Cobalt electrodeposition onto polycrystalline gold from ... - Springer Link

Cent. Eur. J. Chem. • 11(8) • 2013 • 1381-1392 DOI: 10.2478/s11532-013-0269-5

Central European Journal of Chemistry

Cobalt electrodeposition onto polycrystalline gold from ammoniacal solutions Research Article

Luis H. Mendoza-Huizar1*, Clara H. Rios-Reyes2 1 Academic Area of Chemistry, Autonomous University of Hidalgo State, CP. 42186 Mineral de la Reforma, Hidalgo, Mexico

2 Academic Academic Area of Earth Sciences and Materials, Autonomous University of Hidalgo State, CP. 42186 Mineral de la Reforma, Hidalgo, Mexico

Received 10 November 2012; Accepted 22 April 2013

Abstract: In the present work, the cobalt electrodeposition onto polycrystalline gold electrodes from aqueous solutions containing 0.01M CoSO4 + 1 M (NH4)2SO4 at pH=7 was analyzed. Linear voltammetry results suggested a change in the kinetic of the cobalt electrodeposition. In all cases, the nucleation rate (A), the number of active nucleation sites (N0) and the saturation number of nuclei (Ns ) values were potential dependent. The calculated Gibbs free energy (ΔG) for this system was 1.88×10-20 J nuclei-1 and the transfer coefficient for the Hydrogen Electroreduction Reaction (HER) was 0.47. Keywords: Cobalt • Gold • Electrodeposition • Sulfate • Kinetic © Versita Sp. z o.o.

1. Introduction Cobalt deposits have attracted considerable attention due to their potential applications in high-density information storage devices [1-5]. Most of these Co deposits have been prepared by using pulsed laser deposition, molecular beam epitaxy, sputtering, vacuum deposition [6-9], thermal-chemical evaporation [10] and recently, electrodeposition methods [11-48]. Cobalt has been electrodeposited mainly onto carbon [11-22], gold [23-38], platinum [39-41], palladium [42], stainless steel [43-46], nickel [47] and copper [48] substrates. Chloride plating baths have been the preferred systems for studying the cobalt electrodeposition [14-16,25,30,31,37,38,47], rather than sulfate [12,20,24,26,43], perchlorate [21], acetate [36] or citrate [46] solutions. By applying nucleation and growth models to existing data, it has been found that cobalt electrodeposition begins through progressive nucleation, which may change to instantaneous nucleation when the cobalt concentration

is increased [12]. Also, the cobalt electrodeposition may occur through a nucleation process under a charge transfer [12]. From the available results, it can be seen that an increment in temperature, overpotential and cobalt concentration has a favorable effect in increasing the values of the kinetic and nucleation parameters of cobalt electrocrystallization. Also, it has been reported that the cobalt nucleation rate is lower in acid baths in comparison to basic plating baths. This is associated with the competition for the active sites on the surface by H ions with the Co cations [17,20]. Recently, the cobalt electrodeposition on gold has attracted interest due to its magnetic properties which can be used to fabricate high-density magnetic recording media. Thus, some efforts have been directed to analyze the cobalt electrodeposition process on gold substrates from sulfate [23-25,33], acetate [35], and chloride solutions [25,30,31,38]. It has been found that the nucleation of Co on Au depends on the crystallographic orientation of the substrate [49] and on the electrochemical conditions

* E-mail: [email protected] 1381

Cobalt electrodeposition onto polycrystalline gold from ammoniacal solutions

[11-48]. By using basic chloride and sulfate solutions, a cobalt underpotential deposition process (upd) has been observed on gold [25,30,31,38], while in acid chloride and neutral sulfate solutions upd has not been reported [26]. From citrate plating baths, it was observed that the morphology of the cobalt electrodeposits on gold was strongly affected by the magnetic field [46]. However, by using acetate plating baths, it was found that the experimental conditions have significant effects on the structure and electrochemical capacitance of the Co deposits [36]. Allongue et al. [28] studied the electrodeposition of Co from acidic sulfate solutions at overpotentials and found that at small overpotentials, it is possible to synthesize Co nanostructures while at larger overpotentials, Co films grow epitaxially. On the other hand, Krause et al. have found that the magnetic field strongly affects the morphology of Co electrodeposits on Au in acid sulfate solutions [24]. Although, the electrodeposition technique is a reliable, economic and simple technique, it requires a good knowledge of the nucleation and growth parameters in order to get homogeneous deposits with specific morphological and chemical properties. Thus, a good understanding of the kinetic of cobalt electrodeposition will provide a good control of magnetic and electronic properties of cobalt deposits. Despite the increasing interest to produce cobalt deposits by electrodeposition [11-48]; as far as we know, there is little information regarding the nucleation kinetic of the cobalt electrodeposition onto polycrystalline gold surfaces from sulfate solutions [23-25,33]. Therefore, in this paper, a study of the cobalt electrodeposition onto polycrystalline gold by using cyclic voltammetry and chronoamperometry to gain a deeper insight into this process is reported.

2. Experimental procedure The cobalt electrodeposition on polycrystalline gold electrode was carried out from a plating bath containing 0.01M CoSO4 + 1 M (NH4)2SO4 at pH=7. In this system the predominant chemical species corresponds to the [Co(H2O)6]2+ complex with an equilibrium potential at -0.550 V vs Ag/AgCl [16]. All plating baths were prepared using analytic grade reagents (without extra purification) and ultra pure water (Millipore-Q system) and they were deoxygenated by bubbling N2 for 15 minutes before each experiment. The working electrode was a gold tip provided by BAS™ and its exposed surface was polished to a mirror finish with Alfa Aesar™ alumina and subsequently ultrasonically cleaned before the experiments. In all cases a graphite bar was used as counter electrode while an Ag/AgCl 1382

electrode (in saturated KCl), with a Luggin capillary was used as reference electrode. All experiments were carried out at 25oC in unstirred solutions. The electrochemical experiments were carried out in an EPSILON potentiostat with the BASi-EpsilonEC software. To verify the electrochemical behavior of the electrode in the electrodeposition baths, cyclic voltammetry was carried out in the 0.600 V to -1.200 V potential range at different scan rates ([10 mV s-1 - 200 mV s-1]). The kinetic of nucleation of cobalt deposits were studied under potentiostatic conditions by the analysis of the experimental current density transients obtained with the potential step technique. The potential perturbation on the electrode always started at 0.600 V. The potential step was imposed at different potentials determined from the voltammetric study.

3. Results and discussion Fig. 1 shows a typical cyclic voltammogram obtained from the Au/0.01M CoSO4 + 1 M (NH4)2SO4 system at 20 mV s-1. From this figure, two peaks at -0.840 V (peak A) and -1.07 V (peak B) are observed in the direct scan. In order to investigate if peaks A and B correspond to a cobalt electrodeposition process, a cyclic voltammetry measurement on the gold electrode surface immersed in an aqueous solution only containing the supporting electrolyte (1 M (NH4)2SO4 was performed. Fig. 1 shows a comparison of the experimental voltammograms obtained with and without Co2+ ions. From the comparison, it is clear that peaks A and B correspond to the cobalt electrodeposition. It is interesting to note that following peak B, there is a slope change that has been associated to hydrogen evolution after the cobalt formation on the electrode surface [17,20]. In the inverse potential scan, two anodic density peak currents C (shoulder) and D appeared at potentials -0.585 V and -0.350 V, respectively. These peaks are associated to the dissolution of cobalt electrodeposited during the direct scan. To identify the correspondence between the reduction and oxidation peaks, a voltammetric study at different inversion potentials (Eλ) was undertaken, see Fig. 2. In this plot, a cathodic current decrease is seen at -0.800 V, while the formation of peak A is clear at Eλ= -0.850 V. When the scan potential was inverted towards the anodic zone at -0.850 V, an anodic peak C was recorded at -0.525 V. When the Eλ reached more negative potentials than -0.900 V, a new reduction in the cathodic current at around -1.07 V was recorded (peak B). Finally, at higher negative potential than -0.900 V for Eλ a peak D appeared in the anodic zone. These results

L. H. Mendoza-Huizar, C. H. Rios-Reyes

Figure 1.

A typical cyclic voltammogram obtained in the system 0.01M CoSO4 + 1 M (NH4)2SO4 at pH=7 at 20 mV s-1. The potential scan was started at 0.6 V towards the negative direction. Arrows indicate the potential scan direction. Cathodic density peak currents (A and B) and anodic peaks (C and D) are indicated.

suggest the presence of two cobalt phases deposited in opd conditions [22]. However, Grujicic et al. [20] have shown that the overall cobalt oxidation mechanism is given by two peaks in the anodic zone. Therefore, these peaks involve hydroxide forms either as an intermediate or as a final oxidation product. Since Grujicic et al. showed that cobalt electrodeposited in direct scan is passivated in aqueous solutions by forming a hydrated cobalt oxide layer, Co(OH), the peak C, therefore corresponds to the cobalt metal oxidation to cobalt hydroxide [20]. However, peak D appears when the cobalt hydroxide in the surface film and ammonium ions from the ammonium sulfate solution react to produce soluble cobalt sulfate and ammonia which redissolves in the plating bath. Also, the cathodic charge and the anodic charge were compared from the voltammograms in Fig. 2. In all cases, the cathodic charge was larger than the anodic charge. Since it has been reported that cobalt aqueous plating baths shows hydrogen codeposition [50], the additional charge at direct scan can be associated with the Hydrogen Electroreduction Reaction (HER). In Fig. 3, the charge associated to each process is shown. It is interesting to note that the HER charge values are larger in comparison to the cobalt charge. Results suggest a competition for the active sites on the surface by the proton and cobalt ions during the reduction processes. In this case, the hydrogen charge is larger because of the presence of metallic cobalt on the electrode surface,

which serves as a reaction site for hydrogen evolution [20]. The hydrogen reduction products are hydrogen gas bubbles that formed on the surface of cobalt nuclei. These bubbles grow and eventually prevent further cobalt reduction from taking place on their surface. In order to investigate the origin of the two cobalt reduction peaks (peaks A and B), we evaluate the kinetic and thermodynamic parameters of the Co0/Co2+ couple by using a linear cathodic voltammetry (LCV) study. In Fig. 4, a typical LCV (solid line) for the Au/0.01M CoSO4 + 1 M (NH4)2SO4 system is shown. These data were analyzed using the model proposed by Compton et al. [51]. If the presence of HER is considered; then it is necessary to include an additional contribution to the total current recorded during the LCV. Therefore, the total current should be predicted by using Eq. 1: (1) In this model, the current flowing at the electrode surface for cobalt deposition can be calculated using Eq. 2: (2) Where is the standard rate constant for deposition, aCo is the cobalt transfer coefficient and Eeq,Co is the conditional equilibrium potential, R is the ideal gas constant (8.314 J K-1 mol-1), F is the Faraday constant (96485 C mol-1), T is the absolute temperature in K 1383


Figure 2.

Cyclic voltammograms obtained in the system 0.01M CoSO4 + 1 M (NH4)2SO4 at pH=7 at different inversion potentials. In all cases, the potential scan started at 0.600 V towards the negative direction with a potential scan rate of 20 mV s-1. Arrows indicate the potential scan direction. Cathodic and anodic density peak currents are indicated.

Figure 3. Cathodic charges associated to the cobalt and hydrogen reduction processes at different inversion potentials.

1384


Figure 4.

Comparison of an experimental linear cathodic voltammogram recorded during cobalt electrodeposition at the potential range [-600 mV to -1200 mV] onto a polycrystalline gold from an aqueous in the system 0.01M CoSO4 + 1 M (NH4)2SO4 at pH=7 and a theoretical LCV (open circles) obtained by non-linear fit of Eq. 1. rd=8×10-4 cm.

and r is the radius of the cobalt microdisk electrode where the deposition takes place at initial stages [51]. Furthermore, the hydrogen discharge on the electrode surface may be modeled by Eq. 3 (3) where qs is the surface coverage by adsorbed hydrogen atoms, aH is the symmetry factor, k H is the reaction rate of HER, and C H is the concentration of H+ at the interface [52]. Fig. 4 shows a comparison between the experimental data and the model predictions from Eq. 1. For clarity sake, the LCV plot in Fig. 4 has been divided into three zones. The first zone corresponds to peak A, the second one is associated to peak B and the third one is the region where the HER process dominates. From this figure, it is clear that the proposed model accurately describes the whole experimental LCV study. Note that if the pH of the plating bath is considered, it is possible that there is more than one ionic Co species that possibly reduce at different potential causing the apparition of peaks A and B. This agreed with the data reported by Grujicic et al. [20].

By using Eq. 2, the best-fitting parameters for the first zone were aCo =0.31 and =3.18×10-6 cm s-1 while -2 aCo =0.18 and = 6.37×10 cm s-1 were the bestfitting parameters for zone 2. These values suggest a change in the kinetic transfer process during the cobalt electrodeposition. From these results, it is clear that the electrodeposition process in the first zone is slow, and probably there is an increase of cobalt concentration in the double layer. This fact will cause a reduction on the solution diffusion coefficient value in comparison to the second zone. Finally, the best-fit values for the third zone were aH = 0.47 and the k H =1.0×10-5 cm s-1. Usually the transfer coefficient for the HER is assumed to be between 0 and 1 [53]. Elumalai et al. [54] reported an aH value of 0.42 of HER on Co which is very similar to the value found in this work. By using the Berzins-Delahay equation [55], Eq. 4, it is possible to identify the limiting stage of the cobalt electrodeposition process associated to peaks A and B. (4) In this equation, j p is the peak current density value in Amperes, n is the number of electrons transferred, 1385


Figure 5. Plot of the experimental cathodic density peak current (jp) as a function of the scan rate (ν ½) for a) peak A and b) peak B (see Fig. 1). The solid line represents an uncorrected linear fit. The broken line corresponds to the corrected linear fit according to [39].

A is the area in cm2,

is the molar concentration in the bulk, is the cobalt diffusional coefficient cm2s-1, and v is the potential scan rate in V s-1. In 1/ 2 Fig. 5, the j p vs v plot by using equation 4 is shown. Here, a linear relationship is found which suggests a cobalt ion diffusion control in our experiment. According to the Berzins-Delahay equation, the current should vanish as the potential scan rate approaches zero. However, the linear fitting of the data shown in Fig. 5 has a slightly negative intercept for peaks A and B (see solid line in Fig. 5). It has been suggested that to use this equation it is necessary to perform a linear fitting of the peak current considering that at higher scan rates, the behavior is the one predicted by Eq. 4. Thus, the inception of the plot jp vs v1/2 has to be located in the 1386

axes interception in order to correctly apply this equation (see broken line in Fig. 5) [56]. By using the approach by Mendoza-Huizar [56], it was possible to calculate the cobalt diffusion coefficient values as 3.7×10-8 cm2 s-1 for peak A and 5.7×10-6 cm2 s-1 for peak B. A very interesting result from the comparison between both diffusion coefficient values is that the diffusion coefficient from peak A is almost 3 orders of magnitude lower than the typical diffusion coefficients value in aqueous solutions. This fact suggests that the diffusion control in peak A is associated to the diffusion of the electrodeposited species on the metal surface rather than the diffusion of Co2+ ions in solution [57]. Furthermore, these values agreed with the results derived from the linear cathodic voltammogram simulation as shown in Fig. 4.


Figure 6.

A set of current transients obtained from an aqueous solution 0.01M CoSO4 + 1 M (NH4)2SO4 at pH=7 on a polycrystalline gold by using the potential step technique for different potential step values (mV) is shown. In all cases, the initial potential was 600 mV.

3.1. Chronoamperometric study

It is well known that potentiostatic current transients provide information about the kinetic of the electrodeposition of cobalt. Thus, in this work, a kinetic study was performed employing the chronoamperometric technique in order to evaluate the kinetic parameters of the cobalt nucleation and growth on polycrystalline gold. Fig. 6 shows a set of current density transients recorded with a step potential technique. These transients were obtained by applying a 0.600 V initial potential to the gold surface electrode. Then, a series of negative potentials were applied on the gold surface within the [-0.920 – -1.110] V potential range. The obtained transients (see Fig. 6) showed at shorter times a current decrease. The j vs t plot then shows a maximum and then approaches to the planar electrode limiting diffusion current. This behavior has been associated to multiple 3D nucleation and growth processes controlled by a mass transfer reaction [58,59]. When the nucleation and growth of cobalt nuclei centers occurs simultaneously with the proton reduction process; the total current transient can be predicted by Eq. 5 [60]:

(9)

,

(10)

(11) In these equations, zR F is the molar charge transferred during HER, kPR is the rate constant of the proton reduction process, N0 is the number of active nucleation sites on the gold surface electrode, A is the nucleation rate, is the diffusion coefficient, and all others parameters have their electrochemical conventional meanings. Also, if one considers that the double layer contributes to the initial current, the total contribution to the current transient is given by: (12)

, (5) Where

where ,

,

(6)

(7)

(8)

(13) Here k ads ,1 = k ads , 2Qads and Qads is the charge density due to the adsorption process [61]. In Fig. 7, a comparison of an experimental current transient with a theoretical current transient generated by a non-linear fitting to the experimental data by using Eq. 12 is shown. It is clear that the proposed model accurately describes the 1387


Figure 7.

Comparison of an experimental current density transient (—) during cobalt electrodeposition at -1060 mV onto a polycrystalline gold from an aqueous solution containing 0.01M CoSO4 + 1 M (NH4)2SO4 at pH=7 and a theoretical transient (open circles) obtained by non-linear fit of Eq. 5.

whole experimental current transient behavior. Similar fittings were obtained for the other transients shown in Fig. 6. The parameters obtained from the adjustments are summarized in Table 1. Since kPR, A and N0 values increase as the applied potential becomes more negative, the proton and cobalt reduction processes become favored at higher overpotentials. Also, the Gibbs free energy of the nucleation process ( ∆G ) was calculated with Eq. 14 [62-64]: ,

(14)

where is the Boltzmann constant KB (1.38066×10-23 J mol-1), k1 = N 0wn Γ . Here, wn +c +c is the frequency of attachment of single atoms to the critical nucleus and Γ is the non-equilibrium Zeldovich factor and depends on the overpotential [65]. where g is the interfacial tension of nucleus with its motherphase, is a function of the contact angle between the nucleus and the substrate and all others parameters have their conventional meanings [65]. In order to calculate the Gibbs free energy’s value, a graph of ln A vs. η-2 was plotted according to Eq. 16) then from the slope value of the observed linear behavior ( k 2 ), ∆G was calculated by using Eq. 15: , 1388

(15)

The plot ln A vs showed a linear relationship (see Fig. 8); the ∆G calculated for this system was 1.88×10-20 J nuclei-1, since this energy corresponds to the ∆G value requirements for the formation of a stable nucleus [63,64]. The ∆G value obtained is similar to the values obtained for the cobalt electrocrystallization on GCE and HOPG electrodes [66]. The efficiency of the nucleation process was evaluated by the calculation of the saturation number of nuclei ( N S ). The N S estimation was made by employing Eq. 16 [58]: ,

(16)

Where . The N S data are reported in Table 2. As we can see from these numbers, the N S values increased with the applied overpotential. It is important to mention that because of the exclusion zones of the deposit, caused by the hemispherical diffusional gradients of 3D nucleus, N S will be always lower than N 0 at the same applied overpotential. In Table 2, the N S / N 0 ratio, which can be defined as the efficiency regarding the surface available nucleation sites, is shown. Since the N S / N 0 ratio is relatively low we can infer a competition of the


Figure 8. ln

A vs. η-2 plot used to calculate the Gibbs nucleation energy according to Eq. 15. The broken straight line corresponds to the linear fit to the experimental data.

Figure 9. ln A vs. η plot, used to calculate the critical nuclei’s size according to Eq. 17. The broken straight line corresponds to the linear fit to the

experimental data.

active sites by H3O+ ions adsorbed on the electrode surface. The critical size of the cobalt nucleus was calculated using the nucleation rate values reported in Table 1 and Eq. 17 [67]: (17) The plot vs. showed a linear behavior (see Fig. 9) according to Eq. 17. The value of d( )/d( ) was 21.77 and the critical cluster’s size calculated was nc =0. This value suggests that each active site is a critical nucleus on the surface [67]. Also, the values reported in Table 1 were modeled following a Butler– Volmer type relationship [68], by using Eq. 18 (18)

From the slope of the ln vs. E plot, it is possible to estimate the value as to 0.475. This value is similar to the one found in the LCV study. In summary, it is interesting to note that in our experimental conditions, the presence of upd processes was not recorded in comparison with chloride and sulfate solutions at basic conditions. However, it could be seen that there is a change in the value of the kinetic parameters of the cobalt electrodeposition at overpotential conditions. This change may be related to the existence of two predominant cobalt complexes in the plating bath in our experimental conditions. The average ΔG calculated for the stable nucleus formation was 1.88×10-20 which is lower than the necessary to form a critical nucleus on carbon substrates but bigger that the energy required on polycrystalline platinum. Also, the nucleation rate was lower than that obtained from basic chloride solutions onto gold or platinum 1389


Table 1.

Potential dependence observed for the nucleation parameters during cobalt deposition. The values were obtained from best-fit parameters of the experimental j-t plots using Eq. 12.

Table 2.

Potential dependence of Ns from an aqueous solution containing 0.01M CoSO4 + 1 M (NH4)2SO4 at pH=7 calculated from physical constants reported in Table 1 and Eq. 16.

-E / V

A / s-1

N0X10-5 / cm-2

kPRX104 mol / cm−2s−1

-E / V

NsX10-5 / cm-2

Ns /No

0.92

0.10

5.11

0.00

0.92

0.51

0.10

0.94

0.14

6.99

0.00

0.94

0.71

0.10

0.96

0.16

7.28

0.29

0.96

0.76

0.10

0.98

0.27

7.45

0.21

0.98

1.02

0.14

1.00

0.41

14.44

1.19

1.00

1.74

0.12

1.02

0.24

25.70

1.51

1.02

2.35

0.09

1.04

0.57

59.26

2.65

1.04

4.16

0.07

1.06

1.60

81.94

3.07

1.06

8.18

0.10

1.08

4.10

140.34

3.90

1.08

17.13

0.12

1.10

4.36

232.98

4.62

1.10

22.75

0.10

polycrystalline. This is caused by the existence of a proton reduction process which interferes with the cobalt electrodeposition.

4. Conclusions In this work, the cobalt electrodeposition on polycrystalline gold electrode was analyzed. The results indicate the presence of two peaks at overpotential conditions. These peaks were associated to the cobalt reduction from two predominant cobalt complexes. From the cyclic voltammetric study, it was evident that there is a competition for the active nucleation sites between the cobalt and proton ions. The LVC results, suggested a change in the kinetic transfer during the cobalt electrodeposition process characterized by =0.31 at E >-0.850 V and =0.18 for

E < -0.900 V. From the potentiostatic study, the A , N 0 and N S values were calculated. In all cases these parameters were potential dependent. For HER on gold, an of 0.47 was obtained from LCV and potentiostatic study which agrees with other reported values.

Acknowledgment C.H.R.R. is grateful for a postdoctoral fellowship from CONACYT. We gratefully acknowledge financial support from CONACyT project APOY-COMPL2008 No. 91261 and to the Universidad Autónoma del Estado de Hidalgo in projects PIFI 200813M8U0017T-04-01 y PIFI-2009-13MSU0017T-0401. We acknowledge Professor M. Rivera for fruitful comments.

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