Passivation phenomena during sonovoltammetric studies on copper in strongly alkaline solutions

Journal of

Electroanalytical Chemistry Journal of Electroanalytical Chemistry 568 (2004) 379–390 www.elsevier.com/locate/jelechem

Passivation phenomena during sonovoltammetric studies on copper in strongly alkaline solutions J.P. Lorimer, T.J. Mason, Mario Plattes, D.J. Walton

*

School of Science and the Environment, Sonochemistry Centre, Coventry University, Priory Street, Coventry CV1 5FB, UK Received 18 July 2003; received in revised form 20 August 2003; accepted 12 February 2004 Available online 9 April 2004

Abstract In alkaline solution a strongly passivating layer is formed close to a copper metal surface during a positive sweep to potentials beyond approximately +0.5 V (vs. SCE). Simultaneous ultrasonic irradiation during the voltammetric scan promotes the anodic dissolution of copper and greatly increases anodic currents in the less-positive potential region 0–0.5 V (vs. SHE), which corresponds to the formation of a thicker, more porous oxide/hydroxide layer, but does not prevent this passivation process from occurring at potentials above 0.5 V in sodium hydroxide solution up to 3 mol dm
3 concentration. The passivating layer requires maintenance of oxidising conditions, and so the removal of the protective oxide/hydroxide coating, which is effectively performed by ultrasound, facilitates reductive loss of the passivating layer sufficiently readily that an anodic current can be seen during the reduction scan between +0.5 and 0 V while scanning from the positive limit in a negative direction. In 1 M NaOH a curve-crossing phenomenon occurs in silent voltammetry in which scan reversal before the onset of passivation produces a higher anodic current in the reverse (reductive) direction than during the forward positive scan before reversal. Also at this alkali concentration, when the positive range is scanned into the passivation region the maximum anodic peak current in silent voltammetry is independent of scan rate over the range from 5 to 80 mV s
1 . Presonication of the copper surface prior to silent voltammetry greatly increases the peak currents and enhances peak definition in the copper voltammogram, suggesting general activation of the metal surface by ultrasound. The oxidation processes continue to occur at similar potentials, but there is a new peak in the reduction scan, with some shift of reduction potentials. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Sonovoltammetry; Copper electrode; Sodium hydroxide; Curve crossing; Passivation

1. Introduction The surface chemistry of copper is complex and there have been many studies of its voltammetry in various pH regimes. Numerous workers have studied the electrochemistry of copper in basic media under silent conditions where a feature of the surface processes involves the formation of ‘oxide/hydroxide’ (or ‘hydrous oxides’) species, which have consequences for electrocatalytic phenomena on this relatively reactive metal [1–7]. However, sonovoltammetry (the use of ultrasonic irradiation with voltammetry) of this metal in strongly alkaline media has been little reported. Copper has been *

Corresponding author. Fax: +44-24-7688-8702. E-mail address: [email protected] (D.J. Walton).

0022-0728/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.02.010

studied in some sonoelectrochemical systems, for example in acetic and oxalic acids [8], and as copper nickel alloy to study oxygen evolution in base [9]. Here we give sonovoltammetric results from copper metal in NaOH up to 3 mol dm
3 concentration. Sonoelectrochemistry is a subject attracting increasing attention at present, and a number of effects of insonation upon electrochemical systems have been observed. These include: a general improvement of hydrodynamics and movement of species; the alteration of concentration gradients at various points in the reaction profile with consequent switching of kinetic regimes and effects on mechanism and reaction products; the sonochemically induced reactions of intermediate species that have been generated electrochemically; the abrasion and roughening of electrode surfaces and also effects upon

380

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

electrode coatings, which may become evident as the diminution of electrode fouling and loss of species from the electrode surface, or sometimes as an enhancement of formation of a coating, perhaps with altered morphology, adhesion or other properties. Numerous electrochemical systems have been studied that variously demonstrate these effects and there have been several reviews on the topic, which include some mention of copper [10–15]. Ultrasound can cause inertial and non-inertial effects involving cavitation, streaming, microjetting and other phenomena, and an encompassing account of the origin(s) of sonoelectrochemical effects remain a matter for debate [16]. We have studied a number of systems that employ copper as a cheap and readily available electrode material, useful for practical electrosynthetic applications such as the degradation of azo-dyes [17,18], and for the selective electrochemical modification of proteins [19]. This latter work involves the use of mildly alkaline aqueous solutions, and since all our studies may incorporate simultaneous ultrasonic irradiation (see review in [15]) we have now undertaken a more extensive review of the sonoelectrochemistry of copper in alkaline solution. Our preliminary results in buffered solutions of pH 7, 9, and 11 have been reported [20], and now we extend the pH range to beyond 14, where we observe some unusual phenomena.

potential in the positive direction with the first currents arising from the passage of positive charge. Voltammetry was performed in a Compton-type cell. This is a one-compartment cell, which has an electrode adapter on the bottom. The adapter allows immersion of a disc electrode into the electrolyte solution from underneath. The electrode surface is ‘‘looking up’’, which enables sonication with an ultrasonic probe from above, i.e. ‘‘face-on geometry’’. The cell was provided with a glass cooling coil and had a volume of 300 cm3 . The cell temperature was maintained constant by circulating water from a thermostatic bath. Electrolyte solutions were degassed with argon for at least 15 min prior to an experimental run. Voltammetry and sonovoltammetry were performed using buffer solutions of different pH and different composition, i.e. pH 7 (dibasic sodium phosphate + monobasic potassium phosphate), pH 9 (sodium bicarbonate + sodium carbonate), and pH 11 (tribasic sodium phosphate + sodium bicarbonate), and using sodium hydroxide solutions of different concentration, i.e. 0.1, 1 and 3 mol dm
3 . For sonovoltammetry a 20 kHz ultrasonic probe system (Sonics & Materials, Vibracell) was used in the Compton-type electrochemical cell arrangement. The probe allowed directed irradiation of a copper disc electrode, which was placed ‘‘face-on’’ at a 15 mm distance from the probe tip, which had a diameter of 3 mm.

2. Experimental 3. Results and discussion Voltammetry and chronoamperometry of copper were performed using either a Radiometer Copenhagen potentiostat/galvanostat PGP 201 (for sweep rates of up to 83.3 mV s
1 (500 mV min
1 )) or an EG&G PAR potentiostat/galvanostat 273 for a wider range of sweep rates. Both potentiostats were computer controlled using either VoltaMaster1 (Radiometer) or Research Electrochemistry (EG&G PAR) software. For graphical presentation, raw data was transferred into Origin and Microsoft Excel. Copper disc electrodes were made by sealing a copper rod (£ ¼ 5 mm, 99.999%, Alfa Aesar) into a larger teflon rod (£ ¼ 10 mm) using an epoxy resin (Araldite). The disc electrode was polished using diamond slurries (15, 3 and 1 lm) and polishing pads (BAS Polishing Kid, A–1302). All electrodes were rinsed with acetone and deionised water. A similar copper electrode made from a commercial copper wire (£ ¼ 3:2 mm) was used for preliminary experiments. Platinum foils (1 cm2 and larger) were used as counter electrodes and a saturated calomel electrode (SCE) as the reference, at a potential of 0.242 V vs. the standard hydrogen electrode (SHE). In graphic representations most potentials are reported vs. the SHE. All voltammetric scans are started from the most negative

First we repeated earlier work [1] in dilute (0.05 M) NaOH in silent conditions, and obtained the same trace, as shown in Fig. 1.

Fig. 1. Silent voltammetry of copper in 0.05 mol dm
3 NaOH using a disc electrode from commercial copper wire (scan rate ¼ 83.3 mV s
1 (500 mV min
1 ), A ¼ 0:08 cm2 , 30 °C). All scans are performed from the furthest negative potential, in the positive direction first.

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

381

It has been proposed [1] that the first anodic peak A1 in Fig. 1 corresponds to the formation of Cu2 O Eq. (1) and the second anodic peak A2 is due to formation of a passivating oxide/hydroxide layer (due to overlap of processes in Eqs. (2)–(4). This layer is a mixture of cuprous oxide and cupric oxide/hydroxide and its structure is duplex, i.e. Cu/Cu2 O/CuO, Cu(OH)2 . It has been shown that the passive layer has an inner Cu2 O part and an outer CuO/Cu(OH)2 part [1,21]. Composite oxide/ hydroxide species have been termed ‘hydrous oxides’ and are thought to be more porous non-equilibrium thinfilms, sometimes termed ‘b forms’ than the more compact equilibrium ‘a forms’ of thin oxide coatings. There remains debate about the role of these species in electrocatalysis at the surfaces of copper and other metals [3]. 2Cu þ H2 O $ Cu2 O þ 2Hþ þ 2e
Cu þ H2 O $ CuO þ 2Hþ þ 2e


E0 ¼ 0:057 V: E0 ¼ 0:156 V:

Cu2 O þ H2 O $ 2CuO þ 2Hþ þ 2e


ð1Þ ð2Þ

E0 ¼ 0:255 V: ð3Þ

Cu2 O þ H2 O þ 2OH
$ 2CuðOHÞ2 þ 2e
E0 ¼ 0:097 V: ð4Þ According to the Pourbaix diagram of copper [22], three main positive transitions are expected, but these equilibria and the observed peak potentials are all dependent upon hydroxide-ion concentration, and the ease of distinguishing these peaks depends on alkalinity. At higher base concentrations there is clearer discrimination of these peaks. The first cathodic peak C1 (in Fig. 1) is attributed to reduction of CuO/Cu(OH)2 to Cu2 O and the second cathodic peak corresponds to reduction of Cu2 O to Cu [1]. However, it has been pointed out [3,4] that the cathodic reduction of oxide films is complicated because cuprous oxide (Cu2 O) is not greatly conductive and acts as a barrier layer to hinder reduction of cupric oxide/ hydroxide. Therefore the assignment of individual cathodic peaks is problematic. 3.1. pH buffers 7, 9, 11 Initial studies were performed in neutral and alkaline buffer solutions at pH 7, 9, and 11. A typical voltammogram recorded at pH 7 shows only one anodic peak (A1) and two cathodic peaks (C1 and C2) in Fig. 2. A single anodic peak indicates the oxidation of copper metal to cupric oxide Eq. (2). A separate peak for cuprous oxide formation is not distinguished during the positive sweep at this pH. The voltammogram reveals that the copper electrode does not passivate during the positive sweep and this fits with dissolution of cupric oxide into the bulk solution as the main process. Cuprous oxide has been reported to have a lower conduc-

Fig. 2. Silent voltammetry of copper in buffer solution of pH 7 using a disc electrode of commercial copper wire (A ¼ 0:08 cm2 , scan rate ¼ 83.3 mV s
1 , 30 °C).

tivity [9], which can interfere with accompanying processes, but this is not observed in the positive sweep of the voltammogram. However, the intermediacy of cuprous species has been proposed on the return scan where there are two cathodic processes visible. Peaks C1 and C2 are ascribed to reduction of cupric oxide to cuprous oxide and the further reduction of the cuprous oxide and remaining cupric oxide to metallic copper. This is thought to be because the low conductivity of firstformed cuprous oxide may impede further reduction of cupric oxide to cuprous oxide as well as direct reduction of cupric oxide to copper. Therefore once first reduction of cupric to cuprous species occurs then further reduction of the remaining cupric oxide can take place only after the reduction of this cupric oxide layer to copper takes place. This could give the observed single peak, because after passivation by formation of cuprous oxide, then reactivation by its removal, a significant overpotential for the reduction of cupric oxide to copper builds up during the potential sweep. This overpotential causes fast reduction of cupric oxide resulting in a single peak. It is clear from the voltammogram that the sequence of events on the oxidation scan is different from that on the reduction scan, and the different involvement of cuprous species may reflect the behaviour of a semiconducting oxide layer, which can act in the manner of a rectifier. The complexity of surface processes at copper is revealed by changing the positive limit of the scan at pH 7, all other parameters remaining constant, as is shown in Fig. 3. The cathodic peak potentials and the relative reduction currents are altered. The noise on one of the oxidation waves is an artefact. It can be seen that the more negative of the two cathodic peaks is more affected by extension of this scan limit. This fits with the known compaction of hydrous oxides (i.e. the interplay of a and

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

b forms) [3], which would affect follow-up processes (in this case the final reduction of all oxides to copper metal). Increasing the pH from 7 to pH 9 then 11 using carbonate/bicarbonate/phosphate buffers shifts the positive part of the voltammogram to more negative potentials in agreement with the Pourbaix-diagram [22], and allows discrimination of some anodic processes, although the waves are broad as shown in Fig. 4. On the reduction scans peak C2 moves to substantially more negative potentials, at first increasing the potential difference between the two cathodic peaks, since peak C1 moves little. However, at pH 11 peak C1 also moves negatively, and between pH values 9 and 11 both C1 and C2 each move some 300 mV negatively. In pH 11 buffer, the final reduction back to copper metal (at )500 mV vs. SHE) is substantially more difficult than at pH 7 (0.0 V vs. SHE), and the separation between the (broad) anodic peak and the cathodic peak C2 has become over 500 mV from 200 mV respectively.

Fig. 4. The effect of increasing pH from 7 to 11 on silent voltammograms of copper in buffered aqueous solutions, copper disc electrode (A ¼ 0:08 cm
2 , scan rate 83.3 mV s
1 , 30 °C).

1.50

sonovoltammetry silent

-2

Fig. 3. Effect of upper scan limit on the voltammetry of copper in pH 7 buffer solution (all other conditions the same as for Fig. 2).

The effect of ultrasound at pH 7 is to cause an increase in the anodic current and a decrease in the cathodic current (Fig. 5). The positive charge increases from 5.09 to 98.41 mC cm
2 . The considerable current enhancement that is characteristic of increased hydrodynamics under sonication is clearly evident, and the shape of the trace shows a limiting current. This is an often-reported effect of insonation upon voltammetry (see for example the review [12]). At pH 9 there is a different shape to the sonovoltammogram. Here there is the expected increase in current during the positive scan, but at potentials over +0.6 V (vs. SCE) the current dies away without reaching a steady limit, as shown in Fig. 6. A further interesting feature of this sonovoltammogram is the appearance of an anodic peak during the reverse (negative) sweep. This shows reactivation of the electrode during the negative sweep, such as would accompany removal of a kinetically stable inhibitory species on the electrode once the positive potential

j / mA cm

382

1.00 0.50

0.00

0.00

0.20

0.40

0.60

E / V vs SHE Fig. 5. Effect of ultrasound upon voltammetry of copper at pH 7 using a disc electrode from commercial copper wire; 20 kHz probe system, A ¼ 0:08 cm2 , scan rate ¼ 83.3 mV s
1 , 30 °C).

Fig. 6. Effect of ultrasound upon voltammetry of copper at pH 9 using a disc electrode of commercial copper lead (20 kHz probe system, A ¼ 0:08 cm2 , scan rate ¼ 83.3 mV s
1 , 30 °C).

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

(diminishing on the return sweep) is insufficient to maintain its presence. The electrode would then be active again for positive processes until the continuing diminishment of positive potential stopped all anodic processes. There is a large envelope enclosed by the positive and negative scans in the sonovoltammogram at pH 9, unlike the sonovoltammetry at pH 7, suggesting that non-equilibrium processes pertain at the higher pH. There is a visibly large increase in current between the silent and insonated traces. This is clear from the figure, which shows all currents plotted on the same scale. The increase in positive charge is 50-fold. There is also the beginning of a significant reduction current at )0.4 V (vs. SHE). This is noticeable in sonovoltammograms at pH values nearer to neutrality, but disappears at higher alkalinities. 3.2. In 0.05 M NaOH The silent scans have been discussed above (see Fig. 1). We also noted the effect of presonicating (15 min) the copper electrode in 0.05 mol dm
3 NaOH before running the silent voltammogram in this medium. As shown in Fig. 7 there are marked increases in the positive and cathodic peaks, and sharper definition of all peaks in the scan, compared to the silent scan without ultrasonic electrode-pretreatment. This may be due to increased surface roughness and hence effective electrode area. However, we note the appearance of an additional cathodic peak CS, which might reflect separation of the two cathodic processes that are normally superimposed (vide supra). This may be due to the formation of active sites during presonication of the copper surface. It is known that active sites play important roles in electrochemical processes and electro-

Fig. 7. Effect of ultrasonic pretreatment upon silent voltammetry of copper in 0.05 mol dm
3 NaOH solution using a disc electrode made of a copper wire (A ¼ 0:08 cm2 , scan rate ¼ 83.3 mV s
1 , 30 °C, 20 kHz ultrasonic probe). Note that the scans are performed under silent conditions. The scan without pretreatment is the same as that in Fig. 1.

383

catalysis [6–9] and they can be produced through various methods of pre-activation of the electrode, e.g. by potential cycling. It would appear that sonication gives a more active copper electrode surface. We have calculated the number of layers of copper atoms involved in the anodic processes. Given that copper atoms in the cubic face-centred lattice have an  [19], the number of copper atatomic radius of 1.17 A oms per surface square centimetre is 3.50  10
9 mol cm2 . This corresponds to a charge of 0.338 mC cm
2 when one electron is transferred per copper atom, i.e. for the first anodic peak, at )150 mV in Fig. 7, which corresponds to the transformation of copper to cuprous oxide. The charge underneath the first peak is 1.861 mC cm
2 without and 9.607 mC cm
2 with presonication. Therefore approximately six monolayers of copper are apparently oxidised under conventional pre-treatment, i.e. polishing, whereas 28 monolayers are oxidised upon positive scanning after the electrode was also presonicated for 15 min. However, the fact that the second anodic peak is also significantly increased by ultrasonic electrode treatment suggests that pre-treatment increases the effective electrode surface area by a factor of 4.5, although no obvious change is evident from visual inspection. The ratio of negative charge to positive charge was found to be 92% using the conventional and 81% after ultrasonic pre-treatment. 3.3. In 0.1 M NaOH At increasing alkalinities silent voltammograms of copper in 0.1 mol dm
3 sodium hydroxide solution clearly show the three anodic peaks, A1, A2, and A3, and two cathodic peaks C1 and C2 as shown in Fig. 8 (which were recorded at a scan rate of 60 mV s
1 as were most of the voltammograms in Figs. 8–20). In line with the published proposal for 0.05 M alkali [1] the first anodic peak A1 corresponds to the formation of Cu2 O; while peak A2 is associated with the conversion of cuprous oxide to cupric oxide/hydroxide and peak A3 is associated with the direct conversion of copper to cupric oxide. There is now a more evident loss of current under silent conditions at the positive extreme, suggesting passivation, but the passivating layer is not robust and scanning to higher positive potentials produces solvent oxidation. Despite the greater complexity of the anodic peaks, the negative scan still shows only two. This is altered from the situation at pH 7 where there was only a single anodic peak, with two cathodic peaks (cf Fig. 3). The first cathodic peak C1 is associated with the reduction of CuO/Cu(OH)2 to Cu2 O, while the second cathodic peak corresponds to the combined reductions of Cu2 O to Cu and remaining CuO/Cu(OH)2 to Cu. This is plausible since the charge corresponding to C2 is larger than for

384

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

j / mA cm -2

1.500

A1 A2

0.500

A3

-0.500

C1 -1.500

C2

-2.500 -1.000

-0.500

0.000

0.500

1.000

E / V vs SHE

Fig. 8. Silent voltammetry of copper in 0.1 mol dm
3 NaOH solution using a copper disc electrode (99.999% Cu, A ¼ 0:196 cm2 , scan rate ¼ 60 mV s
1 , 30 °C).

2

A1

Fig. 11. Plot of peak currents of cathodic peaks C1 and C2 as a function of the square root of scan rate in 0.1 mol dm
3 NaOH solution (99.999% copper, A ¼ 0:196 cm2 , 30 °C).

A2

jmax / mA cm

-2

1.5

1

0.5

0 0

50

100

150 200 -1 v / mV s

250

300

350

Fig. 9. Plot of peak currents of anodic peaks A1 and A2 as a function of scan rate in 0.1 mol dm
3 NaOH solution (99.999% copper, A ¼ 0:196 cm2 , 30 °C).

Fig. 10. Plot of peak currents of anodic peak A3 as a function of the square root of scan rate in 0.1 mol dm
3 NaOH solution (99.999% copper, A ¼ 0:196 cm2 , 30 °C).

Fig. 12. Effect of ultrasound upon copper voltammetry in 0.1 mol dm
3 NaOH solution using a disc electrode (99.999% Cu, A ¼ 0:196 cm2 , scan rate ¼ 60 mV s
1 , 30 °C).

Fig. 13. Silent voltammetry of copper in 1 mol dm
3 NaOH solution using a disc electrode (99.999% Cu, A ¼ 0.196 cm2 , scan rate ¼ 10 mV s
1 , 30 °C).

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

385

Fig. 14. Effect of upper potential limit for scan reversal on copper silent voltammetry in 1 mol dm
3 NaOH solution using a disc electrode (99.999% Cu, A ¼ 0:196 cm2 , scan rate ¼ 60 mV s
1 , 30 °C).

Fig. 15. (a,b) Effect of changing the scan rate upon copper silent voltammetry in 1 mol dm
3 NaOH solution using a disc electrode (99.999% Cu, A ¼ 0:196 cm2 , 30 °C).

Fig. 16. Repetitive silent voltammetry of copper in 1 mol dm
3 NaOH solution using a disc electrode (99.999% Cu, A ¼ 0:196 cm2 , scan rate ¼ 10 mV s
1 , 30 °C).

Fig. 17. Effect of ultrasound upon copper voltammetry in 1 mol dm
3 NaOH solution using a disc electrode (99.999% Cu, A ¼ 0:196 cm2 , scan rate ¼ 83.3 mV s
1 , 30 °C, 20 kHz ultrasonic probe).

C1 (and the positive charge under A3 is greater than that for A1 and A2 combined). Since the anodic processes are sufficiently distinguished at this pH (although there is some overlap of

peaks), we studied the effect of scan rate on this system. Under silent conditions, all peak currents increased with scan rate, but the various anodic and cathodic processes showed different scan rate dependences. The peak

386

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

Fig. 18. Comparison of silent voltammetry of copper in 1 and 3 mol dm
3 sodium hydroxide solution (99.999% Cu, 60 mV s
1 , 30 °C).

Fig. 19. Comparison of silent and sonovoltammetry of copper in 3 mol dm
3 sodium hydroxide solution (99.999% copper, 60 mV s
1 , 30 °C).

Fig. 20. Effect of sonication time at 0.542 V upon the reverse scan during voltammetry of copper in 3 mol dm
3 sodium hydroxide solution; all positive scans are silent (99.999% copper, 60 mV s
1 , 30 °C, 20 kHz probe).

current densities (jmax ) of A1 and A2 gave linear plots vs. scan rate as shown in Fig. 9. This indicates that the processes associated with peak A1 and A2 are surface processes, which is plausible if A1 is due to formation of a thin cuprous oxide layer by conversion of copper atoms on the copper surface and A2 is due to onward conversion of the so-formed cuprous oxide layer. The fact that the straight line for peak A2 did not intercept at zero is due to overlap between peaks A1 and A2, giving a false zero-line for A2. The plot is not corrected for this. The peak current density of A3 plots linearly with the square root of the scan rate, as shown in Fig. 10, which indicates a diffusion-controlled process. This is plausible if peak A3 is due to formation of a thick oxide/hydroxide layer, through which the diffusion of species is likely to control the overall process. Both the cathodic peaks appear to be under diffusion control since the peak current densities were found to plot linearly vs. v1=2 , as shown in Fig. 11. Therefore it is suggested that diffusion of hydroxide and/or water in the oxide/hydroxide layer is controlling the reduction processes. Ultrasound did not have a large effect upon peak A1, as shown in Fig. 12, which is reasonable if A1 is associated with a surface-controlled process and there is not much change in the surface within the timescale of insonation during the scan, (which is substantially shorter than for the pre-insonation discussed above). Peaks A2 and A3 become harder to distinguish under sonication and the current density is significantly increased in this region of the combined peaks. This fits with a diffusioncontrolled nature for A3 (cf Fig. 10) since ultrasound enhances mass transport phenomena. In the potential region from 0.1 to 0.4 V (vs. SCE) the sonovoltammogram shows a limiting-current plateau, similar to the sigmoidal shape, which is typical of sonovoltammograms (cf Fig. 5). However, the passivation in the potential region E ¼ 0:4–0:9 V (cf Fig. 6) could not be avoided by sonication. In fact, passivation is achieved at slightly lower potentials under ultrasound compared with the silent case. The nature of the main reduction processes appears to be similar under silent and ultrasonic conditions because two cathodic peaks were observed in both experiments, and peak C1 remained surprisingly unchanged although peak C2 was smaller in the sonovoltammogram. This infers that ultrasound removes some of the oxide/hydroxide material mechanically, thus resulting in a smaller voltammetric peak C2, since this peak has a component from reduction of the outer layers of the coating (the inner layers nearest the electrode having been reduced to cuprous species in peak C1). Fig. 12 also shows that the passivation process on the oxidation scan occurs more readily under ultrasound, and this would fit with the higher oxidation currents

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

upon insonation for the combined peak A2 and A3, since we believe the passivating species is metastable and exists in the prevailing microscopic environment closest to the electrode, under the more diffuse oxide/hydroxide (‘hydrous oxide’) coating that forms during peaks A2 and A3. We note that the current increase under ultrasound is only some 2.5 times in 0.1 M NaOH, as opposed to some 50-fold in pH 9 buffer (compare Figs. 12 and 6). The highest sonicated anodic current density is 1.5 mA cm
2 in Fig. 12 whereas it is 2.5 mA cm
2 in Fig. 6, showing that the onset of passivation is enhanced at the higher pH, so that a higher current density is not achieved. The respective silent current densities are 0.5 mA cm
2 in Fig. 12 vs. 0.1 mA cm
2 in Fig. 6 showing that greater alkalinity favours higher anodic currents in the absence of passivation, as is expected. 3.4. In 1 M NaOH Silent voltammograms conducted in 1 mol dm
3 NaOH are shown in Fig. 13, which is recorded at a scan rate of 10 mV s
1 . There are also three anodic peaks and a greater complexity of cathodic peaks, although the shapes are slightly different from those obtained in 0.1 mol dm
3 NaOH with A1 being of similar size to those in earlier figures from lower pH values but A2 and A3 are much larger. Note the current scale compared, for example, to Fig. 8 where there is a 10-fold lower hydroxide ion concentration. A greater loss of copper from the surface would be expected at the higher hydroxide ion concentration. However it should be noted that now there is almost complete passivation of the electrode at the positive limit. If the passivation is due to local conditions near the electrode surface underneath rapidly forming cupric oxide/hydroxide species, then passivation should be enhanced by both increased pH, or by sonication, both of which increase the current during the first part of the joint A2/A3 peak. This is what is found. More information about the nature of the various peaks in 1 mol dm
3 NaOH solution was obtained by changing the positive potential limit, although this also shows the complexity of the system. The results are given in Fig. 14. For example, in silent scans when the positive limit was set into the potential region between A1 and A2 (i.e. before peak A2 commenced), then a single cathodic peak appeared during the reverse sweep corresponding to reduction of the cuprous species. This is as expected because no cupric species are involved (this trace is not shown in Fig. 14). When the positive limit was shifted into the region of A2, two cathodic peaks, C1 and C10 were observed during the reverse scan. Further increase of the positive limit in A2 towards the region of A3 at first did not change C10 and C1 significantly, but a new broad peak C2 emerged on the negative side of C1, becoming larger as the positive

387

limit was increased. However, when the positive limit was clearly within the region of A3, an unusual anodic peak A0 appeared in the return reduction scan. This oxidation current during the reduction sweep was at its highest, and actually gave a current on the return scan in silent conditions greater than the anodic current on the forward sweep (see inset in Fig. 14) when the scan was reversed at approximately 0.15 V (vs. SHE). If the scan was reversed at higher oxidative potentials then the unusual oxidative peak in the reduction scan A0 became smaller and eventually disappeared as the potential limit approached 0.5 V (cf Fig. 13, which represents Fig. 14 with the highest positive sweep limit before reversal). This shows that the passivation process occurs at potentials beyond 0.15 V (vs. SCE) and that incursion into this passivation region insulates the electrode during the first part of the reverse sweep. It is likely that different growth processes pertain for the formation of the various species, and we shall report chronoamperometric results elsewhere [23]. Thus different oxidation peaks might represent growth processes with different energetics (e.g. two-dimensional vs. threedimensional nucleation), rather than different oxidation processes leading to grossly different species. This is supported by a study of the scan rate dependence of the silent voltammetry shown in Fig. 13. A striking observation under these conditions is that increasing the scan rate had very little effect upon the large combined anodic peak A2/A3. (At this pH the current of this peak is over 1000 times greater than that from the formation of the cuprous oxide layer, peak A1). Peak A2/A3 is effectively scan rate independent over a 16-fold range from 5 to 80 mV s
1 , as shown in Fig. 15, parts A and B. This result indicates that the positive processes are neither under diffusion control nor control from surface-immobilised species giving a saturation current at this pH. There was also very little change in the size of the cathodic peaks as the scan rate was increased, although some reorganisation of combined-peak structures may be seen. It seems that the factors influencing current-dependence may chance to cancel out. This result must show that a fixed rate of electron-transfer per unit time is maintained across a constant surface-area of copper. The voltammetry is performed in a still solution, without appreciable convection, and the process creating the current is electrodissolution, although from the charge passed on the return scan a substantial concentration of oxidised species remains in the vicinity of the electrode to be reduced. A plausible explanation for the limiting current density is that it represents the maximum rate of migration of copper atoms from the crystal lattice under the conditions employed, although it must be noted that a further increase in alkalinity (see below) produces a further substantial increase in current.

388

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

Another noticeable effect at the maximum scan rate of 80 mV s
1 is that A3 now appears as a shoulder of A2 (although at lower scan rates this is reversed). This may reflect a competition between a three-dimensional growth process that is sufficiently slow not to follow the changing potential at the higher scan speed, and a twodimensional growth mechanism. The effect of repetitive cycling using a slow scan rate, i.e. 10 mV s
1 is shown in Fig. 16. Here there is an increase in both the anodic current and charge of peaks A1 and A2; while peaks A2 and A3 merged during the repetitive cycle to give a smooth single peak A2. It has been suggested [8] that this effect may be due to an increase of the number of active sites, since cycling of the potential has been shown to be an effective method of activating the metal surface: ‘‘During the positive sweep, especially with a high upper limit value, the oxidation of the surface is assumed to be accompanied by (atom) place-exchange, so that on reduction in the negative sweep, some metal atoms are re-deposited in an active, poorly lattice-stabilised form. Repetition of this process leads to significant accumulation of active copper on the surface’’ [8]. Our results concur with this observation.When the electrode was subjected to sonication, the sonovoltammogram of copper in 1 mol dm
3 sodium hydroxide solution changed significantly, as shown in Fig. 17. The first peak A1 was little affected by ultrasound, as noted before, but the anodic peaks A2 and A3 merged into one large anodic peak (AS). The overall positive charge was much larger under ultrasound than under silent conditions (cf Fig. 13, from which the silent trace is taken, on the same current scale). Two interesting anodic peaks (AS0 and AS100 ) appear during the reverse (negative) sweep and all the cathodic peaks now disappear under sonication. This result is evidence for substantial although not necessarily complete removal of the cupric oxide/hydroxide layer during sonication. Note that the electrode still goes into a passive state in the potential region 0.2– 0.6 V (shown as P in Fig. 17). However, on scan reversal for reduction there is reactivation through an positive reaction during the first part of the negative scan (AS0 ). A similar anodic peak, although less pronounced, has been observed by other authors when starting copper voltammetry with a negative sweep from an oxidising potential [9,24,25]. Thus it appears that the sonicated copper electrode is activated more towards a cathodic reaction during the negative sweep, despite reaching the region of greater passivation at +0.5 V (vs. SHE) before reversal. In comparison the silent trace in Fig. 17 (which is the same as Fig. 13) shows very little anodic current on the reduction scan, and it is recalled that the traces in Fig. 14 that show marked anodic currents during reduction are those where scan reversal has occurred before extensive passivation has occurred. In fact the peak AS00 in Fig. 17 is in the same potential region as peak A0

observed during the silent scan (cf Fig. 14). The sonicated trace in Fig. 17 suggests electrodissolution with loss of oxidised species from the vicinity of the electrode, but even here the high anodic current is followed abruptly by passivation, suggesting that the local microenvironment of the copper surface during the high rate of oxidation processes allows the stabilisation of a passivating species. In the silent scan the presence of reducible species shows that there is no removal of oxidation products from the electrode region. 3.5. In 3 M NaOH Silent voltammograms recorded using a 3 mol dm
3 NaOH solution feature the same general peak structure as the silent voltammogram in 1 mol dm
3 NaOH solution. However, the peak currents are considerably larger as shown in the comparison in Fig. 18. The current is clearly dependent upon hydroxide concentration, but this is to be expected since all processes in Eqs. (1)– (4) involve either direct reaction with hydroxide ion, or the production of protons that would be quenched by hydroxide. Note that there are no adventitious ions from buffers in these experiments, but in consequence there is an increase in ionic strength with concentration of alkali. However, although there is greater availability of hydroxide ion at the start of an oxidative scan, there can be local depletion of hydroxide as the scan progresses, if species are formed that consume this ion and which remain near the electrode. We note in Fig. 18 that even in 3 M sodium hydroxide a substantial reduction current (peak C1) is observed in the region of )0.9 V (vs. SHE). This shows that reducible species remain near the electrode even in strong base, and also further emphasises the shift with alkalinity of the reduction to more negative potentials, since the reductive formation of copper at pH 7 is at 0.0 V (cf Fig. 2). In the oxidation regime the compactness of oxide/hydroxide layers (i.e. a and b forms – see above) affects the properties of these hydrous oxides, and since the rate of formation of oxidised species must affect the compaction of surface layers the increased anodic current may be due to enhanced formation of a cupric hydroxide layer that is more conductive than that formed at lower pH values, thus allowing this layer to grow thicker. No gas evolution is observed during the positive scan suggesting that evolution of oxygen could therefore be ruled out as a cause for the large anodic current, which in any case is followed by passivation at higher oxidation potentials. From comparison of data under silent conditions at the various sodium hydroxide concentrations in Table 1 it can be seen that for silent scans the overall positive charge Qa and negative charge Qc become larger with increased pH (Table 1). The relationship between these charges is roughly linear with activity (although somewhat less linear at the highest concentration). In 0.1

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390 Table 1 Anodic and cathodic charge ratios obtained during silent voltammetry of copper in strongly alkaline solutions [NaOH] (mol dm
3 ) pHa (Qc =Qa ) (%)

0.1 12.9 100

1 13.8 87

3 14.3 78

a

pH calculated using the activity coefficient c values for [NaOH] from [26].

mol dm
3 sodium hydroxide solution the positive charge is 7.88 mC cm
2 , and the 100% ratio of positive and negative charges suggests a compact layer of oxide/hydroxide since little dissolution of this layer into the bulk solution occurs. At higher alkalinities the positive to negative charge ratios lessens, becoming 78% at 3 M NaOH, but this still reflects a considerable retention of positively produced species near the electrode during the reduction scan, and thus not a substantial dissolution of surface species, despite the high concentration of hydroxide ion and the reputed solubility of copper species such as the cuprate anion. Sonication had a similar effect upon the voltammetry in 3 mol dm
3 sodium hydroxide solution as it did in 1 M sodium hydroxide solution, as given in Fig. 19. Two large anodic peaks AS1 and AS2 appear during the forward scan with now a substantial anodic peak AS0 during the reverse scan once the potential of approximately +0.15 V has been crossed. Note also the absence of any appreciable reduction processes under ultrasound within the reduction range to )1.3 V. Here the ratio of positive to negative charges within the same positive and negative range as in Table 1 is far from unity, and shows that ultrasound does remove oxidised species from the electrode, but without stopping the formation of the passivating layer on the positive sweep. Finally we note the effect when the copper electrode is sonicated for different periods of time at the maximum positive potential of +0.54 V (vs. SHE) after a silent forward scan. The aim was to discover whether the passivating layer is affected by insonation. Some effect of the length of sonication time upon peak A0 was observed, shown in Fig. 20. Peak A0 increased with sonication time, but only to a small maximum and no further increase was observed when the electrode was sonicated for more than 30 s. Note that this anodic peak has a much lower current than the one obtained during an entirely sonicated scan (cf Fig. 19). This suggests that the passive film cannot be removed completely by sonication, i.e. the electrode remains partially in a passive state at 0.542 V, even when sonicated for 60 s (not shown). Note that all scans were performed silently in Fig. 20, and the ultrasound was applied only during the delay while holding at the positive limit before reversing the scan. This is a different situation from that of the fully sonicated scans in Fig. 19.

389

In all scans where there is passivation, we have looked for a reduction current that might represent loss of the passivating layer during the return scan. In sonicated scans and those silent ones where the unusual anodic current is obtained during reduction. It is possible that the small reduction current from the passivating layer is subsumed in the larger anodic current that begins immediately once passivation diminishes. However, in Fig. 20 we also note the appearance after sonication of small reduction currents at )0.4 V (vs. SHE). A small reduction current is found here in many silent scans, which show the passivation phenomenon, but this could also be the reduction part of the cuprous to copper couple (i.e. the reverse of peak A1). It becomes increasingly difficult to distinguish this couple at higher alkalinities because of the disproportionate increase in the oxidation peaks A2/3 and reductive peaks C1/2. It is therefore possible that this small reductive peak in Fig. 20 reflects reduction of the residual passivation layer that remains after sonication.

4. Conclusions During copper voltammetry an increased concentration of hydroxide is accompanied by a considerable increase in anodic current in the potential region for formation of cupric species, but in silent scans there is little dissolution of oxidised species and even in 3 M NaOH, some 80% of charge is recovered during the reduction scan. However, a passivation process at potentials beyond approximately +0.5 V (vs. SHE) is also favoured at higher hydroxide concentrations. If the oxidation scan is reversed before this passivation occurs then the silent trace shows striking curve crossing, and on the return (reduction) scan there can be an anodic current higher than on the first oxidation. This suggests a nucleation process and change of effective electrode area. If the scan is reversed after passivation has taken place then a small reduction current at approximately )0.4 V (vs. SHE) is seen, which may represent reduction of the passivating species. After this, the more normal reduction peaks may be observed in the reduction scan, showing that the electrode has been reactivated. Under insonation at high alkalinities there is increased positive charge and also considerable loss of reduction charge on the return scan, suggesting that ultrasound removes the more porous oxide/hydroxide (‘hydrous oxide’) species formed during oxidation. However, ultrasound does not prevent passivation from occurring. Indeed a greater positive charge under insonation seems to hasten the passivation process. However if ultrasound is maintained during reduction then the passivating layer is removed more readily at

390

J.P. Lorimer et al. / Journal of Electroanalytical Chemistry 568 (2004) 379–390

sufficiently high potentials to allow oxidation currents to be observed during the reduction scan. We believe that the passivating layer is a nonequilibrium species that is transiently stable underneath the oxide/hydroxide layer, under conditions where the microenvironment does not reflect that of bulk solution due to the consequences of oxidation upon local stoichiometry. The greater the rate of oxidation, at higher pH or by current increase due to insonation then the more effective is the passivation. The passivating species requires maintenance of a sufficiently oxidising potential for its existence or else it is readily reduced, with reactivation of the electrode. Ultrasound facilitates removal of the thicker protective oxide/hydroxide coating, and then the insulating species is more readily reduced upon diminishment of the positive potential, but this species is not itself greatly removed from the electrode by sonication alone. When its protective layer is present then it resists ready electroreduction. Since Pourbaix diagrams refer to thermodynamic equilibria they may not assist the identification of nonequilibrium species. In general, higher alkali concentrations favour higher oxidation states of metals [22], but here the local hydroxide ion concentration after significant oxidation may not reflect that in the bulk. There are numerous copper species that may be implicated, including those with copper atoms playing roles in either cationic or anionic components. At any event ultrasound has again provided a time-dependent means to perturb electrochemical systems to provide mechanistic discrimination. The existence of further unusual species on copper surfaces has implications for electrocatalysis using this metal.

Acknowledgements M.P. thanks Coventry University for a studentship. The authors are grateful for the help of other members of the Coventry Sonochemistry Centre, and also I.R. Peterson, J. Iniesta and other members of the Coventry Centre for Molecular and Biomedical Science.

References [1] H.H. Strehblow, V. Maurice, P. Marcus, Electrochim. Acta 46 (2001) 3755. [2] S.M. Wilhelm, Y. Tanizawa, C.-Y. Liu, N. Hackerman, Corros. Sci. 28 (1982) 791. [3] L.D. Burke, M.A. Murphy, J. Solid, State Electrochem. 5 (2001) 43. [4] L.D. Burke, J.A. Collins, M.A. Horgan, L.M. Hurley, A.P. O’Mullane, Electrochim. Acta 45 (2000) 4127, and other papers in this series. [5] M.C. Kang, A.A. Gewirth, J. Phys. Chem. B 106 (2002) 12211, and references therein. [6] A.M. Zaky, F.H. Assaf, Br. Corros. J. 37 (2002) 48. [7] S. Nakayama, M. Shibata, T. Notoya, T. Osakai, Bunseki Kagaku 51 (2002) 1145. [8] M. Atobe, T. Nonaka, Ultrason. Sonochem. 4 (1997) 17. [9] L.B. Nei, F. Marken, Q. Hong, R.G. Compton, J. Electrochem. Soc. 144 (1997) 3019. [10] D.J. Walton, S.S. Phull, in: T.J. Mason (Ed.), Advances in Sonochemistry, 4, JAI Press, London, 1996, and references therein. [11] D.J. Walton, T.J. Mason, in: J.-L. Luche (Ed.), Synthetic Organic Sonochemistry, Plenum Press, New York, 1998, and references therein. [12] A.J. Saterlay, R.G. Compton, Fresenius, J. Anal. Chem. 367 (2000) 308, and references therein. [13] R.G. Compton, J.C. Eklund, F. Marken, Electroanalysis 9 (1997) 509. [14] J.L. Hardcastle, R.G. Compton, Electroanalysis 14 (2002) 753, and other papers in the series. [15] D.J. Walton, Arkivoc. (2002) 198. [16] P.R. Birkin, T.G. Leighton, Y.E. Watson, J.F. Power, Acoustics Bull. 26 (2001) 24. [17] J.P. Lorimer, T.J. Mason, M. Plattes, S.S. Phull, Ultrason. Sonochem. 7 (2000) 237. [18] J.P. Lorimer, T.J. Mason, M. Plattes, S.S. Phull, D.J. Walton, Pure Appl. Chem. 73 (2001) 1957. [19] D.J. Walton, J. Heptinstall, Prep. Biochem. Biotechnol. 30 (2000) 1. [20] M. Plattes, J.P. Lorimer, T.J. Mason, D.J. Walton, Ultrason. Sonochem., in press. [21] H.H. Strehblow, B. Titze, Electrochim. Acta 25 (1980) 839. [22] N. De Zoubov, C. Vanleugenhaghe, M. Pourbaix, in: M. Pourbaix (Ed.), Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE, Houston, TX, 1974, p. 384. [23] M. Plattes, J.P. Lorimer, T.J. Mason, D.J. Walton, in preparation. [24] A.M. Castro Luna de Medina, S.L. Marchiano, A.J. Arvia, J. Appl. Electrochem. 8 (1978) 121. [25] J.-B. He, X.-L. Li, D.-R. Lu, Y.-G. Zhu, J.-Z. Lin, Trans. Nonferrous Metals Soc. China 10 (2000) 824. [26] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, 74th Ed., 1993–1994, p. 5–93.