H2SO4 (Cl) + Cu2+ from Distance Tunneling Spectroscopy - American

Institute of Thin Films and Interfaces, ISG 3, Research Centre Ju¨lich,. D-52425 Ju¨lich, Germany. Received May 31, 2003. In Final Form: September 2...
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Langmuir 2003, 19, 10271-10280

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Double Layer Properties of Au(111)/H2SO4 (Cl) + Cu2+ from Distance Tunneling Spectroscopy Gabor Nagy† and Thomas Wandlowski* Institute of Thin Films and Interfaces, ISG 3, Research Centre Ju¨ lich, D-52425 Ju¨ lich, Germany Received May 31, 2003. In Final Form: September 29, 2003 We performed a series of distance tunneling characteristics measurements for the system Au(111)/0.05 M H2SO4 + 1 mM Cu2+ in the absence and in the presence of small amounts of chloride to understand if it is possible to map the liquid part of the double layer perpendicular to the electrode surface. We found that we probed the double layer in a distance range where we do not penetrate into the inner Helmholtz layer. Nevertheless, the tip is sufficiently close to the metal surface to address adlayer features showing sensitivity toward long-range-ordered structures. The bias between the scanning tunneling microscope tip and the sample drops in the inner Helmholtz layer, and strong electronic overlap exists between the adsorbed layer and the metal surface. A detailed analysis showed that our approach is capable of detecting molecular contributions to the electronic structure of ordered adlayers. At larger distances from the surface, the average barrier height was found to be about 1 eV, practically independent of the electrode potential. Our results represent a further step to advance the application of scanning tunneling spectroscopic methods in electrochemistry and open up a new, exciting field of studying not just the structure and electronic properties but also the reactivity of adsorbed adlayers at the atomic level.

1. Introduction During the past decade, a molecular/atomic scale understanding is starting to emerge on the structure of “bare” and adsorbate-covered electrode surfaces and of processes taking place there.1,2 These advances may be attributed to four main areas: (i) the use of well-defined (single-crystal) electrodes instead of mercury and polycrystalline materials;3,4 (ii) the remarkable progress in the development and application of a wide variety of structure-sensitive in situ techniques, such as electroreflectance,5 second harmonic generation,6 vibrational spectroscopy and microscopy (infrared (IR),7 Raman (SERS),8 visible-infrared sum frequency (SFG) and difference frequency (DFG)9 spectroscopy), scanning probe microscopy,10 and surface X-ray techniques;11 (iii) the use of ex situ ultrahigh vacuum (UHV) techniques for structural investigations of simulated electrochemical double layers12 or of emersed electrodes;13 (iv) the progress in advanced * Corresponding author. E-mail: [email protected]. Fax: 49 2461 61 3462. Tel: 49 2461 61 3907. † New address: KFKI Atomic Energy Research Institute, P.O. Box 49, H-1525 Budapest, Hungary. (1) Interfacial Electrochemistry; Wieckowski, A.; Ed.; Marcel Dekker: New York, 1999. (2) Kolb, D. M. Angew. Chem. 2001, 113, 1198. (3) Hamelin, A. In Modern Aspects of Electrochemistry; Conway, B. E., White, R. E., Bockris, J. O. M., Eds.; Plenum Press: New York, 1985; Vol. 16, p 1. (4) Clavilier, J. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 231. (5) Kolb, D. M. In Spectroelectrocemistry: Theory and Practice; Gale, R. J., Ed.; Plenum: New York, 1988; p 87. (6) Pettinger, B.; Bilger, Ch.; Lipkowski, L.; Schmickler, W. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 373. (7) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271. (8) Tian, Z. Q.; Ren, B. In Encyclopedia of Analytical Chemistry; Meyer, R. A., Ed.; J. Wiley & Sons Ltd.: Chichester, 2000; p 9162. (9) Tadjeddine, A.; Peremans, A. In Spectroscopy for Surface Science; Clark, R. J. H., Hester, R. H., Eds.; J. Wiley: New York, 1998; p 159. (10) Itaya, K. Prog. Surf. Sci. 1997, 56, 121. (11) Wang, J. X.; Adzic, R. R.; Ocko, B. M. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 175.

computer simulations14 and theoretical concepts to describe the electrochemical double layer and/or electrode reactions.15,16 The combination of classical electrochemical techniques with in situ structure sensitive methods provided a fairly detailed understanding of electrified interfaces. Among these approaches, scanning tunneling microscopy (STM) advanced tremendously our knowledge about surface structure effects in electrochemistry because imaging of electrode surfaces was now possible in situ, in real space and with atomic resolution.17,18 Although in situ STM measurements at constant height or constant current reveal electronic structure information, separating the contributions of electronic and geometric properties is not straightforward. Electronic information is more directly accessible from scanning tunneling spectroscopy (STS) experiments.19 With the so-called I-V spectroscopy (modulating the bias between the tip and the sample), also applicable to solid/liquid interfaces, the local surface density of states can be measured as a function of the tunneling voltage.20-23 Alternatively, the effective (12) Pirug, G.; Bonzel, H. P. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH Chemie: New York, 1993; p 153. (13) Kolb, D. M. Z. Phys. Chem. NF 1987, 154, 179. (14) Spohr, E. In Advances in Electrochemical Science and Engineering; Alkire, R. C., Kolb, D. M., Eds.; Wiley-VCH: Weinheim, 1999; Vol. 6, p 1. (15) Guidelli, R.; Schmickler, W. Electrochim. Acta 2000, 45, 2317. (16) Koper, M. T. K.; Schmickler, W. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; p 291. (17) Kolb, D. M. Surf. Sci. 2002, 500, 722. (18) Tao, N. J.; Li, C. Z.; He, H. X. J. Electroanal. Chem. 2000, 492, 81. (19) Scanning Probe Microscopy and Spectroscopy; Bonnell, D. A., Ed.; Wiley: New York, 2001. (20) Azumi, K.; Araki, K.; Seo, M. J. Electroanal. Chem. 1997, 427, 15. (21) Hiesgen, R.; Krause, M.; Meissner, D. Electrochim. Acta 2000, 45, 3213. (22) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67. (23) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sanky, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571.

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tunneling barrier can be estimated with the so-called barrier height or I-S characteristics measurements (S is the relative distance between tip and sample). This approach yields the height of the potential barrier between the tip and the sample, coupled with the local work function of the sample. Several applications have been published on measurements in vacuum or in dry atmosphere.19 The first I-S experiment in an electrochemical system was reported by Siegenthaler et al.24 for Pb underpotential deposition (UPD) on Au(111) in perchlorate solution, who found discrete jumps in the exponential tunneling characteristics. The authors attributed their observations to reaching point contact. Lindsay et al.25,26 suggested that the local barrier height measurements are characteristic to the liquid part of the interface. Halbritter and coworkers found for Ag(111)/0.01 M HClO4 exponential I-S curves with barrier heights φ depending on the electrode potential.27 In more recent contributions, exponential I-S characteristics have been reported for bare and iodinemodified Au(111) and Pt(111) in aqueous electrolyte,28 for Au(100) covered with a monolayer of Ag,29 for Cu-(1 × 1) UPD on Au(111),30 and Au(111)-(p × x3) in humid air.31 The corresponding values of the tunneling barrier were estimated as 0.4-1.2 eV,28 1.0 eV,29 1.2 and 1.5 eV,30 and 0.95 eV.31 Despite the scatter, these values for electrochemical systems are significantly smaller than barrier heights measured under UHV conditions.19 Nonexponential tunneling characteristics were found for highly oriented pyrolytic graphite (HOPG)/water and Pt(100)/water interfaces.32 Similar results were obtained by Kang et al.33 for HOPG and Au(111). Kobusch et al. explored the application of I-S spectroscopy to oxidecovered titanium electrodes.34 Schindler35 found weak modulations in the distance tunneling characteristics for Au(111)/HClO4. Baltruschat et al.36 were able to detect the point of contact at different bias values for Cu UPD on Au(111) in copper-containing sulfuric acid solution. The tunneling resistance at the point of contact was found to be close to the quantum resistance. Using quantum chemical methods, Schmickler37,38 and Nitzan et al.39 addressed successfully theoretical aspects of STM/STS in electrolyte solutions. Nagy et al. calculated I-S characteristics for Pt(100)/water interfaces from molecular dynamics (MD) simulations via the electron-water (24) Bingelli, M.; Carnal, D.; Nyffenegger, R.; Siegenthaler, H. J. Vac. Sci. Technol., B 1991, 9, 1985. (25) Pan, J.; Jing, T. W.; Lindsay, S. M. J. Phys. Chem. 1993, 98, 4205. (26) Vaught, A.; Jing, T. W.; Lindsay, S. M. Chem. Phys. Lett. 1995, 236, 305. (27) Halbritter, J.; Repphun, G.; Vinzelberg, S.; Staikov, G.; Lorenz, W. J. Electrochim. Acta 1995, 40, 1385. (28) Nagani, Y.; Hayasi, T.; Yamada, T.; Itaya, K. Jpn. J. Appl. Phys. 1996, 35, 720. (29) Vinzelberg, S. Ph.D. Thesis, University of Karlsruhe, Karlsruhe, Germany, 1995. (30) Engelmann, G. E.; Ziegler, J. C.; Kolb, D. M. Surf. Sci. 1998, 401, L420. (31) Song, M. B.; Jang, J. M.; Bae, S. E.; Lee, C. W. Langmuir 2002, 18, 2780. (32) Nagy, G. Electrochim. Acta 1995, 40, 1417. Nagy, G. J. Electroanal. Chem. 1996, 409, 19. (33) Hahn, J. R.; Hong, Y. A.; Kang, H. Appl. Phys. A 1998, 66, S467. Hong, Y. A.; Hahn, J. R.; Kang, H. J. Chem. Phys, 1998, 108, 4367. (34) Kobusch, C.; Schultze, J. W. Electrochim. Acta 1995, 40, 1395. (35) Schindler, W. Personal communication. (36) Nielinger, M.; Baltruschat, H. Phys. Chem. Chem. Phys., submitted. (37) Schmickler, W. Surf. Sci. 1995, 335, 416. (38) Schmickler, W. In Imaging of Surfaces and Interfaces; Lipkowski, J., Ross, P. N., Eds.; Wiley: New York, 1999; p 305. (39) Peskin, U.; Edlund, A.; Bar-On, I.; Galperin, M.; Nitzan, A. J. Chem. Phys. 1999, 111, 7558 and references therein.

Nagy and Wandlowski

pseudopotential formalism.40 Recently, Nitzan et al.41 performed ab inito molecular dynamics calculations of a realistic STM configuration predicting nonexponential distance dependence and linear bias dependence of the tunneling current at moderate fields. The above examples demonstrate that in situ STS is capable of providing valuable information about the electronic states of bare and adsorbate-covered solid/liquid interfaces. Motivated by these results, we aimed to understand if it is possible to map the liquid part of the double layer perpendicular to the electrode surface by measuring distance tunneling characteristics. We have chosen Au(111)/0.05 M H2SO4 + 1 mM Cu2+ in the absence or in the presence of small amounts of chloride for our investigations. This well-known electrochemical system enables us to systematically investigate in situ I-S characteristics of the bare and the oxidecovered Au(111) surface,42 as well as of several wellordered adlayer structures by tuning the substrate potential.43-47 The UPD of Cu on Au(111) in sulfuric acid electrolyte has been studied using (i) classical electrochemical methods, such as voltammetry48-50 and chronocoulometry,51 (ii) in situ techniques such as STM,52-54 atomic force microscopy (AFM),55,56 Fourier transform infrared (FTIR) spectroscopy,57,58 X-ray absorption spectroscopy,59,60 surface X-ray scattering (SXS),61,62 and quartz crystal microbalance (QCM),63,64 and (iii) ex situ UHV techniques (40) Nagy, G.; Denault, G. J. Electroanal. Chem. 1997, 437, 37. (41) Galperin, M.; Nitzan, A.; Benjamin, I. J. Phys. Chem. A 2002, 106, 10799. (42) Schneeweiss, M. A.; Kolb, D. M.; Liu, D.; Mandler, D. Can. J. Chem. 1997, 75, 1703. (43) Magnussen, O. M.; Hagebo¨ck, J.; Hotlos, J.; Behm, R. J. Faraday Discuss. 1992, 94, 329. (44) Batina, N.; Will, T.; Kolb, D. M. Faraday Discuss. 1992, 94, 93. (45) Matsumoto, H.; Inukai, J.; Ito, I. J. Electroanal. Chem. 1994, 379, 223. (46) Hotlos, J.; Magnussen, O. M.; Behm, R. J. Surf. Sci. 1995, 335, 129. (47) Schneeweiss, M. A.; Kolb, D. M. Phys. Status Solidi 1999, 173, 51. (48) Schultze, J. W.; Dickertmann, D. Faraday Symp. 1977, 12, 36; Surf. Sci. 1976, 54, 489. (49) Ho¨lzle, M. H.; Zwing, V.; Kolb, D. M. Electrochim. Acta 1995, 40, 1237. (50) Omar, I. H.; Pauling, H. P.; Ju¨ttner, K. J. Electrochem. Soc. 1993, 140, 2187. (51) Shi, Z.; Lipkowski, J. J. Electroanal. Chem. 1994, 364, 289; 1994, 365, 303. (52) Magnussen, O. M.; Hotlos, J.; Nichols, R. J.; Kolb, D. M.; Behm, R. J. Phys. Rev. Lett. 1990, 64, 2929. (53) Hachiya, T.; Honbo, H.; Itaya, K. J. Electroanal. Chem. 1991, 315, 275. (54) Will, T.; Dietterle, M.; Kolb, D. M. In Nanoscale Probes of Solid/ Liquid Interfaces; Gewirth, A. A., Siegenthaler, H., Eds.; NATO ASI Series E, Vol. 288; Kluwer: Dordrecht, 1995; p 137. (55) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183. (56) Ikemiya, N.; Miyaoka, S.; Hara, S. Surf. Sci. 1994, 311, L641; 1995, 327, 261. (57) Ataka, K.; Nishina, G.; Cai, W. B.; Sun, S. G.; Osawa, M. Electrochem. Commun. 2000, 2, 417. (58) Parry, D. B.; Samant, M. G.; Seki, H.; Philpott, M. R. Langmuir 1993, 9, 1878. (59) Blum, L.; Abruna, H. D.; White, J.; Gordon, J. G.; Borges, G.; Samant, M. G.; Melroy, O. R. J. Chem. Phys. 1986, 85, 6732. (60) Melroy, O. R.; Samant, M. J.; Borges, J. L.; Gordon, J. G.; Blum, L.; White, J. H.; Arabelli, M. J.; Millan, McM.; Abruna, H. D. Langmuir 1988, 4, 728. (61) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R. Phys. Rev. Lett. 1995, 75, 4472. (62) Gordon, J. G.; Melroy, O. R.; Toney, M. F. Electrochim. Acta 1995, 40, 3. (63) Borges, G. L.; Kanazawa, K. K.; Gordon, J. G.; Ashley, K.; Richer, J. J. Electroanal. Chem. 1994, 364, 281. (64) Uchida, H.; Hiei, M.; Watanabe, M. J. Electroanal. Chem. 1998, 452, 97.

Properties of Au(111)/H2SO4 (Cl) + Cu2+

(LEED, RHEED, AES, etc.).65-67 Cu UPD proceeds in two steps: Randomly adsorbed copper and (hydrogen-) sulfate ions transform first into a 2D ordered adlayer. The resulting (x3 × x3)R30° structure44,45,52-54,61,66 is composed of copper atoms, which form a commensurate honeycomb lattice (occupation of 3-fold hollow sites, 2/3 coverage, electrosorption valency γ ∼ 1.851) with (hydrogen-) sulfate ions adsorbed in the center (C3v symmetry, 1/3 coverage, γ ∼ 0.851) above the plane of the copper atoms.61 Subsequently this structure transforms into a pseudomorphic monolayer of Cu(1 × 1) on Au(111) (occupation of 3-fold hollow sites) with disordered sulfate on top (0.2 monolayer (ML)).51,54,59,60 The anions of the supporting electrolyte have a profound influence on the UPD of Cu on Au(111).45,59,60,68-72 The presence of chloride, as an example, results in a bilayer with the halide (γ ∼ 1.071) coadsorbed on top of the UPD copper (γ ∼ 271), both arranged in two, (quasi-) hexagonal, potential-dependent and incommensurate adlayer structures, which were initially assigned to “(5 × 5)” patterns.44-46,69,72,73 Finally, the chosen test system, Au(111)/0.05 M H2SO4 + 1 mM Cu2+, forms, in the absence of chloride and at sufficiently positive electrode potentials, an ordered (x3 × x7)R19.1° superstructure43,74-77 composed of (hydrogen-) sulfate and coadsorbed hydronium74,77 ions and/or water molecules.75,78 Infrared data revealed a C3v symmetry of the sulfate species with three oxygen atoms in contact with the substrate surface.78,79 The present contribution is organized as follows: We start with a brief description of experimental aspects of the in situ STM/STS measurements and of the strategy for data analysis. The next chapter summarizes the experimental results for Au(111)/H2SO4/Cu2+ in the absence and in the presence of small amounts of chloride, followed by a comparative discussion. The paper ends with conclusions. 2. Experimental Aspects and Data Analysis 2.1. Distance Tunneling Characteristics. The Au(111) electrodes were single-crystal disks of 10 mm diameter and 2.5 mm thickness. They were flame-annealed in a hydrogen flame at red heat for 5 min and then cooled slowly in a high-purity argon stream. The reference electrode was a copper wire (99.999%, Aldrich), and the counter electrode was a platinum wire (Goodfellow). We typically used aqueous electrolyte solutions containing 1 mM Cu2+ and 50 mM H2SO4. In selected experiments, we added chloride as HCl, NaCl, or KCl to reach final concentrations between 10-4 and 10-6 M. All potentials are referred to the Cu/Cu2+ couple. The solutions were prepared with (65) Kolb, D. M. Z. Phys. Chem. NF 1987, 154, 179. (66) Nakai, Y.; Zei, M. S.; Kolb, D. M.; Lehmpfuhl, G. Ber. BunsenGes. Phys. Chem. 1984, 88, 340. (67) Zhang, J.; Sung, Y. E.; Rikvold, P. A.; Wieckowski, A. J. Chem. Phys. 1996, 104, 5699. (68) Horanyi, G.; Rizmayer, E. M.; Joo, P. J. Electroanal. Chem. 1983, 152, 211. (69) Zei, M. S.; Qiao, G.; Lehmpfuhl, G.; Kolb, D. M. Ber. BunsenGes. Phys. Chem. 1987, 91, 349. (70) Michaelis, R. Ph.D. Thesis, Freie Universita¨t, Berlin, 1991. (71) Shi, Z.; Wu, S.; Lipkowski, J. Electrochim. Acta 1995, 40, 9. (72) Herrero, E.; Buller, L. J.; Abruna, H. D. Chem. Rev. 2001, 101, 1897. (73) Wu, S.; Lipkowski, J.; Tyliszczak, T.; Hitchcock, A. Prog. Surf. Sci. 1995, 50, 227. (74) Edens, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357. (75) Inukai, J.; Sugita, S.; Itaya, K. J. Electroanal. Chem. 1996, 403, 159. (76) Dretschkow, Th.; Wandlowski, Th. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 749. (77) Cuesta, A.; Kleinert, M.; Kolb, D. M. Phys. Chem. Chem. Phys. 2000, 2, 5684. (78) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951. (79) Shingaya, Y.; Ito, M. Electrochim. Acta 1998, 44, 745.

Langmuir, Vol. 19, No. 24, 2003 10273 Milli-Q water (18 MΩ, 2-3 ppb TOC (total organic carbon)) and with suprapure Merck reagents, except for CuO, which was purchased from Sigma-Aldrich (99.9999%). The glassware and the STM cell (Kel-F) were cleaned by soaking in caroic acid, followed by multiple rinsing cycles with Milli-Q water. The steady-state images and the distance tunneling characteristics were measured with a Molecular Imaging Pico-SPM under potential control. The I-S curves were recorded as the regime tunneling current (iT) versus relative tunneling distance perpendicular to the surface (∆s). The absolute tip-sample distance is not known a priori. After reaching a desired xyz position, the z-piezo feedback was disabled temporarily, and the tunneling current was measured in the range between 0.01 and 100 nA with a rate of 2 nm s-1. One thousand data points were recorded. The reproducibility was tested by repeating the experiments typically 3-6 times and measuring the I-S curves usually 5-10 times in each experiment. The results were considered as acceptable only if atomic resolution of the surface/ adlayer had been achieved, both before and after taking the I-S characteristics. The experimental iT-∆s characteristics plotted in this paper represent averaged curves of 10-20 individual traces. The exact numbers are given in the respective figure captions. The relative zero distance was chosen at the point where the tunneling current reached a value of 100 nA. This assignment allows a simple comparison of our measurements. Electrochemically etched W and Pt/Ir (70/30%) tips were used. The tips were coated with polyethylene resulting in leakage currents below 10 pA. The tip potential was held at 0.02 V for W and at 0.4 V for Pt/Ir, except when we investigated the bias dependence of the I-S characteristics. The W tip cannot be polarized at potentials more positive than 0.2 V due to the onset of oxidation. All measurements were carried out at room temperature. 2.2. Potential Barrier Height Profiles (O vs ∆s). According to the theory of tunneling,19 the shape of distance tunneling characteristics depends on the form and magnitude of the potential barrier the tunneling electrons are exposed to. In the simplest case, for the low-bias regime, we may consider a rectangular potential barrier. The corresponding I-S curve has an exponential form (if the barrier width is not too small):

iT ) i0 exp(-κ xφrect ∆s)

(1)

iT is the tunneling current, φrect is the height of the rectangular barrier height, and κ ) 10.12 eV-1/2 nm-1;19 i0, a normalization current, accounts for the electronic properties of the metal surface and the bias voltage. According to this equation, we can relate a single barrier value to exponentially decaying I-S characteristics. Obviously, the properties of the tunneling barrier φ should depend on the medium between tip and sample. A rectangular tunneling barrier represents a good approximation for a metal tip|metal sample arrangement in vacuum.19 The barrier may be quite different if there is liquid in the tunneling gap.24,32,33,41 We used a simple fitting procedure32 to determine the potential barrier profile perpendicular to the Au(111) surface (φ versus ∆s). The method is based on solving the one-dimensional, timeindependent Schro¨dinger equation for a series of thin, equidistant and rectangular barriers of nonequal height.80 The number of these consecutive rectangular barrier elements was chosen to be the same as the number of the measured current values of the I-S curves. Therefore, their width is equal to the sampling distance in our measurements. The shape and the magnitude of the overall potential barrier height profiles were obtained by fitting the calculated transmittance profile to the experimentally obtained I-S curve by changing the heights of the rectangular barrier elements. This method involves two adjustable parameters: (i) the kinetic energy of the tunneling electron and (ii) a normalization factor for the tunneling current to convert it to the tunneling transmittance, which should be chosen as the current at point of contact for calculating the whole tunneling barrier profile. From our experimental results, however, only a part of the profile can be calculated, since we could not extend the measurements up to the point of contact. Therefore, we have chosen the highest tunneling current value (the upper limit of our preamplifier, iT ) 100 nA), which represents the (80) Ando, Y.; Itoh, T. J. Appl. Phys. 1987, 61, 1497.

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Figure 1. Steady-state cyclic voltammogram of Au(111)|50 mM H2SO4 + 1 mM Cu2+ as measured in the STM cell, with a scan rate of 10 mV/s. Five potential regions, separated by characteristic current peaks Pi/Pi′, are labeled I to V, and they are illustrated with typical in situ STM images (iT ) 1 nA): (A) Cu-(x3 × x3)R30° UPD at 0.15 V; (B) Au(111)-(1 × 1) at 0.50 V; (C) (H)SO4(2)--(x3 × x7)R19.1° at 0.80 V; (D) large scale of the bare Au(111)-(1 × 1) surface at 0.50 V; (E) oxidized Au surface at 1.35 V. lowest measured tip-sample distance, as the normalization factor. The kinetic energy of the tunneling electron was approximated by kT (≈0.03 eV at room temperature), which is a reasonable assumption for our system. Other values for the latter will not change the shape of the barrier profile; only the height of the peaks in the profile will be different. The validity of our choice is supported by the fact that the calculated barrier profiles in the long-distance limit are converging to the same barrier height values one can obtain from fitting the long-distance tail of our I-S curves with an exponential function according to eq 1. Although it is sufficiently proven that the magnitude of the calculated barrier height profile is correct within the frame of our model, we restrict ourselves to discussing only the peak positions. This method represents a more advanced procedure80 to obtain barrier height profiles from the measured I-S characteristics, in comparison to the conventional derivation method according to the WKB (Wentzel-Kramers-Brillouin) approximation81 by calculating d ln(iT)/ds based on eq 1. The WKB method (i) fails to explain resonance phenomena and (ii) is inaccurate in the regions where the electron energy is close to the actual value of the potential barrier or the potential barrier profile varies abruptly.82 Moreover, we can avoid the inaccuracy of a numerical differentiation. Nevertheless, the used method is still an approximation. The following limitations have to be taken into account: (i) The calculated barrier profiles are one-dimensional, and thus three-dimensional effects are averaged. (ii) The tip is considered as a dimensionless object, and its interaction with the measured system is neglected. Only a full (3D) quantum mechanical treatment of the problem would give correct quantitative results, but the necessary tools are beyond the scope of this work and exceed our current possibilities.

3. Results 3.1. Au(111)|0.05 M H2SO4 + Cu2+. Figure 1 shows a typical cyclic voltammogram for Au(111)|0.05 M H2SO4 + (81) Wiesendanger, R. Scanning Probe Microscopy and Spectroscopy; Cambridge University Press: Cambridge, 1994. (82) Messiah, A. Quantum Mechanics; North-Holland: Amsterdam, 1961.

1 mM Cu2+ measured in the STM cell with 10 mV s-1. Five potential regions, labeled I to V, which are separated by four pairs of peaks, can be readily distinguished. Highresolution in situ STM images are added for illustration. The most negative region I, just before the onset of Cu bulk deposition, represents the stability range of the pseudomorphic Cu-(1 × 1) UPD adlayer.44 The wellknown Cu-(x3 × x3)R30° UPD phase is formed between the peak pairs P1/P1′ and P2/P2′ (Figure 1A).44,52,53 Region III represents the unreconstructed Au(111)-(1 × 1) surface covered with randomly adsorbed sulfate species and, at the negative edge, with coadsorbed Cu2+. The bare Au(111) surface (Figure 1B,D) can be imaged with atomic resolution in this potential range. A disorder-order phase transition triggers the formation of an ordered (x3 × x7)R19.1° adlayer composed of sulfate species and water molecules/hydronium ions when passing P3/P3′ (Figure 1C).43,74-77 Upon the onset of terrace oxidation, peak P4′, the ordered sulfate structure dissolves, and an array of irregularly distributed small clusters of 1-3 nm size and approximately monatomic height is formed (region V, Figure 1E).42 This structure disappears during the subsequent negative potential excursion after passing the main reduction peak P4. In the following, we present I-S characteristics as iT versus ∆s. We emphasize that the zero distance was chosen at the point where the tunneling current reached the value of 100 nA. We are using a relative distance scale. We also show the corresponding distance dependence of the calculated potential barrier height profiles (φ vs ∆s). The simplest I-S characteristics were found for the potential region III (between the peak pairs P2/P2′ and P3/P3′), where no ordered adlayer exists (Figure 2). The I-S curve (iT vs ∆s) is exponential, and the corresponding potential barrier profile (φ vs ∆s) approaches a featureless, almost horizontal line. As we discussed in section 2.2, it is well-known81 that a single barrier height value can be

Properties of Au(111)/H2SO4 (Cl) + Cu2+

Figure 2. Averaged I-S characteristics (iT, lines and open circles) and the calculated one-dimensional potential barrier height profile (φ, lines and filled circles) at E ) 0.50 V vs Cu/ Cu2+. The shown I-S curve was obtained from averaging 20 individual traces of independent experiments. The zero of the distance scale is set to the point where iT ) 100 nA. The tipsample bias is -0.48 V. A model of the adlayer structure is also shown (side view).

Figure 3. Averaged I-S characteristics (iT, lines and open circles; 15 individual traces) and the calculated one-dimensional potential barrier height profile (φ, lines and filled circles) as obtained for the ordered (H)SO4(2)--(x3 × x7)R19.1° structure at 0.80 V (vs Cu/Cu2+). The zero of the distance scale is set to the point where iT ) 100 nA. The tip-sample bias is -0.78 V. A model of the adlayer structure is depicted as a side view (ref 91).

related to the exponentially decaying I-S curves. By fitting eq 1 to the I-S characteristics, the barrier height φrect ) 1.1 ( 0.1 eV was obtained. This value is also estimated from the calculated potential barrier profile at larger distances (φ∆sf∞ ≈ 1.1 eV, cf. Figure 2). Similar results were obtained for the oxide-covered Au(111) surface in 0.1 M H2SO4.83 Both observations are in accordance with earlier results.31,33 In the potential range of the ordered (x3 × x7)R19.1° sulfate adlayer (region IV), I-S characteristics with a “breaking” point and nonexponential segments were obtained (Figure 3). The corresponding barrier profile is not featureless anymore. The breaking point in the I-S curve is represented as a peak in the barrier profile at ∆s ≈ 0.17 ( 0.03 nm. Furthermore, φ increases steeply upon approaching ∆s f 0. The I-S characteristics are exponential at sufficiently larger distances ∆s, that is, the tunneling tip being further away from the adsorbatecovered electrode surface. A single value of the barrier height, φ∆sf∞ ≈ 1.0 eV, can be obtained in this range, which is in agreement with the result of the fit of eq 1 to the (83) Nagy, G.; Wandlowski, Th. Phys. Chem. Commun. 2000, 5, 112.

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Figure 4. Set-point dependence of the topographic images of the (H)SO4(2)--(x3 × x7)R19.1° structure at 0.80 V (vs Cu/ Cu2+). The tip-sample bias is -0.78 V.

Figure 5. Averaged I-S characteristics (iT, open circles and lines; 10 (W) + 10 (Pt/Ir) individual traces averaged) and the calculated one-dimensional potential barrier height profile (φ, filled circles and lines) as obtained for the ordered Cu-(x3 × x3)R30° UPD structure at 0.15 V (vs Cu/Cu2+). The zero of the distance scale is set to the point where iT ) 100 nA. The tipsample bias is -0.13 V. A model of the adlayer is depicted as a side view (ref 61).

exponential tail (φrect ) 1.0 ( 0.2 eV). We also measured topographic images with increasing set-point currents in order to obtain information about the tip-sample interaction (Figure 4). The images are distorted at set-points iT g 40 nA, indicating that there is a strong interaction between the tip and the adlayer. This result underlines that the existence of the ordered adlayer is directly reflected in the shape of the I-S curves. The I-S characteristics in region II, where the wellknown Cu-(x3 × x3)R30° UPD structure exists,57,61 show steplike features (Figure 5). They give rise to a distinct peak in the barrier height profile at ∆s ≈ 0.16 ( 0.04 nm and a sharp increase of φ at ∆s f 0. The shape at ∆s ) 0 nm cannot be calculated exactly due to the limited number of data points. Considering only the exponential part of the I-S curve at larger distances, a single barrier height value, φ∆sf∞ ≈ 0.8 eV, was obtained, which agrees with φrect ) 0.8 ( 0.2 eV estimated from eq 1. Since these measurements showed the most structured profiles, we discuss the reproducibility of our I-S experiments for this case in more detail. The I-S characteristics, obtained from six independent experiments, are plotted in Figure 6. We have used W and Pt/Ir tips. No distinct qualitative differences were observed. Although the curves

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Figure 6. Compiled series of I-S characteristics as measured for the ordered Cu-(x3 × x3)R30° UPD structure at 0.15 V (vs Cu/Cu2+) in independent experiments either with W or Pt/ Ir tips. The tip-sample bias is -0.13 V.

Figure 7. Bias dependence (∆E) of I-S characteristics as measured for the ordered Cu-(x3 × x3)R30° UPD structure. Pt/Ir tips were used, and the substrate potential was fixed to 0.15 V (vs Cu/Cu2+).

Figure 8. Dependence of the topographic image of the Cu(x3 × x3)R30° structure at 0.15 V (vs Cu/Cu2+) upon a steplike change of iT from 1 to 10 nA. The tip-sample bias is -0.13 V.

plotted in Figure 6 are not completely congruent, they always show the same characteristic features: We found two regions where the current increases steeply by more than 10-20 nA, at very small values of ∆s ≈ 0 nm and at ∆s ranging between 0.12 and 0.20 nm (arrows in Figure 6). The long-distance tail (∆s > 0.4 nm) is always exponential. This indicates a reasonable reproducibility. The bias dependence of the I-S curves was investigated in a series of measurements employing Pt/Ir tips (Figure 7). We found no qualitative changes by changing the bias in the range between 0.2 and 0.6 V. We will discuss this observation in more detail in section 4.3. Finally, we measured topographic images as a function of the setpoint current (Figure 8). The image of the Cu-(x3 × x3)R30° UPD structure is strongly distorted at set-point currents of g10 nA, indicating a perturbation of the adsorbed layer by the tip. We may conclude that the stepwise shape of the I-S characteristics is directly related to the complex UPD structure (cf. model in Figure 5).61

Figure 9. Steady-state cyclic voltammogram of Au(111)|50 mM H2SO4 + 1 mM Cu2+ + 0.1 mM Cl- as measured in the STM cell, with a scan rate of 10 mV/s. Three potential regions, separated by characteristic current peaks Pi/Pi′, are labeled I to III, and the two ordered phases are illustrated with typical in situ STM images (iT ) 1 nA): (A) Cu-Cl bilayer I at 0.02 V; (B) Cu-Cl bilayer II at 0.15 V.

3.2. Au(111)|0.05 M H2SO4 + Cl-. The above results motivated further systematic experiments on related systems to explore more general features of our observations. We decided to modify the sulfate electrolyte by adding small amounts of chloride to Au(111)|0.05 M H2SO4 + Cu2+, which changes significantly the double layer properties in the UPD region44,45,46,73 as demonstrated in Figure 9. The voltammogram shows two pairs of peaks (P1/P1′ and P2/P2′) separating three distinct potential regions. The pair (P2/P2′) at high underpotentials is reversible and appears at more positive potentials than the pendant in sulfuric acid. The second pair of peaks almost overlaps with the bulk Cu deposition. Highresolution STM experiments revealed in region I (negative of peak P1/P1′) a long-range hexagonal modulation pattern with a characteristic distance of 1.2 nm and a corrugation of approximately 0.05 nm. The individual “atoms” are not clearly visible; nevertheless, a nearest neighbor spacing of 0.35 ( 0.01 nm could be estimated. Based on XPS,70 chronocoulometric,71 and extended X-ray absorption fine structure (EXAFS)73 studies, these observations could be attributed to a Cu-Cl bilayer, with the chloride on top of Cu and both subphases arranged in (quasi-) hexagonal incommensurate adlayers. The STM contrast pattern is attributed to Cl-, being nearly discharged.45,46 The exact composition of phase I is not yet resolved and still a matter of controversy.72 At more positive potentials, in region II between the two peak pairs P1/P1′ and P2/P2′, a nearly hexagonal STM contrast pattern is observed. The latter is sometimes approximated by a (5 × 5) structure (cf. refs 44-46 and 84). The STM contrast of this bilayer is attributed to Cl- adsorbed in hollow sites between three (84) Nagy, G.; Wandlowski, Th. In preparation.

Properties of Au(111)/H2SO4 (Cl) + Cu2+

Figure 10. I-S characteristics (iT, lines and open circles; 10 individual traces averaged) and the calculated one-dimensional potential barrier height profile (φ, lines and filled circles) as obtained for the ordered Cu-Cl UPD bilayer II at 0.15 V (vs Cu/Cu2+). The zero of the distance scale is set to the point where iT ) 100 nA. The tip-sample bias is -0.13 V. A model of the adlayer is depicted as a side view (ref 46).

copper atoms with an identical Cu-Cu and Cl-Cl spacing.46 The nearest neighbor distance of the “small atomic features” is estimated as 0.35 ( 0.01 nm. In addition, there exists a long-range Moire structure with an average distance between adjacent modulation maxima of 1.2 ( 0.1 nm. The corrugation is approximately 0.03 nm. The quasi-hexagonal incommensurate atomic adlayer and the Moire pattern are rotated by 13.9°.84 Figure 10 shows the I-S characteristics for the Cu-Cl bilayer in region II, which is stable between P1/P1′ and P2/P2′. The curve is exponential upon approaching the surface, and suddenly there is a jump in the tunneling current exceeding 100 nA at ∆s f 0. The discontinuity in the I-S curve is highly reproducible and is clearly related to the UPD structure. The corresponding barrier height profile exhibits a large increase at ∆s f 0. The barrier height is much lower than 1 eV in the range 0 < ∆s < 0.2 nm. At larger distances, where the I-S characteristics are exponential, φ∆sf∞ ≈ 0.9 eV is obtained, which is in agreement with φrect ) 0.8 ( 0.2 eV from eq 1. Somewhat related I-S characteristics were found for the Cu-Cl bilayer in region I, at potentials more negative than P1/P1′, just before the onset of Cu bulk deposition (Figure 11). There is a current jump in the curve at ∆s ≈ 0.17 ( 0.05 nm and a steep increase of iT upon approaching ∆s f 0. One observes correspondingly a large peak in the potential barrier height profile around ∆s ≈ 0.17; the barrier height drops close to zero at smaller distances and rises again at ∆s f 0. The barrier height at larger distances, φ∆sf∞ , approaches 1 eV. The fit of eq 1 to the exponential tail of the I-S curve results in values of φrect, which scatter around 1.0 eV. Our results clearly demonstrate that I-S characteristics measurements are sensitive and capable of addressing properties of ordered adlayers in the double layer between metal electrodes and electrolyte solutions.

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Figure 11. I-S characteristics (iT, lines and open circles; 10 individual traces averaged) and the calculated onedimensional potential barrier height profile (φ, lines and filled circles) as obtained for the ordered Cu-Cl UPD bilayer I at 0.02 V (vs Cu/Cu2+). The zero of the distance scale is set to the point where iT ) 100 nA. The tip-sample bias is -0.02 V. A tentative model of the adlayer is depicted as a side view (ref 84).

4. Discussion

contact. Orientational changes of interfacial water have been also related to nonexponential I-S characteristics.31-33,35 In most of the experiments reported, the effective barrier heights, typically estimated at rather large distances from the electrode surface, scatter around 1 eV. Schmickler proposed a microscopic model for the tunneling of electrons through thin layers of water, based on a combination of MD simulations and numerical calculations of the scattering of electrons by a 3D potential energy barrier, to account for the lowering of the tunneling barrier in solution.37,38 Alternative explanations were proposed based on intermediate states, which involve either “hydrated electrons”85 or water molecules themselves.27,86,87 Recent ab initio molecular dynamics results of Nitzan et al.41 on electron tunneling in a Pt(111)/water/Pt-tip system demonstrated that the effective barrier height appears to be rather independent of the applied bias at moderate electrical fields. Comparison with literature data assured us that our results are meaningful and characteristic of the double layer structures we investigated. 4.1. Absolute Tip-Sample Distance. Before starting to discuss the distance tunneling characteristics in detail, we will consider the question of absolute tip-sample distance. I-S measurements are always relative; only the distance change, ∆s, is controlled accurately. One strategy to estimate the absolute tip-sample separation is the recording of topographic images for the 2D-ordered adlayers with different set-point currents. As seen in Figures 4 and 8, the images became distorted if rather large tunneling currents were applied. This indicates a strong interaction, allowing the conclusion that the tipsample distance should be rather small. Thus, at larger tunneling currents the tip appears to probe the inner part of the double layer. Another approach to calibrate the z-distance comprises the extrapolation to quantum-point contact between the

One can find examples for both exponential and nonexponential tunneling in the literature.24-35 Regarding the reliability of our results, especially for the Cu UPD, we like to point out that Siegenthaler et al.24 reported discrete jumps in the I-S curves for Pb UPD on Au(111). The authors attributed this phenomenon to reaching point

(85) Sass, J. K.; Gimzewski, J. J. Electroanal. Chem. 1991, 308, 333. (86) Lindsay, S. M.; Jing, T. W.; Pan, J.; Lampner, D.; Vaught, A.; Lewis, J. P.; Sankey, O. F. In Nanoscale Probes of the Solid/Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; NATO ASI Series E, Vol. 288; Kluwer: Dordrecht, 1995; p 25. (87) Boussaad, S.; Xu, B. Q.; Nagahara, L. A.; Amlani, I.; Schmickler, W.; Tsui, R.; Tao, N. J. J. Chem. Phys. 2003, 118, 8891.

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tip and the sample. The corresponding conductance is given by88

G0 ) 2e2/h ) 7.75 × 10-5S

(2)

In eq 2, e is the elementary charge and h is Planck’s constant. One estimates with typical bias values ranging between 0.1 and 0.5 V according to

G0 ) |icontact|/|Ebias|

(3)

contact currents, |icontact|, between 8 and 40 µA. These values are significantly larger than the maximum range of our measurements. Based on eq 1, we estimated for the simplest case of exponentially shaped I-S characteristics (Figure 2) the gap distance between reaching point contact at 12.9 kΩ (eq 2) and our arbitrary zero. The corresponding value (in our relative scale) for the gold surface (Ebias ) 0.48 V) was obtained as ca. -0.6 nm. This implies that we are not penetrating into the inner layer in these cases. (The bias voltages applied in our experiments were typically chosen between -0.01 and -0.80 V, which suggests that the zero in our relative ∆s scale corresponds to an absolute offset ranging between 0.4 and 0.6 nm.) Similar conclusions can be drawn for the nonexponential I-S characteristics. Assuming that the discrete jumps in the I-S characteristics of the investigated UPD adlayers (cf. Figures 5, 11, and 12) would correspond to the point of contact, one estimates, with reference to our experimental conditions, theoretical bias voltages Ebias ranging between 0.13 and 1.0 mV. These values are not realistic. They are 2-3 orders of magnitude smaller than the experimentally used bias. Recent results of Baltruschat et al.36 provided additional experimental evidence that, under our experimental conditions, we do not have direct contact with the surface. According to their measurements, the bias value to bring the STM tip in contact with the sample is less than 1.5 mV for a tunneling current of 100 nA. This experimental evidence and our simple estimations suggest that the observed current jumps are not related to the tip touching the metal sample. There still exists a gap between tip and surface. 4.2. Adlayer Effect. Since the measured distance tunneling characteristics show deviations from the ideal exponential behavior, it is clear that they are sensitive to certain double layer properties. Nonexponential segments in the I-S curves were found in the presence of ordered adlayers on Au(111). The nonexponential decay is dominant up to a distance range of ∆s ≈ 0.2 nm. These results imply that the measurements are sensitive only to longrange-ordered structures. If no such layers form on the surface, the measured I-S curves are exponential within the entire current range. Therefore, we may conclude that the I-S characteristics are indeed reflecting the actual adlayer structure. We found that the I-S curves are different for different structures. In the case of the (x3 × x7)R19.1° sulfate adlayer, a breakpoint (∆s ) 0.17 ( 0.03 nm) is observed in the I-S curve, and the current increases steeply at ∆s f 0 (Figure 3). For the Cu-(x3 × x3)R30° UPD, two characteristic steps were found at ∆s f 0 and at ∆s ) 0.16 ( 0.04 nm (Figure 5), while for the Cu-Cl II UPD only one large step was observed at ∆s f 0 (Figure 10). Two characteristic steplike features were found for Cu-Cl I UPD at ∆s f 0 and at ∆s ) 0.17 ( 0.05 nm (Figure 11). These differences point out the possibility of relating our measurements to the actual double layer structure by considering some basic principles of electron tunneling. (88) Landauer, R. Philos. Mag. 1970, 21, 863.

4.3. Electronic Properties of the Double Layer. As a working hypothesis, we assume that the tunneling process is governed by the interaction of the tunneling electron with the liquid environment, that is, the tunneling barrier is the result of the local charge density changes in the double layer. For a single tunneling event, the liquid structure can be considered as frozen, since the tunneling time is much shorter than the characteristic fluctuation times. The overall tunneling current, however, represents a time average and is strongly influenced by local structural changes. This argument supports the understanding of why it is possible to observe nonexponential I-S curves only in the presence of ordered adlayers composed of individual molecules/ions with a low degree of freedom to move independently. The local charge density distribution, even as a time average, is not uniform in these configurations, and this may lead to nonuniform potential barrier distributions (cf. ref 89). Consequently, the potential barrier profiles might open a new avenue to describe properties of ordered adlayers. As a limitation, we need to mention that our current approach neglects structural consequences of the tip penetration into the double layer region.90 We pointed out in Experimental Aspects and Data Analysis that the calculation of one-dimensional potential barrier profiles from the I-S curves is meaningful, although this procedure may represent a considerable simplification. This approach provides direct information on the charge density distribution in the liquid part of the interface, perpendicular to the metal electrode. One may expect the tunneling process to be also modified by the bias between tip and sample, that is, by an external electric field across the double layer. Experimentally, we did not find a marked bias dependence at moderate fields, as is shown explicitly in Figure 7 for Cu-(x3 × x3)R30° UPD. One possible explanation of this observation might be that the bias almost completely drops within the inner Helmholtz layer, that is, within the Cu UPD layer, while the tunneling process takes place between the tip and the UPD phase. A generalization of this idea would lead to a double layer picture of the tip-sample bias dropping in the inner Helmholtz layer and of a strong electronic overlap between the adsorbed layer and the metal surface. In other words, the tunneling process “starts” from the adsorbed species, and not from the metal itself. This picture would explain why we usually observe a constant effective barrier at longer distances, independent of the electrode potential. Our results are in agreement with recent ab initio calculations of Nitzan et al.41 on water between a Pt(111) surface and a Pt tip. These authors found that the effective barrier height was practically independent of the bias between sample and tip. This indicates that the presence of water or electrolyte solution in the tunneling gap affects the tunneling electron mostly through local interactions. Changes of surface properties upon the formation of certain adlayers, such as an UPD phase, might modify the barrier height at larger distances, reflecting changes in the work function. This reasoning may explain why we found lower values of the effective barrier height (0.8 ( 0.2 eV) for Cu UPD in the presence of partially charged coadsorbed anions (Cu-(x3 × x3)R30° and Cu-Cl II) and values around 1 eV for all the other cases. 4.4. Double Layer Structure. By relating the peaks in the potential barrier profile to the charge density distribution within the liquid part of the interface, we attempted to suggest a possible connection between the (89) Nagy, G.; Denault, G. J. Electroanal. Chem. 1997, 433, 161. (90) Xie, Z. X.; Kolb, D. M. J. Electroanal. Chem. 2000, 481, 177.

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of an adjacent water molecule.91 This alternating arrangement of first layer hydronium ions and second layer water molecules, interconnected to each other as well as to sulfate species via hydrogen bonds, is consistent with all experimental observations and may also explain the nonuniform interatomic distances in the (x3 × x7) arrangement.91 Knowing that approximately the same (x3 × x7) structure exists on the Cu(111) surface,92 we assume that the distances in the (H)SO4δ- ion on Au(111) are similar to those of the sulfate species in the Cu UPD adlayer. Attributing the rise of the barrier height profile upon ∆s f 0 to a strong interaction between the tip and the gold surface, where the respective double layers start to overlap, we tend to attribute the peak in the barrier height profile at 0.17 ( 0.03 nm to the average plane of the sulfate species. The fine structure in the tail of the potential barrier height profile of the averaged iT-∆s traces does not allow us to derive further details of electronic/ structural properties of coadsorbed water species from the currently available data sets.43,74-78,91 In the presence of chloride, copper forms on Au(111) two Cu-Cl bilayers in the UPD region with chloride on top of copper (3-fold hollow sites) and both subphases arranged in (quasi-) hexagonal, incommensurate adlayers.44-46,70-73,84 Chronocoulometric measurements revealed that both adlayer components are nearly completely discharged. The exact chloride coverage in the more positive Cu-Cl phase II is approximated by (5 × 5)44-46,84 with an identical Cu-Cu and Cl-Cl spacing (we found 0.35 ( 0.01 nm). EXAFS studies suggested that copper species are in registry with chloride, and not with the underlying gold substrate, and that this bilayer could be represented by a saltlike CuCl phase.73 The anion coverage in the Cu-rich Cu-Cl phase I appears to be smaller, while “discharged” copper approaches the coverage of a complete monolayer.71,73 The exact composition and structure of this more negative Cu-Cl phase I are still a matter of controversy.72 The shape of the I-S characteristics may be understood by considering that both Cu-Cl UPD structures I and II are bilayers with chloride on top of copper. Since the chloride layers appear to be rather compact and the Cu is nearly discharged, one can relate the sharp jump in the I-S curve (Figure 10) to the tip strongly interacting with the Cl as the electron-rich species. The magnitude of the jump for the more negative Cu-Cl phase I is smaller, and with decreasing tip-sample distance a further monotonic increase of the tunneling current was found (Figure 11). This evolution of the I-S characteristics indicates that the chloride layer is not so rigid anymore. This may mean that either there are fewer and not strongly localized particles in the layer or the partial charge on the chloride ions is even smaller than in the Cu-Cl phase II.

double layer structure and the I-S characteristics. We should emphasize that our 1D evaluation method is rather simplified and therefore allows only qualitative deductions (cf. discussion in ref 39). From the shape of the I-S characteristics and the corresponding potential barrier profiles, we may conclude, in general, that we cannot see steady-state charge density changes extending toward the bulk beyond the adsorbed layer (∆s > 0.2 nm). Thus, we cannot confirm, based on our measurements of I-S characteristics with iT e 100 nA and the chosen STM configuration, the predictions of numerous theoretical works about the existence of a “stable” multilayer water structure next to the electrode (cf. review in ref 14). We found characteristic nonexponential I-S curves and, correspondingly, peaks in the potential barrier profiles or a steep increase of φ upon ∆s f 0 for ordered adlayer structures only. We consider first the well-known Cu-(x3 × x3)R30° UPD structure in sulfuric acid solution.44,52-54,57,61 The Cu atoms form a honeycomb lattice of 2/3 ML coverage on the Au(111) surface, while sulfate anions (1/3 ML) are adsorbed in the hexagonal centers above the Cu atoms. Both copper and sulfate species occupy 3-fold hollow sites. The sulfate anions are bonded chemically to the Cu adatoms with three oxygens, and thus they are really part of the adlayer structure. This model is shown in Figure 5 as a side view. Since the exact particle-particle distances are also available,61 we can establish particle planes parallel to the Au(111) surface, such as Au-Cu ) 0.23 ( 0.05 nm, Cu-S ) 0.15 ( 0.01 nm, and S-O ) 0.16 ( 0.01 nm, with O referring to the oxygen atom of SO42- pointing toward the solution. We compare the distance differences of these planes with those of the peaks in the barrier profile. We suggest that the jump in the tunneling current around ∆s f 0 (Figure 5) is due to a strong interaction between the tip and the copper plane (cf. ref 24). The copper in the (x3 × x3)R30° UPD phase is positively charged,51 which could explain why the interaction of the copper layer with the tip occurs at relatively low values of the tunneling current. Further support for this assumption is provided by the fact that the image of the Cu-(x3 × x3)R30° UPD structure appears to be strongly distorted at set-point currents of g10 nA (cf. Figure 8) indicating a perturbation of the adsorbed layer by the tip. However, the tip is not in the point of contact at this stage, as discussed in section 4.1. With the above assignment, we suggest attributing the peak in the barrier height profile at 0.16 ( 0.04 nm to the average plane of the sulfate species (Cu-S ) 0.15 ( 0.01 nm, as obtained from ref 61). Possible contributions from “oxygen atoms” of the coadsorbed sulfate or water species (cf. refs 57, 78, and 91) do not appear as significant features in the averaged potential barrier profile. We have less knowledge about the structure of the (x3 × x7)R19.1° sulfate adlayer. Considering recent STM43,74-77 and spectroscopic works,78,91 the (x3 × x7) unit cell consists of four (hydrogen-) sulfate ions located in 3-fold hollow sites (at the corner positions of the unit cell) and coordinated to three gold atoms via three oxygens. Coadsorbed hydronium ions with their C3 axis rather parallel to the electrode surface bridge sulfate species via hydrogen bonds along the diagonal x7 direction, while water molecules in a second layer form hydrogen bridges between adjacent sulfates along the x3 direction. A further hydrogen bridge is, eventually, being formed between the third hydrogen atom of H3O+ and one lone pair electron

We performed a series of STM and STS measurements with Au(111)/0.05 M H2SO4 + 1 mM Cu2+ in the absence and in the presence of small amounts of chloride, to understand if it is possible to map the liquid part of the double layer perpendicular to the electrode surface by measuring distance tunneling characteristics. This wellknown electrochemical system enables us to investigate in situ I-S characteristics of the bare and the oxidecovered Au(111) surfaces and of several ordered adlayer structures by tuning the substrate potential.

(91) Wandlowski, Th.; Ataka, K.; Pronkin, S.; Diesing, D. Electrochim. Acta, in press.

(92) Wilms, M.; Broekmann, P.; Stuhlmann, C.; Wandelt, K. Surf. Sci. 1998, 416, 121.

5. Conclusions

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We found that the measurements are sensitive to longrange ordered structures, which is reflected in the I-S characteristics as nonexponential profiles for iT e 100 nA. If no ordered adlayer existed on the surface, as for the bare Au(111)-(1 × 1) and the AuOx electrode, we obtained exponential I-S curves. In the potential range of the ordered (x3 × x7)R19.1° sulfate adlayer, I-S characteristics with breaking points and nonexponential segments were obtained. If an ordered UPD copper layer, coadsorbed with anions (sulfate, chloride), existed on the gold surface, current steps were found in the I-S curves, which appear to correlate with the actual adlayer structure. Based on additional topographic experiments and theoretical considerations, we could conclude that we investigate the double layer in a distance range where we do not penetrate into the inner Helmholtz layer, but the tip is close enough to the metal surface to probe the adlayer structure. Our results indicate that the bias between tip and sample drops in the inner Helmholtz layer, and strong electronic overlap exists between the adsorbed layer and the metal surface. A detailed analysis suggests, by relating the results to the respective adlayer structures, that I-S characteristics are capable of detecting molecular and possibly atomic/ ionic contributions of the electronic structure of ordered adlayers. However, we could not conclude on the existence of a “rigid”, multilayered interfacial water structure. At

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larger distances from the surface, the I-S characteristics are always exponential, giving average barrier heights of about 1 eV, practically independent of the electrode potential. We believe that we could demonstrate that I-S characteristics measurements are capable of providing insight into the structure of ordered adlayers potentiostatically deposited onto single-crystal electrode surfaces. We found that the method provides information on the submolecular level about the electronic structure of the Helmholtz layer. These results represent an additional step to advance the application of scanning tunneling spectroscopic methods in electrochemistry and open up a new, exciting field of studying not just the structure and electronic properties but also the reactivity of adsorbed adlayers at the atomic level. Acknowledgment. G.N. is indebted to the Alexander von Humboldt Stiftung for a research fellowship. The work was also supported by the Volkswagen Stiftung under Grant No. I75-262 and the Research Center Ju¨lich and the National Science Research Fund of Hungary under Contract T29894. The authors are thankful to Ju¨rgen Halbritter, Dirk Mayer, and Jens Ulstrup for stimulating discussions. LA034950K