mokht-2 _1

Effect of pH at Early Formed Structures in Cobalt Electrodeposition
Mokhtari Salim,
SAHARI ALI
[email protected]
University of Bouira 10000 Algeria
Introduction
In metals iron group, cobalt electrocrystallization has been far less studied compared to
nickel, although electrodeposited cobalt and cobalt alloys are eligible materials for applications
ranging from magnetic media and devices to wear and corrosion resistant coatings. [1]. The
solution pH was shown to be the most important parameter in determining the structure of
electrodeposited cobalt [2]. Pangarov and Rashkov [3] determined the relations between the
main operative conditions and electrodeposits structure. They found that the cubic structure of
cobalt (βCo) formation is favoured by low temperature, high current density and low pH.
Polukarov [4] came to similar conclusions.. Most of the works found in the literature show that
by increasing the pH solution the structure becomes completely the hexagonal closed packed
structure (αCo)[5-6].
The purpose of the present work was to study, the influence of boric acid, and the effect of
the pH solution on the electrochemical behaviour and on the electrodeposits morphology. In
second hand the present paper gives the results of an investigation of the early stages of cobalt
electrodeposition (the nucleation).
I.
Experimental details
Electrochemical experiments were carried out in
conventional three-electrode cell. The
reference electrode was a saturated calomel electrode mounted in a Lugging capillary. All
potentials are referred to this electrode. The counter electrode was platinum wire. A rectangular
plate of platinum was used as working electrode. The exposited surface of this later electrode is
about 0.4 cm2. Prior to each experiment, the working electrode was cleaned ultrasonically and
sequentially in acetone, methanol and distilled water for 15 min, and finally activated in
hydrochloric acid for 2 mn. Cobalt was deposited from two baths: The N°1 is used for studying
the influence of H3BO3, the N°2 is for studying the effect of pH solution. The composition of
each bath was shown in Table1.The electrodeposition of cobalt was performed at overpotential
deposition (OPD) using a Volta Lab 40 potentiostat/galvanostat controlled by a PC. All baths
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were stirred by nitrogen gas flow before electrodeposition of cobalt. Voltammetric experiments
were carried out at 20 mV/s.
X-ray
ray diffraction (XRD) phase analysis was performed
performed on a INEL CPS120 diffractometer. The
CuKα radiation ( = 154066 pm) was used. The temperature was fixed at 20°C during all
experiments. The surface morphology of the film was analyzed ex situ using a JEOL JSM6400
Scanning Electron Microscop equipped with an Oxford EDS system.
TABLE I
MOLAR COMPOSITION OF USED BATHS.
Bath N°1
CoCl2
NaCl
pH
H3BO3
C/Mol/l
0.1
1.0
4.0
Varied
Bath N°2
CoCl2
NaCl
H3BO3
pH
C/Mol/l
0.1
1.0
0.5
Varied
1 Surface morphology of electrodeposited Co films.
The surface morphology of the Co deposited on Pt surface is shown in fig.5. The presence of Co
was revealed by energy dispersive spectroscopy analysis (EDS). The SEM image corresponding
to early stage of deposit at pH = 4. is schown in fig.5. The surface appears occupied by dispersed
sub grains were seemed to grow uniformly with the same sizes, thus this distribution of grains
proves the instantaneous nucleation
nucleatio mechanism.
a
Fig.1: EDS analysis (a) for electrodeposits of Co at applied potential -1.0
1.0 V.
2
b
Fig.2: SEM image (b) for electrodeposits of Co at applied potential -1.0 V.
2 Structural properties
The effect of bath pH on the phase composition and microstructure of Co films was
investigated by selecting the bath temperature at 20°C, and the applied potential was fixed at -1.0
V. Fig. 3 a, b and c shows the X-ray diffraction patterns of the films deposited at bath pH. 2, 3.1,
and 4.0 respectively.
At all selected pH range, several peaks were detected, indicating the polycrystalline structures.
As can be seen at pH =2 the electrodeposited film consisted of mixture of Cohcp and Cofcc
hcp (110)
Pt (220)
hcp (100)
Pt (111)
crystallographic phases, however their peaks were very small (Fig.3a).
hcp (110)
Pt (200)
Intensity (arb. Units)
hcp (100)
c
cfc(220)
cfc (111)
hcp (100)
b
a
40
50
60
70
80
2 θ (deg.)
Fig.3: X-ray diffraction (XRD) patterns of electrodeposited Co from from solution shown in
table 1.on Pt substrate. a: pH=2.0 b: pH = 3.1 c: pH = 4.0.
If electrodeposition was conducted at pH = 3.1, and 4 the Cocfc variety was a tendency to
disappear. The related Cohcp was considerably more pronounced, (Fig. 3 c, d),
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Conclusion
The influence of the boric acid and the effect of pH on Co electrodeposition have been evaluated
in this present study. It was found that the boric acid alters strongly the polarization curves of Co
electrodeposition. The major change in the voltammetry was a decrease in the hydrogen
electrosorption activity. As a result, the potential region of proton discharge was extended to
more negative potentials. At vertex potential region where proton discharge was under charge
transfer control, the presence of boric acid leads to delay the current density of HER. This effect
was attributed to adsorption of boric acid species. On the other hand, boric acid was found to
affect the nucleation potential of Co. At low pH, Co structure is biphasic Cohcp + Cocfc with
predominance of Cohcp depending on the rate of HER. The metastable Cocfc slowly disappeared
with increasing the pH. The Co electrodeposition with operating potential of -1.0 V in pH bath =
4 was clearly under diffusion control. The nucleation mechanism was instantaneous diffusion
controlled having a 3D-growth pattern.
References
[1] T. Osaka, Electrodeposition of highly functional thin films for magnetic recording devices of
the next century Electrochim. Acta.45
(2000) 3311-3321,
[2] H. Kersten, Influence of Hydrogene concentration on the Crystal Structure of
Electrodeposited Cobalt. Physics. 2(1932).274-275.
[3] N.A. Pangarov, St. Rashkov, Electrolytic deposition of alpha and beta cobalt,Compt. Rend.
Acad. Bulgare Sci.13(1960) 439-442.
[4] Yu.M. Polukarov, the structure and magnetic caracteristics of electrolytic deposits of
ferromagnetic metals and alloys prepared under different conditions Russ. J. Phys. Chem.34
(1960) 68-71.
[5] S. Armyanov, S.D. Vitkova,, Structure and magnetic properties of electrodeposited cobalt
Surf. Technol. 7(1978) 319-329 .
[6] G.I. Finch, H. Wilman, L. Yang, Disc. Faraday Soc. 1 (1947) 144-158
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