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EQCM Study of the [AuIIICl4]--[AuICl2]--Au(0) Redox System in 1-Ethyl-3-methylimidazolium Tetrafluoroborate Room-Temperature Ionic Liquid Taku Oyama,† Shuichiro Yamaguchi,‡ Mohammad Rezaur Rahman,† Takeyoshi Okajima,† Takeo Ohsaka,*,† and Noboru Oyama*,‡ †
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259-G1-5 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan, and ‡ Department of Applied Chemistry, Graduate School of Science and Technology, Tokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo 184-8588, Japan Received November 27, 2009. Revised Manuscript Received February 4, 2010 The electrochemical behavior of the [AuIIICl4]--[AuICl2]--Au(0) redox system in room temperature ionic liquid (RTIL) of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) has been investigated quantitatively using an in situ electrochemical quartz crystal microbalance (EQCM) technique based on a Pt film-coated quartz crystal electrode (Pt-QCE). A series of two-electron (2e) and one-electron (1e) reductions of the [AuIIICl4]- to [AuICl2]- and [AuICl2]to Au metal were recognized at the Pt surface. Besides, the disproportionation reaction of [AuICl2]- (i.e., the 2ereduction product of [AuIIICl4]-) to [AuIIICl4]- and Au metal was also observed. Electro-dissolution of the Au deposited on the Pt electrode through a 1e-oxidation reaction in the presence of chloride ions was also confirmed using the Pt-QCE based EQCM technique. A 2e-oxidation reaction of [AuICl2]- (i.e., the dissolved product) to [AuIIICl4]along with the oxidation of Cl- ion on the Pt surface was also realized at high anodic potential. The results demonstrate that in situ EQCM technique is applicable and powerful in elucidating electrochemical surface phenomena accompanying a mass change in RTIL.
1. Introduction Recently, the electrochemistry of gold in aqueous solutions and common organic solvents has been studied to prepare the nanoscale structures of high-index gold surfaces onto other metal or carbon electrodes1-12 as well as to investigate the behavior of its corrosion and electro-refining process.12 So far, in aqueous solutions and common organic solvents a single cathodic wave has been observed for one-step reduction of [AuIIICl4]- to Au metal at platinum (Pt) and glassy carbon (GC) electrodes.13,14 The oxidation of the deposited Au to [AuICl2]- can be observed in anodic potential region only if a suitable complexing agent such as chloride ion is present.13 Since the [AuICl2]- species is not *To whom correspondence should be addressed. Telephone: þ81-45-9245404. Fax: þ81-45-9245489. E-mail: (T. Ohsaka)
[email protected]; (N. Oyama)
[email protected]. (1) Zhong, C. -J.; Maye, M. M. Adv. Mater. 2001, 13, 1507. (2) Henry, C. R. Appl. Surf. Sci. 2000, 164, 252. (3) Tsodikov, M. V.; Rostovshchikova, T. N.; Smimov, V. V.; Kiseleva, O. I.; Maksimov, Suzdalev, I. P.; Ikorski, V. N. Catal. Today 2005, 105, 634. (4) Biswas, P. C.; Nodasaka, Y.; Enyo, M.; Haruta, M. J. J. Electroanal. Chem. 1995, 381, 167. (5) Maye, M.; Lou, Y.; Zhong, C. -J. Langmuir 2000, 16, 7520. (6) Katz, E.; Willner., I. Angew. Chem., Int. Ed. 2004, 43, 6042. (7) Pingarron, J. M.; Yanez-Sedeno, P.; Gonzalez-Cortes, A. Electrochim. Acta 2008, 53, 5848. (8) Chen, W.; Cai, W. P.; Liang, C. H.; Zhang, L. D. Mater. Res. Bull. 2001, 36, 335. (9) Sagara, T.; Kato, N.; Nakashima, N. J. Phys. Chem. B 2002, 106, 1205. (10) Raj, C. R.; Abdelrahman, A. I.; Ohsaka, T. Electrochem. Commun. 2005, 7, 288. (11) Raj, C. R.; Okajima, T.; Ohsaka, T. J. Electroanal. Chem. 2003, 543, 127. (12) (a) Abdelrahman, A. I.; Mohammad, A. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. B 2006, 110, 2798. (b) Cao, Z.; Xiao, Z. -L.; Gu, N.; Gong, F. -C.; Yang, D. -W.; Zhu, Z. -P. Anal. Lett. 2005, 38, 1289. (c) Alamarguy, D.; Bertoglio, M.; Lecaude, N.; No€el, S.; Ruaut, L.; Tristani, L. Surf. Interface Anal. 2004, 36, 780. (13) Goolsby, A. D.; Sawyer, D. T. Anal. Chem. 1968, 40, 1978. (14) Komsiyska, L.; Staikov, G. Electrochim. Acta 2008, 54, 168.
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chemically stable,14 the detailed electrochemical study of this species has not been conducted so far in aqueous solution. Recently, various kinds of room temperature ionic liquids (RTILs) have been employed as a new possible media for the electrodeposition of Au,15-17 because RTILs have many advantages such as a wide electrochemical potential window, acceptable ionic conductivity, high thermal stability, and negligible vapor pressure.18,19 Previously, we have reported a series of 2e- and 1e-redox processes of the [AuIIICl4]--[AuICl2]--Au(0) system at the GC electrode in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) RTILs.15 The disproportionation reaction of [AuICl2]- (i.e., the 2e-reduction product of [AuIIICl4]-) to [AuIIICl4]- and Au metal was also found to occur significantly. Villagran et al. reported a two-step oxidation of Cl - on the Au electrode in BMIBF4,20 wherein they did not rule out the oxidative dissolution of Au from the electrode surface. Recently, Aldous et al. analyzed the electrode reaction processes of [AuIIICl4]- on GC, Pt and Au electrodes in 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide (BMI-NTf2) RTIL by cyclic voltammetry.17 They considered a two-step reduction process for the deposition of Au (i.e., [AuIIICl4]- f [AuICl2]- f Au(0)) and explained the observed anodic peaks as the oxidation (15) Oyama, T.; Okajima, T.; Ohsaka, T. J. Electrochem. Soc. 2007, 154, D322. (16) Oyama, T.; Okajima, T.; Ohsaka, T.; Yamaguchi, S.; Oyama, N. Bull. Chem. Soc. Jpn. 2008, 81, 726. (17) Aldous, L.; Silvester, D. S.; Villagran, C.; Pitner, W. R.; Compton, R. G.; Lagunas, M. C.; Hardacre, C. New J. Chem. 2006, 30, 1576. (18) Ionic Liquids in Synthesis; Wasserscheid, P.; Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2002. (19) Ionic Liquids Industrial Applications to Green Chemistry, Rogors, R. D.; Seddon, K. R., Eds.; American Chemical Society: Washington, DC, 2002. (20) Villagran, C.; Banks, C. E.; Hardacre, C.; Compton, R. G. Anal. Chem. 2004, 76, 1998.
Published on Web 02/17/2010
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of Cl- species on the deposited Au at GC and Pt electrodes rather than the dissolution of the deposited Au.17 Previous studies also have demonstrated that the electro-deposition and dissolution of Au largely depends on the electrode substrate in RTILs.15,17,20,26 With a view to clarifying these ambiguous points, we have recently introduced the in situ electrochemical quartz crystal microbalance (EQCM) method using a Au film-coated quartz crystal electrode (Au-QCE) to quantitatively analyze the attribution of the cyclic voltammetric response to the mass change of the Au deposition and dissolution on the Au surface in RTILs by means of the differential frequency responses in comparison with the current responses, and have found that this technique can be employed as a highly sensitive in situ probe of changes in mass of electrode surfaces in RTILs.16 In this case, however, the dissolution of Au was found to occur at the Au film-coated QCE surface in the examined potential range. Thus, in the present study, a combined use of electrochemical and in situ EQCM techniques using a Pt film-coated quartz crystal electrode (Pt-QCE) has been attempted to investigate quantitatively and comprehensively the [AuIIICl4]--[AuICl2]--Au(0) redox system at the Pt electrode in EMIBF4, and the differences between our results16 and those by Villagran et al.20 and Aldous et al.17 have been clarified. Unlike the Au-QCE, even in the presence of Cl- ions, the Pt-QCE itself dose not dissolve in the examined potential range. The electrodeposition of Au via a disproportionation reaction of [AuICl2]- (i.e., the 2e-reduction product of [AuIIICl4]-) as well as a two-step reduction of [AuIIICl4]- to metallic Au is realized. The electro-dissolution of the metallic Au from the Pt electrode and further oxidation of the dissolved product in the anodic scan are also clarified together with the oxidation of Cl- at the Pt electrode.
2. Experimental Section RTIL, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) with a purity of more than 99% and less than 30 ppm (i.e., ca. 2.1 mM) water, was purchased from Stella Chemifa Co. Ltd., Japan, and dried under reduced pressure at 120 °C for 12 h prior to use. 1-Ethyl-3-methylimidazolium chloride (EMICl) was available from Kanto Chemicals Co. Ltd., Japan. All of these chemicals were treated in the glovebox. Sodium tetrachloroaurate(III) (Na[AuIIICl4], Kanto Chemical Co. Ltd., Japan) and sulfuric acid (Wako Pure Chemical Industries, Japan) were of analytical grade and were used as received without further purification. All aqueous solutions were prepared with Milli-Q water purified by Millipore Milli-Q system (Millipore, Japan). All electrochemical measurements were carried out in a threeelectrode fashion using a potentiostat/galvanostat (HZ 5000, Hokuto Denko Co., Japan). A Pt wire was used as a counter electrode and a silver wire as a quasi-reference electrode (Ag(QRE)). In addition to the Pt-quartz crystal oscillator electrode, platinum (Pt) (diameter, φ = 3 mm) and gold (Au) (diameter, φ = 1.6 mm) disk working electrodes were used. These disk working electrodes were electrochemically pretreated in 0.1 M H2SO4 solution before use. Prior to each measurement, argon (Ar) or nitrogen (N2) gas was bubbled directly into the cell solution at least for 15 min to obtain a Ar or N2-saturated solution, and during every measurement the inert gas was flushed over the cell solution because molecular oxygen is an electroactive species in the examined electrode potential region.15 All potentials in the text are referred to a Ag(QRE) for the ionic liquids. The redox potential of the ferrocene/ferricinium (Fc/Fcþ) redox couple in EMIBF4 was 0.180 V vs Ag(QRE). All the measurements were conducted at 20 ( 1 °C. Before electro-deposition of gold, Pt electrode except for the Pt-QCE was polished on a polishing microcloth (Marumoto Struers Kogyo Co. Ltd.) with alumina powder (particle diameter, 1.0 and 0.06 μm (Marumoto))/water slurries. After polishing, the 9070 DOI: 10.1021/la904483y
Figure 1. CVs obtained at the Pt electrode in Ar-saturated EMIBF4 containing (a) 10 mM EMICl, (b) 10 mM EMICl and 10 mM Na[AuIIICl4], and (c) 45 mM EMICl and 10 mM Na[AuIIICl4]. Potential scan rate: 100 mV s-1. Pt electrode was sonicated in Milli-Q water for 10 min, and then electrochemically pretreated in 0.1 M H2SO4 solution, and kept in Milli-Q water. Gold deposition onto the Pt electrodes was carried out in a bath of EMIBF4 containing 10 mM Na[AuIIICl4] under Ar or N2 gas atmosphere by sweeping the potential from 1.20 (or 1.30) to -1.20 V at a scan rate of 100 mV s-1. The EQCM measurement system is composed of a 6 MHz AT-cut quartz crystal electrode (QCE) (working electrode area: 1.45 cm2), which is driven at its resonant frequency by a feedback oscillator.16 For a 6 MHz crystal operating in the fundamental mode, the approximate sensitivity of the EQCM is 12.3 ng cm-2 Hz-1, which is an absolute measurement of mass per unit area.21,22 Each side of the crystal was coated with Ti (thickness: 3 nm) as an adhesion layer and then with Pt (thickness: 300 nm) by vacuum deposition with a standard keyhole electrode configuration. One side of the crystal was kept out of the electrolyte solution by using a Teflon O-ring. The other side was used as the working electrode. All the EQCM measurements were performed in the Faraday cage. The resonant frequency was measured to the nearest 0.2 Hz using HQ-101B (Hokuto Denko Co.). Time differentiation of the frequency data and the amount of charge (i.e., the calculation of λ values; see eq 5) was accomplished by the least-squares methods based on “Excel slope function of Excel programs (Microsoft)” using the seven points of the values of the frequency and the amount of charge measured at back and forth each second.16 In order to examine the roughness effect upon the resonant frequency at the surface of Pt-QCE during potential scan, the admittance measurements of the quartz crystal oscillator were performed with 4194A impedance/gainphase analyzer (Hewlett-Packard Co. Ltd.).23,24 Prior to each measurement, the Pt-QCE was electrochemically activated by holding the applied potential at 1.60 V for 100 s in RTILs containing 10 mM EMICl.
3. Results and Discussion 3.1. Redox Behavior of Chloride Ion at the Pt Electrode. Figure 1a shows the typical cyclic voltammogram (CV) obtained at the Pt electrode in EMIBF4 solution in the presence of 10 mM (21) Sauerbrey, G. Z. Phys. 1959, 155, 206. (22) Applications of Piezoelectric Quartz Crystal Microbalances, Lu, C.; Czanderna, A. W.; Eds.; Elsevier Science, Ltd.: Amsterdam, 1984. (23) Okajima, T.; Sakurai, H.; Oyama, N.; Tokuda, K.; Ohsaka, T. Electrochim. Acta 1993, 38, 747. (24) Ikeda, S.; Oyama, N. Anal. Chem. 1993, 65, 1910.
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Figure 3. Potential dependence of the full width at half height (Δfwhh) and the conductance maximum frequency (Δf (Gmax)) in conductance spectra measured at the Pt-QCE in Ar-saturated EMIBF4 solution containing 10 mM Na[AuIIICl4] and 10 mM EMICl. The potential was scanned from the rest potential (0.46 V) to -0.60 V and then from -0.60 to 1.20 V and further from 1.20 V to the initial potential (0.46 V) at a potential scan rate of 10 mV s-1.
Figure 2. (a) CVs and frequency responses obtained at the PtQCE in EMIBF4 containing 5.7 mM Na[AuIIICl4] and 11.7 mM EMICl. Potential scan rate: 10 mV s-1. (-, •): first potential scan; (---, O): second potential scan. (b) CVs and frequency responses obtained under the same experimental condition as those in part a except that the potential was held for 100 s at 0.10 V in the course of the first potential scan.
EMICl. The assignment for each peak in CVs in Figures 1, 2, 4a, 5b, and 6 is given in Table 1. The observed cathodic peak I (0.80 V vs Ag (QRE)) and the anodic peak V (1.00 V) in the reverse scan correspond to the reduction of Cl2 or Cl3- to Cl- (the forward reactions of eq 1 or eq 2) and the oxidation of Cl- to Cl2 (the backward reaction of eq 1) or Cl3- (the backward reaction of eq 2), respectively. Cl2 þ 2e - ¼ 2Cl -
ð1Þ
Cl3 - þ 2e - ¼ 3Cl -
ð2Þ
This is consistent with the findings originally obtained by Villagran et al.20 and Sun et al.25 The anodic peak current was found to be proportional to the square root of potential scan rate
(25) Sun, H.; Yu, L.; Jin, X.; Hu, X.; Wang, D.; Chen, G. Z. Electrochem. Commun. 2005, 7, 685. (26) Xu, X. -H.; Hussey, C. L. J. Electrochem. Soc. 1992, 139, 3103.
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and also to the concentration of Cl-. In the absence of free Cl-, these redox peaks were not observed. 3.2. Redox Behavior of [AuIIICl4]- at Pt and GC Electrodes: Cyclic Voltammetry and in situ EQCM Study. Figure 1b demonstrates the typical CV obtained at the Pt electrode in EMIBF4 solution containing the same concentration (10 mM) of [AuIIICl4]- and EMICl. Two new reduction peaks II (0.10 V vs Ag (QRE)) and III (- 0.40 V) were obtained in the potential scan from 1.30 to -1.20 V, and two oxidation peaks IV (0.40 V) and VI (1.00 V) appeared in the reverse potential scan. The current of the reduction peak II was about twice that of the reduction peak III. Similar current responses have been observed at GC electrode in the same system.15,17,26 The anodic peak IV (0.40 V) at the Pt electrode was negligibly small when the potential scan was reversed at 0 V, just after the first reduction peak (Figure 2(a). However, this anodic peak current increased clearly when the electrode potential was held at 0.10 V, e.g., for 100 s and then scanned in the positive direction of potential (Figure 2(b). These behaviors are essentially the same as those reported previously.15,17,26 Thus, the cathodic peaks II and III are considered to correspond to the reduction of [AuIIICl4]- to [AuICl2]- and further reduction of [AuICl2]- to Au metal, respectively (i.e., the forward reactions of the eqs 3 and 4), and the anodic peak IV is ascribable to the oxidation of the deposited Au metal to [AuICl2](i.e., the backward reaction of eq 4). ½AuIII Cl4 - þ 2e - ¼ ½AuI Cl2 - þ 2Cl -
ð3Þ
½AuI Cl2 - þ e - ¼ Auð0Þ þ 2Cl -
ð4Þ
The anodic peak VI at the Pt electrode, which has been considered to correspond to the oxidation of [AuICl2]- to [AuIIICl4]- (i.e., the backward reaction of eq 3), requires to be discussed in detail. In order to ascertain the reaction processes regarding the two oxidation and three reduction peaks observed at the Pt electrode in the EMIBF4 containing EMICl in the potential range between -0.50 and 1.20 V, in situ EQCM technique was employed. In our DOI: 10.1021/la904483y
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Figure 5. (a) CV and the frequency responses obtained at the Pt-QCE in Ar-saturated EMIBF4 solution containing 6.3 mM Na[AuIIICl4]. In the course of an electrode potential scan at 10 mV s-1, the potential was held at 0.1 V for 314 s. (b) CV and the plot of λ vs potential obtained during the potential scan: (O) cathodic scan; (b) anodic scan. Data were taken from part a.
Figure 4. (a) CV and the plot of frequency vs potential obtained at the Pt-QCE in Ar-saturated EMIBF4 solution containing 3.2 mM Na[AuIIICl4] and 10.1 mM EMICl. Potential scan rate: 10 mV s-1. (b) Plot of frequency vs Q during the potential scan: (b) 1st scan; (O) 2nd scan. (c) CV and the plot of λ vs potential during the 1st potential scan: (b) cathodic scan; (O) anodic scan. Data were taken from parts a and b.
previous study,16 we have successfully applied in situ EQCM technique for studying the dissolution and deposition of Au (i.e., 1e-redox reaction of the Au/AuI redox couple) at Au film-coated QCE in EMIBF4 containing chloride ions, and have demonstrated that it can be employed as a highly sensitive in situ probe of changes in mass of electrode surfaces in RTILs. In general, however, the frequency response at the EQCM is highly sensitive not only to mass change but also to viscosity change of the 9072 DOI: 10.1021/la904483y
interface layer at the Pt-QCE which has been revealed by us and other groups.23,24,27-37 Therefore, first of all, we confirmed whether frequency changes at the Pt-QCE can be interpreted mainly in terms of the rigid mass changes or not, since there is a possibility of the change in surface roughness by the electro-deposition and (27) Moustafa, E. M.; El Abedin, S. Z.; Shkurankov, A.; Zschippang, E.; Saad, A. Y.; Bund, A.; Endres, F. J. Phys. Chem. B 2007, 111, 4693. (28) Oyama, N.; Ohsaka, T. Prog. Polym. Sci. 1995, 20, 761. (29) Buttry, D. A. In Electroanalytical Chemistry; Bard, A. J. Ed.; Marcel Dekker: New York, 1990; Vol. 17, p 1. (30) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. (31) Kanazawa, K. K.; Gordon, J. G., II Anal. Chem. 1985, 57, 1770. (32) Kanazawa, K. K. Faraday Discuss. 1997, 107, 77. (33) Martin, S. J.; Granstaff, V. E.; Frye, G. C. Anal. Chem. 1991, 63, 2272. (34) Martin, S. J.; Bandey, H. L.; Cernosek, R. W.; Hillman, A. R.; Brown, M. J. Anal. Chem. 2000, 72, 141. (35) Hillman, A. R. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds.; WILEY-VCH: Weinheim, Germany, 2003, Vol. 3 (Unwin, P., Vol. Ed.), p 230. (36) Hillman, A. R.; Daisley, S. J.; Bruckenstein, S. Electrochim. Acta 2008, 53, 3763. (37) Tsionsky, V.; Daikhin, L.; Urbakh, M.; Gileadi, E. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 2001; Vol. 22, p 1.
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Figure 6. CVs obtained at (a) Au and (b) Pt electrodes in Arsaturated EMIBF4 containing 30 mM EMICl. Potential scan rate: 100 mV s-1. Table 1. Summary of the Proposed Reactions and Peak Assignments peaks I II III IV V VI
proposed reactions Cl2 þ 2e- f 2Clor Cl3- þ 2e- f 3Cl[AuIIICl4]- þ 2e- f [AuICl2]- þ 2Cl[AuICl2]- þ e- f Au(0) þ 2ClAu(0) þ 2Cl- f [AuICl2]- þ e2Cl- f Cl2 þ 2eor 3Cl- f Cl3- þ 2e[AuICl2]- þ 2Cl- f [AuIIICl4]- þ 2eand 2Cl- f Cl2 þ 2eor 3Cl- f Cl3- þ 2e-
dissolution of Au on the Pt electrode. To evaluate this contribution, piezoelectric admittance measurement of the Pt-QCE is one of the best methods which allow the estimation of the resistance (R), inductance (L), and capacitance (C) in the electrical equivalent circuit for piezoelectric devices. The changes in R, L, and C components correspond to the changes in viscosity, mass, and elasticity of the interface of the QCE, respectively, on the basis of a mechanical model.22-24,28-37 Therefore, the determination of these parameters, especially resistance R which can be obtained directly from the maximum value (Gmax) of conductance (G) spectrum, provides useful information about the viscosity change at the interface of the QCE.23,24 The quantitative data for these admittance measurements during the potential scan of the Pt-QCE are demonstrated in Figure 3. The full width at half height (Δfwhh) was found to be almost the same for the oscillator in the potential range between -0.60 and 1.20 V. This result suggests that the viscosity at the interface of the Pt film is affected only a little by the electro-deposition and dissolution of Au. Therefore, it can be concluded that the mass change on the PtQCE is the main factor causing the frequency change observed in the EQCM measurements. That is, the frequency change during the redox reaction at the Pt-QCE in EMIBF4 is proportional to the mass change as reported previously for the Au-QCE in EMIBF4.16 Langmuir 2010, 26(11), 9069–9075
Figure 4a demonstrates the CV and potential-frequency curves obtained at the Pt-QCE in EMIBF4 containing 10.1 mM EMICl and 3.2 mM [AuIIICl4]- in the potential range between 0.60 and -0.50 V vs Ag (QRE). The potential was first swept from the open circuit potential (0.40 V) to 0.60 V and then from 0.60 to -0.50 V and finally from -0.50 to 0.60 V. On the first negativegoing potential sweep, two cathodic peaks II (0.11 V) and IIIa (-0.01 V) and on the reverse anodic sweep an anodic peak IV (0.35 V) were observed. As shown in Figure 4a, no change in the frequency was recognized in the potential range of the first reduction peak. But a decrease in the frequency was found to start around 0 V suggesting the Au deposition at this potential. The frequency continued to decrease up to 0.20 V during the reverse anodic sweep and then increased in the potential range from 0.20 to 0.60 V. However, at the potential of 0.60 V, the frequency did not return to its initial value, since all the deposited Au was not dissolved. The remaining amount of the Au deposited on the Pt surface can be estimated as 370 ng cm-2 from the difference in the frequency between the first and second potential cycles at 0.60 V. On successive potential cycles, in the first cycle, two cathodic peaks, i.e., II and IIIa, were observed together with the decrease in frequency from the onset of the second cathodic peak (from ca. 0.02 V), while in the second one only one broad peak IIIb was observed in harmony with the decrease in frequency from ca. 0.2 V. For this reason, it is considered that the deposition of Au takes place on the different electrode surfaces in the first and second cycles, i.e., on the Pt electrode surface and the surface of the Au previously deposited on the Pt electrode (in the first potential cycle), respectively. That is, the overpotential for the nucleation and deposition of Au at the Au electrode is by ca. 200 mV reduced compared to that at the Pt electrode. Figure 4b shows the frequency response vs Q plots corresponding to the CVs (Figure 4a) obtained at the first and second potential cycles. At a glance we can see that the frequency response, as expected, decreases and increases in the cathodic and anodic scans, respectively, in the continuous potential scan: the frequency response vs Q plots seem to be linear. However, exactly speaking, these plots are not linear and show the potential dependency. Therefore, a more detailed examination based on the estimation of the so-called apparent equivalent molar mass (λ), defined by eq 5,16 allowed us to elucidate the whole gravimetric phenomena of the electrochemical deposition and dissolution of Au in the present system. λ ¼ FAðjΔmj=jΔQjÞ
ð5Þ
where A and F show the area of electrode surface and the Faraday constant, respectively, and the unit of λ is gram per mol of electrons, i.e., g (mol e)-1. As mentioned above (Figure 3), the frequency change (Δf) of quartz crystal electrode during the electrolysis can be interpreted in terms of rigid mass change (Δm) based on the Saubrey equation: Δf = -K Δm (K is 8.14 107 s-1 g-1 cm2 for 6 MHz quartz crystal).21 The plots of λ as a function of the electrode potential, obtained on the basis of eq 5, are shown in Figure 4c together with the synchronously measured CV. Here it should be noted that the values of λ were omitted in the potential ranges of 0.40 f 0.60 f ca. 0.3 V and ca. 0.14 f 0.17 V, because the calculation of λ is obviously uncertain, that is, in the former potential range, no oxidation and reduction take place actually and the current change (in other words ΔQ) is close to 0, and Δf should be also close to 0, but in fact it was not so due to some extrinsic factor (noise), resulting in meaningless and anomalously scattered values of λ. In the small potential range of ca. 0.14 f 0.17 V, DOI: 10.1021/la904483y
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on the other hand, the current is changed from the cathodic to anodic one. Therefore the estimation method of λ mentioned in the Experimental Section could not be applied for this potential region. As clearly seen from Figure 4c, in the potential range of 0.3 f 0.1 V (in the cathodic scan), the value of λ is 0 g (mol e)-1, and in the potential range of ca. 0.1 to -0.50 V the λ raises from 0 to (100 ( 10) g (mol e)-1 as a maximum value at -0.05 V and then decreases and reaches a constant value of λ = (70 ( 5) g (mol e)-1. Here we recall that the values of λ can be expected to be 65.7 or 98.5 g (mol e)-1 for the Au deposition under the 3e-reduction of the Au(III) species or under the disproportionation of the 2e-reduction products of the Au(III) species, respectively, considering that the atomic weight of Au is 196.97 g.16 The change in λ seems to reflect that the reduction proceeds through the forward reactions expressed by eqs 3, 4, and 6, when the potential is applied to a value at which the 3e-reduction of [AuIIICl4]- takes place. The Au deposition from the [AuICl2]- produced by the 2e-reduction of the [AuIIICl4]- can be also induced by the disproportionation process of eq 7. Our results obtained from the present EQCM measurements strongly support the data obtained previously by the cyclic voltammetric measurements regarding the cathodic peaks II and III.15,17,26 ½AuIII Cl4 - þ 3e - ¼ Auð0Þ þ 4Cl -
ð6Þ
3½AuI Cl2 - h 2Auð0Þ þ ½AuIII Cl4 - þ 2Cl -
ð7Þ
On the anodic sweep (-0.50 f þ0.60 V), one oxidation peak was observed at 0.35 V (Figure 4c). A large mass change was recognized during the oxidation process, i.e., an increase in the frequency started at ca. 0.25 V and continued until 0.50 V. In this case, the increase of frequency corresponds to the mass loss by the oxidation process. In the potential range of 0.20 to 0.40 V, the plot of frequency response vs Q seems to give a straight line with a slope of 1.49 1010 Hz mol-1 cm2 (indicated by symbol * in Figure 4b), corresponding to the λ value of 183 g (mol e)-1. However, in this case, we can not conclude that the observed mass change can simply be attributed to the dissolution process of Au by the 1e-oxidation with an apparent current efficiency of 93%, because the situation is not simple, i.e., we can see from Figure 4c that the λ significantly changes from ca. 120 to 210 g (mol e)-1 when the electrode potential is changed from 0.2 to ca. 0.5 V. Only 1e-oxidative dissolution process without other reactions should give a λ value of 197 g (mol e)-1 with 100% current efficiency. Thus, this observation of the potential dependence of λ suggests that the oxidation process in this potential range is complicated and is not due to only 1e-oxidation of Au to form [AuICl2]-. At potentials more positive than 0.5 V, the values of λ trended to decrease gradually to ca. 125 g (mol e)-1 at 0.6 V. This is presumed to result from the oxidations of [AuICl2]- and/or Cl-, because they are considered not to be involved in the mass change on the electrode surface and thus the λ value becomes smaller as they proceed (i.e., ΔQ becomes larger). Previously, Aldous et al.17 have ascribed the oxidation peak at 0.23 V vs Ag/Agþ to the 2e-oxidation of Cl- to Cl3- (the backward reaction of eq 2) at the GC electrode in the BMINTf2 solution containing [AuIIICl4]- as reported previously by Villagran et al.20 They also mentioned that the chemical dissolution of Au by the electro-generated chlorine species cannot be ruled out.20 However, the present EQCM experiments at the Pt electrode clearly allows us to propose that the oxidation peak at 0.35 V corresponds to the dissolution of the Au. In addition, as another supporting evidence for our proposal, the Pt and GC 9074 DOI: 10.1021/la904483y
electrodes in the EMIBF4 solution containing EMICl without [AuIIICl4]- species did not show the oxidation peak around 0.35 V and also the frequency response with the Pt-QCE maintained a constant value. Therefore, an increase in the frequency in the potential sweep from 0.20 to 0.50 V in the solution of [AuIIICl4]can be ascribed to the dissolution of Au. Of course, as mentioned previously,15 the anodic peak corresponding to the oxidation of the Au was not obviously observed at 0.25 V in the absence of free chloride ion, when the negative-going potential sweep is scanned back at 0.10 V. The oxidation starting at ca. 0.20 V may result in the Au dissolution as [AuICl2]- ion (the backward reaction of eq 4) in RTIL.16 Figure 5a demonstrates the CV and frequency-potential curves obtained at the Pt-QCE in EMIBF4 containing 6.3 mM [AuIIICl4]- in the potential scan of 1.20 f 0.10 f 1.20 V vs Ag (QRE). The anodic peak current at 0.25 V and the frequency change at 0.10 V were observed significantly when the potential was held at 0.10 V for 314 s and then was scanned in the positive direction of potential, as reported previously at the GC electrode.15 Here, it should be noticed that the anodic peak current at 0.25 V and the frequency change at 0.10 V were negligibly small when the potential scan was reversed at 0 V without holding the potential after observing the first reduction peak, as shown in Figure 2a. The frequency decrease at 0.10 V can be attributed to the Au deposition induced by the disproportionation process of eq 7. When the potential was swept from 0.10 to 0.50 V after holding for 314 s at 0.1 V, the absolute value of λ raised to ∼200 g (mol e)-1 (Figure 5b), where the value of λ is expected to be 197 g (mol e)-1 for the dissolution of Au. On going to more positive potential, the value of λ decreased and became 0 at 0.8 V. However, at the potential of 0.8 V the frequency did not return to its initial value (see Figure 5a), since all of the deposited Au was not dissolved in the solution during the positive scan in the examined potential range. The potential dependence of the frequency observed at more than 0.8 V will be discussed in the next section. 3.3. Anodic Processes at High Positive Potential. Figure 6 shows the CVs obtained at bare Au and Pt electrodes in EMIBF4 solution containing 30 mM EMICl in which the potential scan was started from 1.30 or 1.20 V vs Ag (QRE) to the cathodic direction. Here it is worth noting that the reduction of Cl2 or Cl3to Cl- could be observed at 0.80 V at the Pt electrode, but not at the Au electrode. Interestingly, two cathodic peaks II and III corresponding to the forward reactions of eqs 3 and 4, respectively, were observed at the Au electrode when the potential was scanned toward the negative direction of potential. This fact confirms that at the initial potential of 1.30 V, Au was dissolved from the surface of the electrode in the presence of chloride ions to finally produce [AuIIICl4]- which was reduced at 0.10 V when the potential was swept toward the cathodic direction. Therefore, the two anodic waves IV and VI observed at the Au electrode are ascribable to the oxidation of the deposited Au metal to [AuICl2]and further oxidation of [AuICl2]- to [AuIIICl4]-, respectively. Figure 1c demonstrates the typical CV obtained at the Pt electrode in EMIBF4 solution containing 45 mM EMICl and 10 mM [AuIIICl4]-. Potential scan toward the cathodic direction resulted in an increase in the current of the peak I (0.80 V) compared to that in Figure 1b, while the concentration of the chloride ion was increased. But the absolute currents of the cathodic peaks II and III remained unchanged. In the reverse potential scan, two oxidation peaks IV (0.40 V) and VI (1.00 V) appeared with a higher current response than observed in Figure 1b. At this high concentration of chloride ion, the anodic peak VI was observed as a broad one. The anodic peak at 1.00 V Langmuir 2010, 26(11), 9069–9075
Oyama et al.
has been also observed by other researchers at GC and Pt electrodes in different RTILs.17,20,25 In the present study, it was found that the anodic peak VI was observed even when the potential scan was reversed at 0.10 V. At this potential both [AuICl2]- and Cl- are present in the vicinity of the electrode surface. Thus the observed broad oxidation peak VI at 1.00 V (Figure 1c) can be attributed to the oxidation of [AuICl2]- to [AuIIICl4]- as well as the oxidation of Cl- to Cl2 or Cl3- at the Pt electrode. As shown in Figure 5b, on the positive-going potential sweep above 0.50 V, the change of λ with the electrode potential is complicated: λ decreased from 200 g (mol e)-1 at 0.5 V to 0 g (mol e)-1 at ca. 0.8 V and then increased to 75 g (mol e)-1 (maximum) at ca. 0.9 V and then again decreased to 0 g (mol e)-1. When the oxidation wave at ca. 0.9 V corresponds to the oxidation of [AuICl2]- to [AuIIICl4]- and/or the oxidation of Cl- to Cl2 (Cl3-), which are considered not to be involved in the mass change on the electrode surface, the λ values can be expected to decrease simply with increasing the electrode potential to the positive direction. The unexpected observation of the potential dependence of λ suggests that any phenomena, which cause the mass change on the electrode surface, take place on the positive-going potential sweep about 0.5 V, for example, the formation of AuICl(ads) and AuIIICl3(ads) by the adsorption of Cl- anions on the electrode surface. The adsorption of these Cl- species on the metal electrode surfaces in RTILs and in aqueous solution has been proposed previously.17,38,39 (38) Vinals, J.; Nunez, C.; Herreros, O. Hydrometallurgy 1995, 38, 125. (39) Diaz, M. A.; Kelsall, G. H.; Welham, N. J. J. Electroanal. Chem. 1993, 361, 25.
Langmuir 2010, 26(11), 9069–9075
Article
4. Conclusions The cyclic voltammetric and EQCM frequency-potential responses for the redox reaction of [AuIIICl4]- at the Pt electrode in EMIBF4 have been studied. The results show that a series of 2e- and 1e-reductions of the [AuIIICl4]--[AuICl2]--Au(0) redox system occurs at the Pt electrode. Once the metallic Au was formed on the Pt electrode surface, the subsequent reduction of [AuIIICl4]- could occur on the Au deposit with a lower overpotential for the electrodeposition. In addition, in situ EQCM experiments proved that the Au deposited at 0.10 V was produced by the disproportionation of [AuICl2]- to [AuIIICl4]- and Au metal. The dissolution of the Au deposit was also investigated by in situ EQCM method using Pt-QCE. The oxidation peak observed at ca. 0.35 V can result from the dissolution of the Au as a result of its 1e-oxidation in the presence of chloride ions. A 2e-oxidation of the dissolved product [AuICl2]- to [AuIIICl4]- along with the oxidation of Cl- to Cl2 or Cl3- can be considered to occur at 1.0 V. A combined use of electrochemical and in situ EQCM techniques can be expected to be very powerful in elucidating electrochemical surface phenomena accompanying a mass change in RTILs as well as in aqueous and organic media studied previously.22,28-37 Acknowledgment. The present work was financially supported by Grant-in-Aid for Scientific Research (A) (No. 19206079) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.
DOI: 10.1021/la904483y
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