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Scanning Tunneling Microscopy of Sulfur and Benzenethiol Chemisorbed on Ru(0001) in 0.1 M HClO4 Liang-Yueh Ou Yang,†,‡ Shueh-Lin Yau,*,†,§ and Kingo Itaya*,‡,§ Department of Chemistry, National Central University, Chungli, Taiwan 320; Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 04, Sendai 980-8579, Japan; and CREST, JST, 4-1-8 Kawaguchi, Saitama 332-0012, Japan Received December 17, 2003. In Final Form: February 18, 2004 In situ scanning tunneling microscopy (STM) combined with linear sweep voltammetry was used to examine spatial structures of sulfur adatoms (SA) and benzenethiol (BT) molecules adsorbed on an ordered Ru(0001) electrode in 0.1 M HClO4. The Ru(0001) surface, prepared by mechanical polishing and electrochemical reduction at -1.5 V (vs RHE) in 0.1 M HClO4, contained atomically flat terraces with an average width of 20 nm. Cyclic voltammograms obtained with an as-prepared Ru(0001) electrode in 0.1 M HClO4 showed characteristics nearly identical to those of Ru(0001) treated in high vacuum. Highquality STM images were obtained for SA and BT to determine their spatial structures as a function of potential. The structure of the SA adlayer changed from (2 × x3)rect to domain walls to (x7 × x7)R19.1° and then to disordered as the potential was scanned from 0.3 to 0.6 V. In contrast, molecules of BT were arranged in (2 × x3)rect between 0.1 and 0.4 V, while they were disordered at all other potentials. Adsorption of BT molecules was predominantly through the sulfur headgroup. Sulfur adatoms and adsorbed BT molecules were stable against anodic polarization up to 1.0 V (vs RHE). These two species were adsorbed so strongly that their desorption did not occur even at the onset potential for the reduction of water in 0.1 M KOH.
Introduction The adsorption of sulfur adatoms (SA) on Ru(0001) has been extensively examined to reveal its interfacial structure1-6 for its applications in catalysis.6,7 Five ordered structures, p(2 × 2), (x3 × x3)R30°, c(4 × 2), (73 06), and (x7 × x7)R19.1°, have been identified to form at gassolid interfaces with increasing coverage from 0.25 to 0.57.5 Depending on the coverage, interaction between SA can be repulsive or attractive. In particular, repulsive interaction was observed for p(2 × 2) and (x3 × x3)R30°, as SA were adsorbed as individual atoms at hcp threefold hollow sites at the coverages 0.25 and 0.33, respectively. In contrast, pairwise or three-body interaction prevails to produce apparent atomic clusters in c(4 × 2) and (x7 × x7)R19.1° at the coverages 0.5 and 0.57, respectively. From the perspective of catalysis, Ru is of great importance for its role in reducing the poisonous effect of carbon monoxide on the Pt anode of a fuel cell, but this property is greatly obstructed by the adsorption of SA.7,8 The passivation effect of SA on transition metals seems to arise from the formation of a strong covalent bond between S and metal surfaces, which depletes the charge density * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: 81-22-2145380. Fax: 81-22-2145380. † National Central University. ‡ Tohoku University. § CREST, JST. (1) Denner, R.; Sokolowski, M.; Pfnur, H. Surf. Sci. 1992, 271, 1. (2) Jurgens, D.; Held, G.; Pfnur, H. Surf. Sci. 1994, 303, 77. (3) Schwennicke, C.; Jurgens, D.; Held, G.; Pfnur, H. Surf. Sci. 1994, 316, 81. (4) Sklarek, W.; Schwennicke, C.; Jurgens, D.; Pfnur, H. Surf. Sci. 1995, 330, 11. (5) Muller, T.; Heuer, D.; Pfnur, H.; Kohler, U. Surf. Sci. 1996, 347, 80. (6) Jurgens, D.; Schwennicke, C.; Pfnur, H. Surf. Sci. 1997, 381, 174. (7) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 275. (8) Chrzanowski, W.; Wieckowski, A. Langmuir 1998, 14, 1967.
of d orbitals needed for the metal atoms to bind with other species. Furthermore, sulfides of Ru can selectively perform hydrodesulfurization (HDS) on dibenzothiophene, a molecule particularly difficult to desulfurize. HDS is an important reaction in processing petroleum.9,10 The electrochemistry of sulfur at a few single-crystal electrodes such as Au(111), Cu(111), and Pt(111) has been reported.11-14 Results show that the redox process of sulfur varies substantially with the chemical identity of the electrode. On Au(111) sulfur is changed slowly from (x3 × x3)R30° to S8 octomers at potentials positive of -0.7 V in 0.1 M NaOH.11,12 On Cu(111) ordered adlattices of sulfur adatoms, (x3 × x3)R30° and (19 × 19), are produced before the formation of sulfide at the onset potential of Cu dissolution.13 On Pt(111) SA are oxidized to sulfate, without being converted to the S8 octomer or sulfide species.14 Since ruthenium is a member of the platinum group, it is possible that sulfur on Ru would behave similarly to that on Pt. However, the electrochemistry of sulfur at a Ru electrode has not been investigated. Because sulfur can act as a poison to Pt and Ru electrodes, it is important to gain a better understanding of sulfur adsorption at electrified interfaces of these materials. In this study we also explore the adsorption of benzenethiol (BT) at a Ru electrode to gain insight into the effect of molecular structure on the interaction between adsorbates and their spatial arrangement. The adsorption of BT on Au(111), Pt(111), and Rh(111) has been examined (9) Kuo, Y.-J.; Tatarchuk, B. J. J. Catal. 1988, 112, 229. (10) Aray, Y.; Rodriguez, J.; Vega, D.; Coll, S.; Rodriguez-Arias, E. N.; Rosillo, F. J. Phys. Chem. 2002, 106, 13242. (11) Martin, H.; Vericat, C.; Andreasen, G.; Herna’ndez Creus, A.; Vela, M. E.; Salvarezza, R. C. Langmuir 2001, 17, 2334. (12) Vericat, C.; Andreasen, G.; Vela, M. E.; Salvarezza, R. C. J. Phys. Chem. 2000, 104, 302. (13) Sugimasa, M.; Inukai, J.; Itaya, K. J. Electrochem. Soc. 2003, 150, E110. (14) Sung, Y.-E.; Chrzanowski, W.; Zolfaghari, A.; Jerkiewicz, G.; Wieckowski, A. J. Am. Chem. Soc. 1997, 119, 194.
10.1021/la036379v CCC: $27.50 © 2004 American Chemical Society Published on Web 04/21/2004
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in the past.15-18 In addition to the prominent M-S surface bonds, the phenyl group can also interact with d orbitals of metal substrates through its π-electrons. Thus, BT can lie either in parallel or in tilted orientation on the substrate, depending on which molecular configuration is thermodynamically more stable. On Au(111), BT is adsorbed mainly in parallel orientation at the solid-gas interface, and the orientation is insensitive to coverage.15 In contrast, BT is tilted at the liquid-solid interface of Au(111) with its ring inclined by 30° from the surface normal.16 On dry Rh(111) reorientation of BT from a parallel to a tilted configuration occurs with increasing coverage. In addition, decomposition of BT to benzene and sulfur occurs at temperatures higher than 300 K.17 Benzenethiol was found to bind with Pt(111) mainly through the S atom, but no ordered structure was observed with LEED.18 The surface chemistry of BT on ordered Ru surfaces has not been reported thus far. Results of thermal desorption experiments show that benzene is weakly adsorbed on Ru(0001), as benzene desorbs at 350 K.19 This weak benzene-ruthenium interaction implies that the strong covalent bond of Ru-S could control the adsorption of BT on Ru(0001). Experimental Section The Ru(0001) single-crystal electrode used was disk-shaped, 8 mm in diameter, and 1 mm in thickness. It was obtained from the Fritz Haber Institute (Berlin, RFG), and it was polished to a mirror finish with diamond paste. Preparation of a Ru(0001) electrode with an ordered atomic lattice was a challenging task, because oxidation occurs so rapidly at high temperatures that the traditional annealing and quenching process useful for Pt, Rh, Pd, Ir, and Au single-crystal electrodes is not applicable to Ru.20-24 Ru is chemically so stable that electrochemical polishing is unlikely to work. On the other hand, it has been demonstrated that annealing in a hydrogen saturated environment is an alternative technique for preparing ordered Ru(0001) surfaces.25 Meanwhile, some studies have been conducted with Ru(0001) electrodes prepared by simple mechanical polishing.26,27 In the present study the typical pretreatment method was used involving mechanical polishing with successively finer aluminum oxide powers measuring from 1.0 to 0.1 µm. The electrode surface was likely to be covered with a layer of oxide or organic material at the end of this process. Therefore, to remove the contaminated layer and expose the bare Ru surface, a potential of -1.5 V was applied for 5 min in a conventional electrochemical cell containing 0.1 M HClO4. Ultrapure nitrogen was used to purge the cell continuously to remove the hydrogen gas generated in the reduction process, and the supporting electrolyte (0.1 M HClO4) was replaced several times before cyclic voltammetric experiments were performed. The Ru electrode appeared to be hydrophilic, and it was covered with a thin water film upon emersion. This water film was expected to shield the Ru surface from contaminants in air, which (15) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. Langmuir 1999, 15, 116. (16) Wan, L.-J.; Terachima, M.; Noda, H.; Osawa, M. J. Phys. Chem. 2000, 104, 3563. (17) Bol, C. W. J.; Friend, C. M.; Xu, X. Langmuir 1996, 12, 6083. (18) Stern, D. A.; Wellner, E.; Salaita, G. N.; Laguren-Davidson, L.; Lu. F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. Soc. 1988, 110, 4885. (19) Koschel, H.; Held, G.; Steinru¨ck, H.-P. Surf. Sci. 2000, 454456, 83. (20) Clavilier, J.; Rodes, A.; El Achi, K.; Zamakhchari, M. A. J. Chim. Phys. 1991, 88, 1291. (21) Wan, L.-J.; Yau, S.-L.; Itaya, K. J. Phys. Chem. 1995, 99, 9507. (22) Wan, L.-J.; Suzuki, T.; Sashikata, K.; Okada, J.; Inukai, J.; Itaya, K. J. Electroanal. Chem. 2000, 484, 189. (23) Yang, L. M.; Yau, S.-L. J. Phys. Chem. B 2000, 104, 1769. (24) Hamlin, A. J. Electroanal. Chem. 1996, 407, 1. (25) Lu, P.-C.; Yang, C.-H.; Yau, S.-L.; Zei, M.-S. Langmuir 2001, 18, 754. (26) Ikemiya, N.; Senna, T.; Ito, M. Surf. Sci. 2000, 464, L681. (27) Nakamura, M.; Shingaya, Y.; Ito, M. Surf. Sci. 2002, 502, 474.
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Figure 1. Cyclic voltammograms of Ru(0001) recorded at 50 mV/s in 0.1 M HClO4. The Ru(0001) electrode was prepared by mechanical polishing with aluminum oxide particles measuring down to 0.1 µm (dotted trace), followed by cathodic polarization at -1.5 for 5 min (solid trace). could be important to preserve the clean Ru surface required for in situ STM experiments. Ultrapure nitrogen gas was used to purge all solutions used in this study, while in situ STM experiments were conducted in ambient. As illustrated below, this procedure was effective in minimizing the formation of the oxide layer and the contamination with organics on the surface. The resultant Ru(0001) surface appeared to be electrochemically clean with some degree of ordering. Details of STM and linear sweep voltammetric experiments are described elsewhere.28 All STM images were acquired in the constant-current (height) mode, and typical imaging conditions were 50-200 mV in bias voltage and 1-10 nA in feedback current. Ultrapure perchloric acid and sodium sulfide (Na2S) purchased from Merck Inc. (Darmstadt, Germany) were used after dilution with triple-distilled Millipore water. Benzenethiol was purchased from Aldrich (Saint Louis, MO). BT was not very soluble in aqueous solution. According to ref 29, its solubility is 7.6 mM at room temperature. The dosing solution of BT was a 0.1 M HClO4 solution saturated with BT. This concentration appeared to be sufficient to generate a full monolayer of BT on Ru(0001).
Results and Discussion Cyclic Voltammograms of Ru(0001). Figure 1 shows cyclic voltammograms recorded at 50 mV/s with a Ru(0001) electrode in 0.1 M HClO4. The dotted trace was obtained with a sample treated simply by mechanical abrasion. This CV profile is poorly defined, possibly because the electrode surface was rough and contaminated. This is in strong contrast to the solid trace obtained with a Ru(0001) electrode subjected further to electrochemical reduction at -1.5 V for 5 min. This i-E curve contains two pairs of peaks centered at ∼0.18 (A1/C1) and 0.55 V (A2/C2), respectively. These pairs of peaks are associated with the adsorption/desorption couples of hydrogen and oxygen, respectively, at terrace sites on ordered Ru(0001).25 All ordered, electrochemically clean Ru(0001) electrodes exhibited these features.30-33 The above results were somewhat surprising to us, because the as-prepared Ru(0001) electrode was expected to contain a high density of defects. As evidenced by the in situ STM results, the average width of terraces on the as-prepared Ru(0001) was 20 nm. It was thought that surface redox processes such as hydrogen adsorption/ (28) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (29) Howard, P. H., Meylan, W. M., Eds. Handbook of Physical Properties of Organic Chemicals; CRC Press: New York, 1997. (30) Wang, W. B.; Zei, M. S.; Ertl, G. Phys. Chem. Chem. Phys. 2001, 3, 3307. (31) Wang, W. B.; Zei, M. S.; Ertl, G. Chem. Phys. Lett. 2002, 355, 301. (32) Marinkovic, N. S.; Wang, J. X.; Zajonz, H.; Adzic, R. R. J. Electroanal. Chem. 2001, 500, 388. (33) El-Aziz, A. M.; Kibler, R. A. Electrochem. Commun. 2002, 4, 866.
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Figure 2. Cyclic voltammograms of Ru(0001) recorded at 50 mV/s in 0.1 M HClO4. The electrode was first coated with a monolayer of sulfur adatoms. The sulfur adlayer was stable between 0.1 and 1.0 V, and its oxidation commenced at more positive potentials.
desorption at steps would occur at potentials different from those at terraces, as is well-known for platinum electrodes.20 In other words, a rough Ru electrode was expected to produce a poorly defined voltammogram, similar to the dotted trace in Figure 1. However, only one cathodic peak (C1) was observed at 0.18 V in the negative scan, which suggests that terrace and step sites on Ru(0001) were energetically similar to each other for hydrogen atoms. Because the current density of peak C1 is comparable to that obtained with the Ru(0001) electrode prepared by using a rigorous UHV technique,30-32 it can be stated that all Ru(0001) electrodes might have similar surface characteristics. Thus, it is plausible that the cathodic polarization at -1.5 V causes the reduction of oxide, and the removal of organic contaminants, resulting in exposure of the bare Ru(0001) surface. The solid and dotted traces in Figure 2 show cyclic voltammograms of Ru(0001) coated with a monolayer of SA, irreversibly adsorbed from solutions containing Na2S. Both cyclic voltammograms are featureless between 0.05 and 1.0 V, indicating that SA were stable within this potential range. The lack of features between 0.3 and 0.1 V indicates that SA completely blocked the adsorption of hydrogen. This finding agrees with the results previously observed in a vacuum, where no hydrogen adsorption was possible, when the substrate was predosed with one-third of a monolayer of SA.34 In other words, this result infers that the coverage of SA deposited onto an as-prepared Ru(0001) surface was likely to be more than 0.33. Potential excursion to 1.2 V, however, produced a slight increase in current density, possibly resulting from the oxidation of sulfur to some unknown products such as sulfate anions. This is in strong contrast to the characteristics of the cyclic voltammogram for the S/Pt(111) system, where SA undergo irreversible oxidation to sulfate, producing a sharp current spike near 0.93 V.14 As indicated by the STM results described below, the spatial structures of these adsorbates changed along with the potential sweep, but apparently those changes were not detected by voltammetry. Furthermore, we attempted to reductively desorb S in 0.1 M KOH, as in the case of Au(111).11 The result (not shown here) revealed that no desorption occurred prior to the onset of water reduction, which implies that S was strongly held on Ru(0001). The Ru-S surface bond could be covalent in nature, as was noted previously by LEED studies.1-5 The cyclic voltammograms of benzenethiol-modified Ru(0001) were also obtained, which showed that they bear a strong resemblance to those (34) Huang, H. H.; Seet, C. S.; Xu, G. Q.; Hrbek, J. A. Surf. Sci. 1994, 319, 344.
Figure 3. In situ STM topography scans revealing the surface morphology of Ru(0001) pretreated with mechanical polishing and reduction at -1.5 V for 5 min. The potential of Ru(0001) was 0.2 V, and the STM imaging conditions were 200 mV and 2 nA.
of SA. It appears that BT was adsorbed mainly through its sulfur end on Ru(0001) after the sulfhydryl group was removed. In Situ STM Imaging of Sulfur Adlayers on Ru(0001) in 0.1 M HClO4. Ordered Sulfur Adlattices. The following STM results were obtained with Ru(0001) electrodes prepared by mechanical polishing and electrochemical reduction at -1.5 V for 5 min. The constantcurrent STM images in Figure 3 show the typical surface morphology of a Ru(0001) sample potentiostated at 0.2 V in 0.1 M HClO4, which was recorded with STM at 200 mV in bias voltage and 2 nA in feedback current. As expected, the surface contained deep scratches (marked S) formed by the abrasive process. However, many terraces as wide as 30 nm separated by monatomic steps were still present. It is not clear what proportion of the surface had these
Sulfur and Benzenethiol Chemisorbed on Ru(0001)
Figure 4. In situ STM images of Ru(0001) (2 × x3)rect 2S (a and b) and a corresponding model (c). These images were acquired at 0.2 V with 100 mV bias voltage and 5 nA feedback current.
features. As reported previously, an atomic resolution STM scan of the Ru(0001) substrate was obtained at 0.2 V, showing that the substrate was indeed atomically flat.25 The presence of kinks (K) and vacancy defects (D) was unavoidable, and their densities were notably higher than those on Ru(0001) prepared by annealing in hydrogen.25 Prolonged annealing at 800 °C was beneficial to remove thermodynamically unfavorable defects. It is also nearly impossible to avoid the presence of a trace of contamination on the surface, which resulted in defects in the sulfur adlattice (vide infra). Figure 4 shows high-resolution STM images of a terrace site on a sulfur-coated Ru(0001) electrode. The first image reveals ordered arrays on a terrace with a width of 20 nm.
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These structures were determined to be rotational domains of a sulfur adlattice. The close-up view shown in Figure 4b of one of the ordered domains highlights the internal atomic arrangement of this structure, indicating a (2 × x3)rect (or c(4 × 2)) symmetry with two sulfur atoms per unit cell (θ ) 0.5). This ordered adlattice will be referred to as (2 × x3)rect below. The two unit vectors with a 90° angle are 5.5 and 4.6 Å long, respectively. This structure can be arranged in multiple rotational domains, as is actually seen in the STM image of Figure 4a. The rectangle and orthorhombus drawn in the image denote the unit cells. Multiple rotational domains of (2 × x3)rect frequently coexisted on a terrace, with preferences to some certain specific rotational domains. The origin of discrimination could stem from the interaction of (2 × x3)rect with neighboring steps, as noted previously by others.5 Direct evidence for this interaction is clearly seen in the time-dependent STM images, showing that the (2 × x3)rect structure preferentially emerged in regions near step ledges (vide infra). The sulfur adatom inside the (2 × x3)rect unit cell appears to be situated 0.03 Å lower than the S atoms at the corners of the cell (or lattice points), and it is not located at the center of the cell. These findings imply that this S adatom is located at a site different from where the S atoms of lattice points are situated. These results lead to the ball model drawn in Figure 4c. It is essentially identical to that proposed in a previous UHV study.3 A thorough dynamic LEED analysis shows that SA at the corners of a cell reside at hcp-type hollow sites, whereas the S atom in the middle occupies an fcc-type of site.3 LEED results further show that these two types of sulfur atoms shift from their symmetric positions laterally by 0.16 Å in opposite directions along the mirror plane (x3 direction). The uppermost layer of Ru is buckled with an atomic corrugation of 0.19 Å. These atomic relocations might not be detectable by STM. It is intriguing to note that the relative corrugation heights of sulfur atoms do not agree with their physical heights. Sulfur atoms at fcc sites seen as dim spots in the STM images actually lie 0.02 Å higher than those at hcp sites.3 It was the electronic configuration, rather than the physical height, of adsorbates that dominated the tunneling process. Figure 5 shows another ordered structure of SA observed at 0.4 V. The degree of ordering of this structure was always poor, irrespective of the surface state of the Ru(0001) substrate. Figure 5a highlights a 62 × 62 Å STM scan of this new structure, while Figure 5b presents a higher resolution scan which has been treated with a 2D Fourier transform technique to remove noise signals with spatial spacing of less than 2.5 Å. The unit vectors 7.2 Å in length are rotated 19° from the atomic rows of the substrate. This ordered array is characterized as (x7 × x7)R19.1° with four sulfur atoms per cell, corresponding to a coverage of 0.57. Close examination of the atomic image in Figure 5b reveals that protrusions on the edges of the cell are not centered, but they appear to be shifted sideways by about 0.2 Å. The interatomic spacings between atoms #1 and #2 (d12) and atoms #2 and #3 (d23) are 2.7 and 4.5 Å, respectively. Similarly, the sulfur adatom 2′ does not lie between 1 and 3′. These interatomic spacings indicate the occurrence of clustering phenomenon, which was also noted in previous studies.4 On the average, all protrusions exhibited the same intensity, although sulfur adatom 4 was lower in height than the other S atoms by 0.2 Å. These findings led to the real-space model presented in Figure 5c, which is essentially identical to that based on LEED results.4 All SA, except for 4 in the middle of the unit cell, reside at hcp threefold hollow sites. The nearest
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Figure 6. In situ constant-current STM images of sulfur adatoms on Ru(0001) at 0.35 (a) and 0.4 V (b). These images were recorded at 50 mV in bias voltage and 5 nA in feedback current.
Figure 5. In situ STM images (a and b) and a ball model (c) of Ru(0001) (x7 × x7)R19.1° 4S, acquired at 0.4 V with 100 mV in bias voltage and 5 nA in feedback current.
neighbor spacing of this adlattice, 2.7 Å, is considerably smaller than the van der Waals diameter (3.7 Å) of a sulfur atom,35 suggesting the existence of a substrate-mediated interaction between SA. This phenomenon was also observed for SA on Re(0001).36 Potential-Induced Phase Transition of a Sulfur Adlayer on Ru(0001). The S/Ru(0001) system is known for its rich (35) Dean, J. A., Ed. Lange’s Handbook of Chemistry; McGraw-Hill, Inc.: New York, 1979; pp 3-123. (36) Hwang, R. Q.; Zeglinski, D. M.; Vazquez-de-Parga, L. A.; Ogletree, D. F.; Somorjai, G. A.; Salmeron, M.; Denley, D. R. Phys. Rev. B 1991, 44, 1914.
structural variations, which result in the formation of five different ordered structures.1-5 However, these results might not be valid at solid-liquid interfaces, because the chemical nature of sulfur could vary with electrochemical potential. The in situ STM experiment performed in 0.1 M HClO4 containing 0.1 mM Na2S produced the results shown in Figure 6. First, the STM discerned a well-ordered (2 × x3)rect adlattice similar to that shown in Figure 4 at potentials negative of 0.3 V. No structural change was noted even at the onset potential of hydrogen evolution, indicating that SA barely desorbed even at 0 V in 0.1 M HClO4. It seems that the formation of ordered arrays with a coverage lower than 0.5, such as p(2 × 2) and (x3 × x3)R30°, could be possible only in alkaline solutions, because the high pH allows application of more negative potentials with minimal gas evolution. However, stepping potential positively from 0.3 to 0.35 V resulted in marked
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Figure 7. Time-dependent in situ STM images of sulfur adatoms on Ru(0001). The first image was acquired at 0.6 V, while the images in parts b-d were collected 3, 6, and 31 min after the potential was stepped from 0.6 to 0.4 V. The imaging conditions were 100 mV and 5 nA.
changes of this (2 × x3)rect adlattice to generate local domain walls, which are pointed out by arrows in the STM image of Figure 6a. The inset of Figure 6a shows a close-up view of a domain wall, sandwiched between two (2 × x3)rect domains. This domain wall gradually developed into local (x7 × x7)R19.1° at the expense of (2 × x3)rect, leading to the formation of patches of these ordered adlattices or a “glass phase”, as seen in Figure 6b. Muller et al. also observed this structure in a vacuum.5 The latter structure eventually predominated at 0.4 V, despite the fact that the range of ordering was usually less than 100 Å. At potentials positive of 0.5 V, disordering occurred and predominated potentials equal to or more positive than 0.6 V. We have not observed SA clustered to form well-defined S8-type features such as those seen in Au(111). Instead, in situ STM revealed random nucleation of protruding islands on Ru(0001) at even more positive potentials, which could be associated with the commencement of sulfur bulk deposition. To illustrate that the potential-induced phase transition described above was reversible, real-time in situ STM was used to follow structural changes as the potential was stepped from 0.6 to 0.3 V. The results are presented in Figure 7, which shows the time-dependent phase transition of SA. The identical surface morphology seen in these images ensures that the tip was scanned in the same area.
These images are presented in a z-offset mode to enable a clear view of atomic structures on all terraces. The relative corrugations of terraces are unfortunately lost in this mode. In reality the height of terraces follows the sequence I > II > III, and they are separated by monatomic steps marked by dotted lines in Figure 7a. The first image was recorded at 0.6 V, where the sulfur adlayer was mostly disordered, with a local (x7 × x7)R19.1° structure remaining at the upper right half of the image. This STM image also reveals the presence of many pits (P) and kinks (K). Since the coverage of sulfur adatoms increases with potential, these phase transitions could be ascribed to the potential-induced changes in coverage. Figure 7b, obtained 3 min after applying the potential step, reveals the formation of ordered (2 × x3)rect, particularly at the upper two terraces, I and II. Locally ordered (2 × x3)rect domains pointed out by arrows also emerged on terrace III. It is clear that ordered sulfur adlattices nucleated preferentially near the defects of steps and pits, followed by growing outwardly into disordered domains on the terraces. Longer range ordering was finally observed in Figure 7c and d, acquired 6 and 31 min, respectively, after the potential step was applied. These results indicate that steps influence the growth of the (2 × x3)rect structure, which might act to pin the orientation of imminent (2 × x3)rect domains. This interaction
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Figure 8. In situ STM images (a and b) and ball models (c and d) of Ru(0001) (2 × x3)rect benzenethiol. The images were recorded at 0.2 V with imaging conditions of 50 mV in bias voltage and 5 nA in feedback current. Note that the model in part c is different from that of sulfur, in that the middle adsorbates are located at different sites. The solid and dotted circles denote the sites of a benzenethiol molecule and a sulfur adatom, respectively. The model in part d illustrates the arrangement of the phenyl groups of BT and the possible π-π stacking attractive interactions between BT admolecules.
between steps and ordered structures could lead to unequal weights of the three rotational domains of (2 × x3)rect. Nucleation of ordered structures occurred not only at the upper end but also at the lower end of step ledges, as exemplified by the region highlighted by a dotted circle in Figure 7c. These concurrent events inevitably led to the formation of multiple ordered domains on a terrace, but one of the rotational domains eventually prevailed. These results resemble those observed in UHV.5 In Situ STM Imaging of a Benzenethiol Adlayer on Ru(0001). BT was allowed to adsorb from a 0.1 M HClO4 solution saturated with BT. Similarly to the case for sulfur, the potential exerted marked influence on the structure of the BT adlayer. Only between 0.1 and 0.3 V was BT arranged in an ordered structure, which became permanently disordered once the potential was raised above 0.4 V. BT was bonded to the Ru substrate, mainly through the S end after the S-H bond was cleaved. On the other hand, it is difficult to determine whether BT underwent further decomposition upon its adsorption on Ru(0001). Since the STM results obtained (see below) differ from those of sulfur, it is unlikely that BT decomposed to produce SA on the Ru(0001) electrode under the present experimental conditions. Molecular adsorption was al-
ready reported for BT chemisorbed on Au(111) and Pt(111) electrodes.15-18 Figure 8 presents in situ STM images obtained with a Ru(0001) electrode at 0.2 V in 0.1 M HClO4 saturated with BT. The 250 × 250 Å2 scan in Figure 8a reveals that the BT adlayer was well ordered. The internal atomic arrangement is seen in the higher resolution scan of Figure 8b. This structure is identified as (2 × x3)rect, with two spots per unit cell. With the given definition of the (2 × x3)rect unit cell in Figure 8b, the internal spot appears lower in height than the corner spots by 0.03 Å. These STM results bear strong resemblance to those of sulfur atoms seen in Figure 4, which suggests that BT is attached to Ru(0001) mainly through the sulfur headgroup, leaving the phenyl group pendant in solution. This binding configuration has been reported for BT on Pt, Rh, and Au at high saturation.15-18 On the other hand, close examination of Figures 4b and 8b shows that the (2 × x3)rect structures of sulfur and BT are actually different, in that the protrusions inside the unit cells are located at different locations. The real-space model depicted in Figure 8c illustrates this difference, where the empty solid circles represent S headgroups of BT molecules. All BT molecules are believed to reside at hcp threefold hollow sites, the
Sulfur and Benzenethiol Chemisorbed on Ru(0001)
most favorable registries identified for SA.1-5 This is at variance with that observed for SA (Figure 4c), where the protrusion inside the cell resides at an fcc threefold hollows site, marked by the dotted circle in Figure 8c. It is difficult to interpret these STM results precisely, because it is not clear how BT would appear in the molecular resolution STM image. Although a BT molecule appears as a single protrusion on Au(111), as reported by Wan et al.,16 there is no reason it should give an identical image on Ru(0001). In the analogous system of iodobenzene on Cu(111), the admolecule gives rise to an ellipsoidal protrusion or two spots, the former being associated with the sulfur headgroup while the latter is associated with the phenyl group.37 Consequently, it is possible that not every spot in the STM image of Figure 8b corresponds to a BT molecule. Knowledge of the coverage is certainly helpful in this regard, but this information is not available. We then considered the nearest neighbor spacing of BT adsorbates. In the case of BT/Ru(0001) it is 2.7 Å, as compared to 5.2 and 4.8 Å observed for Au(111) and Pt(111), respectively. This value is smaller than that (3.2 Å) observed for SA. Apparently, there must be an abnormally strong attractive force between BT molecules to pull them close to each other. One can adapt an idea of a “π-π stacking” interaction, which has been shown to account for the conformation of biological and porphyrin molecules.38,39 This aromatic interaction was also noted for chemical systems at interfaces. For example, a SAM of BT on Au(111) results in the formation of protruding mesas,40 rather than pits, which are commonly seen for SAMs of alkanethiol molecules. To put this π-π stacking intermolecular interaction in perspective, a tentative model is depicted and presented in Figure 8d, where only the phenyl groups of BT admolecules are shown for clarity. One can see that BT admolecules are arranged in a zigzag pattern along the [121] or the [110] direction, and each (37) Morgenstern, K.; Hla, S. W.; Rieder, K.-H. Surf. Sci. 2003, 523, 141. (38) Waters, M. L. Curr. Opin. Chem. Biol. 2002, 6, 736. (39) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525. (40) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J.Surf. Sci. 1999, 425, 101.
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molecule can have the π-σ attraction with its two nearest neighbors, as reported by Hunter et al.39 However, it is difficult to judge if this interaction is strong enough to pull BT admolecules to such a close spacing as 2.7 Å. Structural investigations using other techniques, such as glancing angle X-ray scattering techniques, would be helpful. Conclusion Results of in situ STM and electrochemical measurements show that it is possible to prepare ordered and electrochemically clean Ru(0001) electrodes by mechanical polishing combined with electrochemical reduction in acidic environment. High-quality STM results allowed identification of a (2 × x3)rect structure for both SA and BT in the potential range between 0.1 and 0.3 V, suggesting that BT molecules are adsorbed through their S headgroups and that the Ru-S interaction predominates the molecular adsorption. The arrangement of sulfur is not identical to that of BT in (2 × x3)rect, as the spacings between two nearest neighbors are 3.2 and 2.7 Å, respectively. It is not clear whether all protrusions observed in the molecular resolution STM image of (2 × x3)rect are associated with BT molecules. Potential modulation resulted in a reversible phase transition for SA, whereas irreversible changes were observed for BT. The coverages of SA and BT increase with increasing positive potential. The former produces (2 × x3)rect, domain wall, (x7 × x7)R19.1°, and disordered adlattices between 0.3 and 0.6 V, but only one ordered structure, (2 × x3)rect, forms for BT. Acknowledgment. S.L.Y. acknowledges the financial support from the National Science Council, Taiwan (NSC 93-2113-M-008-009). This work was partially supported by the Ministry of Education, Culture, Sport, Science and Technology, with a Grant-in-Aid for the COE project Giant Molecules and Complex Systems, 2003. The authors thank Dr. Y. Okinaka for his help in writing this manuscript. LA036379V
