and O elements were detected on the surface of the
complex particles. The Mn element was not detected clearly due to
its low concentration ͑but it can be detected by the emission spectra,
see next part͒. This provides additional evidence for the formation of
coatings of crystalline Zn SiO :Mn on the SiO particles.
The excitation spectrum ͑Fig. 5a͒ contains an intense broad band
with a maximum at 250 nm ͑5.0 eV͒ and a narrow shoulder at
2
23 nm ͑5.6 eV͒ extended to the VUV region ͑below 200 nm, not
shown͒. The former corresponds to the charge-transfer band ͑CTB͒
2
4
2
2
+
of Mn ion, and the latter originates from the absorption of the host
lattice of Zn SiO . This indicates that an energy transfer occurs from
In order to see the core-shell structure of Zn SiO :Mn@SiO
2
4
2
particles, TEM was performed. Representative TEM micrographs
2
4
2
+ 20-26
the Zn SiO host lattice to Mn .
Excitation into the CTB of
are shown for the SiO particles coated by one and two layers of
2
4
2
2
+
Mn at 250 nm or host band at 172 nm yields the emission spec-
Zn SiO :Mn shell in Fig. 4a and b, respectively. In both cases, the
2
4
trum with a maximum at 521–522 nm ͑Fig. 5b and c͒, which corre-
core-shell structure for the Zn SiO :Mn@SiO particles can be seen
2
4
2
4
4
6
6
2+
clearly due to the different electron penetrability for the cores and
shells. The cores are black and the shells have gray color. The core-
shell Zn SiO :Mn@SiO particles remain spherical. The average
sponds to the typical T1͑ G͒- A1͑ S͒ transition of Mn . The green
2
+
emitting can be explained when it is assumed that Mn ions occupy
a part of Zn sites with four oxygen coordinated. The weak crystal
field around Mn results in the low splitting width of its 3d energy
2
+
2
4
2
2
+
thickness for a single-layer shell is about 150 nm ͑Fig. 4a͒, and that
for a double-layer shell is about 220 nm ͑Fig. 4b͒. Obviously, the
shell thickness increases with the increase of the number of coating
times. As a comparison, no such kind of core-shell structure can be
levels, and an emission at high energy ͑green͒ is observed. The
2
+
mechanism involved in the green emission of Mn in Zn2SiO4 is
shown in Fig. 6. Under excitation of 250 nm UV, an electron from
observed in bare SiO particles, as shown in Fig. 4d. Fig. 4c shows
2
the electron-diffraction pattern performed on the surface of the par-
ticles in Fig. 4a, which clearly demonstrates the formation of crys-
talline phase. Combined with the results of XRD in Fig. 1a, we can
know that this crystalline phase must be Zn SiO :Mn. In summary,
2
4
from the results of FE-SEM, EDS, and TEM, it can be concluded
that Zn SiO :Mn layers have crystallized on the surface of SiO
2
4
2
spheres to form Zn SiO :Mn@SiO core-shell particles, which
2
4
2
mainly keep the original spherical morphology of the SiO particles.
2
Luminescent and kinetic properties of Zn SiO :Mn@SiO core-
2
4
2
shell particles.—Upon UV and VUV light excitation, the
Zn SiO :Mn@SiO core-shell particles exhibit strong green lumi-
2
4
2
nescence. Figure 5 shows the ͑a͒ excitation and ͑b, c͒ emission
spectra of the core-shell particles annealed at 1000°C, respectively.
Figure 6. The PL mechanism for Mn2+ in Zn SiO .
2
4
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H150
Journal of The Electrochemical Society, 152 ͑9͒ H146-H151 ͑2005͒
Figure 8. The PL emission intensity of Mn2+ in Zn SiO :Mn@SiO sample
2
4
2
as a function of the coating number ͑N͒: ͑a͒ N = 1, ͑b͒ N = 2, and ͑c͒ N
= 3.
4
4
6
6
Figure 7. The decay curve for the T ͑ G͒– A ͑ S͒ ͑521 nm͒ emission of
1
1
2
+
Mn in ͑single layer͒ Zn SiO :Mn@SiO sample annealed at 1000°C.
2
4
2
and accelerating voltage, as shown in Fig. 9a and b, respectively.
Under 4 kV electron-beam excitation, the CL intensity increases
with increasing the filament current from 14 to 18 mA ͑Fig. 9a͒.
Similarly, when the filament current is fixed at 14 mA, the CL in-
tensity also increases with raising the accelerating voltage from
the Mn2+ ground state A ͑ S͒ is excited to the conduction band
6
6
1
2
+
͑
CB͒ of Zn SiO . The process can be expressed as Mn + h␯
Mn + e, where h␯ is the energy required in the photoexcitation
of Mn . The free electrons in the CB can relax to the T ͑ G͒
excited states of Mn through a nonradiative process, then radiative
2 4
3
+
→
2
+
4
4
1
2
+
1
to 6 kV ͑Fig. 9b͒.
4
4
6
6
transition from T ͑ G͒ excited state to the ground state A ͑ S͒
1
1
2
+
35
occurs, resulting in the green emission of Mn at 521 nm.
In order to study the decay behavior of Mn2+ luminescence in
more detail in the Zn SiO :Mn@SiO core-shell particles,
2
4
2
2
+
the kinetic curve for the representative emission of Mn
T ͑ G͒- A ͑ S͒ ͑521 nm͒ was measured, as shown in Fig. 7. The
decay curve can be well fitted into a double exponential function as
4
4
6
6
1
1
I = A exp͑−t/␶ ͒ + A exp͑−t/␶ ͒, and the fitting results are shown
1
1
2
2
inside Fig. 7. Two lifetimes, a fast one ␶ = 1.96 ms and a slow one
1
2
+
␶2
= 19.7 ms have been obtained for the emission of Mn . The
lattice of zinc silicate ͑Zn SiO ͒ with willemite structure consists of
2
4
corner-joined tetrahedral groups of ͓SiO ͔ and ͓ZnO ͔. This struc-
4
4
ture contains two kinds of Zn2+ sites, both having four nearest-
neighbor oxygens in a slightly distorted tetrahedral configuration.
2
+
Accordingly, it can be assumed that the Mn ions occupy the two
2
+
different Zn sites in the crystalline layer of Zn SiO in the
2
4
Zn SiO @SiO core-shell particles, and the two different lifetimes
2
4
2
2
+
36
arise from the Mn ions occupied in the two different sites. The
average lifetime of Mn
␶͘ = ͑A ␶ + A ␶ ͒/͑A ␶ + A ␶ ͒, can be determined to be
7 ms. The long lifetime of Mn is due to the forbidden proper-
2
+
4
4
6
6
T ͑ G͒- A ͑ S͒ emission, defined as
1 1
2
1
2
2
͗
1
1
2
1
1
2 2
3
7
2+
5
2+ 20
ties of the electronic transitions within 3d configuration of Mn .
The PL intensity of Zn SiO :Mn@SiO core-shell particles can
2
4
2
be tuned by the number of coatings. Figure 8 shows PL intensity as
a function of the number of coatings. Clearly the PL intensity of
Zn SiO :Mn@SiO core-shell particles increases with raising the
2
4
2
coating number. Obviously this can be attributed to the increase of
the thickness of Zn SiO :Mn shells on the SiO spheres. At present
2
4
2
the PL intensity of three-layer Zn SiO :Mn@SiO particles is still
2
4
2
not as strong as that of the pure Zn SiO :Mn sample, but this might
2
4
be overcome by further coating and optimizing other experimental
conditions such as annealing temperature and time as well as the
2
+
doping concentration of Mn , which are underway.
The Zn SiO :Mn@SiO core-shell particles also show a strong
2
4
2
green emission under low-voltage cathode ray excitation, and the
resulted CL spectra are identical with those of the PL emission spec-
4
4
6
6
tra shown in Fig. 5b and c due to the T ͑ G͒- A ͑ S͒ transition of
1
1
2
+
Figure 9. The CL emission intensity of Mn2+ as a function of ͑a͒ filament
current and ͑b͒ accelerating voltage.
Mn . The CL emission intensity for one-layer Zn SiO :Mn-coated
2
4
SiO particles has been investigated as a function of filament current
2
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Journal of The Electrochemical Society, 152 ͑9͒ H146-H151 ͑2005͒
H151
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Conclusions
Spherical core-shell structured Zn SiO :Mn@SiO particles with
6
2
4
2
8
9
uniform size distribution have been successfully prepared by the
sol-gel method followed by annealing at high temperature. Upon
UV, VUV, and cathode ray excitation, the luminescence properties
of the Zn SiO :Mn@SiO core-shell particles are typical of those
1
1
2
4
2
Langmuir, 17, 7145 ͑2001͒.
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Newport, J. Vac. Sci. Technol. B, 17, 750 ͑1999͒.
of Zn SiO :Mn, i.e., green band emission at 521 nm due to
2
4
1
4
4
6
6
2+
T ͑ G͒- A ͑ S͒ transition of Mn . The PL intensity of the core-
1
1
shell particles can be tuned by the number of coatings. With the
increase of the number of coatings, the PL intensity increases. The
14. K. Y. Jung, D. Y. Lee, Y. C. Kang, and H. D. Park, J. Lumin., 105, 127 ͑2003͒.
5. Y. C. Kang, I. W. Lenggoro, S. B. Park, and K. Okuyama, Mater. Res. Bull., 35,
89 ͑2000͒.
1
7
CL intensity of the Zn SiO :Mn@SiO particles increases with rais-
2
4
2
16. A. Celikkaya and M. Akinc, J. Am. Ceram. Soc., 73, 2360 ͑1990͒.
ing the filament current and cathode voltage. The advantages of the
phosphors prepared by this process are the easy availability of ho-
mogeneous spherical morphology in different size, and its wide
practicality for other phosphor materials.
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1
9. M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, H. J. Zhang, and Y. C. Han, Chem. Mater.,
1
4, 2224 ͑2002͒.
2
0. G. Blasse, and B. C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin,
Heidelberg ͑1994͒.
Acknowledgments
2
2
2
2
1. X. Ouyang, A. H. Kitai, and T. Xiao, J. Appl. Phys., 79, 3229 ͑1996͒.
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This project is financially supported by the foundation of “Bairen
Jihua” of the Chinese Academy of Sciences, the MOST of China
͑
no. 2003CB314707͒, and the National Natural Science Foundation
25. A. Roy, S. Polarz, S. Rabe, B. Rellinghaus, H. Zähres, F. E. Kruis, and M. Driess,
Chem.-Eur. J., 10, 1565 ͑2004͒.
of China ͑NSFC; 50225205, 20271048, 20431030͒. Professor J.
Fang is grateful for the financial support by the foundation of a
two-base program for international cooperation of NSFC
2
2
6. R. P. Sreekanth Chakradhar, B. M. Nagabhushana, G. T. Chandrappa, K. P.
Ramesh, and J. L. Rao, J. Chem. Phys., 121, 10250 ͑2004͒.
7. X. Wang, C. J. Summers, and Z. L. Wang, Adv. Mater. (Weinheim, Ger.), 16, 1215
͑2004͒.
͑
00310530͒ related to project 50225205, and NSF DMR-0449580.
28. M. Kakihana and K. Domen, MRS Bull., 25͑9͒, 27 ͑2000͒.
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We are grateful for Professor Yingnimg Yu and Liaohai Ge for mea-
suring the decay curve and TEM, respectively.
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