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Langmuir 2008, 24, 2564-2568
Oscillatory Electrodeposition of Metal Films at Liquid/Liquid Interfaces Induced by the Large Surface Energy of Growing Deposits Shuji Nakanishi,*,†,‡ Tomoyuki Nagai,† Kazuhiro Fukami,† Kentarou Sonoda,† Naohiro Oka,† Daisuke Ihara,† and Yoshihiro Nakato§ DiVision of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan, PRESTO, Japan Science, and Technology Agency, Osaka 560-8531, Japan, and Graduate School of Science and Technology, Kwansei Gakuin UniVersity, Sanda, Hyogo 669-1337, Japan ReceiVed October 27, 2007. In Final Form: December 3, 2007 Electrodeposition of zinc (Zn) at an aqueous ZnSO4/n-butylacetate (BuAc) interface (liquid/liquid (LL) interface) showed a potential oscillation in the region of the current density exceeding the diffusion-limited one, accompanied by formation of two-dimensional Zn film with a concentric pattern at the LL interface. In-situ optical microscopic inspections revealed that the oscillatory growth of the Zn film synchronized with meniscus oscillation of the LL interface. The vigorous growth of the deposits occurs only when the shape of the meniscus becomes hollow on the negative potential side of the potential oscillation. On the other hand, on the positive side, the meniscus becomes almost flat and the deposits formed in the preceding stage are thickened. A mechanism is proposed to explain the oscillatory Zn electrodeposition coupled with the meniscus oscillation, on the basis of the fact that the interfacial tension at the growing metal/aqueous solution interface is extremely large.
Introduction It is now widely known that electrochemical reactions with nonlinear kinetics give rise to various dynamic self-organization phenomena such as oscillations and spatiotemporal pattern formation.1-3 Of the various self-organization phenomena, oscillatory electrodeposition is very interesting from the point of view of production of micro/nanostructured materials because it has the possibility to produce ordered electrodeposits by recording ever-changing self-organized spatiotemporal patterns during the oscillation. They can be divided into two groups: one is treated with the formation of layered structures4-8 and the other the formation of ordered dendrites.9-18 As to the latter group, the oscillation appears during the electrodeposition of some metals, including Sn,10-12 Zn,13-17 and Cd,18 under diffusion-limited conditions. The formation of the ordered * To whom correspondence should be addressed. E-mail: shuji@chem. es.osaka-u.ac.jp. † Osaka University. ‡ PRESTO. § Kwansei Gakuin University. (1) Christoph, J.; Eiswirth, M. Theory of electrochemical pattern formation. Chaos 2002, 12 (1), 215-230. (2) Fafiday, T. Z.; Hudson, J. L. Modern Aspects of Electrochemistry; Plenum: New York, 1995; Vol. 27, p 383. (3) Krischer, K. AdVances in Electrochemical Science and Engineering; WileyVCH: Weinheim, 2003; Vol. 8, p 90. (4) Switzer, J. A.; Hung, C. J.; Huang, L. Y.; Switzer, E. R.; Kammler, D. R.; Golden, T. D.; Bohannan, E. W. Electrochemical self-assembly of copper/cuprous oxide layered nanostructures. J. Am. Chem. Soc. 1998, 120 (14), 3530-3531. (5) Wang, Y.; Cao, Y.; Wang, M.; Zhong, S.; Zhang, M. Z.; Feng, Y.; Peng, R. W.; Hao, X. P.; Ming, N. B. Spontaneous formation of periodic nanostructured film by electrodeposition: Experimental observations and modeling. Phys. ReV. E 2004, 69 (21), 021607-1-021607-7. (6) Nakanishi, S.; Sakai, S.; Nagai, T.; Nakato, Y. Macroscopically uniform nanoperiod alloy multilayers formed by coupling of electrodeposition with current oscillations. J. Phys. Chem. B 2005, 109 (5), 1750-1755. (7) Sakai, S.; Nakanishi, S.; Nakato, Y. Mechanisms of oscillations and formation of nanoscale layered structures in induced co-deposition of some irongroup alloys (Ni-P, Ni-W, and Co-W), studied by an in situ electrochemical quartz crystal microbalance technique. J. Phys. Chem. B 2006, 110 (24), 1194411949. (8) Nakanishi, S.; Sakai, S.; Nishimura, K.; Nakato, Y. Layer-by-layer electrodeposition of copper in the presence of o-phenanthroline, caused by a new type of hidden NDR oscillation with the effective electrode surface area as the key variable. J. Phys. Chem. B 2005, 109 (40), 18846-18851.
microstructures has occurred in synchronization with cycles of the oscillations.9,11-13 Our previous studies have revealed that the autocatalytic electrodeposition under diffusion-limited conditions plays the key role for the electrochemical oscillation.9,11-13 On the other hand, metal electrodeposition at liquid/liquid (LL) or liquid/air (LA) interfaces is known to form 2-dimensional (2D) films at the interfaces.19-25 This phenomenon has attracted a lot of interest from the point of view of morphogenesis in (9) Fukami, K.; Nakanishi, S.; Yamasaki, H.; Tada, T.; Sonoda, K.; Kamikawa, N.; Tsuji, N.; Sakaguchi, H.; Nakato, Y. General mechanism for the synchronization of electrochemical oscillations and self-organized dendrite electrodeposition of metals with ordered 2D and 3D microstructures. J. Phys. Chem. C 2007, 111 (3), 1150-1160. (10) Piron, D. L.; Nagatsugawa, I.; Fan, C. Cathodic potential oscillation of Sn(II)/Sn electrodes in KOH solution under constant current conditions. J. Electrochem. Soc. 1991, 138 (11), 3296-3299. (11) Nakanishi, S.; Fukami, K.; Tada, T.; Nakato, Y. Metal latticeworks formed by self-organization in oscillatory electrodeposition. J. Am. Chem. Soc. 2004, 126 (31), 9556-9557. (12) Tada, T.; Fukami, K.; Nakanishi, S.; Yamasaki, H.; Fukushima, S.; Nagai, T.; Sakai, S.; Nakato, Y. Tuning of the spacing and thickness of metal latticeworks by modulation of self-organized potential oscillations in tin (Sn) electrodeposition. Electrochim. Acta 2005, 50 (25-26), 5050-5055. (13) Fukami, K.; Nakanishi, S.; Tada, T.; Yamasaki, H.; Sakai, S.; Fukushima, S.; Nakato, Y. Self-organized periodic growth of stacked hexagonal wafers in synchronization with a potential oscillation in zinc electrodeposition. J. Electrochem. Soc. 2005, 152 (7), C493-C497. (14) St-Pierre, J.; Piron, D. L. Mechanism of cathodic potential oscillations of the zinc electrode in alkaline solutions. J. Electrochem. Soc. 1990, 137 (8), 2491-2498. (15) Shinohara, N.; Kaneko, H.; Nezu, H. Potential Oscillations during Electrocrystallizaiton of Zinc from Alkaline Cyanide Solutions. Electrochemistry 1993, 61 (10), 1211-1213. (16) Wang, S.; Zhang, K. Q.; Xu, Q. Y.; Wang, M.; Peng, R. W.; Zhang, Z.; Ming, N. B. Oscillations in electrochemical deposition of zinc. J. Phys. Soc. Jpn. 2003, 72 (6), 1574-1580. (17) Suter, R. M.; Wong, P.-z. Nonlinear oscillations in electrochemical growth of Zn dendrites. Phys. ReV. B 1989, 39 (7), 4536. (18) Kaneko, N.; Nezu, H.; Shinohara, N. Potential Oscillations During the Electrocrystallization of Cadmium from Alkaline Cyanide Solutions under Galvanostatic Conditions. J. Electroanal. Chem. 1988, 252 (2), 371-381. (19) Tamamushi, R.; Kaneko, H. Morphological and electrochemical study of a zinc-leaf electrodeposited at the butyl acetate/zinc sulfate solution interface. Electrochim. Acta 1980, 25 (4), 391-397. (20) Zeiri, L.; Younes, O.; Efrima, S.; Deutsch, M. Ring morphology in interfacial electrodeposition. Phys. ReV. Lett. 1997, 79 (23), 4685-4688. (21) Saliba, R.; Mingotaud, C.; Argoul, F.; Ravaine, S. Spontaneous oscillations in gold electrodeposition. Electrochem. Commun. 2002, 4 (8), 629-632.
10.1021/la7033565 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/02/2008
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Figure 1. (a) Schematic illustration of the experimental set up used in this work. (b, c) Schematic expansions of the circled area in (a).
Laplacian fields26,27 because growth patterns such as diffusionlimited aggregates (DLA) and dense branching morphologies (DBM), which are commonly observed in Laplacian fields, appear very easily and quickly in electrodeposition of some metals at the LL (or LA) interfaces. Although the electrodeposition of metal films at the LL (or LA) interfaces has thus been an object of study for a long time, little is known about the mechanism of the anisotropic 2D growth of the deposits along the interfaces. In addition, the electrodeposition of metals at LL (or LA) interfaces shows, in some cases, another notable phenomenon that the potential or current oscillates spontaneously.21-23 Interestingly, the oscillations appear only when the electrodepositions proceed at LL (or LA) interfaces and never occur in bulk solutions, implying the 2D LL interface plays an essential role for the electrochemical instability. In the present work, we have studied the mechanisms for the oscillatory Zn electrodeposition at an aqueous ZnSO4/n-butylacetate (BuAc) interface, which was first reported by Tada, et al.22 Detailed experiments have revealed that the large interfacial tension of the growing deposits/aqueous solution interface (γEA) leads to the 2D anisotropic film growth along the LL interface, which strongly assist the emergence of the oscillatory instability. It is shown that the electrochemical oscillation arises from cooperation of various processes, such as autocatalytic electrodeposition under diffusion-limited conditions, increase and decrease of the γEA, and meniscus oscillation. Experimental Section Figure 1 is a schematic of the experimental setup. Polycrystalline Pt wire (99.97% pure, 0.5 mm in diameter) was used as the working electrode. An aqueous solution of 3.0 M ZnSO4 was used as the electrolyte for Zn deposition. BuAc was poured on the aqueous ZnSO4 solution as an organic phase to form a LL interface. In some cases, a surfactant (trimethylstearylammonium chloride, C18TAC) (22) Tada, E.; Oishi, Y.; Kaneko, H. Electrochemical Oscillation during Electrodeposition of Zinc at the Interface between Two Immiscible Liquids (E). Electrochemistry 2007, 75 (9), 731-733. (23) Fukami, K.; Nakanishi, S.; Sawai, Y.; Sonoda, K.; Murakoshi, K.; Nakato, Y. In situ probing of dynamic nanostructural change of electrodeposits in the course of oscillatory growth using SERS. J. Phys. Chem. C 2007, 111 (8), 32163219. (24) Nakabayashi, S.; Aogaki, R.; Karantonis, A.; Iguchi, U.; Ushida, K.; Nawa, M. Two-dimensional metal deposition at the liquid/liquid interface; Potential Magnetohydrodynamic Pattern Transition 1999, 473 (1), 54-58. (25) Tada, E.; Oishi, Y.; Kaneko, H. Ultrathin copper films grown at the interface between two immiscible liquids. Electrochem. Solid-State Lett. 2005, 8 (2), C26-C29. (26) Ihle, T.; Mo¨ler-Krumbhaar, H. Fractal and compact growth morphologies in phase transitions with diffusion transport. Phys. ReV. E 1994, 49 (4), 2972. (27) Arneodo, A.; Argoul, F.; Couder, Y.; Rabaud, M. Anisotropic Laplacian growths: From diffusion-limited aggregates to dendritic fractals. Phys. ReV. Lett. 1991, 66 (18), 2332.
Figure 2. iap vs U curves for (a) surfactant-free and (b) surfactantadded solutions, observed with the iap being scanned from 0 to the negative at a rate of -0.1 mA/s. was added to the aqueous electrolyte. Electrochemical measurements were carried out using a normal three-electrode system, together with a beaker-type cell. A Pt plate (1 cm2) was used as the counter electrode. The reference electrode was a “Ag|AgCl|saturated KCI” electrode. Current (iap) versus electrode potential (U) and U versus time (t) curves were measured with a commercial potentiogalvanostat (Nikko-Keisoku, NPGS-301) and recorded with a data-storage system (Keyence, NR-2000) with a sampling frequency of 100 Hz. The electrolytes were kept stagnant in all measurements. Pictures and movies of the LL interface were taken using an optical microscope with a digital CCD camera (OM, VH-5000, Keyence), which was placed at the side of the electrode. The contact angles and the interfacial tensions at the LL interface were measured using a Drop Master 300 (Kyowa Interface Science).
Results Figure 2a and b shows iap vs U for the first current scan from zero to absolutely higher values under the current-controlled condition in the absence and presence of the surfactant, respectively. Although the amplitudes of the oscillations are largely different than each other, a potential oscillation appeared spontaneously with increasing the iap for both cases. Figure 3 indicates how the electrodeposition proceeds in the absence of the surfactant under the potential oscillation. Figure 3a shows a wave shape of the oscillation (U vs t curve) observed when the iap was scanned to negative values at a constant rate of -0.1 mA/s. It is to be noted here that the stable oscillation appeared only when the iap was scanned toward the negative in an appropriate rate. Pictures in Figure 3b were taken under insitu conditions with an optical microscope (OM) during the electrodeposition. Under the oscillatory condition, the deposit had a 2D disklike shape with a concentric-circle pattern. The numbers added to the pictures mean that they were taken at stages of the potential oscillation marked by the same numbers in Figure 3a. We can see that the Zn film shows anisotropic 2D growth along the LL interface and the diameter of the film largely increases in the negative side of the potential oscillation (stage 2-3) (see also Supporting Information, movie 1). The meniscus oscillation with small amplitude was also observed in synchronization with the potential oscillation (Supporting Information, movie 2). Similar oscillations and the formations of metal films
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Figure 3. (a) Potential oscillation observed in the absence of the surfactant. (b) Optical microscopic (OM) images of Zn deposits taken at various stages of the potential oscillation. The number, 1-4, added the the OM images means that they were obtained at stages of the potential oscillation marked by the same numbers in (a). See also Supporting Information, movies 1 and 2. (c) SEM images of the aqueous-phase side of the Zn deposits obtained in (1) negative side and (2) positive side of the potential oscillation.
with a concentric-circle pattern have also been observed in electrodeposition of Au at a LA interface.21,23 In addition, we inspected pieces of the Zn deposit with a scanning electron microscope (SEM) by pulling out the electrode from the electrolyte at negative-side and positive-side potentials of the potential oscillation. Figure 3c shows the SEM images of the aqueous-phase side of the Zn deposit. At negative-side potential, the deposits have a structure of thin leaflike wafers (Figure 3c1). On the other hand, at the positive-side potential, we can see that the wafers become thick and rounded (Figure 3c2). The above results were well reproduced in repeated experiments. We performed a similar experiment to Figure 3 in the presence of the surfactant (Figure 4). The notable features of the electrodeposition from surfactant-added solution, in contrast to the surfactant-free solution, are that (1) the 2D disk-shaped film was not obtained and (2) the amplitude of the meniscus oscillation is much larger (Supporting Information, movies 3 and 4). At stage 1, the positive side of the potential oscillation, the meniscus was almost flat and the deposit growth could not be seen. However, when the system reaches stage 2 (negative potential side), the shape of the meniscus became hollow. While the potential stayed in the negative side (stages 2-3), the vigorous growth of deposits occurred together with the downward shift of the LL interface. Then, the position of the LL interface was backed to the original position with the positive potential shift, and the vigorous growth of deposits ceased (stage 4). Thus, very interestingly, the
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Figure 4. (a) Potential oscillation observed in the presence of the surfactant. (b) Optical microscopic images of Zn deposits taken at various stages of the potential oscillation. See also Supporting Information, movies 3 and 4. (c) SEM images of the aqueous-phase side of the Zn deposits obtained in (1) negative side and (2) positive side of the potential oscillation.
Figure 5. Optical microscopic images of 3.0 M ZnSO4/BuAc interface in the presence of surfactant taken at (a) -2.0 and (b) -1.0 V vs Ag|AgCl.
electrodeposition in the presence of the surfactant still occurs along the LL interface, though the 2D film is not formed in this case due to the large-amplitude oscillation of the meniscus. The microscopic structure of the deposits was inspected by SEM. In a similar way to Figure 3c, the thin deposits grown at the negativeside of the potential oscillation (Figure 4c1) become thick and rounded at the positive-side potential (Figure 4c2). The appearance of the meniscus oscillation coupled with the potential oscillation implies that interfacial tension (free energy) of the electrode/aqueous phase interface (γEA) strongly depends on U. In order to visualize the U dependence of the γEA more clearly, we observed the shape of the LL interface in the presence of surfactant. Panels a and b of Figure 5 are the pictures of the LL interface taken at -2.0 and -1.0 V, which correspond to the negative-side and the positive-side of the potential oscillation, respectively. The contact angle (θ), which characterizes the shape
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Table 1. Interfacial Tensions (γLL) of 3.0 M ZnSO4/BuAc and Pure Water/BuAc Interfaces in the Absence and Presence of the Surfactant by Young-Laplace Methods concentration of ZnSO4 (CZn) ru (mNm-1)
with C18 TAC without C18 TAC
0M
3.0 M
9.6 ( 0.4 13.5 ( 0.4
22.6 ( 0.4 24.0 ( 0.4
of the LL interface, is generally determined by the following Young equation (eq 1):
γOA cos θ + γEO ) γEA
(1)
where γOA and γEO are the interfacial tensions of the organic phase/aqueous phase and the electrode/organic phase interfaces, respectively. Since the γOA and γEO are independent of U, the U dependence of θ is determined by γEA. Namely, the change of θ with U directly represents the U-dependence of the γEA. As shown in Figure 5, the θ value decreased with negative U shift, clearly indicating that the γEA increases with the negative U shift. Note that the θ was almost independent from the U in case the Zn(II) ion was not added in the solution so that electrodeposition of Zn did not take place. This fact rules out the electrocapillary as the origin of the large θ change in Figure 5. We also measured the interfacial tensions (γLL) of 3.0 M ZnSO4/ BuAc and pure water/BuAc interfaces in the absence and presence of the surfactant by Young-Laplace methods (Table 1), which will be discussed later.
Discussion First, it will be better to consider why γEA increases with the negative U shift. As mentioned above, θ (or equivalently γEA) changes with U only when the electrodeposition takes place on the electrode. This fact indicates that the growing (nonequilibrium) surface during deposition has larger γEA than the static one. Since the electrodeposition rate is higher in more negative U, the γEA becomes larger, i.e., the θ becomes smaller with the negative U shift. Importantly, the formation of 2D metal films at the LL (or LA) interfaces by electrodeposition, which mechanism had not been clarified so far, can also be explained on the basis of the above fact. Namely, when the tip of the WE is put just at the LL (or LA) interface, the electrodeposition tends to proceed along the LL (or LA) interfaces to minimize the effective area of the metal surface exposing to the aqueous phase with larger γEA. In agreement with this explanation, the 2D film growths at LL (or LA) interfaces proceed in higher over-potential region,19-24 where the electrodeposition occurs more vigorously. On the basis of the above argument, we consider the mechanism for the potential oscillation coupled with the meniscus oscillation with the aid of schematic illustrations of Figure 6. It is important to keep in mind that these phenomena are observed under the externally regulated current iap, which is higher than the diffusionlimited current for Zn deposition. Let us first consider the situation of stage 1, i.e., the positive side of the potential oscillation. At this stage, the electrodeposition proceeds under reaction-limited condition. The effective current density in the absolute value (|jeff|) increases when iap is scanned to the negative. When |jeff| exceeds the diffusion-limited current density in the absolute value (|jdl|), U suddenly shifts to the negative in order to maintain the externally controlled iap. Thus, the system reaches to the negative side of the oscillation (stage 2). At the negative end of the potential oscillation (stage 2), the electrodeposition rate of Zn is very high and, hence, this reaction becomes diffusion limited. Since γEA is very large at this stage, the shape of the meniscus becomes hollow. Under the diffusionlimited conditions, a circular or spherical diffusion layer for
Figure 6. Schematic drawings for explaining the mechanism of potential oscillation coupled with meniscus oscillation.
Zn(II) ions is formed at the edged (or peaked) parts of deposits, and this leads to much faster diffusion of the Zn(II) ions to the edged (or peaked) part, resulting in much faster growths of the deposits at these parts.9,11-13 Thus, electrodeposition proceeds autocatalytically and vigorous crystal growths occurred together with the further downward shift of the meniscus position (stage 3). The vigorous growth of the deposits at stages 2 and 3, which is actually seen in Figures 3b and 4b and Supporting Information, movies 1-4, leads to a large increase in the effective surface area (Seff) of the electrode (Zn deposits) and hence leads to a large decrease in the |jeff|. The decrease in |jeff| leads to a positive shift in U, which continues until the diffusion-limited conditions disappear. At the high-potential state (stage 4), where the reactionlimited conditions are attained, the vigorous crystal growth stops, and instead, the electrodeposition proceeds in an isotropic manner, resulting in the thickening of the deposits formed in the preceding stages. By repeating these processes, the potential oscillates spontaneously together with the meniscus oscillation and the periodic growth of the electrodeposits. As mentioned earlier, the amplitude of the meniscus oscillation in the presence of surfactant is much larger than that in the absence of it, which leads to the difference in the resultant morphology of the electrodeposits. Why the addition of the surfactant drastically changes the behavior of the meniscus oscillation? The key for the answer is the difference of the dependence of γLL on surface concentration of Zn(II) ion (CZn). On the basis of the above mechanism for the potential oscillation, the CZn also oscillates in synchronization with the potential oscillation. In the negative side of the potential oscillation, the reaction proceeds under diffusion-limited conditions, implying the CZn becomes almost zero, whereas CZn has a certain value in the positive side of the oscillation. As shown in Table 1, although there is almost no difference in the γLL between the surfactant-added and the surfactant-free solutions in the case of CZn ) 3.0 M, the γLL value in the presence of the surfactant under the CZn ) 0 condition is lower than that in the absence of it. Due to the smaller γLL value at CZn ) 0 for the surfactantadded solution, the meniscus is downwardly dragged at lowpotential state by large γEA due to active Zn electrodeposition,
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which leads to the appearance of the meniscus oscillation with large amplitude. In conclusion, we have shown that electrodeposition of Zn at an aqueous ZnSO4/n-butylacetate interface showed a potential oscillation coupled with the periodic crystal growth and the meniscus oscillation. Detailed experiments revealed that the large interfacial tension of the growing deposits/aqueous solution interface is the key factor for this phenomenon. In addition, very importantly, the present work has revealed the mechanism of the 2D metal film growths at the LL (or LA) interfaces by electrodeposition, which mechanism had not been clarified so
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far. We expect that the novel insights revealed in the present work will give deeper understanding of the interfacial chemistry at three-phase boundaries, as well as the morphogenesis in interfacial Laplacian fields. Supporting Information Available: Movies showing the oscillatory electrodeposition coupled with the meniscus oscillation taken under in-situ condition by optical microscope with a CCD camera (sampling frequency: 30 frame/s). This material is available free of charge via the Internet at http://pubs.acs.org. LA7033565
