Effect of Electroplating Variables on Electrodeposition of Ni Rich Ni-Ir Alloys from Citrate Aqueous Solutions

According to preliminary studies, the low pH values of 2.0–3.0 led to black precipitate formed on the surface of the substrates. Raising pH value could form the silvery deposits from aqueous solution. Therefore, the pH was varied from 4.0 to 8.0, while keeping all other variables constant. The temperature and current density were 70°C and 50 mA·cm⁻², respectively. The effect of electroplating variables on the characteristics of as-deposited samples is shown in Table II. The mass gain and film thickness increased with increasing pH from 4.0 to 5.0, and then steadily decreased with increase in pH from 5.0 to 8.0. Figure 1 shows the plots of the Me-content (Me = Ir and Ni) in the deposit, the FE and the measured potential as a function of pH. In Fig. 1a, the Ir-content kept in a range of 10–15 at% with increasing pH from 4.0 to 8.0. The abundant of Ni-content in the deposit was obtained. In Fig. 1b, the FE reached a maximal value of 35% at pH 5.0, but the FE kept stable in the range of 20–30% at high pH. The measured cathodic potential increased from –0.704 to –1.323 V, resulting in an increase of 0.619 V with an increase in pH. This variation corresponds well to the change of the reversible potential for hydrogen evolution according to the Nernst equation. The measured cathodic potential shifted toward more cathodic potential with increase in pH, indicating the equilibrium potential shift of oxidation reaction. According to above results, the electrolyte pH 5.0 is the best suitable for the deposition of Ni-Ir alloys.

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The bath temperature was varied from 50 to 90^°C, while keeping all other variables constant. The pH and current density were 5.0 and 50 mA·cm⁻², respectively. The mass gain and film thickness increased with increasing temperature from 50 to 70^°C, and then remained stable (Table II). The increase of temperature could result in the increase in the rates of both diffusion and charge transfer. However, there are different complexing ions in the solution, the equilibrium constant could be changed with an increase in the temperature. At T = 90^°C, the vaporization of the electrolyte was accelerated, leading to the degeneration of the electrolyte. Figure 2 presents the plots of the Me-content in the deposits, the FE and the measured potential as a function of plating temperature. In Fig. 2a, the Ir-content had little change with increasing the temperature, except the minimal Ir-content of 6 at% at T = 80^°C. However, the Ni-content was higher than Ir-content in the deposits, which is independent of the change of the temperature. As shown in Fig. 2b, the FE increased steadily with increasing temperature from 50 to 70^°C, and then slightly decreased with increasing the temperature. The FE had a maximal value of 35% at T = 70^°C. The difference between the measured cathodic potentials from 50 to 90^°C is about – 0.26 V, indicating that the temperature has a large effect on the cathodic overpotential. A rise of bath temperature shifted deposition potential to less negative values, suggesting that the overpotential needed to start the electrodeposition process of the alloy is lower at higher temperatures, which is in accordance with the Nernst Equation.²⁶ An increase in temperature enhanced the concentration of the reducible species in the diffusion layer as a result of increasing their diffusion rates. This behavior could be relative to the decrease in the overpotential of both hydrogen evolution and Ni-Ir deposition reaction. To sum up, the bath temperature 70^°C seems to be the best choice for deposition of Ni-Ir alloys, since above it heavy evaporation take place.

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The current density was in the range of 30 – 80 mA·cm⁻², while keeping all other variables constant. The optimal pH and temperature were 5.0 and 70^°C, respectively. As shown in Table II, the mass gain and deposition rate increased with increasing current density from 30 to 60 mA·cm⁻², and then decreased with increase in current density from 60 to 80 mA·cm⁻². At j = 60 mA·cm⁻², the mass gain and the rate of the film deposition were maximal, corresponding to 43.6 mg and 25.7 μm·h⁻¹, respectively. Figure 3 inllustrates the plots of the Me-content in the deposit, the FE and the measured potential as a function of current density. In Fig. 3a, the Ir-content was minimal at j = 30 mA·cm⁻². With the increase of current density, the Ir-content increased. The rich Ni-content in the alloys (up to 93 at%) was obtained, indicating that the deposition of Ni²⁺ ions was favored. In Fig. 3b, the FE steadily increased with increasing current density from 30 to 60 mA·cm⁻², and then dramatically decreased with increase in current density from 60 to 80 mA·cm⁻². At j = 60 mA·cm⁻², the FE got a maximal value of 36%. In the present work, the optimized current density was 60 mA·cm⁻² for the deposition of Ni–Ir films. An increase in current density enhanced the amount of hydrogen evolved during electrodeposition, resulting in a decrease in the FE for alloy deposition.²⁴ On the other hand, Ir is much better catalysts for hydrogen evolution than Ni, so that increasing the proportion of Ir in the alloy would result in an increase in the hydrogen evolution rate and, consequently, a decrease of the FE. The existance of a maximal FE is in agreement with the diffusion control of the Ni deposition process from citrate electrolytes at high current density.³² The increase of current density beyond 60 mA·cm⁻² caused intensively hydrogen evolution, while the deposition of Ni is limited by partial mass transport control.

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The measured cathodic potential decreased from –1.051 to –1.390 V, resulting in an increase of 0.34 V, as the current density increased from 30 to 80 mA·cm⁻². The measured cathodic potential shifted toward more negative cathodic potential with increasing current density. Figure 4 shows the evolution of the deposition potential during Ni-Ir alloy electrodeposition. A further increase in current density led to a notable shift of deposition potential toward low values. Ni-Ir alloys at 30, 40 60, 80 mA·cm⁻² exhibited an average deposition potential −1.05, −1.07, −1.29 and −1.42 V vs. Ag/AgCl, respectively. Cohen-Sagiv et al.¹³ pointed out that increasing current density above 50 mA·cm⁻² led to the increase of Ni-content in the Re-Ir-Ni alloys. However, the above data indicated that the increase in current density is beneficial for enhancing Ir-content in the Ni-Ir alloy. The increase of Ir-content could be explained by the fact that the current density is applied where Ni²⁺ ions are reduced under diffusion limiting current density. Thus, a further increase in current density could enhance the Ir deposition and hydrogen evolution Reaction.³³

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Figure 5 shows the SEM images of the top surface of Ni-Ir films deposited at different electroplating conditions. The deposits appeared visibly bright and shiny. At low pH of 4.0, the surface was fully covered by the deposits (Fig. 5a). With the increase of pH, the surface was composed of cauliflower-shaped spherical aggregates (Fig. 5b). The silvery deposit was obtained, as visually observed. Some micropores were evident on the surface, but microcracks were not observed due to the abundant of Ni-content in the deposit. Ohsaka et al.⁵ studied that the surface-cracking size of electrodeposited Ir-Co alloys could be reduced with increasing Co-content in the deposits. The pH above 5.0 had little or no effect on the surface morphology of the film (Figs. 5c, 5d and 5e).

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The temperature had a significant influence on the surface morphology of the alloys. At T = 50^°C, many large aggregates and some microcracks were present (Fig. 5f). The intercolony boundaries and grooves were clearly visible. With the increase in the temperature, the aggregates were composed of small nanoparticles, and the intercolony boundaries became blurry (Fig. 5g). At T = 80^°C (Fig. 5h), some large aggregates look like the cherry-shaped structure. The film consisted of agglomerated mesoscale colony and nodules with clear intercolony boundaries and surface grooves, which met and coalesced each other to form a compact layer. At T = 90^°C (Fig. 5i), the top surface was composed of some long nodulars. However, the deposit under the nodulars were dense. The appearance of these islands indicated a Volmer-Weber growth mode.^34,35 This growth mode was preferred, regardless of the crystallographic misfit between the film and the substrate. The formation of nodules might be the outcome of nucleation extending laterally from an existing structural defect and sweeping deposit substances ahead of the growth front of the films.

Furthermore, the surface morphology of the alloys was evolved from rough into smooth with increase in current density. At j = 30 mA·cm⁻², the surface was rough. The large aggregates and microcracks were observed (Fig. 5j). As the applied current density increased, the surface morphology was evolved into relatively small aggregates, which were composed of many nanoparticles (Fig. 5k). At j = 60 mA·cm⁻², the surface became relatively dense, and consisted of many small aggregates (Fig. 5l). At j = 70 mA·cm⁻², the surface became smooth and some fine cracks were present (Fig. 5m). At j = 80 mA·cm⁻², a low density of micro-crack was observed on the smooth surface (Fig. 5n). The difference in the surface morphologies arises, because low current density allows only a few crystals to nucleate, and long deposition time allows the grain growth leading to their impingement.³⁶ Hence, the large aggregates were present at j = 30 mA·cm⁻². As the current density increased, the gain size became small and the number of nucleation increased.

In this work, the ratio of Cit³⁻/Ni²⁺ concentration is 0.78, free nickel ions are the predominant specie. Here citrate is abbreviated to Cit. Both at pH = 5.0 and 8.0, the rate of Ni deposition decrease in the order of Ni²⁺ > NiCit > NiCit₂.³⁷ Thus, the ratio between the concentrations of citric acid and Ni²⁺ in solution is an important factor, controlling the deposition rate, the FE and the composition of the alloy. Naor et al.³⁸ calculated that the relative abundance of citrate-containing species as a function of pH for 343 mM analytical concentration of citric acid at the temperature of 70^°C. At pH = 4.0, there are four anions H₂Cit⁻, HCit²⁻, H₃Cit and Cit³⁻, corresponding to the concentrations of 58%, 35%, 5% and 2%, respectively. When the pH was increased to 5.0, the three anions of H₂Cit⁻, HCit²⁻ and Cit³⁻ exist at significant concentrations of 11.6%, 67.7% and 20.7% respectively. With increase in pH, the concentration of Cit³⁻ increased significantly. The content of Cit³⁻ ion is ∼98% at pH = 8.0. The equilibrium constants of nickel-citrate complexes were logk₁ = 5.72, logk₂ = 3.362, logk₃ = 1.748 provided for [NiCit]⁻, NiHCit and [NiH₂Cit]⁺, respectively.³⁹ Because the concentration of the citrate ion is lower than that of Ni²⁺ ion, there will be significant amounts of free Ni²⁺ present in the solution at all values of pH. Thus, the [NiCit]⁻ is the predominant complex species in the solution at high pH, which could inhibit the parallel paths for deposition of Ni. According to the above data, the concentration of Cit³⁻ in the solution increased significantly with increasing pH. Therefore, the deposition rate and the FE of Ni-Ir alloy decreased with increase in pH. The following electroreduction reactions are possible, therefore with hydrogen evolution from an independent side reaction at high pH value:

However, the reduction of Ni²⁺ ions and hydrogen evolution reaction at pH 5.0 occurred according to the following Reactions.³⁷

The formation constant of the [Ni(Cit)₂]⁴⁻ complex cannot sustain the high concentration of the [Ni(H₂O)₃Cit]⁻ complex.³⁷ Therefore, following reaction Eqs. 4 and 7, the [Ni(Cit)₂]^4- complex dissociates releasing free Ni²⁺ ions, which promptly reduces to metallic Ni. The hydrogen evolution reaction causes the interfacial pH to increase, the Ni-citrate complexes start to dissociate releasing free Ni²⁺ ions. So, the reduction of Ni is always confluent with hydrogen evolution reaction.²¹ In an acid chloride solution, the reduction of [IrCl₆]³⁻ ion takes place in parallel with Reactions 2–7,

However, in an alkaline solution, the Ir³⁺/hydroxyl species play the role in the electrodeposition process. Karthik et al.⁴⁰ suggested that the complex [Ir(OH)₆]²⁻ was present in a dilute solution of 10 mM IrCl₃·3H₂O at pH = 13, which was attributed to the presence of Ir⁴⁺–O–Ir⁴⁺ linkages. Cusanelli et al.⁴¹ studied on complexation reactions and water exchange, and found that hexa-aqua Ir complex [Ir(H₂O)₆]³⁺ had a high degree of inertness toward ligand substitution. Its hydrolysis in water was a kinetically important process due to the production of a significant amount of the conjugate base [Ir(H₂O)₅(OH)]²⁺, which could lead to an alternative pathway for water exchange. It was indicated that [Ir(H₂O)₅(OH)]²⁺ species was much more reactive and had more dissociative character than the hexa-aqua ion. The reduction of Ir was obtained through the following reaction, simultaneously the hydrogen evolution reaction also occurred.

The formation of metallic Ir immediately increases the rate of the hydrogen evolution reaction. Furthermore, the FE and deposition rate for pH 4.0 was less than these for pH 5.0. This reason is that the hydrogen evolution reaction occured significantly at low pH. Li et al.³⁹ pointed out that a decrease in pH increased the concentration of the uncompleted Ni²⁺ ions in the bath and allowed decreasing in the overpotential of the simultaneous hydrogen evolution reaction. At the same time, the release of hydrogen could result in the reduction of Ir³⁺ ions. Therefore, Ir-content in the deposit for pH 4.0 was higher than that for pH 5.0.

The dependence of composition of Ni-Ir alloys on mole concentration ratio of [Ni²⁺]/([Ir³⁺]+[Ni²⁺]) in the electrolyte is show in Figure 6. In one bath chemistry of 115 mM Ni²⁺, 35 mM Ir³⁺ and 90 mM Cit⁻, the mole concentration ratio of [Ni²⁺]/([Ir³⁺]+[Ni²⁺]) in the electrolyte is about 0.77. The electrodeposition of Ni-Ir alloy was performed with changing deposition conditions. The chemical composition of the alloys was almost independent of the deposition conditions. However, the Ni-content in the deposit is always higher than that in this electrolyte. It could be inferred that the deposition of Ni²⁺ was favored during electrodeposition. In this work, the Ni content enriched in the deposits, indicating that the deposition of Ni²⁺ was favored. Therefore, it could be concluded that the co-deposition of Ni-Ir alloy is a normal deposition. This result was good in agreement with our recent work.⁹

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The crystallographic structures of Ni-Ir alloys with 5 at%, 10 at% and 15 at% Ir-content are shown in Figure 7. The diffraction peaks from Cu substrate are apparent (JCPDS file #04-0836), thus indicating a thin deposit. The (111), (200) and (220) three reflections from Ni-Ir deposits were present. These diffraction peaks appear to shift toward low 2-theta values, according to the reflections of pure Ni (JCPDS file #87-0712), which was due to the incorporation of Ir into the deposit. Ni and Ir have identical crystal structure and the close atomic radius (Ni = 0.125 nm and Ir = 0.136 nm), which is approximate to 8.8% misfit. Therefore, it is feasible to systhesize solid solution of Ni-Ir micro-strain alloys by electrodeposition. As per Vegard's law,³⁶ with increasing Ir-content in the deposit, the lattice parameter reaches from that of pure Ir (3.839 Å) toward that of Ni (3.523 Å), indicating the formation of Ni-Ir solid solution. The Ni-Ir alloy was composed of nanocrystalline phase. To further estimate the average grain size (d) for the deposits using the following Debye-Scherrer Equation :11

where K is a constant, λ is the wavelength, β is the full width at half maximum (FWHM) of the (111) diffraction peak of the deposit and θ is the incident angle of the X-ray beam. The average crystal size of the deposits was about 12 nm, respectively. To obtain a more accurate assessment of the grain size, further work would need to be done using a transmission electron microscopy.

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The cyclic voltammograms measured in citrate aqueous solutions containing either or both Ir and Ni salts are shown in Figure 8. In Fig. 8, the curves show that these deposition processes for Ni, Ir and Ni–Ir deposits were irreversible, which were controlled by thermodynamic driving forces due to the complex formation, such as Ni-citrate or Ir-sulfamate.⁴² The electrodepositions have mostly been carried out around −1 V. Furthermore, the Ni is deposited with Ir in the citrate electrolyte at more negative potential in the presence of Ir³⁺ ion than that of the deposition of only Ni in the –1 V region. The both of nickel reduction features are enhanced in the presence of iridium in the –0.2 V region, with Ir addition causing increased currents at the same potentials. The deposition of Ir and Ni appears at the potential of –0.125 V and –0.10 V vs. Ag/AgCl, respectively. However, the co-deposition of Ni and Ir appears at –0.225 V vs. Ag/AgCl, which is more negative than that of the deposition of Ni alone in the solution without Ir salt. The standard electrode potential of Ni²⁺/Ni system is negative, –0.25V. Citric acid as a complexant could form complex ions with Ni²⁺ in the solution, resulting in the increase in the reduction potential. The potential shift was initially explained by the decrease in the activity of the Ni²⁺ ions due to the formation of Ni-citrate complex species. There is a cathodic shoulder on Ni CV at –0.8 V. The current wave for Ni²⁺ reduction was not distinguishable because it was convoluted with the current due to hydrogen evolution. The potential at which the hydrogen evolution reaction occured was significantly shifted anodically indicating that Ni²⁺ ions catalyzed the proton reduction.³⁷ The Ni²⁺ ions increased the citric acid buffer capacity by making the alcoholic proton of the organic molecules acidic and generating additional acidic protons from the water molecules directly coordinated to Ni²⁺ ions. The reduction of Ni²⁺ ion depends on the reduction of protons rather than controlled by activity of Ni²⁺ in solution through the Nernst equation.

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Figure 9 shows current transient for initial stage of Ni–Ir electrodeposition on Cu at cathodic potentials range between –0.2 and –0.35 V vs. Ag/AgCl. It can be observed that the transient could be divided into three regions.³³ Region I: an initial decrease in current density relates to the charging of the double layer. Region II: an increase in current density until a maximum current value j_max reached at a time t_max, which is due either to the growth of a new phase or to an increase of the number of nuclei. Region III: a plateau of current density, which is a diffusion–controlled process. As the deposition potential was shifted to be more negative, j_max increased, denoting the increase in the nucleation and growth rate. Scharifker and Hills⁴³ developed a theoretical model to analyze the current-time transient result of chronoamperometric experiment. According to this model, there are two limiting nucleation mechanisms: the instantaneous and the progressive. Instantaneous nucleation corresponds to a slow growth of nuclei on a small number of active sites. The number of nuclei would be constant during the deposition process and all nuclei have similar age and size. However, progressive nucleation corresponds to fast growth of nuclei on many active sites. The number of nuclei increases during electrodeposition process. The experimental chronoamperometric data, shown in Fig. 9, are normalized to a non-dimensional plot (j/j_max)² vs. t/t_max and compared with the theoretical plots derived for instantaneous and progressive three-dimensional (3D) nucleation and growth models.⁴³ The following equations are given for instantaneous 10 and progressive nucleation 13:

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Non-dimensional plots (j/j_max)² vs (t/t_max) of the chronoamperometric curves for Ni-Ir alloys are presented in Figure 10. Fig. 10 shows the nucleation mechanism of Ni-Ir alloy deposition. An increase in potential deposition until E = –0.35V induces a deviation in the 3D nucleation mechanism. The two curves recorded at potentials of –0.25 and –3.0 V are consistent with the model of diffusion-controlled 3D nucleation. However, the rising parts of the transients (1 > t/t_max > 0) are located near to the progressive nucleation. As soon as the potential turns to –2.5 and –3.0V values, the nucleation process confirms well the progressive nucleation process at 2.3 > t/t_max > 1. But at the time of t > 2.3t_max, the experimental (j/j_max)² is much larger than that calculated from the theoretical model. Two facts can explain this behavior: hydrogen reduction, and Ir incorporated in Ni-Ir deposits.^33,44 Therefore, the applied potential shifted to more negative values could influence the nucleation process.

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