Electro-deposition of Co-La alloy films in urea melt and their performances

Article abstract of DOI:10.1016/j.jallcom.2006.10.091

The Co-La alloy film was electro-deposited in urea melt. The co-deposition behavior, the effect of the cathodic current density on the composition and the surface morphology of the coating were examined, respectively. As a result, lanthanum is co-deposited with cobalt to form Co-La alloy under the inducement effect of cobalt. With the increase of the cathodic current density, La content of the deposit rises at first, then decreases, and reaches a maximal value at the current density of 30 mA cm^-2. Meanwhile, the size of the coating particles becomes small. The more content of Co in the deposit is, the more the saturation magnetization is. The crystallization behavior of the coating was studied by using the differential scanning calorimetry and the X-ray diffraction. The as-plated deposit consists of main amorphous phase and a little amount of hexagon cobalt phase (P6₃/mmc). The amorphous phase was converted into Co-La (Fm3m) phase at 438.9 °C, and hexagon cobalt phase was crystallized into cubic cobalt at 687.1 °C. The electro-catalytic activity of the hydrogen evolution for the Co-La alloy was studied by using electrochemical experiments. The results showed that the electro-catalytic activity of the hydrogen evolution of the alloy is better than that of cobalt.

Full text of DOI:10.1016/j.jallcom.2006.10.091

Journal of Alloys and Compounds 450 (2008) 142–147
Electro-deposition of Co–La alloy films in urea melt
and their performances
∗
Sen-Lin Wang , Ji-Bei Lin, Qiu-Yan Cai, Yan Zhang
Department of Applied Chemistry, College of Materials Science and Engineering, Huaqiao University, Quanzhou 362021, PR China
Received 30 July 2006; received in revised form 11 October 2006; accepted 20 October 2006
Available online 2 January 2007
Abstract
The Co–La alloy film was electro-deposited in urea melt. The co-deposition behavior, the effect of the cathodic current density on the composition
and the surface morphology of the coating were examined, respectively. As a result, lanthanum is co-deposited with cobalt to form Co–La alloy
under the inducement effect of cobalt. With the increase of the cathodic current density, La content of the deposit rises at first, then decreases,
−
2
and reaches a maximal value at the current density of 30 mA cm . Meanwhile, the size of the coating particles becomes small. The more content
of Co in the deposit is, the more the saturation magnetization is. The crystallization behavior of the coating was studied by using the differential
scanning calorimetry and the X-ray diffraction. The as-plated deposit consists of main amorphous phase and a little amount of hexagon cobalt
◦
phase (P6
3
/mmc). The amorphous phase was converted into Co–La (Fm3m) phase at 438.9 C, and hexagon cobalt phase was crystallized into cubic
◦
cobalt at 687.1 C. The electro-catalytic activity of the hydrogen evolution for the Co–La alloy was studied by using electrochemical experiments.
The results showed that the electro-catalytic activity of the hydrogen evolution of the alloy is better than that of cobalt.
©
2006 Elsevier B.V. All rights reserved.
Keywords: Co–La alloy film; Electro-deposition; Urea; Performance
1
. Introduction
plating parameters, the films can be formed on various surface
shapes and the technique is simple and economical. Recently the
electro-deposition of IG–RE alloy films from an organic bath or
low-temperature melt salt was reported. Some rare earth alloys,
such as Sm–Co, Tm–Co, Gd–Co, Nd–Co, etc. were success-
fully deposited from a urea melt salt [6–11]. The urea melt salt
Co–La alloys exhibit interesting magnetic, electric and
hydrogen storage properties and have been used to make func-
tional materials. Iron group (IG)–rare earth (RE) alloys is a type
of important functional material for possessing magnetic, opti-
cal, and electric properties [1,2]. They are commonly prepared
as bulk materials by means of a powder metallurgical sinter-
ing, melt spinning, and mechanical alloying routes, but there is
great demand of film materials for micro-machine technology,
recording media and other applications. Although the sputtering
and vacuum evaporation methods are available for preparing the
IG–RE alloy films, it is difficult to control the film composition
by these methods [3–5]. Furthermore, the equipments are costly
and the technique is complicated. Electrochemical deposition
has been proven to be a good way to prepare these alloy films. It
has many advantages. For examples, the phase and the compo-
sition of the alloy films can be easily controlled by adjusting the
◦
system has lower eutectic points below 134 C, greater solubil-
ity of metallic salt, and a wide electrochemical window. In this
work, the Co–La film by electro-deposition from urea melt salt
was investigated. Moreover, the influence of the operating con-
ditions on the structure and the properties of the deposit, and the
crystallization behavior and the electro-catalytic activity of the
hydrogen evolution of the coating were studied.
2
. Experimental details
All chemical reagents are analytically pure. CoSO4 and LaCl3 were obtained
by the dehydration of CoSO4·7H2O and LaCl3·7H2O under vacuum at 393 K,
respectively. Urea was also dried under vacuum at 393 K. The mixture of urea
(
83.3 wt%) and NaBr (16.7 wt%) was melted at a temperature high than 403 K.
The Co–La alloys were electro-deposited by controlling cathodic current
∗
Corresponding author. Tel.: +86 595 22693746.
E-mail address: slwang@hqu.edu.cn (S.-L. Wang).
density from the plating bath containing cobalt sulphate of 0.31 g, lanthanum
chloride of 0.98 g, urea of 60.00 g and sodium bromide of 10.00 g. The
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.10.091

S.-L. Wang et al. / Journal of Alloys and Compounds 450 (2008) 142–147
experiments were controlled at 403 K by oil bath thermostat. The cathodic
143
−2
current density (Jk) was 10–40 mA cm . The anodic electrode was a graphite
sheet (3.5 cm × 3.0 cm × 0.5 cm).
The electrochemical experiments were carried out using a CHI-660B elec-
trochemical analyzer (CH Instruments). Cyclic voltammetric experiments (CV)
were done in a conventional three-electrode cell (75 ml) and also controlled at
2
4
03 K by oil bath thermostat. The brass disc working electrode (0.1256 cm )
was fitted into a Teflon exposing circular, and its surface was mechanically
polished with 600-grit sand paper, then with 30–50 nm ␣-alumina powder and
was rinsed with alkaline solution and chemically etched before the experiments.
The auxiliary electrode was a platinum sheet (0.9 cm × 1.2 cm × 0.03 cm) and
the reference electrode was platinum wire immersed in urea–NaBr melt in a
−
1
glass tube with a Luggin capillary. The scanning potential rate was 50 mV s
.
Linear sweep voltammetry experiments (LSV) were done in a conventional
three-electrodecell(100 ml)at298 K. Theauxiliaryelectrodewasaplatinumfoil
(
0
1.0 cm × 1.5 cm × 0.05 cm) and the reference electrode was Hg–HgO (NaOH,
−
1
.1 mol l ). LSV experiments employed a carbon steel disc as the working
2
electrode of 0.5 cm in surface area. The Co–La electrode was plated in above
−
2
electroplating melt at the cathodic current density 10.0 mA cm , and cobalt
electrode was deposited in above electroplating melt but not containing lan-
thanum chloride. The electrolyte solution is 10 wt% potassium hydroxide. The
solution was de-aerated with nitrogen gas for 5 min before and during the exper-
Fig. 1. The CV curves in urea–NaBr–CoSO4 (a) and urea–NaBr–CoSO4–LaCl3
(
b). (a) Cobalt sulphate of 0.31 g, urea of 60.00 g and sodium bromide of 10.00 g
and (b) cobalt sulphate of 0.31 g, lanthanum chloride of 0.98 g, urea of 60.00 g
and sodium bromide of 10.00 g.
−1
iments. The scanning potential rate was 1 mV s
.
The heat treatment experiments of the sample in a vacuum (0.67 Pa) were
carried out at various temperatures for 1.0 h. The structure of the deposit was
characterized by using an X-ray diffractometer (Rigaku, D/max-RC) with Cu
more than that of the melt (a). The curves of the melt (a) and
the melt (b) all appeared in the current cross-ring, revealing
that cobalt and cobalt–lanthanum was deposited from the each
melt salt, respectively. In a word, this indicated that Co(II) and
La(III) occurred co-deposition, and the reduction of La(III) may
be induced by cobalt. The molten only containing RE(III) salt
could not be reduced into metallic RE was approved by Liu
et al. and they putted forward the inducement co-deposition of
cobalt–lanthanum [6].
◦
−1
K␣ radiation and a scan rate of 6 min . The samples for the above measure-
2
ments were plated on the carbon steel substrates of 3.0 cm surface area. The
crystallization temperature and the thermogravity analysis of the coating were
measured by a differential scanning calorimeter (DSC-2960, TA Instruments)
in an atmosphere of pure argon gas. To prepare specimens for the DSC and TG
measurements, the alloy was deposited on the stainless steel sheets, and then
mechanically peeled off from the substrate. The magnetic measurements were
made using a vibrating sample magnetometer (VSM). The sample of VSM was
2
plated on the copper substrate whose surface area is about 0.25 cm . The mor-
phology and composition of the Co–La deposit was characterized with SEM
(
scanning electron microscope; Hitachi S3500N) and EDS (energy dispersive
3
.2. Effect of the cathodic current density on the
X-ray spectroscopy; Oxford INCA) attached in the SEM, respectively.
composition of the Co–La films
3
. Results and discussion
The EDX spectrum of the Co–La deposit is shown in Fig. 2.
It is found that the chemical composition is almost the same in
the different zones, and the coating contains Co, La and small
amount of O elements. The content of O in the coating might be
due to the oxidation in the air when the coating was taken out
from the plating bath. Fig. 2 also demonstrates the high purity
of the coating.
3
.1. The co-deposition behavior of Co–La alloy
Fig. 1 presents the cyclic voltammetric (CV) curves obtained
using a brass disc as working electrode in urea–NaBr–CoSO4
a) and urea–NaBr–CoSO4–LaCl3 (b). The voltammetric scan
(
was swept negatively at first. When the potential swept nega-
tively, the cathodic current of the melt (a) increased initially,
appeared a peak at about −0.860 V (versus Pt, urea + NaBr, the
same as below), and then decreased. It is the reason that the
electro-deposition process of cobalt is controlled by diffusion.
When the potential is controlled below −1.000 V, the cathodic
current of the evolution hydrogen appeared. In this time urea
was reduced on working electrode surface covered with cobalt
film, the reaction is
Fig. 3 is the effect of the cathodic current density on the
composition of the Co–La deposit. It can be seen that with the
2
H2N–CO–NH2 + 2e = H2 + 2H2N–CO–NH−
The initiative deposition potential of the melt (b) was more
negative than that of the melt (a). As the potential swept more
negatively than −0.910 V, the cathodic current of the melt (b)
was more than that of the melt (a). The melt (b) also showed
the hydrogen evolution when the potential swept more nega-
tively than −1.000 V. The anodic current of the melt (b) was
Fig. 2. The EDX spectrum of the Co–La deposit obtained from Jk
(10 mA cm 2).
−

1
44
S.-L. Wang et al. / Journal of Alloys and Compounds 450 (2008) 142–147
site to that of lanthanum. The increase of the cathodic current
density favors the reduction of La3 because of the induced
co-deposition mechanism. However, when the cathodic current
density is too high, the evolution of hydrogen occurs, leading to
the decrease of the La content in the coating.
+
3.3. Effect of the cathodic current density on the surface
morphology of the Co–La coatings
The surface morphology of the Co–La deposits plated from
different cathodic current density is shown in Fig. 4. The Co–La
deposits are grey with metallic luster in appearance. To analyze
the effect of the cathodic current density on the surface mor-
phology of the deposit, three deposits were prepared with the
−2
current density of 10, 30 and 40 mA cm , respectively. The
results clearly show the changes of surface morphology with
−2
Fig. 3. Effect of the cathodic current density on the composition of the Co–La
deposit.
the current density. At the current density of 10 mA cm , the
deposit is uniform, homogenous, and has not cracks on the sur-
face. The round particles of the coating were stacked compactly,
but there was some space among particles. With the increase of
the cathodic current density, the amount of space among par-
ticles rises, and the size of the particles decreases initially and
then increases. The surface morphology of the Co–La deposit
increase of the cathodic current density, the La content in the
coating goes up from 2.96 to 38.97 at% at first, reaches the most
−
2
3
8.97 at% at 30 mA cm , and finally decreases to 7.04 at%.
While the change trend of cobalt content in the deposit is oppo-
Fig. 4. The SEM photos of the Co–La deposit plated from different current density (mA cm 2): (a) 10, (b) 30 and (c) 40.
−

S.-L. Wang et al. / Journal of Alloys and Compounds 450 (2008) 142–147
145
Fig. 6. The DSC and TG curves of the Co–La coating obtained from Jk
Fig. 5. The hysteresis curves of the Co–La deposit plated from different current
density.
−2
(
10 mA cm ).
◦
DSC curve, exo-thermal peak at about 264.5 C is broad, and the
◦
is different from that of the cobalt deposit plated by electro-
deposition. The space among particles of the deposit is relative
to the hydrogen evolution. Thus, the more the current density,
the more the extent of the hydrogen evolution reaction, the more
the amount of space among particles of the deposit.
first endo-thermal peak at 428.1–465.9 C is strong, the another
endo-thermal peak at 678.2–699.7 C is weak. There are three
◦
weight loss steps at the corresponding temperature in TG curve.
Fig. 7 is the XRD patterns of the deposit after heat treatment
for 1.0 h at various temperatures. In accordance with the XRD
patterns, the peaks of the XRD pattern for the as-plated deposit
◦
◦
◦
◦
◦
3
.4. Effect of the cathodic current density on the magnetism
at 2θ of 42.02 , 44.56 , 47.75 , 76.05 and 92.68 correspond to
the (1 0 0), (0 0 2), (1 0 1), (1 1 0) and (1 1 2) of planes of hexahe-
dron cobalt (P63/mmc) phase (␣-Co), respectively. There is also
of the Co–La films
◦
The hysteresis curves of the Co–La deposits plated from dif-
ferent current density is presented in Fig. 5. The deposit has
high saturation magnetization (Ms) and coercive force (Hc), and
therefore is suitable to be used as memory magnetic material.
The saturation magnetization and the coercive force were deter-
mined from the magnetic hysteresis curves, respectively. With
a broad peak at about 2θ of 44.6 in the as-plated deposit XRD
pattern, which indicates the deposit contains amorphous phase.
These reveal that the structure of the as-plated deposit consists
of main amorphous phase and a little amount of hexagon cobalt
phase (␣-Co). The deposit XRD pattern after heat treatment at
◦
300 C is basically the same as that of the as-plated deposit.
−
2
the increase of the current density from 10 to 30 mA cm , the
saturation magnetization of the coating moves up from 128.3
Therefore, the first exothermic peak in DSC curve at about
◦
264.5 C is ascribed to the evolution of hydrogen gas. It can be
−
1
to 29.8 emu g , and their coercive force was 53.4, 51.0 and
calculated that the as-plated coating (after removed the coating
containing little water) has 4.4 wt% hydrogen storage from TG
5
3.3 mT, respectively.
According to the magnetic theory, the saturation magnetiza-
tion mainly depends on the composition of the deposit. Co atom
has three lone electrons in 3d atomic orbital, and La atom has one
alone electron in 5d atomic orbital. Therefore, the atomic mag-
netic moment of Co is more than that of La. From above studies
of the deposit composition, with the increase of the cathodic cur-
rent, the cobalt content of the deposit reduces, and the lanthanum
content rises, resulting in the decrease of the saturation magne-
tization. The change scope of the coercive force of the deposit
is less, and the change trend of it is no regular. It is the reason
that the coercive force is sensitive to the structure of the alloy,
which depends on the structure, the disfigurement, the stress, the
thickness of the film, and so on.
3
.5. The crystallization behavior of the Co–La alloy
The differential scanning calorimetry (DSC) and the thermo-
gravity analysis (TG) curves of the Co–La coating are illustrated
in Fig. 6. One exo-thermal and two endo-thermal peaks appear in
−2
Fig. 7. The XRD patterns of the deposit obtained from Jk (10 mA cm ) after
heat treatment for 1.0 h at various temperatures.

1
46
S.-L. Wang et al. / Journal of Alloys and Compounds 450 (2008) 142–147
◦
curve. The crystallization peaks of the deposit heated at 500 C
◦
◦
◦
◦
at 44.45 , 51.38 , 76.15 and 92.45 is the (1 1 1), (2 0 0), (2 2 0)
and (3 1 1) plane diffraction of cubic Co–La (Fm3m) phase (␤-
◦
◦
CoLa), respectively. There are also peaks at 42.02 , 44.56 ,
◦
◦
◦
4
7.75 , 76.05 and 92.68 in XRD pattern of the deposit heated
◦
at 500 C. This indicates that the deposit contains both hexa-
hedron cobalt phase and cubic cobalt phase. This deposit also
has a little amount of amorphous phase. Thus, the strong endo-
◦
thermal peak in DSC curve at 428.1–465.9 C corresponds to the
amorphous phase transformation into cubic Co phase. The phase
◦
structure of the coating after heat treatment at 600 C is similar
◦
to that of the coating at 500 C. The XRD pattern of the coating
heated at 700 C shows peaks at 44.45 , 51.38 , 76.15 , 92.45
and 97.73 , revealing that the coating only contains cubic cobalt
phase. Hence, the endo-thermal peak at 678.2–699.7 C in DSC
◦
◦
◦
◦
◦
◦
◦
curve is ascribed to the phase transformation from hexahedron
cobalt to the cubic cobalt. It should be pointed out that the inter-
metallic compounds of cobalt and lanthanum, such as Co13La,
Fig. 8. The cathodic polarization curves in 10 wt% KOH solution of the Co–La
alloy obtained from Jk (10 mA cm ) and Co electrode.
−
2
Co La, Co7La2 and Co3La2, and the phase of sole lanthanum
were not found in all XRD patterns.
5
Table 1
The Tafel constants and the kinetics coefficient of hydrogen evolution for the
Co–La and Co
The atomic radius of La (0.188 nm) is far larger than that of
Co(0.125 nm), and Lais more active than thatof Co. Itisobvious
that lanthanum may be dissolved in cobalt crystal lattice to form
eutectic alloy, which lanthanum atom replaces cobalt atom in
crystal lattice position. These destroyed the structure of cobalt
phase, and so the amorphous phase was formed. Because the
content of Co of the Co–La coating is far more than that of La, so
the remain cobalt in the deposit itself formed the hexagon cobalt
phase (␣-Co). The structure of the IG–RE alloy prepared by
electro-deposition in the melt is also amorphous phase [10,11].
−
a (V)
b (V)
i0 (mA cm 2)
Co
Co–La
0.869
0.696
0.214
0.192
0.018
0.466
where the over potential (η) is the potential subtract the open
circuit potential, i the cathodic current density, i0 the exchange
current density, R the gas constant, T the Kelvin temperature, α
the transfer coefficient, n the transfer electron number, and F is
the Faraday constant.
3
.6. The electro-catalytic activity of the hydrogen evolution
The over potential (η) versus the natural logarithm of cur-
rent density (ln i) was plotted in accordance with Fig. 8 as the
electrochemical polarization was higher (the figure was omit-
ted). The calculated results were shown in Table 1. It is evident
that the exchange current density of the hydrogen evolution for
the Co–La is far more than that of Co. Therefore, the electro-
catalytic performance of the hydrogen evolution for the Co–La
electrode is better than that of Co.
of the Co–La alloy
Cobalt itself is a good electro-catalytic material of the hydro-
gen evolution. The cathodic polarization curves in 10 wt% KOH
solution of the Co–La alloy and Co electrode were shown in
Fig. 8. As the potential was scanned to about −0.968 V(versus
−
1
−
Hg–HgO, 0.1 mol L OH , the same as below), the Co–La
alloy electrode began to produce hydrogen, and the amount of
the evolution hydrogen and the cathodic current all increased
when the potential shifted more negatively. While the hydrogen
evolution phenomenon for the Co electrode was not obvious in
the scope of the swept potential, and its cathodic current is far
less than that of the Co–La alloy. Thus, the performance of the
hydrogen evolution for the Co–La electrode is better than that
of Co.
4. Summary
The Co–La alloy films were electro-deposited in urea–NaBr
melt. Lanthanum was co-deposited by inducement effect of
cobalt to form Co–La alloy. With the increase of the cathodic
current density, the content of La of the deposit rises at first,
then reduces, and reaches maximal value at current density
When the cathodic polarization curves was negative than
−
2
3
0 mA cm ; at the same time the size of particles of the coat-
−
1.100 V, the relationship between the over potential (η) and the
natural logarithm of current density is Tafel formulary, which is
shown as below:
ing becomes less. The more the content of Co in the deposit
is, the more the saturation magnetization is. The Co–La alloy
film is suitable to be used as memory magnetic material. The
structure of the as-plated coating consists of main amorphous
and a little amount of hexagon cobalt (P63/mmc) phases. Dur-
ing heat treatment, the deposit was converted from amorphous
η = a + b ln i
here, Tafel constants is
RT
RT
◦
phase into cubic Co (Fm3m) phase at 438.9 C, and hexagon
a = −
ln i0 and b =
◦
αnF
αnF
cobalt phase was transformed into cubic cobalt at 687.1 C. The

S.-L. Wang et al. / Journal of Alloys and Compounds 450 (2008) 142–147
147
electro-catalytic performance of the hydrogen evolution for the
Co–La electrode is better than that of Co.
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8
Acknowledgments
[
2
This work was supported by the natural science foundation of
Fujian province, China (Grant No. E0210020). I am grateful to
Prof. Zhi-gao Huang in Fujian Normal University for vibrating
sample magnetometer (VSM) measurements.
[6] L.Z. Liu, Y.X. Tong, Q.Q. Yang, Rare Met. 19 (3) (2000) 237–241.
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