KSCN electrolytes

Journal of Electroanalytical Chemistry, 367 (1994) 7-14

The effect of process parameters into NaCl/KSCN electrolytes

on the anodic dissolution of copper

Z.H. Gu, J. Chen * and T.Z. Fahidy ** Department of Chemical Engineering, University of Waterloo; Waterloo, Ont. N2L 3GI (Canada)

A. Olivier Laboratoire d’Electrochimie et Chimie du Solide, Universitt! de Reims, Champagne Ardenne, Faculte’ des Sciences BP 347, 51062 Reims-C&k (France)

(Received 21 December 1992; in revised form 30 March 1993)

Abstract The effect of pH, KSCN concentration, NaCl concentration and imposed anode potential on oscillation patterns observed in the anodic dissolution of copper into aqueous solutions has been investigated. Surface-enhanced Raman spectroscopy of the anode surface indicates the complexity of interaction between these process parameters and the surface morphology.

1. Introduction

Current oscillations during the anodic dissolution of certain metals have been intensively investigated in recent years [l]. The dissolution of copper into various aqueous acidic solutions [2-91 and acidified sodium chloride solution [lo-171 has received particular attention because of its accessibility to mathematical analysis via the modern theory of chaos [12-171. The discovery of the existence of current oscillations in neutral sodium chloride solutions containing small amounts of thiocyanate ions [18] resulted in studies of certain aspects of the oscillation patterns under the influence of externally imposed magnetic fields and in terms of fractional Brownian motion theory [19]. The presence of CuSCN on the anode surface has also been demonstrated by surface-enhanced Raman spectroscopy (SERS) [20]. Although the mechanism of the oscillatory behaviour is not understood at present, a modification of the hypothetical Brusselator model [21,22] has been tentatively proposed [20] as a qualitative and approximate interpretation of the experimental observations.

Present address: Research Institute of Shanghai, Petrochemical Complex, Shanghai, China. l * To whom correspondence should be addressed. l

0022-0728/94/$7.00 SSDI 0022-0728(93)03003-8

The purpose of this paper is to present a summary of a detailed experimental investigation where oscillation characteristics were examined for various values of the initial pH and thiocyanate ion concentration at a controlled anode potential. As shown below the oscillatory regime is confined to a relatively narrow range of anode potentials when pH and [SCN-] are varied. However, at a certain potential, oscillations can be observed over a relatively wide pH range although their structural characteristics are pH dependent. Exvitro analysis of the anode surface provides additional evidence of the complex chemical and morphological nature of the dissolution process when accompanied by current oscillations. 2. Experimental The experimental apparatus has been described earlier [20,23]. The 3 cm x 4 cm x 6 cm experimental cell contained circular pure copper discs of geometric area 50 mm2 (at the onset of electrolysis) and pure copper plates of active area 1000 mm2 at a distance of 35 mm from the anode disc. The electrolytes consisted of varying amounts of potassium thiocyanate dissolved in aqueous sodium chloride, generally at a concentration of 4 mol dm-3. The working electrodes were polished

0 1994 - Elsevier Sequoia. All rights reserved

8

Z.H. Gu et al. / Anodic dissolution of Cu

TABLE 1. Numerical values of the experimental variables Variable unit KSCN concentration mm01 dmm3 Anode potential mV WE) pH (initial)

Numerical variable 1.0, 2.0,4.0,8.0 - 20.0 to - 90.0 in 10 mV decrements 0.2,0.5, 0.8, 1.1, 1.4, 1.7, 2.5, 2.8,3.2,3.4,3.7, 7.0, 9.0, 9.3, 9.6

by 400 and 600 grade emery paper and washed with water, deionized water and a small amount of acetone prior to each experiment. During potentiostatic electrolysis the temperature was kept at 20 f 1°C via efficient dissipation of Joule heat through the cell walls. Anode potentials were measured versus a saturated calomel electrode (SCE) and were set by a PAR 273 microcomputer-controlled potentiostat. The anode surface was illuminated by a high intensity light source and was monitored using a video camera-video recorder-TV assembly. Tape-recorded images were analysed by a digital computer. SERS was carried out with an OMARS 89 system using the 514.5 nm line of an argon ion laser; the SERS apparatus and technique have been discussed elsewhere [24]. The experimental variables are given in Table 1; the 3.7-7.0 pH range was excluded from this set of experiments since most earlier studies were carried out within this range. 3. Results and discussion 3.1. General observations of the oscillatory behaviour Regardless of the experimental conditions (provided that they induce oscillations), oscillatory currents are invariably observed after a certain time lapse following the imposition of a constant anode potential. During this period the current decreases after an initial sudden increase due to double-layer charging. The onset of oscillations is strongly linked to morphological phenomena, discussed below. The oscillations vanish after a characteristic time period and an essentially steady current is established. The length of the oscillatory period, the dominant oscillation frequency and the oscillation amplitude are determined by the experimental conditions. Replicate experiments produced very similar oscillation patterns (current-time series), onset of oscillations and Hurst exponents (Section 3.6). The reproducibility of the time-span of oscillations varied over several seconds. 3.2. The effect of the imposed (constant) anode potential The primary effect of the imposed potential, which was investigated in detail earlier (ref. 10, Table 2), is a

-100 -r 0

100

200

300

400

500

600

;

0

Time/s

Fig. 1. Current oscillation at pH 3.7, [KSCN] = 1 mm01 dm3, [NaCl] = 4 mol drnm3 and V, = - 60 mV/SCE.

systematic decrease in the oscillation amplitude and a systematic increase in the dominant oscillation frequency as the potential is decreased. However, preliminary experiments have indicated that oscillations exist only in a narrow potential range when the pH and [SCN-] are varied across the domains shown in Table 1. An anode potential V, of -60 mV was found to be optimal for maximizing the width of the oscillation domain, and the experimental results presented here are confined to this specific value of V,. 3.3. The effect of pH on the oscillation structure

Typical current-time series shown in Figs. l-4 indicate clearly that the oscillatory regimes are non-stationary. Although power spectrum analysis of non-stationary signals is unreliable, it has been demonstrated in the literature [2.5-271 that the existence of a high relative amplitude peak at a dominant frequency allows the spectra to be used for analysis provided that the

.1001 0

100

200

300

400

500

600

700

0

Time/s

Fig. 2. Current oscillation at pH 3.4, [KSCN] = 1 mm01 dm3, [NaCl] = 4 mol dmv3 and V, = - 60 mV/SCE.

Z.H. Gu et al. / Anodic dissolution of Cu

I

-eO-

-100, 0 100

-90, 200

300

400 Time/s

500

600

700

0

t

loo

200

300

400

500

800

700

Time/s

Fig. 3. Current oscillation at pH 3.1, [KSCN] = 1 mmol dm3, [NaCl] = 4 mol dmm3 and V, = - 60 mV/SCE.

Fig. 5. Quasi-chaotic oscillation at pH 1.7, [KSCN]= 1 mmol dm3. [NaCl] = 4 mol dmm3 and V, = - 60 mV/SCE.

results are treated with caution. Although the low amplitude portions of such spectra cannot normally be interpreted with the same degree of reliability as in the case of stationary oscillations, the use of the power spectrum is justified if sample spectral distribution function plots [27] yield essentially the same dominant frequencies as the (regular) power spectra. The power spectra associated with Figs. l-4 were found to satisfy these conditions. A dominant frequency exists at each pH investigated, but the spectrum becomes increasingly polychromatic as the pH is decreased. At low pH values the current-time series becomes increasingly irregular, as shown in Fig. 5. 3.4. The effect of the concentration of the thiocyanate ion on the oscillation structure Figures 6 and 7 illustrate the [SCN-I effect. An increase in the thiocyanate ion concentration produces

-1304 0

500

loo0

1500

2000

2500

3

)o

Time/s

Fig. 6. Current oscillation at pH 1.7, [KSCNl= 2 mmol dme3. [NaCl] = 4 mol dmm3 and V, = -60 mV/SCE.

‘1 , , , , , , , , 0

loo

200

300

400

500

600

I

700

Time/s

Fig. 4. Current oscillation at pH 2.8, [KSCN] = 1 mmol dm3, [NaCl] = 4 mol dm-’ and V, = - 60 mV/SCE.

0

loo

200

300

400

500

600

700

800

S K)

Time/s

Fig. 7. Current oscillation at pH 1.7, [KSCN]= 8 mmol dmm3, [NaCl] = 4 mol drne3 and V, = - 60 mV/SCE.

Z.H. Gu et al. / Am&c dissolution of Cu

neutral solutions containing only 3 mol dmm3 NaCl, while at pH 1.7 the threshold value of [NaCl] is 4 mol dmM3.

0

100

200

300

400

500

800

700

eJ30

9G+l

low

Time/s

Fig. 8. The effect of NaCl concentration on current behaviour at pH 1.7; [KSCN] 2 mmol drnT3 and V, = -60 mV/SCE. [NaCl]/mol dmm3: (a) 0.5; (b) 1.0; (c) 2.0; (d) 3.0; (e) 4.0.

a substantial decrease in both the oscillation time-span and the amplitude span. At pH 1.7, the effect of [SCN-] on the oscillation structure is particularly interesting; as [SCN-] is increased, the initially quasimonochromatic frequency behaviour slowly becomes polychromatic but the bandwidth changes very little beyond [SCN-I = 4 mm01 dme3. 3.5. The effect of chloride ion concentration oscillatory behaviour

on the

Figure 8 illustrates the typical effect of [Cl-l on the current behaviour. At a set initial pH and thiocyanate ion concentration, no oscillation is observed below a threshold value of [Cl-l. The chloride ion is provided by sodium chloride dissolved in the electrolyte; the contribution of the small amount of HCl used to set the initial pH is minimal. The more acidic the solution, the higher is the chloride concentration needed to induce oscillations; for example, in the case of [SCN-] = 2 mmol dmm3 oscillations have been observed in

3.6. Characteristic Hurst exponents The exponent H, which was originally proposed by Hurst [28] as a measure of long-term correlation in an oscillatory time series, was formally introduced by Mandelbrot and Van Ness [29] as a a parameter in the kernel of the definition integral of a random function of time. The function generalizes the one-dimensional random walk model of ordinary Brownian motion and is the basis of fractional Brownian motion (FBM) theory. As discussed in the description of the application of FBM theory to anodic copper dissolution [19,20], the domain of existence of H is [O, 11 and H = l/2 represents a purely random (Gaussian) process. A value of H near unity indicates a positive correlation of time series increments. Conversely, as the numerical value of H approaches zero, the chaotic tendency of the oscillation increases. The construction of Pox diagrams for the determination of H has been thoroughly described in the literature [19,20,30-321 and is not repeated here; a typical Pox diagram is shown in Fig. 9. Table 2 summarizes the important characteristics of the experimentally observed oscillations. When all other conditions are kept constant, an increase in the acidity of the solution yields oscillations with gradually weaker positive correlations between instantaneous current values at different times. The tendency towards less ordered behaviour (e.g. Fig. 5) is also indicated by a gradually increasing scatter of the Pox diagrams and gradually decreasing values of their correlation coefficient. Similar conclusions can be reached in the case of increasing KSCN concentration. 3.7. Morphological characteristics of the anode surface The relationship between anode surface morphology and current oscillation was briefly investigated in a

TABLE 2. Characteristics of selected experimentally observed oscillations at V, = - 60 mV/SCE Preset experimental conditions

Experimental variable

Dominant frequency f/Hz

[KSCN] = 1 mm01 dmM3 [NaCl] = 4 mol dme3

pH pH pH pH pH

0.165 0.142 0.165 0.155 0.004

490 529 499 500 780

1.00 0.90

0.90 0.92 0.46

0.96 0.87 0.76 0.74 0.23

0.08 0.16 0.12

1920 316 150

0.94 0.68 0.60

0.94 0.38 0.25

pH 1.7 [NaCl] = 4 mol drne3

3.7 3.4 3.1 2.8 1.7

[KSCN] = 2 mmol drnm3 [KSCN] = 4 mmol drnm3 [KSCN] = 8 mmol drnm3

Time-span of oscillation T/s

Hurst exponent H

Coefficient of determination rz a

a Pertaining to the resealed range versus time lag plot (Pox diagram) [19,20,17,31]; H is the slope of the line obtained by linear regression.

Z.H. Gu et al. / Anodic dissolution of Cu

11

0.80.7OB-

3

0.5

g

0.4

2.

I 0.3

1

I

0.2 0.1 1 04 0.4

I

E

-54 0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

350

1.4

logo

400

450

500

650 Raman

600

650

700

750

preliminary application of the SERS technique [201. The results of a more systematic examination are summarized in Table 3 and typical spectra are shown in Figs. 10 and 11. Since image analysis of the illuminated anode surface [23] indicates clearly that oscillations begin only after the initially film-free anode surface is completely covered with a black film, SERS provides direct evidence for the requirement of copper oxide formation for oscillatory behaviour. In addition, the presence of CuSCN on the anode surface prior to oxide formation appears to be an a-priori condition for oscillation; however, after the oscillatory period its adherence to the anode surface film is weak, as indicated by test R5. In agreement with the results of the preliminary study [20], there is no evidence of CuCl particles in the film. Typical growth of the oxide film on the anode before the onset of oscillations is shown by image analysis of the illuminated anode surface (Fig. 12). As demonstrated in Fig. 13, during the initial phase of film formation a short induction period is followed by a

60 CuSCN

350

do

4&o

GO

550 Raman

6&l

sio

Shift/cm

7bO

750

es0

Fig. 11. SER spectrum, associated with Fig. 1, of an oxide-containing portion of the anode prior to the onset of oscillation. In addition to the CuSCN peaks (Fig. lo), the peaks at shifts of approximately 525 cn and 625 cm correspond to CuO and Cu,O.

Test code

Time of the SERS test

Surface element tested

Qualitative composition of the film on the surface element tested

Rl

Before the onset of oscillation Before the onset of oscillation During oscillation Past the period of oscillation Past the period of oscillation

Free of black film

CuSCN

Black film

CuSCN, CuO, Cu,O

Black film a Black film (unwashed) Washed portion of black film

CuSCN, CuO, Cu,O CuSCN, CuO, Cu,O

R5

&O

-1

TABLE 3. Summary of surface analysis SERS

R3 R4

E‘50

Fig. 10 SER spectrum, associated with Fig. 9, of an oxide-free portion of the anode prior to the onset of oscillation. The peaks at shifts of approximately 430 cm and 750 cm correspond to CuSCN.

Fig. 9. Pox diagram associated with Fig. 11.

R2

80

Shift/cm-l

[KSCN] = 1 mmol dm- 3; V. = -60 mV/SCE, pH 1.7; [NaCl] = 4 mol dm3. a At the onset of oscillation the anode surface is fully covered by a black film [23].

cue,

cu,o

Z.H. Gu et al. / Anodic dissolution of Cu

12

80s

60s

120s

loos

160s

Fig. 12. Typical growth pattern of the oxide-containing layer at pH 3.7, [KSCN] = 1 mmol dmW3, [NaCl] = 4 mol dm-3 and V, = -60 mV/SCE (see Fig. 1.).

period of rapid growth at a linear rate. This period is followed by a considerably slower growth interval with a quasi-linear rate. In the final period full coverage of the surface is gradually achieved. 4. Analysis The experimental observations reported here, covering a wide range of variables which influence the oscillatory anodic dissolution of copper, emphasize the complexity of the process mechanism. The exact role of the thiocyanate ions may remain unclarified until dissolution in aqueous bromide and iodide solutions has

0

20

40

60

60

100

120

140

160

Time/s

Fig. 13. Time series representation Fig. 11.

of the growth pattern shown in

been investigated. In view of the much smaller solubility [331 of CuBr (pKsp = 8.28) and CuI (pK, = 11.96) compared with that of CuCl (pK,, = 5.92), it is conceivable that in NaBr and NaI electrolytes CuSCN formation competes with the formation of these copper compounds in contrast with the consecutive Cu + CuCl + CuSCN overall reaction path. This particular aspect will be investigated in future work. The effect of the apparent interaction of acidic pH with the thiocyanate is to modify the kinetics of the (periodic) surface renewal-blockage process. In addition, the coexistence of cuprous and cupric oxide in the anode film suggests that either the Cu,O --f CuO (or equivalently the CuOH + Cu(OH),) oxidation process is catalysed by the hydronium ions, or the latter induce a direct Cu -+ CuO overall reaction with intermediate steps involving SCN- ions. In neutral solutions CuO has not been found in the anode film nor has oxygen evolution been detected at any pH in any of the experiments (the Cu,O + 30, --) 2CuO reaction is favoured thermodynamically, with a standard Gibbs energy change of - 108 kJ per mol Cu,O at 25°C). The asymmetric growth of the CuSCN + Cu,O + CuO layer, illustrated in Fig. 12, with progressive shrinkage of the eccentric oxide-free and quasi-circular anode portion, suggests anisotropic surface kinetics of intermediate reactions leading to oxide formation. This pattern is not noticeably affected by the solution pH. The growth rate of CuSCN-carrying oxide film on the surface is approximately proportional to the instantaneous fraction of the oxide-free surface. In the spe-

Z.H. Gu et al. / Anodic dissolution of Cu

cific case shown in Fig. 13, non-linear regression analysis of the model equation Y,=l-O.Oly=a

exp(-bt)

(1)

where

D. Irish of the Department of Chemistry, University of Waterloo, and his research team for the use of the SERS apparatus. Nomenclature

a = 1.074

b =

-0.031

has a coefficient of determination r2 of 0.96 and a residual of 4.8 X 10m3. Since the regression parameter u differs only slightly from unity, the regression coefficient b can be considered, at least approximately, as a specific growth rate parameter. At neutral and basic values of the pH, the onset of oscillation is determined by the concentration of SCNions and the imposed anode potential in an apparently irregular pattern. When the electrolyte contains 4 mol dmp3 NaCl + 1 mm01 dmm3 KSCN, oscillations are observed in the potential range from -40 to - 70 mV/SCE at pH 7.0, but only at - 60 mV for pH 9.0 and at -70 mV for pH 9.3. At higher pH values no oscillation is observed even at lower anodic potentials (e.g. -90 mV). At pH 7 oscillatory behaviour is confined to the -50 to -70 mV range when [SCN-] = 2 mm01 dme3, and to the -50 to -60 mV range when [SCN-] = 4 mmol dm -3. Finally, at [SCN-I = 8 mmol dmp3 and pH > 2.5 no oscillation is observed at any imposed potential. These results suggest that, at sufficiently large thiocyanate concentrations, intensive formation of CuSCN (either by the direct reaction Cu++ SCN-+ CuSCN or the indirect reaction Cu++ Cl--+ CuCl + SCN-+ . . . + CuSCN) inhibits the formation of copper oxides. 5. Concluding

13

remarks

The complexity of the interactions determining the structure of oscillations presents a serious challenge to the experimenter. As soon as the working potential is applied, the anode surface is subjected to time- and space-dependent dissolution and deposition phenomena. Understanding these phenomena requires continuous monitoring of their “microprofile fingerprint”. In-situ SERS may be the first step towards achieving this by providing surface-averaged spectra of the anode layer during electrolysis in a continuous manner. The execution of such experiments is one of the future projects planned by this research group. Acknowledgements

This work has been supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and a NATO International Collaborative Research Grant. The authors are grateful to Professor

a b r2 R/S s t K Y YN

regression parameter (eqn. (1)) regression parameter (eqn. (1)) coefficient of determination resealed observation range in a time series [19] time lag [19] time, s anode potential vs. SCE anode surface coverage by an oxide-containing layer, % fraction of anode surface free of an oxide-containing layer (eqn. (1))

References 1 J.L. Hudson and M.R. Bassett, in D. Luss and N.R. Amundson (Eds.), Reviews in Chemical Engineering, Freund, London, 1991. 2 R. Cooper and J.H. Bartlett, .I. Electrochem. Sot., 105 (1958) 109. 3 J.F. Cooper, R.H. Mueller and C.W. Tobias, .I. Ekctrochem. Sot., 127 (1980) 1733. 4 F.N. Albahadily and M. Schell, J. Chem. Whys., 88 (1988) 4312. 5 F.N. Albahadily, J. Ringland and M. Schell, J. Chem. Phys., 90 (1989) 813. 6 M. Schell and F.N. Albahadily, .I. Chem. Phys., 90 (1989) 822. 7 L.T. Tsitsopoulos, T.T. Tsotsis and LA. Webster, Surf. Sci., 191 (1988) 225. 8 L.T. Tsitsopoulos, T.T. Tsotsis and LA. Webster, Surf. Sci., 220 (1989) 391. 9 L. Cicsopoulos and T.T. Tsotsis, American Institute of Chemical Engineering, Annual Meeting, Miami Beach, FL, 2-7 November 1985, Paper 159. 10 H.P. Lee, K. Nobe and A. Pearlstein, J. Electrochem. Sot., 132 (1985) 1031. 11 J.P. Lee and K. Nobe, .l. Eleckochem. Sot., 133 (1986) 2035. 12 M.R. Bassett and J.L. Hudson, Chem. Eng. Commun., 60 (1987) 145. 13 M.R. Bassett and J.L. Hudson, J. Phys. Chem., 92 (1988) 6963. 14 M.R. Bassett and J.L. Hudson, Physica 0, 35 (1989) 289. 15 M.R. Bassett and J.L. Hudson, J. Phys. Chem., 93 (1989) 2731. 16 M.R. Bassett and J.L. Hudson, J. Electrochem. Sot., 137 (1990) 922. 17 M.R. Bassett and J.L. Hudson, J. Elecrrochem. Sot., 137 (1990) 1815. 18 Z.H. Gu, A. Olivier and T.Z. Fahidy, Electrochim. Acta, 35 (1990) 933. 19 P. Fricoteaux, Z.H. Gu and T.Z. Fahidy, J. Electroanal. Chem., 324 (1992) 161. 20 Z.H. Gu, J. Chen and T.Z. Fahidy, Electrochim. Acta, 37 (1992) 2637. 21 G. Nicolis and I. Prigogine, Self-Organization in Non-equilibrium Systems. From Dissipative Structure to Order through Fluctuations, Wiley, New York, 1977. 22 J.M.T. Thompson and H.B. Stewart, Nonlinear Dynamics and Chaos, Wiley, New York, 1986, Section 3.

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Z.H. Gu et al. / Anodic dissolution of CIA

23 Z.H. Gu, J. Chen, A. Olivier and T.Z. Fahidy, J. Ekctrochem. sot., 140 (1993) 408. 24 B.D. Smith, The application of Raman spectroscopy to catalytic systems, MSc. Thesis, University of Waterloo, 1990. 25 B. Abraham and J. Ledolter, Statistical Methods for Forecasting, Wiley, New York, 1983. 26 B. Abraham, private communication, 1992. 27 H.J. Newton, TIMESLAB: A Time Series Analysis Laboratory, Wadsworth and Brooks, Pacific Grove, CA, 1988.

28 E.M. Hurst, Proc. Inst. Ciu. Eng., 5 (1956) 519. 29 B.B. Mandelbrot and J.W. Van Ness, SLAM Reu., 10 (1968) 422. 30 B.B. Mandelbrot and J.R. Wallis, Water Resources Res., 5 (1969) 321. 31 B.B. Mandelbrot and J.R. Wallis, Water Resources Res., 5 (1969) 967. 32 J. Feder, Fractals, Plenum Press, New York, 1988, Section 5. 33 J.A. Dean (Ed.), Lange’s Handbook of Chemistry (13th edn.), McGraw-Hill, New York, 1985, Section 9.