Preparation and characterization of BaO-TeO2 thin films obtained from tellurium(VI) alkoxide by a sol-gel method

Article abstract of DOI:10.1111/j.1551-2916.2009.03280.x

BaO-TeO₂ thin films were prepared from tellurium(VI) alkoxide by a sol-gel method and their structure was investigated by X-ray diffractometry, Fourier transform infrared spectrometry, and ¹²⁵Te static nuclear magnetic resonance. The

Full text of DOI:10.1111/j.1551-2916.2009.03280.x

J. Am. Ceram. Soc., 92 [11] 2619–2622 (2009)
DOI: 10.1111/j.1551-2916.2009.03280.x
r 2009 The American Ceramic Society
ournal
J
Preparation and Characterization of BaO–TeO2 Thin Films Obtained
from Tellurium(VI) Alkoxide by a Sol–Gel Method
Hiroshi Ikeda, Shigeru Fujino,w and Toshihisa Kajiwara
Department of Chemical Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan
BaO–TeO2 thin films were prepared from tellurium(VI) alkox-
ide by a sol–gel method and their structure was investigated by
X-ray diffractometry, Fourier transform infrared spectrometry,
and 125Te static nuclear magnetic resonance. Their crystalliza-
tion temperature (Tc), optical transmittance, and dielectric con-
stant were measured, and their refractive index was calculated
from the transmission spectra. The results indicate that the
BaO–TeO2 thin films were composed of TeO6 and TeO4 units,
and had a Tc of B5201C, refractive index of B1.79, dielectric
constant of B20. These films had a Tc higher than the glass
prepared by a melt-quench method, but their refractive index
and dielectric constant were lower. These differences may be due
to differences in their structural units.
II. Experimental Procedure
Figure 1 shows the flow chart of sample preparation using tel-
lurium(VI) alkoxide. Tellurium(VI) alkoxide was synthesized.12
A clear and viscous liquid was prepared by heating 5 g of or-
thotelluric acid (H6TeO6, purity 499%, Soekawa Chemical,
Tokyo, Japan) in a sealed silica tube at about 1501C for 12 h.
After adding 15 mL of ethanol (C2H5OH, purity 499.5%,
Wako Pure Chemical Industries, Tokyo, Japan) and 15 mL of
benzene (C6H6, purity 499.5%, Wako Pure Chemical Indus-
tries) to the liquid, the solution was refluxed for 4 h at 651C in
air, and then distilled at around 801C. After repeated the reflux
and distillation processes three times, an ethanol solution of tel-
lurium(VI) alkoxide was obtained.
Tellurium(VI) alkoxide was added to a barium acetate
(Ba(OCOCH3)2, purity 499.9%, Wako Pure Chemical Indus-
tries) aqueous solution with a molar ratio of Te:Ba5 8:2. After
stirring the precursor solution at room temperature for 6 h in
air, a wet gel was generated. A dried gel was obtained by keeping
the wet gel in an oven for 12 h at 1001C. The thermal behavior of
the gel was studied by thermogravimetry-differential thermal
analysis (TG-DTA; EXTER6000, Seiko Instrument) with a
heating rate of 51C/min. 20BaO–80TeO2 powders were obtained
by heat treating the gel at 1001, 3001, 4501, and 5501C for 10 min
in air, respectively. The structure of the 20BaO–80TeO2 powders
were analyzed by X-ray diffractometry (XRD; CuKa,
RINT2200, Rigaku, Tokyo, Japan) with a scanning speed of
21/min and Fourier transform infrared spectrometry (FT-IR;
FT/IR-550, Jasco, Tokyo, Japan) using KBr pellet method. To
compare the structure and properties, 20BaO–80TeO2 glass was
prepared by the melt-quench method,2 and its structure was an-
alyzed by FT-IR. 125Te static nuclear magnetic resonance
(NMR; CMX-400, Chemagnetics, Tokyo, Japan) spectra of
the sample prepared by the sol–gel method were measured at
126.3 MHz. The crystallization temperature was measured by
DTA with a heating rate of 51C/min. The crystalline phase of
the sample heat treated at 4301C for 10 min was
examined by XRD.
I. Introduction
ELLURITE glasses have attracted interest due to their excellent
thermal, optical, and electrical properties, such as a low
T
glass transition temperature, high refractive index, wide-band
infrared transmittance, and high dielectric constant.1–8 Tellurite
glasses are promising materials for the photonic switches and
highly efficient ultrabroadband fiber Raman amplifiers because
of their high third-order nonlinear optical susceptibility5 and
higher raman gain,6 respectively.
Bulk tellurite glasses are generally produced by a melt-quench
method.1–8 Because this method requires melting the starting
materials at high temperature, it is difficult to use it to fabricate
thin films. A sol–gel method that easily produces amorphous
phase materials at low temperature is, therefore, an attractive
alternative.
In previous studies, TeO29,10 and TiO2–TeO211 thin films were
prepared from tellurium(IV) alkoxides by a sol–gel method.
These tellurium(IV) alkoxides, e.g., Te(OCH(CH3)2)4 and
Te(OC2H5)4, are very sensitive to water. Therefore, the synthe-
sis of high-quality tellurium materials must be performed in an
inert atmosphere and requires the addition of chemical modifiers
to control the hydrolysis reaction. On the contrary, Weng et al.12
reported that TeO2 glass thin film can be prepared in an air at-
mosphere using tellurium(VI) alkoxide, which is stable in an air
atmosphere. However, there are no reports of the preparation of
multicomponent tellurite materials using tellurium(VI) alkoxide.
This paper reports the preparation of BaO–TeO2 glass thin
films in an air atmosphere using tellurium(VI) alkoxide. The
structure of BaO–TeO2 powders and films was investigated, and
their properties such as crystallization temperature, optical
transmittance, refractive index, and dielectric constant were
characterized.
Precursor solution
H TeO
sealed silica tube
at 150 C for 12h
Spin coated
Dried
Dried
C H OH + C H
Gel
Gel film
Reflux & Distill
Heat-treated at 100~550 C
Te(VI)alkoxide
J. Nino—contributing editor
powders
Thin films
Ba(OCOCH ) + H O
Manuscript No. 25620. Received December 11, 2008; approved June 19, 2009.
This work was financially supported by a grant from the Global-Centre of Excellence in
Novel Carbon Resource Sciences, Kyushu University.
Stirred at room
temperature
wAuthor to whom correspondence should be addressed. e-mail: fujino@chem-eng.
kyushu-u.ac.jp
Fig. 1. Flow chart of sample preparation.
2619

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Journal of the American Ceramic Society—Ikeda et al.
Vol. 92, No. 11
100
Melt-quench
DTA
95
770
90
85
630
TG
80
700
100
200
300
400
500
600
Fig. 2. Thermogravimetry and differential thermal analysis curves of
the 20BaO–80TeO2 dried gel in air at a heating rate of 51C/min.
BaO–TeO2 gel films were deposited on SiO2 glass and Pt sub-
strate by spin coating the precursor solution at 3000 rpm for 20 s.
The gel films were dried at 1001C for 30 min and heat treated at
1001, 4501, and 5501C for 10 min in air, respectively. Thicker BaO–
TeO2 films were obtained by repeating the spin coating process.
The 20BaO–80TeO2 thin films were examined by XRD, and
their surface morphologies of the films were observed by field-
emission scanning electron microscopy (FE-SEM; JSM-6340F,
JEOL, Tokyo, Japan). The film transmission spectrum of the
film was measured by using with ultraviolet visible spectropho-
tometer (UV-Vis; V-550, Jasco) in the range of 200–900 nm. The
film refractive index was determined from transmission spec-
trum following the method of Swanepoel.13 The dielectric con-
stant of the thin films deposited on a Pt substrate was
characterized with an impedance analyzer (HP4129, Hewlett
Packard, Tokyo, Japan) at room temperature. Al circular elec-
trode with a diameter of 1 mm was deposited by vacuum evap-
oration on the top of the thin film.
455
TeO6
TeO4
TeO3
800
605
770
660
720
1200
1000
800
Wavenumber (cm–1
600
400
)
Fig. 4. Fourier transform infrared spectra of the 20BaO–80TeO2 pow-
ders heat treated at 1001, 3001, 4501, and 5501C in air for 10 min.
phase change of the 20BaO–80TeO2 powder with heat-treated
temperatures up to 5501C was investigated by XRD and FT-IR
analyses, and the results are shown in Figs. 3 and 4, respectively.
From the XRD data (Fig. 3), it is obvious that the sample heat
treated at 4501C was amorphous, whereas the sample heat
treated at 5501C contained a-TeO2 and BaTe2O6 crystalline
phases. The FT-IR spectra of these samples (Fig. 4) show ab-
sorption bands at 455, 605, 720, and 800 cmÀ1 for the sample
heat treated at 1001C, which may be assigned to the vibration of
Te–O bond in the TeO6 units with an octahedral structure.12,14
In addition, the bands at 660 and 770 cmÀ1 are assigned to
the asymmetrical stretching vibration of Te–Oax bonds and the
symmetrical stretching vibration of Te–Oeq bonds in the TeO4
III. Results and Discussion
(1) Structure of BaO–TeO2 Materials Prepared from
Tellurium(VI) Alkoxide
Figure 2 shows TG and DTA curves of 20BaO–80TeO2 dried
gel. The exothermic peak at B2301C is attributed to the de-
composition and combustion of organic compounds remaining
in the gel, and the one at 5201C is considered to be associ-
ated with the crystallization of a phase in the material. The
-TeO2
BaTe2O6
10
20
30
40
50
60
70
80
90
2000
1000
0
–1000
–2000
–3000
Chemical shift (ppm)
Fig. 5. 125Te static nuclear magnetic resonance spectrum of the sample
heat treated at 4501C.
Fig. 3. X-ray diffractometric patterns of the 20BaO–80TeO2 powders
heat treated at 1001, 3001, 4501, and 5501C in air for 10 min.

November 2009
Preparation and Characterization of BaO–TeO2 by a Sol–Gel Method
(b) (c)
2621
(a)
Fig. 6. Scanning electron microscopic images of the surface morphologies of 20BaO–80TeO2 thin films heat treated at (a) 1001C, (b) 4501C, (c) 5501C in
air for 10 min.
units with a trigonal bipyramidal structure, respectively.3,15 The
spectra of the samples heat treated at 3001 and 4501C also show
absorption bands that are characteristic of the TeO6 and TeO4
units. The absorption bands for the sample heat treated at 5501C
are stronger because of the phase transfer from an amorphous
to a crystalline phase. In contrast, the spectra of a glass pre-
pared by the melt-quench method exhibits absorption bands at
630 and 770 cmÀ1. According to studies of tellurite glasses,3,15 the
band at 630 cmÀ1 is assigned to the vibration of the Te–Oax bond
in the TeO3 and TeO4 units, and the band at 770 cmÀ1 is assigned
to the vibration of the Te–Oeq bond in the TeO4 units. Figure 5
shows 125Te static NMR spectrum of the sample heat treated at
4501C. There is a broad peak at À670 ppm which is due to six
coordinated tellurium oxygen.16 Therefore, the samples prepared
by the sol-gel method contain TeO6 units. In contrast, this peak
couldn’t be observed in spectra of a glass prepared by the melt-
quench method,17 which contain the TeO3 and TeO4 units.
It was concluded that the samples prepared using tellu-
rium(VI) alkoxide consist of the TeO6 and TeO4 units, and the
structure differs from structure of the glasses prepared by the
conventional melt-quench method.
(2) Characterization of BaO–TeO2 Thin Films
Figure 6 shows SEM images of the surface morphologies of
20BaO–80TeO2 thin films heat treated at 1001, 4001, and 5501C
for 10 min in air, and Fig. 7 shows XRD patterns of these films.
The results demonstrated that the 20BaO–80TeO2 thin films
heat treated at 1001 and 4501C were transparent and amorphous
with a smooth and homogeneous surface. On the contrary, the
film heat treated at 5501C was devitrified by crystallization and
rough surface.
BaTe2O6
BaTe2O5
Figure 8 shows the transmission spectrum of a thin film
(thickness B600 nm) heat treated at 4501C. The thin film was
transparent in the visible region. The transmission curve with
interference fringes indicate the thickness of the thin film was
uniform.13 The refractive index of the film, calculated from the
transmission spectra following the method of Swanepoel,13 was
about 1.79 at around 660 nm.
The dielectric constant and dielectric loss of the thin film heat
treated at 4501C are shown in Fig. 9 as a function of frequency.
The dielectric constant was about 19–22 in the frequency range
of 1 kHz–1 MHz. The dielectric constant was decreased with
increasing frequency; on the contrary, the dielectric loss was
increased.
10
20
30
40
50
60
70
Table I shows the structural units, crystalline phase and crys-
tallization temperature (Tc), refractive index, and dielectric
constant of the glasses prepared by the sol–gel and melt-quench
Fig. 7. X-ray diffractometric patterns of 20BaO–80TeO2 thin films heat
treated at 1001, 4501, and 5501C in air for 10 min.
100
30
20
10
0
10
SiO2 glass substrate
80
20B aO–80TeO2 thin film
60
40
20
0
5
0
1000
1
10
100
200
300
400
500
600
700
800
900
Frequency (kHz)
Wavelength (nm)
Fig. 8. Transmission spectrum of a 20BaO–80TeO2 thin film heat
treated at 4501C in air for 10 min.
Fig. 9. Dielectric constant and dielectric loss of a 20BaO–80TeO2 thin
film heat treated at 4501C in air for 10 min.

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Journal of the American Ceramic Society—Ikeda et al.
Vol. 92, No. 11
Table I. Structural Units, Tc and Crystalline Phase, Refractive Index, and Dielectric Constant of the Glasses Prepared by the Sol–Gel
and Melt-Quench Methods
Structural units
Tc, Crystalline phase
Refractive index
Dielectric constant
Sol–gel method
TeO4, TeO6
5201C
BaTe2O6, a-TeO2
3701C
1.79
(l 5 660 nm)
19–22
(1 k–1 MHz)
Melt-quench method
TeO4, TeO3
2.10 (Takebe et al.2)
(l 5 587.6 nm)
27.5 (Hampton et al.4)
(24BaO–76TeO2)
BaTe4O9, a-TeO2
methods. The glass prepared by the sol–gel method had a Tc of
B5201C and contained a-TeO2 and BaTe2O6 crystalline phases,
whereas that prepared by the melt–quench method had a Tc
of B3701C and contained a-TeO2 and BaTe4O9 crystalline
phases. In the previous work,14 the phase formation temperature
of BaTe2O6 composed of the TeO6 units18 was much higher
than that of BaTe4O9 composed of the TeO4 units. Therefore,
the TeO6 units are thermodynamically more stable relative to
the TeO4 units. This is the reason that the glass produced by the
sol–gel method had a higher Tc than the one by the melt-quench
method.
The BaO–TeO2 glass produced by the sol–gel method had
smaller refractive index and dielectric constant than that pro-
duced by the melt-quench method. The TeO4 unit has a trigonal
bipyramidal structure, in which one equatorial site of the Te sp3d
hybrid orbitals is occupied by a lone pair of electrons and the
other two equatorial sites are occupied by oxygen atoms.3,19 The
TeO3 unit has a trigonal pyramidal structure, in which one of
the Te sp3 hybrid orbitals is occupied by a lone pair of elec-
trons.3,19 The presence of these lone pairs leads to a high po-
larizability.20 Therefore, the tellurite glasses containing the TeO3
and TeO4 units have a high refractive index and high dielectric
constant. On the contrary, the TeO6 unit has a six-coordinated
tellurium(VI) oxygen with an octahedral structure, which does
not contained a lone pair of electrons and does not have a dis-
torted structure.21 Therefore, the polarizability of the TeO6 units
may be low relative to the TeO3 and TeO4 units. As a result, the
refractive index and dielectric constant of the glasses prepared
by the sol–gel method, which contain the TeO6 and TeO4 units,
are lower than those of the glasses prepared by the melt-quench
method, which contain the TeO3 and TeO4 units.
References
1H. Burger, W. Vogel, and V. Kozhukharov, ‘‘IR Transmission and Properties
of Glasses in the TeO2–[RnOm, RnXm, Rn(SO4)m, Rn(PO3)m and B2O3] Systems,’’
Infrared Phys., 25 [1–2] 395–409 (1985).
2H. Takebe, S. Fujino, and K. Morinaga, ‘‘Refractive-Index Dispersion of
Tellurite Glasses in the Region from 0.40 to 1.71 mm,’’ J. Am. Ceram. Soc., 77 [9]
2455–7 (1994).
3N. Mochida, K. Nakata, and S. Shibusawa, ‘‘Properties and Structure of
the Binary Tellurite Glasses Containing Mono- and Di-valent Cations,’’ Yogyo–
Kyokai–Shi, 86 [7] 317–25 (1978).
4R. N. Hampton, H. Wang, G. A. Saunders, and R. A. El-mallawany,
‘‘The Dielectric-Properties of Tellurite Glass,’’ Phys. Chem. Glasses, 29 [3] 100–5
(1988).
5S. H. Kim, T. Yoko, and S. Sakka, ‘‘Linear and Nonlinear-Optical Properties
of TeO2 Glass,’’ J. Am. Ceram. Soc., 76 [10] 2486–90 (1993).
6G. S. Murugan, T. Suzuki, and Y. Ohishi, ‘‘Tellurite Glasses for Ultra-
broadband Fiber Raman Amplifiers,’’ Appl. Phys. Lett., 86 [16] 161109, 3pp
(2005).
7Y. W. S. Dai, F. Chen, T. Xu, and Q. Nie, ‘‘Physical Properties and Optical
Band Gap of New Tellurite Glasses within the TeO2–Nb2O5–Bi2O3 System,’’
Mater. Chem. Phys., 113 [1] 407–11 (2009).
8H. M. M. Moawad, H. Jain, and R. El-Mallawany, ‘‘DC Conductivity of Silver
Vanadium Tellurite Glasses,’’ J. Phys. Chem. Solids, 70 [1] 224–33 (2009).
9L. Weng and S. N. B. Hodgson, ‘‘Sol–Gel Processing of Tellurite Materials
from Tellurium Ethoxide Precursor,’’ Mater. Sci. Eng., B87 [1] 77–82 (2001).
10A. Lecomte, E. Bamiere, S. Coste, P. Thomas, and J. C. Champarnaud-
Mesjard, ‘‘Sol–Gel Processing of TeO2 Thin Films from Citric Acid Stab-
ilised Tellurium Isopropoxide Precursor,’’ J. Eur. Ceram. Soc., 27 [2–3] 1151–8
(2007).
11L. Weng, S. N. B. Hodgson, and J. Ma, ‘‘Preparation of TeO2-TiO2 Thin
Films by Sol-Gel Process,’’ J. Mater. Sci. Lett., 18 [24] 2037–9 (1999).
12L. Weng, S. Hodgson, X. Bao, and K. Sagoe-Crentsil, ‘‘Achieving Control-
lable Sol–Gel Processing of Tellurite Glasses Through the Use of Te(VI) Precur-
sors,’’ Mater. Sci. Eng., B107 [1] 89–93 (2004).
13R. Swanepoel, ‘‘Determination of the Thickness and Optical-Constants of
Amorphous-Silicon,’’ J. Phys. E : Sci. Instrum., 16 [12] 1214–22 (1983).
14H. Sieberg, ‘‘Ultrarotspektren Von Tellursauren, Telluraten Und Antimona-
ten,’’ Zeitschrift Fur Anorganische Und Allgemeine Chemie, 301 [3–4] 161–70
(1959).
15M. Dimitrovapankova, Y. Dimitriev, M. Arnaudov, and V. Dimitrov, ‘‘In-
frared Spectral Investigation of the Influence of Modifying Oxides on the Structure
of Tellurite Glasses,’’ Phys. Chem. Glasses, 30 [6] 260–3 (1989).
16M. J. Collins and J. A. Ripmeester, ‘‘CP/MAS in Solids: An Example of
125Te35,37Cl J Coupling,’’ J. Am. Chem. Soc., 109 [13] 4113–5 (1987).
17S. Sakida, S. Hayakawa, and T. Yoko, ‘‘125Te NMR Study of MO–TeO2
(M5 Mg, Zn, Sr, Ba, and Pb) Glasses,’’ J. Ceram. Soc. Jpn., 107 [5] 395–402
(1999).
IV. Conclusions
BaO–TeO2 thin films were prepared in air from tellurium(VI)
alkoxide. The thin films heat treated at 4501C were transparent
in the visible region. Their refractive index was about 1.79 in
the transparent region, dielectric constant was about 20 between
1 kHz and 1 MHz, and Tc was about 5201C. These films had a
higher Tc and lower refractive index and dielectric constant than
the the glass prepared by the by melt-quench method. These
differences may arise from differences of their structural units.
18D. K. Kwon, M. T. Lanagan, and T. R. Shrout, ‘‘Microwave Dielectric Prop-
erties of BaO–TeO2 Binary Compounds,’’ Mater. Lett., 61 [8–9] 1827–31 (2007).
19E. F. Lambson, G. A. Saunders, B. Bridge, and R. A. El-mallawany, ‘‘The
Elastic Behavior of TeO2 Glass under Uniaxial and Hydrostatic-Pressure,’’ J. Non-
Cryst. Solids, 69 [1] 117–33 (1984).
20R. R. Reddy, Y. N. Ahammed, P. A. Azeem, K. R. Gopal, and T. V. R.
Rao, ‘‘Electronic Polarizability and Optical Basicity Properties of Oxide
Glasses Through Average Electronegativity,’’ J. Non-Cryst. Solids, 286 [3] 169–
80 (2001).
Acknowledgment
21M. Kocak, C. Platte, and M. Tromel, ‘‘Barium Hexaoxoditellurate(IV,VI)-
Oxygen Coordination Number 5 with Tetravalent Tellurium,’’ Acta Cryst, B35
The authors would like to thank Dr. Yomei Tokuda of Institute for Chem-
ical Research, Kyoto University for the NMR measurements and thoughtful
comments.
1439–41 (1979).
&