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A model for the flux of the species generated during the electrodissolution of a copper–nickel alloy on Pt in acidic media
Surface & Coatings Technology 200 (2006) 2990 – 2994 www.elsevier.com/locate/surfcoat
A model for the flux of the species generated during the electrodissolution of a copper–nickel alloy on Pt in acidic media A. Manzoli, M.C. Santos*, L.O.S. Bulho˜es Laborato´rio Interdisciplinar de Eletroquı´mica e Ceraˆmica, Centro Multidisciplinar para o Desenvolvimento de Materiais Ceraˆmicos, Departamento de Quı´mica, Universidade Federal de Sa˜o Carlos, Caixa Postal 676, Sa˜o Carlos, SP 13565-905, Brazil Received 22 July 2004; accepted in revised form 6 October 2004 Available online 26 November 2004
Abstract This paper describes a new model to calculate the fluxes of the species generated during the electrodeposition of Cu–Ni alloys on Pt in acidic media. Measurements were carried out using linear sweep voltammetry and electrochemical quartz crystal microbalance (EQCM) techniques. The alloy electrodeposition was performed between +1.0 and 0.9 V versus SCE in a bath containing 0.04 M CuCl2d 2H2O+0.04 M NiCl2d 6H2O+0.32 M H3BO3+1 M NH4Cl. Following this, a sweep in the positive direction (between 0.3 and +1.0 V) to dissolve the alloy was performed using a blank solution (1 M NH4Cl+0.32 M H3BO3). The fluxes of the species versus the potential (Ni2+ (desorption) and Cl (adsorption on the top of Cu+)) generated during the alloy dissolution were calculated using the current and mass responses. In fact, it was observed that the electrodissolution obeys Faraday’s law. The flux calculated (considering a transference of two electrons in the case of Ni and one electron in the case of Cu) multiplied by the Faraday constant and plotted against E (i calculated) presents the same profile as the experimental results for the alloy dissolution. D 2004 Elsevier B.V. All rights reserved. Keywords: EQCM; Nickel; Copper; Electrodissolution; Flux; Alloys
1. Introduction In recent years interest in the electrodeposition of Cu–Ni alloys and their multilayers has increased, mainly due to the interesting properties of these materials, such as: corrosion resistance [1], mechanical [2], magnetic [3] and their catalytic properties [4]. Cu–Ni alloys, principally those that contain 30% to 40% Cu, are resistant to corrosion in acid media, basic media and especially in solutions containing chloride (seawater) [1]. Such alloys have been utilized in the construction of the commercial boats and in heat exchangers that use seawater for refrigeration [5]. Among the interesting mechanical properties displayed by Cu–Ni alloys are their high tensile strength, malleability and ductility [2].
* Corresponding author. Tel.: +55 16 260 8214; fax: +55 16 260 8214. E-mail address: [email protected] (M.C. Santos). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.012
Cu–Ni alloys present unique magnetic properties, Ni being ferromagnetic and Cu paramagnetic [3]. Ni88Cu19/Cu in alternated multilayers and Ni81Cu19 as a homogeneous alloy have both been shown to display giant magnetic resistances (GMR) [6]. Although there are some papers dealing with the technological aspects of the electrodeposition of Cu–Ni alloys, there are few studies that discuss how the species are generated during both the electrodeposition and electrodissolution processes. To the best of our knowledge there is only one paper [7] employing electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and anodic stripping voltammetry (ASV) techniques to investigate in detail the Cu–Ni system. However, the study was performed on gold electrodes. The authors [7] employed a de-aerated Watts bath for the electrodeposition of Ni and Cu–Ni thin films. The combination of ASV and EQCM techniques permitted the analysis
A. Manzoli et al. / Surface & Coatings Technology 200 (2006) 2990–2994
of sequential Au/Cu–Ni–Ni/Ni thin film systems. This is because the Cu–Ni and Ni layers are dissolved under distinctly different potential regimes. The present study has the aim of proposing a new model for the flux of the species generated during the electrodissolution of the Cu–Ni alloys on Pt in acid medium. The study employs a combination of linear sweep voltammetry (LSV) and electrochemical quartz crystal microbalance (EQCM) techniques.
2. Experimental All reagents were of analytical grade and the water used to prepare the solutions was purified by the Milli-Q system (Millipore, nominal resistivity: 18.2 MV cm1). For the electrodeposition of the 1:1 Cu–Ni alloy, solutions of 0.04 M CuCl2d 2H2O, 0.04 M NiCl2d 6H20, 1 M NH4Cl and 0.32 M H3BO3 were used. The electrodeposition of the 1:1 Cu– Ni alloy was performed using the linear sweep voltammetry, applying a potential scan rate of 40 mV s1, in a potential range of +1.0 to 0.9 V. Before the stripping experiments, a diffraction analysis was performed using XRD (Rigaku diffractometer, model Dmax 2500PC), using CuKa radiation (k=1.5406 A). From this analysis the diffraction pattern associated with the formation of a copper–nickel alloy was observed (phases CuNi(111) (2h=43.608) and CuNi(200) (2h=50.798)) in agreement with the corresponding phases in JCPDS file. For the electrodissolution of the 1:1 Cu–Ni alloy 1 M NH4Cl and 0.32 M H3BO3 were used. This was achieved using linear sweep voltammetry coupled to the electrochemical quartz crystal microbalance. The potential scan rate was 40 mV s1 in the potential range 0.3 to +1.0 V. After stripping, the solutions were analyzed using atomic absorption spectroscopy (AAS) with a AA12/1475 spectrophotometer (INTRALAB/GEMINI) and the composition of 50% (m/m) copper and 50% (m/m) nickel was determined. The solutions were de-aerated with pre-purified N2 (White Martins). All experiments were carried out at room temperature and under atmospheric pressure. A quartz crystal analyzer (Seiko EG&G model QCA917) interfaced with a potentiostat model 273 EG&G Parc Instruments and a microcomputer were used for the electrochemical measurements of the resonant frequency of the quartz crystal. An AT-cut quartz crystal of f 0=9 MHz covered on both sides with a platinum film was employed as the working electrode. The geometric area of Pt electrode was approximately 0.2 cm2. A platinum sheet and a saturated calomel electrode (SCE) were used as the counter and the reference electrodes, respectively. The sensitivity factor for the EQCM was determined using the potentiostatic electrodeposition of copper (1 M H2SO4+0.1 M CuSO4), in which the dissolution charge densities were used to calculate the mass values and the resonant frequencies were measured experimentally. The
2991
value of the sensitivity factor thus obtained was 830 Hz Ag1, which is smaller than the calculated value of 915 Hz Ag1, using Sauerbrey’s Equation [8], Eq. (1): ! f02 Df ¼ Cf Dm ¼ Dm ð1Þ N qq where C f is the sensitivity factor, Dm is the mass per unit area (g cm2), f 0 is the resonant frequency in Hz, N is the frequency constant (1.67105 Hz cm) and q q (2.648 g cm3) is the quartz density. The difference observed between the experimental sensitivity coefficient and its theoretical value is attributed to the difference between the resonant frequency with the crystal oscillating in the vacuum and in the solution [9].
3. Results and discussion The 1:1 Cu–Ni alloy was initially electrodeposited in the interval 1.0 to 0.8 V. The deposited alloy then underwent electrodissolution in a blank solution (1 M NH4Cl+0.32 M H3BO3). In Fig. 1, the results for such an electrodissolution are presented. The potential sweep started at 0.3 V and continued positively to 1 V. Fig. 1a displays the profile for the electrodissolution of the 1:1 Cu–Ni alloy, in which can be observed three different regions. Region I (between 0.3 and 0.15 V) is characterized by the dissolution of Ni and adsorption of chloride on top of Cu+ (generated in the same potential region of the Cu0 layer): Ni0 YNi2þ þ 2e
ð2Þ
Cu0 YCuþ þ 1e
ð3Þ
Cuþ þ Cl YCuCl
ð4Þ
The mass profile for region I (between 0.3 and 0.15 V) (Fig. 1b) presents a mass decrease (0.25 Ag), which is related to the contribution of reaction (2), which is the principal process when compared to reaction (4). The charge versus potential profile (Fig. 1c) presents an increase of approximately 0.35 mC, due to the dissolution of Ni and oxidation of Cu (reactions (2) and (3), respectively). The formation of a CuCl complex has already been discussed for copper electrodeposition from 10 mM CuSO4+0.1 M NaCl utilizing the EQCM technique, however this was done in acid medium (0.1 M H2SO4) without the presence of Ni and at gold electrodes [10]. Region II in Fig. 1a (between 0.15 and 0.11 V) is attributed to the dissolution of a weakly adsorbed CuCl species on the substrate: CuClYCu2þ þ e þ Cl
ð5Þ
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accompanied by a high charge variation. These results are attributed to the CuCl complex dissolution. To analyze the process that takes place in region I during the 1:1 Cu–Ni alloy electrodissolution, a new flux model was employed. To elaborate this model, it was considered that the Cu–Ni alloy is a solid solution, which means that the alloy composition is constant at each potential. The mass variation in region I (0.3 and 0.15 V) can be attributed to Ni electrodissolution (reaction (2)) and Cl adsorption (reaction (4)) on the top of the Cu+ species (generated simultaneously with the oxidation of Cu0). Therefore, such mass variation can be described as presented by Eq. (7). Dm ¼ nNi2þ MNi2þ þ nCl MCl
ð7Þ
where n Ni2+ and n Cl are the number of moles of nickel and chloride, respectively, and M Ni2+=58.71 g mol1 and M Cl=35.45 g mol1, the molar masses for nickel and chloride, respectively. The charge variation (Dq) in the same potential region can be expressed by the following Eq. (8): Dq ¼ nNi2þ zNi2þ F þ nCuþ zCuþ F
ð8Þ
where, z Ni2+ and z Cu+ correspond to the valences of the ions, and F is the Faraday constant (97487 C mol1). Considering: Fig. 1. (a) Current variation, (b) mass variation and (c) charge variation as a function of the potential for the 1:1 Cu/Ni alloy stripping, in a 1 M NH4Cl and 0.32 M H3BO3 aqueous solution using the quartz crystal microbalance and potential scan rate of 40 mV s1.
As a consequence of this dissolution mechanism, the mass profile presents a decrease (0.2 Ag) and the charge profile presents an increase (0.17 mC). Region III (between 0.11 up to 1.0 V) is characterized by an oxidation process that takes place at 0.3 V (Fig. 1a). This process has been previously described [11] and it is due to the oxidation of Cu+ to Cu2+ from the previous formation of a CuCl complex at 0.1 V, which occurs with or without nickel in the solution and it is independent of the processes associated with peak I or II: CuClYCu2þ þ e þ Cl
nCuþ ¼ nCl
ð9Þ
zNi2þ ¼ 2
ð10Þ
zCuþ ¼ 1
ð11Þ
we obtain: Dq ¼ 2nNi2þ F þ nCl F; where n Cl is defined by Eq. (9): nCl ¼
Dq 2nNi2þ F
ð13Þ
Substituting Eq. (13) in Eq. (7), we obtain:
ð6Þ Dm ¼ nNi2þ MNi2þ þ
In this case, the dissolution occurs in a potential region that is more positive than that of region II. This is probably because the deposit is more internal and the adsorption strength on the platinum is higher than that of the previous region. In the mass and charge profiles displayed in Fig. 1 (region III), a small mass change can be observed, but this is
ð12Þ
Dq 2nNi2þ MCl ; F
and n Ni2+ becomes: nNi2þ ¼
Dm DqMCl : ð14Þ ð MNi2þ 2MCl Þ Fð MNi2þ 2MCl Þ
A. Manzoli et al. / Surface & Coatings Technology 200 (2006) 2990–2994
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Setting the values of M Ni2+=58.71 g mol1, M Cl=35.45 g mol1 and F=96487 C mol1 and substituting them in Eq. (14): Dm Dqð35:453Þ nNi2þ ¼ ð 58:71 70:906Þ 96487ð 58:71 70:906Þ ð15Þ nNi2þ ¼ 7:72 103 Dm þ 2:8 106 Dq
ð16Þ
Upon differentiating Eq. (16) with respect to time, we obtain the nickel flux in region I (between 0.3 and 0.15 V) (Fig. 1a): dnNi2þ dðDmÞ þ 2:8 106 ið E Þ ¼ 7:72 103 dt dt
ð17Þ
where i(E) is the experimental current. To obtain the flux of chloride in the 1:1 Cu/Ni alloy, it was considered that the mass variations follow Eq. (7), the charge variations follow Eq. (12). Using the values for the molar mass of copper and nickel and the Faraday constant we obtain: nCl ¼ 1:54 102 Dm þ 4:7 106 Dq
ð18Þ
Upon differentiating Eq. (18) with respect to time, we obtain the chloride flux in region I (Fig. 1a): dnCl dDm þ 4:7 106 ið E Þ: ¼ 1:54 102 dt dt
ð19Þ
The fluxes of nickel and chloride represented by Eqs. (17) and (19) are presented in Fig. 2. It can be observed that the flux of nickel is positive (in the same direction of the dissolution process), whereas the flux of chloride is negative (in an opposite direction to the dissolution process (adsorption of chloride on the Cu+ layer)). To indicate whether or not the fluxes of the species presented in Fig. 2 are, in fact, related to the process that
Fig. 3. Experimental current (solid line) and calculated current (solid-square line) using Eq. (21) in potential region I.
takes place between 0.3 and 0.15 V, the current for the system presented in Fig. 2 was calculated. To obtain the calculated current from the fluxes of nickel and chloride, Eqs. (17) and (19), respectively, Eqs. (8)–(12) leading to the equation: Dq ¼ 2nNi2þ þ nCl F ð20Þ Upon differentiating Eq. (20) with respect to time, we obtain the calculated current: dðDqÞ dnNi2þ dnCl ¼ ið EÞ ¼ 2 þ F: ð21Þ dt dt dt The validity of Eq. (21) is corroborated by the profiles presented in Fig. 3. From these results, it can be noticed that the calculated and experimental currents are very similar, showing the validity of the model applied. Thus, between 0.3 and 0.15 V two processes take place simultaneously: (i) dissolution of nickel and (ii) chloride adsorption on the Cu+ sites with the formation of CuClads. These two processes together present an overall mass variation of 0.25 Ag. At potentials higher than 0.15 V there is a partial desorption of CuClads, while another part of it dissolves at 0.3 V. A plot of Dm versus Dq, in the region between 0.15 and 0.11 V presents a slope of 1103 g C1 that multiplied by the Faraday constant gives 97 g mol1. This value is close to that expected for the CuClads desorption (99 g mol1), corresponding to the transference of one electron. Therefore, in the region between 0.15 and 0.11 V the mass variation of 0.2 Ag is ascribed to CuClads desorption.
4. Conclusions
Fig. 2. Nickel flux obtained from Eq. (17) (dashed line) and chloride flux obtained from Eq. (19) (solid line) in potential region I from Fig. 1a (0.30 and 0.15 V).
In this paper a model of the flux of the species generated during the dissolution of a 1:1 Cu–Ni alloy in acidic medium has been presented for the first time. It can be concluded that the dissolution of the alloy occurs in two
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main regions: (I) desorption of Ni2+ and Cl adsorption on the Cu+ sites and (II) desorption of the weakly adsorbed CuClads. The third region (III) was characterized by the desorption of strongly adsorbed CuClads species formed at 0.1 V. These species can be formed with or without Ni in the solution and are independent of processes I and II.
Acknowledgements
[3] [4] [5] [6]
[7] [8] [9] [10]
The authors wish to thank the Brazilian Research Funding Institutions CAPES, CNPq and FAPESP for their financial support. [11]
References [1] E. Beltowska-Lehman, P. Ozga, Electrochim. Acta 43 (1998) 617. [2] D. Tench, J. White, Metall. Trans., A, Phys. Metall. Mater. Sci. 15 (1984) 2039.
C.A. Chang, J. Appl. Phys. 71 (1992) 825. R.Y. Ying, J. Electrochem. Soc. 135 (1988) 2957. H.P. Hack, H.W. Pickering, J. Electrochem. Soc. 138 (1991) 690. E. To´th-Ka´da´r, L. Peter, T. Becsei, J. To´th, L. Poga´ny, T. Tarno´czi, P. ´ . Czira´ ki, W. Schwarzacher, Kamasa, I. Bakonyi, G. La´ ng, A J. Electrochem. Soc. 147 (2000) 3311. M. Zhou, N. Myung, X. Chen, K. Rajeshwar, J. Electroanal. Chem. 398 (1995) 5. G. Sauerbrey, Z. Phys. 155 (1959) 206. C. Gabrielli, M. Keddan, R.M. Torresi, J. Electrochem. Soc. 138 (1991) 2657. T. Hepel. In R. Woods, F.M. Doyle, P. Richardson, eds. Proceedings of the International Symposium on Electrochemistry in Mineral and Metal Processing IV. Pennington, NJ. Vol. 96-6, 1996, pp. 72–78; M. Hepel, in A. Wieckowski (Ed.), Interfacial Electrochemistry, vol. 1, Marcel Dekker, New York, 1999, Chapter 34, p. 599. F.K. Crundwell, Electrochim. Acta 37 (1992) 2707.
A model for the flux of the species generated during the electrodissolution of a copper–nickel alloy on Pt in acidic media A. Manzoli, M.C. Santos*, L.O.S. Bulho˜es Laborato´rio Interdisciplinar de Eletroquı´mica e Ceraˆmica, Centro Multidisciplinar para o Desenvolvimento de Materiais Ceraˆmicos, Departamento de Quı´mica, Universidade Federal de Sa˜o Carlos, Caixa Postal 676, Sa˜o Carlos, SP 13565-905, Brazil Received 22 July 2004; accepted in revised form 6 October 2004 Available online 26 November 2004
Abstract This paper describes a new model to calculate the fluxes of the species generated during the electrodeposition of Cu–Ni alloys on Pt in acidic media. Measurements were carried out using linear sweep voltammetry and electrochemical quartz crystal microbalance (EQCM) techniques. The alloy electrodeposition was performed between +1.0 and 0.9 V versus SCE in a bath containing 0.04 M CuCl2d 2H2O+0.04 M NiCl2d 6H2O+0.32 M H3BO3+1 M NH4Cl. Following this, a sweep in the positive direction (between 0.3 and +1.0 V) to dissolve the alloy was performed using a blank solution (1 M NH4Cl+0.32 M H3BO3). The fluxes of the species versus the potential (Ni2+ (desorption) and Cl (adsorption on the top of Cu+)) generated during the alloy dissolution were calculated using the current and mass responses. In fact, it was observed that the electrodissolution obeys Faraday’s law. The flux calculated (considering a transference of two electrons in the case of Ni and one electron in the case of Cu) multiplied by the Faraday constant and plotted against E (i calculated) presents the same profile as the experimental results for the alloy dissolution. D 2004 Elsevier B.V. All rights reserved. Keywords: EQCM; Nickel; Copper; Electrodissolution; Flux; Alloys
1. Introduction In recent years interest in the electrodeposition of Cu–Ni alloys and their multilayers has increased, mainly due to the interesting properties of these materials, such as: corrosion resistance [1], mechanical [2], magnetic [3] and their catalytic properties [4]. Cu–Ni alloys, principally those that contain 30% to 40% Cu, are resistant to corrosion in acid media, basic media and especially in solutions containing chloride (seawater) [1]. Such alloys have been utilized in the construction of the commercial boats and in heat exchangers that use seawater for refrigeration [5]. Among the interesting mechanical properties displayed by Cu–Ni alloys are their high tensile strength, malleability and ductility [2].
* Corresponding author. Tel.: +55 16 260 8214; fax: +55 16 260 8214. E-mail address: [email protected] (M.C. Santos). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.012
Cu–Ni alloys present unique magnetic properties, Ni being ferromagnetic and Cu paramagnetic [3]. Ni88Cu19/Cu in alternated multilayers and Ni81Cu19 as a homogeneous alloy have both been shown to display giant magnetic resistances (GMR) [6]. Although there are some papers dealing with the technological aspects of the electrodeposition of Cu–Ni alloys, there are few studies that discuss how the species are generated during both the electrodeposition and electrodissolution processes. To the best of our knowledge there is only one paper [7] employing electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) and anodic stripping voltammetry (ASV) techniques to investigate in detail the Cu–Ni system. However, the study was performed on gold electrodes. The authors [7] employed a de-aerated Watts bath for the electrodeposition of Ni and Cu–Ni thin films. The combination of ASV and EQCM techniques permitted the analysis
A. Manzoli et al. / Surface & Coatings Technology 200 (2006) 2990–2994
of sequential Au/Cu–Ni–Ni/Ni thin film systems. This is because the Cu–Ni and Ni layers are dissolved under distinctly different potential regimes. The present study has the aim of proposing a new model for the flux of the species generated during the electrodissolution of the Cu–Ni alloys on Pt in acid medium. The study employs a combination of linear sweep voltammetry (LSV) and electrochemical quartz crystal microbalance (EQCM) techniques.
2. Experimental All reagents were of analytical grade and the water used to prepare the solutions was purified by the Milli-Q system (Millipore, nominal resistivity: 18.2 MV cm1). For the electrodeposition of the 1:1 Cu–Ni alloy, solutions of 0.04 M CuCl2d 2H2O, 0.04 M NiCl2d 6H20, 1 M NH4Cl and 0.32 M H3BO3 were used. The electrodeposition of the 1:1 Cu– Ni alloy was performed using the linear sweep voltammetry, applying a potential scan rate of 40 mV s1, in a potential range of +1.0 to 0.9 V. Before the stripping experiments, a diffraction analysis was performed using XRD (Rigaku diffractometer, model Dmax 2500PC), using CuKa radiation (k=1.5406 A). From this analysis the diffraction pattern associated with the formation of a copper–nickel alloy was observed (phases CuNi(111) (2h=43.608) and CuNi(200) (2h=50.798)) in agreement with the corresponding phases in JCPDS file. For the electrodissolution of the 1:1 Cu–Ni alloy 1 M NH4Cl and 0.32 M H3BO3 were used. This was achieved using linear sweep voltammetry coupled to the electrochemical quartz crystal microbalance. The potential scan rate was 40 mV s1 in the potential range 0.3 to +1.0 V. After stripping, the solutions were analyzed using atomic absorption spectroscopy (AAS) with a AA12/1475 spectrophotometer (INTRALAB/GEMINI) and the composition of 50% (m/m) copper and 50% (m/m) nickel was determined. The solutions were de-aerated with pre-purified N2 (White Martins). All experiments were carried out at room temperature and under atmospheric pressure. A quartz crystal analyzer (Seiko EG&G model QCA917) interfaced with a potentiostat model 273 EG&G Parc Instruments and a microcomputer were used for the electrochemical measurements of the resonant frequency of the quartz crystal. An AT-cut quartz crystal of f 0=9 MHz covered on both sides with a platinum film was employed as the working electrode. The geometric area of Pt electrode was approximately 0.2 cm2. A platinum sheet and a saturated calomel electrode (SCE) were used as the counter and the reference electrodes, respectively. The sensitivity factor for the EQCM was determined using the potentiostatic electrodeposition of copper (1 M H2SO4+0.1 M CuSO4), in which the dissolution charge densities were used to calculate the mass values and the resonant frequencies were measured experimentally. The
2991
value of the sensitivity factor thus obtained was 830 Hz Ag1, which is smaller than the calculated value of 915 Hz Ag1, using Sauerbrey’s Equation [8], Eq. (1): ! f02 Df ¼ Cf Dm ¼ Dm ð1Þ N qq where C f is the sensitivity factor, Dm is the mass per unit area (g cm2), f 0 is the resonant frequency in Hz, N is the frequency constant (1.67105 Hz cm) and q q (2.648 g cm3) is the quartz density. The difference observed between the experimental sensitivity coefficient and its theoretical value is attributed to the difference between the resonant frequency with the crystal oscillating in the vacuum and in the solution [9].
3. Results and discussion The 1:1 Cu–Ni alloy was initially electrodeposited in the interval 1.0 to 0.8 V. The deposited alloy then underwent electrodissolution in a blank solution (1 M NH4Cl+0.32 M H3BO3). In Fig. 1, the results for such an electrodissolution are presented. The potential sweep started at 0.3 V and continued positively to 1 V. Fig. 1a displays the profile for the electrodissolution of the 1:1 Cu–Ni alloy, in which can be observed three different regions. Region I (between 0.3 and 0.15 V) is characterized by the dissolution of Ni and adsorption of chloride on top of Cu+ (generated in the same potential region of the Cu0 layer): Ni0 YNi2þ þ 2e
ð2Þ
Cu0 YCuþ þ 1e
ð3Þ
Cuþ þ Cl YCuCl
ð4Þ
The mass profile for region I (between 0.3 and 0.15 V) (Fig. 1b) presents a mass decrease (0.25 Ag), which is related to the contribution of reaction (2), which is the principal process when compared to reaction (4). The charge versus potential profile (Fig. 1c) presents an increase of approximately 0.35 mC, due to the dissolution of Ni and oxidation of Cu (reactions (2) and (3), respectively). The formation of a CuCl complex has already been discussed for copper electrodeposition from 10 mM CuSO4+0.1 M NaCl utilizing the EQCM technique, however this was done in acid medium (0.1 M H2SO4) without the presence of Ni and at gold electrodes [10]. Region II in Fig. 1a (between 0.15 and 0.11 V) is attributed to the dissolution of a weakly adsorbed CuCl species on the substrate: CuClYCu2þ þ e þ Cl
ð5Þ
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accompanied by a high charge variation. These results are attributed to the CuCl complex dissolution. To analyze the process that takes place in region I during the 1:1 Cu–Ni alloy electrodissolution, a new flux model was employed. To elaborate this model, it was considered that the Cu–Ni alloy is a solid solution, which means that the alloy composition is constant at each potential. The mass variation in region I (0.3 and 0.15 V) can be attributed to Ni electrodissolution (reaction (2)) and Cl adsorption (reaction (4)) on the top of the Cu+ species (generated simultaneously with the oxidation of Cu0). Therefore, such mass variation can be described as presented by Eq. (7). Dm ¼ nNi2þ MNi2þ þ nCl MCl
ð7Þ
where n Ni2+ and n Cl are the number of moles of nickel and chloride, respectively, and M Ni2+=58.71 g mol1 and M Cl=35.45 g mol1, the molar masses for nickel and chloride, respectively. The charge variation (Dq) in the same potential region can be expressed by the following Eq. (8): Dq ¼ nNi2þ zNi2þ F þ nCuþ zCuþ F
ð8Þ
where, z Ni2+ and z Cu+ correspond to the valences of the ions, and F is the Faraday constant (97487 C mol1). Considering: Fig. 1. (a) Current variation, (b) mass variation and (c) charge variation as a function of the potential for the 1:1 Cu/Ni alloy stripping, in a 1 M NH4Cl and 0.32 M H3BO3 aqueous solution using the quartz crystal microbalance and potential scan rate of 40 mV s1.
As a consequence of this dissolution mechanism, the mass profile presents a decrease (0.2 Ag) and the charge profile presents an increase (0.17 mC). Region III (between 0.11 up to 1.0 V) is characterized by an oxidation process that takes place at 0.3 V (Fig. 1a). This process has been previously described [11] and it is due to the oxidation of Cu+ to Cu2+ from the previous formation of a CuCl complex at 0.1 V, which occurs with or without nickel in the solution and it is independent of the processes associated with peak I or II: CuClYCu2þ þ e þ Cl
nCuþ ¼ nCl
ð9Þ
zNi2þ ¼ 2
ð10Þ
zCuþ ¼ 1
ð11Þ
we obtain: Dq ¼ 2nNi2þ F þ nCl F; where n Cl is defined by Eq. (9): nCl ¼
Dq 2nNi2þ F
ð13Þ
Substituting Eq. (13) in Eq. (7), we obtain:
ð6Þ Dm ¼ nNi2þ MNi2þ þ
In this case, the dissolution occurs in a potential region that is more positive than that of region II. This is probably because the deposit is more internal and the adsorption strength on the platinum is higher than that of the previous region. In the mass and charge profiles displayed in Fig. 1 (region III), a small mass change can be observed, but this is
ð12Þ
Dq 2nNi2þ MCl ; F
and n Ni2+ becomes: nNi2þ ¼
Dm DqMCl : ð14Þ ð MNi2þ 2MCl Þ Fð MNi2þ 2MCl Þ
A. Manzoli et al. / Surface & Coatings Technology 200 (2006) 2990–2994
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Setting the values of M Ni2+=58.71 g mol1, M Cl=35.45 g mol1 and F=96487 C mol1 and substituting them in Eq. (14): Dm Dqð35:453Þ nNi2þ ¼ ð 58:71 70:906Þ 96487ð 58:71 70:906Þ ð15Þ nNi2þ ¼ 7:72 103 Dm þ 2:8 106 Dq
ð16Þ
Upon differentiating Eq. (16) with respect to time, we obtain the nickel flux in region I (between 0.3 and 0.15 V) (Fig. 1a): dnNi2þ dðDmÞ þ 2:8 106 ið E Þ ¼ 7:72 103 dt dt
ð17Þ
where i(E) is the experimental current. To obtain the flux of chloride in the 1:1 Cu/Ni alloy, it was considered that the mass variations follow Eq. (7), the charge variations follow Eq. (12). Using the values for the molar mass of copper and nickel and the Faraday constant we obtain: nCl ¼ 1:54 102 Dm þ 4:7 106 Dq
ð18Þ
Upon differentiating Eq. (18) with respect to time, we obtain the chloride flux in region I (Fig. 1a): dnCl dDm þ 4:7 106 ið E Þ: ¼ 1:54 102 dt dt
ð19Þ
The fluxes of nickel and chloride represented by Eqs. (17) and (19) are presented in Fig. 2. It can be observed that the flux of nickel is positive (in the same direction of the dissolution process), whereas the flux of chloride is negative (in an opposite direction to the dissolution process (adsorption of chloride on the Cu+ layer)). To indicate whether or not the fluxes of the species presented in Fig. 2 are, in fact, related to the process that
Fig. 3. Experimental current (solid line) and calculated current (solid-square line) using Eq. (21) in potential region I.
takes place between 0.3 and 0.15 V, the current for the system presented in Fig. 2 was calculated. To obtain the calculated current from the fluxes of nickel and chloride, Eqs. (17) and (19), respectively, Eqs. (8)–(12) leading to the equation: Dq ¼ 2nNi2þ þ nCl F ð20Þ Upon differentiating Eq. (20) with respect to time, we obtain the calculated current: dðDqÞ dnNi2þ dnCl ¼ ið EÞ ¼ 2 þ F: ð21Þ dt dt dt The validity of Eq. (21) is corroborated by the profiles presented in Fig. 3. From these results, it can be noticed that the calculated and experimental currents are very similar, showing the validity of the model applied. Thus, between 0.3 and 0.15 V two processes take place simultaneously: (i) dissolution of nickel and (ii) chloride adsorption on the Cu+ sites with the formation of CuClads. These two processes together present an overall mass variation of 0.25 Ag. At potentials higher than 0.15 V there is a partial desorption of CuClads, while another part of it dissolves at 0.3 V. A plot of Dm versus Dq, in the region between 0.15 and 0.11 V presents a slope of 1103 g C1 that multiplied by the Faraday constant gives 97 g mol1. This value is close to that expected for the CuClads desorption (99 g mol1), corresponding to the transference of one electron. Therefore, in the region between 0.15 and 0.11 V the mass variation of 0.2 Ag is ascribed to CuClads desorption.
4. Conclusions
Fig. 2. Nickel flux obtained from Eq. (17) (dashed line) and chloride flux obtained from Eq. (19) (solid line) in potential region I from Fig. 1a (0.30 and 0.15 V).
In this paper a model of the flux of the species generated during the dissolution of a 1:1 Cu–Ni alloy in acidic medium has been presented for the first time. It can be concluded that the dissolution of the alloy occurs in two
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main regions: (I) desorption of Ni2+ and Cl adsorption on the Cu+ sites and (II) desorption of the weakly adsorbed CuClads. The third region (III) was characterized by the desorption of strongly adsorbed CuClads species formed at 0.1 V. These species can be formed with or without Ni in the solution and are independent of processes I and II.
Acknowledgements
[3] [4] [5] [6]
[7] [8] [9] [10]
The authors wish to thank the Brazilian Research Funding Institutions CAPES, CNPq and FAPESP for their financial support. [11]
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