Linear Arrays Formed by Interaction of Thiocyanate with Pb

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Linear Arrays Formed by Interaction of Thiocyanate with Pb Monolayers Show-Jon Hsieh and Andrew A. Gewirth* Department of Chemistry and the Frederick Seitz Materials Research Laboratory, 600 South Mathews Avenue, University of Illinois, Urbana, Illinois 61080 Received March 4, 2003. In Final Form: June 17, 2003 We used scanning tunneling microscopy to examine the effect of introducing SCN- to the Pb monolayer island structure formed during the underpotential deposition (upd) of Pb on Au(111). In the absence of SCN-, upd of Pb initially leads to the formation of Pb islands exhibiting the same hexagonal structure and atom-atom spacing (0.34 nm) as does the Pb bulk deposit. Upon addition of SCN-, a new type of structure is formed, featuring linear rows of Pb adatoms, held apart from each other by SCN-, which are lying flat on the electrode surface. These linear Pb structures reflect templating by the ambidentate SCNmolecule.

I. Introduction Underpotential deposition (upd) is an electrochemical process wherein a monolayer or submonolayer of a foreign metal adatom is deposited onto an electrode surface at potentials positive of the reversible or Nernst potential.1,2 The driving force for upd arises from electronegativity or work function differences between the adatom and the surface. Interest in upd stems not only from the unique structural features found in these monolayers, but also from the efficacy of some of these monolayers to act as catalysts for important electrochemical processes, such as the four electron reduction of dioxygen to water.3 Underpotential deposition of Pb onto Au(111) is one of the best characterized upd systems, in part because Pb in submonolayer form acts as a catalyst for the electroreduction of peroxide to water. In the case of Pb upd on Au(111), electrochemical characterization of this system in the presence of peroxide has shown that the catalytic activity occurs in a narrow potential region associated with the most positive Pb upd peaks.4,5 Our AFM structural studies further showed that the catalytic peroxide reduction potential region is associated with the presence of Pb islands.6 Recently, we examined the way in which Pb islands are responsible for the catalytic activity toward peroxide reduction. By monitoring the change in electroreduction activity as a function of added ethanethiol7 or I-,8 we showed that the locus of electroreduction activity was very likely to be found at the island edges. Scanning tunneling microscopy (STM) images of thiol-poisoned Pb surfaces revealed thiol decoration only at Pb island step edges.7 Alternatively, STM images obtained following introduction of I- showed only features associated with * To whom correspondence should be addressed. Phone: 217333-8329. Fax: 217-333-2685. E-mail: [email protected]. (1) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, pp 125-271. (2) Herrero, E.; Buller, L. J.; Abruna, H. D. Chem. Rev. 2001, 101, 1897-1930. (3) Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley-Interscience: New York, 1984; Vol. 13, pp 159-260. (4) Juttner, K. Electrochim. Acta 1986, 31, 917-927. (5) Juttner, K. Electrochim. Acta 1984, 29, 1597-1604. (6) Chen, C.-h.; Washburn, N.; Gewirth, A. A. J. Phys. Chem. 1993, 97, 9754-9760. (7) Oh, I.; Gewirth, A. A.; Kwak, J. J. Catal. 2003, 213, 17-22. (8) Hsieh, S.-J.; Gewirth, A. A. Surf. Sci. 2001, 498, 147-160.

an I adlattice, which is likely a consequence of the much stronger Au-I interaction relative to Pb. Other aspects of Pb upd on Au(111) have been structurally characterized by STM.31 Briefly, the first phase of Pb upd features the formation of close-packed monolayer-high islands of Pb on the Au(111) surface. These islands then condense to a full monolayer, followed by bulk deposition. Electrochemical characterization of Pb upd on gold absent peroxide has also been extensively reported.9-15 The structure of the Pb system is always found to be close packed, even in the region where islands are found to form. The reason for the close-packed nature of the upd adlattice likely resides in the relatively negative potentials required to form the upd system, especially when compared with those for Ag or Cu which occur at much more positive potentials by virtue of their higher formal potential. At these more positive potentials, the surface can interact more strongly with anions, and anion coadsorption is understood to provide the driving force yielding open adlattice structures.16 The negative potentials required for upd of Pb should inhibit the association of most anions with the electrode surface. The synthesis of linear metallic structures has been of recent interest, because of the putative use of these materials in nanofabrication schemes involving interconnect, grating, sensor, and other structures. In the electrochemical environment, synthesis of linear (nanowire) materials on surfaces is effected by using the substrate to template deposition17,18 or by exploiting strain between adatom and substrate.19 There is a substantial effort directed at using bifunctional ligands to connect metal centers in order to synthesize cubic and other polygonic structures.20,21 The use of a bifunctional or ambidentate (9) Schultze, J. W.; Dickertmann, D. Surf. Sci. 1976, 54, 489-505. (10) Hamelin, A. J. Electroanal. Chem. 1979, 101, 285-290. (11) Hamelin, A.; Katayama; Picq, G.; Vennereau, P. J. Electroanal. Chem. 1980, 113, 293-300. (12) Hamelin, A.; Lipkowski, J. J. Electroanal. Chem. 1984, 171, 317-330. (13) Engelsmann, K.; Lorenz, W. J. J. Electroanal. Chem. 1980, 114, 1-10. (14) Engelsmann, K.; Lorenz, W. J.; Schmidt, E. J. Electroanal. Chem. 1980, 114, 11-24. (15) Deakin, M. R.; Melroy, O. J. Electroanal. Chem. 1988, 239, 321331. (16) Chen, C.-H.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 451-458.

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ligand to direct electrochemical assembly has not yet been reported. In this report, we examine the interaction of a Pb upd monolayer with SCN-, a linear molecule which can coordinate Pb by both S and N moieties. By exploiting this bifunctionality, we show that we can generate linear structures on the Au(111) surface. II. Experimental Section Solutions for Pb adsorption studies were prepared from ultrapure water (Milli-Q UV plus, Millipore Inc., 18.2 MΩ cm), KSCN (Alfa Aesar, ACS, 99.0% minimum), and PbO (Aldrich, 99.999%). The supporting electrolyte was 0.1 M HClO4 (J.T. Baker, Ultrex II). The working electrode for cyclic voltammetric and chronocoulometric measurements was a Au(111) single crystal (Monocrystals) with a diameter of 0.91 cm and a nominal area of 0.657 cm2. The orientation of the crystal was confirmed with Laue´ backscattering. The crystal was annealed for 3 min in a hydrogen flame prior to use and quenched in ultrapure water. Oxide formation and stripping voltammetry of the surface in 0.1 M HClO4 were found to be similar to those reported in the literature.22 Voltammetric data were collected using a platinum wire counter electrode and a saturated Hg|Hg2SO4 or Ag|AgCl|saturated KCl reference electrode connected to the electrochemical cell via a capillary salt bridge. All potentials in this paper are reported relative to the Ag|AgCl saturated KCl electrode, unless otherwise specified. The solutions were purged with Ar prior to use, and a positive pressure of Ar was maintained over the solution in the cell during all electrochemical measurements. Potential control and voltage sweeps were established using a Pine AFRDE-5 potentiostat. Voltammetric data were digitized and collected by computer using a Data Translation DT-2821 analogue I/O board and software written at the University of Illinois. STM images were collected in constant current mode with Digital Instruments Nanoscope III instrumentation (Digital Instruments, Santa Barbara, CA). The STM was calibrated by imaging a highly oriented pyrolytic graphite (HOPG) surface in air. STM tips were Pt/Ir wires (Digital Instruments) which were coated with polyethylene to minimize faradaic currents at the tip-electrolyte interface. For STM studies, the Au(111) substrates were gold evaporated onto borosilicate glass (Metallhandel Schro¨er GmbH). The substrates were annealed prior to use according to a published procedure.23 Atomic resolution images of the Au(111) lattice exhibiting hexagonal symmetry and Au-Au spacings of 0.29 ( 0.02 nm were obtained prior to performing upd measurements. Pt was used as the counter electrode, while Pb or Au wires were employed as the reference electrode.

III. Results 3.1. Voltammetry. Figure 1A shows the cyclic voltammogram (CV) of Pb upd on Au(111) in perchloric acid electrolyte. This voltammogram is essentially identical to that reported previously.10,13 The broad peaks a and a′ are related to the deposition and desorption of Pb, respectively, on Au step edges.10 The broad deposition peaks b and c, around -140 and -200 mV, are associated with the irreversible desorption peaks b′ and c′, around 200 and 120 mV, respectively. Peak c is associated with the formation of Pb islands on the Au(111) terraces. The (17) Lay, M. D.; Stickney, J. L. J. Am. Chem. Soc. 2003, 125, 13521355. (18) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 21202123. (19) Moller, F. A.; Magnussen, O. M.; Behm, R. J. Phys. Rev. Lett. 1996, 77, 3165-3168. (20) Seidel, S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972-983. (21) Contakes, S. M.; Rauchfuss, T. B. Angew. Chem., Int. Ed. 2000, 39, 1984. (22) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1-11. (23) Will, T.; Dietterle, M.; Kolb, D. M. In Nanoscale Probes of the Solid-Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; Kluwer: Dordrecht, The Netherlands, 1995; Vol. 228, pp 137-162.

Figure 1. (A) Cyclic voltammogram of Pb upd on Au(111) at 20 mV/s in 1 mM PbO and 0.1 M HClO4. (B) Cyclic voltammogram at 20 mV/s of Au(111) in 1 mM PbO, 0.1 M HClO4, and 1 mM KSCN.

sharp reversible peak at -245 mV, peak d, is associated with the coalescence of the islands to a Pb monolayer. The associated desorption process for peak d is a set of a pair of reversible peaks, d1′ and d2′. 3.2. Introduction of KSCN. To perturb the Pb upd system, we introduced KSCN to the active surface. SCNwas chosen because it is known to interact with gold24-27 and also is known to poison the Bi upd surface toward peroxide reduction.28 SCN is also known to coordinate to Pb through both S and N, potentially leading to some interesting structural rearrangements on the electrode surface. Figure 1B shows the CV obtained from a solution containing 1 mM PbO and 1 mM KSCN in 0.1 M perchloric acid electrolyte. Peaks e and f are at -167 and -244 mV, respectively, with the related desorption processes e′ and f′ at -133 and -217 mV. Charge integration of peaks e and e′ yields values of 72 and 61 µC/cm2, respectively. Charge integration of peaks f and f′ yields values of 142 and 136 µC/cm2, respectively, which corresponds to a Pb coverage of 0.32 and 0.31 monolayer (ML), respectively, assuming 2 e-/Pb adatom. Comparison of the CV with KSCN to the CV without KSCN in Figure 1 shows that the potential of deposition peak associated with the coalescence of Pb islands to a full monolayer, peak d in Figure 1A, has not changed with the addition of SCN-. Charge integration of peaks d and d′ yielded values of 167 and 157 µC/cm2, respectively, which corresponds to a Pb coverage of 0.38 and 0.35 ML, assuming 2 e-/Pb adatom. Comparison of the calculated Pb coverages for the main Pb deposition peak with and without KSCN shows that the Pb coverage is only slightly diminished upon addition of SCN- (θ ) 0.35 ML decreases to θ ) 0.31 ML). We also performed measurements in which SCN- was introduced to the system at the potential where the Pb upd island structure was already formed with the aim of possibly trapping some kinetic intermediates in the process. However, subsequent voltammetry revealed no differences regardless of when SCN- was introduced to the system. (24) Parry, D. B.; Harris, J. M. Langmuir 1990, 6, 209-217. (25) McCarley, R. L.; Kim, Y.-T.; Bard, A. J. J. Phys. Chem. 1993, 97, 211-215. (26) Gao, P.; Weaver, M. J. J. Phys. Chem. 1986, 90, 4057-4063. (27) Ong, T. H.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1993, 97, 12047-12050. (28) Oh, I.; Biggin, M. E.; Gewirth, A. A. Langmuir 2000, 16, 13971406.

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Figure 2. (A) 114 × 114 nm in situ STM image of Pb islands on Au(111) in 1 mM PbO and 0.1 M HClO4. Ewkg ) -200 mV, Ebias ) 111 mV, Itip ) 1.8 nA. (B) 121 × 121 nm in situ STM image of the initial atomic structure of Au(111) exposed to 1 mM PbO, 0.1 M HClO4, and 1 mM KSCN. Ewkg ) -200 mV, Ebias ) 101 mV, Itip ) 1.8 nA.

As with the case of I-, introduction of SCN- leads to a loss in activity of the Pb island structure toward the electroreduction of peroxide to water. In related work, we plotted the fractional change in peroxide reduction current with [KSCN]. This plot was fit to a Langmuir isotherm, and from this fit the equilibrium constant, Keq, for SCNadsorption to Pb upd was found to be 8.74 × 102 M-1. From this value, the Gibbs free energy of adsorption, ∆Ga, is calculated to be -17 kJ/mol.29 Values between -18 and -24 kJ/mol were determined for the adsorption of octadecanethiol on Au in organic solvents.30 The similarity in these values indicates that the strength of interaction of KSCN to the Pb upd adlayer is similar to the association between octadecanethiol and the bare Au surface. However, this interaction is quite a bit weaker than that estimated for the interaction of I- with the Pb upd adlattice, where we estimate ∆Ga for I-/Pb upd to be ca. -130 kJ/mol.8 I- is known to exhibit a much stronger interaction with Au than does SCN-.1 3.3. STM Results. We performed in situ STM measurements in order to follow changes in structure occurring following the addition of KSCN to the Pb-upd island system. These measurements also show the structure of the Pb upd deposit prior to KSCN addition. 3.3.1. Pb upd on Au(111) Absent KSCN. Figure 2A is a 114 nm × 114 nm in situ STM image obtained at a potential of -0.20 V versus Ag|AgCl. The image shows a number of monolayer-high islands of what is known to be Pb decorating the Au(111) terrace. These islands have (29) Hsieh, S. J. Ph.D. Thesis, University of Illinois, Urbana, IL, 2001. (30) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 33153322.

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been observed previously.6,31 Atomic force microscopy (AFM) measurements on top of the islands revealed a hexagonal close-packed spacing which was identical to that expected for Pb.6 As the potential is swept to more negative values, the island structure condenses into a full Pb monolayer. This full monolayer evinces a hexagonal structure exhibiting an adatom-adatom spacing of 0.34 ( 0.02 nm, as well as a larger repeat spacing of 1.7 nm. This larger spacing is associated with a Moire´ pattern created by the mismatch between the Pb layer spacing and that of the underlying Au substrate and has been observed previously.6,31 3.3.2. Pb upd on Au(111) with KSCN. Figure 2B is a 121 nm × 121 nm in situ STM image obtained from a solution containing 1 mM PbO, 0.1 M HClO4, and 0.5 mM KSCN on Au(111) obtained at a potential of -0.2 V. KSCN was added to the solution at this potential, which is associated with Pb island formation as described above. Following SCN- addition, islands are still visible on the surface. However, these islands are different from the Pb upd islands described above in two ways. First, the islands are of two heights. The lower islands (darker in the image) have heights of 0.17 ( 0.02 nm above the bare terrace, a decrease from the 0.30 ( 0.02 nm high islands observed before SCN- addition. The higher islands (lighter in the image) have heights of 0.30 ( 0.02 nm, identical to the 0.30 ( 0.02 nm high islands observed before SCNaddition. While the higher islands are Pb islands, we do not associate the lower islands with the same species that is found in the SCN--free case. The second difference between the islands observed with SCN- addition and those seen without SCN- is the presence of an ordered structure atop the lower-height islands. Figure 3A shows a 15 nm × 15 nm STM image of dislocated paired rows obtained on top of the lower islands. This structure was observed from -0.21 to -0.34 V in solutions containing 1 mM PbO, 0.1 M HClO4, and 0.5 or 1 mM KSCN. The image shows that the rows occur in pairs with a relatively short distance between paired rows and a somewhat longer distance between row pairs. Analysis of the image reveals that the vector between spots in different rows makes an angle of 13° with the row perpendicular. This means that the spots between rows must be offset from one another. The nearest neighbor distance along the row is 0.33 ( 0.02 nm. The nearest neighbor distance between spots rotated by 13° between adjacent rows is 0.73 nm. (The distance between rows, at 0° rotation, is 0.70 nm.) The distance from one pair of spots to the next is 1.25 nm. (The distance between a spot in one row of a pair at a perpendicular distance to the nearest row of another pair is 1.21 nm.) These measurements are detailed in Figure 3B. Pb-Pb adatom distances are 0.34 nm, which could be the spots along the row of 0.33 nm. Since the structure of the upd adlattice changes upon introduction of SCN-, it is reasonable to anticipate that this anion is coadsorbed with the Pb. We thus look to the dimensions of the SCNanion to understand the spacings between the rows. Hyperchem (Release 4 for Windows Molecular Modeling System; Hypercube, Inc.; 1994) calculations using molecular mechanics optimization determined molecular lengths of 0.287 and 0.743 nm, respectively, for a SCN monomer and a SCN dimer (SCN‚‚‚SCN). If the van der Waals radii of S (0.104 nm) and N (0.070 nm) are added to these distances, then the lengths of a SCN monomer and a SCN dimer are 0.461 and 0.917 nm, respectively. (31) Tao, N. J.; Pan, J.; Li, Y.; Oden, P. I.; DeRose, J. A.; Lindsay, S. M. Surf. Sci. Lett. 1992, 271, L338-L344.

Interaction of Thiocyanate with Pb Monolayers

Figure 3. (A) 15 × 15 nm in situ STM image of the initial atomic structure of Au(111) exposed to 1 mM PbO, 0.1 M HClO4, and 1 mM KSCN. Ewkg ) -200 mV, Ebias ) 123 mV, Itip ) 5.0 nA. (B) Model of the structure observed in (A). Long dark ovals represent SCN dimer (SCN‚‚‚SCN); smaller light ovals represent SCN monomer; circles represent Pb.

If a SCN monomer is found lying at an angle between the paired rows, then the SCN would keep the Pb adatoms from forming a close-packed structure. Similarly, if the SCN monomer associates with another SCN monomer to form a dimer, then the presence of the dimer lying down on the surface would account for the larger distance separation of 1.21 nm. This situation is illustrated in Figure 3B. The STM image does not reveal the SCN molecules. We note that these rows are substantially different than the rows of Au atoms observed in the (x3 × 23) reconstruction of the Au(111) surface which has been observed at negative potentials in solution by STM.32 The structure shown in Figure 3A subsequently condenses to a closer-packed structure. This closer-packed structure is shown in Figure 4A. Figure 4A shows a smallscale STM image of atomic features of Pb upd on Au(111) with 1 mM SCN-. This structure was observed from -0.19 to -0.35 V and was also obtained in the same Pb upd system with a lower [SCN-] ) 0.5 mM. The spot-spot distance along the rows is 0.34 ( 0.02 nm. The image shows clearly that spots in adjacent rows are displaced relative to each other by an angle of 13° and the distance between spots at this angle is found to be 0.73 nm. This structure is similar to that observed in Figure 3A except that all the rows are now evenly spaced relative to each other. A model of the structure shown in Figure 4A is presented in Figure 4B. The Pb-Pb adatom distances are 0.34 nm, which could be the spots along the row of 0.34 nm. The length of a SCN- anion lying down on the surface is 0.46 nm. If a SCN monomer is lying at an angle between the (32) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129-1162.

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Figure 4. (A) 7.5 × 7.5 nm in situ STM image of the atomic structure of Au(111) exposed to 1 mM PbO, 0.1 M HClO4, and 1 mM KSCN. Ewkg ) -208 mV, Ebias ) 52 mV, Itip ) 3.2 nA. (B) Model of the structure observed in (A). Ovals represent SCN monomer; circles represent Pb.

rows, then the SCN- would keep the Pb adatoms from forming a close-packed structure. In this model, the structure in Figure 4A would represent condensation of the less well-formed structure seen in Figure 3A. Again, the STM image does not reveal the SCN molecules. Moving the potential toward more negative values from the region where the islands were initially observed leads to island coalescence. Figure 5A-D is a series of 176 nm × 176 nm in situ STM images obtained at a potential of -232, -235, -237, and -254 mV, respectively. Figure 5A-C clearly reveals the presence of islands with two different heights, as described previously. The lower height overlayer evinces the row structure described above. At potentials past the maximum of peak f in Figure 1, the islands coalesce to a smooth layer, as observed in Figure 5D. The coalescence of islands into a smooth terraced surface is also found in the Pb system absent KSCN. Shortrange STM images obtained from the smooth terraces in Figure 5D (not shown) reveal a hexagonal structure with spot-spot distances of 0.34 nm. The hexagonal structure is attributed to close-packed Pb adatoms. IV. Discussion STM images show that the Pb islands are altered as to both type and number by addition of SCN-. The images further show development of a new type of atomic-scale structure on top of the islands. 4.1. Structural Effects. 4.1.1. Atomic Structure with Pb and SCN-. Addition of KSCN to the Pb upd on Au(111) system initially generates a paired row pattern as seen in Figure 4A. Between the paired rows is additional area absent any resolvable features. Although linear structures are observed in the case of substrate metal

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Figure 5. (A-D) 176 × 176 nm in situ STM images showing the coalescence of islands of 1 mM PbO and 0.1 M HClO4 following exposure to 0.5 mM KSCN. (A) Ewkg ) -232 mV, Ebias ) 103 mV, Itip ) 1.8 nA. (B) Ewkg ) -235 mV, Ebias ) 103 mV, Itip ) 1.8 nA. (C) Ewkg ) -237 mV, Ebias ) 103 mV, Itip ) 1.8 nA. (D) Ewkg ) -254 mV, Ebias ) 103 mV, Itip ) 1.6 nA.

reconstruction,33-35 they are uncommon for overlayers on substrates of hexagonal symmetry. Previously reported examples of this kind of behavior include the linear structures observed in the upd of Cd on Au(111)36,37 and the linear needlelike Ni islands observed on the reconstructed Au(111) surface.38 In these cases, the anisotropic growth pattern is believed to arise from an electronic effect. In neutral pH solutions, spectroscopic studies in the absence of Pb indicated that the thiocyanate anion is adsorbed on bare gold via the S terminus at potentials between -0.5 and +0.5 V versus Ag|AgCl.24,26,27 At potentials below ca. -0.5 V in neutral solution, the lack of a spectroscopic signal has been interpreted as an indication that the SCN- lies flat on the Au surface, but no indication of N-bound SCN- has been observed. With Pb present, other SCN- geometries become available due to the facile coordination Pb exhibits with both the N and S termini of the SCN molecule. In particular, we propose that SCN- is lying flat on the surface when it is coadsorbed with Pb. (33) Gao, X.; Hamelin, A.; Weaver, M. J. Phys. Rev. B 1992, 46, 70967102. (34) Magnussen, O. M.; Hotlos, J.; Behm, R. J.; Batina, N.; Kolb, D. M. Surf. Sci. 1993, 296, 310-332. (35) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986, 31, 929-936. (36) Bondos, J. C.; Gewirth, A. A.; Nuzzo, R. G. J. Phys. Chem. 1996, 100, 8617-8620. (37) Hsieh, S. J.; Gewirth, A. A. Langmuir 2000, 16, 9501-9512. (38) Moller, F. A.; Kintrup, J.; Lachenwitzer, A.; Magnussen, O. M.; Behm, R. J. Phys. Rev. B 1997, 56, 1250-12518.

The linear structure obtained in the Pb upd on Au(111) system with SCN- might reflect the linear chain structures reported for lead thiocyanate complexes, which exhibit Pb complexation with both the N and S termini of the ambidentate SCN- moiety.39 By way of contrast, SCN coordination in L-Au-thiocyanate complexes (where L is a ligand) is most often found via the S termini (i.e., L-Au-SCN). However, if L is a strong trans directing ligand, some formation of L-Au-NCS is possible.40 Comparison of the Au-thiocyanate complexes to Pb-thiocyanate complexes reveals that SCNis bound to Au via either the N or S termini, whereas SCN- is bound to Pb via both the N and S termini. In contrast to the situation with Pb, there are no ambidentate complexes of Au and SCN- known. Support for the flat orientation model comes from analysis of the STM images. First, SCN- adsorbed on Au,25 Bi-modified Au(111),28 Rh,41 and Pt surfaces,42,43 where the SCN is known to be oriented vertically, results in a single feature in the image. These features were not observed here. Second, the anisotropy of Pb growth (39) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford University Press: New York, 1984. (40) Akhtar, M. N.; Isab, A. A.; Al-Arfaj, A. R.; Hussain, M. S. Polyhedron 1997, 16, 125-132. (41) Wan, L.-J.; Yau, S.-L.; Itaya, K. J. Solid State Electrochem. 1997, 1, 45-52. (42) Peng, Q.; Breen, J. J. Electrochim. Acta 1998, 43, 2619-2626. (43) Yau, S.-L.; Kim, Y.-G.; Itaya, K. Anal. Sci. Technol. 1995, 8, 723-730.

Interaction of Thiocyanate with Pb Monolayers

requires some nonspherically symmetric structure present on the surface and this structure would not be presented by vertically oriented SCN-. In the flat orientation suggested in our model, SCNtemplates the structures observed in Figures 3 and 4. The 0.73 and 1.25 nm spacings in Figure 3A are due to thiocyanate monomers and dimers (SCN‚‚‚SCN), respectively, lying flat on the surface. Incorporation of additional Pb into the slowly growing chain structures disrupts the dimer coordination, resulting in the evenly spaced rows observed in Figure 4A. The dissociation of the thiocyanate dimer most likely results from the stronger Pb-SCN interaction relative to that presented by the SCN‚‚‚SCN complex alone. 4.2. Reactivity Effects. Addition of SCN- to the Pb upd system leads to loss of reactivity toward peroxide reduction. This loss of reactivity is modeled by a Langmuir isotherm, which yielded a value of ∆Ga ) -16.8 kJ/mol for the adsorption of SCN- to the Pb upd adlayer. This ∆Ga value is similar to that determined for the adsorption of octadecanethiol on Au in organic solvents (ca. -18 to -24 kJ/mol).30 Consequently, we can infer that the strength of interaction between SCN- and Pb is considerable. The SCN- must adsorb on the surface so as to preclude Pb reactivity with H2O2. The enthalpy of formation, ∆H, of Pb(NCS)2 in 3 M LiClO4 was reported to have a value of -12.6 ( 10.1 kJ/mol,44 which is at least consistent with the ∆Ga value found in this study. The STM images shown in Figure 2 also reveal a substantial change in the number of islands. We found (44) Fedorov, V. A.; Samsonova, N. P.; Moronov, V. E. Zh. Neorg. Khim. 1969, 14, 3264-3268.

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previously that the island edges were the locus of peroxide electroreduction activity.7 For the concentration of SCNused in Figure 2, the peroxide reduction current was found to be ca. 60% of that found without SCN-.29 From an analysis of Figure 2 and related images, we estimate that the Pb island circumference has decreased to ca. 40% of its initial value. The discrepancy suggests that not all island edge sites on the SCN-free surface are catalytically active. V. Conclusion Following the addition of SCN- to the Pb upd system, new islands exhibiting an open linear adlattice structure are observed to form. The linear motif is explained as arising from interactions between an ambidentate SCNand Pb, yielding a chain structure not dissimilar from that found in bulk Pb(SCN)2. The interaction of SCNwith the Pb upd system was modeled with a Langmuir isotherm, yielding a free energy of interaction between the Pb-upd system and SCN- of -16.8 kJ/mol. This ∆Ga value is similar in magnitude to that measured for the interaction of octadecanethiol and a bare Au surface in organic solvents,30 suggesting strong interaction of the SCN- and the metal surface. However, this interaction is much smaller than that estimated for the interaction of I- with the same system. The corresponding STM images reveal a surface diminished in the metallic Pb islands known to be the locus of catalytic activity. Acknowledgment. Support of this work from the National Science Foundation (CHE-98-20828) is gratefully acknowledged. LA034380O