A Study of Copper Passivity by Electrochemical Impedance Spectroscopy

Cyclic voltammograms (CVs) provide a very instructive overview of the electrode processes. CVs obtained by different workers are not identical, mainly due to surface contamination as well as differing methods of surface preparation.¹⁸ In this work, reproducible results were obtained for freshly polished electrodes, which were polarized for 45 s at −1.4 V.

Figure 1 represents the CV obtained for the copper electrode in a borate buffer solution, pH 8.4, at a sweep rate of 3 mV s⁻¹. Three anodic and two cathodic current peaks can be observed in the CV. According to Pourbaix,¹ the following oxidation equilibria are possible in the copper/aqueous solution system at pH 8.4

Accordingly, the current peak has been assigned to the electroformation of a layer (Eq. 1), whereas the current peaks and are associated with the anodic formation of a complex hydrous CuO film (Eq. 2 3 4 5), resulting in a duplex structure of the passive film.^{4 5 6 7 8 9 12 17 18 19 20 21 22 23 24 25} Apparently, such a structure represented by an outer layer overlaying a barrier is responsible for a decrease in anodic dissolution, i.e., a decrease in anodic current up to approximately 0.9 V, where oxygen evolution commences. The electroreduction scan shows the current peaks and which are related to the electroreduction of CuO and/or to and to Cu, respectively.

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CVs obtained when the potential sweep was reversed at various positive potential limits showed that no cathodic peak was observed unless the potential had reached a value more positive than the current peak Only the cathodic peak appeared when the potential was set between the current peaks and during the anodic part of the potentiodynamic curve. Both peaks and were observed when the electrode was polarized to more positive potentials than the maximum potential of peak At a sweep rate of 10 mV s⁻¹, the cathodic current peak appeared when the electrode was polarized above −0.065 V, but a more significant increase of occurred when the electrode was polarized above 0.17 V. However, a potential sweep reversal above had a pronounced influence on the structure of the cathodic polarization curve (Fig. 2). First, the height of the cathodic current peaks increased with a shift in the sweep reversal toward more positive values, suggesting that electroformation of both copper oxides, and CuO, occurred during anodic polarization up to the sweep reversal. Second, an increase in the anodic potential sweep reversal significantly shifted the potential of toward negative potential values. The same changes in the cathodic polarization curve were observed when the electrode was stabilized for a longer period of time (a few minutes) at anodic potentials more positive than the current peak, All of these observations are supposed to be a consequence of the oxide layer composition. According to Seo et al. ,¹² the average composition of the outer oxide layer could conventionally be represented as The value of x is variable in the range of corresponding to the composition range of to CuO. It was found¹² that the value of x increases with increasing anodic potential and oxidation time. When we apply these considerations to the system under investigation, it follows that the outer part of the Cu(II) oxide layer formed during increased anodic polarization and longer oxidation time is less hydrated and its reduction potential is more negative than the reduction potential of the more hydrated Cu(II) oxide layer, the formation of which is favored at shorter oxidation times. The observed reduction potentials of the more and less hydrated Cu(II) oxides are in accordance with the corresponding reversible potentials (Eq. 4 5).

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The total charges of the anodic, and cathodic, processes are determined by integration of the anodic and cathodic current peaks. They are analyzed as dependent on the positive potential sweep limit and the sweep rate. It must be pointed out that the charges and are almost equal in the potential range of electroformation and its reduction. Once the Cu(II) generation starts, the charges associated with the anodic processes become larger than those for cathodic processes (the inset in Fig. 2) and the observed difference increases with increasing positive potential limit. For example, at a sweep rate of 10 mV s⁻¹, the ratio changes almost linearly from 1.0 at −0.15 V to 1.8 at 0.7 V. Much greater values for the charge ratio are found for copper in slightly acidic solution²⁴ due to increased solubility of copper(II) oxide. In spite of the fact that solubility of copper(II) oxide is quite low in slightly alkaline solution (due to electrode reactions) (Eq. 1 2 3 4 5), the pH at the electrode/electrolyte boundary decreases and may cause increased solubility of copper(II) oxides. Therefore, it has to be concluded that a part of the oxidized Cu(II) dissolves as the corrosion product. The alternative possibility that could explain the difference between and is the accumulation of unreduced Cu(II) oxide or hydroxide at the surface. However, this assumption is not acceptable, because successive potentiodynamic examination with the same specimen, and under the same conditions, resulted in almost identical polarization curves. This would not be observed if some of the corrosion products remained at the surface. It is also observed that the ratio increases with increasing sweep rate. At a low sweep rate (3 mV s⁻¹) is slightly larger than but at the sweep rates between 10 and 100 mV s⁻¹, their ratio lies between 1.3 and 1.5. It seems that almost all dissolved copper species can be reduced during a slow potential sweep.

The total cathodic charge obtained by integration of and can be correlated with the thickness of the total oxide layer d, which is equal to the sum of the inner and outer parts of the duplex film, where is the charge corresponding to represents the difference of the charges corresponding to and is the molar volume of CuO and /or is the molar volume of is the roughness factor, and F is the Faraday constant. In both equations, the reduction efficiency was assumed to be 1 and the molar volumes of CuO, and were 23.9, 12.4, and 29.0 cm³ mol⁻¹, respectively. When the potential sweep reversal was complete at −0.065 V (the potential up to which the formation of copper(II) oxide was not observed), the amount of charge under was 0.358 mC cm⁻². This amount of charge very closely corresponds to a monolayer of the film whose thickness is equal to 0.44 nm. In fact, the charge required to form a unit cell monolayer oriented in the basal plane is 0.353 mC cm⁻², calculated from the unit cell dimensions. crystallizes in a cubic structure built up from a body centered cubic lattice of oxide ions and a face centered cubic lattice of copper(I) ions.¹⁸ For several potential sweep reversals up to 0.9 V, the charges were determined and the corresponding film thickness' calculated and presented in Fig. 3. For example, at the anodic potential sweep reversal of 0.9 V, the charges and amount to 1.34 and 1.87 mC cm⁻², respectively. The corresponding thicknesses of oxide layers, under the assumption that their outer parts are composed of are: and the total thickness Figure 3 shows that the thickness of both parts of the oxide layer increases with increasing anodic polarization.

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Voltammograms represented in Fig. 4 show the influence of the sweep rate on the shape of CVs. As the sweep rate increases, the following observations can be made: (i) the height of all current peaks increases; (ii) the anodic peak potentials shift toward more positive potentials, (iii) at the sweep rates above 10 mV s⁻¹, the first and the second anodic current peaks overlap; (iv) the cathodic peak potential shifts toward more negative potentials, while shifts in the opposite direction. The opposite shift of the potential values with increasing sweep rates is probably caused by a partial disproportionation of during its reduction. Voltammograms were used to analyze the dependence of peak potentials and peak currents on the sweep rate. A linear relationship was observed between the peak potentials and the square root of the sweep rate. Extrapolation of these straight lines to zero scan rate gives a spontaneous oxidation/reduction potential, i.e., anodic (cathodic) nucleation potential The absence of the polarization effect makes these the most suitable values for comparison with thermodynamic data. The difference between these experimental values and the equilibrium potentials from Eq. 1 2 3 4 5 gives the anodic and cathodic overpotentials. It was found that the anodic overpotential of Cu to oxidation is 0.232 V, while the cathodic overpotentials of to and to Cu reduction are 0.318 and 0.285 V, respectively.

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A linear relationship was also observed between the peak currents and the square root of the sweep rate. For the current peaks and the straight lines pass almost through the origin. The intercepts slightly different than zero indicate that the induction rate is required before the electrode reactions become limited by diffusion. For the current peak the against dependence showed two segments. At lower sweep rates the slope of vs. was the same as that for the current peak while at higher sweep rates the slope was much greater. The most probable reason for the observed behavior is disproportionation of

Wide frequency range impedance measurements, from 100 kHz to 30 kmHz, were applied to investigate the nature of the Cu/borate buffer system, pH 8.4, in the potential range from −0.3 to 0.9 V. Prior to the EIS measurements, the samples were cathodically polarized to −1.4 V for 45 s, then potentiodynamically polarized to the desired potential value at a sweep rate of 10 mV s⁻¹. After a 10 min wait period, impedance data were collected. Impedance measurements typically took about 30 min due to the low frequencies involved. The potential was then progressively stepped in the positive direction to the desired value and the process was repeated. In order to present the impedance data completely (in the entire potential and frequency range applied), and to give a more general overview of the influence of potential on the impedance response of the Cu/oxyhydroxide electrolyte interface, Bode plots have been presented as 3-D surface plots in Fig. 5. Several characteristic potential regions could be distinguished, quite comparable to those determined from the CV presented in Fig. 1.

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The potential region up to −0.07 V corresponds to Cu(I) oxyhydroxide nucleation and formation. The first step is adsorption of hydroxide ions, which could be related to the initial electrooxidation step forming the new phase according to the reaction

According to Strehblow and co-workers,²⁶ this process begins ca. 200 mV millivolts before the layer can be observed in a diluted alkaline solution, and the corresponding charge and thickness are approximately 55 μF cm⁻² and 0.2 monolayer, respectively. The maximum of the low frequency absolute impedance and the minimum of the phase angle at about −0.07 V most probably correspond to the monolayer (see the results of cyclic voltammetry). The growth of a surface layer, above a monolayer, is followed by a further increase in the low frequency absolute impedance up to the potential value of 0.5 V. Between 0.0 and 0.5 V electrode resistance of the order of is observed. According to Fig. 3, the passive surface overlayer is between 1.1 and 2.6 nm thick. This suggests that the surface overlayer exhibits ionic resistivity where is the oxide thickness) of Above 0.5 V, |Z| decreases and the phase angle increases with potential. This correlates to the onset of structural transformations in the surface film because surface film thickness is a linear function of the potential in the entire potential range, as can be seen from Fig. 3.

For frequencies ranging up to nearly 1 Hz, the electrode impedance may be precisely described by a constant phase element in series with a resistor ^{27 28}

where is the absolute impedance at denotes the complex variable for sinusoidal perturbation with and n is the slope of the vs. Bode plot. The factor n is an adjustable parameter that usually lies between 0.5 and 1. The constant phase element (CPE) only describes an ideal capacitor when Otherwise, for the CPE describes a distribution of dielectric relaxation times in frequency space. A constant phase element has been used to model oxide film formation.^{23 24 25 27 28 29 30} The symbol represents the resistive component measured at the high frequency limit. This resistance is attributed to the solution between the Luggin capillary and the working electrode.

Figure 6 shows how the n and electrode capacitance C (calculated at 0.16 Hz) depend on the electrode potential in the potential range from −0.3 to 0.9 V. For illustration, the anodic branch of the CV is presented in all cases.

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It is clearly evident that the monolayer formation (the current peak is accompanied by a sharp increase in and a decrease in the electrode capacitance to the value characteristic for a passive copper electrode. Electrochemical polarization curves, XPS, and ion scattering examination⁸ show that copper forms a layer at potentials above 0.58-0.058 pH vs. SHE, in weakly acidic and alkaline solutions. According to Strehblow and co-workers,¹⁰ the initial stage of layer formation is characterized by a competition between dissolution and nucleation of the oxide grains locally blocking the dissolution. Keeping in mind the results of Arvia et al. ,³¹ the first step in copper passivation and formation corresponds to ion discharge on the Cu surface, i.e., the electrochemical reaction associated with the redox couple. This type of reaction is considered to be the first step in the electro-oxidation of many metals in aqueous solution and is generally a fast electrochemical reaction. undergoes dissolution in a reaction with ions, and forms according to The results showed that the oxide formation proceeds by a preferential nucleation of the oxide grains whose dimensions are 2-3 nm laterally and 0.25-0.3 nm vertically.¹⁰ Further growth of the oxide film is expected to proceed by 3-D thickening with consumption of Cu from the substrate below the oxide nuclei. The film growth can be considered with respect to the electrochemical reactions occurring both at the metal/oxide interface and at the oxide/electrolyte interface. The oxidation of metal atoms at the metal/oxide interface generates a driving force for formation of anion vacancies (see Ref. ³² and references therein)

where is a copper atom in a regular metal site, is a copper cation in a regular site of the oxide film, is a positively charged oxygen vacancy, and represents the electron. At the oxide/electrolyte interface, anion vacancies become occupied by anions

where is an oxide ion in a regular site of the oxide film and is the hydrogen ion in the aqueous solution. Cation vacancies formed by dissolution of cations into the electrolyte diffuse toward the inner surface where they can exchange with cations

where is a positively charged copper cation in the aqueous electrolyte, is a negatively charged cation vacancy in the oxide film, and is a neutral vacancy in a regular metal site. Photoelectrochemical measurements of passive copper in the potential range of formation describe its electronic properties.^{26 33 34 35 36} According to these investigations, the oxide has properties similar to those of a p-type semiconductor with a high density of local states within the bandgap. Therefore, the increasing electrode potential during anodic polarization leads to an upward bending of the energy bands, and accumulation of positive holes. At the potential where the valence band of crosses the Fermi level, a large accumulation of positive holes causes the formation of a higher valency Cu oxide and subsequently duplex film formation with a redistribution of the potential drop inside the two layers. The simple band structure model for thin Cu oxide films on copper for increasing electrode potential is depicted in the inset in Fig. 1.

In the potential range corresponding to the electroformation of copper(II) oxide (from the current peak to a certain decrease in and an increase in C can be observed. However, the formation of a duplex structure of the passive layer is followed by an increase in and a decrease in C. Electrochemical polarization curves, XPS and ion scattering examination show that copper forms a duplex film at potentials above 0.78-0.059 pH vs. SHE in weakly acidic and alkaline solutions.⁸ Assuming that film initially forms before the growth of a duplex oxide film, the outer layer will be formed by a direct reaction of and water

Anodic oxidation of ions previously dissolved from the copper surface may contribute to the formation of an outer layer

The formation of these thin oxide layers by precipitation from a supersaturated electrolyte adjacent to the metal surface does not seem very likely. Therefore, a mechanism of direct oxide growth on the electrode (metal) surface is accepted. The growth rate increases with increasing anodic potential.

In the potential range between 0.0 and 0.5 V, the passive electrode behaves like a simple nonideal dielectric with the capacitance diminishing with the potential. In this potential region, the dielectric constant of the oxide layer, was examined using the equation for the parallel plate capacitor

where is the capacity of the electrogenerated oxide film, and is the oxide thickness calculated from the cathodic charge using the Faraday law (Fig. 3). For the roughness factor, σ equal to 2, the corresponding dielectric constant of the total oxide film is 105. A sharp departure from simple dielectric behavior occurs above 0.5 V, where reciprocal capacitance begins to decrease, while at the same time the dielectric constant begins to increase.

Above the current peak prior to the onset of oxygen evolution, the current density decreases, while an increase in capacitance and a decrease in can be observed (Fig. 6). Similar behavior was observed by Speckmann et al.⁷ in an alkaline solution, and was attributed to the formation of new species, A further increase in the electrode potential, at potentials >0.9 V, causes the crossing of the CuO layer valence band and the Fermi level and leads to a transpassive region. This increasing potential drop in the Helmholtz layer accelerates the rate of the electrochemical reactions at the oxide/electrolyte interface, i.e., the dissolution of ions from the oxide surface and oxygen evolution. For transpassive dissolution and oxygen evolution are expected according to experimentally determined band structure model in the inset in Fig. 3.

Using a simple resistivity model, it is possible to estimate the concentration of oxygen vacancies. Their concentration, within the oxide film is given by the equation³⁷

where is the charge carrier velocity (equal to where D is the diffusion coefficient of metallic cations in the oxide film taken as and d is the film thickness), q is the related charge is the film resistivity, and represents the electric field strength. With increasing potential of oxide formation from 0.00 to 0.90 V, the film resistivity, linearly decreases from a value of to A linear decrease in correlates with an increase in concentration of charge carriers from to as is shown in Fig. 7. The obtained values are similar to those referred for many metal/metal oxide/electrolyte systems such as iron/iron oxide,³⁸ tin/tin oxide,³⁹ and tungsten/tungsten oxide.⁴⁰

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The potentiodynamic behavior of copper in alkaline electrolyte reveals that the process involves a series of complex reactions including electrochemical and chemical stages occurring both consecutively and competitively. This reaction formalism allows the participation of processes controlled by phase growth associated with conductivity changes in the oxide layer. During the oxide layer growth under potentiodynamic conditions differential dehydration processes yield a changing OH/O ratio through the oxide layer. In essence, an increase in the potential increases the ratio of oxide to hydroxide ions, which accompanies the structural changes within the oxide film, i.e., the transformation from an amorphous, glassy to a crystalline structure.⁴¹ The amorphous oxides all occupy larger volumes than the same mass of crystalline material.⁴² Crystallization produces electron-conductive paths through the anodic oxide film and oxygen can be sited in these crystalline regions during film growth up to the potential value of about 1.0 V, where oxygen evolution begins.