Materials Chemistry and Physics 72 (2001)

Transcription

1 Materials Chemistry and Physics 72 (2001) Kinetic and morphological investigation of copper electrodeposition from sulfate electrolytes in the presence of an additive based on ethoxyacetic alcohol and triethyl-benzyl-ammonium chloride S. Varvara a, L. Muresan a,, A. Nicoara a, G. Maurin b, I.C. Popescu a a Faculty of Chemistry and Chemical Engineering, Department of Physical Chemistry, Babes-Bolyai University, Str. Arany Janos 11, 3400 Cluj-Napoca, Romania b Laboratoire de Physique des Liquides et Electrochimie, Université Pierre et Marie Curie, Paris, France Received 18 May 2000; received in revised form 24 July 2000; accepted 16 September 2000 Abstract A comparative study of the influence of a new additive (IT-85), based on ethoxyacetic alcohol and triethyl-benzyl-ammonium chloride, and thiourea on copper electrodeposition from acidic sulfate solutions has been performed in order to obtain information about the kinetics of the cathodic process. In spite of their different chemical nature, both additives were found to be efficient as leveling agents, leading to fine-grained cathodic deposits. The study was based on scanning electron microscopy and X-ray dispersive analysis of the Cu deposits, coupled with an electrochemical investigation using cyclic voltammetry (CV) and steady-state polarization measurements at a rotating disc electrode. CV results and kinetic parameters obtained from Tafel plots led to the conclusion that both additives have a pronounced inhibiting effect on Cu 2+ discharge, the strongest inhibition being observed for thiourea. An induction period related to a slow nucleation, increasing with additive concentration, was clearly put on evidence on the polarization curves, for both additives Elsevier Science B.V. All rights reserved. Keywords: Additives; Cyclic voltammetry; Copper electrowinning; Ethoxyacetic alcohol; Polarization curves; Thiourea; Triethyl-benzyl-ammonium chloride 1. Introduction In modern electrodeposition practice, it is well known that the introduction of small amounts of proper additives in the plating bath results in beneficial changes in the quality of the cathodic deposits. Practically, all commercial electroplating baths contain one or more addition agents, which lead to leveled and bright deposits. However, in spite of this extensive use, there are still many unknown aspects concerning the mechanism of action of additives. Organic additives, such as thiourea [1 12], gelatine [5,6], polyacrylamide [13,14] and mixtures of different additives [15 17], are commonly used in copper electrorefining and electrowinning in order to produce smooth and bright copper deposits. In the present context, it is interesting to observe that all the above mentioned additives have beneficial effects upon copper electrodeposition, although their mechanism of action is rather different. Thus, gelatine and polyacrylamide act upon the mass transport because they Corresponding author. Tel.: /25; fax: address: limur@chem.ubbcluj.ro (L. Muresan). could form a viscous film near to the electrode. Contrarily, thiourea (Tu) influences the charge transfer, either via S 2 generation, followed by CuS precipitation [5,6], or by forming adsorbed [Cu(Tu) n ] + species, which block the active electrode sites and deliver copper slowly by dissociation [3,7,10]. Moreover, when different mixtures of additives are used, synergetic effects may appear, leading to more leveled cathodic deposits as compared with the case when the different components of the mixture were used alone [15 17]. Recently, a mixture of ethoxyacetic alcohol and triethyl-benzyl-ammonium chloride (TEBA) (IT-85), which has been successfully used as leveling agent in zinc electrowinning from acidic sulfate electrolytes [18], was found to be an efficient leveling agent in copper electrodeposition process [11]. Taking into account that both IT-85 components, TEBA and ethoxyacetic alcohol, separately behave as efficient blocking agents in metal electrodeposition [19,20] it was interesting to investigate their combined effect on copper electrodeposition from sulfate electrolytes. In order to gain better understanding of the fact that, in case of copper, two different additives (IT-85 and thiourea) /01/$ see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S (01)

2 S. Varvara et al. / Materials Chemistry and Physics 72 (2001) have similar effects on the morphology and structure of cathodic deposits, the aim of the present study was to evaluate the kinetic parameters of the cathodic process in presence of IT-85, as the first step in the explanation of the inhibition mechanism exerted by this additive. The effect of IT-85 on the kinetics and mechanism of the copper discharge process as well as upon the quality of copper deposits were compared with those exerted by thiourea. X-ray diffraction, scanning electron microscopy (SEM) and X-ray dispersive analysis were used to investigate the structure, the morphology, and the purity of the copper deposits, respectively. These results were correlated with those obtained from electrochemical measurements. Cyclic voltammetry (CV) and steady-state polarization curves were used to characterize the effects of IT-85 on cathodic polarization and to estimate the kinetic parameters for the copper electrodeposition process. 2. Experimental details 2.1. Reagents The inhibitor IT-85 was supplied by the Institute of Physical Chemistry of the Bulgarian Academy of Sciences from Sofia. It contains 300 g l 1 hydroxyethylated-2-butyne-1,4- diol ( Ferasine ) and 20 g l 1 TEBA [11,18]. The CuSO 4 stock solution used in this study contained 30gl 1 Cu 2+ and 100 g l 1 H 2 SO 4. All chemicals were Merck products of analytical grade, excepting thiourea (Reactivul Bucharest) Electrochemical measurements The experimental set-up for polarization curves and CV consisted of a three-electrode cell, a potentiostat (PS 3 Meinsberg, Germany) and a data acquisition system (Olivetti AT 486 DX computer with a National Instruments AT MIO16 T5 acquisition board). The working electrode was a copper disc electrode ( = 2mm). To ensure reproducibility between experiments, the exposed surface was polished with 600 grit paper, alumina, and rinsed with distillated water. Then it was introduced into an ultrasonic bath for 5 min. The counter electrode was a platinum foil separated from the solution by a ceramic diaphragm and a saturated calomel electrode (SCE) was used as reference electrode. The steady-state polarization curves for copper electrodeposition were recorded under potentiostatic conditions in the potential range from to 0.95 V vs. SCE, using a rotating working electrode (1000 rpm) in order to diminish the influence of mass transport. The cyclic voltammograms were recorded under potentiostatic conditions without ohmic drop compensation. The potential range was between +0.8 and 0.75 V vs. SCE and the potential scan rate was 20 mv s Preparative electrolysis Small-scale galvanostatic electrolysis was performed in the absence and in the presence of various amounts of additives, employing a 0.1-l plexiglass cell equipped with one vertical planar brass cathode between two parallel lead anodes. The interelectrode distance was 2 cm and the working temperature was 25 C. In all cases, the current density was held constant at 2.5 A dm 2 during the deposition time of 180 min. Copper was deposited on both sides of cathode onto a total area of 2.25 cm Deposit examination Morphological examination of the copper deposits involved visual inspection and SEM observation. The deposit purity was determined using an energy dispersive X-ray analyzer (Tracor-Voyager) coupled with a scanning electron microscope (Cambridge S 250). The preferential orientation of the crystals was determined by comparing the relative peak intensities from X-ray diffraction diagrams of the copper deposits with those of copper powder according to a method described in a previous study on lead electrodeposition [21]. 3. Results and discussion 3.1. Effect of additives on deposit morphology As it can be seen from Fig. 1a and b, the tested additives changed significantly the morphology and structure of the copper deposits as compared with those obtained from solutions without additives (Fig. 1c). The analysis of the Cu deposits by X-ray diffraction showed a change of the texture from [1 1 1] observed in the absence of additives to [1 1 0] noticed in the presence of both investigated additives. In both cases, the [1 1 0] texture showed clearly that an inhibition of the electrocrystallization process took place. The action of thiourea and IT-85 is one of inhibition of the crystal growth process, so that a relative enhancement of the nucleation process is induced. This results in a finer grained deposit. The grain size is smaller in the case of thiourea, but the Cu deposits obtained when IT-85 was used in a concentration of 50 ml l 1 were very satisfactory as well being smooth and consisting of uniform crystals. The X-ray dispersive analysis of copper deposits obtained from electrolytes containing thiourea did not indicate the presence of sulfur, proving that in our experimental conditions no incorporation of thiourea took place. This is in accordance with some results from the literature [1,8] Influence of the additives on the cathodic process The voltammograms presented in Fig. 2 show clearly that the tested additives influenced the cathodic process. In both cases, the maximum of the cathodic peak corresponding to

3 334 S. Varvara et al. / Materials Chemistry and Physics 72 (2001) Fig. 1. SEM micrographs of copper deposits obtained from an electrolyte containing 30 g l 1 Cu 2+ and 100 g l 1 H 2 SO 4 in the presence of: (a) 50 mg l 1 thiourea; (b) 50 ml l 1 IT-85; (c) no additives. Cu 2+ reduction is slightly shifted towards more negative values, simultaneously with the reduction of its area, denoting an inhibition of the electrocrystallization. The strongest inhibition is observed in the presence of thiourea. As mentioned before, the morphology and structure of the copper deposits obtained in the presence of thiourea or IT-85 are very similar, which means that both additives determine an inhibition of the crystal growth process, having comparable beneficial effects on copper electrocrystallization. In spite of this fact, the shapes of the voltammograms are not the same. Thus, in the case of thiourea a small shoulder on the cathodic wave, placed at 0.2 V vs. SCE can be ascribed to a reduction of an adsorbed [Cu(Tu) n ] + species [3]. In the presence of IT-85 the shoulder does not appear, denoting a different mechanism of action of this additive. In order to minimize the influence of mass transport, the cathodic linear polarization curves were recorded using the rotating disc electrode, without ohmic drop compensation. The ohmic drop could be neglected because of the high conductance of the electrolyte, the small distance between the working and reference electrode (<1 cm), and the low electrolysis current. The polarization curves obtained from solutions without and with different amounts of additives (thiourea and IT-85) are shown in Fig. 3a and b. As expected, it can be seen from Fig. 3a and b that the presence of both tested additives changes the cathode polarization, due to their adsorption at the electrode interface. The length of the initial part of the polarization curve, corresponding to the activation of electrocrystallization process, could be considered as a measure of the nucleation inhibition degree. The strongest inhibition was in the case of thiourea. As the additive concentration increases, the inhibiting action increases, too. The inhibition of nucleation observed in the presence of additives led to finer grained copper deposits. These facts are in good agreement with the deposit morphology (Fig. 1). For copper electrodeposition from acid sulfate solutions Mattson and Bockris [22] proposed the following mechanism: Cu 2+ + e Cu + Cu + + e Cu (R1) (R2) Fig. 2. Influence of additives on cyclic voltammograms. Experimental conditions: electrolyte containing 30 g l 1 Cu 2+ and 100 g l 1 H 2 SO 4 ; scan rate: 20 mv s 1. The redox process between Cu 2+ and Cu + (R1) is rate controlling, while Cu + exists in reversible equilibrium with Cu at the cathode surface. The above presented mechanism was used to estimate the cathodic transfer coefficient (α c ) and the exchange current density (i 0 ), by means of a Tafel plot [2,4]. For activation of overpotentials higher than 120 mv, it was found that the cathodic Tafel slope is varying from 115 to 134 mv per decade. Using the same modality for data interpretation, the electrochemical kinetic parameters, α c and i 0, corresponding to our experimental conditions, were calculated using the experimental data plotted in Fig. 4, and the obtained results are given in Table 1.

4 S. Varvara et al. / Materials Chemistry and Physics 72 (2001) Fig. 4. The cathodic Tafel plots for copper electrodeposition from electrolytes containing various IT-85 concentrations. in agreement with the decay of the current intensity observed on the polarization curves. In the same time, the presence of IT-85 changes the mechanism of the copper electrodeposition as it can be seen from the decreasing of the cathodic transfer coefficient. A possible explanation for this fact consists in the increasing role of an additional reaction that produces the same chemical species (Cu + ) as those involved in the rate-determining reaction. Thus, for copper electrodeposition, additional Cu + ions could be generated by the equilibrium Cu 2+ + Cu 2Cu + (R3) Fig. 3. Polarization curves for copper deposition on Cu cathode: (a) in absence and presence of different concentration of thiourea; (b) in absence and presence of different concentration of IT-85. Experimental conditions: scan rate, 20 mv s 1 ; rotation speed, 1000 rpm; electrolyte containing CuSO 4 (30 g l 1 Cu 2+ ) + H 2 SO 4 (100 g l 1 ). The obtained results showed that the presence of IT-85 has an inhibiting effect on the kinetics of the copper discharge process, pointed out by the decrease of the exchange current density. The inhibition enhancing due to increasing of IT-85 concentration could be related to the strong adsorption of IT-85 constituents on the copper electrode surface, which is The cathodic global transfer coefficient determines the process activation caused by the electrode potential. Thus, taking into account that the reaction (R3) does not involve charge transfer, α c should decrease when the ratio between the rate of the first electronation reaction (R1) and the reaction (R3) will decrease, as a consequence of the additive presence. This hypothesis about the influence of additives on the mechanism of copper electrodeposition requires further experimental work, which is actually in progress in our laboratory. In the literature, there are different opinions about the possibility to obtain cathodic Tafel plots in the presence of thiourea. In our experiments, in the investigated concentra- Table 1 Kinetic parameters of the reaction Cu + + e Cu in solutions without and with IT-85 IT-85 concentration (ml l 1 ) Tafel slope (V per decade) Corr. Coeff./No. of exp. points Transfer coefficient, α c ± a / ± a / ± a / ± a / a Standard deviation. Exchange current density, i 0 (ma cm 2 )

5 336 S. Varvara et al. / Materials Chemistry and Physics 72 (2001) tion range, a well-defined Tafel region in the presence of thiourea was not evidenced. This is in accordance with results of Suarez and Olson [7], who attributed this fact to a decreasing of the activity of cathodic surface in the presence of thiourea, due to its strong blocking effect of the surface active sites. This phenomenon along with diffusion of cupric ions through an adsorbed complex may account for the absence of Tafel region. Another possibility could be the strong inhibition exerted by thiourea on the nucleation process. The nucleation becomes the rate-determining step and leads to a flat region at low overpotentials in the polarization curves, which disables the Tafel treatment. 4. Conclusions Recently, the IT-85 additive was successfully used as leveling agent in zinc and copper electrowinning. A preliminary study about the influence of this additive and a widespread additive in copper electrodeposition, thiourea, was performed in order to obtain information about the kinetics and reaction mechanism. Thiourea and IT-85 were found to be efficient leveling additives in copper electrodeposition, leading to fine-grained cathodic deposit with a preferential growth orientation [1 1 0]. The results of electrochemical investigation led to the conclusion that both additives have an inhibiting effect on the copper ions discharge. The strongest inhibition is observed in the presence of thiourea. An induction period related to a slow nucleation, increasing with additive concentration, was clearly put on evidence in all cases. Based on steady-state polarization curves, Tafel plots were used in order to calculate the cathodic transfer coefficient and the exchange current density for IT-85 additive. The decrease of the cathodic transfer coefficient allowed the assumption that the electrodeposition mechanism is slightly modified in the presence of IT-85. It was observed that with increasing concentration of IT-85 the exchange current density decreases due to the additive adsorption. A possible explanation of the cathodic transfer coefficient decreasing was related to an additional chemical reaction that generates Cu +, species involved in the rate-determining reaction. In the case of thiourea, limited information was obtained, due to the lack of a well-defined Tafel region. The additional cathodic peak on cyclic voltammograms, evidenced in the presence of thiourea, led to the assumption of a different electrodeposition mechanism than in the case of IT-85 additive. The electrochemical impedance investigations, performed on the same experimental system, will be presented in a future paper giving additional data on the mechanism of inhibition. Acknowledgements We are grateful to Dr. Ts. Dobrev and Dr. I. Ivanov from the Institute of Physical Chemistry of the Bulgarian Academy of Sciences, Sofia, for supplying the inhibitor IT-85 to be tested in our laboratory. Financial support from CNCSIS (Grant No. 110/1999) is gratefully acknowledged. References [1] B. Ke, J.J. Hoekstra, B.C. Sison Jr., D. Trivich, J. Electrochem. Soc. 106 (1962) [2] Z.D. Stankovic, Erzmetall 38 (1985) [3] P. Cofré, A. Bustos, J. Appl. Electrochem. 24 (1994) [4] Z.D. Stankovic, M. Vukovic, Electrochim. Acta 41 (1996) [5] D.R. Turner, G.R. Johnson, J. Electrochem. Soc. 109 (1962) [6] D.R. Turner, G.R. Johnson, J. Electrochem. Soc. 109 (1962) [7] D.F. Suarez, F.A. Olson, J. Appl. Electrochem. 22 (1992) [8] M. Alodan, W. Smyrl, Electrochim. Acta 44 (1998) [9] S. Mendez, G. Andreasen, P. Schilardi, M. Figueroa, L. Vazquez, R.C. Salvarezza, A.J. Arvia, Langmuir 14 (1998) [10] G. Fabricius, K. Kontturi, G. Sundholm, Electrochim. Acta 39 (1994) [11] L. Muresan, S. Varvara, G. Maurin, S. Dorneanu, Hydrometallurgy 54 (2000) [12] M. Alodan, W. Smyrl, J. Electrochem. Soc. 145 (1998) [13] S. Goto, C. Oshima, JP 7,580,914 (1975). [14] J. Verecken, R. Winand, Surf. Technol. 4 (1976) [15] M. Loshkarev, L.M. Boichenko, Ukr. Khim. Zh. 36 (1970) [16] V.I. Sorokin, L.E. Sribnyi, Ukr. Khim. Zh. 44 (1978) [17] L. Mirkova, N. Petkova, I. Popova, St. Rashkov, Hydrometallurgy 36 (1994) [18] St.R. Stefanov, I.St. Ivanov, Hr.P. Bozhkov, V.V. Mircheva, Commun. Dept. Chem., Bulg. Acad. Sci. 24 (1991) [19] C. Cachet, R. Wiart, I. Ivanov, Y. Stefanov, S. Rashkov, J. Appl. Electrochem. 24 (1994) [20] E.D. Eliadis, R.C. Alkire, J. Electrochem. Soc. 145 (1998) [21] L. Muresan, L. Oniciu, M. Froment, G. Maurin, Electrochim. Acta 37 (1992) [22] E. Mattson, O.M. Bockris, Trans. Faraday Soc. 55 (1960)