Electroplating of Ni /W Alloys : I. Ammoniacal Citrate Baths

The chemistry of tungstate in aqueous solution is rather complicated.³³ When tungsten is dissolved in an aqueous alkaline solution, the resulting solution contains tetrahedral ions and simple tungstates, such as can be crystallized from it. If this solution is acidified strongly, a white precipitate of tungstic acid is formed. At intermediate pH values, polymeric anions, made up almost invariably of octahedra are formed. In aqueous solution, equilibration among the tungstate species is rather slow, and can take several weeks.

The predominant species produced by the progressive acidification of tungstate solutions at pH values below 6 are the paratungstates, which are followed, upon further acidification by metatungstates

Metal tungstates of the type can also be formed in solutions containing divalent cations. The solubility of these salts in water is rather low. Complexes of tungstate with anions of organic acids such as citrate,³⁴ tartarate,³⁵ and oxalate³⁵ are well known. In the case of citrate, complexes in the form of are formed, where Cit stands for the triply deprotonated anion of citric acid, and the number of protons, , depends on the pH.

The main reaction studied in this work is the reduction of tungstate ions to metallic W

The standard potential given in the Pourbaix diagrams³⁶ for this reaction is

In a typical system used in the present work, at pH 8.0 and a tungstate concentration of 0.1 M, the reversible potential is

where SHE is standard hydrogen electrode and RHE is reversible hydrogen electrode. The reversible potential is not very sensitive to the concentration of tungstate. Thus, if the concentration of this ion is increased to 0.5 M, for example, the reversible potential will shift by less than 7 mV. It should be borne in mind, however, that Eq. 3 does not necessarily represent the reversible potential in the system studies here. In this case, both nickel and tungstate ions are complexed, and the stability constant of the ternary complex of the two metal ions with citrate, which we believe is the precursor for deposition of the W/Ni alloy, is not known.

Deposition of W/Ni alloys was studied using solutions that contain and as the electroactive species. was used as the complexing agent. Ammonium hydroxide and sulfuric acid were used to adjust the pH. No additives were used, to keep the system as simple as possible. The concentrations of the above species were varied over a wide range, to test the effect of each on the concentration of W, and on the crystal structure of the alloy.

Experiments were conducted at A constant current density of and a pH of 8 were used, except when their effects were being tested. Mass transport was controlled with a rotating cylinder working electrode The rotation rate was adjusted to ensure turbulent flow at the surface. It was maintained at 2,000 rpm, except when the effect of mass transport was tested. A one-compartment cell, deaerated with purified nitrogen, was used. The faradaic efficiency was calculated from the charge passed and the added weight.

A gold cylinder, 0.2 cm diam and 1 cm long, was used as the working electrode. A platinum wire counter electrode and Ag/AgCl/1 M KCl reference electrode were used.

The composition of the alloy was measured using an energy dispersive spectroscopy (EDS) probe (Link Corp.) attached to a JOEL, model 6300 SEM. ZAF calculation was used to determine the composition. Each sample was measured at five different points along the cylinder, to confirm uniformity. The thickness of the deposit was large enough (more than 1 μm), so that no signal from the underlying gold substrate was observed. In some of the samples the composition was also confirmed by secondary ion mass spectrometry and by X-ray photoelectron spectroscopy measurements.

The current density was controlled with a Pine potentiostat model AFCBP. An analytical rotator (Pine instruments) was used to control the rotation rate of the cylindrical working electrode.

In Fig. 1 we show the effect of the concentration of in solutions containing 0.10 M and 0.40 M citrate. At equal molar concentrations of nickel and tungstate ions, the concentration of W in the alloy is quite low, about 5 atom %. As the concentration of nickel in solution is decreased, the W content of the alloy increases, but the faradaic efficiency decreases dramatically. The deposition potential changes in the positive direction (i.e., to lower overpotential for hydrogen evolution), in parallel with the increase in faradaic efficiency, as expected. Note that the partial current density for W deposition increases by a factor of about 10 with increasing concentration of nickel in solution, while its concentration in the alloy decreases by a factor of about 7.5 over the same range of Ni concentration. This lends very strong support to the reaction pathway suggested here and in Part II of this paper, claiming that W is deposited from a complex with Ni, while Ni itself can also be deposited in a parallel route, independent of the existence of tungstate in solution.

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The effect of changing the concentration of tungstate ion in solution is shown in Fig. 2. The results of two sets of experiments are shown, in which the concentrations of were 0.1 and 0.2 M. The concentration of citrate was 0.60 M in both cases. The W content of the alloy initially increases sharply with increasing concentration of tungstate in the solution, but seems to level off at about 12 and 8 atom %, in 0.1 and 0.2 M respectively. The faradaic efficiency seems to decrease first and then to increase with increasing tungstate concentration. The deposition potential changes in the positive direction over the whole range. The partial current for W deposition increases with the concentration of tungstate, which is to be expected. When the concentration of nickel in solution is doubled, the W content of the alloy decreases, but the partial current for W deposition increases, as seen in Fig. 2d, supporting the behavior shown in Fig. 1d and the conclusions derived from it. The faradaic efficiency is significantly higher and the plating potential is more positive in 0.2 M solutions, except at very low tungstate concentrations.

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The effect of changing the concentration of citrate, in a solution containing equal amounts of nickel and tungstate ions, is shown in Fig. 3. The W content of the alloy increases with increasing concentration of citrate up to 0.50 M, but then starts to decrease. The faradaic efficiency decreases dramatically when the concentration of citrate exceeds 0.20 M, i.e., when the molar concentration of citrate exceeds the sum of concentrations of nickel and tungstate ions. It levels off in the region of 0.50-0.80 M, and seems to increase a little above that. The deposition potential changes in the negative direction with increasing citrate concentration, first sharply, and then moderately. Note that the partial current density for the deposition of both metals decreases with increasing concentration of citrate. It can be concluded qualitatively that citrate, being a strong complexing agent for both metal ions in solution, tends to sequester these ions in stable complexes, making it more difficult to deposit the metal. The relative effect on Ni is apparently stronger up to a concentration of 0.50 M citrate and weaker at higher concentrations, leading to a maximum W content of the alloy at a citrate concentration of about 0.5 M.

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The effect of mass transport can be examined in two ways: by varying the rotation rate at constant current density, or changing the current density at constant rotation rate. In Fig. 4, we show the effect of rotation rate on the deposition process. Two solutions were examined: one in which the ratio was 10 and the other in which it was 0.1, in order to determine which of the two ions is limiting the rate of W deposition in this system. The limiting current at a rotating cylinder electrode, operating in the region of turbulent flow, is given by

where is the radius of the rotating cylinder, and are the diffusion coefficient and bulk concentration of the electroactive species, respectively, ν is the kinematic viscosity, and ω is the angular velocity.

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For the systems used in our experiments, the lowest rotation rate ensuring turbulence is 2,000 rpm. At this rotation rate, the limiting current densities for 0.04 M and are approximately 24 and respectively, using The corresponding limiting current densities at 5,000 rpm, (the highest value shown in Fig. 4) is 90% higher (since ), namely, 46 and respectively. Note that plating was conducted at a current density of in this set of experiments (compared to in the previous sets), to enhance the effect of mass-transport limitation, if any.

The W content of the alloy is higher in the tungstate-rich solution, as expected, but its dependence on rotation rate is rather small in both solutions. The faradaic efficiency and the deposition potential are essentially constant in Ni-rich solutions. In tungstate-rich solutions, the faradaic efficiency increased somewhat with increasing rate of mass transport, and the deposition potential shifts in the positive direction.

The most important information obtained from this series of experiments is in the examination of the partial currents for the deposition of the two metals. The rate of W deposition increases with increasing rotation rate in both solutions by almost the same amount (about 40%). Increasing the concentration of tungstate by a factor of ten, leads to a moderate increase in the rate of deposition of this metal, by a factor of about two. The data for the tungstate-rich solution are particularly significant for the following reasons: (i) While the rate of mass transport is increased by a factor of 1.90 in the range of rotation rates shown, the partial current for W deposition increases by a factor of 1.4 in both solutions. (ii) In the solution containing 0.040 M tungstate the calculated limiting current densities are 72 and at 2,000 and 5,000 rpm, respectively. The corresponding values of are 0.03 and 0.02, respectively. This is too small to justify a 40% increase in current density with increasing rotation rate, indicating that a different species (probably a ternary complex containing both and with citrate, as is discussed below) is determining the rate of reaction. (iii) The results for the tungstate-rich solution shown in Fig. 4d are even more striking. Here, the calculated values of are 0.0057 and 0.0034, respectively, and one would not expect to observe any mass-transport limitation, based on the concentration of tungstate. If the species limiting mass transport for the deposition of W is the ion in solution (which makes sense, considering that W cannot be deposited from a solution of tungstate in the absence of ions), the calculated values of the ratio of are ten times larger, namely, 0.057 and 0.034, respectively, enough to explain some dependence of the partial current of W deposition on the rotation rate, but not all of it.

The effect of rotation rate on the partial current for nickel deposition is very minor, as seen in Fig. 4e. It is also noted, however, that as the concentration of nickel in solution is increased tenfold, the partial current density for its deposition is only increased by a factor of about three. This behavior is addressed in detail in the Discussion section.

We now turn to describe the effect of current density, shown in Fig. 5. Comparison was made in the same two solutions as shown in Fig. 4, one rich in tungstate and the other rich in nickel. All measurements were taken at a rotation rate of 2,000 rpm. The W content of the alloy is found to decrease with increasing current density, which is consistent with the increase of the same quantity with increasing rotation rate, both showing some degree of mass transport limitation. For the Ni-rich solution, the dependence of W content in the alloy disappears above about consistent with an apparent plateau in the partial current density for W deposition. In the tungstate-rich solution, the concentration of W in the alloy is almost independent of current density.

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The potential is shifting in the cathodic direction with increasing current density, as expected. The potential is found to change by about 0.35 V over an increase of current density by a factor of 20. This could formally lead to an average Tafel slope of about 0.27 V. However, we do not think that one should attempt to interpret these results in terms of the apparent Tafel slopes observed, since the system is very complex and at least four reactions can occur in parallel, as is explained also in the Discussion section.

Ammonia is commonly added to plating baths for W/Ni alloys, to increase the faradaic efficiency, and sometimes to fine-tune the pH of the solution to a desired value, usually around pH 8. However, it is well known that is a strong ligand for and can form a series of complexes of the type where can assume several values in the range of 2-6. The role of ammonia as a complexing agent, in addition to citrate, was not considered until recently.^{15 16} In Fig. 6, we show the effect of the concentration of ammonia as the parameter in the electroplating on Ni/W alloys. In all experiments shown above, there was a large excess of ammonia in solution, of the order of 2 M, hence its effect of W/Ni deposition was studied in the same range.

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The first observation is that adding ammonia decreases the concentration of W in the alloy dramatically, from about 30 atom % in the absence of ammonia to 20 atom % in a solution that is 0.3 M with respect to ammonia. Increasing the concentration of ammonia further had only a moderate effect, decreasing the W content of the alloy to about 15 atom % in 2 M solution of ammonia. In parallel, the faradaic efficiency is more than doubled in the particular solution composition shown in Fig. 6, and the deposition potential shifts in the positive direction. The partial current densities for both metals first increases and then decreases again with increasing concentration of ammonia.

There is no doubt that the marked effect of ammonia on the deposition of W/Ni alloys is associated with its ability to form complexes with This apparently reduces the concentration of the ternary Ni/W/citrate complex, which is the precursor for W deposition. It may also influence the rate of Ni deposition, as is discussed below.

Since eliminating ammonia from the plating bath allows the plating of W/Ni alloys having a high concentration of W, this system is of interest both from the fundamental and the practical point of view. The detailed behavior of plating baths from which ammonia has been eliminated will be discussed in detail in a following paper (Part II).

The ammoniacal citrate bath for deposition of W/Ni alloys is commonly operated at pH 8.0. It is well known in the literature, however, that increasing the pH can increase the faradaic efficiency. It was therefore of interest to determine the effect of pH on this process in more alkaline media. In Fig. 7, we show the effect of pH on deposition of W/Ni alloys, in the range of pH 8.0 to 12.0. The solutions chosen have equal amounts of nickel and tungstate ions, and an excess of citrate. A dramatic decrease of the W content of the alloy (from 9 to about 3 atom %) is detected as the pH is increased from 8.0 to 9.0. Beyond that, the decrease is moderate, reaching about 1 atom % at a pH of 12.0. The faradaic efficiency is indeed found to increase, from about 38% at pH 8.0 to 83% at a pH of 9.0, and approaches 100% at a pH of 12.0. The partial current density for W deposition declines almost linearly with increasing pH, while that for Ni rises sharply between 8.0 and 9.0 and moderately at higher pH values. It should be noted that this experiment was not performed under ideal conditions, since the pH was increased by adding Thus, the variation with pH of the measured parameters is also affected by the added ammonia. Nevertheless, the sharp changes between pH values of 8.0 and 9.0 must be attributed mainly to the effect of pH, since more is needed to change the pH from, say 10.0 to 11.0 than from 8.0 to 9.0, yet the effect on the concentration of W in the alloy is much larger in the latter case.

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The effect of pH on the W content and other parameters of W/Ni alloy deposition in ammoniacal citrate plating baths can be explained on the basis of the equilibrium between and as is shown in the Discussion section below. We note, however, that a strong dependence on the pH is observed, even in solutions where ammonia has been excluded. This effect will be discussed in detail in our next paper on this subject.

This acid has three active hydrogen atoms on acidic groups and a less active hydrogen on an alcohol group. The corresponding values are 2.96, 4.38, 5.68, and 10.82.³⁵ The distribution of the different ions as a function of pH is shown in Fig. 8a. The important point is that in the range of pH 7-10, the predominant species is the ion with three negative charges, which is assumed to be the main ligand in all the complexes discussed here. This anion forms two types of complexes with nickel, and The distribution of these species as a function of the concentration of citrate is shown in Fig. 8b. We note that the concentration of free ions falls to very low values as soon as the concentration of citrate reaches that of The ion exists only in a fairly narrow range of citrate concentration. When the ratio of about 95% of the nickel ions are in the highly charged complex It should be mentioned that the nickel citrate complex is partially deprotonated to form The concentrations of these two complexes are equal over a range of concentrations of hence the concentration of in Fig. 8b reaches a maximum of only 50%.

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Citrate also forms a complex with the tungstate ion, in spite of the fact that they are both negatively charged.³⁴ In the simplest case, this ion should have the formula This is, however, unstable. Several protonated forms of this complex are known, such as where can assume values of 1-3. The values for the equilibria

are 10.21, 17.21, and 21.67 for 2, and 3, respectively. The distribution of different complexes as a function of pH is given in Fig. 8c. Note that around pH 8, the predominant species is the complex containing only one proton, which can be written as for short. Since is not protonated at this pH, as seen in Fig. 8a, it may be assumed that in the hydrogen atom originates from the partially protonated ion. The first dissociation constant of tungstic acid is not given in the literature, probably because the corresponding value is low, and formation of polyions makes measurements difficult.

It is usual to ignore the effect of ammonia on the composition of the alloy, considering this compound only as a mild base, used to adjust the pH of the solution and to increase the faradaic efficiency of plating in this system. However, it is well known that which is at equilibrium with in alkaline media, is a ligand, forming several complex ions with nickel, all having the composition where can assume values of 2-6. The values for the equilibria of these complexes are 5.0, 6.82, 7.14, and 8.55 for 3, 4, and 6, respectively.³⁵ The concentration of the different complexes, as a function of the concentration of is shown in Fig. 8d. At the concentration of about 2 M used in the present work, the predominant species is and the concentration of free is close to zero. It is obvious that will compete with citrate for nickel ions in solution.

In previous publications,^{15 16} we have shown that it is possible (in a citrate bath from which ammonia was excluded) to deposit a Ni/W alloy with equal amounts of the two metals. It is reasonable to assume that the precursor for the deposition of such an alloy is a ternary complex of the type The overall reduction of this complex requires eight electrons, and can be written as

The ternary complex shown in Eq. 6 could be formed in the reaction

Unfortunately, this complex has not been detected directly, either in solution or as an adsorbed species, even though its existence is strongly implied by the deposition of a 1:1 Ni/W alloy. The rate of the reaction shown in Eq. 7 is expected to be quite low, in view of the fact that it involves interaction between two negative ions, one of them highly charged, and requires the removal of a citrate ligand from one of the two complexes. This may also lead to a low concentration of the ternary complex at steady state, which is the reason that this complex has not yet been detected. The likelihood of a similar reaction involving the complex of nickel with two citrate ions is low, because it would be difficult to get two anions, each having a charge of to interact with each other.

The fact that, under most circumstances, the concentration of Ni in the alloy is much higher than that of W, and in ammoniacal citrate baths an alloy with 50 atom % W cannot be obtained at all, is easily understood if we remember that there are parallel paths through which Ni can be deposited alone, from its complexes with citrate or with ammonia, as shown in Fig. 8b and d, respectively.

The addition of ammonia causes a sharp decrease in the concentration of W in the alloy, as shown in Fig. 6. This is easy to understand, in terms of the model suggested above. Since competes with citrate as a ligand for nickel ions, the concentration of will be lower in the presence of ammonia. This shifts the reaction shown in Eq. 7 to the left, decreasing the concentration of the ternary complex from which W is plated. On the other hand, the rate of deposition of Ni may not be decreased at all; since it can be deposited from its complexes with as well as from its complexes with citrate. Indeed, we notice in Fig. 6 that the partial current for Ni deposition increases in the same region where the concentration of W in the alloy decreases drastically.

It should be pointed out that the addition of ammonia should decrease the concentration of W in the alloy for any mechanism, as long as there is a synergistic effect of in solution upon the deposition of W from aqueous which is a well-established experimental fact. In the presence of in solution, some of the ions are sequestered in the form of one or more of the complexes shown in Fig. 8d, and less will be available to assist in the deposition of W, whatever the mechanism of that assistance may be. The model given above is a highly plausible model, but it may not be the only one that explains the behavior observed.

The effect of pH is shown in Fig. 7. A sharp decline of the concentration of W in the alloy, and a corresponding increase in the faradaic efficiency, are observed as the pH is increased from 8 to 9. The partial current density for deposition of W declines almost linearly, while that for Ni rises sharply between these two pH values. Thus, the sharp decline in W content of the alloy is caused by the combined effects of the decrease in the rate of deposition of W and the concurrent increase in the rate of deposition of Ni.

This change in the rate of deposition of W can be associated with the complexing of with The for the dissociation ammonium hydroxide, corresponding to the equilibrium

is 9.25. Hence, the concentration of equals that of at pH 9.25. At a pH of 8, only 5.3% of the ammonium hydroxide remains undissociated, while at pH 9, this is increased to 36%. Thus, in a 2 M solution of the concentration of undissociated ammonium hydroxide will increase from about 0.1 to 0.7 M as the pH is increased from 8 to 9. The ligand is formed in the reaction

Hence, increasing the pH in this range has an effect similar to increasing the concentration of ammonium hydroxide from 0.1 to 0.7 M, thus decreasing the concentration of W in the alloy.

The effect of rotation rate was studied in the range of 2,000 to 5,000 rpm, which represents a 90% increase in the rate of mass transport for a rotating cylinder electrode. The partial current densities for deposition of both W and Ni show a rather irregular behavior. In Fig. 4d, we noted that increasing the concentration of tungstate tenfold, from 0.04 to 0.4 M increases the current density by a factor of only two. The limiting current, calculated on the basis of the concentration of the ion in solution, is much higher than the partial currents for deposition of this metal ( 1, cf. the Results section), so that one would not expect a 40% increase of the rate of deposition of W with the increase of the rate of mass transport, as found experimentally.

The explanation of these anomalous observations lies in the formation of the ternary complex shown in Eq. 7 above. The concentration of the ternary complex is low and its rate of formation is also expected to be low, because it entails the reaction between two negatively charged species, one having a high charge of From the dependence of the partial current density for W deposition shown in Fig. 4d, we can estimate the activation-controlled and the mass-transport-limited current densities, using the well-known relationship

bearing in mind that the limiting current at a rotating cylinder electrode is proportional to

For solutions containing 0.04 and 0.4 M the values of the activation controlled current increases from 5.0 to a ratio of only 1.8 for a tenfold increase in concentration. The corresponding limiting currents at 2,000 rpm increase in the same proportion, from 4.1 to Moreover, we recall that the limiting current for deposition of W from a 0.04 M solution was calculated to be Thus, the limiting current estimated from the data in Fig. 4d is only 5.7% of that calculated for a concentration of 0.04 M.

From the above value of the limiting current, it is possible to estimate the concentration of the electroactive species that is the precursor for deposition of W. Neglecting the difference in diffusion coefficients, this turns out to be 2.3 mM. This is the order of magnitude of the concentration of the ternary complex in a solution containing 0.04 M 0.4 M 0.4 M and about 2.0 M Considering that the diffusion coefficient of this ternary complex is probably smaller than that of the complex the above concentrations of the ternary complex is probably a slight underestimate.

When the concentration of tungstate is increased tenfold and that of nickel is decreased by the same factor, both the activation controlled current density and the limiting current density increase by a factor of 1.8. It may be concluded that the concentration of the precursor for the deposition of W increased by the same factor, to a value of 4.1 mM. Considering only the equilibrium shown in Eq. 7, there would seem to be no reason for the concentration of the ternary complex to change. However, it must be remembered that ions are also in equilibrium with complexes containing and the concentration of the nickel-citrate complex shown in Eq. 7 is not necessarily a linear function of the nominal concentration of in solution.

The partial current density for Ni deposition was found to be essentially independent of the rotation rate. An anomaly was observed in this case too, since increasing the concentration of in solution by a factor of 10 causes an increase of partial current density only by a factor of 2-3, as seen in Fig. 4e. We recall, however, that the nickel ion can exist in solution in many different forms. There are four possible complexes with and two complexes with citrate, in addition to the ternary complex with tungstate and citrate. The overpotential for deposition depends on the nature of each complex, and the relative abundance of the various complexes depends on the concentration of nickel. Hence, the resulting partial current density for Ni deposition may depend on concentration in a complex manner, which we shall not discuss here.

The plots of partial current density vs. the applied current density indicate a limiting current behavior for W deposition, as shown in Fig. 5d. For 0.04 and 0.4 M solutions of tungstate, the values of the limiting current densities observed are about 2.5 and respectively. These are in fairly good agreement with the values of 4.1 and estimated above from the data given in Fig. 4d.