Electrodeposition of cobalt selenide thin films

Article abstract of DOI:10.1149/1.3468675

Cobalt selenide thin films have been prepared onto tin oxide glass substrates by electrodeposition potentiostatically from an aqueous acid bath containing H₂SeO₃ and Co (CH₃COO)₂ at 50°C. The electrodeposition m

Full text of DOI:10.1149/1.3468675

Journal of The Electrochemical Society, 157 ͑10͒ D523-D527 ͑2010͒
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0013-4651/2010/157͑10͒/D523/5/$28.00 © The Electrochemical Society
Electrodeposition of Cobalt Selenide Thin Films
*
**,z
Zhian Zhang, and Yexiang Liu
Fangyang Liu, Bo Wang, Yanqing Lai, Jie Li,
School of Metallurgical Science and Engineering, Central South University, Changsha,
Hunan 410083, China
Cobalt selenide thin films have been prepared onto tin oxide glass substrates by electrodeposition potentiostatically from an
aqueous acid bath containing H2SeO3 and Co͑CH3COO͒ at 50°C. The electrodeposition mechanism was investigated by cyclic
2
voltammetry. The morphological, compositional, structural, and optical properties of the deposited films have been studied using
scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and optical absorption techniques, respec-
tively. The formation of cobalt selenide was confirmed to proceed via an underpotential deposition mechanism. Se-rich CoSe thin
films with compact and homogeneous morphology and hexagonal crystal structure were obtained at a deposition potential of Ϫ0.5
V vs saturated calomel electrode. The electrodeposited CoSe film exhibits an optical absorption coefficient of higher than
105 cm−1 and an optical bandgap of 1.53 Ϯ 0.01 eV.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3468675͔ All rights reserved.
Manuscript submitted March 2, 2010; revised manuscript received June 30, 2010. Published August 11, 2010.
Late transition-metal selenides have received considerable atten-
tion in the past few years due to their unusual structures and elec-
tronic properties.1,2 These materials, in thin-film form, have found
many applications such as in solar cells,3 light emitting devices,4
catalysts,5 superionic conductor,6 etc. Several methods have been
attempted to prepare these selenides thin films: molecular beam
epitaxy,7 metallorganic chemical vapor deposition,8 evaporation,9
chemical bath deposition,10 electrodeposition,3,11 and spray
pyrolysis.12 Compared with the other methods, electrodeposition has
numerous advantages3,13 including ͑i͒ a low cost, high rate process
involving very simple and inexpensive equipment; ͑ii͒ a large-area,
continuous, multicomponent, low temperature deposition method;
͑iii͒ deposition of films on a variety of shapes and forms; and ͑iv͒ no
use of toxic gases, effective material use, and minimum waste gen-
eration ͑solution can be recycled͒. Many attempts have been made
for the electrodeposition of late transition-metal selenide thin films
such as NixSe,11 FexSe,3,14 ZnSe,15 and CuxSe.16 However, the
preparation of cobalt selenide by this effective, low cost technique
has not been reported.
ning electron microscope ͑SEM, JSM-6360LV͒, and an X-ray dif-
fractometer ͑XRD, Rigaku3014͒, respectively. The optical properties
of the films were determined by a Shimadzu UV-2450 spectropho-
tometer.
Results and Discussion
Figure 1 shows the cyclic voltammograms for the SnO2 electrode
in the 100 mM LiCl, pH 2.0 solution in the presence of 2 mM
Co͑CH3COO͒2 and that corresponding to the blank solution. For the
blank solution, it is observed that there are one cathodic peak at
about Ϫ0.76 V and one anodic peak at about Ϫ0.41 V, which can be
assigned to a diffusion-limited reduction of protons and hydrogen
oxidation ͑Eq. 1͒, respectively.17,18 For the Co͑CH3COO͒ solution,
2
the appreciably negative shift of the cathodic peak, suggesting that
hydrogen evolution is somewhat inhibited, may be due to the de-
crease in H+ concentration near the cathode surface caused by com-
petitive adsorption of Co2+ and H+ ions. After this cathodic peak,
there is a gradual increase in current with further shifting the poten-
tial negatively up to a particular potential ͑Ϫ0.82 V in our experi-
ment͒ at which the current shows a much steeper increase abruptly
than that in the blank solution. Therefore, this particular potential is
considered as the deposition potential of cobalt according to Eq. 2.19
At potentials equal to, or below this deposition potential, both depo-
sition of cobalt and evolution of hydrogen occur simultaneously.
Anodic branch of cyclic voltammogram can also provide informa-
tion about the deposit formed. The anodic peak located at about
In this work, cobalt selenide thin films were prepared by elec-
trodeposition from an aqueous solution. The results of the investi-
gation of the deposition mechanism by cyclic voltammetry ͑CV͒, as
well as film composition, morphology, structure, and optical prop-
erties, were presented.
Experimental
The electrochemical experiments, including CV and electrodepo-
sition, were carried out in a three-electrode cell configuration with a
SnO2-coated glass substrate ͑20 ⍀/ᮀ͒ as the working electrode, a
purity graphite plate as the counter electrode, and a saturated
calomel electrode ͑SCE͒ as a reference electrode. All potentials are
reported with respect to this reference. All substrates were ultrasoni-
cally cleaned with acetone, rinsed with deionized water
͑18.2 M⍀ cm−1͒, and then subsequently dried. The electrolyte so-
lution consisted of 2 mM H2SeO3, 2 mM Co͑CH3COO͒2, and 100
mM LiCl. The pH of the solution was adjusted to 2.0 using concen-
trated HCl. Residual oxygen in the bath was removed by bubbling
N2 for 20 min before each experiment. A Princeton Applied Re-
search 2273A potentiostat was used for all electrochemical experi-
ments. The cyclic voltammograms were measured at a scan rate of
10 mV/s and were first scanned in the negative direction. All experi-
ments were performed in a stagnant bath at 50°C.
The chemical composition, surface morphology, and crystalline
properties of the prepared films were characterized by an energy-
dispersive X-ray spectroscope ͑EDS, EDAX-GENSIS60S͒, a scan-
Figure 1. ͑Color online͒ Cyclic voltammograms for SnO2 electrode at 10
mV/s scan rate and 50°C in 100 mM LiCl, pH 2.0 solution with ͑curve a͒
and without ͑curve b͒ 2 mM Co͑CH3COO͒2.
*
Electrochemical Society Student Member.
Electrochemical Society Active Member.
**
z E-mail: csulightmetals@126.com
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Journal of The Electrochemical Society, 157 ͑10͒ D523-D527 ͑2010͒
Figure 3. ͑Color online͒ Cyclic voltammograms for SnO2 electrode at 10
mV/s scan rate and 50°C in 100 mM LiCl + 2 mM H2SeO3, pH 2.0 solution
with ͑curve a͒ and without ͑curve b͒ 2 mM Co͑CH3COO͒2.
Figure 2. ͑Color online͒ Cyclic voltammograms for SnO2 electrode at 10
mV/s scan rate and 50°C in 100 mM LiCl, pH 2.0 solution with ͑curve a͒
and without ͑curve b͒ 2 mM H2SeO3.
Ϫ0.48 V relates to hydrogen oxidation, as already shown in the
cyclic voltammogram of the blank solution. Another anodic peak at
Ϫ0.10 V with a shoulder at about Ϫ0.38 V, which has been fre-
quently observed by others,20-23 can be associated either with the
dissolution of cobalt from the electrode to the solution in the form of
different ionic species or with the dissolution of different cobalt
phases, previously formed during the cathodic scan.20,21,24 The cur-
rent remains cathodic upon the sweep reversal, crossing over the
current recorded during the negative sweep at Ϫ0.17 V. This current
loop is attributed to the fact that the deposition of metals onto its
own, in this case cobalt on cobalt, occurs at lower overpotentials
than the deposition of metal onto different nature substrates, in this
H2SeO3 + 4H+ + 4e− ꢀ Se + 3H2O
H2SeO3 + 6H+ + 6e− ꢀ H2Se + 3H2O
H2SeO3 + 2H2Se ꢀ Se + 3H2O
͓3͔
͓4͔
͓5͔
Figure 3 presents the cyclic voltammograms for the SnO2 elec-
trode in the 100 mM LiCl + 2 mM Co͑CH3COO͒ + 2 mM
2
H2SeO3, pH 2.0 solution. Additionally, a voltammogram corre-
sponding to the 100 mM LiCl + 2 mM H2SeO3, pH 2.0 solution is
shown for comparison. The initial reductive feature beginning from
0.15 V and corresponding to the predeposition of selenium is ob-
served again from the inset of Fig. 3. However, the current of this
reductive feature in the binary Co–Se system is slightly larger than
that in the unitary 2 mM H2SeO3 solution, and this increase in
current continues to extend to the four-electron bulk Se deposition
potential region. The small increase in current can be reasonably
attributed to the contribution of additional reduction reaction with
very slow deposition kinetics, which involves cobalt forming cobalt
selenide proceeded by Eq. 6. The deposition of Co thus begins much
earlier on a Se surface than on the SnO2 surface. Whatever the
cobalt selenide stoichiometry is, CoxSe is presented here for sim-
plicity. This is characteristic of the induced underpotential deposi-
tion mechanism, known as Kröger’s mechanism,34 which is caused
by the large energy release in the formation of cobalt selenides. For
instance, the value of the Gibbs free energy of the formation of
25
case cobalt on SnO2
2H+ + 2e ꢀ H2
Co2+ + 2e ꢀ Co
͓1͔
͓2͔
Figure 2 illustrates the cyclic voltammograms for the SnO2 elec-
trode in the 100 mM LiCl, pH 2.0 solution in the presence of 2 mM
H2SeO3 and that corresponding to the blank solution. For the
H2SeO3 solution, the curve displays two cathodic peaks at Ϫ0.36
and Ϫ0.65 V. In combination with previous studies,26-32 the assign-
ment of these two peaks is as follows: The weak cathodic peak at a
potential of about Ϫ0.36 V corresponds to bulk selenium deposition
through the four-electron reduction of Se͑IV͒ to Se͑0͒ proceeded by
Eq. 3. Another cathodic peak at about Ϫ0.65 V corresponds to the
six-electron reduction of Se͑IV͒ to Se͑ϪII͒ according to Eq. 4. The
charge involved in the peak at Ϫ0.36 V is much smaller than that
involved in the peak at Ϫ0.65 V, also contrasting with four-electron
and six-electron processes, respectively. The product Se͑ϪII͒ then
undergoes a comproportionation reaction with Se͑IV͒ in the solution
͑Eq. 5͒, leading to the chemical formation of Se͑0͒.29,32 This process
of selenium deposition according to the reactions in Eq. 4 and 5
looks like the four-electron reduction ͑Eq. 3͒ when the concentration
of H2SeO3 is high enough. It is also observed from the inset in Fig.
2 that there is an initial reductive feature before the peak at Ϫ0.36 V
in the potential range from 0.15 to Ϫ0.05 V identified from a very
weak reduction current, similar to that reported by other groups31,32
and us.33 This weak reduction current may correspond to the four-
electron predeposition of selenium on SnO2 substrate before the
overpotential deposition of bulk selenium caused by the deposit–
substrate interaction according to Eq. 3.29-33 No anodic peaks are
seen, although the anodic current is detected obviously because the
potential is not positive enough for the development into an anodic
peak for selenium oxidation.
35
CoSe is Ϫ40.74 kJ/mol and, therefore, the redox potential of the
reaction in Eq. 6 is shifted by an amount of −⌬G/2F = +0.211 V
theoretically with respect to the standard deposition potential of me-
tallic cobalt. This mechanism has also been employed for the elec-
trodeposition of many other compound semiconductors.36-39 Obvi-
ously, the peak for the six-electron reduction reaction shows a
significant positive shift, revealing that Co2+ helps in promoting the
six-electron reduction of H2SeO3. This is because the generated
H2Se immediately reacts with Co2+ in the solution according to Eq.
7 due to the large free energy of formation of cobalt selenide again,
which can facilitate the reaction in Eq. 4 in the forward direction.40
By combining Eq. 4 and 7 to form Eq. 8, it is indicative once more
that Co2+ promotes the six-electron reduction of H2SeO3. This posi-
tive shift leads to the overlapping of four-electron and six-electron
Se͑IV͒ reduction potential regions. Therefore, the much greater in-
crease in current when the potential reaches about Ϫ0.35 V can be
due to the contribution of two reduction reactions proceeded by Eq.
6 and 8 together. Moreover, the increase in current becomes more
and more significant with further shifting the potential toward the
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Journal of The Electrochemical Society, 157 ͑10͒ D523-D527 ͑2010͒
D525
negative direction, which can be due to the growing reaction rates of
both reactions in Eq. 6 and 8 under the enlarged reaction driving
force with the negative shift of the cathodic potential
Se + xCo2+ + 2xe ꢀ CoxSe
͓6͔
͓7͔
H2Se + xCo2+ ꢀ CoxSe + 2H+
H2SeO3 + xCo2+ + 4H+ + ͑4 + 2x͒e + ꢀ CoxSe + 3H2O ͓8͔
All the above results allow us to conclude that the deposition of
Co into the CoxSe solid phase can proceed through two different
routes: surface-induced reduction by Se and reaction with H2Se. In
any case, the underpotential deposition of cobalt as cobalt selenide
is apparent.
Figure 4 shows the EDS composition of cobalt selenide films
deposited at different potentials between Ϫ0.3 and Ϫ0.7 V from the
2 mM Co͑CH3COO͒ + 2 mM H2SeO3, pH 2.0 solution. The films
2
electrodeposited at potentials more negative than Ϫ0.7 V show
many pinholes and poor adherence because of hydrogen evolution
and have not been considered. For films deposited at Ϫ0.3 V or
more positive potentials, the content of Co is very low, making it
difficult to confirm the incorporation of Co into films by an EDS
Figure 4. ͑Color online͒ The EDS composition of electrodeposited cobalt
selenide films at 50°C and different potentials between Ϫ0.3 and Ϫ0.7 V
from 2 mM Co͑CH3COO͒ + 2 mM H2SeO3, pH 2.0 solution.
2
Figure 5. The SEM micrograph of elec-
trodeposited cobalt selenide thin films at
50°C: ͑a͒ Deposition potential = −0.3 V,
͑b͒ deposition potential = −0.4 V, ͑c͒
deposition potential = −0.5 V, ͑d͒ depo-
sition potential = −0.6 V, ͑e͒ deposition
potential = −0.7 V, and ͑f͒
a cross-
sectional view of cobalt selenide film elec-
trodeposited at Ϫ0.5 V.
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Journal of The Electrochemical Society, 157 ͑10͒ D523-D527 ͑2010͒
Figure 6. ͑Color online͒ X-ray diffraction
pattern of cobalt selenide thin film elec-
trodeposited at Ϫ0.5 V and 50°C on SnO2
substrate.
͑the EDS measurement error is within 3–5 atom %͒, which is attrib-
uted to the extremely slow kinetics of the underpotential deposition
of cobalt selenide when the deposition potential is not negative
enough based on the above CV studies. It is observed that, as the
deposition potential shifts negatively, the cobalt content increases
rapidly and the selenium content decreases in the film, leading to a
composition of 56.6 atom % Se and 43.4 atom % Co obtained at
Ϫ0.5 V. Further negative shift of the deposition potential, however,
does not change the composition of the films significantly, which
indicates that the electrodeposition process is mainly controlled by a
diffusion independent of the deposition potential. This diffusion-
controlled process has been maintained until the onset of the evolu-
tion of hydrogen, which disturbs the double layer forming on the
electrode surface, leading to the instability of electrodeposition and
accordingly film composition fluctuation.41
Figure 5a-e shows the dramatic difference of surface morpholo-
gies of electrodeposited Co–Se films at varied deposition potentials
from Ϫ0.3 to Ϫ0.7 V. The film deposited at Ϫ0.3 V consists mainly
of selenium and shows some clusters with sizes between 0.2 and
1 ␮m and some agglomerates with size larger than 2 ␮m ͑Fig. 5a͒.
Similar morphologies of Se films were observed by Solaliendres et
al.42 The Se clusters are eliminated significantly with incorporation
of a certain amount of Co with the deposition potential shifting
negatively to Ϫ0.4 V ͑Fig. 5b͒. When the deposition potential
reaches Ϫ0.5 V, the film shows a very compact and homogeneous
surface morphology having isolated grains with uniform size and
well-defined boundaries ͑Fig. 5c͒. With further negative shift of the
deposition potential, the films became rough and porous ͑Fig. 5d͒ or
even a loose structure with flocculent appearance ͑Fig. 5e͒. This is
due to the high electrode reaction rate and excessive concentration
polarization.43Generally, compact and smooth films are needed for
application. Therefore, the deposition potential of Ϫ0.5 V was con-
sidered to be optimum for cobalt selenide thin-film electrodeposi-
tion. To get insight into the microstructure profile of the electrode-
posited cobalt selenide thin film at Ϫ0.5 V, the cross-sectional SEM
image is also present, as shown in Fig. 5f. The compact and uniform
cobalt selenide thin film consisting of large grains extending from
the bottom to the top of the film is obtained, and the thickness of the
film is ϳ400 nm.
the peaks of SnO2 ͑JCPDS card no. 41-1445͒, which come from the
substrate, perfectly match with the hexagonal phase of CoSe
͑JCPDS card no. 89-2004͒. No characteristic peaks were observed
for other impurities. The excess Se according to the EDS composi-
tion cannot be indexed by an XRD due to its amorphous nature.
Therefore, the deposits on the surface of the SnO2 film consist of a
compact film of the Se-rich CoSe sample.
Figure 7 shows the optical absorption coefficient ͑␣͒ of the elec-
trodeposited CoSe thin film as a function of photon energy ͑h␯͒,
converted from the transmission spectra recorded in the range of
300–900 nm. The sharp line near 300 nm ͑4.13 eV͒ is due to the
change in source and the lower wavelength absorption by the
substrate.44 The absorption coefficient is larger than 10−5 cm−1 in
the visible region, which supports the direct bandgap nature of the
material3 and reveals that the CoSe film can be considered to be a
suitable material for photovoltaic solar energy conversion. Based on
the allowed direct interband transition, the bandgap is determined to
be 1.53 Ϯ 0.01 eV by extrapolating the linear ͑␣h␯͒2 vs h␯ plots to
Figure 6 displays the XRD patterns measured for the electrode-
posited cobalt selenide thin film at Ϫ0.5 V and the SnO2/glass sub-
strate. The results clearly indicate that all diffraction peaks, except
Figure 7. Optical absorption coefficient ͑␣͒ of the electrodeposited CoSe
2
thin film at Ϫ0.5 V and 50°C. The inset shows ͑␣h ͒ vs h for CoSe film;
v
v
the estimated bandgap is 1.52 eV.
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Journal of The Electrochemical Society, 157 ͑10͒ D523-D527 ͑2010͒
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2
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