Combinatorial Electrodeposition of Cobalt-Copper Material ... - JKU

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-´a-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.

Downloaded on 2016-09-21 to IP 140.78.97.11 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

D3070

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

Downloaded on 2016-09-21 to IP 140.78.97.11 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

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

Downloaded on 2016-09-21 to IP 140.78.97.11 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

D3072

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

Downloaded on 2016-09-21 to IP 140.78.97.11 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

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.

Downloaded on 2016-09-21 to IP 140.78.97.11 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

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. References 1. T. H. Muster, A. Trinchi, T. A. Markley, D. Lau, P. Martin, A. Bradbury, A. Bendavid, and S. Dligatch, Electrochim. Acta, 56, 9679 (2011). 2. S. D. Beattie and J. R. Dahn, J. Electrochem. Soc., 150, C802 (2003). 3. S. D. Beattie and J. R. Dahn, J. Electrochem. Soc., 150, C457 (2003). 4. S. D. Beattie and J. R. Dahn, J. Electrochem. Soc., 152, C549 (2005). 5. P. Srinivas, S. Hamann, M. Wambach, M. Kieschnick, A. Ludwig, and S. R. Dey, Trans. Indian Inst. Met., 66, 429 (2013). 6. P. Srinivas, S. Hamann, M. Wambach, A. Ludwig, and S. R. Dey, J. Electrochem. Soc., 161, D504 (2014). 7. C. D. Grill, J. P. Kollender, and A. W. Hassel, Phys. Status Solidi A, 212, 1216 (2015). 8. R. R. McGowan, S. Roberts, R. P. Foster, A. J. Boyce, and D. Coller, Geology, 31, 497 (2003). 9. J. Cailteux, A. B. Kampunzu, C. Lerouge, A. K. Kaputo, and J. P. Milesi, J. Afr. Earth Sci., 42, 134 (2005). 10. R. R. McGowan, S. Roberts, and A. J. Boyce, Miner. Deposita, 40, 617 (2006). 11. W. X. Pan, R. Cao, and G. L. Griffin, J. Catal., 114, 447 (1988). 12. P. Courty, D. Durand, E. Freund, and A. Sugier, J. Mol. Catal., 17, 241 (1982). 13. J. E. Baker, R. Burch, and S. E. Golunski, Appl. Catal., 53, 279 (1989). 14. A. Kiennemann, C. Diagne, J. P. Hindermann, P. Chaumette, and P. Courty, Appl. Catal., 53, 197 (1989). 15. V. Mahdavi and M. H. Peyrovi, Catal. Commun., 7, 542 (2006). 16. A. Berkowitz, J. Mitchell, M. Carey, A. Young, S. Zhang, F. Spada, F. Parker, A. Hutten, and G. Thomas, Phys. Rev. Lett., 68, 3745 (1992). 17. V. M. Fedosyuk, O. I. Kasyutich, D. Ravinder, and H. J. Blythe, J. Magn. Magn. Mater., 156, 345 (1996). 18. Y. Ueda, N. Hataya, and H. Zaman, J. Magn. Magn. Mater., 156, 350 (1996). 19. H. El Fanity, K. Rahmouni, M. Bouanani, A. Dinia, G. Shmerber, C. M´eny, P. Panissod, A. Cziraki, F. Cherkaoui, and A. Berrada, Thin Solid Films, 318, 227 (1998).

Downloaded on 2016-09-21 to IP 140.78.97.11 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 163 (12) D3069-D3075 (2016) 20. R. J. Highmore, W. C. Shih, R. E. Somekh, and J. E. Evetts, J. Magn. Magn. Mater., 116, 249 (1992). 21. H. Holloway and D. J. Kubinski, J. Appl. Phys., 83, 2705 (1998). 22. A. Blondel, B. Doudin, and J.-P. Ansermet, J. Magn. Magn. Mater., 165, 34 (1997). 23. H. Matsushima, A. Bund, W. Plieth, S. Kikuchi, and Y. Fukunaka, Electrochim. Acta, 53, 161 (2007). 24. G. Mutschke, K. Tschulik, T. Weier, M. Uhlemann, A. Bund, and J. Fr¨ohlich, Electrochim. Acta, 55, 9060 (2010). 25. A. Krause, J. Koza, A. Ispas, M. Uhlemann, A. Gebert, and A. Bund, Electrochim. Acta, 52, 6338 (2007). 26. T. Nishizawa and K. Ishida, Bull. Alloy Phase Diagrams, 5, 161 (1984). 27. S. Gu, P. Atanasova, M. J. Hampden-Smith, and T. T. Kodas, Thin Solid Films, 340, 45 (1999). 28. J. Childress and C. Chien, Phys. Rev. B, 43, 8089 (1991). 29. R. L´opez Ant´on and M. L. Fern´andez-Gubieda Ruiz, Sens. Actuators, A, 106, 203 (2003). 30. H. Zaman, A. Yamada, H. Fukuda, and Y. Ueda, J. Electrochem. Soc., 145, 565 (1998). 31. E. G´omez, A. Llorente, and E. Vall´es, J. Electroanal. Chem., 495, 19 (2000). 32. E. G´omez, A. Labarta, A. Llorente, and E. Vall´es, J. Electroanal. Chem., 517, 63 (2001). 33. E. G´omez, A. Labarta, A. Llorente, and E. Vall´es, Surf. Coat. Technol., 153, 261 (2002). 34. P. Bradley and D. Landolt, Electrochim. Acta, 45, 1077 (1999). ´ 35. R. Ant´on, M. Fdez-Gubieda, A. GarcıFa-Arribas, J. Herreros, and M. Insausti, Mater. Sci. Eng., A, 335, 94 (2002). 36. J. P. Kollender, A. I. Mardare, and A. W. Hassel, ChemPhysChem, 14, 560 (2013). 37. T. Suter and H. B¨ohni, Electrochim. Acta, 47, 191 (2001). 38. A. I. Mardare, A. Ludwig, A. Savan, and A. W. Hassel, Electrochim. Acta, 110, 539 (2013). 39. M. Sakairi, F. Sato, Y. Gotou, K. Fushimi, T. Kikuchi, and H. Takahashi, Electrochim. Acta, 54, 616 (2008). 40. A. W. Hassel, K. Fushimi, and M. Seo, Electrochem. Commun., 1, 180 (1999). 41. A. I. Mardare and A. W. Hassel, Rev. Sci. Instrum., 80, 046106 (2009).

D3075

42. A. J. Bard, R. Parsons, and J. Jordan, Standard potentials in aqueous solution, New York, M. Dekker (1985). 43. A. Brenner, Electrodeposition of Alloys Vol. 1: Principles and practice, New York, Academic Press (1963). 44. M. Paunovic and M. Schlesinger, Fundamentals of electrochemical deposition, Hoboken, N.J., Wiley-Interscience (2006). 45. A. C. Frank and P. Sumodjo, Electrochim. Acta, 132, 75 (2014). 46. G. R. Pattanaik, D. K. Pandya, and S. C. Kashyap, J. Electrochem. Soc., 149, C363 (2002). 47. E. Chassaing, K. V. Quang, and R. Wiart, J. Appl. Electrochem., 17, 1267 (1987). 48. C. Bonhˆote and D. Landolt, Electrochim. Acta, 42, 2407 (1997). 49. S. Rode, C. Henninot, and M. Matlosz, J. Electrochem. Soc., 152, C248 (2005). 50. I. Rodriguez-Torres, G. Valentin, S. Chanel, and F. Lapicque, Electrochim. Acta, 46, 279 (2000). 51. S. Rode, C. Henninot, C. Valli`eres, and M. Matlosz, J. Electrochem. Soc., 151, C405 (2004). 52. F. I. Lizama-Tzec, L. Canch´e-Canul, and G. Oskam, Electrochim. Acta, 56, 9391 (2011). . 53. A. Survila, Z. Mockus, and S. Kanapeckait=, Electrochim. Acta, 46, 571 (2000). 54. B. D. Cullity and S. R. Stock, Elements of X-ray diffraction, Upper Saddle River, NJ, Prentice Hall (2001). 55. D. Golodnitsky, Y. Rosenberg, and A. Ulus, Electrochim. Acta, 47, 2707 (2002). 56. W. N. Hansen and G. J. Hansen, Surf. Sci., 481, 172 (2001). 57. D. R. Lide, CRC Handbook of chemistry and physics, 89th Edition: A ready-reference book of chemical and physical data, Boca Raton [etc.], CRC Press-Taylor & Francis (op. 2008). 58. H. Lu, G. Hua, and D. Li, Appl. Phys. Lett., 103, 261902 (2013). 59. S. Saito, K. Takeda, T. Soumura, T. Tani, and T. Maeda, Phys. Status Solidi A, 142, K29 (1994). 60. W. Li and D. Y. Li, Acta Mater., 54, 445 (2006). 61. Y. Wan, Y. B. Li, Q. Wang, K. Zhang, and Y. H. Wu, Int. J. Electrochem. Sci., 7, 5204 (2012). 62. W. Li and D. Y. Li, Appl. Surf. Sci., 240, 388 (2005). 63. T. Hong and M. Nagumo, Corros. Sci., 39, 1665 (1997). 64. R. Walter and M. B. Kannan, Mater. Des., 32, 2350 (2011).

Downloaded on 2016-09-21 to IP 140.78.97.11 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).