Electrodeposition of bismuth telluride films from a nonaqueous solvent

Article abstract of DOI:10.1016/j.electacta.2009.07.008

The author examines Bi₂Te₃ deposition from a DMSO solution containing TeCl₄ and Bi(NO₃)₃ × 5H₂O by means of cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). Accumu

Full text of DOI:10.1016/j.electacta.2009.07.008

Electrochimica Acta 54 (2009) 7167–7172
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrodeposition of bismuth telluride films from a nonaqueous solvent
Wen-Jin Li∗
Material and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 310, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 April 2009
Received in revised form 4 July 2009
Accepted 4 July 2009
Available online 14 July 2009
The author examines Bi2Te3 deposition from a DMSO solution containing TeCl4 and Bi(NO3)3 × 5H2O
by means of cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). Accumulated
charges and related mass changes for Bi2Te3 deposition on working electrodes are measured in situ. The
deposit composition is more dependent on Te4+ concentrations in DMSO solution than on the potential. In
a DMSO solution containing 0.01 M Te4+ and 0.0075 M Bi3+, Bi2Te3 deposits were obtained in the potential
range between −0.2 and −0.8 V vs. Ag/AgCl. In a DMSO solution containing 0.05 M Te4+ and 0.0375 M Bi3+
Te-rich deposits were formed from −0.2 to −0.8 V vs. Ag/AgCl.
,
Keywords:
Bi2Te3 film
EQCM
© 2009 Elsevier Ltd. All rights reserved.
DMSO
Electrodeposition
Cyclic voltammetry
1. Introduction
mechanism in aqueous solution, consisting of an 18-electron reduc-
tive reaction, has been proposed as follows [5]:
Bismuth telluride (Bi2Te3) and its doped derivative compounds
are attracting strong research interest because of their excellent
thermoelectric refrigeration characteristics for room temperature
applications. The thermoelectric properties of Bi2Te3 and its doped
derivative compounds are dependent on their composition. For
example, increasing the telluride concentration to more than
62.5 at.% can change Bi2Te3 polarity from p-type to n-type [1]; the
electrical conductivity for bulk form Bi2Te3 raises to a maximum
value if the ratio of Bi/Te is approach to 2/3 [2]. Accordingly, it is
important to control chemical composition in order to synthesize
bismuth telluride compounds with high Z values.
Electrodeposition technology is an easy and a low cost method
to obtain Bi2Te3 and its doped derivative compounds. Numer-
ous investigations have been reported for Bi2Te3 electrodeposition
[3–10]. Magri et al. have synthesized n-type Bi2Te3 films from acidic
aqueous solution using galvanostatic methods and report that Te
atom percentage in the compound decreases when the applied
current density is increased [3]. Miyazaki’s and Martin-González’s
groups have respectively prepared Bi2Te3 films on Ti and Pt sub-
strate using a potentiostatic process [4,5], and suggest that the
potential for forming p-type Bi2Te3 films is more negative than that
for n-type films.
3HTeO2+ + 9H+ + 12e− → 3Te + 6H2O
2Bi3+ + 3Te + 6e− → Bi2Te3
(1)
(2)
In acid aqueous solution, the solubility of tellurium and bismuth
is determined by the pH value. The working pH has be approxi-
mately zero in order to dissolve enough of the cationic species of Bi
and Te. But this condition is not good for the electrodeposition of
Bi2Te3 on several substrates. For example the electrodeposition of
Bi2Te3 nanowires from acidic solution into AAO template, the AAO
template is easy to dissolve in very acidic baths.
For this project we deposited Bi2Te3 films from dimethyl sulfox-
ide (DMSO) solutions containing TeCl4 and Bi(NO3)3 × 5H2O. DMSO
could be used as an alternative to water. It has higher solubility
for several Te, Bi and Sb salts than in water. Moreover, more nega-
tive potentials can be used in DMSO for electrodeposition without
reduction of the solvent. Martin-González’s group has successfully
synthesized Bi1−xSbx films and nanowires from DMSO solutions
containing high concentration Sb cations without any complex-
ing agents [11,12]. The DMSO solution containing Te and Bi salts
is suitable for Bi2Te3 nanowire deposition in porous alumina tem-
plates due to its low corrosion for the AAO template. We examined
the Bi2Te3 deposition mechanism using cyclic voltammetry and
electrochemical quartz crystal microbalance (EQCM). Accumulated
charges and related mass changes for Bi2Te3 deposition on working
electrodes were measured in situ and the results used to propose a
reaction mechanism. Bi2Te3 films were prepared by potentiostatic
and pulsed electrodeposition methods to study the concentration
effect on the film composition. The morphology and crystallinity
Most bismuth telluride composites were deposited from aque-
ous acidic solution with pH ≤ 1. The Bi2Te3 electrodeposition
∗
Tel.: +886 3 5918241; fax: +886 3 5820207.
E-mail address: WenJinLi@itri.org.tw.
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.07.008

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W.-J. Li / Electrochimica Acta 54 (2009) 7167–7172
of bismuth telluride films were determined by scanning electron
microscopy (SEM) and X-ray diffraction (XRD).
Acceleration voltage and collection time were 20 kV and 100 s,
respectively. X-ray diffraction analysis was performed using an X-
ray source diffractometer (Siemens-D5000; Cu K␣ radiation).
2. Experimental
3. Results and discussion
EQCM and cyclic voltammetry were simultaneously used to
measure current, charge, and related mass change on working elec-
trodes. All electrochemical measurements were performed using
an EQCM-equipped CHI-440 potentiostat (CH Instruments). The
working electrode consisted of an 8 MHz AT-cut quartz crystal
(0.205 cm2) coated with 300 nm Au film on both sides. Gold adhe-
sion to the crystal was enhanced by a 100 nm Cr sublayer. According
to Sauerbrey’s equation, quartz crystal frequency change (ꢀf) is
proportional to the mass change (ꢀm) that occurs on the crystal,
expressed as ꢀf = −K ꢀm, where K represents the crystal’s charac-
teristic constant whose value is affected by such factors as driver
electronics and the viscoelastic interaction between the solution
and electrode surface. This mandates EQCM calibration prior to
each measurement [13,14]. Calibration was performed via Te elec-
trodeposition from a solution containing 0.01 M Te4+ in DMSO at
E = −0.4 V vs. Ag/AgCl. Following Faraday’s law and assuming 100%
electrodeposition efficiency, the K value was 622.2 Hz/␮g.
All electrochemical measurements and electrodeposits were
performed under inert atmospheric (N2) conditions, with solutions
deaerated with N2 gas for 15 min prior to each measurement. The
electrolytes used in cyclic voltammetry experiments were DMSO
solutions containing (a) 0.0075 M Bi3+; (b) 0.01 M Te4+; and (c)
0.01 M Te4+ plus 0.0075 M Bi3+. These solutions were prepared by
dissolving TeCl4 and Bi(NO3)3 × 5H2O in DMSO. 0.1 M NaNO3 was
added as supporting electrolyte to increase the solution conduc-
tivity. A scan rate of 10 mV/s was used in the cyclic voltammetry
experiment. A platinum wire served as the counter electrode; the
reference electrode consisted of Ag/AgCl (3N KCl).
3.1. Cyclic voltammograms and EQCM
Cyclic voltammograms was analyzed for DMSO solutions con-
taining Bi3+ and Te4+ individually as well as DMSO solutions
containing a mixture of both. Possible reaction mechanisms for
Bi2Te3 electrodeposition were evaluated by plotting observed
frequency change (ꢀf) as a function of consumed charge (Q). Com-
bining Sauerbrey’s equation and Faraday’s law results in
ꢀ
ꢁꢀ
ꢁ
M
z
KQ
F
ꢀf = −
(3)
where M is the molar mass of the deposit, z the number of electrons
involved in the reaction, and F the Faraday constant (96,500 C/mol).
The M/z values obtained from the slope of ꢀf vs. KQ/F can be used to
examine reaction mechanisms. Furthermore, theoretic M/z values
can easily be calculated for any electrochemical reaction, making it
possible to determine a dominant single reaction or several parallel
reactions.
Fig. 1a presents the cyclic voltammetric scan observed for
0.0075 M Bi3+ in DMSO. The reduction reaction begins at E = −0.2 V
vs. Ag/AgCl, and one reduction wave is observed at Epc = −0.35 V.
The M/z value 69.35 g/mol is corresponded to the theoretical value
of a 3-electron Bi reduction.
Bi3+ + 3e− → Bi,
M/z = 69.6 g mol−1
(4)
Bi is the only one phase in the reduction reaction as identified by
XRD. The corresponding oxidation wave is observed at Epa = 0.2 V in
the anodic scan.
Potentiostatic electrodeposition was performed in different
DMSO solutions containing (a) 0.01 M Te4+ and 0.0075 M Bi3+; (b)
0.05 M Te4+ and 0.0375 M Bi3+. Bismuth telluride deposit composi-
tion was calculated from the results of EQCM and analyzed using
inductively coupled plasma mass spectroscopy (ICP-MS, Agilent
7500A), with bismuth telluride film deposited at different potential
and samples dissolved in HNO3.
The electrochemical reaction for Te4+ dissolved in DMSO is pre-
sented in Fig. 1b. Reduction reaction occurred at 0 V. There are
three reduction peaks in the cathodic scan. The first reduction peak
(Epc = −0.15 V) and the second reduction peak (Epc = −0.45 V) had
M/z values of 33 and 32 g/mol, respectively. They are consistent with
the theoretical M/z value of the Te reduction:
Bismuth telluride film morphology was characterized via JEOL
2100 scanning electron microscopy (20 kV) equipped with an
energy-dispersive X-ray spectrophotometer (EDX), which was used
to determine the composition of films obtained from pulsed plat-
ing method. For EDX analysis, Bi and HgTe were used as standards.
Te4+ + 4e− → Te,
M/z = 31.9 g mol−1
(5)
The first peak could be attributed to the Te reduction occurring
on the Au electrode. The second reduction peak is assigned to the
Te reduction occurring on the Te substrate since the substrate is
Fig. 1. (a) Cyclic voltammetric scan of 0.0075 M Bi3+in DMSO. (b) Cyclic voltammetric scan (solid line) and changes in frequency (dashed line, from 0 to −1.2 V) of 0.01 M Te4+
in DMSO, scan rate, 10 mV/s.

W.-J. Li / Electrochimica Acta 54 (2009) 7167–7172
7169
Fig. 2. (a) Cyclic voltammetric scan. (b) Changes in frequency (solid line) and charge (dashed line). (c) Corresponding ꢀf vs. (KQ/F) curve of Bi2Te3 in DMSO containing 0.01 M
Te4+ and 0.0075 M Bi3+; scan rate, 10 mV/s.
covered completely with Te layer. The third reduction peak cou-
pling with the mass loss on the Au electrode is observed at about
E = −0.80 V, as shown in Fig. 2b. It is Te dissolution reaction in DMSO
according to the following reaction:
The deposit was identified to be Te by XRD. Following the Te reduc-
tion, another reduction reaction occurred with the measured M/z
value as 44.1 g mol−1. It is corresponding to the 18-electron reduc-
tion reaction for Bi2Te3 deposition:
2Bi3+ + 3Te4+ + 18e− → Bi2Te3,
M/z = 44.5 g mol−1
(7)
Te + 2e− → Te2−
,
M/z = 63.8 g mol−1
(6)
According to results from our XRD analysis, the Bi2−xTe3+x phase
was found during the cathodic potential scanning ranged between
−0.2 and −0.8 V vs. Ag/AgCl.
The working electrode became no deposit and clean. The mea-
sured M/z value is about 46.33 g mol−1, it indicated that reduction
reaction (5) and (6) occurred simultaneously in the potential range
between −0.7 and −1.2 V. In the anodic scan, Te deposition occurs
between−1.2and −0.1 V. Theoxidationreactionoccurs at E = 0.25 V.
The reduction reaction occurred at the beginning of the anodic
scan (from −0.8 to −0.075 V). The parallel mass (ꢀf) and accu-
mulated charges (Q) were increased. The calculated M/z value
37.8 g mol−1 means Te-rich deposit was formed at the initail stage
of the anodic scan. Bi3+ depletion near the electrode makes that
Te-rich deposit was obtained. We observed the beginning of the oxi-
dation reaction at E = −0.075 V, corresponding to a decrease of mass
(ꢀf) and accumulated charges, as shown in Fig. 2a and b. Two broad
oxidation peaks could be obtained in the anodic scan with Epc = 0.27
and 0.72 V, respectively. According to the M/z values measured from
−0.07 to 0.8 V in the anodic scan, there are three oxidation reactions
occurring in sequence under these two oxidation peaks: Bi oxida-
tion(from−0.07to0.20 V, M/z = 71.5), Bi2Te3 oxidation(from0.20to
0.50 V, M/z = 48.4) and Te oxidation (from 0.50 to 0.70 V, M/z = 32.2),
as shown in Fig. 2c. Bi oxidation is due to the top layer of Bi on the
Bi2Te3 films. Since surface Bi is removed prior to Bi2Te3 oxidation,
it is expected that the excess Te and Bi2Te3 oxidation occurs to pro-
The wave with Epa = 0.6 V is assigned to be the oxidation of Te to Te4+
.
Results from cyclic voltammetric scans of the Au electrode in
DMSO solution containing 0.01 M Te4+ and 0.0075 M Bi3+ are shown
in Fig. 2a. Simultaneous changes in frequency (ꢀf) and charge (Q)
are presented in Fig. 2b and the corresponding plot of ꢀf vs. KQ/F
for bismuth telluride deposition-oxidation is shown in Fig. 2c. Elec-
trodepotentialscanningrangedbetween0.8and−0.8 Vvs. Ag/AgCl.
In the cathodic scan, reduction reactions began at 0 V; we observed
the first reduction peak with peak potential at −0.16 V. Reduction
reaction M/z was measured as 31 g mol−1, which is close to the the-
oretical value of a 4-electron charge transfer (Fig. 2c, line from 0 to
−0.2 V):
Te4+ + 4e− → Te,
M/z = 31.9 g mol−1
(5)

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W.-J. Li / Electrochimica Acta 54 (2009) 7167–7172
Fig. 3. Composition analysis of Bi2Te3 film deposited on gold electrode at different
time. Solution: (᭹) 0.01 M Te4+ and 0.0075 M Bi3+ in DMSO; (ꢀ) 0.05 M Te4+ and
0.0375 M Bi3+ in DMSO, applied potential E = −0.6 V vs. Ag/AgCl.
Fig. 4. Composition as a function of applied potential for Bi–Te film deposited on
gold electrode for solutions. (ꢁ) 0.01 M Te4+ and 0.0075 M Bi3+ in DMSO. Calculated
from M/z values; (×) 0.01 M Te4+ and 0.0075 M Bi3+ in DMSO. Analyzed by ICP-MS;
(᭹) 0.05 M Te4+ and 0.0375 M Bi3+ in DMSO. Calculated from M/z values.
duce a second broad peak. This behavior is similar to what has been
reported regarding Bi2Te3 oxidation in aqueous solutions [5,10].
with the adsorption of Te4+ on the electrode surface. Because Te4+
adsorbs strongly onto the electrode surface and the onset poten-
tial for Te reduction is more positive, Te formation is more feasible.
When the Te4+ concentrations near electrode are high, the possi-
bility of Te4+ adsorption on the electrode is higher than that of Bi3+
adsorption on the electrode. So Te reduction is the dominant reac-
tion. As the potential shifts in the negative direction, the probability
of Bi3+ contact with the electrode is increased such that the reduc-
tion of Te was accompanied by Bi2Te3 deposition. Te-rich deposit
was obtained. If Te4+ concentrations near the electrode are low, Bi3+
ions have more possibilities to contact with Te and to form Bi2Te3,
so Bi2Te3 deposition will dominate the reduction reaction and ion
diffusion will determine the film composition at high polarization
potentials.
3.2. Potentiostatic and pulse deposition of Bi2Te3 films
The simultaneous application of potentiostatic method and
EQCM was used to study the deposition behavior of the deposit.
The Te atomic percentage in the deposit was calculated by the M/z
value as
20, 898 − 300 × (M/z)
Te atomic percentage =
%
(8)
81.38 + (M/z)
The composition of deposit obtained at different time during the
deposition is shown in Fig. 3. The data indicate that during a brief
initial stage a thin layer of Bi2−xTe3+x with inhomogeneous com-
position forms on the Au surface until it is completely covered
with the deposit. As deposition time increases, the deposition sub-
strate changes from Au to Bi2−xTe3+x and the deposit compositions
gradually approach a constant value.
Te-rich deposit also can be obtained by pulse electrodeposition
from a DMSO solution containing 0.01 M Te4+ and 0.0075 M Bi3+
.
Single potential step mode with different pulse time at E = −0.6 V
was performed for pause duration of 1 s. The amount of Te in the
compound increased as the period of the pulse time decreased (as
listed in Table 1). When the pulse time was 10 ms, result from EDX
measurement indicated a resultant deposit containing more than
88% Te. This result could be explained by the deposition mechanism
of Bi2Te3 in DMSO. The first step of the mechanism is Te4+ adsorbing
on electrode and reducing to be Te. Then Bi3+ reacts with Te to form
Bi2Te3. It is similar to the mechanism of Bi2Te3 deposition in acidic
aqueous solution [4]. When the pulse time is short that only little
Bi3+ could react with Te. Consumed Te4+ was replenished during
the subsequent pause stage. Repeated cycles resulted in a buildup
of a Te-rich deposit.
In a DMSO solution containing 0.01 M Te4+ and 0.0075 M Bi3+
,
films were deposited at different potentials between 0 and −0.8 V.
The films composition was analyzed by ICP-MS and calculated from
the M/z values. Te atomic percentage calculated from M/z value is
close to that analyzed by ICP-MS, as shown in Fig. 4. Bi–Te com-
pounds were formed at the potentials negative than −0.2 V. Te
atomic percentage in the films approach to a constant value of 60%
in the range between E = −0.4 and −0.8 V. Bi-rich bismuth telluride
compounds were obtained at E = −0.5 and −0.6 V.
Keeping the same relative concentrations of the cations in solu-
tion, in a DMSO solution containing 0.05 M Te4+ and 0.0375 M Bi3+
,
Te-rich compounds were synthesized when the applied potential
is more negative than −0.2 V. Te atomic percentage in the films
deposited at potentials between −0.2 to −0.6 V is higher than 70%.
After E = −0.7 V, Te atomic percentage in the films approach to a
constant value of 67%. According to binary alloy phase diagrams,
large excesses of Te indicate there are two phases, Bi2Te3 and Te,
in deposits at low temperature. It suggests the reduction of Te was
accompanied by a small amount of Bi2Te3 deposition in the poten-
tial range from −0.2 to −0.8 V for DMSO solution containing 0.05 M
Table 1
Te atomic percentages for deposits obtained via single potential step
pulse plating using 0.01 M Te4+ and 0.0075 M Bi3+ in DMSO solution.
Epulse = −0.60 V. Film composition obtained by EDS.
Pulse time/pause time (ms)
Te at.% in Bi2Te3 film
10/1000
50/1000
100/1000
500/1000
88.6
84.9
76.0
56.2
Te4+ and 0.0375 M Bi3+
.
The deposition content is more dependent on Te4+ concentra-
tions in solution than on the applied potential. It is correlated

W.-J. Li / Electrochimica Acta 54 (2009) 7167–7172
7171
Fig. 5. XRD pattern (Cu K␣ radiation, 1.5406 Å wavelength) of deposits obtained at different potential using 0.01 M Te4+ and 0.0075 M Bi3+ in DMSO. (a) −0.1 V; (b) −0.4 V; (c)
−0.7 V.
3.3. X-ray diffraction pattern and surface morphology of Bi2Te3
films
films deposited at E = −0.1 V are consistent with Te according to
the JCPDF No. 36-1452 [15]. Results from EDX indicated a resul-
tant deposit containing more than 96% Te. The (1 0 1) diffraction
peak is the largest in the diffraction pattern for the films, as shown
in Fig. 5a. It indicates the films have highly preferential orientation
along the [1 0 1] direction. The peaks in the XRD patterns were sharp
and narrow, indicating that the films are highly crystalline and con-
sist of only one phase. Similar pattern could be obtained for films
X-ray diffraction patterns and SEM micrographs of deposits
obtained by potentiostatic method at different potentials from
DMSO solution containing 0.01 M Te4+ and 0.0075 M Bi3+ are shown
in Figs. 5 and 6. Deposition time was fixed at 4500 s to ensure suf-
ficient deposits for surface characterizations. The XRD pattern for
Fig. 6. SEM micrographs of deposits obtained at different potential using 0.01 M Te4+ and 0.0075 M Bi3+ in DMSO. (a) −0.1 V; (b) −0.4 V; (c) −0.7 V.

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W.-J. Li / Electrochimica Acta 54 (2009) 7167–7172
depositedfromDMSOsolutioncontaining0.05 MTe4+ and0.0375 M
Bi3+ at E = −0.1 and −0.5 V. The film morphology was identified as
a polycrystalline assembly of hexagonal needles on the substrate
surface, as shown in Fig. 6a. The feature sizes are on the order of
200 nm × 20 nm.
When the Te4+ concentration near electrode is high (e.g. 0.05 M
Te4+), Te deposition is the dominant reaction. Te-rich deposits are
obtained in the potential range between −0.2 and −0.8 V. When the
Te4+ concentration near electrode is low (e.g. 0.01 M Te4+), Bi2Te3
deposition is the dominant reaction, and the atomic ratio of Bi to
Te in the films is close to 2/3 at potential negative than −0.4 V.
Te-rich composites are obtained via single potential step mode
with a pulse of 10–100 ms at E = −0.6 V followed by a pause of 1 s.
Short pulse time causes only little Bi3+ could reacts with Te.
The XRD patterns for films deposited at E = −0.4 and −0.7 V from
DMSO solution containing 0.01 M Te4+ and 0.0075 M Bi3+ are pre-
sented in Fig. 5b. The diffraction peaks are corresponding to the
(0 1 5), (1 1 0), (1 0 1 0) planes of Bi2Te3 compound according to the
powder diffraction file for JCPDF No. 15-0863 [16]. From the Harris
texture analysis, we can find the films obtained at E = −0.7 V has a
highly preferential orientation along the [0 1 5] direction compared
with the films obtained at E = −0.4 V. The film deposited at −0.4 and
−0.7 V was dense with granular structure, as shown in Fig. 6b and
c. The films deposited at −0.7 V were rougher because of the fast
growth.
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We examined Bi2Te3 deposition in DMSO solutions by means of
simultaneous cyclic voltammetry and EQCM plus measured charge
accumulations and changes in deposit mass in situ. Bi2−xTe3+x films
could be obtained from DMSO solution containing 0.01 M Te4+
and 0.0075 M Bi3+in the potential range between −0.2 and −0.8 V.
The results obtained from simultaneous application of potentio-
static method and EQCM were used to propose a mechanism
about Bi2−xTe3+x deposition in DMSO solution. Firstly a thin layer
of Bi2−xTe3+x with inhomogeneous composition forms on the Au
surface until it is completely covered with the deposit. Then a
homogenous deposit eventually appears on the Bi2−xTe3+x surface.
Te films are obtained in the potential range between 0 and
−0.2 V, independent of the Te4+ concentration in DMSO. When
the applied potential is negative than −0.2 V, the film composition
is more dependent on Te4+ concentrations than on the potential.