TeO2 nanoparticles synthesized by evaporation of tellurium in atmospheric microwave-plasma torch-flame

Article abstract of DOI:10.1016/j.cplett.2006.08.026

Tellurium dioxide (TeO₂) nanoparticles were synthesized directly by evaporation of tellurium (Te) granules in an atmospheric microwave-plasma torch-flame with possibility for a direct continuous preparation and mass production of TeO₂ nanoparticles. The mean size of the synthesized TeO₂ particles was 108.2 nm observed from the TEM image and was 112.0 nm estimated by the Williamson-Hall plot using XRD data. The surface area and pore distribution properties of the particles were also analyzed by making use of a nitrogen adsorption apparatus.

Full text of DOI:10.1016/j.cplett.2006.08.026

Chemical Physics Letters 429 (2006) 214–218
www.elsevier.com/locate/cplett
TeO nanoparticles synthesized by evaporation of tellurium
2
in atmospheric microwave-plasma torch-flame
Soon Cheon Cho, Yong Cheol Hong, Han Sup Uhm *
Department of Molecular Science and Technology, Ajou University, San 5, Wonchon-Dong, Youngtong-Gu, Suwon 443-749, Republic of Korea
Received 23 June 2006; in final form 27 July 2006
Available online 9 August 2006
Abstract
Tellurium dioxide (TeO
wave-plasma torch-flame with possibility for a direct continuous preparation and mass production of TeO
of the synthesized TeO particles was 108.2 nm observed from the TEM image and was 112.0 nm estimated by the Williamson–Hall plot
2
) nanoparticles were synthesized directly by evaporation of tellurium (Te) granules in an atmospheric micro-
2
nanoparticles. The mean size
2
using XRD data. The surface area and pore distribution properties of the particles were also analyzed by making use of a nitrogen
adsorption apparatus.
ꢀ
2006 Elsevier B.V. All rights reserved.
The tellurium dioxide (TeO2) nanoparticles have
attracted much attention lately both in fundamental and
and Weng [9] have found the conditions to produce TeO2
powders from tellurium alkoxides by precipitation.
In this Letter, we present a simple method for the synthe-
application research areas [1–3], because TeO have various
2
outstanding physical properties. For example, TeO -based
sis of TeO nanoparticles by directly heating Tellurium (Te)
2
2
glasses manifest remarkable dielectric properties including
granule samples in an oxygen microwave-plasma torch-
flame at atmospheric pressure. Atmospheric microwave
plasmas generally produce a high-temperature plasma flame
due to ohmic heating by high frequency electric fields,
revealing a maximum central temperature of 6500 ± 350 K
[10] and atomic and molecular oxygen of high density [11].
Therefore, from the synthesis standpoint of nanoparticles,
such characteristics are favorable to instantaneously evapo-
rate metal and explosively oxidize metal vapors. We also
remind the reader that there is no earlier research work for
very high hyper-polarization [4]. Also, TeO glasses are less
2
hygroscopic than borate and phosphate glasses [5,6]. The
TeO -based glasses are also very promising for non-linear
2
optical device due to their high non-linear refractive indi-
ces, which could be 100 times more than that of SiO [7].
2
So far, pure tellurium oxide glasses present the highest
non-linear index. However, it is very difficult to obtain
TeO glasses under conventional process of melting and
2
air-quenching mechanism. Only small amounts of pure
TeO glass and powder samples have been successfully pre-
synthesis of TeO nanoparticles by directly heating Te gran-
2
2
pared by melting it at 800 ꢁC and then quenching it in a
freezing mixture consisted of ice, ethanol and NaCl at tem-
peratures less than ꢀ11 ꢁC [8]. Therefore, the sol–gel pro-
ule samples in the microwave plasma flame.
A schematic diagram for the preparation of TeO nano-
particles is shown in Fig. 1. The microwave synthesis sys-
tem has been presented elsewhere [12–16]. As shown in
2
cess has been investigated in order to produce pure TeO
2
amorphous materials at low temperature, without melting
mechanism. Until now, only few investigative researches
of sol–gel process have been done. However, Hodgson
Fig. 1, the synthesis system of TeO nanoparticles consists
2
mainly of the microwave plasma torch operated at
2.45 GHz and gas feeders controlled by mass flow control-
lers (MFCs). A microwave generator supplies the reactive
energy to the flowing gas at the atmospheric pressure. An
efficient power transfer through the microwave from the
*
Corresponding author. Fax: +82 31 219 2969.
E-mail address: hsuhm@ajou.ac.kr (H.S. Uhm).
0
009-2614/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.08.026

S.C. Cho et al. / Chemical Physics Letters 429 (2006) 214–218
215
2 2
Fig. 1. Schematic diagram showing the experimental set-up for preparation of TeO nanoparticles by O microwave plasma flame.
microwave generator to the reactive gas is achieved
through a matching network, which basically consists of
an isolator, directional coupler, and three stub tuner. The
center axis of quartz tube (170 mm inner diameter,
to 5 lpm, the length and diameter of the plasma torch-flame
increases and then an explosive oxidation reaction of Te
occurs owing to the contact between Te granules and the
plasma torch-flame. Namely, Te granules in solid melt
immediately and are vaporized by the O2 microwave-
1
.5 mm thickness, and 500 mm length) is located at 1/4
wavelength from the short end of the waveguide where
the induced electric field is peaked before the plasma is ini-
tiated and is perpendicular to the wide waveguide walls.
The plasma initiation is accomplished by inserting a tung-
sten wire into the quartz tube in the waveguide and spark-
ing in it. The microwave-induced electric field inside the
quartz tube can be maximized by adjusting the three stub
tuner. This produces a plasma torch with high temperature
plasma torch-flame. The TeO nanoparticles are synthe-
2
sized by oxidation reactions of the Te vapors and oxygen
*
radicals (O ) produced in pure O plasma. It may take a
2
few seconds that TeO nanoparticles are formed in the
2
microwave plasma torch and are deposited in the inner wall
of the quartz tube.
X-ray diffraction (XRD) patterns were obtained on a
RIGAKU 12KW diffractometer, using Cu Ka radiation
1
3
3
ꢀ1
(
6500 ± 350 K) and high plasma density (ꢁ10 /cm ).
at a scan rate of 0.02ꢁ 2h s . In order to observe the sur-
Therefore, the microwave plasma torch provides a highly
unusual and reactive chemical environment in which sev-
eral plasma-molecular reactions occur.
face morphology and structure of our samples obtained,
a scanning electron microscope (SEM) and a transmission
electron microscopy (TEM) were employed. The Bru-
nauer–Emmett–Teller (BET) surface area of the as-pro-
duced sample was analyzed by nitrogen adsorption in a
Micromertics ASAP 2010 nitrogen adsorption apparatus.
The BET surface area was determined by the multipoint
BET method using the adsorption data. The pore size dis-
tribution from the desorption branch of the isotherm was
calculated, according to the method developed by Bar-
rett–Joyner–Halender (BJH) [17].
Fifteen liters per minute (lpm) O as a swirl gas injected
2
to the tangential direction of the quartz tube upstream via
four small holes (swirl gas inlets) in Fig. 1 keeps the plasma
flame off the inside walls of the quartz tube, confining the
plasma to the center of the tube. In other words, O swirl
2
gas plays pivotal roles not only in plasma stabilization by
vortex flows but also in chemical reactions by providing
radicals and ions. Te granules (99.9%, Aldrich) of 2 mm
average size were placed inside the quartz tube, in which
a high-temperature oxygen plasma flame is generated.
Actually, as the swirl gas flow increases, the diameter and
length of the microwave plasma decrease [16]. Fifteen
Fig. 2 shows the selected time sequence images of the
plasma emission color during the synthesis of TeO nano-
2
particles. The images were captured by a digital camera
at 16 Hz frame rate. Fig. 2a is the picture of microwave
lpm O as a swirl gas was injected into the microwave
plasma operated at 15 lpm O and 1 kW plasma power.
2
2
plasma torch initiated. Te granules placed inside the quartz
tube are not in contact with the plasma torch-flame gener-
Once the synthesis of TeO nanoparticles starts by the con-
2
tact between Te granules and the plasma flame, as shown in
Fig. 2b–e, a pale blue light like a flesh is emitted from the
torch flame. Fig. 2f shows the deposits inside the quartz
ated at flow rate of 15 lpm O and do not melt yet. Remem-
2
ber that Te has a melting point of ꢁ722 K and a boiling
point of ꢁ1263 K. However, when O flow rate decreases
tube after synthesis finish, revealing a typical TeO powder.
2
2

216
S.C. Cho et al. / Chemical Physics Letters 429 (2006) 214–218
Fig. 2. Selected time-sequence images of the color of the microwave-plasma torch-flame during the synthesis of TeO
2 2
: (a) Plasma flame at 15 lpm O and
1
kW plasma power. (b–e) Plasma emissions showing explosive oxidation-reactions between plasma and Te species. (f) Deposits inside the quartz tube
after synthesis finish. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
All the XRD reflections in Fig. 3a can be indexed to
the standard powder pattern for the pure sphere phase of
TeO [space group: p4 2 2 (92)] with lattice constants a =
2
1 1
˚ ˚
4
.808 A, c = 7.612 A, which are in good agreement with
the reported data (JCPDS file No. 84-1777) [18]. Apart
from a small peak of residual Te in Fig. 3a, most of the
Te granules close to 100% were converted to TeO . The
2
strong intensities relative to the background signal indicate
the high purity of the TeO sphere phase of the resulting
2
products. The Willamson–Hall method [19] can be used
to determine the crystallite size and lattice strain when
there are three more reflections available for measurement.
Contributions of the crystallite size and lattice strain are
given by the following equation:
kk
b cos h ¼ L þ g sin h
ð1Þ
´
˚
where k is the wavelength of the X-rays (k = 1.54 A in this
experiment), h is the diffraction angle, g is the lattice strain,
L is the crystallite size, k is a constant (0.94 for GAUSSIAN
line profiles and small cubic crystals of uniform size), and
b is the full width at half maximum (FWHM). In Eq. (1),
a plot of bcosh vs sinh yields a straight line with a slope
g and intercept of kk/L. Fig. 3b shows a Williamson–Hall
plot using XRD data in Fig. 3a. The crystallite size was
determined to be approximately 112 nm from the intercept
in Fig. 3b the and the almost flat slope indicates minimal
ꢀ
3
strain (g = 1.3 · 10 ) in the powder.
The SEM image in Fig. 4a shows a group of the TeO2
clusters aggregated with several particles composed of a
few hundreds of nanometers in size. The aggregates were
made in the plasma region due to a high-temperature
2
Fig. 3. (a) XRD pattern of the as-produced TeO nanoparticles. (b)
Williamson–Hall plot of bcosh vs sinh for XRD pattern corresponding to
(a).

S.C. Cho et al. / Chemical Physics Letters 429 (2006) 214–218
217
Fig. 4. SEM and TEM images of the as-synthesized TeO
2
nanoparticles: (a, b) The SEM images showing aggregated and sphere-shaped TeO
2
particles,
respectively. (c) The TEM image corresponding to the SEM image of (a). (d) A close-up image of a rectangular box in (c). (e) The TEM image
corresponding to the SEM image of (b). (f) A magnified view of a rectangular box in (d).
plasma flame and a low melting point (ꢁ1000 K) of TeO
ual layers of 0.456 nm and 0.266 nm thicknesses with high
crystallinity, respectively. The findings means that two
independent particles are aggregated each other. Fig. 4e
2
[
2], relatively. However, Fig. 4b shows the TeO nanopar-
2
ticles near perfect sphere in the range of 37.5–340 nm.
The TeO nanoparticles with sphere shape were taken from
shows the TEM image of TeO nanoparticles with sphere
2
2
the region away from the plasma flame, a relatively cool
region, showing a quenching effect. Fig. 4c is the TEM
image of aggregated TeO2 particles corresponding to
Fig. 4a. Fig. 4d is a magnified view of a part marked with
a rectangular box in Fig. 4c, revealing well-ordered individ-
shape corresponding to Fig. 4b. As shown in the SEM
image of Fig. 4b, near perfect TeO spheres were observed.
2
During TEM measurement, however, we observed that
TeO nanoparticles were damaged severely and were aggre-
2
gated by electron beam. A large TeO sphere (ꢁ180 nm in
2

218
S.C. Cho et al. / Chemical Physics Letters 429 (2006) 214–218
diameter) in center part of Fig. 4e was attached with two
adsorption, demonstrating that the TeO powder contains
2
small TeO spheres due to electron beam. The average par-
micropores. However, at the relative pressure P/P in the
2
0
ticle size measured from Fig. 4e was approximately
range of 0.82–1.0, there is a close hysteresis loop, which
can be observed in the pores with narrow necks and wider
bodies such as an ink-bottle, and can be associated with the
aggregates of particles.
1
08.2 nm consistent with the average particle size estimated
from the Williamson–Hall equation in Fig. 3b. Fig. 4f is a
magnified view of a rectangular box (d) in Fig. 4e, revealing
well-ordered layers of 0.167 nm thickness. In the synthesis
method by evaporation of metal in flame, metallic Te may
In summary, TeO nanoparticles have been synthesized
2
by directly heating Te granules in the O2 microwave-
plasma torch-flame at atmospheric pressure. The mean size
be located at the core of the TeO or form separate particle.
2
From energy dispersive X-ray (EDX) spectrum not shown
in this Letter, Te and oxygen species were detected with
atomic fraction of 66.6% oxygen and 33.4% tellurium con-
sistently. If the small amount of Te in Fig. 3a is ignored, we
expect that the as-produced sample is the oxide particle
close to 100%. Therefore, metallic Te is not incorporated
into the core of the oxide particle but exists as separate
particle.
of the synthesized TeO particles was 108.2 nm observed
2
from the TEM image and was 112.0 nm estimated by the
Williamson–Hall plot using XRD data. From the BET
measurements, the sample also showed the specific surface
2
area of about 5.75 m /g, porosity of 53.2%, pore volume of
3
0.011 cm /g, and pore size of 7.17 nm. Although the syn-
thesis approach by the microwave plasma torch in the
experiment is not finely controlled, this synthesis method
may be suitable for the direct continuous preparation and
In order to characterize the surface area and pore distri-
bution properties, the as-produced TeO2 nanoparticles
were analyzed by a nitrogen adsorption method. The as-
mass production of TeO nanoparticles.
2
produced sample had the specific surface area of about
Acknowledgements
2
5
.75 m /g at P/P = 0.2, porosity of 53.2%, pore volume
0
3
of 0.011 cm /g at P/P = 0.97, and pore size of 7.17 nm.
0
We acknowledge the financial support from the Nuclear
Fusion Research Center (NFRC) in Korea and BK 21 pro-
gram in part. TEM data were obtained from CCRF in
Sungkyunkwan University.
Fig. 5 shows the pore size distribution curve calculated
from the desorption branch of nitrogen isotherm by the
BJH method using the Halsey equation [17,20]. The TeO2
powders in Fig. 5 reveal tri-modal pore size distributions
consisting of mesopores (<20 nm), macropores (about
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of types I and IV of adsorption isotherms. At a very-low
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