- Email: [email protected]
Electrodeposition of palladium–silver in a Lewis basic 1-ethyl-3-methylimidazolium chloride-tetrafluoroborate ionic liquid
Electrochimica Acta 50 (2005) 5504–5509
Electrodeposition of palladium–silver in a Lewis basic 1-ethyl-3-methylimidazolium chloride-tetrafluoroborate ionic liquid Chia-Cheng Tai, Fan-Yin Su, I-Wen Sun ∗,1 Department of Chemistry, National Cheng Kung University, Tainan, Taiwan, ROC Received 17 February 2005; received in revised form 16 March 2005; accepted 19 March 2005 Available online 24 May 2005
Abstract The electrodeposition of palladium–silver alloys was investigated in a basic 1-ethyl-3-methylimidazolium chloride/tetrafluoroborate ionic liquid containing Pd(II) and Ag(I). Cyclic voltammetry experiments showed that the reduction of Ag(I) occurs prior to the reduction Pd(II). Both electrodeposition processes require nucleation overpotential. Energy-dispersive spectroscopy data indicated that the composition of the Pd–Ag alloys could be varied by deposition potential and concentrations of Pd(II) and Ag(I) in the solution. The Pd content in the deposited Pd–Ag alloy increased with decreasing deposition potential and the Pd mole fraction in the plating bath. At potentials where the deposition of both Pd and Ag was mass-transport limited, the Pd/Ag ratio in the electrodeposited alloys was slightly less than the Pd(II)/Ag(I) ratio in the ionic liquid due to the smaller diffusion coefficient of Pd(II). Scanning electron micrographs of the electrodeposits showed that in general, the Pd–Ag alloys were nodular and become more compact upon increasing the temperature up to 120 ◦ C. © 2005 Elsevier Ltd. All rights reserved. Keywords: Electrodeposition; Palladium; Silver; Ionic liquid; Molten salt
1. Introduction Room-temperature ionic liquids have attracted intensive interests in recent years due to their wide applications in synthesis, catalysis, chemical separations, and electrochemistry [1–4]. In comparison with conventional molecular solvents, the ionic liquids are advantageous because they are nonvolatile, thermally stable over a wide temperature range, and recyclable. Because of their wide electrochemical window and good conductivity, ionic liquids are suitable for various electrochemical applications. Many examples on the electrodeposition of pure metals and alloys have been published [4]. The earlier works were focused on the water-sensitive chloroaluminate ionic liquid. Recently, the water-stable (either hydrophilic or hydrophobic) ionic liquids are becoming the trend of research because they are much easier to handle [5–11]. For the electrodeposition of metals or alloys, it ∗ 1
Corresponding author. Fax: +886 6 270552. E-mail address: [email protected] (I.-W. Sun). ISE Member.
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.03.045
is necessary to dissolve the corresponding metal ions to the ionic liquids first. Such dissolution process is made possible by introducing excess amount halide ions to form soluble metal-halide complex anions. Palladium and its alloys are important metals for industrial applications because of its excellent wear resistance, good solderability and high catalytic activities for various chemical reactions [12]. To enhance the efficiency of Pd for different applications, it often requires the incorporation of a second metal to Pd to alter the properties of Pd. For example, Pd–Ag solid solution alloys are known to be suitable for electrical contact materials and catalytic applications [13–16]. Computer simulation studies concerning the catalytic behavior of Pd–Ag alloys have also appeared in the literature [17–19]. Among the various methods for the preparation of Pd–Ag alloys coating, electrodeposition would provide an economical and convenient choice because it is easy to operate and requires relatively inexpensive equipments. The thickness, composition, and morphologies of the alloys can be varied easily through the applied deposition potential or current and time. Many examples of the electrodeposition of Pd–Ag in
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
aqueous baths are available nowadays [20–23]. The general difficulty associated with electrodeposition of Pd–Ag from aqueous bath is the low hydrogen overvoltage and the large hydrogen solubility in the Pd metal. On the other hand, the electrodeposition of Pd–Ag alloy in aprotic water-stable ionic liquids may be excused from this difficulty. In the electrodeposition of Pd–Ag alloy in dry ionic liquids there is no hydrogen coevolution, which could alter the quality of the deposits. However, electrodeposition of Pd–Ag from the ionic liquids has not been reported. In view of this, it was necessary to accumulate more fundamental data of the electrochemical investigation of Pd–Ag alloy in the ionic liquids. In this study, the electrodeposition of Pd–Ag was investigated in a hydrophilic water-stable Lewis basic 1-ethyl3-methylimidazolium chloride-tetrafluoroborate (EMI-ClBF4 ) ionic liquid that contained excess amounts of chloride ions. The electrochemical reduction of Pd and Ag was studied with cyclic voltammetry. Thin films of Pd–Ag alloys were electrodeposited with constant potential electrolysis at nickel substrates and characterized. The effects of deposition potential, plating concentrations, and deposition temperature on the composition and morphologies of the electrodeposits were examined.
2. Experimental 2.1. Apparatus All electrochemical experiments were conducted in a N2 -filled glove-box system (Vacuum Atmosphere Co.) in which the moisture and oxygen contents were kept below 1 ppm. The electrochemical experiments were accomplished by an EG&G Princeton Applied Research Corporation (PAR) model 273A potentiostat controlled by EG&G PAR model 270 software. A three-electrode cell was used for electrochemical experiments. For static voltammetry, the working electrode was a glassy carbon (GC) (A = 0.07 cm2 ) disk electrode. The counter electrode was an aluminum (Aldrich, 99.99%) spiral immersed in the pure ionic liquid contained in a glass tube having a fine porosity tip. The reference electrode was an aluminum wire immersed in a 60–40 mol% AlCl3 EMIC ionic liquid contained in the same type of glass tube as the counter electrode. Electrodeposition experiments were conducted on nickel plates (Aldrich 99.99%). A Hitachi S4200 field effect scanning electron microscope (SEM) with an energy dispersive spectroscope (EDS) working at 15 kV was used to examine the surface morphology of the electrodeposits. The crystalline phases of the deposits were studied with a Shimadzu XD-D1 X-ray diffractometer (XRD).
5505
99.9%) were used as received. The EMI-Cl-BF4 ionic liquid was prepared by direct reaction of proper amounts of EMIC and NaBF4 in dry acetone as described previously [5]. The ionic liquid was further dried under vacuum to remove any trace of water.
3. Results and discussion 3.1. Voltammetry Silver(I) was introduced to the melt by dissolution of AgCl at 35 ◦ C to form colorless solutions. A typical cyclic voltammograms of a 10 mM Ag(I) solution recorded at a GC electrode is shown in Fig. 1a. This voltammogram shows that the reduction of Ag(I) to Ag occurs at wave c1 and the anodic stripping of the electrodeposited Ag occurs at wave a1 . The peak potential separation between waves c1 and a1 increases with increasing potential scan rate. Palladium(II) was introduced to the basic EMI-Cl-BF4 melt by dissolution of PdCl2 at 35 ◦ C to produce orange-colored solutions. A typical cyclic voltammogram of a 10 mM Pd(II) solution recorded at a GC electrode is shown in Fig. 1b. This voltammogram shows that the electrodeposition of Pd(II) to Pd occurs at wave c2 and the anodic stripping of the bulk and surface Pd electrodeposits occurs at waves a2 and a2 , respectively [25,26]. Similar to waves c1 and a1 , the peak potential separation between c2 and a2 is fairly large and increases with increasing potential scan rate. Both of the two voltammograms shown in Fig. 1a and b exhibit the nucleation loop that is typical of an electrodeposition process requiring nucleation overpotential. Fig. 1a and b also indicate that the reduction potential of Ag(I) is fairly close to that of Pd(II). Such a small difference in electrodeposition potential is advantageous to the electrodeposition of Pd–Ag alloys. Fig. 1c shows the cyclic voltammogram recorded for a solution of 10 mM Pd(II) and 10 mM of Ag(I). As can be
2.2. Chemicals 1-Ethyl-3-methylimidazolium chloride was prepared and purified according to the literature [24]. Sodium tetrafluoroborate (Aldrich, 98%), anhydrous PdCl2 and AgCl (Aldrich,
Fig. 1. Cyclic voltammograms of (a) 10 mM Ag(I), (b) 10 mM Pd(II), and (c) 10 mM Pd(II) + 10 mM Ag(I) in basic EMI-Cl-BF4 ionic liquid at a GC electrode at 35 ◦ C. Scan rate = 100 mV s−1 .
5506
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
Fig. 2. Rotating disc electrode linear scan voltammogram of 10 mM Pd(II) and Ag(I) in a EMI-Cl-BF4 ionic liquid containing at a GCRDE at 35 ◦ C. Scan rate = 5 mV s−1 .
seen in this figure, the reduction part of this voltammogram is essentially the overlap of Fig. 1a and b, indicating that Ag(I) and Pd(II) does not interfere with each other on reduction. On the anodic potential scan, it can be seen that the anodic wave a1 for the stripping of pure Ag deposits decreases significantly while a new wave a1 appears. Furthermore, anodic wave a2 for the stripping of Pd is replaced by a wave a2∗ at a more negative potential and its peak current is significantly larger than that of wave a2 observed in Fig. 1b. These changes in the voltammetric features strongly indicate the formation of Pd–Ag alloy. Cyclic voltammograms were also collected for solutions containing different concentrations of Ag(I) and Pd(II). It was found that, when the concentration of Ag(I) was kept constant, waves c2 and a2∗ increased but wave a1 decreased with increasing Pd(II) concentration. On the other hand, when the concentration of Pd(II) was kept constant, wave c1 and a1 increased with increasing Ag(I) concentration. These results suggest that wave a2∗ is due to the stripping of the bulk Pd-dominating Pd–Ag alloy while wave a1 is due to the stripping of the Ag-dominating Pd–Ag alloy. A typical RDE voltammograms that was recorded for the electrodeposition of Pd–Ag at a glassy carbon RDE in the EMI-Cl-BF4 at 35 ◦ C is presented in Fig. 2. From the concentrations of Ag(I) and Pd(II) and the limiting current of each, it is realized that the diffusion coefficient of Ag(I) is about 1.5 times larger than that of Pd(II). From separate chronoamperometry experiments and using the Cottrell equation it was estimated that the diffusion coefficient of Ag(I) is 2.64 × 10−7 cm2 s−1 and that of Pd(II) is 2.03 × 10−7 cm2 s−1 at this temperature. 3.2. Preparation and characterization of Pd–Ag alloys Pd–Ag electrodeposits were prepared using constant potential electrolysis on Ni substrates at potentials ranging from −0.53 to −0.73 V in EMI-Cl-BF4 ionic liquid containing different concentrations of Pd(II) and Ag(I). Following each deposition experiment, the Pd–Ag coated nickel substrate was transferred out of the glove box and rinsed with warm deionized water and dried, then the composition of the electrodeposits was estimated by EDS analysis. The compositions of
Fig. 3. Variation of the Pd–Ag alloy composition with deposition potential in a EMI-Cl-BF4 ionic liquid at 35 ◦ C containing: (a) 10 mM Ag(I) as well as: () 10 and () 20 mM Pd(II); (b) () 20 mM Ag(I) + 20 mM Pd(II), () 50 mM Ag(I) + 50 mM Pd(II), and () 100 mM Ag(I) + 100 mM Pd(II).
the Pd–Ag electrodeposits that were prepared at 35 ◦ C from solutions containing 10 mM Ag(I) and different concentrations of Pd(II) are plotted in Fig. 3a as a function of the deposition potential. This figure shows that the Pd metal content (expressed in atomic percentage, a/o) in the deposits increased as the deposition potential became more negative and started to level off as the potential reached −0.65 V where both the reductions of Ag and Pd had reached their mass-transport limited rates. In addition, the Pd content in the Pd–Ag electrodeposits increased with increasing Pd(II))/Ag(I) mole ratio in the plating bath. However, the Pd/Ag mole ratio in the Pd–Ag electrodeposits is less than the Pd(II)/Ag(I) mole ratio in the plating bath because the diffusion coefficient of Pd(II) species is smaller than that of Ag(I) species. Fig. 3b shows the compositions of the Pd–Ag electrodeposits that were prepared at 35 ◦ C from solutions containing different concentrations of Ag(I) and Pd(II) while the concentration ratio of Ag(I)/Pd(II) was kept constant (∼1/1). This figure shows that at potentials where the reduction rate of Pd(II) had not reached mass transport-limited value the Pd atomic percentage in the deposits increased as the concentrations of Ag(I) and Pd(II) increased. However, when the electrodeposition rate was under mass transport limited, the deposited alloy composition became less dependent on the concentrations of Ag(I) and Pd(II) as long as the Ag(I)/Pd(II) concentration ratio was constant. Electrodeposits of Pd–Ag alloy were examined by the Xray diffraction (XRD) method. Fig. 4 shows the typical XRD patterns of Pd–Ag films electrodeposited at 35 ◦ C from EMICl-BF4 containing 10 mM Ag(I) and different concentrations of Pd(II) at −0.56 V. For comparison, XRD patterns of the nickel substrate, pure Pd and Ag deposits are also presented in this figure. As illustrated in this figure, the characteristic
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
5507
Fig. 4. XRD patterns (Cu K␣) of the nickel substrate and the Pd, In, and Pd–Ag samples prepared on nickel substrate from EMI-Cl-BF4 ionic liquid containing 10 mM Ag(I) and: (a) 10, (b) 20, and (c) 30 mM Pd(II) at 35 ◦ C.
patterns of pure Pd and Ag disappeared upon the formation of Pd–Ag alloy, and the characteristic 2θ of the Pd–Ag alloys varied from that of pure Ag toward that of pure Pd with increasing Pd atomic percentage in the electrodeposited alloys. Fig. 4 also shows a broadening in the X-ray reflection with increasing Pd atomic percentage in the Pd–Ag alloy, indicating the decrease in the deposited crystal size. The surface morphologies of bulk electrodeposits were examined by SEM. Typical micrographs of the electrodeposits with different compositions are shown in Fig. 5. This figure indicates that the morphology of the electrodeposits varies with the alloy compositions. As can be seen in Fig. 5a, loose and fluffy structures were observed on the surface of electrodeposits having a low Pd content, which were obtained either by using a plating bath containing a low Pd(II)/Ag(I) concentration ratio or by applying a less negative deposition potential. The formation of loose deposits is because the deposits contained mainly Ag, and the deposition of which was mass-transport limited under these conditions. However, as illustrated in Fig. 5(b and c) more Pd was incorporated into the electrodeposits by using a plating bath having a higher Pd(II)/Ag(I) concentration ratio or a more negative deposition potential; then the deposits turned into nodular and more compact. 3.3. Temperature effect Fig. 6 shows the cyclic voltammograms of a EMI-Cl-BF4 solution containing 20 mM Ag(I) and 20 mM Pd(II) recorded on a GC electrode within the temperature range between 35 and 120 ◦ C. These voltammograms show that increasing temperature increases the electrodeposition rate (current) because the viscosity of the solution is reduced and the mass transport rate of the solutes is enhanced. Furthermore, the nucleation overpotential required for the deposition of Pd and Ag decreased with increasing temperature, shifting the reduction peaks to less negative potentials. Fig. 6 also shows that as the temperature is increased, the size of wave a2 (Agdominating deposits) becomes significantly smaller in comparison to that of wave a2∗ (bulk Pd–Ag alloy), indicating that increasing the temperature increases the interdiffusion between the electrodeposited Pd and Ag.
Fig. 5. SEM micrographs of Pd–Au electrodeposits prepared on nickel substrates from EMI-Cl-BF4 ionic liquid at 35 ◦ C. The Pd content (a/o) in the deposits is: (a) 5.1, (b) 17.7, and (c) 38.4.
The effect of temperature on the deposits composition was also studied with melt solutions containing 20 mM Ag(I) and 20 mM Pd(II). The composition of the electrodeposited Pd–Ag alloys was plotted in Fig. 7 as a function of deposition potential. At potentials where only the deposition of Ag was mass-transport limited but deposition of Pd was not, the Pd content in the deposits increases slightly with increasing
5508
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
Fig. 6. Cyclic voltammograms recorded on a GC disc electrode for a basic EMI-Cl-BF4 ionic liquid containing 20 mM Pd(II) and 20 mM Ag(I) at: (a) 35, (b) 50, and (c) 120 ◦ C. Scan rate = 100 mV s−1 .
Fig. 7. Variation of the Pd–Au alloy composition with deposition potential in a EMI-Cl-BF4 ionic liquid containing 20 mM Ag(I) and 20 mM Pd(II) at: (♦) 35, ()50, and () 120 ◦ C.
temperature, suggesting that the charge transfer rate of Pd reduction was enhanced by temperature. However, temperature does not significantly alter the composition of the deposited alloys when the deposition potential is so negative that both of the electrodeposition of Ag and Pd reach the mass transportlimited rate; the deposits composition is mainly determined by the Pd(II)/Ag(I) concentration ratio in the plating bath and the diffusion coefficients of the two species. Fig. 8 compares the XRD patterns of the electrodeposited Pd–Ag film on the Ni substrates at −0.50 V at 35, 50, and 120 ◦ C. This figure shows that the Pd content in the alloy increases with
Fig. 9. SEM micrographs of Pd–Ag electrodeposits prepared on nickel substrates from EMI-Cl-BF4 ionic liquid containing 20 mM Pd(II) and 20 mM Ag(I) at: (a) 35, (b) 50, and (c) 120 ◦ C. The Pd content in the deposits is: (a) 33.1, (b) 35.1, and (c) 35.8 a/o.
increasing temperature because of the overpotential required for the electrodeposition is reduced. The characteristic 2θ of the alloy varies from that of pure Ag toward that of pure Pd with increasing Pd content in the deposits. The surface morphologies of bulk electrodeposits having similar composition but prepared at different temperatures are shown in Fig. 9. In general, all the deposits preserve the nodular structures and become more compact upon increasing the temperature.
4. Summary and conclusions
Fig. 8. XRD patterns (Cu K␣) of the nickel substrate and the Pd, In, and Pd–Ag samples prepared on nickel substrate from EMI-Cl-BF4 ionic liquid containing 20 mM Pd(II) and 20 mM Ag(I) at (a) 35, (b) 50, and (c) 120 ◦ C. The deposition potential was: −0.5 V.
The electrodeposition of Pd–Ag alloys from the basic EMIC-Cl-BF4 ionic liquid was studied in solutions containing Pd(II) and Ag(I) within a temperature range from 35 to 120 ◦ C. Voltammetric data show that the reduction potentials of Ag(I) and Pd(II) are very close which favors the codeposition of the Pd–Ag alloys. Both the electrodeposition of Ag and Pd involve overpotential driven nucleation process.
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
The diffusion coefficient of Pd(II) species is less than that of the Ag(I) species. The Pd content in the Pd–Ag electrodeposits increases with decreasing applied deposition potential until the deposition of Pd is mass transport limited at which the deposit composition is mainly a function of the plating bath composition. Because the diffusion coefficient of Pd(II) is less than that of the Ag(I), the Pd content in the deposits is less than that in the plating bath. Increase the deposition temperature reduces the overpotential required for the deposition and shifts the deposition processes toward more positive potentials. However, temperature does not significantly alter the deposit composition as long as the deposits are obtained at potentials where both the deposition of Ag and Pd are under mass-transport limited. SEM micrographs show that in general, the Pd–Ag electrodeposits have nodular structures. Acknowledgment This research was supported by the National Science Council of Taiwan. References [1] C.L. Hussey, J.S. Wilkes, in: Mamantov, A.I. Popov (Eds.), Chemistry of Non-aqueous Solutions, Current Progress, VCH, New York, 1994, p. 227. [2] R.D. Rogers, K.R. Seddon (Eds.), Ionic liquids: Industrial Applications to Green Chemistry, ACS, Washington, DC, 2002. [3] P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, WileyVCH, Weinheim, 2002. [4] (a) G.R. Stafford, C.L. Hussey, in: D.M. Alkire, Kolb (Eds.), Advances in Electrochemical Science and Engineering, vol. 7, WileyVCH, 2001, p. 275; (b) M.C. Buzzeo, R.G. Evans, R.G. Compoton, Chem. Phys. Chem. 5 (8) (2004) 1106; (c) F. Endres, Chem. Phys. Chem. 3 (2) (2002) 144. [5] (a) P.-Y. Chen, I.-W. Sun, Electrochim. Acta 45 (1999) 441; (b) P.-Y. Chen, I.-W. Sun, Electrochim. Acta 45 (2000) 3163;
5509
(c) M.-H. Yang, I.-W. Sun, J. Appl. Electrochem. 33 (2003) 1077; (d) F.-Y. Su, J.-F. Huang, I.-W. Sun, J. Electrochem. Soc. 151 (2004) C811. [6] Y. Katayama, S. Dan, T. Miura, T. Kishi, J. Electrochem. Soc. 148 (2001) C102. [7] (a) F. Endres, Phys. Chem. Chem. Phys. 3 (2001) 3165; (b) F. Endres, M. Bukowski, R. Hempelmann, H. Natter, Angewandte Chemie 115 (2003) 3550; F. Endres, M. Bukowski, R. Hempelmann, H. Natter, Int. Ed. 42 (2003) 3428. [8] I. Mukhopadhyay, W. Freyland, Langumuir 19 (2003) 1951. [9] R.F. de Souza, J.C. Padilha, R.S. Goncalves, Electrochem. Commun. 5 (2003) 728. [10] P. He, H. Liu, Z. Li, Y. Liu, X. Xu, J. Li, Langmuir 20 (2004) 10260. [11] J.N. Barisci, G.G. Wallace, D.R. Macfarlane, R.H. Baughman, Electrochem. Commun. 6 (2004) 22. [12] M. Schlesinger, M. Paunovic (Eds.), Modern Electroplating, Wiley and Sons, New York, 2000, p. 483. [13] F.I. Nobel, IEEE Trans. Comp., Hybrids Manuf. Technol. 8 (1985) 163. [14] P. Bertus, U. Halbes, P. Pale, Eur. J. Org. Chem. 23 (2001) 4391. [15] B. Heinrichs, F. Noville, J.P. Schoebrechts, J.P. Pirard, J. Catal. 220 (2003) 215. [16] T.C. Huang, M.C. Wei, H.I. Chen, Separ. Purif. Technol. 32 (2003) 239. [17] K.A. Hitoshi, T. Nakayama, Y. Kobayashi, K. Suzuki, M. Takahashi, S. Takami, M. Kubo, N. Itoh, P. Selvam, S. Miyamoto, Catal. Today 82 (2003) 233. [18] H.H. Kart, M. Uludogan, T. Cagin, M. Tomak, J. Non-crystalline Solids 342 (2004) 6. [19] M. Gados, A. Eichler, J. Hafner, J. Phys.-Condens. Matter 16 (2004) 1141. [20] U. Cohen, F. Kohn, R. Sard, J. Electrochem. Soc. 130 (1983) 1987. [21] (a) P. Hasler, Th. Allimendinger, Surf. Coat. Technol. 58 (1993) 179; (b) P. Hasler, Th. Allimendinger, Surf. Coat. Technol. 58 (1993) 185. [22] N. Kubota, S. Yoshimura, E. Sato, Metal Finishing (1986) 55. [23] K. Ignatov, L. Nikolova, W. Dimov, J. Phys. Chem. B 101 (1997) 6891. [24] J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263. [25] I.-W. Sun, C.L. Hussey, J. Electroanal. Chem. 274 (1989) 325. [26] H.C. De Long, J.S. Wilkes, R.T. Carlin, J. Electrochem. Soc. 141 (1994) 1000.
Electrodeposition of palladium–silver in a Lewis basic 1-ethyl-3-methylimidazolium chloride-tetrafluoroborate ionic liquid Chia-Cheng Tai, Fan-Yin Su, I-Wen Sun ∗,1 Department of Chemistry, National Cheng Kung University, Tainan, Taiwan, ROC Received 17 February 2005; received in revised form 16 March 2005; accepted 19 March 2005 Available online 24 May 2005
Abstract The electrodeposition of palladium–silver alloys was investigated in a basic 1-ethyl-3-methylimidazolium chloride/tetrafluoroborate ionic liquid containing Pd(II) and Ag(I). Cyclic voltammetry experiments showed that the reduction of Ag(I) occurs prior to the reduction Pd(II). Both electrodeposition processes require nucleation overpotential. Energy-dispersive spectroscopy data indicated that the composition of the Pd–Ag alloys could be varied by deposition potential and concentrations of Pd(II) and Ag(I) in the solution. The Pd content in the deposited Pd–Ag alloy increased with decreasing deposition potential and the Pd mole fraction in the plating bath. At potentials where the deposition of both Pd and Ag was mass-transport limited, the Pd/Ag ratio in the electrodeposited alloys was slightly less than the Pd(II)/Ag(I) ratio in the ionic liquid due to the smaller diffusion coefficient of Pd(II). Scanning electron micrographs of the electrodeposits showed that in general, the Pd–Ag alloys were nodular and become more compact upon increasing the temperature up to 120 ◦ C. © 2005 Elsevier Ltd. All rights reserved. Keywords: Electrodeposition; Palladium; Silver; Ionic liquid; Molten salt
1. Introduction Room-temperature ionic liquids have attracted intensive interests in recent years due to their wide applications in synthesis, catalysis, chemical separations, and electrochemistry [1–4]. In comparison with conventional molecular solvents, the ionic liquids are advantageous because they are nonvolatile, thermally stable over a wide temperature range, and recyclable. Because of their wide electrochemical window and good conductivity, ionic liquids are suitable for various electrochemical applications. Many examples on the electrodeposition of pure metals and alloys have been published [4]. The earlier works were focused on the water-sensitive chloroaluminate ionic liquid. Recently, the water-stable (either hydrophilic or hydrophobic) ionic liquids are becoming the trend of research because they are much easier to handle [5–11]. For the electrodeposition of metals or alloys, it ∗ 1
Corresponding author. Fax: +886 6 270552. E-mail address: [email protected] (I.-W. Sun). ISE Member.
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.03.045
is necessary to dissolve the corresponding metal ions to the ionic liquids first. Such dissolution process is made possible by introducing excess amount halide ions to form soluble metal-halide complex anions. Palladium and its alloys are important metals for industrial applications because of its excellent wear resistance, good solderability and high catalytic activities for various chemical reactions [12]. To enhance the efficiency of Pd for different applications, it often requires the incorporation of a second metal to Pd to alter the properties of Pd. For example, Pd–Ag solid solution alloys are known to be suitable for electrical contact materials and catalytic applications [13–16]. Computer simulation studies concerning the catalytic behavior of Pd–Ag alloys have also appeared in the literature [17–19]. Among the various methods for the preparation of Pd–Ag alloys coating, electrodeposition would provide an economical and convenient choice because it is easy to operate and requires relatively inexpensive equipments. The thickness, composition, and morphologies of the alloys can be varied easily through the applied deposition potential or current and time. Many examples of the electrodeposition of Pd–Ag in
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
aqueous baths are available nowadays [20–23]. The general difficulty associated with electrodeposition of Pd–Ag from aqueous bath is the low hydrogen overvoltage and the large hydrogen solubility in the Pd metal. On the other hand, the electrodeposition of Pd–Ag alloy in aprotic water-stable ionic liquids may be excused from this difficulty. In the electrodeposition of Pd–Ag alloy in dry ionic liquids there is no hydrogen coevolution, which could alter the quality of the deposits. However, electrodeposition of Pd–Ag from the ionic liquids has not been reported. In view of this, it was necessary to accumulate more fundamental data of the electrochemical investigation of Pd–Ag alloy in the ionic liquids. In this study, the electrodeposition of Pd–Ag was investigated in a hydrophilic water-stable Lewis basic 1-ethyl3-methylimidazolium chloride-tetrafluoroborate (EMI-ClBF4 ) ionic liquid that contained excess amounts of chloride ions. The electrochemical reduction of Pd and Ag was studied with cyclic voltammetry. Thin films of Pd–Ag alloys were electrodeposited with constant potential electrolysis at nickel substrates and characterized. The effects of deposition potential, plating concentrations, and deposition temperature on the composition and morphologies of the electrodeposits were examined.
2. Experimental 2.1. Apparatus All electrochemical experiments were conducted in a N2 -filled glove-box system (Vacuum Atmosphere Co.) in which the moisture and oxygen contents were kept below 1 ppm. The electrochemical experiments were accomplished by an EG&G Princeton Applied Research Corporation (PAR) model 273A potentiostat controlled by EG&G PAR model 270 software. A three-electrode cell was used for electrochemical experiments. For static voltammetry, the working electrode was a glassy carbon (GC) (A = 0.07 cm2 ) disk electrode. The counter electrode was an aluminum (Aldrich, 99.99%) spiral immersed in the pure ionic liquid contained in a glass tube having a fine porosity tip. The reference electrode was an aluminum wire immersed in a 60–40 mol% AlCl3 EMIC ionic liquid contained in the same type of glass tube as the counter electrode. Electrodeposition experiments were conducted on nickel plates (Aldrich 99.99%). A Hitachi S4200 field effect scanning electron microscope (SEM) with an energy dispersive spectroscope (EDS) working at 15 kV was used to examine the surface morphology of the electrodeposits. The crystalline phases of the deposits were studied with a Shimadzu XD-D1 X-ray diffractometer (XRD).
5505
99.9%) were used as received. The EMI-Cl-BF4 ionic liquid was prepared by direct reaction of proper amounts of EMIC and NaBF4 in dry acetone as described previously [5]. The ionic liquid was further dried under vacuum to remove any trace of water.
3. Results and discussion 3.1. Voltammetry Silver(I) was introduced to the melt by dissolution of AgCl at 35 ◦ C to form colorless solutions. A typical cyclic voltammograms of a 10 mM Ag(I) solution recorded at a GC electrode is shown in Fig. 1a. This voltammogram shows that the reduction of Ag(I) to Ag occurs at wave c1 and the anodic stripping of the electrodeposited Ag occurs at wave a1 . The peak potential separation between waves c1 and a1 increases with increasing potential scan rate. Palladium(II) was introduced to the basic EMI-Cl-BF4 melt by dissolution of PdCl2 at 35 ◦ C to produce orange-colored solutions. A typical cyclic voltammogram of a 10 mM Pd(II) solution recorded at a GC electrode is shown in Fig. 1b. This voltammogram shows that the electrodeposition of Pd(II) to Pd occurs at wave c2 and the anodic stripping of the bulk and surface Pd electrodeposits occurs at waves a2 and a2 , respectively [25,26]. Similar to waves c1 and a1 , the peak potential separation between c2 and a2 is fairly large and increases with increasing potential scan rate. Both of the two voltammograms shown in Fig. 1a and b exhibit the nucleation loop that is typical of an electrodeposition process requiring nucleation overpotential. Fig. 1a and b also indicate that the reduction potential of Ag(I) is fairly close to that of Pd(II). Such a small difference in electrodeposition potential is advantageous to the electrodeposition of Pd–Ag alloys. Fig. 1c shows the cyclic voltammogram recorded for a solution of 10 mM Pd(II) and 10 mM of Ag(I). As can be
2.2. Chemicals 1-Ethyl-3-methylimidazolium chloride was prepared and purified according to the literature [24]. Sodium tetrafluoroborate (Aldrich, 98%), anhydrous PdCl2 and AgCl (Aldrich,
Fig. 1. Cyclic voltammograms of (a) 10 mM Ag(I), (b) 10 mM Pd(II), and (c) 10 mM Pd(II) + 10 mM Ag(I) in basic EMI-Cl-BF4 ionic liquid at a GC electrode at 35 ◦ C. Scan rate = 100 mV s−1 .
5506
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
Fig. 2. Rotating disc electrode linear scan voltammogram of 10 mM Pd(II) and Ag(I) in a EMI-Cl-BF4 ionic liquid containing at a GCRDE at 35 ◦ C. Scan rate = 5 mV s−1 .
seen in this figure, the reduction part of this voltammogram is essentially the overlap of Fig. 1a and b, indicating that Ag(I) and Pd(II) does not interfere with each other on reduction. On the anodic potential scan, it can be seen that the anodic wave a1 for the stripping of pure Ag deposits decreases significantly while a new wave a1 appears. Furthermore, anodic wave a2 for the stripping of Pd is replaced by a wave a2∗ at a more negative potential and its peak current is significantly larger than that of wave a2 observed in Fig. 1b. These changes in the voltammetric features strongly indicate the formation of Pd–Ag alloy. Cyclic voltammograms were also collected for solutions containing different concentrations of Ag(I) and Pd(II). It was found that, when the concentration of Ag(I) was kept constant, waves c2 and a2∗ increased but wave a1 decreased with increasing Pd(II) concentration. On the other hand, when the concentration of Pd(II) was kept constant, wave c1 and a1 increased with increasing Ag(I) concentration. These results suggest that wave a2∗ is due to the stripping of the bulk Pd-dominating Pd–Ag alloy while wave a1 is due to the stripping of the Ag-dominating Pd–Ag alloy. A typical RDE voltammograms that was recorded for the electrodeposition of Pd–Ag at a glassy carbon RDE in the EMI-Cl-BF4 at 35 ◦ C is presented in Fig. 2. From the concentrations of Ag(I) and Pd(II) and the limiting current of each, it is realized that the diffusion coefficient of Ag(I) is about 1.5 times larger than that of Pd(II). From separate chronoamperometry experiments and using the Cottrell equation it was estimated that the diffusion coefficient of Ag(I) is 2.64 × 10−7 cm2 s−1 and that of Pd(II) is 2.03 × 10−7 cm2 s−1 at this temperature. 3.2. Preparation and characterization of Pd–Ag alloys Pd–Ag electrodeposits were prepared using constant potential electrolysis on Ni substrates at potentials ranging from −0.53 to −0.73 V in EMI-Cl-BF4 ionic liquid containing different concentrations of Pd(II) and Ag(I). Following each deposition experiment, the Pd–Ag coated nickel substrate was transferred out of the glove box and rinsed with warm deionized water and dried, then the composition of the electrodeposits was estimated by EDS analysis. The compositions of
Fig. 3. Variation of the Pd–Ag alloy composition with deposition potential in a EMI-Cl-BF4 ionic liquid at 35 ◦ C containing: (a) 10 mM Ag(I) as well as: () 10 and () 20 mM Pd(II); (b) () 20 mM Ag(I) + 20 mM Pd(II), () 50 mM Ag(I) + 50 mM Pd(II), and () 100 mM Ag(I) + 100 mM Pd(II).
the Pd–Ag electrodeposits that were prepared at 35 ◦ C from solutions containing 10 mM Ag(I) and different concentrations of Pd(II) are plotted in Fig. 3a as a function of the deposition potential. This figure shows that the Pd metal content (expressed in atomic percentage, a/o) in the deposits increased as the deposition potential became more negative and started to level off as the potential reached −0.65 V where both the reductions of Ag and Pd had reached their mass-transport limited rates. In addition, the Pd content in the Pd–Ag electrodeposits increased with increasing Pd(II))/Ag(I) mole ratio in the plating bath. However, the Pd/Ag mole ratio in the Pd–Ag electrodeposits is less than the Pd(II)/Ag(I) mole ratio in the plating bath because the diffusion coefficient of Pd(II) species is smaller than that of Ag(I) species. Fig. 3b shows the compositions of the Pd–Ag electrodeposits that were prepared at 35 ◦ C from solutions containing different concentrations of Ag(I) and Pd(II) while the concentration ratio of Ag(I)/Pd(II) was kept constant (∼1/1). This figure shows that at potentials where the reduction rate of Pd(II) had not reached mass transport-limited value the Pd atomic percentage in the deposits increased as the concentrations of Ag(I) and Pd(II) increased. However, when the electrodeposition rate was under mass transport limited, the deposited alloy composition became less dependent on the concentrations of Ag(I) and Pd(II) as long as the Ag(I)/Pd(II) concentration ratio was constant. Electrodeposits of Pd–Ag alloy were examined by the Xray diffraction (XRD) method. Fig. 4 shows the typical XRD patterns of Pd–Ag films electrodeposited at 35 ◦ C from EMICl-BF4 containing 10 mM Ag(I) and different concentrations of Pd(II) at −0.56 V. For comparison, XRD patterns of the nickel substrate, pure Pd and Ag deposits are also presented in this figure. As illustrated in this figure, the characteristic
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
5507
Fig. 4. XRD patterns (Cu K␣) of the nickel substrate and the Pd, In, and Pd–Ag samples prepared on nickel substrate from EMI-Cl-BF4 ionic liquid containing 10 mM Ag(I) and: (a) 10, (b) 20, and (c) 30 mM Pd(II) at 35 ◦ C.
patterns of pure Pd and Ag disappeared upon the formation of Pd–Ag alloy, and the characteristic 2θ of the Pd–Ag alloys varied from that of pure Ag toward that of pure Pd with increasing Pd atomic percentage in the electrodeposited alloys. Fig. 4 also shows a broadening in the X-ray reflection with increasing Pd atomic percentage in the Pd–Ag alloy, indicating the decrease in the deposited crystal size. The surface morphologies of bulk electrodeposits were examined by SEM. Typical micrographs of the electrodeposits with different compositions are shown in Fig. 5. This figure indicates that the morphology of the electrodeposits varies with the alloy compositions. As can be seen in Fig. 5a, loose and fluffy structures were observed on the surface of electrodeposits having a low Pd content, which were obtained either by using a plating bath containing a low Pd(II)/Ag(I) concentration ratio or by applying a less negative deposition potential. The formation of loose deposits is because the deposits contained mainly Ag, and the deposition of which was mass-transport limited under these conditions. However, as illustrated in Fig. 5(b and c) more Pd was incorporated into the electrodeposits by using a plating bath having a higher Pd(II)/Ag(I) concentration ratio or a more negative deposition potential; then the deposits turned into nodular and more compact. 3.3. Temperature effect Fig. 6 shows the cyclic voltammograms of a EMI-Cl-BF4 solution containing 20 mM Ag(I) and 20 mM Pd(II) recorded on a GC electrode within the temperature range between 35 and 120 ◦ C. These voltammograms show that increasing temperature increases the electrodeposition rate (current) because the viscosity of the solution is reduced and the mass transport rate of the solutes is enhanced. Furthermore, the nucleation overpotential required for the deposition of Pd and Ag decreased with increasing temperature, shifting the reduction peaks to less negative potentials. Fig. 6 also shows that as the temperature is increased, the size of wave a2 (Agdominating deposits) becomes significantly smaller in comparison to that of wave a2∗ (bulk Pd–Ag alloy), indicating that increasing the temperature increases the interdiffusion between the electrodeposited Pd and Ag.
Fig. 5. SEM micrographs of Pd–Au electrodeposits prepared on nickel substrates from EMI-Cl-BF4 ionic liquid at 35 ◦ C. The Pd content (a/o) in the deposits is: (a) 5.1, (b) 17.7, and (c) 38.4.
The effect of temperature on the deposits composition was also studied with melt solutions containing 20 mM Ag(I) and 20 mM Pd(II). The composition of the electrodeposited Pd–Ag alloys was plotted in Fig. 7 as a function of deposition potential. At potentials where only the deposition of Ag was mass-transport limited but deposition of Pd was not, the Pd content in the deposits increases slightly with increasing
5508
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
Fig. 6. Cyclic voltammograms recorded on a GC disc electrode for a basic EMI-Cl-BF4 ionic liquid containing 20 mM Pd(II) and 20 mM Ag(I) at: (a) 35, (b) 50, and (c) 120 ◦ C. Scan rate = 100 mV s−1 .
Fig. 7. Variation of the Pd–Au alloy composition with deposition potential in a EMI-Cl-BF4 ionic liquid containing 20 mM Ag(I) and 20 mM Pd(II) at: (♦) 35, ()50, and () 120 ◦ C.
temperature, suggesting that the charge transfer rate of Pd reduction was enhanced by temperature. However, temperature does not significantly alter the composition of the deposited alloys when the deposition potential is so negative that both of the electrodeposition of Ag and Pd reach the mass transportlimited rate; the deposits composition is mainly determined by the Pd(II)/Ag(I) concentration ratio in the plating bath and the diffusion coefficients of the two species. Fig. 8 compares the XRD patterns of the electrodeposited Pd–Ag film on the Ni substrates at −0.50 V at 35, 50, and 120 ◦ C. This figure shows that the Pd content in the alloy increases with
Fig. 9. SEM micrographs of Pd–Ag electrodeposits prepared on nickel substrates from EMI-Cl-BF4 ionic liquid containing 20 mM Pd(II) and 20 mM Ag(I) at: (a) 35, (b) 50, and (c) 120 ◦ C. The Pd content in the deposits is: (a) 33.1, (b) 35.1, and (c) 35.8 a/o.
increasing temperature because of the overpotential required for the electrodeposition is reduced. The characteristic 2θ of the alloy varies from that of pure Ag toward that of pure Pd with increasing Pd content in the deposits. The surface morphologies of bulk electrodeposits having similar composition but prepared at different temperatures are shown in Fig. 9. In general, all the deposits preserve the nodular structures and become more compact upon increasing the temperature.
4. Summary and conclusions
Fig. 8. XRD patterns (Cu K␣) of the nickel substrate and the Pd, In, and Pd–Ag samples prepared on nickel substrate from EMI-Cl-BF4 ionic liquid containing 20 mM Pd(II) and 20 mM Ag(I) at (a) 35, (b) 50, and (c) 120 ◦ C. The deposition potential was: −0.5 V.
The electrodeposition of Pd–Ag alloys from the basic EMIC-Cl-BF4 ionic liquid was studied in solutions containing Pd(II) and Ag(I) within a temperature range from 35 to 120 ◦ C. Voltammetric data show that the reduction potentials of Ag(I) and Pd(II) are very close which favors the codeposition of the Pd–Ag alloys. Both the electrodeposition of Ag and Pd involve overpotential driven nucleation process.
C.-C. Tai et al. / Electrochimica Acta 50 (2005) 5504–5509
The diffusion coefficient of Pd(II) species is less than that of the Ag(I) species. The Pd content in the Pd–Ag electrodeposits increases with decreasing applied deposition potential until the deposition of Pd is mass transport limited at which the deposit composition is mainly a function of the plating bath composition. Because the diffusion coefficient of Pd(II) is less than that of the Ag(I), the Pd content in the deposits is less than that in the plating bath. Increase the deposition temperature reduces the overpotential required for the deposition and shifts the deposition processes toward more positive potentials. However, temperature does not significantly alter the deposit composition as long as the deposits are obtained at potentials where both the deposition of Ag and Pd are under mass-transport limited. SEM micrographs show that in general, the Pd–Ag electrodeposits have nodular structures. Acknowledgment This research was supported by the National Science Council of Taiwan. References [1] C.L. Hussey, J.S. Wilkes, in: Mamantov, A.I. Popov (Eds.), Chemistry of Non-aqueous Solutions, Current Progress, VCH, New York, 1994, p. 227. [2] R.D. Rogers, K.R. Seddon (Eds.), Ionic liquids: Industrial Applications to Green Chemistry, ACS, Washington, DC, 2002. [3] P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, WileyVCH, Weinheim, 2002. [4] (a) G.R. Stafford, C.L. Hussey, in: D.M. Alkire, Kolb (Eds.), Advances in Electrochemical Science and Engineering, vol. 7, WileyVCH, 2001, p. 275; (b) M.C. Buzzeo, R.G. Evans, R.G. Compoton, Chem. Phys. Chem. 5 (8) (2004) 1106; (c) F. Endres, Chem. Phys. Chem. 3 (2) (2002) 144. [5] (a) P.-Y. Chen, I.-W. Sun, Electrochim. Acta 45 (1999) 441; (b) P.-Y. Chen, I.-W. Sun, Electrochim. Acta 45 (2000) 3163;
5509
(c) M.-H. Yang, I.-W. Sun, J. Appl. Electrochem. 33 (2003) 1077; (d) F.-Y. Su, J.-F. Huang, I.-W. Sun, J. Electrochem. Soc. 151 (2004) C811. [6] Y. Katayama, S. Dan, T. Miura, T. Kishi, J. Electrochem. Soc. 148 (2001) C102. [7] (a) F. Endres, Phys. Chem. Chem. Phys. 3 (2001) 3165; (b) F. Endres, M. Bukowski, R. Hempelmann, H. Natter, Angewandte Chemie 115 (2003) 3550; F. Endres, M. Bukowski, R. Hempelmann, H. Natter, Int. Ed. 42 (2003) 3428. [8] I. Mukhopadhyay, W. Freyland, Langumuir 19 (2003) 1951. [9] R.F. de Souza, J.C. Padilha, R.S. Goncalves, Electrochem. Commun. 5 (2003) 728. [10] P. He, H. Liu, Z. Li, Y. Liu, X. Xu, J. Li, Langmuir 20 (2004) 10260. [11] J.N. Barisci, G.G. Wallace, D.R. Macfarlane, R.H. Baughman, Electrochem. Commun. 6 (2004) 22. [12] M. Schlesinger, M. Paunovic (Eds.), Modern Electroplating, Wiley and Sons, New York, 2000, p. 483. [13] F.I. Nobel, IEEE Trans. Comp., Hybrids Manuf. Technol. 8 (1985) 163. [14] P. Bertus, U. Halbes, P. Pale, Eur. J. Org. Chem. 23 (2001) 4391. [15] B. Heinrichs, F. Noville, J.P. Schoebrechts, J.P. Pirard, J. Catal. 220 (2003) 215. [16] T.C. Huang, M.C. Wei, H.I. Chen, Separ. Purif. Technol. 32 (2003) 239. [17] K.A. Hitoshi, T. Nakayama, Y. Kobayashi, K. Suzuki, M. Takahashi, S. Takami, M. Kubo, N. Itoh, P. Selvam, S. Miyamoto, Catal. Today 82 (2003) 233. [18] H.H. Kart, M. Uludogan, T. Cagin, M. Tomak, J. Non-crystalline Solids 342 (2004) 6. [19] M. Gados, A. Eichler, J. Hafner, J. Phys.-Condens. Matter 16 (2004) 1141. [20] U. Cohen, F. Kohn, R. Sard, J. Electrochem. Soc. 130 (1983) 1987. [21] (a) P. Hasler, Th. Allimendinger, Surf. Coat. Technol. 58 (1993) 179; (b) P. Hasler, Th. Allimendinger, Surf. Coat. Technol. 58 (1993) 185. [22] N. Kubota, S. Yoshimura, E. Sato, Metal Finishing (1986) 55. [23] K. Ignatov, L. Nikolova, W. Dimov, J. Phys. Chem. B 101 (1997) 6891. [24] J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263. [25] I.-W. Sun, C.L. Hussey, J. Electroanal. Chem. 274 (1989) 325. [26] H.C. De Long, J.S. Wilkes, R.T. Carlin, J. Electrochem. Soc. 141 (1994) 1000.