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A study by voltammetry and the photocurrent response method of copper electrode behavior in acidic and alkaline solutions containing chloride ions
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Journal of Electroanalytical Chemistry 380 (1995) 63 68
A study by voltammetry and the photocurrent response method of copper electrode behavior in acidic and alkaline solutions containing chloride ions A.D. Modestov ,,1, Guo-Ding Zhou a, H o n g - H u a Ge ", B.H. Loo b a Electrochemical Research Group, Shanghai Institute of Electric Power, Shanghai 200090, People's Republic of China b Department of Chemistry, Uniuersity of Alabama in Huntst:ille, Huntst,ille, AL 35899, USA, and Department of Chemisto', National Unicersity of Singapore, Kent Ridge, S.0511, Singapore Received 4 January 1994; in revised form 18 May 1994
Abstract
The electrochemical behavior of Cu electrodes in Cl solutions was studied in a wide range of pH. The results were compared with those obtained in solutions containing F-, Br-, I - and SO42 ions at pH 8.5, and discussed in terms of the competitive formation of Cu20 and CuC1 films on the Cu surface and the influence of CuCI on the properties of Cu20. At pH 8.5 or higher, Cu20 was formed first, whereas at pH 5.7 or lower the Cu20 film was formed on the Cu surface under the CuC1 layer which was formed initially. It is believed that the Cu20 films doped with C1 ions exhibited poor protective properties against Cu corrosion.
Keywords: Voltammetry; Photocurrent response; Cu electrodes 1. Introduction
Photoelectrochemical methods based on measurements of potential or current changes due to illumination have been widely used in the study of corrosion processes which involve the formation of semiconducting passive layers [1-11]. Identification of the semiconducting film is made possible from its photoresponse spectrum [4-8,10]. The passivity of Cu in basic solutions containing no C1 ions was ascribed to the formation of a thin C u 2 0 semiconducting film covered by a layer of C u O / C u ( O H ) 2 [2-4,6,7,12-19]. Crystalline C u 2 0 is a semiconductor with p-type conductivity and a band-gap e n e r g y (Eg) of 2.2-2.3 eV [20,21]. Studies of the cathodic photocurrent responses of C u 2 0 films of a few nanometers thickness, formed electrochemically in basic solutions [15], yielded band-gaps ( E g ) o f 3 and 2.3 eV for the direct and indirect transitions, respectively. For thin C u 2 0 films (about 1 rim), the influence of indirect transitions on the photoresponse spectrum was negligible. When the film thickness was
1present address: A.N. Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Moscow 117071, Russia. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 4 ) 0 3 5 7 7 - P
increased to 40 nm, the Eg value became 2.4 and 2 eV for the direct and indirect transitions, respectively [16]. The CuzO films formed on Cu by corrosion in neutral and weakly acidic media, in most cases, were characterized by an n-type photoresponse [6]. However, both n and p types of photoresponses have been reported from C u 2 0 film, depending on the way the film was prepared [22,23]. The passivity of Cu electrodes in acidic solutions containing C1 ions was attributed to the formation of a CuC1 surface layer [24-28]. In borax buffer solution containing C1 ions (pH 9), the electrochemical behavior was treated as a competition between the formation of a C u 2 0 film which brought about the passivity, and the nucleation and growth of a CuC1 layer which resulted in pitting [29]. Cu corrosion in neutral and weakly acidic media containing C1 ions resulted in the formation of passive layers with photoresponses essentially of the n type [2,6,13,14]. A semiconducting layer consisting of CuCI or Cu 2O was proposed to be formed on the electrode surface [2]. The Eg value determined for the layer was 2.7 eV, which was less than t h e Eg of 3.3 eV for the crystalline CuCI [21]. It was also proposed that the protective properties of the C u 2 0 film depended on its electronic properties, and they deteri-
64
A.D. Modestov et al. /Journal of Electroanalytical Chemistry 380 (1995) 63-68 ,?,
orated with an increase in the film conductivity [2]. Anodic photocurrents were observed at Cu electrodes in H 3 P O 4 [30,31]. The goal of the present work is to study the nature of Cu passivity in Cl--containing electrolytes in a wide range of p H by monitoring the formation of C u 2 0 with voltammetry and photocurrent measurements. The photoelectrochemical behavior of Cu electrodes in F - , B r - , I - and SO42- electrolytes was also studied, and is compared with that observed in the C1--containing solutions. Spectral measurements were also performed in order to identify the composition of the semiconducting films formed on the electrode surfaces.
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2. Experimental The details of the experimental set-up and sample preparation are given elsewhere [32]. All experiments were performed using fresh electrodes which were activated in a working electrolyte by a potentiostatic treatment for 5 rain in the region of hydrogen evolution at 2 - 5 m A cm 2. The following buffer solutions were used: 0.2 M KHPhth + 0.1 M N a O H , p H 4-5.7; 0.2 M K H 2 P O 4 + 0.1 M N a O H , p H 8; 0.1 M Na2B407 + 0.15 M H3BO3; 0.1 M NazB407 + 0.1 M N a O H , p H 10-12. The second component of the buffer solution was added so as to reach the desired p H value. The p H 2 solution was achieved by adding HC1 to 0.5 M NaC1 solution. The photocurrent measurements were conducted using the light modulation method, and only the real component was recorded. All the photocurrent action spectra are reported in arbitrary units of quantum efficiency. The potentials are reported with respect to a saturated calomel electrode (SCE).
Fig. 1. P h o t o c u r r e n t - p o t e n t i a l curves (1) and v o l t a m m o g r a m s (2) in 0.5 M NaCI (scan rate, 1 m V s - t ; m o d u l a t i o n frequency, 230 Hz; i r r a d i a t i o n w a v e l e n g t h , 400 nm): (a) p H 2.0; (b) p H 8.0. The scans w e r e i n i t i a t e d at - 0 . 5 V.
formation of C u 2 0 o n Cu electrodes in the p H range 5-12.9 [19], it may be concluded that CuCI was formed first in acidic solutions, whereas C u 2 0 was formed first in basic solutions. The voltammograms shown in curves (2) of Figs. 1 and 2 support this conclusion. At p H 5.7 and lower, the voltammograms indicated the formation of a passive layer consisting mostly of CuC1. At p H 8.0, the potentials of formation of both C u 2 0 and CuC1 were about the same, and hence one oxidation peak was observed (curve (2) of Fig. l(b)). At p H 10, a small reduction peak for CuCI could be seen as a shoulder on the reduction peak of CuO (curve (2) in Fig. 2(a)). At p H higher than 10, the voltammograms showed peaks indicating the formation of the C u 2 0 film, followed by its oxidation to CuO or Cu(OH) 2 on
3. Results and discussion 100
The voltammetric and photocurrent measurements were made on Cu electrodes in 0.5 M C1 solutions at different p H values. In acidic solutions, from 0.5 M HCI to p H 5.7, the voltammograms and photocurrent curves were similar to those given in Fig. l(a). The passive layer formed on the Cu electrode in acidic solutions in the presence of C I - ions consisted of a CuC1 layer [24-28]. The potential of the C u 2 0 formation on the Cu electrode was shifted to more negative values with an increase in the p H at 60 mV per p H [17], whereas the potential of CuC1 formation was not affected by the pH. From this and the data on the equilibrium potentials of CuC1 [33], the cathodic peak at - 0.15 V in curve (2) of Fig. l(a) can be attributed to the reduction of CuC1. Accordingly, the potential of the CuC1 formation in acidic electrolytes was at about - 0 . 0 5 V. From these data and the potential of the
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Fig. 2. P h o t o c u r r e n t - p o t e n t i a l curves (1) and v o l t a m m o g r a m s (2) in 0.5 M NaCI (scan rate, 1 mW s t; m o d u l a t i o n frequency, 230 Hz; i r r a d i a t i o n w a v e l e n g t h , 400 nm): (a) p H 10.0, scan i n i t i a t e d at - 0 . 5 V; (b) p H 11.9, scan i n i t i a t e d at - 0 . 6 V.
65
A.D. Modestov et al. / Journal of Electroanalytical Chemistry 380 (1995) 63-68
the oxidation scan. The voltammogram recorded in 1 M N a O H + 0.5 M NaC1 was similar to those recorded in strongly alkaline solution in the absence of C1- ions [16,19]. The voltammogram was more complex at p H 8.5. It showed two sharp oxidation peaks at - 0 . 1 0 and - 0 . 0 7 V, with a broad shoulder at more positive potentials, and three reduction peaks at - 0 . 1 5 , - 0 . 2 and - 0 . 3 V. A study of the voltammograms obtained by reversing the scan at different potential limits has shown that the following oxidation and reduction peaks are related to each other: - 0 . 1 0 V and - 0 . 3 V; - 0 . 0 7 V and - 0 . 1 5 V; the shoulder at potentials more positive than 0.0 V and the reduction peak at - 0 . 2 V. Thus, the sequence of the anodic peaks in curve (2) of Fig. 3(a) can be ascribed to the following processes: the initial formation of Cu20, followed by the formation of CuC1 and CuO at more positive potentials. On the reverse scan, the sequence is the reduction of CuC1 to Cu, CuO to C u 2 0 , and then C u 2 0 to Cu. The photocurrent responses during the potential scan revealed tile presence of photosensitive films in all cases when the passive state of Cu was achieved. The direction (anodic or cathodic) and magnitude of the photocurrent were strongly influenced by the presence of C u 2 0 and CuC1. The presence of the photocurrent may be explained in two ways: it may be caused by C u 2 0 only, or it may be caused by the excitation of the charge carriers in individual n-type CuC1 islands formed on the Cu surface. The use of the first explanation led to the layer-structure passive film on Cu; i.e. the Cu electrode was covered by a C u 2 0 layer and on top of it was a CuC1 layer. Because of a large area of contact between the CuC1 and C u 2 0
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Fig. 3. Photocurrent-potential curves (1) and voltammograms(2) in different solutions at pH 8.5 (scan rate, 1 mV s-i; modulation frequency, 180 Hz; irradiation wavelength, 450 nm): (a) 0.5M NaC1; (b) 0.1M KI; (c) 0.5M KBr. The scans were initiated at -0.6 V.1
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Fig. 4. Spectral dependencies of the real componentof the photocurrent measured at passivated Cu electrodes: (a) pure borax buffer solution, pH 8.5, -0.05 V, 180 Hz, cathodic photocurrent; (b) 4 M HC1, 0.2 V, 180 Hz, anodic photocurrent; (c) 0.5 M NaCl, pH 8.5, 0 V, 230 Hz, anodic photocurrent; (d) 0.1 M KI, pH 8.5, -0.35 V after oxidation at -0.2 V, 180 Hz, cathodic photoeurrent; (e) 0.5 M NaBr, 0.2 V, 180 Hz, anodic photocurrent; (f) 0.5 M NaF, pH 8.5, 0.1 V after oxidation at 2 V, 180 Hz, anodic photocurrent.
layers, the presence of CuCI could substantially affect the properties of the C u 2 0 layer by halide ion doping. The second explanation led to the island structures. The CuC1 islands were formed at the weak points of the C u 2 0 film. In this case, the properties of the C u 2 0 film could not be affected significantly by the presence of CuCI owing to a relatively smaller area of contact. In order to differentiate between these two mechanisms, photocurrent and voltammetric measurements were performed on Cu electrodes in F - , Br , I - and SO 2 solutions at pH 8.5. The photocurrent spectra measured on Cu electrodes in the pure borax buffer solution, 4 M HC1 and 0.5 M NaC1 are shown in Figs. 4(a), 4(b) and 4(c), respectively. For the sample in Fig. 4(c), subsequent reduction of CuO and CuC1 at - 0 . 2 V resulted in a change in the photocurrent direction• The new cathodic photocurrent spectrum coincided with the photoresponse spectrum of the anodic photocurrent, shown in Fig. 4(c). This indicates that both the anodic and cathodic photocurrents were caused by the excitation of the charge carriers in the same semiconductor, namely, C u 2 0 . The potential-dependence of the photocurrent response and voltammogram for Cu electrodes in 0.1 M KI, 0.5 M KBr, 0.5 M NaF and 0.25 M NazSO 4
66
A.D. Modestor et al. /Journal of Electroanalytical Chemistry 380 (1995) 63-68 .
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solutions (all of pH 8.5) are shown in Figs. 3(b), 3(c), 5(a) and 5(b), respectively. In 0.1 M KI solution, the reduction scan shows two peaks which may be attributed to the reduction of CuI and Cu20. The onsets of the oxidation current and photocurrent are at potentials substantially more negative than the corresponding ones in the pure borax buffer solution [17] or in 0.5 M NaCI solution at the same pH (Fig. 3(a)). The photocurrent changed its direction at 0 V, as shown by the dashed lines in curve (1) of Fig. 3(b). The spectrum of cathodic photocurrent at - 0 . 3 5 V is shown in Fig. 4(d). The photosensitive film was prepared by scanning the potential of the Cu electrode initially at - 0 . 6 V to - 0 . 2 V at 10 mV s-l, and then reversed to - 0 . 3 5 V. In 0.5 M KBr solution, the formation of CuBr and C u 2 0 occurred in one oxidation peak. However, analysis of the voltammograms and photocurrent-potential curves at different potential limits has shown that the formation of C u 2 0 occurred at a potential about 10 mV more negative than that of CuBr. The spectral dependence of the anodic photocurrent is shown in Fig. 4(e). In 0.5 M NaF solution, the formation of the oxide films resulted in a weak positive photocurrent response, followed by a strong negative photocurrent. The breakdown of the film at 0.2 V eliminated the photocurrent. However, oscillations of the photocurrent response were observed at potentials more positive than 0.2 V. The electrode repassivation on the reverse scan brought about a strong anodic photocurrent, whose spectral dependence is shown in Fig. 4(0. The spectral measurements were made on a Cu electrode held at 2 V for 30 s. The high value of oxidation potential was chosen so as to obtain a uniform break-
down of the passive film. If the repassivated electrode was held at - 0 . 3 V, a gradual change (in about 2 h) of the direction of the photocurrent could be observed. The spectral dependence of this cathodic photocurrent was similar to that of the anodic current, shown in Fig. 4(f). In 0.25 M NazSO 4 solution, the breakdown of the passive film at about 0.2 V also resulted in the elimination of the photocurrent. The repassivation of the Cu electrode on the reverse scan gave rise to a positive photocurrent whose spectral dependence was similar to that obtained in 0.5 M NaF solution (Fig. 4(f)). The photocurrent became cathodic at negative potentials. If the breakdown potential was not reached, the voltammogram in the N a z S O 4 or NaF solution was similar to that in the pure borax buffer solution [17]. The band-gap energies of the photosensitive materials in the passive layers were determined using a procedure based on the dependence of light absorption coefficient on the photon energy [5,15]. Plots of the spectra in Figs. 4(a)-4(f) in the coordinates ('r/he) 2 vs. he, where r/ and he are photocurrent quantum efficiency and photon energy, respectively, revealed a direct transition of electrons in the light absorption process. The band-gap energies were determined to be 3.1 + 0.1 eV for all the six curves. Moreover, replotting portions of the spectra in Figs. 4(c), 4(e) and 4(f) in the 1.8-3.0 eV region in the coordinates (~lhu) °s vs. h e yielded indirect transitions with the band-gap energies of 2.1 + 0.2 eV. The value Eg = 3.1 _+ 0.1 eV fits well with the bandgap energies of C u 2 0 and CuC1. However, the same band-gap energy value was also observed in the pure borax buffer, KBr and KI solutions, as well as in the F and SO 2- solutions, where both n- and p-type photocurrents were observed. Although Eg values for crystalline CuF, CuBr, and CuI are not available in the literature, it is quite unlikely that all the Cu(I) halides should have the s a m e Eg values. This suggests the presence of the C u 2 0 film in all cases. In addition, the Eg value for the indirect transition, 2.1 _+ 0.2 eV, also leads to the same conclusion. The spectrum of anodic photocurrent with this kind of electron transition was observed in 0.5 M NaCl at pH 8.5 where CuCl was present on the electrode surface (see Fig. 4(c)). Upon reduction of CuC1, the photocurrent changed its direction but not its spectrum. Therefore, it is concluded that the substance that was responsible for the observed photocurrent was Cu20. The change in the direction of the photocurrent is ascribable to a change in the type of conductivity, a consequence of the doping of C u 2 0 by the anions present in the electrolyte or a change in the stoichiometry. The absence of shoulders, which is related to the indirect transitions, in the spectra for the pure borax buffer solution, 4 M HC1 and 0.1 M KI (Figs. 4(a), 4(b)
A.D. Modestot, et al. /Journal of Electroanalytical Chemistry 380 (1995) 63-68
and 4(d), respectively) indicated a low thickness (about 1 nm) of the C u 2 0 films formed in these systems [16]. The appearance of a small peak of positive photocurrent followed by a negative one at the onset of the photocurrent in Figs. 2(a), 3(a) and 5(a) can be related to a dissolution-precipitation mechanism during the first stage of C u 2 0 film formation [25]. The initial portions of C u 2 0 formed on the Cu surface contained some amount of anions from the electrolyte. Further growth of the Cu?O layer proceeded through solid-state reactions, producing C u 2 0 of p-type conductivity. The anodic photocurrent observed in 0.5 M NaF and 0.25 M N a z S O 4 at pH 8.5 from the repassivated electrodes can be ascribed to the doping of the C u 2 0 film by the anions present in the electrolyte or a stoichiometry change. According to the Pourbaix diagram, C u : O is highly soluble in acids [34]. Thus the C u 2 0 film, which was responsible for the observed anodic photocurrent, could only be formed underneath the CuCI layer in acidic solutions. The CuC1 overlayer protected the C u 2 0 film from dissolution and, at the same time, was responsible for the Cu passivity in acidic Cl--containing solutions [25-28]. Hence, the n-type C u 2 0 formed on Cu under these conditions would not exhibit substantial protection against Cu corrosion. An extrapolation of the kinetic data on the peak potentials of the C u 2 0 formation [17] to pH 2 gave a value about 0.3 V more positive than the onset potential of the photocurrent shown in curve (1) of Fig. l(a). This indicates a peculiar behavior of C u 2 0 in its competition with CuC1 in their formation on Cu electrodes. In basic solutions, C u 2 0 was formed on Cu first, in agreement with the potential value of the CuC1 formation and the reported potential values for C u 2 0 formation. In acidic solutions, C u 2 0 was expected to form at substantially more positive potentials than CuCI. It was formed under the CuCI layer immediately after the formation of the CuC1 layer. The same behavior was observed in 0.1 M KI at pH 8.5. The C u 2 0 film formed also exhibited poor protective properties.
4. Conclusions We have shown that a semiconducting film of Cu20 was formed in the passive layer on Cu electrodes in the presence of C1- ions at any pH. C u 2 0 was also formed in the passive films on Cu electrodes in F , Br-, I and SO 2- solutions at pH 8.5. The photocurrent produced by the photoexcitation of electrons in C u 2 0 could be anodic or cathodic, depending on the composition of the solution and the applied electrode potential. The change in the photocurrent direction was likely caused by a change in the type of conductivity related to ion doping. The results indicate a strong
67
influence of the method of passive film preparation on the semiconducting properties of the C u : O film. In basic C1- solutions, CuC1 was formed on the Cu electrode at more positive potentials than Cu20. CuCI affected the semiconducting properties of the C u 2 0 underlayer probably by doping it with C l - ions. The effect of the Cl ions on the voltammograms and photocurrent-potential curves decreased with an increase in pH. This suggests opposite actions of the CIand O H ions on the protective properties of the CI120 films. In acidic C1 solutions, CuCI was formed on Cu at more negative potentials than Cu20. After the formation of the passive CuCI layer, the semiconducting C u 2 0 film was formed under the CuC1 layer. C u 2 0 films were found even in passive layers on Cu in 4 M HC1. These C u 2 0 films did not exhibit significant protective properties against Cu corrosion.
Acknowledgements This work was supported by the National Natural Science Foundation of China and the State Bureau of Foreign Experts of China.
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A.D. Modesto~' et al. /Journal of Electroanalytical Chemistry 380 (1995) 63-68
[19] A.M. Castro Luna de Medima, S.L. Marciano and A.J. Arvia, J. Appl. Electrochem., 8 (1978) 121. [20] G.F.J. Garlick, in H. Brooks (Ed.), Advances in Semiconductor Science, Pergamon, New York, 1959, p. 452. [21] J.A. Duffy, Bonding, Energy Levels and Bands in Inorganic Solids, Wiley, New York, 1990, p. 136. [22] W. Siripala and K.P. Kumara, Semicond. Sci. Technol., 4 (1989) 465. [23] U. Bertocci, J. Electrochem. Soc., 125 (1978) 1598. [24] R.K. Flat and P.A. Brook, Corros. Sci., 11 (1971) 185. [25] G. Faita, G. Fiory and D. Salvadore, Corros. Sci., 15 (1975) 383. [26] A.J. Calandra, N.R. de Tacconi, P. Pereiro and A.J. Arvia, Electrochim. Acta, 19 (1974) 901. [27] H.P. Lee, K. Nobe and A.J. Pearlstein, J. Electrochem. Soc., 132 (1985) 1031.
[28] A.J. Pearlstein, H.P. Lee and K. Nobe, J. Electrochem. Soc., 132 (1985) 219. [29] M.R.G. de Chialvo, R.C. Salvarezza, D.V. Moll and A.J. Arvia, Electrochim. Acta, 30 (1985) 1501. [30] B. Pointu, M. Braizaz, P. Poucet, J. Rouseau and N. Muhlstein, J. Electroanal. Chem., 122 (1981) 111. [31] B. Pointu, M. Braizaz, P. Poucet and J. Rouseau, J. Electroanal. Chem., 151 (1983) 65. [32] A.D. Modestov, G.-D. Zhou, H.-H. Ge and B.H. Loo, J. Electroanal. Chem., 375 (1994) 293. [33] D. Tromans and R.-H. Sun, J. Electrochem. Soc., 138 (1991) 3235. [34] M. Pourbaix, Atlas of Electrochemical Equilibria, Pergamon Press, Oxford, UK, 1966, p. 386.
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Journal of Electroanalytical Chemistry 380 (1995) 63 68
A study by voltammetry and the photocurrent response method of copper electrode behavior in acidic and alkaline solutions containing chloride ions A.D. Modestov ,,1, Guo-Ding Zhou a, H o n g - H u a Ge ", B.H. Loo b a Electrochemical Research Group, Shanghai Institute of Electric Power, Shanghai 200090, People's Republic of China b Department of Chemistry, Uniuersity of Alabama in Huntst:ille, Huntst,ille, AL 35899, USA, and Department of Chemisto', National Unicersity of Singapore, Kent Ridge, S.0511, Singapore Received 4 January 1994; in revised form 18 May 1994
Abstract
The electrochemical behavior of Cu electrodes in Cl solutions was studied in a wide range of pH. The results were compared with those obtained in solutions containing F-, Br-, I - and SO42 ions at pH 8.5, and discussed in terms of the competitive formation of Cu20 and CuC1 films on the Cu surface and the influence of CuCI on the properties of Cu20. At pH 8.5 or higher, Cu20 was formed first, whereas at pH 5.7 or lower the Cu20 film was formed on the Cu surface under the CuC1 layer which was formed initially. It is believed that the Cu20 films doped with C1 ions exhibited poor protective properties against Cu corrosion.
Keywords: Voltammetry; Photocurrent response; Cu electrodes 1. Introduction
Photoelectrochemical methods based on measurements of potential or current changes due to illumination have been widely used in the study of corrosion processes which involve the formation of semiconducting passive layers [1-11]. Identification of the semiconducting film is made possible from its photoresponse spectrum [4-8,10]. The passivity of Cu in basic solutions containing no C1 ions was ascribed to the formation of a thin C u 2 0 semiconducting film covered by a layer of C u O / C u ( O H ) 2 [2-4,6,7,12-19]. Crystalline C u 2 0 is a semiconductor with p-type conductivity and a band-gap e n e r g y (Eg) of 2.2-2.3 eV [20,21]. Studies of the cathodic photocurrent responses of C u 2 0 films of a few nanometers thickness, formed electrochemically in basic solutions [15], yielded band-gaps ( E g ) o f 3 and 2.3 eV for the direct and indirect transitions, respectively. For thin C u 2 0 films (about 1 rim), the influence of indirect transitions on the photoresponse spectrum was negligible. When the film thickness was
1present address: A.N. Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Moscow 117071, Russia. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 4 ) 0 3 5 7 7 - P
increased to 40 nm, the Eg value became 2.4 and 2 eV for the direct and indirect transitions, respectively [16]. The CuzO films formed on Cu by corrosion in neutral and weakly acidic media, in most cases, were characterized by an n-type photoresponse [6]. However, both n and p types of photoresponses have been reported from C u 2 0 film, depending on the way the film was prepared [22,23]. The passivity of Cu electrodes in acidic solutions containing C1 ions was attributed to the formation of a CuC1 surface layer [24-28]. In borax buffer solution containing C1 ions (pH 9), the electrochemical behavior was treated as a competition between the formation of a C u 2 0 film which brought about the passivity, and the nucleation and growth of a CuC1 layer which resulted in pitting [29]. Cu corrosion in neutral and weakly acidic media containing C1 ions resulted in the formation of passive layers with photoresponses essentially of the n type [2,6,13,14]. A semiconducting layer consisting of CuCI or Cu 2O was proposed to be formed on the electrode surface [2]. The Eg value determined for the layer was 2.7 eV, which was less than t h e Eg of 3.3 eV for the crystalline CuCI [21]. It was also proposed that the protective properties of the C u 2 0 film depended on its electronic properties, and they deteri-
64
A.D. Modestov et al. /Journal of Electroanalytical Chemistry 380 (1995) 63-68 ,?,
orated with an increase in the film conductivity [2]. Anodic photocurrents were observed at Cu electrodes in H 3 P O 4 [30,31]. The goal of the present work is to study the nature of Cu passivity in Cl--containing electrolytes in a wide range of p H by monitoring the formation of C u 2 0 with voltammetry and photocurrent measurements. The photoelectrochemical behavior of Cu electrodes in F - , B r - , I - and SO42- electrolytes was also studied, and is compared with that observed in the C1--containing solutions. Spectral measurements were also performed in order to identify the composition of the semiconducting films formed on the electrode surfaces.
(a) pH 2
(b) pH 8
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20
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I ~ I 0.4
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2. Experimental The details of the experimental set-up and sample preparation are given elsewhere [32]. All experiments were performed using fresh electrodes which were activated in a working electrolyte by a potentiostatic treatment for 5 rain in the region of hydrogen evolution at 2 - 5 m A cm 2. The following buffer solutions were used: 0.2 M KHPhth + 0.1 M N a O H , p H 4-5.7; 0.2 M K H 2 P O 4 + 0.1 M N a O H , p H 8; 0.1 M Na2B407 + 0.15 M H3BO3; 0.1 M NazB407 + 0.1 M N a O H , p H 10-12. The second component of the buffer solution was added so as to reach the desired p H value. The p H 2 solution was achieved by adding HC1 to 0.5 M NaC1 solution. The photocurrent measurements were conducted using the light modulation method, and only the real component was recorded. All the photocurrent action spectra are reported in arbitrary units of quantum efficiency. The potentials are reported with respect to a saturated calomel electrode (SCE).
Fig. 1. P h o t o c u r r e n t - p o t e n t i a l curves (1) and v o l t a m m o g r a m s (2) in 0.5 M NaCI (scan rate, 1 m V s - t ; m o d u l a t i o n frequency, 230 Hz; i r r a d i a t i o n w a v e l e n g t h , 400 nm): (a) p H 2.0; (b) p H 8.0. The scans w e r e i n i t i a t e d at - 0 . 5 V.
formation of C u 2 0 o n Cu electrodes in the p H range 5-12.9 [19], it may be concluded that CuCI was formed first in acidic solutions, whereas C u 2 0 was formed first in basic solutions. The voltammograms shown in curves (2) of Figs. 1 and 2 support this conclusion. At p H 5.7 and lower, the voltammograms indicated the formation of a passive layer consisting mostly of CuC1. At p H 8.0, the potentials of formation of both C u 2 0 and CuC1 were about the same, and hence one oxidation peak was observed (curve (2) of Fig. l(b)). At p H 10, a small reduction peak for CuCI could be seen as a shoulder on the reduction peak of CuO (curve (2) in Fig. 2(a)). At p H higher than 10, the voltammograms showed peaks indicating the formation of the C u 2 0 film, followed by its oxidation to CuO or Cu(OH) 2 on
3. Results and discussion 100
The voltammetric and photocurrent measurements were made on Cu electrodes in 0.5 M C1 solutions at different p H values. In acidic solutions, from 0.5 M HCI to p H 5.7, the voltammograms and photocurrent curves were similar to those given in Fig. l(a). The passive layer formed on the Cu electrode in acidic solutions in the presence of C I - ions consisted of a CuC1 layer [24-28]. The potential of the C u 2 0 formation on the Cu electrode was shifted to more negative values with an increase in the p H at 60 mV per p H [17], whereas the potential of CuC1 formation was not affected by the pH. From this and the data on the equilibrium potentials of CuC1 [33], the cathodic peak at - 0.15 V in curve (2) of Fig. l(a) can be attributed to the reduction of CuC1. Accordingly, the potential of the CuC1 formation in acidic electrolytes was at about - 0 . 0 5 V. From these data and the potential of the
_
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(b) pH 11.9
(a) pH 10
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-0.5
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Fig. 2. P h o t o c u r r e n t - p o t e n t i a l curves (1) and v o l t a m m o g r a m s (2) in 0.5 M NaCI (scan rate, 1 mW s t; m o d u l a t i o n frequency, 230 Hz; i r r a d i a t i o n w a v e l e n g t h , 400 nm): (a) p H 10.0, scan i n i t i a t e d at - 0 . 5 V; (b) p H 11.9, scan i n i t i a t e d at - 0 . 6 V.
65
A.D. Modestov et al. / Journal of Electroanalytical Chemistry 380 (1995) 63-68
the oxidation scan. The voltammogram recorded in 1 M N a O H + 0.5 M NaC1 was similar to those recorded in strongly alkaline solution in the absence of C1- ions [16,19]. The voltammogram was more complex at p H 8.5. It showed two sharp oxidation peaks at - 0 . 1 0 and - 0 . 0 7 V, with a broad shoulder at more positive potentials, and three reduction peaks at - 0 . 1 5 , - 0 . 2 and - 0 . 3 V. A study of the voltammograms obtained by reversing the scan at different potential limits has shown that the following oxidation and reduction peaks are related to each other: - 0 . 1 0 V and - 0 . 3 V; - 0 . 0 7 V and - 0 . 1 5 V; the shoulder at potentials more positive than 0.0 V and the reduction peak at - 0 . 2 V. Thus, the sequence of the anodic peaks in curve (2) of Fig. 3(a) can be ascribed to the following processes: the initial formation of Cu20, followed by the formation of CuC1 and CuO at more positive potentials. On the reverse scan, the sequence is the reduction of CuC1 to Cu, CuO to C u 2 0 , and then C u 2 0 to Cu. The photocurrent responses during the potential scan revealed tile presence of photosensitive films in all cases when the passive state of Cu was achieved. The direction (anodic or cathodic) and magnitude of the photocurrent were strongly influenced by the presence of C u 2 0 and CuC1. The presence of the photocurrent may be explained in two ways: it may be caused by C u 2 0 only, or it may be caused by the excitation of the charge carriers in individual n-type CuC1 islands formed on the Cu surface. The use of the first explanation led to the layer-structure passive film on Cu; i.e. the Cu electrode was covered by a C u 2 0 layer and on top of it was a CuC1 layer. Because of a large area of contact between the CuC1 and C u 2 0
0.I
(b) O.l
(c)
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0
~
0
-0.1
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I -0.6
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Fig. 3. Photocurrent-potential curves (1) and voltammograms(2) in different solutions at pH 8.5 (scan rate, 1 mV s-i; modulation frequency, 180 Hz; irradiation wavelength, 450 nm): (a) 0.5M NaC1; (b) 0.1M KI; (c) 0.5M KBr. The scans were initiated at -0.6 V.1
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Fig. 4. Spectral dependencies of the real componentof the photocurrent measured at passivated Cu electrodes: (a) pure borax buffer solution, pH 8.5, -0.05 V, 180 Hz, cathodic photocurrent; (b) 4 M HC1, 0.2 V, 180 Hz, anodic photocurrent; (c) 0.5 M NaCl, pH 8.5, 0 V, 230 Hz, anodic photocurrent; (d) 0.1 M KI, pH 8.5, -0.35 V after oxidation at -0.2 V, 180 Hz, cathodic photoeurrent; (e) 0.5 M NaBr, 0.2 V, 180 Hz, anodic photocurrent; (f) 0.5 M NaF, pH 8.5, 0.1 V after oxidation at 2 V, 180 Hz, anodic photocurrent.
layers, the presence of CuCI could substantially affect the properties of the C u 2 0 layer by halide ion doping. The second explanation led to the island structures. The CuC1 islands were formed at the weak points of the C u 2 0 film. In this case, the properties of the C u 2 0 film could not be affected significantly by the presence of CuCI owing to a relatively smaller area of contact. In order to differentiate between these two mechanisms, photocurrent and voltammetric measurements were performed on Cu electrodes in F - , Br , I - and SO 2 solutions at pH 8.5. The photocurrent spectra measured on Cu electrodes in the pure borax buffer solution, 4 M HC1 and 0.5 M NaC1 are shown in Figs. 4(a), 4(b) and 4(c), respectively. For the sample in Fig. 4(c), subsequent reduction of CuO and CuC1 at - 0 . 2 V resulted in a change in the photocurrent direction• The new cathodic photocurrent spectrum coincided with the photoresponse spectrum of the anodic photocurrent, shown in Fig. 4(c). This indicates that both the anodic and cathodic photocurrents were caused by the excitation of the charge carriers in the same semiconductor, namely, C u 2 0 . The potential-dependence of the photocurrent response and voltammogram for Cu electrodes in 0.1 M KI, 0.5 M KBr, 0.5 M NaF and 0.25 M NazSO 4
66
A.D. Modestor et al. /Journal of Electroanalytical Chemistry 380 (1995) 63-68 .
(a)
l
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1
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~
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< 0
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,
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Potential/V Fig. 5. Photocurrent-potential curves (1) and voltammograms (2) at pH 8.5 (scan rate, 1 mV s 1; modulation frequency, 180 Hz; irradiation wavelength, 400 nm): (a) 0.5 M NaF, scan initiated at -0.6 V; (b) 0.25 M NazSO4, scan initiated at -0.4 V.
solutions (all of pH 8.5) are shown in Figs. 3(b), 3(c), 5(a) and 5(b), respectively. In 0.1 M KI solution, the reduction scan shows two peaks which may be attributed to the reduction of CuI and Cu20. The onsets of the oxidation current and photocurrent are at potentials substantially more negative than the corresponding ones in the pure borax buffer solution [17] or in 0.5 M NaCI solution at the same pH (Fig. 3(a)). The photocurrent changed its direction at 0 V, as shown by the dashed lines in curve (1) of Fig. 3(b). The spectrum of cathodic photocurrent at - 0 . 3 5 V is shown in Fig. 4(d). The photosensitive film was prepared by scanning the potential of the Cu electrode initially at - 0 . 6 V to - 0 . 2 V at 10 mV s-l, and then reversed to - 0 . 3 5 V. In 0.5 M KBr solution, the formation of CuBr and C u 2 0 occurred in one oxidation peak. However, analysis of the voltammograms and photocurrent-potential curves at different potential limits has shown that the formation of C u 2 0 occurred at a potential about 10 mV more negative than that of CuBr. The spectral dependence of the anodic photocurrent is shown in Fig. 4(e). In 0.5 M NaF solution, the formation of the oxide films resulted in a weak positive photocurrent response, followed by a strong negative photocurrent. The breakdown of the film at 0.2 V eliminated the photocurrent. However, oscillations of the photocurrent response were observed at potentials more positive than 0.2 V. The electrode repassivation on the reverse scan brought about a strong anodic photocurrent, whose spectral dependence is shown in Fig. 4(0. The spectral measurements were made on a Cu electrode held at 2 V for 30 s. The high value of oxidation potential was chosen so as to obtain a uniform break-
down of the passive film. If the repassivated electrode was held at - 0 . 3 V, a gradual change (in about 2 h) of the direction of the photocurrent could be observed. The spectral dependence of this cathodic photocurrent was similar to that of the anodic current, shown in Fig. 4(f). In 0.25 M NazSO 4 solution, the breakdown of the passive film at about 0.2 V also resulted in the elimination of the photocurrent. The repassivation of the Cu electrode on the reverse scan gave rise to a positive photocurrent whose spectral dependence was similar to that obtained in 0.5 M NaF solution (Fig. 4(f)). The photocurrent became cathodic at negative potentials. If the breakdown potential was not reached, the voltammogram in the N a z S O 4 or NaF solution was similar to that in the pure borax buffer solution [17]. The band-gap energies of the photosensitive materials in the passive layers were determined using a procedure based on the dependence of light absorption coefficient on the photon energy [5,15]. Plots of the spectra in Figs. 4(a)-4(f) in the coordinates ('r/he) 2 vs. he, where r/ and he are photocurrent quantum efficiency and photon energy, respectively, revealed a direct transition of electrons in the light absorption process. The band-gap energies were determined to be 3.1 + 0.1 eV for all the six curves. Moreover, replotting portions of the spectra in Figs. 4(c), 4(e) and 4(f) in the 1.8-3.0 eV region in the coordinates (~lhu) °s vs. h e yielded indirect transitions with the band-gap energies of 2.1 + 0.2 eV. The value Eg = 3.1 _+ 0.1 eV fits well with the bandgap energies of C u 2 0 and CuC1. However, the same band-gap energy value was also observed in the pure borax buffer, KBr and KI solutions, as well as in the F and SO 2- solutions, where both n- and p-type photocurrents were observed. Although Eg values for crystalline CuF, CuBr, and CuI are not available in the literature, it is quite unlikely that all the Cu(I) halides should have the s a m e Eg values. This suggests the presence of the C u 2 0 film in all cases. In addition, the Eg value for the indirect transition, 2.1 _+ 0.2 eV, also leads to the same conclusion. The spectrum of anodic photocurrent with this kind of electron transition was observed in 0.5 M NaCl at pH 8.5 where CuCl was present on the electrode surface (see Fig. 4(c)). Upon reduction of CuC1, the photocurrent changed its direction but not its spectrum. Therefore, it is concluded that the substance that was responsible for the observed photocurrent was Cu20. The change in the direction of the photocurrent is ascribable to a change in the type of conductivity, a consequence of the doping of C u 2 0 by the anions present in the electrolyte or a change in the stoichiometry. The absence of shoulders, which is related to the indirect transitions, in the spectra for the pure borax buffer solution, 4 M HC1 and 0.1 M KI (Figs. 4(a), 4(b)
A.D. Modestot, et al. /Journal of Electroanalytical Chemistry 380 (1995) 63-68
and 4(d), respectively) indicated a low thickness (about 1 nm) of the C u 2 0 films formed in these systems [16]. The appearance of a small peak of positive photocurrent followed by a negative one at the onset of the photocurrent in Figs. 2(a), 3(a) and 5(a) can be related to a dissolution-precipitation mechanism during the first stage of C u 2 0 film formation [25]. The initial portions of C u 2 0 formed on the Cu surface contained some amount of anions from the electrolyte. Further growth of the Cu?O layer proceeded through solid-state reactions, producing C u 2 0 of p-type conductivity. The anodic photocurrent observed in 0.5 M NaF and 0.25 M N a z S O 4 at pH 8.5 from the repassivated electrodes can be ascribed to the doping of the C u 2 0 film by the anions present in the electrolyte or a stoichiometry change. According to the Pourbaix diagram, C u : O is highly soluble in acids [34]. Thus the C u 2 0 film, which was responsible for the observed anodic photocurrent, could only be formed underneath the CuCI layer in acidic solutions. The CuC1 overlayer protected the C u 2 0 film from dissolution and, at the same time, was responsible for the Cu passivity in acidic Cl--containing solutions [25-28]. Hence, the n-type C u 2 0 formed on Cu under these conditions would not exhibit substantial protection against Cu corrosion. An extrapolation of the kinetic data on the peak potentials of the C u 2 0 formation [17] to pH 2 gave a value about 0.3 V more positive than the onset potential of the photocurrent shown in curve (1) of Fig. l(a). This indicates a peculiar behavior of C u 2 0 in its competition with CuC1 in their formation on Cu electrodes. In basic solutions, C u 2 0 was formed on Cu first, in agreement with the potential value of the CuC1 formation and the reported potential values for C u 2 0 formation. In acidic solutions, C u 2 0 was expected to form at substantially more positive potentials than CuCI. It was formed under the CuCI layer immediately after the formation of the CuC1 layer. The same behavior was observed in 0.1 M KI at pH 8.5. The C u 2 0 film formed also exhibited poor protective properties.
4. Conclusions We have shown that a semiconducting film of Cu20 was formed in the passive layer on Cu electrodes in the presence of C1- ions at any pH. C u 2 0 was also formed in the passive films on Cu electrodes in F , Br-, I and SO 2- solutions at pH 8.5. The photocurrent produced by the photoexcitation of electrons in C u 2 0 could be anodic or cathodic, depending on the composition of the solution and the applied electrode potential. The change in the photocurrent direction was likely caused by a change in the type of conductivity related to ion doping. The results indicate a strong
67
influence of the method of passive film preparation on the semiconducting properties of the C u : O film. In basic C1- solutions, CuC1 was formed on the Cu electrode at more positive potentials than Cu20. CuCI affected the semiconducting properties of the C u 2 0 underlayer probably by doping it with C l - ions. The effect of the Cl ions on the voltammograms and photocurrent-potential curves decreased with an increase in pH. This suggests opposite actions of the CIand O H ions on the protective properties of the CI120 films. In acidic C1 solutions, CuCI was formed on Cu at more negative potentials than Cu20. After the formation of the passive CuCI layer, the semiconducting C u 2 0 film was formed under the CuC1 layer. C u 2 0 films were found even in passive layers on Cu in 4 M HC1. These C u 2 0 films did not exhibit significant protective properties against Cu corrosion.
Acknowledgements This work was supported by the National Natural Science Foundation of China and the State Bureau of Foreign Experts of China.
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A.D. Modesto~' et al. /Journal of Electroanalytical Chemistry 380 (1995) 63-68
[19] A.M. Castro Luna de Medima, S.L. Marciano and A.J. Arvia, J. Appl. Electrochem., 8 (1978) 121. [20] G.F.J. Garlick, in H. Brooks (Ed.), Advances in Semiconductor Science, Pergamon, New York, 1959, p. 452. [21] J.A. Duffy, Bonding, Energy Levels and Bands in Inorganic Solids, Wiley, New York, 1990, p. 136. [22] W. Siripala and K.P. Kumara, Semicond. Sci. Technol., 4 (1989) 465. [23] U. Bertocci, J. Electrochem. Soc., 125 (1978) 1598. [24] R.K. Flat and P.A. Brook, Corros. Sci., 11 (1971) 185. [25] G. Faita, G. Fiory and D. Salvadore, Corros. Sci., 15 (1975) 383. [26] A.J. Calandra, N.R. de Tacconi, P. Pereiro and A.J. Arvia, Electrochim. Acta, 19 (1974) 901. [27] H.P. Lee, K. Nobe and A.J. Pearlstein, J. Electrochem. Soc., 132 (1985) 1031.
[28] A.J. Pearlstein, H.P. Lee and K. Nobe, J. Electrochem. Soc., 132 (1985) 219. [29] M.R.G. de Chialvo, R.C. Salvarezza, D.V. Moll and A.J. Arvia, Electrochim. Acta, 30 (1985) 1501. [30] B. Pointu, M. Braizaz, P. Poucet, J. Rouseau and N. Muhlstein, J. Electroanal. Chem., 122 (1981) 111. [31] B. Pointu, M. Braizaz, P. Poucet and J. Rouseau, J. Electroanal. Chem., 151 (1983) 65. [32] A.D. Modestov, G.-D. Zhou, H.-H. Ge and B.H. Loo, J. Electroanal. Chem., 375 (1994) 293. [33] D. Tromans and R.-H. Sun, J. Electrochem. Soc., 138 (1991) 3235. [34] M. Pourbaix, Atlas of Electrochemical Equilibria, Pergamon Press, Oxford, UK, 1966, p. 386.