COMPARATIVE STUDY ON ELECTRODEPOSITION OF Со-Cu …materials [1, 2]. Materials with size of crystalls from 0,1 to 0,3 µm are classified as submicron materials, and those with size

Katya Ignatova, Daniela Lilova

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Journal of Chemical Technology and Metallurgy, 51, 2, 2016, 173-180

COMPARATIVE STUDY ON ELECTRODEPOSITION OF Со-Cu ALLOYS FROM AMMONIA-SULPHATE AND CITRATE ELECTROLYTE


Department of Inorganic and Electrochemical ProductionsUniversity of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, BulgariaE-mail: [email protected]

ABSTRACT

The kinetics of the еlectrodeposition of Cu and Co in Cо-Cu alloy, chemical composition, cathodic current efficiency and morphology of Co-Cu alloys electrodeposited from an ammonia-sulphate electrolyte (AMSE) and from a weakly acidic citrate electrolyte (CE) without and with addition of saccharin (CE+SHR) at constant potential mode are compared. The results indicate that the deposition of Co-Cu alloys from AMSE occurs without initial polarization, while the deposition from CE is characterized by long initial polarization and the overall polarization is higher compared to that in AMSE as a result of formation of more stable citrate complexes of the metals. The individual deposition of Co as well as Cu from CE is a two-stage process of reduction of their complex ions. It is found that the Co-Cu alloys with Co up to 50 - 60 mass % deposited from AMSE have coarse-grained crystalline structure with comparable amount of elongated and spheroid crystals with average size in the range from 2 to 10 μm, as compared to those deposited from CE with average size of about and less than1μm. The increase of the Co content in the alloys up to 80 mass % results in decrease of the average size of the crystallites in AMSE to less than 5 µm, and in СЕ - of about 500 nm. The addition of saccharin in the citrate electrolyte yields glossy and smooth coatings with close to nano-sized structure with average size of the crystallites in the range from 100 to 300 nm.

Keywords: electrodeposition kinetics, cobalt alloys, nanostructured coatings, morphology, chemical composition, constant potential mode.

Received 11 May 2015Accepted 18 December 2015

INTRODUCTION

Recently the miniaturization of magnetic technol-ogy has drawn the interest to submicron and nano-sized materials [1, 2]. Materials with size of crystalls from 0,1 to 0,3 µm are classified as submicron materials, and those with size less than 100 nm are classified as nanomaterials [2].

Both Со and Co-Cu nanostructured alloys [3, 4 - 7] and multi-layers [8 - 11] obtained through either elec-trodeposition, or another methods, have been intensively investigated recently, due to their specific mechanical,

electrical, magnetic, and corrosion properties varying in wide ranges with the variation of Co/Cu concentration ratio. These coatings find increasingly wider application in such areas as magnetoelectronics, biotechnology, catalysis, metal ceramics and semiconductor technolo-gies [12].

In many cases, the choice of the electrolyte with appropriate composition is decisive for obtaining alloys with structure and content required for their application. There are evidences that the electrodeposition of Со-Cu alloys may be carried out in different electrolytes, e.g. sulphate [3, 7, 13], citrate [8, 12, 14, 17] or glycinate [4]

Journal of Chemical Technology and Metallurgy, 51, 2, 2016

174

electrolytes. It is also found that some organic additives, such as dodecyl sulfate, saccharin and glycine, affect not only the structure of Со-Cu alloys and multi-layers, but their magnetic properties as well [4,11].

The structural and magnetic properties of Со-Сu films depending on the content of Со are investigated [4, 7, 12, 17]. The surface morphology of the films reveals that at low content of Co (less than 3 %) the prevalent structure is dendritic, and at higher content of Co (more than 61 %) the prevalent structure is nanosized, with needle-shaped crystals in some areas. At low Co content the crystalline structure is face-centered cubic α-phase and at high Co content it is hexagonal ε-phase [7].

The pulse electrolysis is successfully applied for preparation of nanocrystalline coatings based on cobalt [15, 16]. The pulse deposition allows to extend the range of properties of the obtained coatings as a result of variation of at least three parameters: amplitude of the current or potential, frequency and duty cycle of the pulses, instead of one, as it is the case in a stationary mode deposition. In the present article the results from comparison of the kinetics of deposition of Cu and Co, as well as the chemical composition and morphology of the Co-Cu alloy deposition from an ammonia-sulphate electrolyte (AMSE), a weakly acidic citrate electrolyte (CE) and СЕ with addition of saccharin (CE+SHR) at constant potential mode are reported.

EXPERIMENTAL

All electrochemical experiments were carried out in a thermostatic three-electrode cell without stirring. The cell with a total volume of 150 dm3 includes a working electrode with a copper substrate (Merck, 99,97 mass % Cu), a platinum counter electrode ar-ranged concentrically around the working electrode and a reference electrode - saturated calomel electrode in a special pan with Lugin capillary (ESCE = 0,241 V). The kinetics of deposition is examined using copper electrodes shaped as discs with surface area 1 cm-2. For electrodeposition of Co-Cu alloys coatings, copper cathode plates with dimensions 1,5 x 4 cm are prepared. Prior to each experiment, the surfaces of the copper cathodes are fully cleaned from oxides by etching in an especially prepared acidic solution (H2SO4 : HNO3 = 1 : 1; 10 % HCl); then the cathodes are rinsed with distilled water several times and dried. The anodes are cleaned in hot 1N HNO3, rinsed with distilled water and dried. The Co-Cu alloys, with thickness 8±2 µm, are dissolved in 30 % HNO3 and the solution is subjected to Atomic Absorption Analysis (ААА). The data from AAA are used to determine the chemical composition of the coatings, which is the basis for calculating the current cathodic efficiency (CCE) upon Faraday’s law of electrolysis:

Table 1. Composition of the working electrolytes: (A) ammonia-sulphate (AMSE); (B) citrate (CE); (C) citrate electrolyte + saccharin (CE+SHR).

Еlectrolyte (A)AMSE (B) CE (C) CE+SCH

CuSO4.5Н2О 0,1-0,4M 0,15-

0,025M

0,025M

CoSO4.7H2O 0,2M 0,3M 0,3M

NH3 0,7M - -

Na3C6H5O7 - 0,4M 0,4M

424 SO)(NH 0,5M - -

33BOH - 0,485M 0,485M

Saccharin,SHR - - 0,2-1,5 g dm-3

pH 10,0 5,5 5,5


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pr

Co-Cu

mCCE .100,%

q .Q∆

=

where Co-Cuq [geq C-1] is the electrochemical equivalent of the alloy, which is determined by the data for chemi-cal composition of the alloy, Co-Cu thq .Q m= ∆ is the theoretically determined mass of Co-Cu alloy and Δmpr is the practical determined mass.

The experimental study is carried out in three work-ing electrolytes with composition given in Table 1. The kinetics of deposition of Со-Cu alloy is studied using Wenking Electrochemical Analysis System (Germany) at linear rate of potential scanning of 30 mV s-1. The morphology of the coatings is studied through SEM, and the elemental composition - by means of Energy Dis-persive Spectral Analysis (EDSA) using JSM-6390-Jeol, Oxford Instruments.

RESULTS AND DISCUSSION

Kinetics of electrodepositionThe conditions for electrodeposition of Co-Cu

alloy are identified through comparison between the polarization dependencies of independent deposition of Cu and Co and the dependencies of their co-deposition. In Fig. 1 a comparison of polarization dependencies of independent and co-deposition in AMSE (composition A with 0,2 M Cu, Table 1), while Fig. 2 displays those dependencies in CE (composition В).

As seen from the kinetics data obtained in composition A and B (Table 1), the electrodeposition of Cu, Co, and

Co-Cu alloy is characterized with specificities that cannot be explained only by different values of Cu/Co ratio in the solution, i.e. Cu/Co = 1, and Cu/Co << 1 for AMSE and CE, respectively (Table 1). The most characteristic difference in the kinetics of deposition is the absence of initial polarization both for independent and co-deposition of Cu and Co in AMSE (Fig. 1), unlike the case with CE (Fig. 2). As seen at comparable concentrations of both metals in ASME, there is similarity in their deposition po-tentials. The deposition of copper in AMSE (Fig.1, curve 1) occurs at more positive potentials and proceeds with lower polarization than that of cobalt (Fig. 1, curve 2), as the limiting current densities are comparable. These data comply with the data of the standard potentials of the two metals. In presence of ammonia at рН 10, however, both Co and Cu ions are involved in complexes and consider-ing the data for their constants of stability (Table 2), the copper-ammonia complexes have higher stability than the cobalt-ammonia complexes. This probably explains the observed similarity of deposition potentials of the two metals from AMSE.

Meanwhile, in AMSE both of Co and Cu deposi-tions occur as two-stage process (Fig. 1). Our previous research [20, 21] and the literature data [3] prove that only the second plateau of the two curves is related to substantial reduction of complex metal ions to metal in AMSE upon the equation: 2Me 2e Me+ −+ ↔ . The first plateau is related to the process: 2Me 1e Me+ − ++ ↔ , as some disproportionation chemical reactions of the type

2 Me Me 2Me+ ++ ↔ possibly occur prior to the actual deposition.

Fig. 1. Comparison of polarization dependencies of Cu, Co and Co-Cu alloy deposition from AMSE.

Fig. 2. Comparison of polarization dependencies of Cu, Co and Cu-Co alloy deposition from CE.


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The comparison between the kinetics of independent deposition and co-deposition of Cu and Co from weakly acidic CE (рН 5,5) with ratio Cu/Co << 1is illustrated in Fig. 2. Unlike AMSE, there is a long initial polarization during the independent deposition of the two metals, as well as of the co-deposition of the alloy. That behavior is observed for all Cu/Co concentration ratios studied. The long initial polarization is possibly associated both with the adsorption of citrate complexes of metals and subse-quent dissociation, and the presence of oxide- or hydroxide products onto the surface of the copper electrode in the weakly acidic environment of CE. In these conditions, the deposition of copper (Fig. 2, curve 1) takes place at almost twice higher cathodic polarization compared to that in AMSE (Fig. 1, curve 1), and with that of Co from СЕ (Fig. 2, curve 2), as quite low values of the limiting cur-rent are reached. The deposition of Co also occurs with a long initial polarization, as the total polarization also increases, and the limiting diffusion current decreases

in comparison with deposition in AMSE. Generally, the higher cathodic polarizations of deposition of Cu and Co reached in CE are possibly associated with the higher stability of their citrate complexes compared to the am-monia ones, as it is seen from the data in Table 2. Both the individual and the co-deposition of Cu and Co from СE occur with only one peak of the current for Co, and one plateau of the current for Cu, unlike the case with AMSE (Fig. 1).

The addition of saccharin in CE (composition С, Table 1) increases the total polarization of deposition of both independent deposition and co-deposition, as the increase does not exceed 100 mV (Fig. 3). Moreover, the nature of the curves remains relatively unchanged, as a whole.

The investigation of the kinetics of deposition of Co and Cu from all three working electrolytes reveals some differences in the kinetics, and possibly in the mecha-nism of deposition of Co-Cu alloy, which is expected to affect the type and the properties of the coatings formed. These data are the base for evaluation of the potentials of deposition of Co-Cu alloy coating.

Chemical composition and cathodic current ef-ficiency

The data of the component composition and cathodic current efficiency depending on the conditions for depo-sition from the three working electrolytes A, B, and C are presented in Figs. 4 - 6. For the purpose of compa-rability of conditions, the deposition of Co-Cu alloys is carried out at potentials corresponding to one specified value from the polarization curve for the respective composition, which is always prior to the beginning of

Table 2. Data about stability constants (pK = -lgK) of Со and Cu complexes with ammonia[18] and with citrate ions [19].

Complexes of Сu рК Complexes of Сo рК +2

3 )]Cu(NH[ 4,27 +23 ])Co(NH[ 2,11

+223 ])Cu(NH[ 7,82 +2

33 ])Co(NH[ 4,79

+243 ])Cu(NH[ 12,90 +2

53 ])Co(NH[ 5,73

CuHCit 9,55 −CoCit 4,38

−222CitCu 14,43 −4

2CoCit 7,103

Fig. 3. Effect of addition of saccharin(SHR) on the kinetics of co-deposition of Cu and Со from CE (Compositions B and С in Table 1).


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the plateau of the limiting current. This value is equal to 15 mA cm-2 for AMSE (Composition A, Table 1), and 28 mA cm-2 in CE and CE+SHR (B and C in Table 1).

The data shown in Fig. 4 indicate that with the in-crease of copper content in AMSE (Composition A), the content of Co in the alloys decreases from 55 mass % to 10 mass % and the cathodic current efficiency slightly increases from 90 % to 98 % (curve 2), which means that at higher cobalt content are registered lower values of cathodic current efficiency in AMSE. The presence of higher quantities of cobalt may result in decrease of the overvoltage for hydrogen evolution, and subsequently to decrease of CCE. The data in Table 2 suggest that the cobalt-ammonia complexes have lower stability com-pared to copper-ammonia complexes. This increases the capability of cobalt to form insoluble hydroxide-or oxide products on the cathode which also leads to lower CCE.

It is seen from Fig. 5 that with increase of Со con-tent in citrate electrolyte (Composition B), the content of cobalt in the alloys increases from 21 mass % to 83 mass %. With the increase of content of Со in СЕ at comparable conditions, CCE decreases from 90 % to 86 % (curve 2), as this result correlates with the results for AMSE (Fig. 4).

The stronger decrease of CCE in citrate electrolyte is probably due to the larger difference in the constants of stability of the complexes of copper and cobalt with citrate ions (Table 2), from where it follows that the stability of copper-citrate ions is almost twice higher

than that of cobalt-citrate ions. Moreover, the above described reasons, i.e. increased capability of cobalt to form hydroxide- and oxide products on the cathode and the decreased overvoltage of hydrogen evolution in presence of more cobalt in AMSE, respectively in Co-Cu alloy in the case of CE, are exhibited in a higher extent.

The effect of the content of SHR in CE (Composi-tion C) on the composition of the alloy and the CCE is shown on Fig. 6. As it is seen, the increase of the content of SHR in the solution from 0,2 to 1,5 g dm-3 leads to decrease of the content of Co in the alloy from 87 mass

Fig. 4. Effect of Cu/Co ratio in the solution on the chemi-cal composition of the alloy (1) and the CCE (2) in AMSE (Composition А, Table 1).

Fig. 5. Effect of Cо/Cu ratio in the solution on the chemi-cal composition of the alloy (1) and the CCE (2) in CE (Composition B).

Fig. 6. Effect of the concentration of saccharin additive on the chemical composition of the alloy (1) and CCE (2) in CE+SHR.


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% to 65 mass %, as well as to decrease of CCE from 82 % to 74 % (Fig. 6, curve 2) in result of the increasingly hampered deposition due to adsorption on the surface and its blocking by saccharine molecules, which is proved by the kinetics of the process.

Morphology of Co-Cu coatingsIn order to examine the morphology of the Co-Cu

coatings in different conditions, (i.e. two complex solu-tions, either with or without addition of saccharin), the magnifications of SEM varied from 2000х to 10000х

Fig. 7. SEM-images of Co-Cu alloy deposited from: (a) Composition A (AMSE) at 0,2 М Cu and E = -1,150 V; (b) Com-position А (AMSE) with 0,1 M Cu and E = -1,150 V; (c) Composition B Cu (CE) at E = - 1,200 V; (d) Composition C (СЕ + 1,225 g dm-3 SHR) at E = -1,200 V; (e) Composition C (СЕ + 1,225 g dm-3 SHR), E = -1,280 V; (f) Composition С (СЕ + 1,5 g dm-3 SHR), E = -1,280 V (for composition A: 0,2 M Co; for B and C: 0,025 M Cu and 0,3 M Co).

(a) 21 % Co - 79 % Cu (b) 56 % Co - 44 % Cu

(c) 85 % Co - 15 % Cu (d) 68 % Co - 32 % Cu

(e) 76 % Co - 24 % Cu (f) 71 % Co - 29 % Cu


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depending on the image obtained.The SEM images of Co-Cu alloys deposited from the

three compositions studied: A (Fig. 7a,b); B (Fig. 7c), and C (Fig. 7d-f) are shown on Fig. 7.

They illustrate: the effect of copper content in AMSE (Fig. 7a,b); the presence of SHR in СЕ (Fig. 7c,d); the increase of potential (Fig. 7d,e) and the increase of SHR content in СЕ (Fig. 7e,f). The data for the composition of the coatings (in mass %), evaluated using AAA and EDSA, are given bellow each image.

As it is seen from the SEM images in Fig. 7a,b, Co-Cu alloy coatings deposited from AMSEhave coarse-grained structure with comparable amount of elongated and sphe-roidal crystals with average size from 2 to 10 µm.

The increase of Co content in AMSE, and cor-respondingly in the alloys (Fig. 7b), results in a much more fine-grained structure, as the average size of the crystallites is in the range from 1 µm to 5 µm. The Co-Cu alloys deposited from CE in compositions B and C (Table 1) at different potentials with addition of saccharin are much more fine-grained.

In absence of SCH mainly needle-shaped crystal-lites are observed, as well as larger ones with spheroidal shape (Fig. 7с), with addition of saccharin, however, more fine-grained coatings are deposited (Fig. 7d-f). From the electrolyte with the same content but with only slight increase of potential from -1,2 V to -1,28 V, both the morphology and the alloy composition change greatly (Fig. 7d-f). At potential -1,2 V the prevalent metal is copper and the coatings have spheroidal grain structure (Fig. 7d).

The increase of potential to -1,28 V leads to increase of the content of Co in the coating up to 80 mass % (Fig. 7e) and makes the coatings more fine-grained. With the increase of the content of saccharin to 1,5 g dm-3 (Fig. 7f) the blocking effect on the growth of crystallites in-creases and the crystallites are smaller and more evenly distributed on the surface. The study proves that both the presence of saccharin in CE and the application of more negative cathode potentials results in formation of finely crystalline Co-Cu alloys with structure close to nanosized structure with average size of the crystals around 100 - 300 nm.

CONCLUSIONSData about the chemical content, cathodic current

efficiency and morphology of Co-Cu alloys with cobalt

content up to 60 mass % deposited at constant potential mode from three types of electrolytes: ammonia-sulphate and citrate without and with addition of saccharin are obtained. It is found that Co-Cu alloys with cobalt con-tent up to 60 mass % deposited from AMSE are coarse-grained with comparable quantities of elongated and spheroidal crystals with average size from 2 to 10 µm, while the crystals deposited from CE have average size about and less than 1 µm. The increase of cobalt content in the alloys up to 80 % leads to decrease of the average size of the crystallites to less than 5 µm in AMSE, and to about 500 nm in CE. The data from SEM-images indicate that in presence of saccharin in СЕ and at more negative cathodic potentials the resulting Co-Cu alloys have fine-grained structure close to nano-sized structure with average size of crystallites up to 100 - 300 nm.

REFERENCES

1. S.C. Tjong, H. Chen, Nanocrystalline materials and coatings, Materials Scince and Engineering, R45, 2004, 1-88.

2. S. Bagheri, M. Guagliano, Review of shot peening processes to obtain nanocrystalline surface in metal alloys, Surface Engineering, 25, 1, 2009, 3-14.

3. M. Gu, Initial stages of the electrocrystallization of Co-Cu alloys on GCE from the Co rich electrolytes, Еlectrochimica Acta, 52, 2007, 4443-4448.

4. A.E. Mohamed, S.M. Rashwan, S.M. Abdel-Wahaab, M.M. Kamel, Electrodeposition of Co-Cu alloy coat-ings from glycinate baths, J.Appl.Electrochem., 33, 2003, 1085-1092.

5. S.K. Gupta, M.G. Gartley, XRD and VSM analysis of nanostructured Cu-Co alloys, J.CPDS International Centre for Diffraction Data, 1999, 688-697.

6. М. Yuasa, H. Nakono, Y. Nakamoto, M. Hakamada, M. Mabuchi, Ferromagnetic рroperties of Co-Cu alloy with nanoscale lamellar structure, Materials Transac-tions, 50, 2, 2009, 419-422.

7. O. Karaagac, H. Kockar, M. Alper, Composition Dependence of Structural and Magnetic Properties of Electrodeposited Co-Cu Films, IEEE Transactions on Magnetics, 46, 12, 2010, 3973-3977.

8. M. Uhlemann, A. Gebert, M. Herrich, A. Krause, A. Cziraki, L. Shulz, Electrochemical deposition and modification of Cu/Co-Cu multilayer, Electrochim. Acta, 48, 2003, 3005-3011.


180

9. S.M.S.I. Dulal, E.A. Charles, S. Roy, Characterisation of Co-Ni(Cu)/Cu multilayers deposited from a citrate electrolyte in a flow channel cell, Electrocimica Acta, 49, 2004, 2041-2049.

10. J.J. Kelly, M. Cantoni, D. Landlt, Tree-Dimensional Structuring of Electrodeposited Co-Cu multilayer al-loys, J.Electrochem.Soc.,148, 9, 2001, C620-C626.

11. E. Chassaing, Еffect of Organic Additives on the Electrocrystallization and the Magnetoresistance of Cu-Co Multilayers, J.Electrochem. Soc., 148, 2001, C690-694.

12. G.R. Pattanaik, D.K. Pandya, S.C. Kashyap, Prepara-tion of Cu-Co alloy thin films on n-Si by galvano-static electrodeposition, J. Electrochem Soc, 149, 2002, C363-369.

13. L. Mentar, M.R. Khelladi, A. Beniaiche, A. Azizi, Influ-ence of the electrodeposition potential on the Co-Cu al-loys thin films properties, Int. J. Nanosci., 12, 2013, 1-8.

14. C.P. Singh, R.K. Shukla, S.C. Srivastava, Electro-deposition of Ni-Co-Cu alloys from a citrate bath, Surface Technology, 16, 1982, 219-225.

15. T. Ohgai, X. Hoffer, L. Gravier, J.-Ph. Anseremet, Electrochemical Surface Modification of Aluminium Sheets for Application to Nano-electronic Devices: Anodization Aluminium and Deposition of Cobalt-Copper, J. Appl. Electrochem., 34, 2004, 1007-1012.

16. B. Tury, G.Z. Radnoczi, G. Radnoczi, M.L. Vasanyi, Microstructure properties of pulse plated Ni-Co al-loy, Surface and Coatings Technology, 202, 2007, 331-335.

17. S.S. Abd El-Rehim, S.M. Abd El-Wahab, S.M. Rashwan, Z.M. Anwar, Electroplating of a Co-Cu alloy from a citrate bath containing boric acid, J. Chem. Technology & Biotechnology, 45, 3, 2000, 237-244.

18. Handbook Electrochemistry, Leningrad, Chemistry, 1981, (in Russian).

19. N.Кotsakis, C.P. Raptopoulou, V. Tangoulis, A. Terzis, J. Giapintzakis, T. Jakusch, Correlation of synthetic, spectroscopic, structural and speciation studies in the biological relevant Co(II)-citrate sys-tem, Inorganic Chemistry, 42, 2003, 22-31.

20. Katya Ignatova, Lyubomir Petkov, Electrodeposi-tion and morphology of Cu, Co and Cu-Co alloy coating in aminosulphate electrolyte, Journal of the University of Chemical Technology and Metallurgy, 44, 2, 2009, 133-138.

21. Katya Ignatova, Daniela Lilova, A kinetic study of the electrodeposited Ni-Co alloy in citrate electro-lyte. Part 1: The kinetic study of the independent electrodeposition of Ni and Co, Journal of Chemical Technology and Metallurgy, 50, 2, 2015, 199-206.

Documents

COMPARATIVE STUDY ON ELECTRODEPOSITION OF Со-Cu …materials [1, 2]. Materials with size of crystalls from 0,1 to 0,3 µm are classified as submicron materials, and those with size