Electrodeposition of bismuth telluride thermoelectric films from a nonaqueous electrolyte using ethylene glycol

Article abstract of DOI:10.1016/j.electacta.2012.01.091

Ethylene glycol was studied as an electrolyte for the electrodeposition of thermoelectric bismuth telluride films by cyclic voltammetry, rotating ring disk electrode and electrochemical quartz crystal microbalance (EQCM). The reduction of both Bi³⁺ and Te⁴⁺ ions proceeds in one step without the formation of intermediates at potentials more negative than +0.2 V and +0.55 V vs. SHE, respectively. The diffusion coefficients and the rate constants for reduction were found to be similar for Bi and Te. Stoichiometric Bi ₂Te₃ films with a uniform composition profile were obtained from solutions containing up to 1 M of Bi(NO₃)₃ and TeCl₄, at current densities up to 5 A dm^-2 (～102 μm h^-1). Both p- and n-type bismuth telluride films could be obtained, as confirmed by Seebeck coefficient measurements.

Full text of DOI:10.1016/j.electacta.2012.01.091

Electrochimica Acta 68 (2012) 9–17
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrodeposition of bismuth telluride thermoelectric films from a nonaqueous
electrolyte using ethylene glycol
Hai P. Nguyen , Minxian Wu , Jiale Su , Ruud J.M. Vullers , Philippe M. Vereeckenc,d, Jan Fransaera,∗
a
a
b
b
a
Department of Metallurgy and Materials Engineering, KU Leuven, Heverlee, Belgium
imec/Holst Centre, Eindhoven, The Netherlands
imec, Heverlee, Belgium
Center for Surface Chemistry and Catalysis, KU Leuven, Heverlee, Belgium
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Ethylene glycol was studied as an electrolyte for the electrodeposition of thermoelectric bismuth telluride
films by cyclic voltammetry, rotating ring disk electrode and electrochemical quartz crystal microbalance
Received 23 November 2011
Received in revised form 25 January 2012
Accepted 25 January 2012
3+
4+
(EQCM). The reduction of both Bi and Te ions proceeds in one step without the formation of interme-
diates at potentials more negative than +0.2 V and +0.55 V vs. SHE, respectively. The diffusion coefficients
and the rate constants for reduction were found to be similar for Bi and Te. Stoichiometric Bi2Te3 films
with a uniform composition profile were obtained from solutions containing up to 1 M of Bi(NO3)3 and
Available online 23 February 2012
Keywords:
−
2
−1
TeCl4, at current densities up to 5 A dm (∼102 ␮m h ). Both p- and n-type bismuth telluride films could
Thermoelectric
Bismuth telluride
Electrodeposition
High rate
be obtained, as confirmed by Seebeck coefficient measurements.
©
2012 Elsevier Ltd. All rights reserved.
Ethylene glycol
1
. Introduction
far, Glatz et al. were the only researchers that reported the
−
1
electrodeposition of thick p-type Bi Te films at a rate of 50 ␮m h
2
3
Bismuth telluride and its derivatives doped with antimony
[9]. However, 2 M nitric acid was required to maintain higher solu-
bility of Te (80 mM) and a complex pulsed deposition method was
utilized.
and selenium are the best-known materials for room tempera-
ture thermoelectric applications [1,2]. Electrodeposition has been
recognized as an effective method for preparing these mate-
rials due to its unique features: simple, fast, low-cost, and
capable of selective deposition. The number of publications on
electrodeposition of thermoelectric materials, particularly bis-
muth telluride based materials, has increased by a factor of
three in the last decade [3]. Most of these studies dealt with
the electrodeposition of bismuth telluride from acidic solutions
using 1 M HNO3 that can dissolve only about 20 mM Te species,
which in turn limits the Bi concentration in order to obtain
the correct alloy composition (Bi Te ) [4–9]. The low metal ion
Non-aqueous electrolytes often provide advantages such as
wide electrochemical window, good thermal stability, high metal
ion solubility. Tellurium and tellurium alloys films have been elec-
trochemically prepared from dimethyl sulfoxide (DMSO) [10,11]
and dimethyl formamide (DMF) [12]. Bismuth antimony thermo-
electric nanowires was prepared from DMSO [13]. Li and coworkers
have used DMSO for the electrodeposition of bismuth telluride
and bismuth antimony telluride materials [14,15]. In our previous
work [16], we have examined the effects of various parameters
such as bath composition, total metal concentration, and tem-
2
3
concentration causes composition inhomogeneities across the
film thickness due to the unbalanced depletion of Bi and Te
species in plating bath. The rather low mass transport in solu-
tion also limits the maximum deposition current and hence
growth rate that can be attained. The highest reported deposi-
perature on the composition and morphology of Bi Te3 films
2
deposited from DMSO electrolytes. We found that film composi-
tion was very sensitive to temperature as almost pure Te films
were obtained if the temperature rose slightly above room temper-
ature, which somewhat limits the use of DMSO-base electrolytes for
Bi Te .
−2
tion current density is 1.6 A dm
which corresponds to about
2
3
−
1
3
4 ␮m h
[8]. In addition, it was reported that it was diffi-
In this work, the use of ethylene glycol (EG) is investigated as an
cult to obtain p-type Bi Te3 films from nitric acid bath [5]. So
electrolyte for Bi Te deposition. Ethylene glycol (EG) is an organic
2
2 3
solvent with a relatively high dielectric constant (ꢀ= 37.7). It is
known to dissolve various metal salts that are only sparingly solu-
ble in water and was used for the electrodeposition of compounds
that are difficult to deposit from aqueous electrolytes such as CdSe
∗ Corresponding author. Tel.: +32 16321239; fax: +32 16321991.
E-mail address: jan.fransaer@mtm.kuleuven.be (J. Fransaer).
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.01.091

1
0
H.P. Nguyen et al. / Electrochimica Acta 68 (2012) 9–17
0
0
0
0
0
.4
.3
.2
.1
.0
a) 1.0
0
0
0
0
0
.8
.6
.4
.2
.0
Bi
NaCl
-0.1
-0.2
-0.3
-0.4
-0.2
-0.4
LiNO3
Te
-
-
0.6
0.8
Bi & Te
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
-
0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
E (V) vs. SHE
E vs. SHE / V
b) 1.0
Fig. 1. Current vs. potential curves obtained from EG solutions containing either
0
0
0
0
.8
.6
.4
.2
◦
−1
0
.5 M LiNO3 or 0.5 M NaCl, at 50 C, scan rate 20 mV s , Pt WE (ꢁ4 mm). Similar
electrochemical windows of 1.7 V are indicated for both solutions.
[
(
17], ZnSb [18], and AuSn [19]. Due to its high boiling temperature
197 C) and low vapor pressure (8 Pa at room temperature), EG
◦
allows electrodeposition at high temperature, which was reported
Te
Bi
to improve crystallization of CdSxSe1 films for solar cell appli-
−x
0.0
cations [20]. In this work, we used ethylene glycol as a solvent
for electrodeposition of bismuth telluride films for thermoelectric
applications. We studied the deposition mechanism of Bi, Te, and
BixTey, the effects of various parameters on the deposits, and very
high rate deposition of stoichiometric bismuth telluride films from
concentrated electrolytes.
-0.2
Bi & Te
-
0.4
-
0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
E (V) vs. SHE
2
. Experimental
Fig. 2. Cyclic voltammograms obtained from EG solutions containing different con-
ducting salts: 0.5 M LiNO3 (a) or 0.5 M NaCl (b) and either 0.05 M Bi(NO3)3 (dashed
◦
line) or 0.05 M TeCl4 (solid line) or both (dash-dotted line), at 50 C, scan rate 20 mV
Bismuth nitrate pentahydrate (Bi(NO ) ·5H O, Alfa Aesar, >98%)
−1
3
3
2
s
, WE Pt ꢁ4 mm. Reductions of Bi and Te are largely different in the solutions
and tellurium tetrachloride (TeCl , Alfa Aesar, 99%) were used as
4
containing TeCl4.
sources of metal ions. Lithium nitrate (LiNO , Sigma–Aldrich, 99%)
3
was added up to 0.5 M to improve electrolyte conductivity. Elec-
trolytes were prepared by dissolving the salts in EG (Alfa Aesar,
9
9%) without further treatment. The solutions were then stirred
electrolyte and surface morphology of the deposits [21], we cal-
ibrated our EQCM electrodes based on the electrodeposition and
stripping of Bi films at several potentials from an EG solution con-
at room temperature until completely transparent. Concentrations
of bismuth and tellurium were fixed at 0.05 M each, unless men-
tioned otherwise. All the experiments were carried out with a three
taining 50 mM Bi(NO ) and 0.5 M LiNO and the C was determined
3
3
3
f
◦
electrode setup in ambient environment at 50 C except for some
−1
2
to be 40.2 Hz ␮g cm .
experiments where the influence of temperature was investigated.
The reference electrode was a Pt wire immersed in an ethylene gly-
col solution containing 0.002 M of ferrocene/ferrocenium and 1 M
LiNO3 as a supporting electrolyte. However, potentials reported
in this work were converted to the standard hydrogen electrode
A rotating ring disk electrode (RRDE) (Ametek, model RDE0071,
ꢁdisk = 4.57 mm, ꢁinnerring = 4.93 mm, ꢁouterring = 5.38 mm, collection
(
SHE) using the potential of our reference electrode, +0.5 V vs.
the SHE. The counter electrode was a Pt mesh. Different working
electrodes (WE) were used in different experiments as mentioned
below.
An electrochemical quartz crystal microbalance (EQCM, Max-
tek) and a potentiostat (EG&G 273) were connected to simultane-
ously monitor potential, current and resonant frequency change
of the quartz crystal (5 MHz Ti/Pt AT cut, ꢁ1.27 cm, Maxtek, CA).
The mass change of the quartz crystals was calculated based on the
Sauerbrey equation:
ꢂf = C ꢂm
(1)
f
where ꢂf (Hz) is the change in the resonance frequency of the
−2
crystal due to the change in mass ꢂm (␮g cm ) of the crystal;
−
1
2
Cf (Hz ␮g cm ) is a conversion coefficient. Because the coeffi-
Fig. 3. Cyclic voltammogram and M/z values obtained for a Bi solution containing
◦ −1
0.05 M Bi(NO3)3 and 0.5 M LiNO3, at 50 C, scan rate 20 mV s , Pt WE (EQCM crystal).
cient C is strongly affected by parameters such as viscosity of the
f

H.P. Nguyen et al. / Electrochimica Acta 68 (2012) 9–17
11
◦
Fig. 4. SEM images of Bi films deposited at different potentials from a Bi solution containing 0.05 M Bi(NO3)3 and 0.5 M LiNO3, 50 C, non-stirred (scale bar 2 ␮m).
efficiency 22%) was used to examine the existence of soluble
reduction products in the Te solution. Polarization curves from
individual Bi and Te solutions were measured at different rota-
tion speeds using a platinum rotating disk electrode (ꢁ4 mm). The
Koutecky–Levich equation was applied following the method intro-
duced in [22] to determine diffusion coefficients of bismuth and
tellurium and rate constants of their reductions.
Bismuth telluride films were prepared in potentiostatic
mode on Au-coated silicon wafers. The morphology of the
deposited films were analyzed by a scanning electron micro-
scope (SEM, Philip XL30). The atomic ratios of Bi and Te in
the deposited films were analyzed by energy-dispersive X-ray
3. Results and discussion
3.1. Electrochemical behaviors
In order to enhance the ionic conductivity of the EG, either 0.5 M
NaCl or 0.5 M LiNO3 was used as supporting electrolytes. As can be
seen from Fig. 1, both solutions have the same electrochemical win-
dow of 1.7 V (from −0.5 V to 1.2 V vs. SHE), which is a bit larger than
the electrochemical window of water (1.24 V). However, the sup-
porting electrolytes were found to have a marked influence on the
reduction potential of bismuth and tellurium. Fig. 2a shows cyclic
voltammograms (CV) of Bi, Te, and mixed solutions in presence of
0.5 M NaCl. Reduction peak for bismuth and tellurium are situated
at −0.08 V and +0.22 V, respectively. When LiNO3 is used as sup-
porting electrolyte, both reduction peaks almost coincide as shown
in Fig. 2b, which is very advantageous for the electrodeposition of
alloys. For this reason, LiNO3 was used in all further experiments.
Cyclic voltammograms and EQCM data from the individual Bi,
Te solutions were analyzed for M/z based on Faraday’s law:
(
EDX).
M
z
ꢂm
jꢂt
=
F
(2)
where M (g mol 1) is the molar mass of the reducing/oxidizing
−
agent; z is the number of electrons involved in the electrochem-
−
1
ical reaction; F = 96,500 (C mol ) is the Faraday constant; ꢂm
−
2
(
g cm ) is the mass change of the working electrode caused by
−
2
current density j (A cm ) passing in a time span ꢂt (s). Because
the theoretical value of this ratio can be calculated for any elec-
trochemical reaction, electrochemical processes can be examined
by comparing the experimental to the theoretical M/z value. Fur-
thermore, the M/z ratio given by Eq. (2) can also be estimated as a
function of applied potential, which allows to examine changes in
reaction mechanisms along a CV. However, when the current in CV
goes through zero, experimentally determined M/z values become
uncertain. Therefore, any conclusion drawn in these zones should
Fig. 5. Cyclic voltammogram and M/z values obtained for a Te solution containing
◦ −1
.05 M TeCl4 and 0.5 M LiNO3, at 50 C, scan rate 20 mV s , Pt WE (EQCM crystal).
0

1
2
H.P. Nguyen et al. / Electrochimica Acta 68 (2012) 9–17
◦
Fig. 6. SEM images of Te films deposited at different potentials from a Te solution containing 0.05 M TeCl4 and 0.5 M LiNO3 at 50 C, non-stirred (scale bar 1 ␮m).
be avoided [23]. In this paper, we use the magnitude of current den-
sity |j| in Eq. (2), so that M/z has the same sign as ꢂm, i.e. positive
for deposition and negative for a dissolution process.
zero. Because this current is not observed when measuring on the
small (bulk) Pt (see Fig. 1), it is believed to relate to the work-
ing electrode rather than decomposition of the solvent. In order
to confirm this phenomenon, we carried out CVs from LiNO3 0.5 M
solution using a piece of Pt-coated Si wafer as working electrode
and observed a similar current. Because both Pt/Si and Pt/quartz
electrode have a thin Ti adhesion layer, the current may be caused
by reactions of Ti or on Ti.
Fig. 3 shows the CV and EQCM data obtained from a solution
containing 0.05 M Bi(NO3)3 and 0.5 M LiNO . The reduction starts
3
around +0.2 V and between +0.2 V and 0.07 V, the M/z value is
−
1
constant and roughly equal to +69 g mol which corresponds to
the theoretical value for the three-electron reduction of Bi3 to Bi :
+
0
ꢀ
ꢁ
Fig. 5 shows the CV obtained for an EG solution containing
M
z
209
3
Bi3 + 3e− → Bi
+
0
=
= 69.6 g mol−1
(3)
0.05 M TeCl4 and 0.5 M LiNO . The first wave (with zero M/z) starts
3
from open circuit potential +0.9 V and ends at +0.55 V. This reduc-
tion current is also observed where a piece of Pt-coated silicon
wafer is used as the working electrode. However, it does not appear
if CVs are carried out on a small-bulk Pt electrode, as shown in
Fig. 2b. We attribute this wave to artifacts of the thin film working
The reduction potential of Bi in the EG electrolyte is similar to the
value +0.2 V obtained in aqueous solution at a pH between −2.0
and +2.0 [24]. Bismuth films deposited in this potential region are
smooth and compact as shown by SEM images in Fig. 4a and b.
At more negative potentials (+0.07 to −0.3 V), M/z increases
−1
up to 130 g mol . It was observed that the films start to growing
dendritically at these more negative potentials, as can be seen in
Fig. 4, which alters the sensitivity of the quartz crystal according to
Vatankhah et al. [21]. Hence, the higher M/z values are not a result
of a change in the three-electron reduction mechanism, but are due
to a decrease of the sensitivity of the QCMB crystal. This was also
confirmed by the increase in the damping of the crystal that was
noticed in this potential range.
1.5
1
0
0
.0
.5
.0
During the anodic scan, oxidation starts at +0.23 V with M/z of
−1
about −69.7 g mol , which corresponds to the dissolution of Bi to
3
+
Bi . The dissolution continues up to +0.77 V where the oxidation
current shows a maximum and M/z increases strongly. The increase
in M/z are observed at the end of the dissolution process, when
some parts of Pt electrode become expose to the solution form-
ing a galvanic couple with the remaining Bi. This leads to galvanic
corrosion which leads to an overestimation of M/z. Hence, Bi also
anodically dissolves in a one-step three-electron process.
2
20
-
0.5
1.0
50
00
-1
Scan rate / mV s
1
2
00
-
0.0
0.2
0.4
0.6
0.8
1.0
As the Bi layer dissolves, the current decreases but then
increases at potentials more positive than +0.9 V. On the backward
scan, the current decreases to zero at +0.55 V vs. SHE. During this
process, no mass change is detected on the EQCM and M/z remains
E vs. SHE / V
−1
Fig. 7. Cyclic voltammograms obtained at different scan rates from 2 to 200 mV s
◦
for a Te solution containing 0.05 M TeCl4 and 0.5 M LiNO3, at 50 C, Pt WE (RRDE).

H.P. Nguyen et al. / Electrochimica Acta 68 (2012) 9–17
13
a) 0.75
0
0
0
.50
.25
.00
Fig. 9. Cyclic voltammogram and M/z values obtained for a mixed solution contain-
ing 0.05 M Bi(NO3)3, 0.05 M TeCl4, and 0.5 M LiNO3, at 50 C, scan rate 20 mV s , Pt
WE (EQCM crystal).
0
50
100
150
200
250
◦
−1
-
1
scan rate / mV s
b) 0.5
The slightly increasing M/z ratio obtained at lower potentials can
be explained by the roughening of the Te surface, as shown in
Fig. 6c and d, and also indicated by the concomitant increase in
the damping of the quartz crystal.
The M/z values corresponding to the reduction peak R are
between 0 and +32 g mol . This can occur if two electrochemi-
cal processes occur in parallel this potential range, one with an
0
0
0
.4
.3
.2
1
−
1
M /z = 0 and one with an M /z = 32, where the latter process
1
1
2
2
corresponds to the 4-electron reduction of tellurium, which is con-
sistent with the EDX result for the films deposited in this potential
range. The side reaction with M/z = 0 implies that the reduction
product is soluble in solution, such as Te2+. We have carried out
RRDE experiments, in which the potential of the disk is scanned
from OCP (+0.9 V vs. SHE) to 0 V while the ring potential is set at
0
.0
0.5
1.0
1.5
2.0
2.5
3.0
+
1.0 V. However, zero oxidation current was detected on the ring,
-
1
2+
implying that Te was not produced. Thus, the peak R1 is assigned
to the reduction of Te, but with a current efficiency lower than 100%.
log(scan rate / mV s )
0
The increasing M/z implies that the reduction to Te becomes more
c)
6
5
4
3
2
1
0
dominant as the potential becomes more negative.
Fig. 7 shows the peak R obtained at various scan rates, from 2 to
1
−1
2
00 mV s . The peak current is found to be linear with scan rate as
shown in Fig. 8a. Fig. 8b shows the variation of peak position (Ep) vs.
log(scan rate), where a linear dependence is found at high scan rate
while the peak position remains constant at low scan rates. Such
a behavior is consistent with formation of a monolayer following
the random adsorption model, irreversible case [25]. In Fig. 8c, as
scan rate increases, the peak charge tends to a constant value of
−
2
2
.1 mC cm , close to the value for two close-packed (0 0 0 1) mono-
−
2
Charge of a Te mono layer (0001)
layers of Te (0.94 mC cm ). At low scan rates, the formation of the
monolayer may change the kinetics of the electrode leading to bulk
deposition of Te, which explains the higher peak charge. Similar for-
mation of monolayers have been found for Bi on GaAs substrates
from aqueous solutions elsewhere in literature [26]. Dissolution
of Te can be observed during the anodic scan with M/z values of
0
50
100
150
200
-
1
scan rate / mV s
−
1
−
32 g mol following the 4-electron oxidation of Eq. (4).
Fig. 8. Dependence of position (a), height (b) and charge (c) of the peak R1 (in Fig. 5)
with respect to scan rate from 2 to 200 mV s for a solution contains 0.05 M TeCl4
and 0.5 M LiNO3, at 50 C, non-stirred, Pt WE (RRDE).
For the deposition of bismuth telluride films, the theoretical
M/z value may vary from +32 to +69.6 g mol depending on the
−1
−
1
◦
alloy composition, from pure Te to pure Bi, respectively. For the
−
1
stoichiometric composition Bi Te , a value of +44.6 g mol can be
2
3
calculated from the reaction:
electrode, probably reactions of the Ti adhesion layer underneath
the Pt layer on the quartz crystal or silicon wafer.
Bi + 3Te4+ + 18e− → Bi Te3
3+
2
(5)
2
Bulk Te deposition can be observed at potentials more nega-
−
1
tive than +0.3 V (peak R ) with M/z of approximately +32 g mol
,
Fig. 9 shows the CV and M/z ratio obtained for a solution con-
taining 50 mM Bi(NO3)3 and 50 mM TeCl . At potentials more
2
4+
corresponding to the four-electron reduction of Te
:
4
negative than +0.2 V, which is the reduction potential of Bi in the
Te4 + 4e− → Te
+
0
(4)
−1
pure Bi solution, the M/z ratio increases from +32 to +48 g mol
.

1
4
H.P. Nguyen et al. / Electrochimica Acta 68 (2012) 9–17
1
00
8
6
4
2
0
0
0
0
Bi Te3
2
0
-
0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
E
vs. SHE / V
Fig. 11. Composition of BixTey films deposited at different potentials from a mixed
solution containing 0.05 M TeCl4, 0.05 M Bi(NO3)3, and 0.5 M LiNO3, at 50 C, non-
stirred.
Fig. 10. Polarization curves obtained at different rotation speeds from 2 solutions
◦
◦
containing either 0.05 M Bi(NO3)3 or 0.05 M TeCl4 and 0.5 M LiNO3, at 50 C, scan
−1
rate 10 mV s , Pt WE ꢁ4 mm. The inset shows the plots and fits according to Eq. (6)
for Bi and Te cases.
◦
Fig. 12. SEM images of BixTey films deposited from a mixed solution containing 0.05 M TeCl4, 0.05 M Bi(NO3)3, and 0.5 M LiNO3, at 50 C, non-stirred (scale bar 1 ␮m).

H.P. Nguyen et al. / Electrochimica Acta 68 (2012) 9–17
15
1
00
100
8
6
4
2
0
0
0
0
0
80
Bi Te
Bi Te3
2
2
3
60
40
20
0
0
100
200
300
400
500
600
30
40
50
60
70
80
90
100 110
o
Rotation rate / rpm
Temperature / C
Fig. 13. Composition of BixTey films deposited at 0.02 V vs. SHE, at different rotation
Fig. 14. Composition of BixTey films deposited at different temperatures from a
mixed solution containing 0.05 M TeCl4, 0.05 M Bi(NO3)3, and 0.5 M LiNO3, non-
stirred.
rates from a mixed solution containing 0.05 M TeCl4, 0.05 M Bi(NO3)3, and 0.5 M
◦
LiNO3, at 50 C, non-stirred (scale bar 1 ␮m).
To study the kinetics of the individual Bi and Te systems, polar-
ization curves were measured with a rotating disk electrode at
different rotation speeds. According to the Koutecky–Levich equa-
tion, the reciprocal of the current density j depends linearly on the
reciprocal of the square root of the rotating speed ω [22]:
This corresponds to deposition of BixTey with increasing Bi con-
tent. At potentials more positive than +0.2 V, the CV and M/z ratio
shows a very similar behavior to that obtained from the pure Te
solution (Fig. 5). During the cathodic scan, M/z remains 0 as the
potential is scanned from open circuit potential +0.9 V to +0.5 V,
while the current increases slightly. Between +0.2 and +0.5 V, a
mixed mechanism of two reduction processes is obtained with
1
j
1
1
1
zFCk
=
+
(6)
0.62zFD2/3
ꢃ
−1/6
C ω1/2
−1
increasing M/z from 0 to +32 g mol . It is likely that pure Te films
are obtained in this potential range. In the anodic scan, the oxi-
dation current increases constantly, starting from +0.6 V. The M/z
2
−1
where D is the diffusion coefficient (m s ), ꢃ is the kinematic
2
−1
−3
viscosity (m s ), C is the concentration (mol m ), ω is the
rotation speed (rad s ), k is the rate constant (m s ). Eq. (6)
will generate straight lines
having slopes proportional to 1/D and intersects with y-axis pro-
portional to 1/k. Fig. 10 shows the polarization curves obtained
−
1
ratio equals about −40 g mol , corresponding to the dissolution of
−1
−1
−
1
BixTey alloys. Because the M/z value is smaller than 44.6 g mol , it
is predicted that the deposited film is probably Te-rich. This varia-
tion of the film composition was then confirmed by EDX analysis.
1/2
predicts that plots of 1/j vs. 1/ω
2/3
Fig. 15. SEM images of BixTey films deposited at different temperatures from a mixed solution containing 0.05 M TeCl4, 0.05 M Bi(NO3)3, and 0.5 M LiNO3, non-stirred (scale
bar 1 ␮m).

1
6
H.P. Nguyen et al. / Electrochimica Acta 68 (2012) 9–17
for Bi and Te solutions containing either 0.05 M Bi(NO ) or
3
3
◦
0
.05 M TeCl4 and 0.5 LiNO3 at 50 C on a Pt RDE (pre-coated
with Bi or Te before each experiment). Dependence of 1/j vs.
1/2
1
/ω
for both Bi and Te cases are plotted in the inset together
with their linear fits following Eq. (6). For the case of bismuth,
−
10
2
−1
a diffusion coefficient D = 1.3 × 10
m s and a rate con-
Bi
−5
−1
stant kBi = 1.0 × 10 m s were estimated. A diffusion coefficient
−10
2
−1
−5
−1
DTe = 0.98 × 10
were found for tellurium. These diffusion coefficients are compa-
m s and a rate constant DTe = 0.8 × 10 m s
−10
2
−1
−10 −1
2
rable with 1.72 × 10
m s [27] and 1.3 × 10
m s [28] for
bismuth and tellurium, respectively, in aqueous solutions.
3.2. Bismuth telluride electrodeposition
Bismuth telluride films were deposited under potentiostatic
conditions from the mixed solution containing 50 mM Bi(NO ) and
3
3
◦
0 mM TeCl4 (0.5 M LiNO ) at 50 C. The same amount of charge
3
5
was used for each deposition, so that the thickness of the deposits
is comparable. Fig. 11 shows the variation of film composition with
respect to applied potential. As predicted from the CV and M/z anal-
ysis (Section 3.1), BixTey films can be deposited at potential more
negative than +0.2 V. A strong influence of the potential on the
film composition can be observed in the potential range between
+0.2 and 0 V, where the Te content decreases from 98 at.% to about
5
8 at.%. At more negative potentials, the film composition is sta-
ble around 50 at.%, which can be explained by the nearly identical
limiting mass transport for both ions as seen in Figs. 3 and 5.
Fig. 12 shows SEM images of the films deposited at different
potentials. The Te-rich films deposited at +0.35 V and +0.2 V are
very compact and have the typical morphology of Te films (com-
pared e.g. to those in Fig. 6a and b). The film deposited at +0.13 V
(
80 at.% Te) shows a smooth background with hills (Fig. 12c). Point
EDX analysis shows that the composition of the background is close
to the stoichiometric ratio Bi Te3 while the hills contained much
2
more Te. A needle-like morphology is obtained for the Bi2.1Te2.9
film deposited at +0.02 V (Fig. 12d). This is a typical morphology for
stoichiometric bismuth telluride films obtained from aqueous solu-
tions [7,8]. At more negative potentials, the films become rougher
with star-like texture.
Agitation is found to have very strong influence on the compo-
sition of the films deposited at constant potentials. Fig. 13 shows
composition variation of the films deposited at +0.02 V vs. SHE
using a rotating disk. Te content increases quickly with increasing
rotation speed and nearly pure Te films are obtained once the sub-
strate rotates faster than 200 rpm. This effect can be understood
Fig. 16. Composition of BixTey films with respect to bath composition and total
concentration at different potentials E = +0.13 V (a), E = +0.02 V (b), and E = −0.1 V (c)
vs. SHE, at 50 ◦C, non-stirred.
Fig. 16 shows the influence of the electrolyte composition (metal
ion ratio and concentration) on the film composition at three differ-
ent potentials. As expected, the Te content of the films increase with
increasing Te concentration in solution. Such behavior is typical for
regular alloy electrodeposition systems. The dependence of film
composition on electrolyte composition is more obvious at lower
total concentration. In more concentrated solutions (0.6 and 1 M of
Bi(NO ) and TeCl ), Te-richer films are obtained. Because the films
(
at least in part) as a result of IR-drop effect in solution. In these
experiments, when rotation speed increases from 0 to 500 rpm,
−
2
the deposition current increases from 0.15 to 0.33 A dm , leading
to increase of IR drop from 0.13 to 0.34 V. Thus, actual deposition
potentials are shifted to more positive range +0.15 to +0.36 V vs.
SHE, which results in Te-rich or pure Te films as observed in Fig. 11
for the variation of film composition with respect to deposition
potential.
3
3
4
were deposited at constant potential, the problem of IR drop might
play a role here as deposition current, so IR drop, was increasing
with increasing total metal concentration. The actual deposition
potential, thus, shifted to more positive potentials, resulting in Te-
richer films. When the applied potentials become more negative
(Fig. 16a–c), the Te content in the film decreases. This shows that
stoichiometric bismuth telluride films can be obtained by opti-
mizing electrolyte composition, total concentration and applied
potential.
The influence of temperature on the morphology and compo-
sition of Bi Te films are presented in Figs. 14 and 15. The films
2
3
were prepared potentiostatically at +0.02 V with the same amount
of charge. EDX shows that the films deposited at temperatures
◦
below 70 C have very similar composition, about 55 at.% Te while
◦
at 100 C, the Te content drops to 18 at.%.
◦
Bi Te films deposited at 50 and 70 C have the same needle-like
2
3
◦
morphology (Fig. 15). The needles in the film deposited at 50 C
have sharp edges while those in the films deposited at 70 C are
bigger. On the film deposited at 35 C, small grains with very little
needles can be observed. The film deposited at 100 C shows a very
◦
High rate electrodeposition of bismuth telluride films was per-
◦
formed under galvanostatic conditions at a current density of
◦
−2
5 A dm
from a solution containing 0.4 M Bi(NO3)3 and 0.2 M
rough surface.
TeCl , with 0.5 M LiNO3 as conducting salt. This current density is
4

H.P. Nguyen et al. / Electrochimica Acta 68 (2012) 9–17
17
equivalent to a growth rate of 108.5 ␮m h−1, which is more than
twice the highest deposition rate reported in literature (50 ␮m h
Acknowledgments
−
1
by Glatz et al. via pulsed electrodeposition from a 2 M HNO3 solu-
tion [9]). EDX analysis shows that the Te content in the films is
This research was funded by a Ph.D. grant of the imec/Holst cen-
ter to HPN. The authors acknowledge financial support by the KU
Leuven (Projects IDO/05/005 and GOA 08/05), by the FWO-Flanders
(research community “Ionic Liquids”), by the IWT-Flanders (SBO-
project IWT 80031 “MAPIL”) and the support of the Belgian Federal
Science Policy Office through the IUAP project INANOMAT (contract
P6/17).
6
7 at.%, which is very close to the best n-type bismuth telluride
materials.
Stability of the EG electrolytes was investigated by long term
electrodeposition. Thick Bi–Te films were prepared in potentio-
static mode. Thickness measured along the 15 mm length of the
film shows unform values of about 67 ␮m. No cracks were observed,
implying low internal stress in the film. The composition of the film
was found to be uniform across the thickness and equal to 75 at.% Te.
The semiconductor type of the deposited films were confirmed
by Seebeck coefficient measurement described in [29]. Positive val-
ues, i.e. p-type behavior, are observed for the films deposited at
constant potential (−0.3 V to −0.9 V vs. SHE) from a solution con-
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2
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