The usefulness of nanosized ceramic functional particles
are increasingly recognized. Due the high surface area
to volume ratio of these particles, processing difficulty
of handling and processing are often encountered. The
demand for nano-size dielectric ceramic particles has
increased due to the development of volume-efficient
multilayer ceramic capacitors (MLCCs). Barium titanate
(BaTiO3 : BT) is one of more important dielectric
material widely used for MLCCs. In an industrial scale,
BT powders are synthesized by a solid state reaction at
high temperatures [1, 2]. BT nanoparticles have been
synthesized by many researchers using hydrothermal
method [3-16]. Hydrothermal synthesis of BT particles
has a special advantage over conventional solid state
reaction due to the quasi-atomic dispersion of Ba2+ and
Ti4+ in a liquid precursor, leading to a nucleation and
crystallization process occurring at low temperatures
under a high pressure, yielding high purity particles.
BT nanocrystals overcoming the processing difficulty
with suitable properties may be synthesized by controlling
the nucleation and growth. This may be achieved
by modifying the surface [17] inhibiting further growth.
The growth inhibitor adsorbate may also be utilized as
a built-in dispersant for processing of the powder.
Luminescent properties of inorganic phosphors have
been extensively investigated for commercial flat paneldisplays (FPDs) in the recent years [18, 19]. Mn2+
doped Zn2SiO4 and zinc sulfide phosphors are considered
for a FPD. Li et al. [20] investigated morphology of
Zn2SiO4 particles with respect to the nature of anion,
silica source, and the NH4OH : Zn2+ ratio in their
hydrothermal processing. They demonstrated the control
of spherical Zn2SiO4 particles using ZnSO4 as zinc
source in an ammonia solution under a hydrothermal
condition. However, the phosphors showed poor crystallinity.
Although the luminescence efficiency of zinc sulfide
is satisfactory, the stability under a cathode ray beam in
high vacuum has been questioned. This problem may
be overcome by a surface passivating agent layer (PAL).
The discovery of a new class of luminescence materials of
doped nanocrystals combining high luminescence efficiency
and decay time shortening, as suggested by
Bhargava et al. [21, 22] has been disputed recently by
Bol and Meijerink [23], and Murase et al. [24]. Indeed,
the high photoluminescence (PL) of Mn2+ doped ZnS
nanocrystals is still remaining as an interesting research
field.
The doping of Mn2+ into ZnS lattice was achieved
during the precipitation at room temperature in the
solution or during the reaction of cations with H2S gas
at an elevated temperature up to 200°C [25]. Methacrylic
acid (MA) has been used as a surfactant in order to
prevent nanoparticle agglomeration in the solution [26].
PL enhancement up to ten-fold has been observed for
polymethyl merthacrylate (PMMA) coated ZnS nanocrystals
doped with Mn2+ ions [27, 28].
This paper describes chemical techniques to modify
the surface for enhanced properties and processabilityfor nanosized particles. The PL enhancement mechanism
for Mn2+ doped ZnS nanoparticles modified with
a surface modifying agent has not been adequately
addressed in the literature, especially the role of a PAL
on PL enhancement.
Experimental
Nano BT was synthesized by using an aqueous
solution of BaCl2 mixed with an aqueous solution of
TiCl4. Polyoxyethylene (20) sorbitan monooleate (Tweena
80) was added as a polymeric stabilizer into above
solution at a concentration of 5.0 wt%. A high pH was
maintained by KOH. The resulting milky sol filled a
high-pressure stainless steel vessel. The sealed vessel
was heated to 100oC ~ 230oC for 10 min to 2 h. The
resultant nanocrystals were washed with deionized
water and dried .
For zinc, silicate tetraethyl orthosilicate (TEOS) sol,
which had been hydrolyzed by a water/TEOS molar
ratio of 2, was added to a stoichiometric solution of
Mn(CH3COO)2 · 4H2O and Zn(CH3COO)2 · 2H2O in
ethanol and deionized water at room temperature for
Mn2+ doping concentration of 2 mole%. Tween80 was
added as a surface modifier to the above solution and
the sol was adjusted to pH = 10 by ammonium hydroxide.
The zinc silicate sol filled a stainless steel vessel
along with Zn2SiO4 : Mn2+ seed particles (5 wt% with
respect to the final products). The seed particles were
obtained by firing a 25 mL volume of the above sol at
1000oC for 2 h in air. The sealed vessel was then
heated to 230oC for 2 h. After cooling down to room
temperature, the resultant precipitate was centrifuged
and washed with deionized water several times followed
by drying at 60oC for 24 h in a vacuum oven.
Mn2+ doped ZnS nanocrystals were synthesized by a
chemical precipitation method at room temperature
using Zn(CH3COO)2 · 2H2O, Mn(CH3COO)2· 4H2O,
and Na2S · 9H2O as starting materials. A 50 mL
ethanol solution was prepared by dissolving 2.195 g
Zn(CH3COO)2 · 2H2O and 0.049 g Mn(CH3COO)2 ·
4H2O with stirring at room temperature. This yielded a
Mn2+ doping concentration of 2 mole%. Then, a 50 mL
aqueous solution of 2.451 g Na2S · 9H2O was added to
the ethanol solution drop by drop with vigorous
stirring. The resultant white precipitate was centrifuged
and washed using deionized water. Finally, 1.987 g of
3-methacryloxypropyl trimethoxysilane (MPTS) was
added to the resultant mixture after centrifuging and
washing. In order to compare the effect of a passivating
additive on luminescence properties, 0.5 g Tweena80
was added to the resultant precipitate in the second
sample. The third sample was prepared without the
addition of the surface modifying agent.
Transmission electron microscopy (TEM) study of
these nanocrystals was carried out at 200 kV using a
Hitachi HF-2000 TEM equipped with a field emissionsource. The TEM specimens were prepared by dispersing
the as-prepared Mn2+ doped ZnS nanoparticles in
methane, and picking up the nanocrystals using a
carbon film supported by a copper grid. The UV-visible
absorption spectrum was obtained in a Hitachi 5000
spectrophotometer for Mn2+ doped ZnS nanoparticles
coated on a silica glass substrate. This sample was
prepared by dip-coating from a colloidal solution of
ZnS and drying at room temperature in air.
The particle size and size distribution were characterized
using a Horiba LA-910 laser scattering particle
size analyzer. The samples were dispersed in distilled
water and ultrasonically treated for 10 min prior to the
size analysis. X-ray diffraction (XRD) measurements
were performed using an XDS X-ray diffractometer
(Model 2000, Scintag, Inc.) from 2q = 20o to 80o with a
scan step of 0.02o. Thermogravimetric analysis (TGA)
measurement was performed by a NETZSCH STA
449C TGA unit in argon atmosphere from 50oC to
1300oC with a heating rate of 10oC · min-1.
For PL measurements, above bandgap excitation was
achieved by using the 275 nm (4.51 eV) line from an
argon ion laser. The laser power was reduced to an
appropriate level by using a set of neutral density filters
to avoid local heating. Typical excitation intensities
were between 300 mW · cm-2 and 1 W· cm-2. The PL
signal was dispersed by a 1000M Spex monochromator
and detected by a thermoelectrically cooled GaAs
photomultiplier tube (PMT) operating in the photon
counting mode.
Results and Discussion
Figure 1 shows a TEM bright field image of BT
nanocrystals synthesized with a pH of 13.5 at 230oC
for 0.5 h. It can be seen that BT particles are well-dispersed and in spherical forms. Some particles are
weakly agglomerated due to the large amount of
particles presented in the TEM image. The EDS results
confirmed that the chemical composition of particles is
BaTiO3.
The mean particle diameter was statistically estimated
to be 77.4 ± 27.6 nm from approximately 250 particles
in the TEM image, as shown in Fig. 1. In order to
compare the results, the mean particle size was also
measured by a Horiba particle size analyzer based on a
laser scattering method. The size distribution from laser
scattering method was 83 ± 19 nm. It is noted that the
mean particle size measured from laser scattering
method is larger than the estimation from the TEM
image. This is due to the fact that polymeric species,
anchored on particle surface, have been taken into
account during the laser scattering method, i.e., the
polymer coated particles are slightly larger than the BT
particles themselves.
In order to determine the phase of BT particles, XRD
analyses were performed. Figure 2 presents the XRD
patterns of BT nanoparticles synthesized at 150oC for
30 min from the precursor with different pH values.
From a solution with a pH of 12.0, there is no crystallization
after hydrothermal treatment. When the precursor
pH was raised to 12.8, BT nanocrystals became
detectable. However, there is an unexpected peak at
2q = 24o (marked with *), whose origin is unclear. It
might belong to an intermediate phase or carbonate.
For the precursor sol with a pH of 13.5, all peaks from
the XRD pattern match well with standard cubic BT
phase JCPDS No. 31-174.
Figure 3 shows the DSC results of BT nanoparticles
synthesized from a precursor of pH = 13.5 at 230oC for
0.5 h. It has been shown in DSC results that there is no
endo-thermal peak between 50oC and 150oC, indicativeof no phase transition around the BT Curie point
between 125oC and 130oC. It is, therefore, suggested
that the BT particles synthesized by the hydrothermal
method have a cubic structure. There are three exothermal
peaks at 171oC, 344oC, and 426oC, which
correspond to the burnout of organic species from BT
nanoparticle surfaces.
Although the phase of BT nanoparticles have been
confirmed as cubic phase from both XRD and DSC
results, a debate on actual phase of BT nanoparticles
remains in the literature. This debate originated from
the detection of Raman-active modes attributing to
tetragonal BT (p4mm) by using Raman spectroscopy
from BT nanoparticles whose XRD patterns showing a
cubic phase (pm3m). During nucleation and crystallization
of BT nanocrystals, Tweena80 acted as a surface
modifier and growth inhibitor for the particles. Tweena
80 has a hydrophilic head group and a hydrophobic tail
as shown in Fig. 4. The presence of these polymeric
species prevents the agglomeration of particles and
hinders the further growth of individual particles.
Tweena80 can also be used as a steric stabilizer while
dispersing resultant BT nanoparticles in aqueous media.The polymeric species on BT particle surface act as a
dispersant in water, leading to a better redispersibility
of the synthesized nanoparticles. The better redispersibility
of BT nanocrystals is actually due to the steric
stabilization of the particles in the presence of Tweena
80 on the particle surface.
Figure 5 shows the X-ray diffraction (XRD) pattern
of the Mn2+ doped Zn2SiO4 phosphor particles synthesized
by the seeded hydrothermal method. In order to
compare the seeding effect on crystallization, the XRD
pattern of samples without seeds after the same hydrothermal
reaction was also measured. The XRD pattern
of the Mn2+ doped Zn2SiO4 seeds after firing at 1000oC
for 2 h in air is also presented for comparison. It is
shown in Fig. 5 that the XRD patterns of both Zn2SiO4 :
Mn2+ seeds and phosphor samples after a seeded hydrothermal
reaction agree very well with those in the literature
for Zn2SiO4 crystals [29]. However, no Zn2SiO4
crystallization, under the same condition, was detected
from a sample without seeds, whose XRD pattern
shows only weak intermediate peak at 2q = 45. In the
present work, Zn2SiO4 crystals were well developed after
only 2 h at 230oC in a seeded hydrothermal method.
Thus, seeding brings a positive effect on heterogeneous
nucleation sites lowering the supersaturation necessary
for crystallization.
Figure 6 displays a transmission electron microscopy
(TEM) image of the Mn2+ doped Zn2SiO4 phosphor
particles. The particles are non-spherical with a mean
length of 700 nm and a mean width of 350 nm. These
particles are uniform without agglomeration. The PL
spectrum of our Mn2+ doped Zn2SiO4 phosphor particles
is shown as curve 1 in Fig. 7(a) with the emission
maximum located at 522 nm, attributable to the 4G-6S
transition in Mn2+ doping centers. The seeding effect is
apparent that no green photoluminescence was observedwas detected from the sample without seeds.
HRTEM images of ZnS in Fig. 8 show well-defined
nanocrystals whose size was estimated to be about 2.8
nm. The lattice fringe is clearly exhibited from an individual
nanocrystal, whose lattice constant d was evaluated
as 3.11 A. This is in good agreement with lattice
constant of cubic ZnS with a d of 3.123 A for {111}
plane. The peak broadening in the XRD pattern clearly
indicates that very small nanocrystals are present in the
samples. From the width of the XRD peak (Fig. 9)
broadening, the mean crystalline size can be calculated
using Scherrer’s equation: D = 57.3 kl/bcosq, where k
is a geometric factor taken to be 1, l is the X-ray wavelength
(for Cu Ka radiation, l = 1.541 A), q is the
diffraction angle, and b is the half-width of the diffraction
peak at 2q. The mean crystal size of Mn2+
doped ZnS nanoparticles is calculated to be 2.6 nm
with a calculation error of 15%, i.e. 2.6 ± 0.4 nm.
This is consistent with the estimated size from nanocrystals
in HRTEM images. No apparent difference was
observed in the XRD peak shape and broadening of
Mn2+ doped ZnS nanoparticles before and after surface
passivation.
PL properties of Mn2+ doped ZnS nanoparticles were
characterized for samples with MPTS, with Tweena80,
and without additives as shown in Fig. 10. An orange
photoluminescence was observed from Mn2+ doped
ZnS nanocrystals without MPTS whose peak is located
at 601 nm. It is also shown in Fig. 10 that the PL
intensity of Mn2+ doped ZnS nanocrystals passivated by
MPTS was enhanced by a 30-fold, in comparison with
the samples without MPTS passivation. No orange
photoluminescence from MPTS alone was observed
after UV excitation at 275 nm. This confirms that the
enhanced PL is not from MPTS itself but from surface
passivated nanoparticles. In contrast, the PL intensity of
Mn2+ doped ZnS nanoparticles coated with Tweena80
remained the same as that without the surface agent.
This indicates that not all surface absorbates effective
enhancing photoluminescence of Mn2+ doped ZnS
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