Langmuir 1997, 13, 5215-5217
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Underpotential Deposition of Copper on Gold Electrodes through Self-Assembled Monolayers of Propanethiol Matsuhiko Nishizawa, Tomohide Sunagawa, and Hiroshi Yoneyama* Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka 565, Japan Received May 27, 1997. In Final Form: August 1, 1997X Electrodeposition of Cu on an Au(111)/mica electrode surface coated with a self-assembled monolayer (SAM) of propanethiol has been studied in a potential region comprising the underpotential deposition (UPD). The UPD of Cu and stripping of the deposited Cu took place reversibly through the SAM layer, and furthermore it was found that the SAM was still present on the electrode surface without significant changes in its amount and structure even after repeating Cu deposition/stripping cycles. The Cu adlayer stabilized the SAM completely toward its reductive desorption in an alkali solution, suggesting that a homogeneous Cu adlayer was formed on the entire electrode surface. A surface structure of the resulting electrode is discussed on the basis of these electrochemical results.
1. Introduction Chemically and structurally well-defined electrochemical interfaces prepared by underpotential deposition (UPD) of metals or by self-assembled monolayers (SAMs) of organic molecules have become important research subjects in recent years.1-6 We report here a surface designing with a combination of UPD of Cu (Cu UPD) and SAM of n-alkanethiol. Recently, Jennings and Laibinis7 prepared alkanethiol SAMs on an Ag UPD layer formed previously on an Au electrode and found that they possessed higher thermal stability than SAMs prepared directly on the Au electrode. In contrast, we have studied the opposite procedure: UPD of Cu or Ag was conducted on an Au electrode coated with a SAM. There have already been a few reports describing UPD and/or bulk deposition of Cu at Au electrodes coated with SAMs of n-alkanethiols (CH3(CH2)nSH),8-12 but most of them are concerned with the use of long chain thiols (n g 10) which are highly resistive to penetration of electrolyte solutions.13 Since UPD reactions involve strong adatom-electrode substrate interactions that are more energetically favorable than adatom-adatom interactions which occur during bulk * Author to whom correspondence should be addressed: e-mail address,
[email protected]. Fax, +81-6-8776802. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109335, and references therein. (2) (a) Ulman, A. An Introduction to ULTRATHIN ORGANIC FILMS From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, p 125. (4) Bard, A. J.; Abrun˜a, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, D.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (5) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Chem. Phys. 1992, 43, 437. (6) Whiteside, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (7) Jennings, G. K.; Laibinis, P. E. Langmuir 1996, 12, 6173. (8) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23. (9) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (b) Steinberg, S.; Rubinstein, I. Langmuir 1992, 8, 1183. (10) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 4823. (11) Gilbert, S. E.; Cavalleri, O.; Kern, K. J. Phys. Chem. 1996, 100, 12123. (12) Eliadis, E. D.; Nuzzo, R. G.; Gewirth, A. A.; Alkire, R. C. J. Electrochem. Soc. 1997, 144, 96. (13) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Chailapakul, O.; Sun, L.; Xu, C.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459.
S0743-7463(97)00545-3 CCC: $14.00
electrodeposition,3 they should not occur unless metal ions reach to the underlying electrode surface. Crooks et al.8 and Rubinstein et al.9 utilized Cu UPD as a probe to evaluate the nature and density of defects in the octadecanethiolate SAM. Gilbert et al.11 observed underpotentially-deposited Cu nanoclusters on a decanethiol SAMcoated Au electrode by STM, while this process was too slow to permit observable deposition/stripping currents flow. More recently, Eliadis et al.12 studied the influence of length of alkanethiols (n ) 4, 6, 9, and 15) on the morphology of bulk electrodeposites of Cu and found that the use of thiols of short chain length resulted in formation of smooth Cu films due to numerous monolayer defects that permit Cu UPD and nucleation of deposited Cu. In all these previous studies, the SAM itself acted as an effective barrier for Cu UPD and the defects in the SAM provided the sites for UPD to take place. We report in this paper that Cu UPD takes place on the entire surface of Au substrate through a SAM of propanethiol (PT) and that the SAM serves as a coadsorbate for the UPD process. 2. Experimental Section All chemicals used were of reagent grade, and water was purified by two distillations of deionized water. Au films of ca. 200 nm thickness were vacuum evaporated on freshly cleaved natural mica sheets (Nilaco Co.) maintained at 350 °C to prepare Au/mica electrode substrates. It is well-established that the vapor-deposited gold film on the heated mica has predominantly a (111) surface crystallinity and serves as a quasi-Au(111) electrode.1,14 The prepared electrodes were treated with H2SO4/ H2O2 (3:1) for 5 min and rinsed thoroughly with water. The electrode coated with SAM of propanethiol, which is denoted in this paper as a PT-Au electrode, was prepared by immersing the Au/mica substrate in 1 mM propanethiol dissolved in ethanol overnight. Voltammograms were measured using a BAS-100B electrochemical analyzer connected to a Gateway 2000 computer. The electrode substrate was placed at a bottom hole of an electrochemical cell with a Teflon-coated O-ring (inner diameter, 0.68 cm). The active electrode area was 0.40 cm2, as determined from the amount of charges involved in anodic oxidation of adsorbed iodine.14c,15 A bright Pt wire was used as a reference electrode for UPD experiments to minimize contamination, but all potentials are reported with respect to an Ag/AgCl electrode in a saturated KCl. (14) (a) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (b) Golan, Y.; Margulis, L.; Rubinstein, I. Surf. Sci. 1992, 264, 312. (c) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (15) (a) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283.
© 1997 American Chemical Society
5216 Langmuir, Vol. 13, No. 20, 1997
Letters
Figure 2. Linear sweep voltammetric curves for reductive desorption of PT SAM from Au/mica substrates before use in the Cu UPD experiment (dashed curve) and after anodic stripping of the deposited Cu in the Cu UPD experiments conducted in sulfuric acid solution (solid curve). Measurements were conducted in 0.5 M KOH aqueous solution at a scan rate of 100 mV s-1.
Figure 1. Cyclic voltammograms of a naked Au/mica electrode (dashed curve) and the PT-Au/mica electrode (solid curve) taken in (a) 1 mM CuSO4/50 mM H2SO4 and (b) 1 mM Cu(ClO4)2/50 mM HClO4 aqueous solutions. Scan rate of electrode potential was 2 mV s-1.
3. Results and Discussion Figure 1 shows typical cyclic voltammograms (CVs) of a naked Au and the PT-Au electrodes taken in (a) 1 mM CuSO4/50 mM H2SO4 and (b) 1 mM Cu(ClO4)2/50 mM HClO4 aqueous solution at a scan rate of 2 mV s-1. The Cu UPD on the naked Au electrode gave two energetically well-separated reversible waves in sulfuric acid solution, while a couple of broad deposition and sharp stripping waves was observed in the perchloric acid solution, as reported previously.16,17 It was discovered in the present study that UPD occurred even on the PT-Au electrode, though the peak potential of the deposition was shifted negatively and that of stripping was shifted positively as compared to that observed at the naked Au electrode. By comparison of the results obtained in the sulfuric acid and those obtained in the perchloric acid solution with each other, it is found that the CV shape of the PT-Au electrode is not significantly influenced by the kind of electrolyte. The amount of charges involved in the stripping wave at around 0.45 V was 283 (( 15%) µC cm-2 for the scan rates of 1, 2, 5, and 10 mV s-1, and the peak current of both deposition and stripping waves depended linearly on the scan rate. We obtained similar results for SAMs of ethanethiol and butanethiol, but for the SAM of pentanethiol the Cu UPD was not clear due exactly to its high resistance for penetration of electrolyte solutions.12 (16) (a) Zei, M. S.; Qiao, G.; Lehmpfuhl, G,; Kolb, D. M. Ber. Bunsenges. Phys. Chem. 1987, 91, 349. (b) Magunussen, O. M.; Hotlos, J.; Beitel, G.; Kolb, D. M.; Behm, R. J. J. Vac. Sci. Technol., B 1991, 9, 969. (c) Hachiya, T.; Honbo, H.; Itaya, K. J. Electroanal. Chem. 1991, 315, 275. (d) Borges, G. L.; Kanazawa, K. K.; Gordon, J. G., II; Ashley, K.; Richer, J. J. Electroanal. Chem. 1994, 364, 281. (17) (a) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1994, 365, 303. (b) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1994, 364, 289. (c) Borges, G. L.; Kanazawa, K. K.; Gordon, J. G.; Ashley, K.; Richer, J. J. Electroanal. Chem. 1994, 364, 281. (d) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Yee, D.; Sorensen, L. B. Phys. Rev. Lett. 1995, 75, 4472.
The CV shape of PT-Au electrodes shown in Figure 1 was unchanged during the course of successive measurements, being in marked contrast to that obtained at the naked Au electrode. The amount of PT molecules on the electrode surface was evaluated from the charge involved in one-electron reductive desorption of the PT SAM in 0.5 M KOH.14c,18 Figure 2 shows typical results obtained before the UPD experiments (dashed curve) and after the Cu deposition/stripping cycles (solid curve). The integrated currents of 74.4 and 76.6 µC cm-2 were obtained for the dashed and the solid voltammograms, respectively, and the difference between these values was within experimental error. Furthermore, the amount of PT molecules calculated from those charges (ca. 7.8 × 10-10 mol cm-2) is in good agreement with that expected for the complete coverage of n-alkanethiol SAM on a Au(111) surface (7.6 × 10-10 mol cm-2).14c,18 An important information derived from these results is that no significant loss of PT molecules occurred during the Cu deposition/ stripping experiments. However, the shape and the position of reduction peaks of these two voltammograms were a little different, suggesting that the ion permeability of SAM was slightly changed by some changes in ordering of SAM during the Cu UPD experiments. We found that the deposited Cu stabilizes the SAM toward its reductive desorption. As shown in Figure 3, the reductive desorption wave of PT did not appear at all at the Cu-deposited PT-Au electrode even if the electrode potential was swept down to -1.2 V in 0.5 M KOH, while “this” electrode gave a clear reductive current wave (71.4 µC cm-2) after the anodic stripping of Cu. The finding that the Cu-deposited SAM gained high stability against reductive desorption suggests that some kind of interaction was operative between Cu and the sulfur head group of PT. At present we have no definite mechanism capable of explaining the stabilization effect, but the observed complete stabilization seems to indicate that all PT molecules interacted with the deposited Cu atoms. If the Cu deposition occurred in such a way as to give Cu islands, the reductive desorption of PT molecules should have taken place at the parts where no Cu deposit was present. Therefore, it would be reasonable to assume that the Cu (18) (a) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (b) Walczak, M. M.; Popenoe, D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (c) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860.
Letters
Figure 3. Cyclic voltammograms of a Cu-deposited PT-Au/ mica electrode in 0.5 M KOH aqueous solution, obtained before (solid curve) and after (dashed curve) the anodic stripping of Cu in 50 mM H2SO4 aqueous solution. Scan rate of the electrode potential was 100 mV s-1.
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composite of PT and Cu was formed with the ratio of 1:2 as illustrated in Figure 4b. The structure proposed here is consistent with the fact that two-thirds of the 3-fold hollow sites are still available for deposition of Cu even if a SAM covers fully the Au(111) surface. However, since the amount of charge estimated from CVs often contains contributions from coadsorbed electrolyte anions to some extent, it is not always rationalized to determine the coverage of UPD metals from the charge obtained from CVs alone.17 If the actual Cu coverage was larger than that estimated from CVs, it may be reasonable to assume the formation of a bilayer as illustrated in Figure 4c, in which Cu UPD monolayer covers the Au substrate with displacement of the SAM layer from the Au substrate to the deposited Cu UPD layer. Such exchange of the thiolate-Au bond was proposed by Tarlov23 for the case of vacuum deposition of an Ag monolayer onto the hexadecanethiol SAM-coated Au substrate. In that case, the sulfur head group of the SAM was thought to detach from the Au substrate and rebond with the Ag atom.23 In order to verify the conversion of the surface structure given by Figure 4 a to that given by Figure 4c, it seems essential to characterize a PT SAM prepared on Cu UPD monolayercoated Au. However, our repeated trials of assembling a PT SAM on Cu monolayer were unsuccessful by the reason that the Cu monolayer prepared by UPD tended to be oxidized by air and to be corroded by thiol.24 4. Conclusion
Figure 4. Schematic illustrations of (a) the (x3×x3)R30° adlayer of alkanethiolate on Au (111) and of (b, c) the proposed surface structures of the Cu UPD adlayer. The open, close, and shaded circles represent the Au, S, and Cu atoms, respectively.
adlayer deposited homogeneously on the entire surface of the PT-Au electrode. In fact, our preliminary ex situ STM observations did not give any evidence suggesting the presence of Cu nanoislands. While the aim of this letter is to report the finding that Cu UPD takes place reversibly on the entire surface of a PT-Au electrode without any significant loss of PT SAM, we add here discussion about the surface structure of the Cu-deposited PT-Au electrode. At first, it has been widely accepted that the sulfur head groups of n-alkanethiol SAM form a commensurate (x3×x3)R30° structure on an Au(111) surface,19-22 as illustrated in Figure 4a regardless of the length of the alkylchain.18a In order for the Cu UPD to take place on the Au surface, the electrolyte solution containing Cu2+ should penetrate into the SAM layer. If we assume that the amount of charge obtained from the Cu stripping wave (ca. 283 µC cm-2) is indebted purely to the two-electron Cu dissolution reaction, the deposited Cu atoms amount to ca. 14.7 × 10-10 mol cm-2, which is approximately twice that of the PT molecules on the surface (ca. 7.8 × 10-10 mol cm-2), suggesting that a (19) (a) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (b) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (c) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719. (20) (a) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (b) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993, 9, 1556. (21) (a) Dubois, H. L.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1933, 98, 678. (b) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (c) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (22) (a) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (b) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 4994.
We have reported for the first time that Cu UPD takes place reversibly on the entire surface of an Au electrode coated with SAM of short alkanethiol without any significant loss of thiol molecules and that the SAM serves as a coadsorbate for the UPD process. Recent studies have revealed that the coadsorption of anionic species such as sulfate and iodide has important roles in metal UPD processes.4,16,17,25 While the amount and the form of adsorption of such species are usually changed by changing the electrode potential, the SAM of alkanethiols is thought to be stable against potential imposition.20b Thus we believe that SAM provides an easy way of preparation of structure-defined electrode surfaces for UPD studies. However, the electrochemical results shown here are insufficient to draw any reliable surface structure of the Cu-deposited PT-Au electrode. Further studies using other surface sensitive techniques such as atomic-resolution scanning tunneling microscopy should be needed to reveal the details. Acknowledgment. This work was supported by Grant-in-Aid for Priority Area No. 09237104 and by Grantin-Aid for the International Scientific Research Program: Joint Research No. 0744150 from the Ministry of Education, Science, Culture and Sports, Japan. LA970545F
(23) Tarlov M. J. Langmuir 1992, 8, 80. (24) (a) Laibinis, P. E.; Whitesides, G. M.; Allara,, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (25) (a) Weaver, M. J. J. Phys. Chem. 1996, 100, 13079. (b) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (c) Soriaga, M. P. Prog. Surf. Sci. 1992, 39, 325. (d) Shi, Z.; Wu, S.; Lipkowski, J. Electrochim. Acta 1995, 40, 9. (e) Gao, X.; Zhang, Y.; Weaver, M. J, J. Phys. Chem. 1992, 96, 4156. (f) Demir, U.; Shannon, C. Langmuir 1994, 10, 2794. (g) Sugita, S.; Abe, T.; Itaya, K. J. Phys. Chem. 1993, 97, 8780.
