Conditions of hydrochemical synthesis, composition, and structure of tellurium films

Article abstract of DOI:10.1134/S1070427212050084

Conditions of the hydrochemical synthesis of thin tellurium fi lms by reduction of an aqueous solution of tellurous acid with hydrazine were determined. The redox potentials of tellurous acid and hydrazine and their difference in relation to pH of the med

Full text of DOI:10.1134/S1070427212050084

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 5, pp. 731−735. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © L.N. Maskaeva, I.V. Zarubin, E.G. Vovkotrub, V.F. Markov, 2012, published in Zhurnal Prikladnoi Khimii, 2012, Vol. 85, No. 5,
pp. 729−734.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Conditions of Hydrochemical Synthesis, Composition,
and Structure of Tellurium Films
a
a
b
a
L. N. Maskaeva , I. V. Zarubin , E. G. Vovkotrub , and V. F. Markov
a
Yeltsin Ural Federal University, Federal State Autonomous Educational Institution of Higher Professional Education,
Yekaterinburg, Russia
Institute of High-Temperature Electrochemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg, Russia
b
e-mail: ivan-carevich85@mail.ru
Received April 18, 2011
Abstract—Conditions of the hydrochemical synthesis of thin tellurium films by reduction of an aqueous solution
of tellurous acid with hydrazine were determined. The redox potentials of tellurous acid and hydrazine and their
difference in relation to pH of the medium and concentration of tellurite ions were calculated. The kinetics of
tellurium layer growth on glass-ceramic substrates and the structure, composition, and morphology of the layers
were studied.
DOI: 10.1134/S1070427212050084
Thin tellurium films are widely used for optical
information recording, in γ-radiation detectors, and for
fabrication of solar batteries and field-effect transistors
mixture. Previously published data on the deposition
of elemental tellurium and tellurides are restricted only
to various formulations of deposition baths [6, 7]. Data
on the chemical mechanism of processes leading to the
tellurium formation are scarce and contradictory [8].
[1, 2]. There is no common opinion on the structure of
tellurium films. Some authors prove that tellurium can
exist only in the crystalline form [3], whereas other
authors admit the existence of amorphous tellurium
The goal of this study was to examine regular trends
in reduction of TeIV compounds to Te with hydrazine,
to predict the conditions of the hydrochemical synthesis
of nanostructured tellurium films, and to determine the
composition, structure, and electrophysical properties of
the films.
0
[4]. The specificity of the structure is responsible for
the dependence of many electrophysical properties of
tellurium on the voltage applied. Various methods for
preparing elemental tellurium films have been described
[3]:sublimation, distillation, thermalvacuumdeposition,
and molecular-beam epitaxy; particles of hexagonal
tellurium can also be grown using microorganisms
EXPERIMENTAL
[
5]. However, preparation of the high-quality material
The investigation objects were tellurium films
prepared by the hydrochemical procedure from reaction
mixtures containing potassium tellurite using a solution
of hydrazine hydrochloride as reductant. The pH was
adjusted with alkali or nitric acid and was monitored with
a pH meter. As substrates we used 30 × 24 mm plates
of ST-150-1 grade glass-ceramic. The substrates were
degreased with a hot (323–343 K) solution of chromic
mixture and etched in dilute (1 : 20) hydrofluoric acid.
remains an intricate program.
A promising method for preparing tellurium,
taking into account its colloid-chemical nature, is
hydrochemical deposition. The method does not require
sophisticated equipment, is simple in implementation,
and allows the conditions of low-temperature synthesis
to be widely varied. However, implementation of this
method requires knowledge of the conditions under
which elemental tellurium is formed in the reaction
7
31

732
MASKAEVA et al.
Then the substrates were placed in a molybdenum glass
reactor. The reactor was arranged in a U-10 thermostat.
The temperature was maintained with an accuracy of
The hydrochemical synthesis of tellurium films
is based on reduction of tellurite ions to elemental
tellurium. Tellurous acid in aqueous solution dissociates
–
2–
±
0.1 K. In all the experiments, the reactants were added
to form НТеО and ТеО ions (pk'Te = 6.29, pk''Te
=
3
in a strictly definite order.
9.43) [10].
The synthesized tellurium films were heat-treated
in air in a PM-1.0-7 electric furnace at 593 K. The
temperature in the zone of the specimen arrangement
was maintained with an accuracy of ±2 K. The furnace
with tellurium film specimens placed in it was heated
to the required temperature and then switched off
and allowed to slowly cool to room temperature. The
working temperature was attained, on the average,
within 15–20 min, and the cooling lasted for 12–14 h.
The analytical concentration of tellurium in solution
is equal to the sum of the concentrations of its species:
–
2–
c = [H TeO ] + [НТеО ] + [ТеО ].
Те
2
3
3
3
The fraction of each species depends on pH and can
be calculated from the ionic equilibria. The tellurous
acid speciation in relation to pH is shown in Fig. 1.
The solubility of tellurous acid is strongly influenced
by pH. In aqueous solutions at рН from 0.7 to 8.7,
a voluminous white precipitate of tellurium(IV)
X-ray diffraction studies were performed with
IV
oxide TeO ·nH O is formed, which affects the Te
2
2
a DRON-UM1 diffractometer using CuK radiation (λ =
α
concentration in solution. Therefore, when calculating
the pH dependence of the concentrations of various
tellurous acid species, it is necessary to take into
account data on the solubility of tellurium(IV) oxide.
According to [11], the solubility of TeO ·nH O is
1
.15418 Å) at room temperature in the 2θ range 20°–80°
in the step-by-step scanning mode with 0.04° step and
0-s signal accumulation in each step. The structural
1
parameters of the synthesized tellurium films were
refined by the Rietveld method of full-profile analysis
using FULLPROF program [9].
2
2
minimal at pH from 4 to 5. The pH dependence of the
IV
Te concentration, taking into account the TeO ·nH O
2
2
The film composition was examined by laser Raman
spectroscopy. Studies were performed with an InVi,
Renishaw-1000 Raman microspectrometer. The spectra
were recorded with a Mole microprobe analyzer (argon
laser with a power of up to 600 mW, λ = 514.5 nm).
Electron microscopic images of tellurium films were
taken with a JEOL JSM-5900 LV scanning electron
microscope.
solubility, is shown in Fig. 2.
Among reagents capable to reduce tellurous acid to
0
Te , hydrazine is advantageous in that its use practically
excludes contamination of the films with decomposition
products. Aqueous solutions of hydrazine exhibit basic
properties. Hydrazine is predominantly a monoacidic
+
base [12, 13] forming the N H ion in acid solutions.
2
5
The total concentration of hydrazine с in solution can
h
+
Photoelectric properties of tellurium films were
be expressed as с = [N H ] + [N H ]. The fraction
h
2
5
2
4
studied in accordance with GOST (State Standard)
of each hydrazine species also depends on pH. The
hydrazine speciation in relation to pH is shown in Fig. 3.
As seen from Fig. 3, at pH 5–10 both hydrazine species
coexist in the solution.
1
7782–79 using a K 54.410 measurement installation.
Fig. 1. Influence of pH on tellurous acid speciation. (α) Frac-
Fig. 2. Influence of pH on the relative content сТе of soluble
−
2–
tion of species. (1) Н ТеО , (2) НТеО , and (3) ТеО .
2
3
3
3
Te species.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 5 2012

CONDITIONS OF HYDROCHEMICAL SYNTHESIS
733
In our study we considered two schemes of TeIV
reduction with hydrazine in acid and alkaline solutions,
i.e., the reactions involving molecular and ionic
hydrazine species. The Te reduction to Te can be
described by the equations
were examined by X-ray phase analysis, electron
microscopy, and laser Raman spectroscopy.
Figure 5 shows an X-ray diffraction pattern of a Te
film deposited from the reaction mixture of the above-
given composition. In the X-ray diffraction pattern, we
identified only the hexagonal structure of the A8 type,
characteristics of tellurium, and reflections of the glass-
ceramic substrate [main component: titanium(IV) oxide].
IV
0
2
–
+
+
ТеО + Н + N H = N ↑ + Te↓ + 3Н О ,
3
2
5
2
2
2
–
–
ТеО3 + N H = Те↓ + N ↑ + Н О + 2OH .
2
4
2
2
The interplanar spacings of the tellurium crystal
lattice in the film prepared from the base composition
are given in the table in comparison with the reference
data. As can be seen, the interplanar spacings in the
structure of the deposited tellurium films practically
coincide with the reference data for the Te single crystal,
suggesting high ordering of the crystal structure.
The possibility of the occurrence of a redox reaction
can be judged from the emf, which depends on the reac-
tant concentrations, pH, temperature, and other factors.
The difference between the redox potentials φ
φTe/TeO3 depends on pH and concentration of tellurite
and
N /N H
2 4
2
2
–
ions as follows:
By the Rietveld method of full-profile analysis, using
the FULLPROF program, we determined the unit cell
parameters: а = 0.44535, b = 0.44535, с = 0.59283 nm.
These values are well consistent with the reference
data: а = 0.4457, b = 0.4457, с = 0.59290 nm [13]. The
2
–
φTe/TeO 32 – – φ
= 0.278 – 0.03pH + 0.0151log [TeO ].
3
N /N H
2 4
2
The results of calculating the potentials of hydrazine
and of tellurite ions and the potential difference for this
couple are shown in Fig. 4. Thermodynamic analysis
IV
0
shows that the Te reduction with hydrazine to Te is
possible in the entire pH range. The emf of the reduction
reactions is positive and varies from 0.486 to 0.676 V.
The dependence has a maximum at pH 9. Hence,
deposition of elemental Te from aqueous solutions with
hydrazine is most probable in alkaline solutions. Taking
into account the parameters affecting the film thickness,
we obtained the optimal composition of the reaction
φ, V
(a)
2−
−2
mixture: [TeO ] = 2 × 10 M, [N H ·2HCl] = 1 M.
3
2
4
The hydrochemical synthesis from alkaline tellurite
solutions allowed us to prepare 240 to 300 nm thick
elemental tellurium layers on glass-ceramic substrates.
The tellurium layers synthesized from this composition
φ, V
(b)
Fig. 4. Influence of pH on the potentials φ of (1) hydrazine,
, and (2) tellurite ions, φTe/TeO 32 –, and on the (3)
φ
N /N H
2 4
2
Fig. 3. Influence of pH on the fraction α of molecular and ionic
difference between them for the reactions involving (a) N H
2
4
+
hydrazine species. (1) α
and (2) α
.
+
N H
N H
and (b) N H .
2 5
2
5
2
4
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 5 2012

734
MASKAEVA et al.
I, %
Glass-ceramic
ν, cm–1
2
θ, deg
Fig. 6. Raman spectrum of a Te film freshly deposited from the
reaction mixture of the base composition. Substrate: ST-150-1
glass-ceramic. (I) Intensity and (ν) wavenumber.
Fig. 5. X-ray diffraction pattern of a Te film. (I) Intensity and
2θ) Bragg angle.
(
reflection half-width was 0.283.
After the annealing, the Te unit cell parameters were as
follows: а = 0.44945, b = 0.44945, с = 0.58862 nm, i.e.,
the parameters а and b increased, whereas с decreased.
The presence of elemental tellurium on the substrates
is confirmed by the Raman spectra (Fig. 6) exhibiting
peaks characteristic of the trigonal tellurium structure
The reflections in the X-ray diffraction patterns of
the heat-treated Te layers are considerably less intense
and are appreciably broadened (0.350) relative to the
freshly deposited films (0.283). The reflection broadening
and an increase in the unit cell parameters may be due to
the following facts: small size of coherent scattering do-
mains (to a first approximation, small crystallite size) and
appearance of stresses and microstrains in the specimen.
–
1
[14, 15]. The peaks at 441, 605, 169, and 243 cm be-
–
long to the glass-ceramic substrates. At 667 and 433 cm
1
,
there are smeared peaks belonging to tellurium(IV)
oxide. The X-ray diffraction data for the films suggest
that tellurium(IV) oxide present in them is X-ray amor-
phous.
The freshly deposited Te films are not photosensitive.
For their sensitization, they were heat-treated at 593 K.
The results of electron-microscopic examination of
Interplanar spacings d of the Te crystal lattice for the film deposited from the reaction mixture of the base composition onto
ST-150-1 glass-ceramic in comparison with ASTM data [10]
Reflection
hkl
ICDD reference data
our data
d , nm
2θ, deg
d , nm
2θ, deg
hkl
hkl
1
00
01
12
00
3.8600
3.2349
2.3511
2.2286
2.0861
1.9300
1.8352
1.7814
1.6174
23.022
27.551
38.250
40.442
43.339
47.046
49.635
51.242
56.881
3.8545
3.2306
2.3435
2.2257
2.0833
1.9292
1.8326
1.7838
1.6174
23.055
27.588
38.378
40.497
43.398
47.067
49.711
51.165
56.883
1
0
1
1
11
200
021
112
2
02
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 5 2012

CONDITIONS OF HYDROCHEMICAL SYNTHESIS
(b)
735
(a)
1
μm
1
μm
Fig. 7. Electron micrograps of a Te film, (a) freshly deposited and (b) heat-treated at 593 K.
the films obtained are of indubitable interest. Figure 7
shows the electron micrographs of the freshly deposited
and heat-treated (593 K) Te films. Comparison of the
images shows that the film morphology changes upon
annealing. As can be seen, the freshly deposited film
is formed of crystallites whose size does not exceed
at 593 K exhibit photosensitivity to IR radiation.
REFERENCES
1
2
3
4
5
6
. Goldobin, A.N. and Lezheiko, L.V., Fiz. Tverd. Tela,
1
961, vol. 3, no. 10, pp. 3247–3249.
. Slavnov, A.G., Izv. Vyssh. Uchebn. Zaved., Fiz., 1967,
vol. 3, pp. 128–130.
1
00 nm. It should be noted that the Te microcrystals are
well-faceted rectangular particles of length 0.2–0.5 and
width 0.1–0.2 μm, fairly uniformly distributed on the
substrate. The mean Te crystallite size increased upon
annealing by a factor of ~2 owing to coarsening as
a result of recrystallization.
. Novoselova, A.V., Redkie metally i ikh primenenie (Rare
Metals and Their Use), Moscow: Znanie, 1966.
. Stishov, S.M. and Tikhomirova, N.A., Zh. Eksp. Teor.
Fiz., 1965, vol. 4, no. 2, pp. 618–620.
. Tucker, F. and Walper, J., J. Bacteriol., 1961, vol. 83,
pp. 1313–1314.
The dark resistance of the Te film after the heat
treatment increased from 35 to 1400 kΩ.The dark current
and photocurrent of the heat-treated 300 nm thick Te
. Kulifay, S.M., J. Am. Chem. Soc., 1961, vol. 83, no. 24,
p. 4916.
–
5
–5
film were 10 × 10 and 11.8 × 10 А, respectively. The
7
8
. US Patent 3058802.
ratio of the photocurrent to the dark current was about 2.
. Belyaev, A.I. and Berchenko, M.A., Zh. Neorg. Khim.,
1
970, vol. 15, p. 2005.
CONCLUSIONS
9
. Rietveld, H.M., J. Appl. Crystallogr., 1969, vol. 2,
pp. 65–71.
(
1) Calculation of the redox potentials in the tellurous
1
0. Lur’e, Yu.Yu., Spravochnik po analiticheskoi khimii
Handbook of Analytical Chemistry), Moscow: Khimiya,
971.
1. Cheng, K.L., Anal. Chem., 1961, vol. 33, no. 6, pp. 761–
64.
acid–hydrazine system at various pH values and tellurite
ion concentrations shows that the reduction of tellurite
ions is most probable in alkaline solutions.
(
1
1
1
(2) Elemental tellurium films 240 to 300 nm thick on
7
glass-ceramic substrates were prepared for the first time
by hydrochemical deposition.
2. Audrieth, L.F. and Ogg, B.A., The Chemistry of
Hydrazine, New York: Wiley, 1951.
(3) The unit cell parameters, composition, and
13. ASTM X-ray Diffraction Data Cards, Philadelphia, 1968,
morphology of the hydrochemically deposited
elemental tellurium films were determined by X-ray
phase diffraction, electron microscopy, and Raman
spectroscopy.
no. 6–0354.
14. Artamonov, V.V., Baidullaeva, A., and Vlasenko, A.I.,
Fiz. Tverd. Tela, 2004, vol. 46, no. 8, pp. 1489–1493.
15. Belogorokhov, A.I., Denisov, I.A., and Smirnova, N.A.,
Fiz. Tekh. Poluprovodn., 2004, vol. 38, no. 1, pp. 84–93.
(4) The chemically deposited Te layers heat-treated
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 5 2012