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Electrodeposition of Cu–Zn thin films from room temperature ionic liquid
Electrochimica Acta 107 (2013) 624–631
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrodeposition of Cu–Zn thin films from room temperature ionic liquid C. Rousse, S. Beaufils, P. Fricoteaux ∗ LISM, EA4695, UFR Sciences Exactes et Naturelles, URCA, BP 1039 51687 Reims Cedex 2, France
a r t i c l e
i n f o
Article history: Received 10 April 2013 Received in revised form 10 June 2013 Accepted 15 June 2013 Available online 24 June 2013 Keywords: Ionic liquid Zinc Copper Alloy Electrodeposition
a b s t r a c t The electrosynthesis of Cu, Zn and Cu–Zn deposits from the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid has been investigated. Anion of this metallic salt (bis(trifluoromethylsulfonyl)imide) is the same as that used as the solvent. The redox potential sequence of copper and zinc in the used ionic liquid was the same as that of aqueous solvent. The obtained voltammograms for single metal deposition exhibits several electrodeposition steps. In the case of copper, the first step does not lead to metal deposition and is attributed to the Cu(II) → Cu(I) transformation. Rest of the steps result in the Cu(0). Contrary, all the steps in zinc deposition, correspond to growth of metallic deposits. In the case of the copper–zinc alloys, their chemical composition exhibits a surprising evolution versus potential and seems to be greatly dependent on the cathodic steps. The morphology and analysis of copper, zinc and their alloys are reported. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Zinc and copper are important materials which play a strategic role in several industrial applications, such as: automotive industry, aeronautics, household appliances, etc. Both metals may be easily produced by the electrochemical technique from an aqueous solution with low fabrication costs. Nevertheless, with the development of air and water stable ionic liquids, new opportunities appear for electrodeposition routes. In the beginning, the use of these solvents concerned essentially materials, which cannot be electroplated from aqueous solutions [1–3]. But other advantages also exist, for example, the absence of hydrogen evolution simultaneously with metal deposition such as: nickel, zinc, etc. Therefore, different studies have been made to describe zinc and zinc alloys deposition. Some of them (Zn–Co, Zn–Cu, Zn–Sn, Zn–Fe, Zn–Ni. . .) were reported by the research group of Sun [4–9], who used the zinc chloride-1-ethyl-3-methylimidazolium chloride as a low temperature molten salt. Zn–Mn alloy has been studied by Chen and Hussey [10] using the tri-1-butylmethylammonium bis((trifluoromethane)sulfonyl)imide as the solvent. These authors used an anodic dissolution of metals, because they did not find Zn salts, which could be dissolved in the used solvent. More recently, Pan et al. [11] investigated the Al–Zn co-deposition from metal chlorides.
∗ Corresponding author. Tel.: +33 3 26 91 85 82. E-mail address: [email protected] (P. Fricoteaux). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.06.053
Another advantage of the use of ionic liquids is the possibility to obtain nanocrystalline deposits without any additive. For example, Zein El Abedin et al. [12] have deposited nanocrystalline copper film using the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide solvent. However, the tested copper salts by authors had a limited solubility in this solvent and they should have introduced the cation by anodic dissolution of a copper electrode. According to their results, the oxidation state of copper species was found to be 1. In this paper, we investigate the electrodeposition of thin film of Cu, Zn and their alloys. The used solvent is the same as in Zein El Abedin et al. [12] works but similar as Zhu et al. [13], we employed metallic salts composed with the bis(trifluoromethylsulfonyl)imide anion. Because the anion is as same as the solvent, the salts were easily dissolvable. 2. Experimental The electrodeposition of copper, zinc and copper–zinc was performed in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([P1,4 ]Tf2 N) ionic solvent. Because of the weak solubility of “classic” metallic salts [10,12,13], copper and zinc were introduced in the solution by dissolution of Cu(Tf2 N)2 and/or Zn(Tf2 N)2 . All the compounds came from the Solvionic manufacturing (France) and had a certified high purity level (99.9% for solvent and 99% for salts). The used concentrations were 0.2 mol L−1 for each metallic salt. After salt dissolution and before the experimental stage, the solutions has been dried under vacuum at 130 ◦ C during 1 week.
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Fig. 1. Current density–potential curves for solvent (dashed curves) and copper solutions (solid curves) (a) scan rate = 2 mV s−1 ; nickel substrate, (b) Zoom of the initial scan of the copper curve, (c) scan rate = 2 mV s−1 ; nickel substrate previously deposited with copper (−1 V/Ag during 30 min), (d) scan rate = 50 mV s−1 ; nickel substrate.
The electrochemical experiments were carried out at room temperature, without stirring, inside an argon-filled glove box with water and oxygen content below 1 ppm. A conventional three electrode setup was used to perform experiments. All of the electrodes were immersed in an unique compartment of a glass cell. Teflon O-ring was used to delimitate the exposed surface (0.2 cm2 ) of the working electrode (WE). Platinum wire was used as a counter
electrode. Silver wire immersed in Ag(Tf2 N) (0.1 mol L−1 ) and dissolved in [P1,4 ]Tf2 N solvent was used as the reference electrode. Silver wire and Ag(I) solution were placed in a glass tube closed by a sintered glass. All values of the potentials are given with a respect to the Ag(I)/Ag system (noted/Ag in the text). Before each experiment, the platinum wire was washed with 99.9% alcohol and then heated by flame for few seconds until glowing. Nickel and copper
Fig. 2. SEM images of the copper deposits after 2 h of electrolysis onto nickel substrate (a) E = −0.75 V/Ag, (b) E = −1.5 V/Ag, (c) E = −2 V/Ag, (d) E = −2.5 V/Ag.
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Fig. 3. EDXS specters of copper deposits after 2 h of electrolyze onto nickel substrate (a) E = −0.75 V/Ag, (b) E = −2 V/Ag.
substrates were used as WE. Before electrodeposition, the samples were mechanically polished with increasing finer grades of silicon carbide paper and rinsed with alcohol before drying. The electrochemical measurements were conducted using a Radiometer potentiostat-galvanostat PGZ 301 controlled by Voltamaster 4 software. The morphology and chemical composition of the samples were analyzed by JEOL JSM 6460LA Scanning Electron Microscope (SEM) coupled with EDS JEOL 1300 microprobe (EDXS). X-ray diffraction investigations have been carried out using BRUKER D8 ADVANCE X-ray diffractometer equipped with a copper anticathode ( Cu K ˚ ˛ = 1.54056 A). 3. Results and discussion 3.1. Cu electrodeposition Fig. 1 displays some current density–potential curves for solvent (dashed curves) and copper deposit (solid curves). The voltammograms in Fig. 1a, b and d were scanned directly on the nickel
Fig. 4. XRD of copper deposit after 2 h of electrolyze at −0.75 V/Ag onto nickel substrate.
Fig. 5. Current density–potential curves for solvent (dashed curves) and zinc solutions (solid curves). (a) Scan rate = 2 mV s−1 ; nickel substrate, (b) scan rate = 2 mV s−1 ; nickel substrate previously deposited with zinc (−1.6/Ag during 30 min), (c) scan rate = 50 mV s−1 ; nickel substrate.
substrate. The curve in Fig. 1c was recorded on the nickel substrate previously deposited with copper (−1 V/Ag during 30 min). The scan rate was 2 mV s−1 for Fig. 1a–c and 50 mV s−1 for Fig. 1d. In order to avoid the co-reduction of organic cations, the copper reduction scan was limited to −3 V/Ag (massive evolution of solvent appears for potentials between −3 and −3.5 V/Ag). The anodic scan was stopped just after the first step of the deposit oxidation. The obtained copper voltammograms are complex and require some comments. Many cathodic steps, more or less visible depending on the scan rate, are presented and are labeled as C1Cu , C2Cu , C3Cu and C4Cu . The first cathodic step, C1Cu takes place on a nickel substrate for a potential superior to the anodic step A1Cu . However, it does not correspond to an underpotential deposition (UPD) phenomenon because this step did not result in any metallic deposit. Hence, the first reduction process is assigned to the reaction (1). Cu(II) + e− → Cu(I)
(1)
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Fig. 6. SEM images of zinc deposits after 2 h of electrolyze onto nickel substrate. (a) E = −1.6 V/Ag, (b) E = −1.9 V/Ag, (c) E = −2.3 V/Ag, and (d) E = −2.7 V/Ag.
According to the results of Zein El Abedin et al. [12], the first oxidation step of metallic copper A1Cu in this solvent results in an oxidation state equal to one (reaction (2)). Cu(0) → Cu(I) + e−
(2)
Consequently, when the curve is scanned after a preliminary copper deposit (Fig. 1c), the step C1Cu becomes almost invisible because of
Fig. 8. XRD of zinc deposit after 2 h of electrolyze at −1.6 V/Ag onto nickel substrate.
Fig. 7. EDXS specters of zinc deposits after 2 h of electrolyze onto nickel substrate. (a) E = −1.6 V/Ag and (b) E = −2.7 V/Ag.
Fig. 9. Current density–potential curves for copper (dashed curve), zinc (dotted curve) and copper–zinc solutions (solid curve). Scan rate = 2 mV s−1 ; nickel substrate.
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Fig. 10. Current density–potential curves for zinc solutions; scan rate = 2 mV s−1 : nickel substrate (solid curve), copper substrate (dashed curve).
the chemical reaction (3) between the chemical species Cu(II) and Cu(0): Cu(II) + Cu(0) → 2Cu(I)
(3)
Reaction (3) leads to a partial or total dissolution of the deposit depending on its thickness or the rest time before the scan initiation. The following cathodic step (C2Cu ) results in metallic deposits and could be attributed to the reactions (4) or (5): Cu(I) + e− → Cu(0)
(4)
Cu(II) + 2e− → Cu(0)
(5)
A slight shift between the beginning of steps C2Cu and A1Cu in Fig. 1a is observed comparing to Fig. 1c, i.e. depending on the presence or absence of a previously copper deposit. This suggests that a
small overpotential deposition (OPD) could be present on the nickel substrate. In the case of steps C3Cu and C4Cu , it cannot be excluded that Cu(II) may be complexed by the bis(trifluoromethylsulfonyl)amide anion under different forms. Note that the C4Cu step is not distinctly visible at the higher scan rate (Fig. 1d). This is probably due to a slow kinetic reaction in relation with adsorbed intermediates. At the end of the step C4Cu (−2.5 V/Ag), a slight increase of current is observed in Fig. 1b. This is attributed to the impurity traces and/or the adsorption of the organic cation from the solvent [14,15]. According to the previous equations, the coulomb integration below the cathodic curve (sum of initial and return) should be about the double of the anodic peak A1Cu because there is only one exchange electron in the reaction (2). For low scan rate (Fig. 1a and c) the obtained value between the reductions steps and oxidation peak gives a ratio superior to 4. This difference compared with the theoretical value (equal to 2) is most likely explained by the reaction (3), which generates a partial dissolution of the metallic copper. At the higher scan rate (Fig. 1d), reaction (3) becomes negligible and an approximate ratio of 2 is obtained (88 mC cm−2 for cathodic reactions and 39 mC cm−2 for anodic reaction). Note that the reduction of impurities (or beginning of the organic cation reduction) at the end of C4Cu could be also explained by the fact that the calculated ratio is not exactly equal to 2. Fig. 2 shows SEM microphotographs of electrodeposited copper during 2 h in the zones C2Cu , C3Cu and C4Cu . The morphology of plate surfaces of the deposition zones C2Cu and C3Cu (Fig. 2a and b) is very similar to each other. The morphological characteristic of samples reveals nodular shapes. On the other hand, the samples plated in the zone C4Cu have clearly different morphology. Flower-like structure begins to appear at −2 V/Ag (Fig. 2c) before to be transformed in aggregated spherical particles at −2.5 V/Ag (Fig. 2d). Note that the obtained deposits at lower
Fig. 11. Zinc atomic percentage (black points) and current density (solid curve) versus applied potential for copper–zinc solution.
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Fig. 12. SEM images of copper–zinc deposits after 2 h of electrolyze onto nickel substrate. (a) E = −1.4 V/Ag (Zn 24 at%), (b) E = −1.6 V/Ag (Zn 45 at%), (c) E = −2.2 V/Ag (Zn 26 at%), (d) E = −2.5 V/Ag (Zn 43 at%).
potentials (−3 V/Ag) were brown with a non adhesive gelatinous layer on the top of the plated surface. This is related with the beginning of the solvent reduction (even if this potential value is clearly inferior to the massive evolution of the solvent). A weakly pink deposit, characteristic for copper was often found under this gelatinous layer but was too thin to be correctly analyzed and these deposits were consequently excluded from our study. EDXS and XRD analyses were performed on each sample. Some EDXS results are presented in Fig. 3. In each case, nickel and copper signals in the EDXS spectrums are detected. The first ones are associated with the substrate and the seconds are assigned to the presence of copper deposit. The XRD analysis (Fig. 4) of the film deposited at −0.75 V/Ag proves that the obtained copper is in metallic form.
The presence of two reduction waves suggests, as in the case of copper, the possibility of two different reduction mechanisms or the presence of two different forms of complexed Zn(II), which are formed by the bis(trifluoromethylsulfonyl)amide anion. The first C1Zn step (Fig. 5a), takes place on the nickel substrate at a potential clearly inferior to the anodic step, indicating an overpotential phenomenon for the zinc nucleation. This is confirmed in Fig. 5b, where the beginning of the reduction process is similar between the initial and the return scans when zinc is previously deposit before experiment. Regardless on the scan rate, the area below the initial and return cathodic peaks and the anodic peak is almost equivalent to a 1.1 ratio. Although this value is higher than one (probably due to the reduction of impurities or the beginning of the organic cation reduction), it shows that the oxidation of zinc involves a two electron mechanism (Eq. (7)).
3.2. Zn electrodeposition In Fig. 5, zinc voltammetry on nickel substrate (solid curves) were plotted and compared with the current density–potential curves of the solvent reduction (dashed curves). The voltammogram in Fig. 5a was scanned directly on the nickel substrate while the one in Fig. 5b was scanned after achieving a weak zinc deposit. In these two cases, the scan rate was equal to 2 mV s−1 . For Fig. 5c, the scan rate was increased to 50 mV s−1 and no deposit has been made before. Two distinct cathodic steps, labeled C1Zn and C2Zn , are presented regardless of the scan rate. Contrary to the Cu reduction, metallic deposit appears on the working electrode immediately after the beginning of the first reduction process. This involves a two electron transfer, which depends on the reaction (6). Zn(II) + 2e− → Zn(0)
(6)
Zn(0) → Zn(II) + 2e−
(7)
In Fig. 6, SEM micrographs of Zn deposits at different potentials are presented. The surface morphologies of zinc deposits are very different than those obtained for copper deposits. Fig. 6a shows plated Zn at the applied potential equal to −1.6 V/Ag corresponding to the beginning of the first reduction C1Zn step. For this polarization, the deposit consists of well-defined hexagonal shapes crystals. At the end of the zone C1Zn (−1.9 V/Ag), the edges of crystals are much less defined and composed of needle-shaped clusters (Fig. 6b). This evolution progress with the overpotential, thus no distinguished crystallites are observed at the beginning of C2Zn (−2.3 V/Ag; Fig. 6c). However, a distinct morphology can be observed at −2.7 V/Ag (Fig. 6d), where the presence of granular shaped crystals is noticed.
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Fig. 13. EDXS specters of copper–zinc deposits after 2 h of electrolyze onto nickel substrate. (a) E = −1.6 V/Ag and (b) E = −2.5 V/Ag.
As in the case of copper, EDXS (Fig. 7) and XRD (Fig. 8) analyses were performed to confirm the presence of metallic zinc in the deposits.
3.3. Cu–Zn electrodeposition Fig. 9 presents the current density–potential curve recorded in the solution containing 0.2 mol L−1 Cu(Tf2 N)2 and 0.2 mol L−1 Zn(Tf2 N)2 (solid curve). This curve was overlaid with that of copper (dashed curve) and zinc (dotted line). Similar to zinc or copper curves, several reduction waves appear before the solvent reduction. By comparing Fig. 1 (Cu alone) and Fig. 5 (Zn alone), we have attributed the different waves either to copper or to zinc reduction.
Because zinc deposition takes place after the copper deposition, we have firstly confirmed that the comportment of zinc reduction on the copper substrate is as same as on the nickel substrate and that no shift in the Zn reduction potential is observed (Fig. 10). The Cu–Zn deposits were plated at different values of polarization during 2 h. Fig. 11 shows the evolution of zinc atomic percentage (black points) inside the plated Cu–Zn alloy versus applied potential. The Cu–Zn current density–potential curve (solid curve) was overlaid in the same graph. The obtained evolution does not correspond to a logical progression versus overpotential i.e. with a regular increase of zinc. At the beginning of the zinc reduction, the evolution of zinc content seems to correspond to a classical progression versus polarization. The atomic percentage of zinc rises up until potential value equal to about −1.6 V/Ag. For more negative potential values, the Zn content begins to decrease. It is interesting to note that the beginning of decreasing Zn percentage corresponds to the beginning of C4Cu process. At this potential, zinc is under diffusion control for the C1Zn step and the beginning of C4Cu step leads to an increase of copper content in the deposit. Nevertheless, an inhibition of the zinc reduction due to a partial occupation of adsorption sites by copper reduced metal intermediates from the C4Cu step could be also possible. Following several hundred of millivolts of overpotential a new increase in the zinc percentage is observed. Finally, the obtained content of copper and zinc in the deposit at −2.5 V/Ag reaches almost an equivalent value of each metal, i.e. close to the solution composition. The SEM micrographs of deposits (Fig. 12) exhibit two different forms of crystals. Circular shapes (Fig. 12a and b) are obtained when potential is equal to −1.6 V/Ag, i.e. until the beginning of C4Cu process. For more negative potentials (Fig. 12c and d), nodular shapes appear. In each case, no fundamental difference in the EDXS analyses is observed (Fig. 13). However, the XRD patterns are completely different before and after the C4Cu step (Fig. 14a). At the beginning of zinc reduction up to −1.6 V/Ag, the Cu–Zn alloy signals are observed. For more negative potentials, the coating becomes amorphous and the Cu–Zn signals rapidly disappears and only the nickel substrate peaks are clearly visible. Note that in each case, no pure copper and no pure zinc peaks are detected in the diffractograms. Fig. 14b shows fragments of the XRD patterns for different polarization. It exhibits that more or less important mixture of
Fig. 14. XRD and fragments of XRD of copper–zinc deposit after 2 h of electrolyze onto nickel substrate. (a) E = −1.6 V/Ag and E = −2.5 V/Ag (b) XRD fragments for applied potential between −1.4 V/Ag and −2.5 V/Ag.
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different Cu–Zn phases (PDF No.25-1228, 19-0179, 41-1435) for lower polarization is presented, which is not observed at higher potential. 4. Conclusions This study shows the possibility to plate Cu, Zn and Cu–Zn deposits from ionic liquid without use of chloride anions. The copper deposition takes place after a one electron transfer step, while zinc is directly reduced by a two electron transfer. The kinetic of deposition process is complicated and leads to surprising evolution of the Cu–Zn deposit composition versus polarization. At the beginning of zinc reduction up to −1.6 V/Ag, a regular increase of the zinc percentage in the deposit is obtained. The plated deposits at this potential range results in XRD patterns corresponding to a mixture of different Cu–Zn phases. All deposits obtained in this potential range (potential superior to −1.6 V/Ag) exhibit same morphology composed with spherical shapes crystallites. Between −1.6 V/Ag and −2.25 V/Ag, the zinc content decreases and the Cu–Zn alloy signals disappear, indicating an amorphous coating. For potential more negative than −2.25 V/Ag, the zinc content increases again, but no Cu–Zn alloy signal appears. For these two last potential range, the alloy morphology changes and nodular shapes crystallites appears. Acknowledgment The authors thank Patrick Baudart for the electrochemical cell conception. References [1] P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, Wiley-VCH, Verlag GmpH, 2003.
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[2] F. Endres, S. Zein El Abedin, Air and water stable ionic liquids in physical chemistry, Phys. Chem. Chem. Phys. 8 (2006) 2101–2116. [3] F. Endres, A.P. Abbott, D.R. MacFarlane (Eds.), Electrodeposition from Ionic Liquids, Wiley-VCH, Weinheim, 2008. [4] Y.-F. Lin, I.-W. Sun, Electrodeposition of zinc from a Lewis acidic zinc chloride-1ethyl-3-methylimidazolium chloride molten salt, Electrochim. Acta 44 (1999) 2771–2777. [5] P.-Y. Chen, M.-C. Lin, I.-W. Sun, Electrodeposition of Cu-Zn alloy from a Lewis acidic ZnCl2 -EMIC molten salt, J. Electrochem. Soc. 147 (2000) 3350–3355. [6] P.-Y. Chen, I.-W Sun, Electrodeposition of cobalt and zinc-cobalt from a Lewis acidic zinc chloride-1-ethyl-3-methylimidazolium chloride molten salt, Electrochim. Acta 46 (2001) 1169–1177. [7] J.-F. Huang, I.-W. Sun, Electrochemical studies Tin in Zinc chloride-1-ethyl3-methylimidazolium chloride ionic liquids, J. Electrochem. Soc. 150 (2003) E299–E306. [8] J.-F. Huang, I.-W. Sun, Nonanomalous electrodeposition of zinc-iron alloys in an acidic zinc chloride-1-ethyl-3-methylimidazolium chloride ionic liquid, J. Electrochem. Soc. 151 (2004) C8–C14. [9] S.-P. Gou, I.-W. Sun, Electrodeposition behavior of nickel and nickel-zinc alloys from the zinc chloride-1-ethyl-3-methylimidazolium chloride low temperature molten salt, Electrochim. Acta 53 (2008) 2538–2544. [10] P.-Y. Chen, C.L. Hussey, The electrodeposition of Mn and Zn-Mn from the room-temperature tri-1-buthylmethylammonium alloys bis((trifluoromethane)sulfonyl)imide ionic liquid, Electrochim. Acta 52 (2007) 1857–1864. [11] S.-J. Pan, W.-T. Tsai, J.-K. Chang, I.-W. Sun, Co-deposition of Al-Zn on AZ91D magnesium alloy in AlCl3 -1-ethyl-3-methylimidazolium chloride ionic liquid, Electrochim. Acta 55 (2010) 2158–2162. [12] S. Zein El Abedin, A.Y. Saad, H.K. Farag, N. Borisenko, Q.X. Liu, F. Endres, Electrodeposition of selenium, indium and copper in an air-and water- stable ionic liquid with varying water content, Electrochim. Acta 52 (2007) 2746–2754. [13] Y.-L. Zhu, Y. Kozuma, Y. Katayama, T. Miura, Electrochemical behavior of Ni(II)/Ni in a hydrophobic amide-type room-temperature ionic liquid, Electrochim. Acta 54 (2009) 7502–7506. [14] P.C. Howlett, E.I. Izgorodina, M. Forsyth, D.R. MacFarlane, Electrochemistry at negative potentials in bis(trifluoromethanesulfonyl) amide ionic liquids, Z. Phys. Chem. 220 (2006) 1483–1498. [15] N. Borisenko, S. Zein El Abedin, F. Endres, In situ STM investigation of gold reconstruction and of silicon electrodeposition on Au(111) in the room temperature ionic liquid 1-Buthyl-1-methylpyrrolidinium J. Phys. Chem. B 110 (2006) bis(trifluoromethylsulfonyl)imide, 6250–6256.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrodeposition of Cu–Zn thin films from room temperature ionic liquid C. Rousse, S. Beaufils, P. Fricoteaux ∗ LISM, EA4695, UFR Sciences Exactes et Naturelles, URCA, BP 1039 51687 Reims Cedex 2, France
a r t i c l e
i n f o
Article history: Received 10 April 2013 Received in revised form 10 June 2013 Accepted 15 June 2013 Available online 24 June 2013 Keywords: Ionic liquid Zinc Copper Alloy Electrodeposition
a b s t r a c t The electrosynthesis of Cu, Zn and Cu–Zn deposits from the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid has been investigated. Anion of this metallic salt (bis(trifluoromethylsulfonyl)imide) is the same as that used as the solvent. The redox potential sequence of copper and zinc in the used ionic liquid was the same as that of aqueous solvent. The obtained voltammograms for single metal deposition exhibits several electrodeposition steps. In the case of copper, the first step does not lead to metal deposition and is attributed to the Cu(II) → Cu(I) transformation. Rest of the steps result in the Cu(0). Contrary, all the steps in zinc deposition, correspond to growth of metallic deposits. In the case of the copper–zinc alloys, their chemical composition exhibits a surprising evolution versus potential and seems to be greatly dependent on the cathodic steps. The morphology and analysis of copper, zinc and their alloys are reported. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Zinc and copper are important materials which play a strategic role in several industrial applications, such as: automotive industry, aeronautics, household appliances, etc. Both metals may be easily produced by the electrochemical technique from an aqueous solution with low fabrication costs. Nevertheless, with the development of air and water stable ionic liquids, new opportunities appear for electrodeposition routes. In the beginning, the use of these solvents concerned essentially materials, which cannot be electroplated from aqueous solutions [1–3]. But other advantages also exist, for example, the absence of hydrogen evolution simultaneously with metal deposition such as: nickel, zinc, etc. Therefore, different studies have been made to describe zinc and zinc alloys deposition. Some of them (Zn–Co, Zn–Cu, Zn–Sn, Zn–Fe, Zn–Ni. . .) were reported by the research group of Sun [4–9], who used the zinc chloride-1-ethyl-3-methylimidazolium chloride as a low temperature molten salt. Zn–Mn alloy has been studied by Chen and Hussey [10] using the tri-1-butylmethylammonium bis((trifluoromethane)sulfonyl)imide as the solvent. These authors used an anodic dissolution of metals, because they did not find Zn salts, which could be dissolved in the used solvent. More recently, Pan et al. [11] investigated the Al–Zn co-deposition from metal chlorides.
∗ Corresponding author. Tel.: +33 3 26 91 85 82. E-mail address: [email protected] (P. Fricoteaux). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.06.053
Another advantage of the use of ionic liquids is the possibility to obtain nanocrystalline deposits without any additive. For example, Zein El Abedin et al. [12] have deposited nanocrystalline copper film using the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide solvent. However, the tested copper salts by authors had a limited solubility in this solvent and they should have introduced the cation by anodic dissolution of a copper electrode. According to their results, the oxidation state of copper species was found to be 1. In this paper, we investigate the electrodeposition of thin film of Cu, Zn and their alloys. The used solvent is the same as in Zein El Abedin et al. [12] works but similar as Zhu et al. [13], we employed metallic salts composed with the bis(trifluoromethylsulfonyl)imide anion. Because the anion is as same as the solvent, the salts were easily dissolvable. 2. Experimental The electrodeposition of copper, zinc and copper–zinc was performed in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([P1,4 ]Tf2 N) ionic solvent. Because of the weak solubility of “classic” metallic salts [10,12,13], copper and zinc were introduced in the solution by dissolution of Cu(Tf2 N)2 and/or Zn(Tf2 N)2 . All the compounds came from the Solvionic manufacturing (France) and had a certified high purity level (99.9% for solvent and 99% for salts). The used concentrations were 0.2 mol L−1 for each metallic salt. After salt dissolution and before the experimental stage, the solutions has been dried under vacuum at 130 ◦ C during 1 week.
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Fig. 1. Current density–potential curves for solvent (dashed curves) and copper solutions (solid curves) (a) scan rate = 2 mV s−1 ; nickel substrate, (b) Zoom of the initial scan of the copper curve, (c) scan rate = 2 mV s−1 ; nickel substrate previously deposited with copper (−1 V/Ag during 30 min), (d) scan rate = 50 mV s−1 ; nickel substrate.
The electrochemical experiments were carried out at room temperature, without stirring, inside an argon-filled glove box with water and oxygen content below 1 ppm. A conventional three electrode setup was used to perform experiments. All of the electrodes were immersed in an unique compartment of a glass cell. Teflon O-ring was used to delimitate the exposed surface (0.2 cm2 ) of the working electrode (WE). Platinum wire was used as a counter
electrode. Silver wire immersed in Ag(Tf2 N) (0.1 mol L−1 ) and dissolved in [P1,4 ]Tf2 N solvent was used as the reference electrode. Silver wire and Ag(I) solution were placed in a glass tube closed by a sintered glass. All values of the potentials are given with a respect to the Ag(I)/Ag system (noted/Ag in the text). Before each experiment, the platinum wire was washed with 99.9% alcohol and then heated by flame for few seconds until glowing. Nickel and copper
Fig. 2. SEM images of the copper deposits after 2 h of electrolysis onto nickel substrate (a) E = −0.75 V/Ag, (b) E = −1.5 V/Ag, (c) E = −2 V/Ag, (d) E = −2.5 V/Ag.
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Fig. 3. EDXS specters of copper deposits after 2 h of electrolyze onto nickel substrate (a) E = −0.75 V/Ag, (b) E = −2 V/Ag.
substrates were used as WE. Before electrodeposition, the samples were mechanically polished with increasing finer grades of silicon carbide paper and rinsed with alcohol before drying. The electrochemical measurements were conducted using a Radiometer potentiostat-galvanostat PGZ 301 controlled by Voltamaster 4 software. The morphology and chemical composition of the samples were analyzed by JEOL JSM 6460LA Scanning Electron Microscope (SEM) coupled with EDS JEOL 1300 microprobe (EDXS). X-ray diffraction investigations have been carried out using BRUKER D8 ADVANCE X-ray diffractometer equipped with a copper anticathode ( Cu K ˚ ˛ = 1.54056 A). 3. Results and discussion 3.1. Cu electrodeposition Fig. 1 displays some current density–potential curves for solvent (dashed curves) and copper deposit (solid curves). The voltammograms in Fig. 1a, b and d were scanned directly on the nickel
Fig. 4. XRD of copper deposit after 2 h of electrolyze at −0.75 V/Ag onto nickel substrate.
Fig. 5. Current density–potential curves for solvent (dashed curves) and zinc solutions (solid curves). (a) Scan rate = 2 mV s−1 ; nickel substrate, (b) scan rate = 2 mV s−1 ; nickel substrate previously deposited with zinc (−1.6/Ag during 30 min), (c) scan rate = 50 mV s−1 ; nickel substrate.
substrate. The curve in Fig. 1c was recorded on the nickel substrate previously deposited with copper (−1 V/Ag during 30 min). The scan rate was 2 mV s−1 for Fig. 1a–c and 50 mV s−1 for Fig. 1d. In order to avoid the co-reduction of organic cations, the copper reduction scan was limited to −3 V/Ag (massive evolution of solvent appears for potentials between −3 and −3.5 V/Ag). The anodic scan was stopped just after the first step of the deposit oxidation. The obtained copper voltammograms are complex and require some comments. Many cathodic steps, more or less visible depending on the scan rate, are presented and are labeled as C1Cu , C2Cu , C3Cu and C4Cu . The first cathodic step, C1Cu takes place on a nickel substrate for a potential superior to the anodic step A1Cu . However, it does not correspond to an underpotential deposition (UPD) phenomenon because this step did not result in any metallic deposit. Hence, the first reduction process is assigned to the reaction (1). Cu(II) + e− → Cu(I)
(1)
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Fig. 6. SEM images of zinc deposits after 2 h of electrolyze onto nickel substrate. (a) E = −1.6 V/Ag, (b) E = −1.9 V/Ag, (c) E = −2.3 V/Ag, and (d) E = −2.7 V/Ag.
According to the results of Zein El Abedin et al. [12], the first oxidation step of metallic copper A1Cu in this solvent results in an oxidation state equal to one (reaction (2)). Cu(0) → Cu(I) + e−
(2)
Consequently, when the curve is scanned after a preliminary copper deposit (Fig. 1c), the step C1Cu becomes almost invisible because of
Fig. 8. XRD of zinc deposit after 2 h of electrolyze at −1.6 V/Ag onto nickel substrate.
Fig. 7. EDXS specters of zinc deposits after 2 h of electrolyze onto nickel substrate. (a) E = −1.6 V/Ag and (b) E = −2.7 V/Ag.
Fig. 9. Current density–potential curves for copper (dashed curve), zinc (dotted curve) and copper–zinc solutions (solid curve). Scan rate = 2 mV s−1 ; nickel substrate.
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Fig. 10. Current density–potential curves for zinc solutions; scan rate = 2 mV s−1 : nickel substrate (solid curve), copper substrate (dashed curve).
the chemical reaction (3) between the chemical species Cu(II) and Cu(0): Cu(II) + Cu(0) → 2Cu(I)
(3)
Reaction (3) leads to a partial or total dissolution of the deposit depending on its thickness or the rest time before the scan initiation. The following cathodic step (C2Cu ) results in metallic deposits and could be attributed to the reactions (4) or (5): Cu(I) + e− → Cu(0)
(4)
Cu(II) + 2e− → Cu(0)
(5)
A slight shift between the beginning of steps C2Cu and A1Cu in Fig. 1a is observed comparing to Fig. 1c, i.e. depending on the presence or absence of a previously copper deposit. This suggests that a
small overpotential deposition (OPD) could be present on the nickel substrate. In the case of steps C3Cu and C4Cu , it cannot be excluded that Cu(II) may be complexed by the bis(trifluoromethylsulfonyl)amide anion under different forms. Note that the C4Cu step is not distinctly visible at the higher scan rate (Fig. 1d). This is probably due to a slow kinetic reaction in relation with adsorbed intermediates. At the end of the step C4Cu (−2.5 V/Ag), a slight increase of current is observed in Fig. 1b. This is attributed to the impurity traces and/or the adsorption of the organic cation from the solvent [14,15]. According to the previous equations, the coulomb integration below the cathodic curve (sum of initial and return) should be about the double of the anodic peak A1Cu because there is only one exchange electron in the reaction (2). For low scan rate (Fig. 1a and c) the obtained value between the reductions steps and oxidation peak gives a ratio superior to 4. This difference compared with the theoretical value (equal to 2) is most likely explained by the reaction (3), which generates a partial dissolution of the metallic copper. At the higher scan rate (Fig. 1d), reaction (3) becomes negligible and an approximate ratio of 2 is obtained (88 mC cm−2 for cathodic reactions and 39 mC cm−2 for anodic reaction). Note that the reduction of impurities (or beginning of the organic cation reduction) at the end of C4Cu could be also explained by the fact that the calculated ratio is not exactly equal to 2. Fig. 2 shows SEM microphotographs of electrodeposited copper during 2 h in the zones C2Cu , C3Cu and C4Cu . The morphology of plate surfaces of the deposition zones C2Cu and C3Cu (Fig. 2a and b) is very similar to each other. The morphological characteristic of samples reveals nodular shapes. On the other hand, the samples plated in the zone C4Cu have clearly different morphology. Flower-like structure begins to appear at −2 V/Ag (Fig. 2c) before to be transformed in aggregated spherical particles at −2.5 V/Ag (Fig. 2d). Note that the obtained deposits at lower
Fig. 11. Zinc atomic percentage (black points) and current density (solid curve) versus applied potential for copper–zinc solution.
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Fig. 12. SEM images of copper–zinc deposits after 2 h of electrolyze onto nickel substrate. (a) E = −1.4 V/Ag (Zn 24 at%), (b) E = −1.6 V/Ag (Zn 45 at%), (c) E = −2.2 V/Ag (Zn 26 at%), (d) E = −2.5 V/Ag (Zn 43 at%).
potentials (−3 V/Ag) were brown with a non adhesive gelatinous layer on the top of the plated surface. This is related with the beginning of the solvent reduction (even if this potential value is clearly inferior to the massive evolution of the solvent). A weakly pink deposit, characteristic for copper was often found under this gelatinous layer but was too thin to be correctly analyzed and these deposits were consequently excluded from our study. EDXS and XRD analyses were performed on each sample. Some EDXS results are presented in Fig. 3. In each case, nickel and copper signals in the EDXS spectrums are detected. The first ones are associated with the substrate and the seconds are assigned to the presence of copper deposit. The XRD analysis (Fig. 4) of the film deposited at −0.75 V/Ag proves that the obtained copper is in metallic form.
The presence of two reduction waves suggests, as in the case of copper, the possibility of two different reduction mechanisms or the presence of two different forms of complexed Zn(II), which are formed by the bis(trifluoromethylsulfonyl)amide anion. The first C1Zn step (Fig. 5a), takes place on the nickel substrate at a potential clearly inferior to the anodic step, indicating an overpotential phenomenon for the zinc nucleation. This is confirmed in Fig. 5b, where the beginning of the reduction process is similar between the initial and the return scans when zinc is previously deposit before experiment. Regardless on the scan rate, the area below the initial and return cathodic peaks and the anodic peak is almost equivalent to a 1.1 ratio. Although this value is higher than one (probably due to the reduction of impurities or the beginning of the organic cation reduction), it shows that the oxidation of zinc involves a two electron mechanism (Eq. (7)).
3.2. Zn electrodeposition In Fig. 5, zinc voltammetry on nickel substrate (solid curves) were plotted and compared with the current density–potential curves of the solvent reduction (dashed curves). The voltammogram in Fig. 5a was scanned directly on the nickel substrate while the one in Fig. 5b was scanned after achieving a weak zinc deposit. In these two cases, the scan rate was equal to 2 mV s−1 . For Fig. 5c, the scan rate was increased to 50 mV s−1 and no deposit has been made before. Two distinct cathodic steps, labeled C1Zn and C2Zn , are presented regardless of the scan rate. Contrary to the Cu reduction, metallic deposit appears on the working electrode immediately after the beginning of the first reduction process. This involves a two electron transfer, which depends on the reaction (6). Zn(II) + 2e− → Zn(0)
(6)
Zn(0) → Zn(II) + 2e−
(7)
In Fig. 6, SEM micrographs of Zn deposits at different potentials are presented. The surface morphologies of zinc deposits are very different than those obtained for copper deposits. Fig. 6a shows plated Zn at the applied potential equal to −1.6 V/Ag corresponding to the beginning of the first reduction C1Zn step. For this polarization, the deposit consists of well-defined hexagonal shapes crystals. At the end of the zone C1Zn (−1.9 V/Ag), the edges of crystals are much less defined and composed of needle-shaped clusters (Fig. 6b). This evolution progress with the overpotential, thus no distinguished crystallites are observed at the beginning of C2Zn (−2.3 V/Ag; Fig. 6c). However, a distinct morphology can be observed at −2.7 V/Ag (Fig. 6d), where the presence of granular shaped crystals is noticed.
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Fig. 13. EDXS specters of copper–zinc deposits after 2 h of electrolyze onto nickel substrate. (a) E = −1.6 V/Ag and (b) E = −2.5 V/Ag.
As in the case of copper, EDXS (Fig. 7) and XRD (Fig. 8) analyses were performed to confirm the presence of metallic zinc in the deposits.
3.3. Cu–Zn electrodeposition Fig. 9 presents the current density–potential curve recorded in the solution containing 0.2 mol L−1 Cu(Tf2 N)2 and 0.2 mol L−1 Zn(Tf2 N)2 (solid curve). This curve was overlaid with that of copper (dashed curve) and zinc (dotted line). Similar to zinc or copper curves, several reduction waves appear before the solvent reduction. By comparing Fig. 1 (Cu alone) and Fig. 5 (Zn alone), we have attributed the different waves either to copper or to zinc reduction.
Because zinc deposition takes place after the copper deposition, we have firstly confirmed that the comportment of zinc reduction on the copper substrate is as same as on the nickel substrate and that no shift in the Zn reduction potential is observed (Fig. 10). The Cu–Zn deposits were plated at different values of polarization during 2 h. Fig. 11 shows the evolution of zinc atomic percentage (black points) inside the plated Cu–Zn alloy versus applied potential. The Cu–Zn current density–potential curve (solid curve) was overlaid in the same graph. The obtained evolution does not correspond to a logical progression versus overpotential i.e. with a regular increase of zinc. At the beginning of the zinc reduction, the evolution of zinc content seems to correspond to a classical progression versus polarization. The atomic percentage of zinc rises up until potential value equal to about −1.6 V/Ag. For more negative potential values, the Zn content begins to decrease. It is interesting to note that the beginning of decreasing Zn percentage corresponds to the beginning of C4Cu process. At this potential, zinc is under diffusion control for the C1Zn step and the beginning of C4Cu step leads to an increase of copper content in the deposit. Nevertheless, an inhibition of the zinc reduction due to a partial occupation of adsorption sites by copper reduced metal intermediates from the C4Cu step could be also possible. Following several hundred of millivolts of overpotential a new increase in the zinc percentage is observed. Finally, the obtained content of copper and zinc in the deposit at −2.5 V/Ag reaches almost an equivalent value of each metal, i.e. close to the solution composition. The SEM micrographs of deposits (Fig. 12) exhibit two different forms of crystals. Circular shapes (Fig. 12a and b) are obtained when potential is equal to −1.6 V/Ag, i.e. until the beginning of C4Cu process. For more negative potentials (Fig. 12c and d), nodular shapes appear. In each case, no fundamental difference in the EDXS analyses is observed (Fig. 13). However, the XRD patterns are completely different before and after the C4Cu step (Fig. 14a). At the beginning of zinc reduction up to −1.6 V/Ag, the Cu–Zn alloy signals are observed. For more negative potentials, the coating becomes amorphous and the Cu–Zn signals rapidly disappears and only the nickel substrate peaks are clearly visible. Note that in each case, no pure copper and no pure zinc peaks are detected in the diffractograms. Fig. 14b shows fragments of the XRD patterns for different polarization. It exhibits that more or less important mixture of
Fig. 14. XRD and fragments of XRD of copper–zinc deposit after 2 h of electrolyze onto nickel substrate. (a) E = −1.6 V/Ag and E = −2.5 V/Ag (b) XRD fragments for applied potential between −1.4 V/Ag and −2.5 V/Ag.
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different Cu–Zn phases (PDF No.25-1228, 19-0179, 41-1435) for lower polarization is presented, which is not observed at higher potential. 4. Conclusions This study shows the possibility to plate Cu, Zn and Cu–Zn deposits from ionic liquid without use of chloride anions. The copper deposition takes place after a one electron transfer step, while zinc is directly reduced by a two electron transfer. The kinetic of deposition process is complicated and leads to surprising evolution of the Cu–Zn deposit composition versus polarization. At the beginning of zinc reduction up to −1.6 V/Ag, a regular increase of the zinc percentage in the deposit is obtained. The plated deposits at this potential range results in XRD patterns corresponding to a mixture of different Cu–Zn phases. All deposits obtained in this potential range (potential superior to −1.6 V/Ag) exhibit same morphology composed with spherical shapes crystallites. Between −1.6 V/Ag and −2.25 V/Ag, the zinc content decreases and the Cu–Zn alloy signals disappear, indicating an amorphous coating. For potential more negative than −2.25 V/Ag, the zinc content increases again, but no Cu–Zn alloy signal appears. For these two last potential range, the alloy morphology changes and nodular shapes crystallites appears. Acknowledgment The authors thank Patrick Baudart for the electrochemical cell conception. References [1] P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, Wiley-VCH, Verlag GmpH, 2003.
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