Journal of The Electrochemical Society, 163 (12) D3069-D3075 (2016)
D3069
FOCUS ISSUE ON ELECTROCHEMICAL DEPOSITION AS SURFACE CONTROLLED PHENOMENON
Combinatorial Electrodeposition of Cobalt-Copper Material
Libraries
Carina Daniela Grill,a Jan Philipp Kollender,a and Achim Walter Hassela,b,∗,z
a Institute for Chemical Technology
b Christian Doppler Laboratory for
of Inorganic Materials, Johannes Kepler University Linz, 4040 Linz, Austria
Combinatorial Oxide Chemistry, Institute for Chemical Technology of Inorganic
Materials, Johannes Kepler University Linz, 4040 Linz, Austria
A full cobalt-copper material library was successfully prepared from a single experiment by galvanostatic electrodeposition using a
modified Hull cell. X-ray fluorescence spectroscopy (XRF) measurements showed, that a composition gradient of 28–96 at.% copper
could be achieved. The change of the surface morphology and topography along the material library was examined by scanning
electron microscopy (SEM) and atomic force microscopy (AFM), respectively. X-ray diffraction (XRD) revealed a Vegard-like
behavior of the face-centered cubic (fcc) cobalt-copper solid solution, as the lattice constant a can be linearly correlated with the
atomic copper ratio. The work function, determined from scanning Kelvin probe (SKP) measurements, was expected to decrease
gradually with increasing copper content, but was found to be highest (4.86 eV) for a shiny area within the material library (48–62
at.% Cu), indicating higher nobility. This result was confirmed by localized corrosion potential (Ecorr ) determinations performed by
scanning droplet cell microscopy (SDCM). The unexpected high nobility in work function and Ecorr for this region go hand in hand
with a minimum in surface roughness within the material library.
© The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/),
which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any
way and is properly cited. For permission for commercial reuse, please email: [email protected]. [DOI: 10.1149/2.0101612jes]
All rights reserved.
Manuscript submitted May 19, 2016; revised manuscript received August 4, 2016. Published September 20, 2016. This paper is part
of the JES Focus Issue on Electrochemical Deposition as Surface Controlled Phenomenon: Fundamentals and Applications.
Combinatorial and high throughput methods have recently attracted enormous research interest, due to the numerous possibilities of developing multicomponent materials, which can subsequently
be screened with respect to physical, chemical or electrochemical
properties.1 Combinatorial pulse and galvanostatic plating using a
modified Hull cell have already been utilized to prepare binary material libraries. Beattie et al. electrodeposited copper-zinc,2 copper-tin,3
tin-zinc4 libraries. More recently Srinivas et al. applied the Hull cell to
produce copper-nickel alloys.5,6 Grill et al. investigated the influence
of electrolyte composition and current density on the composition of
cobalt-nickel material libraries fabricated in a modified Hull cell.7
Copper-cobalt orebodies occur naturally together, and can be
found in significant amounts in the Nchanga deposit of the Zambian Copperbelt.8–10 Cobalt-copper11 and their mixed oxides12–15 are
industrially used in multicomponent catalysts for alcohol synthesis
from syngas.
As a possible material for data storage and magnetic sensing cobaltcopper alloys,16,17 multilayers18–21 and multilayered nanowires22 have
been subject of research with respect to their magnetoresistive properties. Moreover copper23–25 and cobalt25 electrodeposition in a simultaneously applied magnetic field have been studied. Although the cobaltcopper phase diagram26 suggests essentially no solubility of copper in
cobalt at ambient temperature, certain preparation methods obviously
present a way to overcome this problem. Hence, homogeneous cobaltcopper alloys and multilayers can be prepared by means of chemical
vapor deposition (CVD),27 magnetron sputtering,28,20 laser ablation29
and electrodeposition.30–35,18
In this work the combinatorial electrodeposition strategy using a
modified Hull cell is employed in order to prepare a wide compositional spread material library of cobalt-copper alloys. In order to
enable codeposition of both metals, trisodium citrate was added to
the electrolyte bath as complexing agent, giving rise to thermodynamic and kinetic considerations about cobalt and copper codeposition, which are discussed. Moreover, the appearance, surface morphology and crystallographic structure for different compositions on the
material library are described. From surface potential measurements,
∗ Electrochemical Society Member.
z
E-mail: [email protected]
performed by scanning Kelvin probe (SKP), the effective work function along the material library was calculated and compared to the
corrosion potential Ecorr , in correlation with surface roughness. Ecorr
values were determined by means of scanning droplet cell microscopy
(SDCM),36–39 which allows localized electrochemical measurements
on different positions, offering the unique opportunity for a detailed
electrochemical mapping.
Experimental
For electrodeposition of the cobalt-copper material library a modified Hull cell was manufactured, which has also been presented in
Ref. 7. The original Hull cell geometry was rotated by 90◦ due to
preliminary experiments suggesting an improved surface appearance
of the deposits. Suitable PVC plates with a thickness of 6 mm were cut
to the desired shape, pre-treated with a mixture of acetone: butanone
(1:1) and fixed with adhesive. The anode was positioned at the side
perpendicular to the ground. The cathode side vis-á-vis was inclined
toward the z-axis by an angle of 50◦ . A rendered image of the modified Hull cell and a schematic of the inner dimensions are shown in
Figs. 1a and 1b, respectively.
The cobalt-copper material library was deposited in the modified
Hull cell with a simple two-electrode setup using a dimensionally
stable anode and a steel cathode.
The steel substrate was ground with successively finer abrasive
paper (Struers SiC Foil, grit 320, 500, 800, 1200, 2400, and 4000),
rinsed with deionized water, degreased with acetone and isolated on
the back side using lacquer. The area immersed in the electrolyte
solution was 25 × 75 mm2 .
The electrolyte bath was freshly prepared by dissolving the metal
sulfate salts and the trisodium citrate complexing agent in deionized water giving respective concentrations of 0.3 M CoSO4 · 7 H2 O
(pure, AppliChem), 0.02 M CuSO4 · 5 H2 O (p.a., Merck) and 0.2 M
Na3 C6 H5 O7 · 2 H2 O (p.a., Merck).
The electrodeposition was performed galvanostatically at room
temperature for 3.5 h at an overall applied current density of 100
mA dm−2 using a Keithley 2400 SourceMeter. After electroplating
the substrate was rinsed with deionized water and dried with gaseous
nitrogen.
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Journal of The Electrochemical Society, 163 (12) D3069-D3075 (2016)
robot with three linear stages. A detailed description of the SDCM
setup and preparation process is given elsewhere.36
Corrosion potentials were determined from potentiodynamic polarization curves measured in 5 wt% NaCl (p. a., Merck) solution using
an Ivium CompactStat potentiostat (Ivium Technologies). The potential range was set to ±250 mV vs. the open circuit potential (OCP)
with a scan rate of 5 mV s–1 . All investigations were performed on the
as-deposited material library, without any further treatment.
Results and Discussion
Figure 1. (a) Rendered image indicating the positions of the anode and the
cathode using arrows and (b) side view schematic including dimensions of the
modified Hull cell (see also7 ).
Using scanning X-ray fluorescence spectroscopy (SXRF) the composition of the cobalt-copper material library was determined (Oxford
Instruments X-Strata 980-A). X-ray diffraction was performed in order to identify the crystallographic structure and its changes depending on the composition of the cobalt-copper material library (Philips
X’Pert Pro PW 3040/60 diffractometer, Cu-Kα radiation, 45 kV and
40 mA, X’Celerator RTMS detector). Scanning electron microscopy
(SEM) images were taken to examine the surface morphology of the
material library (Zeiss Gemini 1540 XB) using 10 kV acceleration
voltage and SE2 detector. Surface topography investigations were
performed by atomic force microscopy (AFM), scanning an area of
50 × 50 μm at each selected position on the sample by using a
Nanosurf easyScan 2 device. The average root mean square roughness Rsq was determined by means of Gwyddion software.
From surface potential measurements by scanning Kelvin probe
(SKP) the effective work function of the cobalt-copper material library
was calculated. The probe tip and the core software were supplied by
K & M Softcontrol, the platform and measurement chamber were both
developed and constructed in house. A gold standard was chosen as
reference for calculating the work function of the material library.
Localized corrosion studies at different positions on the material library were performed using scanning droplet cell microscopy (SDCM). To manufacture the scanning droplet cell (SDC) a
Ag/AgCl/3 M KCl micro-reference electrode with a 2 M KNO3 salt
bridge40 and a gold wire (100 μm diameter, 99.999%, Wieland Dentaltechnik) as counter electrode were mounted in a borosilicate glass
capillary (Hilgenberg GmbH), which was shaped using a capillary
puller and ground with SiC paper (1200-grit) to the desired tip size in
the range of 600 μm. A silicone (Momentum, Albany, USA) gasket
at the capillary tip was applied to prevent leakage of the electrolyte
solution when the SDC is in contact with the sample. The effective
tip diameter (577 μm) of the scanning droplet cell was determined
by optical microscopy of colored TiO2 spots after anodization of a
Ti sample using the SDC, like it has been shown previously for Hf.41
The automated positioning of the SDC was performed using a gantry
Cobalt-copper material library electrodeposition approach.—In
alloy electrodeposition the crucial factor is the difference between the
deposition potentials of the involved metal ion species. Considering
the standard electrode potentials E0 , which are 0.34 V for the Cu/Cu2+
and −0.277 V for the Co/Co2+ redox couple,42 might give a first estimation of their deposition behavior. As the deposition potentials have
to be similar to enable alloy electrodeposition, it becomes obvious
from the E0 values that further conditioning is required to achieve
codeposition of cobalt and copper. According to the Nernst equation
the reversible electrode potential E can be shifted to more negative
values as the metal ion activity in the electrolyte solution is decreased.
Thus, the Cu2+ :Co2+ ratio in the electrolyte bath was set to 1:15, to
use the low copper concentration of 0.02 M to shift the reversible
electrode potentials of cobalt and copper closer together.
Nevertheless, this shift of approximately 50 mV is not enough
to have a significant influence on the codeposition system. Another
common and straightforward method of bringing deposition potentials closer together is the addition of a complexing agent to the
solution, which reduces the apparent concentrations43,44 viz the activity. In this work trisodium citrate (Na3 Cit) was used as a complexing
agent. It is widely employed in electrodeposition of cobalt,45 coppercobalt alloy35,34,31,32,46 multilayers33 and copper-nickel.47,48,6,49 Using
the complexation constants of the formed complexes the standard
potential for metal deposition from complexing solution can be calculated according to Eq. 1.44,50
0
z+
E 0 (MLz+
m M) = E (M M) −
RT
ln K m
zF
[1]
where, E0 (MLz+
m /M) is the standard potential for metal deposition
from complexing solution, E0 (Mz+ /M) is the standard potential for
metal deposition from non-complexing solution, R is the gas constant,
z is the number of transferred electrons per formula unit, F is the
Faraday constant and Km is the complexation constant of the particular
complex.
According to51,49 and52 the predominant copper citrate complexes
in the electrolyte solution are the dimer species [Cu2 Cit2 H–2 ]4– and
[Cu2 Cit2 H–1 ]3– . The main cobalt citrate complexes are CoCit– and
CoCitH.53,45 Table I shows the complexation constants K of the citrate
complexes, the calculated standard potential for metal reduction from
these complexes and the potential shift compared to the standard
potential from non-complexing solution.
Table I. Complexation reactions and stability constants for the
predominant copper(II) citrate51,52 and cobalt(II) citrate53,45
complexes, calculated standard potentials of copper and cobalt in
complexing solution and potential shift referring to the standard
electrode potentials.
Complexation reaction
log K
E0 (MLz+
m /M) / V
E0 shift / V
2 Cu2+ + 2 Cit3– ↔
[Cu2 Cit2 H−2 ]4– + 2 H+
2 Cu2+ + 2 Cit3– ↔
[Cu2 Cit2 H−1 ]3– + H+
Co2+ + Cit3– ↔ CoCit–
Co2+ + Cit3– + H+ ↔
CoCitH
5.9
0.17
−0.17
10.9
0.02
−0.32
16.3
20.1
−0.76
−0.87
−0.48
−0.60
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Journal of The Electrochemical Society, 163 (12) D3069-D3075 (2016)
D3071
Figure 2. XRF analysis of the cobalt-copper material library showing (a) the copper content changing over the sample area and (b) the composition averaged over
the y-position, the film thickness and a photograph of the material library depicted in the background.
From the calculations it can be noticed that the potential shift
to more negative values is more significant for the cobalt(II) citrate
complexes, which is leading to the result that the standard potentials for copper and cobalt reduction from complexing solution are
actually further apart from each other. Nevertheless, in studies about
copper electrodeposition from solutions containing citrate it has been
reported that the [Cu2 Cit2 H–2 ]4– complex considerably inhibits electrodeposition of copper due to kinetic effects, involving reduction in
two separate steps and intermediate adsorption of the dimer ion.51
The discharge and adsorption processes described influence the total
overpotential, and thus the deposition potential, for copper reduction.
Consequently, although the calculated estimations of the potentials for
the copper and cobalt citrate complexes do not show a shift to more
similar potentials, kinetic effects are considered to make a crucial contribution to the copper deposition potential, permitting cobalt-copper
codeposition.
Beattie and Dahn described the theory on using a Hull cell for electrodeposition of material libraries.2 The suggested deposition mechanism was adopted in this work for the normal cobalt-copper plating
system. Firstly, the concentration of the more readily deposited metal
ion species (Cu2+ ) in the plating bath is significantly lower as compared to the Co2+ concentration (Cu2+ :Co2+ = 1:15). In the modified
Hull cell at the geometrically established current density gradient the
plating rate is higher where the electrodes are closer together, corresponding to a higher current density. Here Cu2+ ions are reduced
immediately, resulting in a depletion in the vicinity of the cathode
surface for this ionic species. As a result of the missing bath agitation,
Cu2+ ions have to diffuse toward the cathode from the bulk solution,
while the Co2+ ions are more readily available for reduction with
respect to their excess in the electrolyte solution. At the low current
density side on the other hand, the lower plating rate leaves more time
for diffusion of Cu2+ from the bulk solution, inducing a higher copper
concentration in the deposited film. Hence, in the very first stage of
electrodeposition copper is more likely to be deposited until the Cu2+
diffusion limitation at each specific position on the cathode, corresponding to the local current density, is reached. Therefore it is likely
that a few nm of pure copper are initially deposited on the Fe substrate.
The higher the current density, the sooner Co2+ ions are reduced instead of copper cations (due to Cu2+ depletion) and in addition to
copper (due to Cu2+ reduction under limiting current density). After
establishing the steady state mass transport conditions no changes in
the locally deposited compositions over time are expected to happen
due to the relatively high ion concentrations in the solutions, and the
overall low current densities used in combination with an unstirred
electrolyte solution.
Composition, film thickness and appearance.—The cobaltcopper material library exhibits a color gradient, correlating with the
composition. At the high current density end, which is rich in cobalt,
the coating is light gray. Close to the middle of the sample the color
starts to change to red, which is a result of the higher copper content.
The deposit appears dull, except the area just before the color transition from gray to red, which is bright and reflective (approximately
48–62 at.% Cu). At this particular area the surface roughness was
found to show a minimum. Accordingly, the work function shows a
maximum and the corrosion potential a local maximum within in the
material library.
The composition as well as the thickness of the cobalt-copper material library was determined by scanning X-ray fluorescence spectroscopy (SXRF). Figure 2a shows the results of the SXRF measurements over the whole material library, revealing a uniform and almost
linear variation of about 28–96 at.% for copper along the sample
length. As the composition was found to differ only slightly along the
y-position on the sample these values were averaged in Figure 2b. All
error bars were included, however as the size of the data points takes
approximately 2.3 at.%, the error bars indicating a smaller confidence
interval (95%) cannot be identified in the graph. As a result of the current density gradient establishing during electrodeposition in the Hull
cell, a thickness gradient of the cobalt-copper deposit was obtained
(see Fig. 2b), ranging between 1.8 and 9.5 μm. For film thicknesses
in the micrometer range a significant influence of the thickness variation on the surface sensitive measurements discussed is unlikely. The
global current efficiency for deposition of the total material library
was 70.3%, taking into account an overall copper content of 66 at.%
for the entire material library.
Crystallographic structure.—X-ray diffractograms were taken for
different compositions on the cobalt-copper material library and are
depicted in Figure 3. At the low current density end (95 at.% Cu)
only peaks of copper and the steel substrate (Fe) underneath can be
noticed. With increasing current density and cobalt content, the copper
peaks are shifted toward the cobalt peaks, suggesting formation of a
face-centered cubic (fcc) cobalt-copper solid solution, which has also
been found by.30,31,35 Hence, cobalt is not forming the usual hexagonal
close-packed (hcp) structure, which is confirmed by the absence of
hcp peaks for cobalt in the diffractograms, but electrocrystallizes in
fcc structure in presence of copper under the applied experimental
conditions. The formation of a cobalt-copper solid solution strongly
indicates a uniform film buildup without deposition of multilayers,
apart from the probable enrichment of copper at the first stage of
electrodeposition as discussed above. Nevertheless, after deposition
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Journal of The Electrochemical Society, 163 (12) D3069-D3075 (2016)
Figure 3. X-ray diffractograms measured at different compositions of the
cobalt-copper material library.
of a thin copper layer cobalt and copper are reduced at the same time,
resulting in a cobalt-copper alloy with a composition gradient.
The most intense peak obtained from XRD, shifting from the
Cu(111) toward the Co(111) line, was further investigated in order to
describe the lattice expansion using Vegard’s law, according to which
- in a first approximation - the lattice constant a in a binary solid
solution depends linearly on the composition.54
The ideal expression for this relation is given by Eq. 2,
aCo−Cu = aCo + xCu · (aCu − aCo ) = 354.49 + xCu · 7.15
[2]
where aCo-Cu describes the lattice expansion of the cobalt-copper solid
solution depending on the atomic ratio of copper xCu , aCo and aCu are
the lattice constants of pure cobalt and copper, determined from their
respective powder diffraction file (PDF) reference patterns 15–0806
and 04–0836 via ICDD database.
Figure 4 shows the change of the lattice constant depending on the
atomic ratio of copper. For a higher copper content (95–74 at.%) the
measurements revealed a positive deviation from Vegard’s law, which
Figure 4. Lattice expansion of the fcc cobalt-copper phase depending on the
composition.
Figure 5. SEM images of the cobalt-copper material library for different
compositions.
is indicated by the dashed line. As the copper atomic ratio decreased a
transition to a negative deviation from the ideal correlation occurred.
Linear fitting of the data points resulted in the experimentally found
Equation 3.
aCo−Cu = 353.19 + xCu · 9.13, R2 = 0.953
[3]
From these crystallographic measurements no indication for stress inside the deposited thick film could be detected, otherwise a significant
distortion of the lattice parameter within the material library would be
observed. Additionally, the low applied current density and presence
of citrate in the electrolyte bath ensure minimization of stress during
electrodeposition.55 Furthermore, the cobalt-copper coating thickness
is in the micrometer range, reducing the probability for building up of
film stress.
Surface morphology and topography.—Scanning electron micrographs at different positions; and thus varying copper content, of the
cobalt-copper material library are shown in Figure 5. At the lower
current density end of the sample (96 and 88 at.% Cu), the surface
morphology is smooth and the crystallites are small and possess a uniform spherical shape. With increasing current density (69 at.% Cu) the
crystallites grow slightly larger, but still keep their spherical shape and
uniformity. The SEM image at 56 at.% copper was taken at the shiny
area on the sample, where the color changed from gray to copper red.
Here, the deposit basically appears more compact. In addition, some
larger crystallites grow on the surface. At the high current density
end of the material library (50 and 30 at.% Cu) the surface is rougher
with nodular crystallites, and between them exhibiting needle-shaped
crystallites. The results from AFM surface topography measurements
are depicted in Fig. 6, the corresponding root mean square roughness Rsq is plotted against the copper content at the specific positions
on the sample in Fig. 7. What becomes obvious immediately is, that
the lowest surface roughness was determined for 59 at.% Cu to be
236 ± 64 nm. This composition is located right in the middle of the
previously described bright and shiny area on the material library,
which has been identified to exhibit increased compactness by SEM.
From this position on in the direction of higher cobalt content Rsq increases up to 558 ± 171 nm (35 at.% Cu), which matches again with
the impressions from SEM imaging. Towards the opposite direction
(higher Cu content), the surface roughness increases significantly to
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Journal of The Electrochemical Society, 163 (12) D3069-D3075 (2016)
D3073
Figure 6. AFM topographic studies on different positions of the cobalt-copper material library.
569 ± 265 nm. The broader distribution of the roughness is caused
by the appearance of larger dendritic crystallites in the size of 3–5
μm on the still rather compact deposit. From there on in the direction
of higher copper content Rsq shows a declining trend, although the
surface roughness remains relatively high. However, the topography
becomes considerably more uniform, which is also indicated by the
smaller error bars in Fig. 7 for higher copper contents.
Work function and corrosion potential.—From surface potential measurements performed by scanning Kelvin probe, the effective
work function of the cobalt-copper material library was calculated
according to Hansen and Hansen56 using the following equation:
WF = (V0 − V ) + WF0
[4]
where WF and WF0 are the respective work functions of the material
library and the gold reference in eV, V and V0 are the respective surface
potentials of the sample and the reference in V, determined by SKP.
As56 point out that the work function of gold in air is 4.8 eV, this value
was taken as WF0 .
Fig. 8 depicts the calculated effective work function of the cobaltcopper material library depending on the copper content averaged
over the y-position as shown in Fig. 2b. At the particular region (approximately 48–62 at.% Cu) where the material library exhibits a
shiny surface in contrast to the dull rest of the film, the work function
shows a significant maximum of about 4.86 eV. With increasing copper content the effective work function decreases to approximately
4.75 eV. The curve shape, apart from the maximum, reflects the trend
Figure 7. Roughness Rsq determined from AFM measurements in dependence
of the composition of the material library.
of the work functions for the pure metals. Those are reported to be
5.0 eV (Co, polycrystalline) and 4.48 eV (Cu, (111)),57 though determined by means of photoelectrical measurements and the fact that
work function values reported in literature often differ considerably.
For different compositions of copper-nickel alloys Lu et al.58 found
the work function to depend linearly on the atomic ratios of the metals, increasing with higher nickel content, which is comparable to
our findings, as like copper-nickel also copper-cobalt has been shown
to form a solid solution here, and nickel as well as cobalt exhibits
a higher work function than copper. However, the above described
bright and reflective area on the material library stands out from the
linear trend.
It is well known in literature, that the work function of a material is highly affected by its surface roughness in the sense, that
a higher surface roughness causes a decrease in work function.59–61
This can be explained by an increased likelihood of an electron to
escape to the vacuum level from a peak on a rough metal surface,
than from a valley.60 Comparing the results from SKP measurements
to the Rsq values obtained from AFM in Fig. 6 and Fig. 7, we see
that the lowest surface roughness has been determined for a composition of 59 at.% Cu, which is located right within the area of the
work function maximum. Therefore, the increase in effective work
function for the outstanding shiny area between 48–62 at.% on the
Figure 8. Effective work function of the cobalt-copper material library determined from SKP measurements. The copper content was averaged over the
y-position on the sample.
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D3074
Journal of The Electrochemical Society, 163 (12) D3069-D3075 (2016)
potential, which we also observe for our measurements. The local
maximum in Ecorr for a copper content of 59 at.% is matching with
the peak in effective work function for the shiny region containing
about 48–62 at.% Cu, and therefore both methods suggest improved
corrosion behavior for this composition range on the material library.
Conclusions
Figure 9. Potentiodynamic polarization curves at different positions on the
cobalt-copper material library obtained from SDCM measurements.
cobalt-copper material library could be attributed to a local decrease
in surface roughness.
As the work function of a metal can also be correlated to the
corrosion potential,62 localized corrosion studies were performed
by SDCM. Fig. 9 shows the recorded potentiodynamic polarization
curves for different copper atomic ratios within the material library.
The Ecorr value of the uncoated steel substrate (not shown here) was
determined to be −336 mV vs. SHE. For better readability the polarization curve for 90 at.% Cu was also excluded from the graph.
In Fig. 10 the Ecorr values from the polarization curves are plotted
against the respective copper atomic ratios corresponding to a particular position on the sample. For higher copper contents of 81 and
90 at.%, respectively, the higher thermodynamic nobility of copper
with respect to cobalt might become the determining factor, thus here
we find the most noble Ecorr values despite of relatively high surface
roughness. In the region of higher cobalt contents the corrosion potential increases consistently with increasing copper percentage until
a local Ecorr maximum of −61 mV vs. SHE is reached at 59 at.% Cu.
This value is only 6 mV less noble than the highest obtained corrosion
potential at 81 at.% Cu. For the positions with 63 and 70 at.% Cu,
a significant decline of the corrosion potential was observed, which
would not be expected considering the higher amount of copper in
the alloy. However, according to the surface roughness measurements
(see Figs. 6 and 7), in this region Rsq changes abruptly to higher values. A correlation between surface roughness and corrosion potential
has been reported for example for stainless steel,63 Mg alloy64 and
Cu,60 stating that a higher roughness leads to a less noble corrosion
Figure 10. Ecorr (SHE) values as a function of the copper content on the
material library.
By galvanostatic electrodeposition, a cobalt-copper material library was prepared using a modified Hull cell and a sulfate based
electrolyte bath with trisodium citrate as complexing agent. A composition gradient was obtained due to the Hull cell geometry, the
complex formation with citrate and kinetic effects hindering copper
reduction. The composition was found to be varying almost linearly
over the sample length and ranged from 28 to 96 at.% of copper. The
change in composition could also be noticed optically due to the color
transition from gray to reddish with increasing copper content. At
this transition area the sample surface appears shiny (48–62 at.% Cu),
in contrast to the dull main part of the coating. The effective work
function at this particular area shows a maximum, indicating higher
nobility, which is supported by the SEM micrographs revealing a
noticeably compact surface and AFM measurements, from which a
minimum surface roughness of 236 nm was determined for a copper
content of 59 at.%. At this position the corrosion potential Ecorr was
found to exhibit a local nobility maximum, confirming the results
from SKP measurements together with the dependence of corrosion
potential and work function on the roughness.
In this particular region XRD measurements suggested the formation of a cobalt-copper solid solution, where the cobalt adopts the fcc
structure of copper. Additionally, the usage of Vegard’s law allowed
concluding a linear dependency between the lattice constant and the
copper content.
Acknowledgments
The authors are indebted to W. Burgstaller for his assistance with
the SKP measurements. The financial support by the Austrian Federal
Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development through the Christian
Doppler Laboratory for Combinatorial Oxide Chemistry (COMBOX)
is gratefully acknowledged.
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