Current-potential curves copper

The co-reduction of copper and selenium is considered as an exception to Kroger s theory. Current-potential curves in the literature show that deposition of copper is rather compulsory to make the deposition of selenium possible. In fact, although the standard potential for Se(IV) reduction is more positive than that of copper (0.741 and 0.340 V vs. SHE, for selenous acid and cupric ion, respectively), it turns out that Se(IV) alone is reduced at more negative potentials than Cu(II). In the presence of copper, the order is reversed. 

Figure 7.18. Current-potential curve showing the correlation between overpotential 17 and growth forms of electrodeposited copper from WCUSO4 and WH2SO4 at 25°C. (From Ref 40, with permission from Elsevier.)...

Electroless Deposition of Copper. The basic ideas of the mixed-potential theory were tested by Paunovic (10) for the case of electroless copper deposition from a cupric sulfate solution containing ethylenediaminetetraacetic acid (EDTA) as a complexing agent and formaldehyde (HCHO) as the reducing agent (Red). The test involved a comparison between direct experimental values for and the rate of deposition with those derived theoretically from the current-potential curves for partial reactions on the basis of the mixed-potential theory. 

Interaction Between Partial Reactions. The original mixed-p)otential theory assumes that the two partial reactions are independent of each other (1). In some cases this is a valid assumption, as was shown earlier in this chapter. However, it was shown later that the partial reactions are not always independent of each other. For example, Schoenberg (13) has shown that the methylene glycol anion (the formaldehyde in an alkaline solution), the reducing agent in electroless copper deposition, enters the first coordination sphere of the copper tartrate complex and thus influences the rate of the cathodic partial reaction. Ohno and Haruyama (37) showed the presence of interference in partial reactions for electroless deposition of Cu, Co, and Ni in terms of current-potential curves. 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

Secondly, if the first oxidation wave cannot be attributed to metallic copper oxidation, only one oxidisable compound is left, namely Cu(I). Indications for this can be found in the fact that in Cu(I)-containing styrene solutions, the limiting-current of this first oxidation wave is much higher than for Cu(II)-containing solutions. As a matter of fact, the first oxidation wave is expected to be absent in Cu(II) solutions. Apparently, the presence of this wave has to be attributed to the fact that some Cu(I) is present in the vicinity of the electrode surface. When the position of the current-potential curves in Fig. 12.2 reflects the standard potentials of the... 

It was found [285, 291] that there exist four different kinds of copper species on the Ti02 surface, which correspond to peaks at the potentiodynamic current-potential curves (Fig. 8.20) ... 

As mentioned in Sec. 2.1, simple superposition of the respective current-potential curves for the two partial reactions does not always yield the curve obtained with a complete electroless bath. This is illustrated in Fig. 21 [126], in which the current-potential curve obtained with a complete electroless copper bath containing EDTA (curve 1) is compared with the curve obtained in solution in the absence of formaldehyde (curve 2) and with that obtained in solution in the absence of Cu(II) (curve 3). All three curves were recorded at room temperature with a copper disk electrode rotating at 2100 rpm, while scanning the potential in the positive direction... 

Fig. 23 Evans diagram for electroless deposition of copper. Current-potential curves for the reduction of Cu2+ ions and for the oxidation of reducing agent Red, formaldehyde, combined into one graph. Solution for the Tafel line for the reduction of Cu2+ ions 0.1 M CuS04, 0.175 M EDTA, pH 12.50, Eeq (Cu/Cu2+) = -0.47 V versus SCE for the oxidation of formaldehyde 0.05 M HCHO and 0.075 M EDTA, pH 12.50, Eeq(HCHO) = —1.0 V versus SCE temperature 25°C ( 0.5°C) (from Ref. 43 with permission from American Electroplaters and Surface Finishers Society).

Fig. 31 Current-potential curve showing correlation between overpotential rj and growth forms of electrodeposited copper from N CUSO4 and NH2SO4, 25 °C (from Ref. 80 with permission from Elsevier Science Inc.).

This particular characteristic of current-potential curves is linked to the steady character, which is chosen for this description. In transient experiments such as voltammetry, using a copper electrode with no CcF ions in the solution, one would see a reduction current during the reverse scan of the... 

Let us return to the example of the interface between a copper electrode and an aqueous electrolyte now containing Cu ions. With both elements of the Cu VCu couple present, the two branches of the current-potential curve are then observed. For a cathodic working point, Cu ions must be carried to the cathode to be reduced. As shown in figure 2.19, a reduction limiting current is observed, with a value proportional to the Cu concentration in the bulk solution. 

On the other hand, the shape of the current-potential curve is different in the oxidation branch because metallic copper is not concerned by any mass transport phenomenon, since copper is always present at the interface. The zone where the current undergoes large variations is close to the open-circuit potential, which is, in this case, equal to the equilibrium potential of the system. The latter, which can be calculated using the Nernst law, is shifted slightly from the standard potential. To give an order of magnitude for a concentration in Cu ions equal to 10" mol L , there is the following ... 

Figure 2.19- Current-potential curve of a copper electrode in an acidic aqueous solution containing Cu ions...

For example, in the case of a copper foil immersed in an acidic solution (with no Cu ions at the outset) which has not been deaerated, dioxygen can be reduced. The resulting current-potential curve, which is shown by a dashed and dotted line in figure 2.33, is the sum of the three grey curves which relate to the three couples (supposedly fast) present in the system Cu VCu, H H20/H2 and 02/H20,0H". It is important to emphasise again that the resulting curve shown by a dashed and dotted line is the only one accessible through experiments. As far as the open-circuit potential is concerned, the overall current-potential curve for such a simple system is the sum of... 

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