Electrodeposition of thin Re-Se films

Article abstract of DOI:10.1023/A:1022178109180

Re, Se, and Re-Se electrodeposition from alkaline electrolytes was studied. The effects of process variables were examined, and the nature of polarization in the course of rhenium and selenium deposition from alkaline electrolytes was assessed. Re-Se elec

Full text of DOI:10.1023/A:1022178109180

Inorganic Materials, Vol. 39, No. 2, 2003, pp. 99–102. Translated from Neorganicheskie Materialy, Vol. 39, No. 2, 2003, pp. 142–146.
Original Russian Text Copyright © 2003 by Salakhova.
Electrodeposition of Thin Re–Se Films
E. A. Salakhova
Institute of Inorganic and Physical Chemistry, Academy of Sciences of Azerbaijan,
pr. Dzhavida 29, Baku, 370143 Azerbaijan
e-mail: itpcht@itpcht.ab.az
Received May 18, 2001; in final form, August 13, 2002
Abstract—Re, Se, and Re–Se electrodeposition from alkaline electrolytes was studied. The effects of process
variables were examined, and the nature of polarization in the course of rhenium and selenium deposition from
alkaline electrolytes was assessed. Re–Se electrodeposition was investigated by the potentiodynamic polariza-
tion technique. The codeposition process was shown to be attended by depolarization, which is due to the
energy release upon the formation of the alloy.
INTRODUCTION
added to the solution, which was then diluted to 50 ml
in a volumetric flask. Next, rhenium was separated
from selenium by selective extraction with isoamyl
alcohol. Rhenium and selenium were determined by
photometric analysis (FEK-56M instrument) of
rhodanide and thiourea complexes, respectively [6].
The Re–Se films were characterized by x-ray diffrac-
tion (XRD) using a 57.3-mm Debye–Scherrer camera
(CuKα radiation, URS-55 x-ray generator).
Rhenium–selenium alloys possess semiconducting
properties and are employed as photosensitive elements
for the visible range. In most semiconductor devices,
rhenium alloys are used in thin-film form [1–3].
In this paper, we report our efforts to prepare thin
Re–Se films by electrochemical means. Electrodeposi-
tion offers the advantages of simple equipment, relative
ease, and low cost and, in some cases, allows one to
produce films of controlled composition by adjusting
the electrolyte composition and electrolysis conditions.
RESULTS AND DISCUSSION
Earlier [4, 5], Re–Se alloys were prepared using
electrolytes consisting of NH4ReO4 (10–50 g/l), SeO2
(10–50 g/l), and H2SO4 (200 g/l).
At the same time, it is of interest to use electrolytes
containing rhenium and selenium compounds as major
constituents in order to produce alloys close in proper-
ties to those employed in modern technology.
We studied the effects of electrolyte composition, cur-
rent density, and temperature on the electrolysis process.
jc, mA/cm2
6
Here, we describe electrodeposition from 0.01–0.1 M
NH4ReO4 + 0.01–0.1 M SeO2 + NaOH alkaline electro-
lytes.
We prepared Re, Se, and Re–Se films. Particular
attention was paid to electrodeposition of thin Re–Se
films from alkaline electrolytes. The process was fol-
lowed by the potentiodynamic polarization technique.
4
6
5
7
4
2
EXPERIMENTAL
3
Potentiodynamic polarization curves were recorded
using a P-5827 potentiostat and PDP4-002 chart
recorder. Pt wire and a 0.15-cm2 Pt platelet were uti-
lized as the anode and cathode, respectively. All poten-
tials were measured with respect to an Ag/AgCl refer-
ence. The electrolyte temperature was maintained con-
stant using a thermostat. The cathode deposit was
analyzed for Re and Se. To this end, the deposit was dis-
solved in concentrated HNO3 (10 ml) during heating.
After long-tern boiling on a water bath, 5 N H3PO4 was
1
2
–0.6
–0.8
–1.0
ϕ, V
Fig. 1. Polarization curves for (1) a blank solution and
(2, 3) Re, (4, 5) Se, and (6, 7) Re–Se electrodeposition at
75°C. Electrolytes: (1) NaOH, (2) NaOH + 0.05 M
NH ReO , (3) NaOH + 0.08 M NH ReO , (4) NaOH +
0.05 M SeO , (5) NaOH + 0.08 M SeO , (6) NaOH +
4
4
4
4
2
2
0.05 M NH ReO + 0.05 M SeO , (7) NaOH + 0.08 M
4
4
2
NH ReO + 0.08 M SeO .
4
4
2
0020-1685/03/3902-0099$25.00 © 2003 MAIK “Nauka/Interperiodica”

100
SALAKHOVA
ϕ, V
ϕ, V
1
1
2 3 4
–0.75
–0.70
2
–1.1
–1.0
–0.9
3
–0.65
–0.60
–4
–3
–2
–1
log j [A/cm2]
–4.0 –3.5 –3.0 –2.5 –2.0
log j [A/cm2]
Fig. 3. Polarization plots for solutions containing 1 M
Fig. 2. Polarization plots for solutions containing 1 M
NaOH and (1) 0.02, (2) 0.05, (3) 0.06, and (4) 0.09 M
NaOH and (1) 0.03, (2) 0.05, and (3) 0.08 M SeO ; t =
2
75°C.
NH ReO ; t = 75°C.
4
4
Figure 1 shows the polarization curves of a blank more rapidly than the rate of rhenium deposition.
solution (curve 1) and Re (curves 2, 3), Se (curves 4, 5), Under such conditions, the current efficiency in terms
and Re–Se (curves 6, 7) electrodeposition. The NaOH of rhenium is 4%. The low current efficiency is due to
concentration and temperature (75°C) were maintained the low overpotential of hydrogen on rhenium and plat-
constant. It can be seen in Fig. 1 that increasing the rhe- inum. Owing to the appreciable hydrogen sorption by
nium concentration in the electrolyte increases the dep- rhenium during electrodeposition, rhenium acts as a
osition rate and shifts the cathode potential to positive hydrogen electrolyte, and the potential (–1.0 V) is
values.
determined by the hydrogen saturation of the metal [3].
The current efficiency versus current density data
for rhenium electrodeposition are summarized in
Table 1. Figure 2 demonstrates that, with increasing
rhenium concentration, the cathodic polarization plot
shifts to higher potentials, with no change in slope
At higher rhenium concentrations in the electrolyte,
we obtain lustrous, gray, uniform coatings up to 10 µm
thick on the cathode surface. At low rhenium concen-
trations and a current density of 1 mA/cm2 µm, elec-
trodeposition yields lustrous, gray, uniform coatings
≤2 µm in thickness.
(∂ϕc/∂ log jc = 0.1 V).
Rhenium deposition is accompanied by hydrogen
To gain detailed insight into the mechanism of sele-
release, and hydrogen actively adsorbs on the electrode nium cathodic deposition in alkaline solutions, we
surface. The rate of hydrogen release depends on the examined the effect of selenium concentration in the
electrolyte composition and current density. The rhe- electrolyte on cathodic polarization. As seen in Fig. 3,
nium deposition and hydrogen adsorption on the cath- the semilog plot of cathodic polarization shifts to
ode begin at –1.0 V. Increasing the magnitude of the higher potentials with increasing Se concentration in
potential increases the rate of hydrogen release much the electrolyte, which points to an increase in the rate of
Table 1. Current efficiency vs. current density data for rhenium electrodeposition from a 0.05 M NH4ReO4 + NaOH solution
at t = 75°C
j, mA/cm2
Re deposition
Current efficiency, %
rhenium hydrogen
4.0 96
External appearance of the coating
total
2
5
0.08
0.17
0.32
0.33
Light gray, lustrous, uniform
Light gray, lustrous, smooth
Dark gray, dull, smooth
3.5
3.2
2.2
96.5
96.8
97.8
10
15
Dark gray, lustrous, nonuniform
INORGANIC MATERIALS Vol. 39 No. 2 2003

ELECTRODEPOSITION OF THIN Re–Se FILMS
101
log j [A/cm2]
Ea, kJ/mol
–2.5
–3.0
–3.5
40
30
6
5
4
20
10
3
2
1
–4.0
–2.4 –2.6 –2.8 –3.0 –3.2 –3.4 –3.6
103/T, K–1
–0.65 –0.75 –0.80
ϕ, V
Fig. 4. Temperature dependences of current density at Pt
electrode potentials of (1) –0.65, (2) –0.68, (3) –0.70,
(4) −0.72, (5) –0.75, and (6) –0.80 V.
Fig. 5. Apparent activation energy as a function of potential.
3
2
jc, mA/cm2
wt % Re
1
60
10
3
2
40
20
1
5
4
2
4
6
8
jc, mA/cm2
–0.70
–0.75 –1.00
–1.10
ϕ, V
Fig. 7. Rhenium content of the Re–Se alloy as a function of
current density at 75°C. Electrolytes: (1) 0.01 M NH ReO +
4
4
Fig. 6. Partial polarization curves for rhenium and selenium
deposition from alkaline electrolytes at 75°C: (1) rhenium
in the alloy, (2) elemental selenium, (3) selenium in the
alloy, (4) elemental rhenium.
0.09 M SeO + 1 M NaOH, (2) 0.03 M NH ReO + 0.07 M
2
4
4
SeO + 1 M NaOH, (3) 0.05 M NH ReO + 0.05 M SeO +
2
4
4
2
1 M NaOH.
nium, the formation of Se2–, and hydrogen release:
SeO32– + 4e + 3H2O
Se + 6OH–,
3Se + 6OH–
2Se2– + SeO32– + 3H2O.
the cathodic process. The slope of the plot is
∂ϕc/∂ log jc = 0.056–0.07 V. These data suggest that, at
low current densities, selenium deposition is accompa-
nied by chemical polarization.
The formation of Se2– was also reported by Baeshov
et al. [7], who showed that, with increasing NaOH con-
centration, the wave in polarization curves disappears.
To identify the rate-limiting step of the cathodic pro-
cess and the nature of cathodic polarization upon the
reduction of SeO32– in alkaline electrolytes, we exam-
The polarization curves of selenium electrodeposi-
tion from alkaline electrolytes (Fig. 1, curves 4, 5) dis-
play a prominent peak at –0.8 V. The polarization
curves of Se4+ reduction on the platinum cathode in
alkaline media consist of three distinct portions, which
correspond to the reduction of Se4+ to elemental sele-
INORGANIC MATERIALS Vol. 39 No. 2 2003

102
SALAKHOVA
Table 2. Composition and quality of Re–Se coatings as functions of current density in a 1 M NaOH solution at t = 75°C
Electrolyte, mol/l
NH4ReO4 SeO2
0.05 0.05
Alloy composition, wt %
j, mA/cm2
Light gray, nonuniform
(deposition)
Re
Se
2
4
5
7
38.2
48.4
54.0
72.1
61.8
51.6
46.0
27.9
Dark gray, uniform, smooth
Black, lustrous, smooth
0.05
0.05
0.05
0.05
0.05
0.05
Dull black, smooth
External appearance of the coating
ined the temperature effect on the rate of the electrode Se. During the electrolysis of mixtures of alkaline Se and
Re solutions, the rate of ReO4– and SeO32– reduction is
process at a constant cathode potential. The results are
displayed in Fig. 4. From the slope of the lines thus
higher. This seems to be due to the depolarization effect,
obtained, we evaluated the apparent activation energy
associated mainly with the formation of the alloy. Con-
of the electrode process (Fig. 5). At potentials from
sequently, codeposition of rhenium and selenium must
−0.65 to –0.725 V, the apparent activation energy
lead to the formation of a compound or solid solution.
attains 30–35 kJ/mol. With increasing cathodic polar-
Our results demonstrate that, as the current density
rises from 2 to 8 mA/cm2, the rhenium content of the
alloy increases from 20 to 80 wt % (Fig. 7) and the
quality of the coating improves (Table 2).
At a cathode current density of 4 mA/cm2, using an
electrolyte of composition 0.05 M NH4ReO4 + 0.05 M
ization, the apparent activation energy drops to
20 kJ/mol at –0.8 V and then varies little.
Thus, at cathodic polarizations above –0.785 V,
selenium electrodeposition from alkaline electrolytes is
accompanied mainly by chemical polarization.
Since selenium deposition on the cathode is
attended by the release of a substantial amount of SeO2 + 1 M NaOH, we obtained lustrous ReSe2
hydrogen, we obtained partial polarization curves in
order to gain detailed insight into the electrode pro-
cesses involved.
Figure 6 shows the partial polarization curves for Re,
Se, and Re–Se deposition. From the curves of Re depo-
sition in elemental form and in the Re–Se alloy, it follows
that depolarization in the latter case is 350 mV (Fig. 6).
For selenium in the alloy, depolarization is 20–30 mV.
(54 wt % Re) coatings. The formation of ReSe2 was
confirmed by XRD. This compound crystallizes in the
triclinic system with lattice parameters a = 6.7275 Å,
b = 6.6065 Å, and c = 6.7196 Å. The XRD intensities
and d spacings for ReSe2 are listed in Table 3.
CONCLUSION
As seen in Fig. 1, the curves for Re–Se deposition
(curves 6, 7) lie at higher potentials than those for Re and
Codeposition of rhenium and selenium was shown
to be attended by depolarization, which is due to the
energy release upon the formation of the alloy.
Table 3. XRD data for ReSe2 obtained in an alkaline elec-
trolyte of composition 0.05 M NH4ReO4 + 0.05 M SeO2 +
NaOH (at jc = 5 mA/cm2 and t = 75°C)
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