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Adsorption—Nucleation and growth mechanism of CuSCN monolayer formation on a copper amalgam electrode in thiocyanate solutions
J. Electroanal. Chem., 195 (1985) 137-149 Elsevier Sequoia !%A., Lausanne - Printed in The Netherlands
ADSORPTION-NUCLEATION MONOLAYER FORMATION THIOCYANATE SOLUTIONS
RENATA
BILEWICZ
Department (Received
and ZENON
of Chemistry,
AND GROWTH MECHANISM OF CuSCN ON A COPPER AMALGAM ELECTRODE
137
IN
KUBLIK
University of Warsaw, 02093 Warsaw, Pasteura I (Poland)
27th March 1985)
ABSTRACT
Using voltammetric, galvanostatic and potentiostatic techniques, the electrodeposition of cuprous thiocyanate on a copper amalgam electrode has been studied in acidic solutions containing various thiocyanate ion concentrations. Under the conditions used, the formation of two successive monolayers of CuSCN could be studied before the onset of bulk deposition. It was found that the formation of the first monolayer proceeds according to a complex adsorption-nucleation and growth mechanism. In the first stage, cuprous thiocyanate is adsorbed on the-amalgam electrode; in the second, the disordered layer of adsorbate rearranges to form two-dimensional crystalline centres; and in the third stage, these centres grow, completing the formation of the entire monolayer. The second monolayer is formed in a much simpler way, according to a mechanism of two-dimensional, instantaneous nucleation and growth. At high thiocyanate ion concentration, the formation of the CuSCN layer is obscured by the extensive formation of solution soluble species.
INTRODUCTION
The corrosion of copper in various solutions is an important problem from an engineering point of view [l]. The corrosion rate is, under certain conditions, inhibited in the presence of thiocyanate ions [2]. However, there have been few attempts to elucidate the mechanisms of copper corrosion and copper anodic oxidation in solutions containing thiocyanate ions [2-41. The mechanisms of the formation of CuSCN deposit on a solid substrate are also interesting from the point of view of solar energy conversion [5]. In order to facilitate the investigation of the mechanisms of the anodic dissolution of metals, samples of solid metals are sometimes replaced by the appropriate amalgams [6]. The aim of the present work was to elucidate the mechanism of the formation of anodic deposits on a copper amalgam electrode in solutions containing SCN- ions. As yet, such a problem has not been discussed in the literature, although the system Cu(II)-Cu(I)-Cu(0) has been studied in thiocyanate solutions under polarographic [7] and voltammetric [S] conditions. Voltammetric, galvanostatic and potentiostatic techniques were used to obtain the characteristics of the CuSCN anodic deposits. 0022-0728/85/%03.30
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138 EXPERIMENTAL
The voltammetric curves were recorded using a set-up consisting of an Elpan Model EG-200 voltage scan generator, a PAR Model 173 potentiostat equipped with a Model 176 current-to-voltage converter and a Riken Denshi F-3C X-Y recorder. In potentiostatic and coulometric experiments the potentiostat PAR Model 173 was used in conjunction with a Model 179 coulometer. Galvanostatic experiments were performed with a home-made galvanostat [9]. A saturated calomel electrode was used as the reference electrode and the potentials are referred to this electrode. The counter-electrode was a 2 cm2 platinum foil. An HMDE of the type described by Kemula and Kublik [lo], filled with copper amalgam, was used as the indicating electrode. The surface area of the drop electrode used was 0.024 cm2. Copper amalgam was prepared by dissolving a weighed sample of A.R. copper in an appropriate volume of mercury. Copper amalgam is not stable even in deaerated solutions, where only traces of oxygen are present [ll]. The concentration of copper in the amalgam was checked on the basis of the Sevtiik-Randles equation from the height of the copper dissolution peak. The resistance of the system, with 1 mol dmW3 sodium perchlorate solution as the supporting electrolyte, was close to 12 Q. The currents flowing in the experiments described in this paper were not higher than 1 PA. Thus, the iR drop components should be insignificant in our experiments. In some cases a positive feedback arrangement was used to compensate for the iR drop component but in no case did the use of this arrangement lead to improvement in the shapes of the curves. All solutions used were prepared from A.R. chemicals and triply-distilled water. In some cases the thiocyanate solutions were additionally purified by electrolysis at a large mercury cathode. Solutions were deoxygenated prior to recording the curves. All the experiments were carried out at 25 f 0.5”C. RESULTS
General characteristics of the Cu(Hg)-Cu(I) system in thiocyanate media For this study cyclic voltammetry was chosen as the most convenient technique. In the first series of the experiments cyclic voltammetric curves were recorded with a constant concentration of copper in the amalgam and with different concentrations of thiocyanate ions in the solution. Some of these curves are presented in Fig. 1. Curve 1, obtained at low thiocyanate ion concentration, shows a single system of peaks labelled a, and cl. Under the conditions used, the anodic process is controlled by the diffusion of SCN- ions. On the other hand, the cathodic process shows a significant deviation from the diffusion-controlled shape. The cathodic current is markedly enhanced, in comparison with the anodic one, and after crossing the maximum it decays abruptly. A slight increase of SCN- concentration (curve 2) leads to enhancement of peak a, and to the occurrence of two new effects. First, a very small anodic spike, a;,
139
-cl6
-a5
-0.4
-0.3
-‘I’
E[V
Fig. 1. Cyclic voltammograms obtained for _a 1 X 10m4 mol drnF3 copper amalgamelectrode in the presence of increasing concentrations of SCW ion: (1) 1 X 10e5; (2) 2X 10e5; (3) 5 X 10w3; (4) 5 X lo-’ mol dmw3. Supporting electrolyte: 1 mol dmm3 NaClO,, pH 2. Voltage scan rate = 5mV s-l.
occurs at the descending portion of the anodic branch and just after this spike the current decays abruptly to zero, i.e. the electrode becomes passive. Then, at more positive potentials, a second, markedly drawn out, system of peaks, a2-c2, occurs. At still higher thiocyanate ion concentration (curve 3) the anodic branch of the curve becomes less drawn out. The spike a; is now higher and is located very close to peak aI. Under these conditions, peak a2 forms a shoulder on the markedly higher and broader peak a3. Under the conditions used, the height of peak a3 starts to become controlled by diffusion of copper in the amalgam, whereas the heights of peaks aI and a2 are controlled by other factors. The product formed in peak a3 is reduced irreversibly but the charges passing in the anodic and cathodic processes are the same. Thus, at this thiocyanate ion con~ntration during the positive scan, only insoluble species are formed. Moreover, a ten-fold increase of SCN- concentration (curve 4) only slightly affected the height of peak a3 but led to better separation of peaks a2 and a3 and to better development of spike a;. Also, under these conditions, the species formed during the anodic process were completely reduced during the subsequent negative run. This finding is consistent with the data reported by Altukov et al. 141,who, using a rotating ring-disc electrode, studied the properties of the products of copper oxidation in thiocyanate solutions. At SCN- concentrations higher than 0.1 mol dmV3, soluble products start to occur during the positive scan; investigation of these processes was beyond the scope of the present work.
140
In parallel with the increase of SCN- ion concentration, a systematic shift of the entire curve towards negative potentials was observed. The slope of the dependence of log i vs. E, for increasing SCN- concentration, was equal to 60 mV when the potential was measured at the foot of the first anodic peak. Such behaviour confirms the literature results that the first anodic peak obtained at a copper amalgam electrode in thiocyanate solutions corresponds to the formation of copper (I) species. An increase of copper concentration in amalgam from 1 X 10d4 to 2 X 10m3 mol dmv3 led to a shift of the entire cyclic curve towards more negative potentials without essentially changing the shape and height of the al-c, and a2-c1 systems. The behaviour described above did not alter essentially when the pH of the solution was varied in the range l-5. Investigation in solutions containing low thiocyanate ion concentrations The cathodic peak ci occurring in curve 1 of Fig. 1 is significantly higher than the anodic peak a,, indicating that the product formed in the anodic process accumulates on the electrode surface. The charges involved in the formation of the anodic and cathodic peaks are the same, even on the curves obtained in stirred solutions, and this means that no solution soluble species are formed during the anodic process. With a decrease of SCN- concentration, peak a, becomes obscured by the background current but in spite of this, peak ci is further seen on the cathodic branch of the cyclic curve. Moreover, cathodic stripping experiments show that a CuSCN deposit accumulates on the electrode surface, even from solutions with a SCN- concentration as low as 2 X 10-s mol dme3. This phenomenon was exploited for the determination of traces of thiocyanate ions [12]. Figure 2 shows in detail the influence of small variations in SCN- concentration on the shape of the peak corresponding to the formation of the first monolayer. With increasing SCN- concentration, the area under peaks 2-5 was approximately constant corresponding to a charge of 60 PC cmP2. Simultaneously the peak width decreased, the peak height increased, and the spike became better developed. In this range of SCN- concentration, the appearance of the spike depended on the extent of coverage of the electrode surface by the deposit. In solutions with SCN- concentrations equalto 2 X 10e5, 6 X 10-’ and 5 X 10e4 mol dme3, the spike appeared when the coverage attained 84, 70 and 61%, respectively. Figure 3 shows the potentiostatic transients obtained for the solution giving, under voltammetric conditions, curve 2 in Fig. 2. All the transients presented reveal an initial fall. On some of the curves the fall is followed by a shoulder and on others by a small maximum. Note that the characteristic feature of pure nucleation and growth transients is that their current vs. time curves start from zero and exhibit a maximum. Because no oxidized material goes into solution under the conditions used, the oxidation products must be adsorbed on the electrode surface. It is very likely that the maxima and shoulders observed on the potentiostatic transients and the spikes on the voltammograms have the same origin. This origin may be phase transition. Thus, the complex shape of peak a, may be explained by a mechanism
141
Oj6 JJ*
-
I+
_____-__-_____
-Ok
xi.3
61”
Fig. 2. Anodic voltammograms obtained for a 1 x 10v4 mol dme3 copper amalgam electrode in the presence of increasing concentrations of SCN- ion: (1) 1 X 10w5; (2) 2 X 10e5; (3) 3 X 10W5;(4) 4 X lo-‘; (5) 6 x 10K5 mol dmm3. Supporting electrolyte: 1 mol dme3 NaC104, pH 2. Voltage scan rate = 5 mV s-‘. (- - -) Blank test.
that takes into account the adsorption process at the beginning of film formation followed by phase transition and further expansion of crystalline patches. Under conditions where the formation of the first monolayer is completed, the --
t
i ,.
1
2
3
t/s
Fig. 3. -Anodic potentiostatic transients for cuprous thiocyanate formation. Copper amalgam: 1.8 X low4 mol dmd3. Solution: 1 mol dmm3 NaClO,, pH 2, +2X10-’ mol dm-3 NaSCN. Potential steps from -0.9 V to: (1) - 0.305; (2) -0.300; (3) -0.292; (4) -0.290 V.
142
second system of peaks a*-c2 appears on the cyclic curves. This system is markedly drawn out, but attempts to improve its shape by the use of a positive feedback were unsuccessful. Also, the estimations, taking into account the resistance of the circuit and the current that flows, showed that the iR drop component in potential cannot be responsible for the disturbed shape of system a,+. The charges involved in the formation of peaks a2 and c2 are, in practice, the same. Thus, during the formation of peak a2, only solution insoluble products are formed. The charge involved in the formation of peak a2 increased initially with the rise of thiocyanate ion concentration but it attained rather quickly a constant value equal to 70 PC cmP2. Thus, the formation of the second monolayer requires a slightly higher charge than the formation of the first monolayer. The occurrence of hysteresis in system al-c2 clearly indicates that the second monolayer starts to grow at a certain overpotential and under the conditions used, this overpotential must be the nucleation overpotential. Voltammetric
investigation
in solutions with moderate concentrations
of thiocyanate
ions
The voltammetric results obtained for the copper amalgam electrode in solutions with a SCN- concentration in the range 5 X 10W3-5 X 10e2 mol dme3 were similar, apart from the marked displacement of the entire curve towards negative potentials as the SCN- concentration was increased. In this SCN- concentration range no solution soluble products are formed anodically and the heights of peaks a, and a2 are not controlled by diffusion. Therefore, in the experiments described below, this SCN- concentration range was treated as a whole. The effect of varying the potential limit on the shape of the voltammograms obtained in this SCN- concentration range is shown in Fig. 4. For low positive limits (curve 1) the cyclic curve reveals a symmetric deposition-stripping behaviour. Such behaviour fulfils the diagnostic criteria for an adsorption process. Note the quite different shape of curve 1’ obtained under the same conditions but in the absence of SCN- ions. In the latter case, only solution soluble species are formed. Curves 2 and 3 reveal in detail the proper shapes of the spikes. It is evident that in this case the cathodic spike is even higher than the anodic one. In contrast to the symmetric shape of curve 1, the spikes occurring on curves 2 and 3 show some asymmetry. The potentials of the anodic and cathodic spikes are separated by about 10 mV. The separation increases when the potential scan rate is enhanced. The occurrence of spike separation is well explained by the phase transition concept, whereas there are serious difficulties in explaining it on the basis of the pure adsorption mode. The charge calculated from the area under peak a, was equal to 58 PC cmv2. This value was independent of the voltage scan rate. On the other hand, the charge passed before the onset of the spike depended on the voltage scan rate. At scan rates equal to 2.5 and 25 mV s-i, this charge was 50 and 70% of the total charge passed during the formation of the complete layer, respectively. As curve 3’ shows, increasing the copper concentration in amalgam to 2 x 10e3 mol dm -3 led to a shift of the a,-+ system to more negative potentials without essentially
143
I
-0.6
-0.5
-0.6
E/V
Fig. 4. Effect of varying the potential limit on the cyclic voltammograms obtained for a 1 x 10e4 mol dmP3 copper amalgam electrode. Solution: 1 mol dmv3 NaClO,, pH 2,+0.05 mol dme3 NaSCN. (1’) Without NaSCN. (3’) As (3) but with the copper concentration in amalgam enhanced to 2~10~~ mol dm-‘. Voltage scan rate = 5 mV s-l.
changing the shape and height of the peaks. The surface area under peak a, in curve 3’ corresponds to a charge of 60 PC cm-‘. Curve 4 was obtained under conditions where the potential was reversed just before the foot of peak a3. Peaks a, and c2 obtained in this case are markedly better developed than peaks a, and c2 shown earlier in Fig. 1. The differences cannot be caused by the change in the iR drop because in both cases the resistance of the system was the same. The maximal charge involved in the formation of peak a, was equal to 70 PC cmp2 and the same charge was involved in the formation of peak c2. The separation of peaks a2-c2 is noticeably higher (48 mV) than that observed for the spikes in system at-c,. The occurrence of hysteresis indicates that the separation of peaks is mainly connected with the overpotential of the anodic process. Under the conditions used, this overpotential must be the nucleation overpotential. Galvanostatic experiments in solutions with moderate thiocyanate ion concentrations Figure 5 shows the galvanostatic transients obtained under the conditions used previously for obtaining curves 3 and 4 in Fig. 1. It is evident that transients a and b are similar in shape. In both curves the first and second plateaus correspond to the
144 E/V
[
a
4
8
12
'6 t/s
Fig. 5. Galvanostatic transients obtained for anodic formation of the CuSCN deposit. Concentration of copper in amalgam: 1 x 10m4 mol dme3. Solution: 1 mol dme3 NaClO,, pH 2, containing: (a) 5 x 10W3 mol dme3 NaSCN, current density = 5.26 ALAcm -*: (b) 5 x lo-’ mol dmP3 NaSCN, current density = 7.89 pA cm-*. (- - -) Open-circuit transients; (A-F) points of current interruption.
of the first and second monolayers of the CuSCN deposit. Both these monolayers are formed in the underpotential region in comparison with the potential of the bulk CuSCN layer. In both curves the first plateau reveals two striking features. First, no nucleation spike occurs at the beginning of the plateau, whereas such nucleation spikes are clearly apparent at the beginning of the second plateau. Second, the small but distinct “potential spike”, labelled X on the curves, is apparent on the first plateau. The charges passed through the system during the formation of the first and second monolayers are equal to 58 and 75 PC cm-‘, respectively. The charge passed up to the occurrence of the potential spike corresponds to 67% of the total charge involved in the formation of the first monolayer. With an increase of current density, the potential spike is displaced to the position corresponding to higher coverages of the electrode surface by adsorbate. It is reasonable to suppose that the current spikes occurring in the voltammograms and the potential spikes X appearing in the galvanostatic transients have the same origin, i.e. they correspond to rearangement of the disordered adsorbate layer to an ordered crystalline layer. Additional information on the properties of the system studied may be obtained by recording the open-circuit transients [13]. Figure 5b shows several transients of this type. When the circuit is interrupted at point A, i.e. prior to the occurrence of the potential spike, no decay of the potential is observed. Such behaviour supports the opinion that the initial portion of the first plateau is adsorptive in nature. On the other hand, when the current is interrupted beyond the potential spike, a small decay of the potential becomes apparent. Moreover, even for longer periods of time the formation
145
potential does not decay to the equilib~um potential characteristic of the adsorption layer. The difference between the two equilibrium potentials is equal to 8 mV. Decay transients obtained after interruption of the current at points D, E and F, i.e. on the second plateau, tend towards more positive potential values. Potentiostatic investigations in solutions with moderate concentrations of thiocyanate ions Figure 6a shows several potentiostatic transients whereas Fig. 6b gives, for comparison purposes, the voltammetric curve obtained under the same conditions. A potential step into the region before the spike produced monotonic, falling current-time transients of the type shown by curve 1. When the potential step was extended into the region coincident with the potential of the spike the transients developed new features. An initially falling transient was followed by a distinct maximum (curve 2). With further increase of the potential step, the maximum moved to progressively shorter times (curves 2 and 3). Finally a transient with a monotonic decay was again obtained. A similar sequence of transients was also observed for a solution with SCN- concentration equal to 5 X 10m3 mol dmv3 (Fig. 6~). The charge measured during the formation of the entire potentiostatic transient was close to 58 PC cm2. The charge corresponding to the m~mum observed on curve 2 in Fig. 6c was 20 PC cm-‘. Figure 7a shows the potentiostatic transients obtained for the formation of the second CuSCN monolayer. In these transients the maxima are significant, i.e. nearly all charge passed corresponds to the nucleation and growth process. Curves of this type are comparable with the predictions of the theory developed by Bewick et al. [14] for the case of instantaneous or progressive nucleation and growth of a
Fig. 6. Potentiostatic transients obtained for anodic CuSCN formatfon. Concentration of copper in amalgam: 1 x 10m4 mol dmm3. Solution: 1 mol dm-3 NaClO,, pH 2, containing: (a,b) 5 x lo-’ mol dme3 NaSCN; (c) 5x10-’ mol dm-3 NaSCN. (a) Potential steps from -0.8 V to: (1) -0.522; (2) - 0.520; (3) -0.518; (4) -0.516 V. (c) Potential steps from -0.8 V to: (1) - b.460; (2) -0.450; (3) -0.429; (4) -0.420 V.
146
1
2
3
tls
Fig. 7. (a) Potentiostatic transients obtained for the formation of the second CuSCN monolayer. Concentration of copper in amalgam: 1.8~10-~ mol dmm3. Solution: 1 mol dme3 NaClO,, pH 2, +5 ~10~~ mol dme3 NaSCN. Potential steps from -0.8 V to: (1) -0.370; (2) -0.360; (3) -0.348; (4) - 0.340; (5) - 0.326 V. (b) Comparison of experimental points (. . . . ) taken from transient 4 in (a) with the reduced variable plots predicted for: (1) instantaneous and (2) progressive two-dimensional nucleation and growth process.
two-dimensional monolayer. Such a comparison is presented in Fig. 7b. The experimental results obtained for the initial portion of the transient are in good agreement with the theoretical plot valid for an instantaneous two-dimensional nucleation and growth process. On the other hand, the experimental points obtained for longer times deviate markedly from theoretical predictions. The deviations undoubtedly result from the overlap of the current corresponding to the formation of the multilayer. As shown in curve 4 in Fig. 1, the peak corresponding to formation of the second monolayer is located close to the multilayer peak. DISCUSSION
The mechanisms of the formation of anodic films on mercury and amalgam electrodes have been studied many times and the results obtained have been interpreted mainly in terms of nucleation and growth of crystallization centres [15]. However, recently Peter et al. [13] found that the formation of the first monolayer of mercuric sulphide proceeds according to a more complex adsorption-nucleation and growth mechanism. Such an adsorption-nucleation mechanism was proposed earlier by Bewick and co-workers [16-191 for the deposition of some metals on single-crystal silver and copper electrodes. Theoretical models, taking into account the adsorption coupled with nucleation and growth processes, have recently been proposed by Bosco and Rangarajan [20]. These models are capable of predicting the potentiostatic transients with such characteristics as initial fall followed by a maximum or a shoulder. The combined evidence from our voltammetric, galvanostatic and potentiostatic
147
experiments shows conclusively that anodic formation of a single CuSCN monolayer on a copper amalgam electrode is quite consistent with the complex adsorption-nucleation and growth mechanism. The essential feature of this mechanism is that the initial adsorption step is followed by a sharp phase transition process. Adsorption of the CuSCN deposit on the mercury surface seems to be facilitated by the specific adsorption of SCN- ion on this substrate [21,22]. Bilewicz et al. [8], studying the anodic formation of CuSCN deposit by normal and reverse pulse voltammetry, showed that this process may be qualified as SCN- ion induced adsorption of Cu(1) on the mercury surface. With an increase of positive potential, the adsorbed layer transforms, forming a crystal plane by two-dimensional nucleation and growth. The phase transition is potential-dependent in the sense that after reversal of the scan direction the crystal plane transforms again into the adsorption layer. Under voltammetric conditions the phase transition manifests itself by the occurrence of anodic and cathodic spikes. In our case the spikes occurred at moderate coverages of the electrode surface by adsorbed material and because of this they were well developed. On the other hand, in the case studied by Peter et al. [13], the phase transition occurred at high coverage and under these conditions the spikes can easily be overlooked. In the cases studied by Bewick and Thomas [16-181, the phase transition spikes were poorly defined when a polycrystalline silver electrode was used, but they were well defined on a single-crystal silver cathode. Under galvanostatic conditions the nucleation and phase transition processes manifest themselves by the appearance of overpotential spikes. The lack of such a spike at the start of the galvanostatic transient and its appearance after partial coverage of the electrode surface by deposited material may serve as a criterion of the complex adsorption-nucleation and growth mechanism. The overpotential spike appeared on our galvanostatic transients at longer times, i.e. after the electrode surface was covered by some amount of oxidized material, and such behaviour is consistent with the criterion given above. The lack of a nucleation and growth overpotential on the galvanostatic transients can also be detected by recording open-circuit transients. Our open-circuit transients were also consistent with the proposed mechanism. Under potentiostatic conditions a potential step to the adsorption region gave the monotonic, falling transients, whereas the transients obtained for the nucleation region revealed an initial fall followed by a maximum. These results are in excellent accordance with the adsorption-nucleation and growth mechanism. Another example, ascribed in the literature to this mechanism, i.e. the anodic formation of mercuric sulphide [13,23], does not give a lucid, potentiostatic picture of the process. Transients with a shoulder obtained for this case in the adsorption region were explained by Peter et al. [13] as an artefact due to ohmic error in potentiostatic control. None of the three techniques used revealed any evidence of the adsorption step in the formation of the second CuSCN monolayer. This monolayer is formed according to a two-dimensional, instantaneous nucleation and growth mechanism. The charge
148
required for each monolayer of CuSCN deposit is not the same, being smaller for the first (59 f 5 PC cm-*) than for the second (70 f 5 PC cmd2). This difference suggests that the structure of the first monolayer is less closely packed than the second one. Unfortunately, these charges are not comparable with the values calculated from the crystallographic data. Approximate calculations taking into account the density of CuSCN (2.843 g cmw3) gave the value 93 PC cme2. The charge found by Wey et al. [3] for the first anodic peak occurring on a polycrystalline copper electrode in solutions containing moderate concentrations of SCN- ions was approximately 6 mC cme2. This value is two orders of magnitude higher than the value found in our work for a single monolayer of CuSCN. It is apparent that the first anodic peak occurring at the polycrystalline copper electrode corresponds to the formation of the bulk CuSCN deposit. Summing up, it should be mentioned that some of our conclusions based on the drawn-out peaks obtained at low SCN- concentration may raise some objections. Simply, the curves obtained under such conditions have an irreversible shape or a shape distorted by the iR drop. However, under the conditions used, such a drawn-out shape is a natural consequence of the deficiency of the ligands. At higher ligand concentrations the distortion disappears although the resistance of the system, defined mainly by the high concentration of the supporting electrolyte, changes only slightly. The polarographic curves obtained by Kivalo [24] and Schlapp et al. [25] in solutions with low ligand concentrations exhibited similar behaviour. The drawn-out shape of the cyclic voltammetric curves is also postulated by the theory developed by Shuman [26] for a reversible charge transfer process followed by a reversible chemical reaction of order different from 1. ACKNOWLEDGEMENT
This work was done as a partial fulfiment of Project MR-I-11. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
L.H. Leidheiser, Jr., The Corrosion of Copper, Tin and Their Alloys, Wiley, New York, 1971. 2. Dinnappa, H.B. Rudresh and N. Mayamra, Surf. Technol., 10 (1980) 363. A.Wey, M. Abramovich and C.V. D’Alkaine, J. Electroanal. Chem., 165 (1984) 147. V.K. Altukov, I.K. Vorontsov, I.K. Karshikor and T.N. Klepinina, Izv. Vyssh. Uchebn. Zaved. Kbim. Kbim. Tekhnol., 18 (1975) 1338. K. Tennakone, M. Kahanda, C. Kasige, P. Abeysooriya, R.H. Wijayanayaka and P. Kaviratna, J. Electrochem. So-c., 131 (1984) 1574. S. Fletcher, R.G. Barradas and J.D. Porter, J. Electrochem. Sot., 125 (1978) 1960. I.M. Kolthoff and Y. Okinaka, J. Am. Chem. Sot., 82 (1960) 3528. R. Bilewicz, Z. Stojek, Z. Kublik and J. Osteryoung, J. Electroanal. Chem., 137 (1982) 77. G. Geblewicz, Thesis, Department of Chemistry, Warsaw University, 1983. W. Kemula and Z. Kubhk, Anal. Chim. A_cta, 18 (1958) 104. S. Glodowski and Z. Kublik, Anal. Chim. Acta, 149 (1983) 137. R. Bilewicz and Z. Kublik, Anal. Chim. Acta, 123 (1981) 201. L.M. Peter, J.D. Reid and B.R. Scharifker, J. Electroanal. Chem., 119 (1981) 73.
149 14 A. Bewick, M. Fleishmarm and H.R. Thirsk, Trans. Faraday Sot., 58 (1962) 2200. 15 J.A. Harrison and H.R. Thirsk in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 5, Marcel Dekker, New York, 1971, p. 67. 16 A. Bewick and B. Thomas, J. Electroanal. Chem., 65 (1975) 911. 17 A. Bewick and B. Thomas, J. Electroanal. Chem., 84 (1977) 127. 18 A. Bewick and B. Thomas, J. Electroanal. Chem., 85 (1977) 329. 19 A. Bewick, J. jovicevic and B. Thomas, Faraday Symp. Chem. Sot., 12 (1977) 24. 20 E. Bosco and S.K. Rangarajan, J. Chem. Sot., Faraday Trans. 1, 77 (1981) 1673. 21 S. Mint and J. Andrzejczak, J. Electroanal. Chem., 17 (1968) 101. 22 R. Parsons and P.C. Symons, Trans. Faraday Sot., 64 (1968) 1077. 23 ic.D. Armstrong, D.F. Porter and H.R. Thirsk, J. Phys. Chem., 72 (1968) 2300. 24 P. Kivalo, J. Am. Chem. Sot., 77 (1955) 2678. 25 O.E. Schlapp, T.Y. Youness and J.F. Wotles, J. Am. Chem. Sot., 84 (1962) 505. 26 M.K. Shuman, Anal. Chem., 42 (1970) 521.
ADSORPTION-NUCLEATION MONOLAYER FORMATION THIOCYANATE SOLUTIONS
RENATA
BILEWICZ
Department (Received
and ZENON
of Chemistry,
AND GROWTH MECHANISM OF CuSCN ON A COPPER AMALGAM ELECTRODE
137
IN
KUBLIK
University of Warsaw, 02093 Warsaw, Pasteura I (Poland)
27th March 1985)
ABSTRACT
Using voltammetric, galvanostatic and potentiostatic techniques, the electrodeposition of cuprous thiocyanate on a copper amalgam electrode has been studied in acidic solutions containing various thiocyanate ion concentrations. Under the conditions used, the formation of two successive monolayers of CuSCN could be studied before the onset of bulk deposition. It was found that the formation of the first monolayer proceeds according to a complex adsorption-nucleation and growth mechanism. In the first stage, cuprous thiocyanate is adsorbed on the-amalgam electrode; in the second, the disordered layer of adsorbate rearranges to form two-dimensional crystalline centres; and in the third stage, these centres grow, completing the formation of the entire monolayer. The second monolayer is formed in a much simpler way, according to a mechanism of two-dimensional, instantaneous nucleation and growth. At high thiocyanate ion concentration, the formation of the CuSCN layer is obscured by the extensive formation of solution soluble species.
INTRODUCTION
The corrosion of copper in various solutions is an important problem from an engineering point of view [l]. The corrosion rate is, under certain conditions, inhibited in the presence of thiocyanate ions [2]. However, there have been few attempts to elucidate the mechanisms of copper corrosion and copper anodic oxidation in solutions containing thiocyanate ions [2-41. The mechanisms of the formation of CuSCN deposit on a solid substrate are also interesting from the point of view of solar energy conversion [5]. In order to facilitate the investigation of the mechanisms of the anodic dissolution of metals, samples of solid metals are sometimes replaced by the appropriate amalgams [6]. The aim of the present work was to elucidate the mechanism of the formation of anodic deposits on a copper amalgam electrode in solutions containing SCN- ions. As yet, such a problem has not been discussed in the literature, although the system Cu(II)-Cu(I)-Cu(0) has been studied in thiocyanate solutions under polarographic [7] and voltammetric [S] conditions. Voltammetric, galvanostatic and potentiostatic techniques were used to obtain the characteristics of the CuSCN anodic deposits. 0022-0728/85/%03.30
0 1985 E%e.viex Sequoia S.A.
138 EXPERIMENTAL
The voltammetric curves were recorded using a set-up consisting of an Elpan Model EG-200 voltage scan generator, a PAR Model 173 potentiostat equipped with a Model 176 current-to-voltage converter and a Riken Denshi F-3C X-Y recorder. In potentiostatic and coulometric experiments the potentiostat PAR Model 173 was used in conjunction with a Model 179 coulometer. Galvanostatic experiments were performed with a home-made galvanostat [9]. A saturated calomel electrode was used as the reference electrode and the potentials are referred to this electrode. The counter-electrode was a 2 cm2 platinum foil. An HMDE of the type described by Kemula and Kublik [lo], filled with copper amalgam, was used as the indicating electrode. The surface area of the drop electrode used was 0.024 cm2. Copper amalgam was prepared by dissolving a weighed sample of A.R. copper in an appropriate volume of mercury. Copper amalgam is not stable even in deaerated solutions, where only traces of oxygen are present [ll]. The concentration of copper in the amalgam was checked on the basis of the Sevtiik-Randles equation from the height of the copper dissolution peak. The resistance of the system, with 1 mol dmW3 sodium perchlorate solution as the supporting electrolyte, was close to 12 Q. The currents flowing in the experiments described in this paper were not higher than 1 PA. Thus, the iR drop components should be insignificant in our experiments. In some cases a positive feedback arrangement was used to compensate for the iR drop component but in no case did the use of this arrangement lead to improvement in the shapes of the curves. All solutions used were prepared from A.R. chemicals and triply-distilled water. In some cases the thiocyanate solutions were additionally purified by electrolysis at a large mercury cathode. Solutions were deoxygenated prior to recording the curves. All the experiments were carried out at 25 f 0.5”C. RESULTS
General characteristics of the Cu(Hg)-Cu(I) system in thiocyanate media For this study cyclic voltammetry was chosen as the most convenient technique. In the first series of the experiments cyclic voltammetric curves were recorded with a constant concentration of copper in the amalgam and with different concentrations of thiocyanate ions in the solution. Some of these curves are presented in Fig. 1. Curve 1, obtained at low thiocyanate ion concentration, shows a single system of peaks labelled a, and cl. Under the conditions used, the anodic process is controlled by the diffusion of SCN- ions. On the other hand, the cathodic process shows a significant deviation from the diffusion-controlled shape. The cathodic current is markedly enhanced, in comparison with the anodic one, and after crossing the maximum it decays abruptly. A slight increase of SCN- concentration (curve 2) leads to enhancement of peak a, and to the occurrence of two new effects. First, a very small anodic spike, a;,
139
-cl6
-a5
-0.4
-0.3
-‘I’
E[V
Fig. 1. Cyclic voltammograms obtained for _a 1 X 10m4 mol drnF3 copper amalgamelectrode in the presence of increasing concentrations of SCW ion: (1) 1 X 10e5; (2) 2X 10e5; (3) 5 X 10w3; (4) 5 X lo-’ mol dmw3. Supporting electrolyte: 1 mol dmm3 NaClO,, pH 2. Voltage scan rate = 5mV s-l.
occurs at the descending portion of the anodic branch and just after this spike the current decays abruptly to zero, i.e. the electrode becomes passive. Then, at more positive potentials, a second, markedly drawn out, system of peaks, a2-c2, occurs. At still higher thiocyanate ion concentration (curve 3) the anodic branch of the curve becomes less drawn out. The spike a; is now higher and is located very close to peak aI. Under these conditions, peak a2 forms a shoulder on the markedly higher and broader peak a3. Under the conditions used, the height of peak a3 starts to become controlled by diffusion of copper in the amalgam, whereas the heights of peaks aI and a2 are controlled by other factors. The product formed in peak a3 is reduced irreversibly but the charges passing in the anodic and cathodic processes are the same. Thus, at this thiocyanate ion con~ntration during the positive scan, only insoluble species are formed. Moreover, a ten-fold increase of SCN- concentration (curve 4) only slightly affected the height of peak a3 but led to better separation of peaks a2 and a3 and to better development of spike a;. Also, under these conditions, the species formed during the anodic process were completely reduced during the subsequent negative run. This finding is consistent with the data reported by Altukov et al. 141,who, using a rotating ring-disc electrode, studied the properties of the products of copper oxidation in thiocyanate solutions. At SCN- concentrations higher than 0.1 mol dmV3, soluble products start to occur during the positive scan; investigation of these processes was beyond the scope of the present work.
140
In parallel with the increase of SCN- ion concentration, a systematic shift of the entire curve towards negative potentials was observed. The slope of the dependence of log i vs. E, for increasing SCN- concentration, was equal to 60 mV when the potential was measured at the foot of the first anodic peak. Such behaviour confirms the literature results that the first anodic peak obtained at a copper amalgam electrode in thiocyanate solutions corresponds to the formation of copper (I) species. An increase of copper concentration in amalgam from 1 X 10d4 to 2 X 10m3 mol dmv3 led to a shift of the entire cyclic curve towards more negative potentials without essentially changing the shape and height of the al-c, and a2-c1 systems. The behaviour described above did not alter essentially when the pH of the solution was varied in the range l-5. Investigation in solutions containing low thiocyanate ion concentrations The cathodic peak ci occurring in curve 1 of Fig. 1 is significantly higher than the anodic peak a,, indicating that the product formed in the anodic process accumulates on the electrode surface. The charges involved in the formation of the anodic and cathodic peaks are the same, even on the curves obtained in stirred solutions, and this means that no solution soluble species are formed during the anodic process. With a decrease of SCN- concentration, peak a, becomes obscured by the background current but in spite of this, peak ci is further seen on the cathodic branch of the cyclic curve. Moreover, cathodic stripping experiments show that a CuSCN deposit accumulates on the electrode surface, even from solutions with a SCN- concentration as low as 2 X 10-s mol dme3. This phenomenon was exploited for the determination of traces of thiocyanate ions [12]. Figure 2 shows in detail the influence of small variations in SCN- concentration on the shape of the peak corresponding to the formation of the first monolayer. With increasing SCN- concentration, the area under peaks 2-5 was approximately constant corresponding to a charge of 60 PC cmP2. Simultaneously the peak width decreased, the peak height increased, and the spike became better developed. In this range of SCN- concentration, the appearance of the spike depended on the extent of coverage of the electrode surface by the deposit. In solutions with SCN- concentrations equalto 2 X 10e5, 6 X 10-’ and 5 X 10e4 mol dme3, the spike appeared when the coverage attained 84, 70 and 61%, respectively. Figure 3 shows the potentiostatic transients obtained for the solution giving, under voltammetric conditions, curve 2 in Fig. 2. All the transients presented reveal an initial fall. On some of the curves the fall is followed by a shoulder and on others by a small maximum. Note that the characteristic feature of pure nucleation and growth transients is that their current vs. time curves start from zero and exhibit a maximum. Because no oxidized material goes into solution under the conditions used, the oxidation products must be adsorbed on the electrode surface. It is very likely that the maxima and shoulders observed on the potentiostatic transients and the spikes on the voltammograms have the same origin. This origin may be phase transition. Thus, the complex shape of peak a, may be explained by a mechanism
141
Oj6 JJ*
-
I+
_____-__-_____
-Ok
xi.3
61”
Fig. 2. Anodic voltammograms obtained for a 1 x 10v4 mol dme3 copper amalgam electrode in the presence of increasing concentrations of SCN- ion: (1) 1 X 10w5; (2) 2 X 10e5; (3) 3 X 10W5;(4) 4 X lo-‘; (5) 6 x 10K5 mol dmm3. Supporting electrolyte: 1 mol dme3 NaC104, pH 2. Voltage scan rate = 5 mV s-‘. (- - -) Blank test.
that takes into account the adsorption process at the beginning of film formation followed by phase transition and further expansion of crystalline patches. Under conditions where the formation of the first monolayer is completed, the --
t
i ,.
1
2
3
t/s
Fig. 3. -Anodic potentiostatic transients for cuprous thiocyanate formation. Copper amalgam: 1.8 X low4 mol dmd3. Solution: 1 mol dmm3 NaClO,, pH 2, +2X10-’ mol dm-3 NaSCN. Potential steps from -0.9 V to: (1) - 0.305; (2) -0.300; (3) -0.292; (4) -0.290 V.
142
second system of peaks a*-c2 appears on the cyclic curves. This system is markedly drawn out, but attempts to improve its shape by the use of a positive feedback were unsuccessful. Also, the estimations, taking into account the resistance of the circuit and the current that flows, showed that the iR drop component in potential cannot be responsible for the disturbed shape of system a,+. The charges involved in the formation of peaks a2 and c2 are, in practice, the same. Thus, during the formation of peak a2, only solution insoluble products are formed. The charge involved in the formation of peak a2 increased initially with the rise of thiocyanate ion concentration but it attained rather quickly a constant value equal to 70 PC cmP2. Thus, the formation of the second monolayer requires a slightly higher charge than the formation of the first monolayer. The occurrence of hysteresis in system al-c2 clearly indicates that the second monolayer starts to grow at a certain overpotential and under the conditions used, this overpotential must be the nucleation overpotential. Voltammetric
investigation
in solutions with moderate concentrations
of thiocyanate
ions
The voltammetric results obtained for the copper amalgam electrode in solutions with a SCN- concentration in the range 5 X 10W3-5 X 10e2 mol dme3 were similar, apart from the marked displacement of the entire curve towards negative potentials as the SCN- concentration was increased. In this SCN- concentration range no solution soluble products are formed anodically and the heights of peaks a, and a2 are not controlled by diffusion. Therefore, in the experiments described below, this SCN- concentration range was treated as a whole. The effect of varying the potential limit on the shape of the voltammograms obtained in this SCN- concentration range is shown in Fig. 4. For low positive limits (curve 1) the cyclic curve reveals a symmetric deposition-stripping behaviour. Such behaviour fulfils the diagnostic criteria for an adsorption process. Note the quite different shape of curve 1’ obtained under the same conditions but in the absence of SCN- ions. In the latter case, only solution soluble species are formed. Curves 2 and 3 reveal in detail the proper shapes of the spikes. It is evident that in this case the cathodic spike is even higher than the anodic one. In contrast to the symmetric shape of curve 1, the spikes occurring on curves 2 and 3 show some asymmetry. The potentials of the anodic and cathodic spikes are separated by about 10 mV. The separation increases when the potential scan rate is enhanced. The occurrence of spike separation is well explained by the phase transition concept, whereas there are serious difficulties in explaining it on the basis of the pure adsorption mode. The charge calculated from the area under peak a, was equal to 58 PC cmv2. This value was independent of the voltage scan rate. On the other hand, the charge passed before the onset of the spike depended on the voltage scan rate. At scan rates equal to 2.5 and 25 mV s-i, this charge was 50 and 70% of the total charge passed during the formation of the complete layer, respectively. As curve 3’ shows, increasing the copper concentration in amalgam to 2 x 10e3 mol dm -3 led to a shift of the a,-+ system to more negative potentials without essentially
143
I
-0.6
-0.5
-0.6
E/V
Fig. 4. Effect of varying the potential limit on the cyclic voltammograms obtained for a 1 x 10e4 mol dmP3 copper amalgam electrode. Solution: 1 mol dmv3 NaClO,, pH 2,+0.05 mol dme3 NaSCN. (1’) Without NaSCN. (3’) As (3) but with the copper concentration in amalgam enhanced to 2~10~~ mol dm-‘. Voltage scan rate = 5 mV s-l.
changing the shape and height of the peaks. The surface area under peak a, in curve 3’ corresponds to a charge of 60 PC cm-‘. Curve 4 was obtained under conditions where the potential was reversed just before the foot of peak a3. Peaks a, and c2 obtained in this case are markedly better developed than peaks a, and c2 shown earlier in Fig. 1. The differences cannot be caused by the change in the iR drop because in both cases the resistance of the system was the same. The maximal charge involved in the formation of peak a, was equal to 70 PC cmp2 and the same charge was involved in the formation of peak c2. The separation of peaks a2-c2 is noticeably higher (48 mV) than that observed for the spikes in system at-c,. The occurrence of hysteresis indicates that the separation of peaks is mainly connected with the overpotential of the anodic process. Under the conditions used, this overpotential must be the nucleation overpotential. Galvanostatic experiments in solutions with moderate thiocyanate ion concentrations Figure 5 shows the galvanostatic transients obtained under the conditions used previously for obtaining curves 3 and 4 in Fig. 1. It is evident that transients a and b are similar in shape. In both curves the first and second plateaus correspond to the
144 E/V
[
a
4
8
12
'6 t/s
Fig. 5. Galvanostatic transients obtained for anodic formation of the CuSCN deposit. Concentration of copper in amalgam: 1 x 10m4 mol dme3. Solution: 1 mol dme3 NaClO,, pH 2, containing: (a) 5 x 10W3 mol dme3 NaSCN, current density = 5.26 ALAcm -*: (b) 5 x lo-’ mol dmP3 NaSCN, current density = 7.89 pA cm-*. (- - -) Open-circuit transients; (A-F) points of current interruption.
of the first and second monolayers of the CuSCN deposit. Both these monolayers are formed in the underpotential region in comparison with the potential of the bulk CuSCN layer. In both curves the first plateau reveals two striking features. First, no nucleation spike occurs at the beginning of the plateau, whereas such nucleation spikes are clearly apparent at the beginning of the second plateau. Second, the small but distinct “potential spike”, labelled X on the curves, is apparent on the first plateau. The charges passed through the system during the formation of the first and second monolayers are equal to 58 and 75 PC cm-‘, respectively. The charge passed up to the occurrence of the potential spike corresponds to 67% of the total charge involved in the formation of the first monolayer. With an increase of current density, the potential spike is displaced to the position corresponding to higher coverages of the electrode surface by adsorbate. It is reasonable to suppose that the current spikes occurring in the voltammograms and the potential spikes X appearing in the galvanostatic transients have the same origin, i.e. they correspond to rearangement of the disordered adsorbate layer to an ordered crystalline layer. Additional information on the properties of the system studied may be obtained by recording the open-circuit transients [13]. Figure 5b shows several transients of this type. When the circuit is interrupted at point A, i.e. prior to the occurrence of the potential spike, no decay of the potential is observed. Such behaviour supports the opinion that the initial portion of the first plateau is adsorptive in nature. On the other hand, when the current is interrupted beyond the potential spike, a small decay of the potential becomes apparent. Moreover, even for longer periods of time the formation
145
potential does not decay to the equilib~um potential characteristic of the adsorption layer. The difference between the two equilibrium potentials is equal to 8 mV. Decay transients obtained after interruption of the current at points D, E and F, i.e. on the second plateau, tend towards more positive potential values. Potentiostatic investigations in solutions with moderate concentrations of thiocyanate ions Figure 6a shows several potentiostatic transients whereas Fig. 6b gives, for comparison purposes, the voltammetric curve obtained under the same conditions. A potential step into the region before the spike produced monotonic, falling current-time transients of the type shown by curve 1. When the potential step was extended into the region coincident with the potential of the spike the transients developed new features. An initially falling transient was followed by a distinct maximum (curve 2). With further increase of the potential step, the maximum moved to progressively shorter times (curves 2 and 3). Finally a transient with a monotonic decay was again obtained. A similar sequence of transients was also observed for a solution with SCN- concentration equal to 5 X 10m3 mol dmv3 (Fig. 6~). The charge measured during the formation of the entire potentiostatic transient was close to 58 PC cm2. The charge corresponding to the m~mum observed on curve 2 in Fig. 6c was 20 PC cm-‘. Figure 7a shows the potentiostatic transients obtained for the formation of the second CuSCN monolayer. In these transients the maxima are significant, i.e. nearly all charge passed corresponds to the nucleation and growth process. Curves of this type are comparable with the predictions of the theory developed by Bewick et al. [14] for the case of instantaneous or progressive nucleation and growth of a
Fig. 6. Potentiostatic transients obtained for anodic CuSCN formatfon. Concentration of copper in amalgam: 1 x 10m4 mol dmm3. Solution: 1 mol dm-3 NaClO,, pH 2, containing: (a,b) 5 x lo-’ mol dme3 NaSCN; (c) 5x10-’ mol dm-3 NaSCN. (a) Potential steps from -0.8 V to: (1) -0.522; (2) - 0.520; (3) -0.518; (4) -0.516 V. (c) Potential steps from -0.8 V to: (1) - b.460; (2) -0.450; (3) -0.429; (4) -0.420 V.
146
1
2
3
tls
Fig. 7. (a) Potentiostatic transients obtained for the formation of the second CuSCN monolayer. Concentration of copper in amalgam: 1.8~10-~ mol dmm3. Solution: 1 mol dme3 NaClO,, pH 2, +5 ~10~~ mol dme3 NaSCN. Potential steps from -0.8 V to: (1) -0.370; (2) -0.360; (3) -0.348; (4) - 0.340; (5) - 0.326 V. (b) Comparison of experimental points (. . . . ) taken from transient 4 in (a) with the reduced variable plots predicted for: (1) instantaneous and (2) progressive two-dimensional nucleation and growth process.
two-dimensional monolayer. Such a comparison is presented in Fig. 7b. The experimental results obtained for the initial portion of the transient are in good agreement with the theoretical plot valid for an instantaneous two-dimensional nucleation and growth process. On the other hand, the experimental points obtained for longer times deviate markedly from theoretical predictions. The deviations undoubtedly result from the overlap of the current corresponding to the formation of the multilayer. As shown in curve 4 in Fig. 1, the peak corresponding to formation of the second monolayer is located close to the multilayer peak. DISCUSSION
The mechanisms of the formation of anodic films on mercury and amalgam electrodes have been studied many times and the results obtained have been interpreted mainly in terms of nucleation and growth of crystallization centres [15]. However, recently Peter et al. [13] found that the formation of the first monolayer of mercuric sulphide proceeds according to a more complex adsorption-nucleation and growth mechanism. Such an adsorption-nucleation mechanism was proposed earlier by Bewick and co-workers [16-191 for the deposition of some metals on single-crystal silver and copper electrodes. Theoretical models, taking into account the adsorption coupled with nucleation and growth processes, have recently been proposed by Bosco and Rangarajan [20]. These models are capable of predicting the potentiostatic transients with such characteristics as initial fall followed by a maximum or a shoulder. The combined evidence from our voltammetric, galvanostatic and potentiostatic
147
experiments shows conclusively that anodic formation of a single CuSCN monolayer on a copper amalgam electrode is quite consistent with the complex adsorption-nucleation and growth mechanism. The essential feature of this mechanism is that the initial adsorption step is followed by a sharp phase transition process. Adsorption of the CuSCN deposit on the mercury surface seems to be facilitated by the specific adsorption of SCN- ion on this substrate [21,22]. Bilewicz et al. [8], studying the anodic formation of CuSCN deposit by normal and reverse pulse voltammetry, showed that this process may be qualified as SCN- ion induced adsorption of Cu(1) on the mercury surface. With an increase of positive potential, the adsorbed layer transforms, forming a crystal plane by two-dimensional nucleation and growth. The phase transition is potential-dependent in the sense that after reversal of the scan direction the crystal plane transforms again into the adsorption layer. Under voltammetric conditions the phase transition manifests itself by the occurrence of anodic and cathodic spikes. In our case the spikes occurred at moderate coverages of the electrode surface by adsorbed material and because of this they were well developed. On the other hand, in the case studied by Peter et al. [13], the phase transition occurred at high coverage and under these conditions the spikes can easily be overlooked. In the cases studied by Bewick and Thomas [16-181, the phase transition spikes were poorly defined when a polycrystalline silver electrode was used, but they were well defined on a single-crystal silver cathode. Under galvanostatic conditions the nucleation and phase transition processes manifest themselves by the appearance of overpotential spikes. The lack of such a spike at the start of the galvanostatic transient and its appearance after partial coverage of the electrode surface by deposited material may serve as a criterion of the complex adsorption-nucleation and growth mechanism. The overpotential spike appeared on our galvanostatic transients at longer times, i.e. after the electrode surface was covered by some amount of oxidized material, and such behaviour is consistent with the criterion given above. The lack of a nucleation and growth overpotential on the galvanostatic transients can also be detected by recording open-circuit transients. Our open-circuit transients were also consistent with the proposed mechanism. Under potentiostatic conditions a potential step to the adsorption region gave the monotonic, falling transients, whereas the transients obtained for the nucleation region revealed an initial fall followed by a maximum. These results are in excellent accordance with the adsorption-nucleation and growth mechanism. Another example, ascribed in the literature to this mechanism, i.e. the anodic formation of mercuric sulphide [13,23], does not give a lucid, potentiostatic picture of the process. Transients with a shoulder obtained for this case in the adsorption region were explained by Peter et al. [13] as an artefact due to ohmic error in potentiostatic control. None of the three techniques used revealed any evidence of the adsorption step in the formation of the second CuSCN monolayer. This monolayer is formed according to a two-dimensional, instantaneous nucleation and growth mechanism. The charge
148
required for each monolayer of CuSCN deposit is not the same, being smaller for the first (59 f 5 PC cm-*) than for the second (70 f 5 PC cmd2). This difference suggests that the structure of the first monolayer is less closely packed than the second one. Unfortunately, these charges are not comparable with the values calculated from the crystallographic data. Approximate calculations taking into account the density of CuSCN (2.843 g cmw3) gave the value 93 PC cme2. The charge found by Wey et al. [3] for the first anodic peak occurring on a polycrystalline copper electrode in solutions containing moderate concentrations of SCN- ions was approximately 6 mC cme2. This value is two orders of magnitude higher than the value found in our work for a single monolayer of CuSCN. It is apparent that the first anodic peak occurring at the polycrystalline copper electrode corresponds to the formation of the bulk CuSCN deposit. Summing up, it should be mentioned that some of our conclusions based on the drawn-out peaks obtained at low SCN- concentration may raise some objections. Simply, the curves obtained under such conditions have an irreversible shape or a shape distorted by the iR drop. However, under the conditions used, such a drawn-out shape is a natural consequence of the deficiency of the ligands. At higher ligand concentrations the distortion disappears although the resistance of the system, defined mainly by the high concentration of the supporting electrolyte, changes only slightly. The polarographic curves obtained by Kivalo [24] and Schlapp et al. [25] in solutions with low ligand concentrations exhibited similar behaviour. The drawn-out shape of the cyclic voltammetric curves is also postulated by the theory developed by Shuman [26] for a reversible charge transfer process followed by a reversible chemical reaction of order different from 1. ACKNOWLEDGEMENT
This work was done as a partial fulfiment of Project MR-I-11. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
L.H. Leidheiser, Jr., The Corrosion of Copper, Tin and Their Alloys, Wiley, New York, 1971. 2. Dinnappa, H.B. Rudresh and N. Mayamra, Surf. Technol., 10 (1980) 363. A.Wey, M. Abramovich and C.V. D’Alkaine, J. Electroanal. Chem., 165 (1984) 147. V.K. Altukov, I.K. Vorontsov, I.K. Karshikor and T.N. Klepinina, Izv. Vyssh. Uchebn. Zaved. Kbim. Kbim. Tekhnol., 18 (1975) 1338. K. Tennakone, M. Kahanda, C. Kasige, P. Abeysooriya, R.H. Wijayanayaka and P. Kaviratna, J. Electrochem. So-c., 131 (1984) 1574. S. Fletcher, R.G. Barradas and J.D. Porter, J. Electrochem. Sot., 125 (1978) 1960. I.M. Kolthoff and Y. Okinaka, J. Am. Chem. Sot., 82 (1960) 3528. R. Bilewicz, Z. Stojek, Z. Kublik and J. Osteryoung, J. Electroanal. Chem., 137 (1982) 77. G. Geblewicz, Thesis, Department of Chemistry, Warsaw University, 1983. W. Kemula and Z. Kubhk, Anal. Chim. A_cta, 18 (1958) 104. S. Glodowski and Z. Kublik, Anal. Chim. Acta, 149 (1983) 137. R. Bilewicz and Z. Kublik, Anal. Chim. Acta, 123 (1981) 201. L.M. Peter, J.D. Reid and B.R. Scharifker, J. Electroanal. Chem., 119 (1981) 73.
149 14 A. Bewick, M. Fleishmarm and H.R. Thirsk, Trans. Faraday Sot., 58 (1962) 2200. 15 J.A. Harrison and H.R. Thirsk in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 5, Marcel Dekker, New York, 1971, p. 67. 16 A. Bewick and B. Thomas, J. Electroanal. Chem., 65 (1975) 911. 17 A. Bewick and B. Thomas, J. Electroanal. Chem., 84 (1977) 127. 18 A. Bewick and B. Thomas, J. Electroanal. Chem., 85 (1977) 329. 19 A. Bewick, J. jovicevic and B. Thomas, Faraday Symp. Chem. Sot., 12 (1977) 24. 20 E. Bosco and S.K. Rangarajan, J. Chem. Sot., Faraday Trans. 1, 77 (1981) 1673. 21 S. Mint and J. Andrzejczak, J. Electroanal. Chem., 17 (1968) 101. 22 R. Parsons and P.C. Symons, Trans. Faraday Sot., 64 (1968) 1077. 23 ic.D. Armstrong, D.F. Porter and H.R. Thirsk, J. Phys. Chem., 72 (1968) 2300. 24 P. Kivalo, J. Am. Chem. Sot., 77 (1955) 2678. 25 O.E. Schlapp, T.Y. Youness and J.F. Wotles, J. Am. Chem. Sot., 84 (1962) 505. 26 M.K. Shuman, Anal. Chem., 42 (1970) 521.