scanning tunneling microscopy of sulfur and benzenethiol chemisorbed on ru(0001) in 0.1 m hclo 4

Scanning Tunneling Microscopy of Sulfur andBenzenethiol 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 toexamine spatial structures of sulfur adatoms (SA) and benzenethiol (BT) molecules adsorbed on an orderedRu(0001) electrode in 0.1 M HClO4. The Ru(0001) surface, prepared by mechanical polishing andelectrochemical reduction at -1.5 V (vs RHE) in 0.1 M HClO4, contained atomically flat terraces with anaverage width of 20 nm. Cyclic voltammograms obtained with an as-prepared Ru(0001) electrode in 0.1M HClO4 showed characteristics nearly identical to those of Ru(0001) treated in high vacuum. High-quality STM images were obtained for SA and BT to determine their spatial structures as a function ofpotential. 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 werearranged in (2×x3)rect between 0.1 and 0.4 V, while they were disordered at all other potentials. Adsorptionof BT molecules was predominantly through the sulfur headgroup. Sulfur adatoms and adsorbed BTmolecules were stable against anodic polarization up to 1.0 V (vs RHE). These two species were adsorbedso strongly that their desorption did not occur even at the onset potential for the reduction of water in0.1 M KOH.

Introduction

The adsorption of sulfur adatoms (SA) on Ru(0001) hasbeen extensively examined to reveal its interfacialstructure1-6 for its applications in catalysis.6,7 Five orderedstructures, p(2 × 2), (x3 × x3)R30°, c(4 × 2), (3

760), and

(x7 × x7)R19.1°, have been identified to form at gas-solid interfaces with increasing coverage from 0.25 to 0.57.5Depending on the coverage, interaction between SA canbe repulsive or attractive. In particular, repulsive inter-action was observed for p(2 × 2) and (x3 × x3)R30°, asSA were adsorbed as individual atoms at hcp threefoldhollow sites at the coverages 0.25 and 0.33, respectively.In contrast, pairwise or three-body interaction prevails toproduce 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 importancefor its role in reducing the poisonous effect of carbonmonoxide on the Pt anode of a fuel cell, but this propertyis greatly obstructed by the adsorption of SA.7,8 Thepassivation effect of SA on transition metals seems toarise from the formation of a strong covalent bond betweenS and metal surfaces, which depletes the charge density

of d orbitals needed for the metal atoms to bind with otherspecies. Furthermore, sulfides of Ru can selectivelyperform hydrodesulfurization (HDS) on dibenzothiophene,a molecule particularly difficult to desulfurize. HDS is animportant reaction in processing petroleum.9,10

The electrochemistry of sulfur at a few single-crystalelectrodes such as Au(111), Cu(111), and Pt(111) has beenreported.11-14 Results show that the redox process of sulfurvaries substantially with the chemical identity of theelectrode. On Au(111) sulfur is changed slowly from (x3× x3)R30° to S8 octomers at potentials positive of -0.7V in 0.1 M NaOH.11,12 On Cu(111) ordered adlattices ofsulfur adatoms, (x3×x3)R30° and (19×19), are producedbefore the formation of sulfide at the onset potential of Cudissolution.13 On Pt(111) SA are oxidized to sulfate,without being converted to the S8 octomer or sulfidespecies.14 Since ruthenium is a member of the platinumgroup, it is possible that sulfur on Ru would behavesimilarly to that on Pt. However, the electrochemistry ofsulfur at a Ru electrode has not been investigated. Becausesulfur can act as a poison to Pt and Ru electrodes, it isimportant to gain a better understanding of sulfuradsorption at electrified interfaces of these materials.

In this study we also explore the adsorption of ben-zenethiol (BT) at a Ru electrode to gain insight into theeffect of molecular structure on the interaction betweenadsorbates and their spatial arrangement. The adsorptionof BT on Au(111), Pt(111), and Rh(111) has been examined

* 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,

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4596 Langmuir 2004, 20, 4596-4603

10.1021/la036379v CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 04/21/2004

in the past.15-18 In addition to the prominent M-S surfacebonds, the phenyl group can also interact with d orbitalsof metal substrates through its π-electrons. Thus, BT canlie either in parallel or in tilted orientation on thesubstrate, depending on which molecular configurationis thermodynamically more stable. On Au(111), BT isadsorbed mainly in parallel orientation at the solid-gasinterface, and the orientation is insensitive to coverage.15

In contrast, BT is tilted at the liquid-solid interface ofAu(111) with its ring inclined by 30° from the surfacenormal.16 On dry Rh(111) reorientation of BT from aparallel to a tilted configuration occurs with increasingcoverage. In addition, decomposition of BT to benzene andsulfur occurs at temperatures higher than 300 K.17

Benzenethiol was found to bind with Pt(111) mainlythrough the S atom, but no ordered structure was observedwith LEED.18 The surface chemistry of BT on ordered Rusurfaces has not been reported thus far. Results of thermaldesorption experiments show that benzene is weaklyadsorbed on Ru(0001), as benzene desorbs at 350 K.19 Thisweak benzene-ruthenium interaction implies that thestrong covalent bond of Ru-S could control the adsorptionof BT on Ru(0001).

Experimental SectionThe Ru(0001) single-crystal electrode used was disk-shaped,

8 mm in diameter, and 1 mm in thickness. It was obtained fromthe Fritz Haber Institute (Berlin, RFG), and it was polished toa 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 thatthe traditional annealing and quenching process useful for Pt,Rh, Pd, Ir, and Au single-crystal electrodes is not applicable toRu.20-24 Ru is chemically so stable that electrochemical polishingis unlikely to work. On the other hand, it has been demonstratedthat annealing in a hydrogen saturated environment is analternative 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 thepresent study the typical pretreatment method was usedinvolving mechanical polishing with successively finer aluminumoxide powers measuring from 1.0 to 0.1 µm. The electrode surfacewas likely to be covered with a layer of oxide or organic materialat the end of this process. Therefore, to remove the contaminatedlayer and expose the bare Ru surface, a potential of -1.5 V wasapplied for 5 min in a conventional electrochemical cell containing0.1 M HClO4. Ultrapure nitrogen was used to purge the cellcontinuously to remove the hydrogen gas generated in thereduction process, and the supporting electrolyte (0.1 M HClO4)was replaced several times before cyclic voltammetric experi-ments were performed.

The Ru electrode appeared to be hydrophilic, and it was coveredwith a thin water film upon emersion. This water film wasexpected to shield the Ru surface from contaminants in air, which

could be important to preserve the clean Ru surface required forin situ STM experiments. Ultrapure nitrogen gas was used topurge all solutions used in this study, while in situ STMexperiments were conducted in ambient. As illustrated below,this procedure was effective in minimizing the formation of theoxide layer and the contamination with organics on the surface.The resultant Ru(0001) surface appeared to be electrochemicallyclean with some degree of ordering. Details of STM and linearsweep voltammetric experiments are described elsewhere.28 AllSTM images were acquired in the constant-current (height) mode,and typical imaging conditions were 50-200 mV in bias voltageand 1-10 nA in feedback current.

Ultrapure perchloric acid and sodium sulfide (Na2S) purchasedfrom Merck Inc. (Darmstadt, Germany) were used after dilutionwith triple-distilled Millipore water. Benzenethiol was purchasedfrom Aldrich (Saint Louis, MO). BT was not very soluble inaqueous solution. According to ref 29, its solubility is 7.6 mM atroom temperature. The dosing solution of BT was a 0.1 M HClO4solution saturated with BT. This concentration appeared to besufficient to generate a full monolayer of BT on Ru(0001).

Results and DiscussionCyclic 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 wasobtained with a sample treated simply by mechanicalabrasion. This CV profile is poorly defined, possiblybecause the electrode surface was rough and contami-nated. This is in strong contrast to the solid trace obtainedwith a Ru(0001) electrode subjected further to electro-chemical reduction at -1.5 V for 5 min. This i-E curvecontains two pairs of peaks centered at ∼0.18 (A1/C1) and0.55 V (A2/C2), respectively. These pairs of peaks areassociated with the adsorption/desorption couples ofhydrogen and oxygen, respectively, at terrace sites onordered Ru(0001).25 All ordered, electrochemically cleanRu(0001) electrodes exhibited these features.30-33

The above results were somewhat surprising to us,because the as-prepared Ru(0001) electrode was expectedto contain a high density of defects. As evidenced by thein situ STM results, the average width of terraces on theas-prepared Ru(0001) was 20 nm. It was thought thatsurface redox processes such as hydrogen adsorption/

(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.; Steinruck, H.-P. Surf. Sci. 2000, 454-456, 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.

(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.

Figure 1. Cyclic voltammograms of Ru(0001) recorded at 50mV/s in 0.1 M HClO4. The Ru(0001) electrode was prepared bymechanical polishing with aluminum oxide particles measuringdown to 0.1 µm (dotted trace), followed by cathodic polarizationat -1.5 for 5 min (solid trace).

Sulfur and Benzenethiol Chemisorbed on Ru(0001) Langmuir, Vol. 20, No. 11, 2004 4597

desorption at steps would occur at potentials differentfrom those at terraces, as is well-known for platinumelectrodes.20 In other words, a rough Ru electrode wasexpected to produce a poorly defined voltammogram,similar to the dotted trace in Figure 1. However, only onecathodic peak (C1) was observed at 0.18 V in the negativescan, which suggests that terrace and step sites on Ru-(0001) were energetically similar to each other forhydrogen atoms. Because the current density of peak C1is comparable to that obtained with the Ru(0001) electrodeprepared by using a rigorous UHV technique,30-32 it canbe stated that all Ru(0001) electrodes might have similarsurface characteristics. Thus, it is plausible that thecathodic polarization at -1.5 V causes the reduction ofoxide, and the removal of organic contaminants, resultingin exposure of the bare Ru(0001) surface.

The solid and dotted traces in Figure 2 show cyclicvoltammograms of Ru(0001) coated with a monolayer ofSA, irreversibly adsorbed from solutions containing Na2S.Both cyclic voltammograms are featureless between 0.05and 1.0 V, indicating that SA were stable within thispotential range. The lack of features between 0.3 and 0.1V indicates that SA completely blocked the adsorption ofhydrogen. This finding agrees with the results previouslyobserved in a vacuum, where no hydrogen adsorption waspossible, when the substrate was predosed with one-thirdof a monolayer of SA.34 In other words, this result infersthat the coverage of SA deposited onto an as-preparedRu(0001) surface was likely to be more than 0.33. Potentialexcursion to 1.2 V, however, produced a slight increase incurrent density, possibly resulting from the oxidation ofsulfur to some unknown products such as sulfate anions.This is in strong contrast to the characteristics of the cyclicvoltammogram for the S/Pt(111) system, where SAundergo irreversible oxidation to sulfate, producing asharp current spike near 0.93 V.14 As indicated by theSTM results described below, the spatial structures ofthese adsorbates changed along with the potential sweep,but apparently those changes were not detected byvoltammetry. Furthermore, we attempted to reductivelydesorb S in 0.1 M KOH, as in the case of Au(111).11 Theresult (not shown here) revealed that no desorptionoccurred prior to the onset of water reduction, whichimplies that S was strongly held on Ru(0001). The Ru-Ssurface bond could be covalent in nature, as was notedpreviously by LEED studies.1-5 The cyclic voltammogramsof benzenethiol-modified Ru(0001) were also obtained,which showed that they bear a strong resemblance to those

of SA. It appears that BT was adsorbed mainly throughits sulfur end on Ru(0001) after the sulfhydryl group wasremoved.

In Situ STM Imaging of Sulfur Adlayers on Ru-(0001) in 0.1 M HClO4. Ordered Sulfur Adlattices. Thefollowing STM results were obtained with Ru(0001)electrodes prepared by mechanical polishing and elec-trochemical reduction at -1.5 V for 5 min. The constant-current STM images in Figure 3 show the typical surfacemorphology of a Ru(0001) sample potentiostated at 0.2 Vin 0.1 M HClO4, which was recorded with STM at 200 mVin bias voltage and 2 nA in feedback current. As expected,the surface contained deep scratches (marked S) formedby the abrasive process. However, many terraces as wideas 30 nm separated by monatomic steps were still present.It is not clear what proportion of the surface had these

(34) Huang, H. H.; Seet, C. S.; Xu, G. Q.; Hrbek, J. A. Surf. Sci. 1994,319, 344.

Figure 2. Cyclic voltammograms of Ru(0001) recorded at 50mV/s in 0.1 M HClO4. The electrode was first coated with amonolayer of sulfur adatoms. The sulfur adlayer was stablebetween 0.1 and 1.0 V, and its oxidation commenced at morepositive potentials.

Figure 3. In situ STM topography scans revealing the surfacemorphology of Ru(0001) pretreated with mechanical polishingand 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 and2 nA.

4598 Langmuir, Vol. 20, No. 11, 2004 Ou Yang et al.

features. As reported previously, an atomic resolution STMscan 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) wasunavoidable, and their densities were notably higher thanthose on Ru(0001) prepared by annealing in hydrogen.25

Prolonged annealing at 800 °C was beneficial to removethermodynamically unfavorable defects. It is also nearlyimpossible to avoid the presence of a trace of contaminationon the surface, which resulted in defects in the sulfuradlattice (vide infra).

Figure 4 shows high-resolution STM images of a terracesite on a sulfur-coated Ru(0001) electrode. The first imagereveals ordered arrays on a terrace with a width of 20 nm.

Thesestructuresweredeterminedtoberotationaldomainsof a sulfur adlattice. The close-up view shown in Figure4b of one of the ordered domains highlights the internalatomic arrangement of this structure, indicating a (2 ×x3)rect (or c(4 × 2)) symmetry with two sulfur atoms perunit cell (θ ) 0.5). This ordered adlattice will be referredto as (2 × x3)rect below. The two unit vectors with a 90°angle are 5.5 and 4.6 Å long, respectively. This structurecan be arranged in multiple rotational domains, as isactually seen in the STM image of Figure 4a. The rectangleand orthorhombus drawn in the image denote the unitcells. Multiple rotational domains of (2 × x3)rect fre-quently coexisted on a terrace, with preferences to somecertain specific rotational domains. The origin of dis-crimination could stem from the interaction of (2 × x3)-rect with neighboring steps, as noted previously by others.5Direct evidence for this interaction is clearly seen in thetime-dependent STM images, showing that the (2 × x3)-rect structure preferentially emerged in regions near stepledges (vide infra).

The sulfur adatom inside the (2 × x3)rect unit cellappears to be situated 0.03 Å lower than the S atoms atthe corners of the cell (or lattice points), and it is not locatedat the center of the cell. These findings imply that this Sadatom is located at a site different from where the Satoms of lattice points are situated. These results lead tothe ball model drawn in Figure 4c. It is essentially identicalto that proposed in a previous UHV study.3 A thoroughdynamic LEED analysis shows that SA at the corners ofa cell reside at hcp-type hollow sites, whereas the S atomin the middle occupies an fcc-type of site.3 LEED resultsfurther show that these two types of sulfur atoms shiftfrom their symmetric positions laterally by 0.16 Å inopposite directions along the mirror plane (x3 direction).The uppermost layer of Ru is buckled with an atomiccorrugation of 0.19 Å. These atomic relocations might notbe detectable by STM. It is intriguing to note that therelative corrugation heights of sulfur atoms do not agreewith their physical heights. Sulfur atoms at fcc sites seenas dim spots in the STM images actually lie 0.02 Å higherthan those at hcp sites.3 It was the electronic configuration,rather than the physical height, of adsorbates thatdominated the tunneling process.

Figure 5 shows another ordered structure of SA observedat 0.4 V. The degree of ordering of this structure wasalways poor, irrespective of the surface state of the Ru-(0001) substrate. Figure 5a highlights a 62 × 62 Å STMscan of this new structure, while Figure 5b presents ahigher resolution scan which has been treated with a 2DFourier transform technique to remove noise signals withspatial spacing of less than 2.5 Å. The unit vectors 7.2 Åin length are rotated 19° from the atomic rows of thesubstrate. This ordered array is characterized as (x7 ×x7)R19.1° with four sulfur atoms per cell, correspondingto a coverage of 0.57. Close examination of the atomicimage in Figure 5b reveals that protrusions on the edgesof the cell are not centered, but they appear to be shiftedsideways by about 0.2 Å. The interatomic spacings betweenatoms #1 and #2 (d12) and atoms #2 and #3 (d23) are 2.7and 4.5 Å, respectively. Similarly, the sulfur adatom 2′does not lie between 1 and 3′. These interatomic spacingsindicate the occurrence of clustering phenomenon, whichwas also noted in previous studies.4 On the average, allprotrusions exhibited the same intensity, although sulfuradatom 4 was lower in height than the other S atoms by0.2 Å. These findings led to the real-space model presentedin Figure 5c, which is essentially identical to that basedon LEED results.4 All SA, except for 4 in the middle of theunit cell, reside at hcp threefold hollow sites. The nearest

Figure 4. In situ STM images of Ru(0001) (2 × x3)rect 2S (aand b) and a corresponding model (c). These images wereacquired at 0.2 V with 100 mV bias voltage and 5 nA feedbackcurrent.


neighbor spacing of this adlattice, 2.7 Å, is considerablysmaller than the van der Waals diameter (3.7 Å) of a sulfuratom,35 suggesting the existence of a substrate-mediatedinteraction between SA. This phenomenon was alsoobserved for SA on Re(0001).36

Potential-Induced Phase Transition of a Sulfur Adlayeron Ru(0001). The S/Ru(0001) system is known for its rich

structural variations, which result in the formation offive different ordered structures.1-5 However, these resultsmight not be valid at solid-liquid interfaces, because thechemical nature of sulfur could vary with electrochemicalpotential.

The in situ STM experiment performed in 0.1 M HClO4containing 0.1 mM Na2S produced the results shown inFigure 6. First, the STM discerned a well-ordered (2 ×x3)rect adlattice similar to that shown in Figure 4 atpotentials negative of 0.3 V. No structural change wasnoted even at the onset potential of hydrogen evolution,indicating that SA barely desorbed even at 0 V in 0.1 MHClO4. It seems that the formation of ordered arrays witha 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 negativepotentials with minimal gas evolution. However, steppingpotential positively from 0.3 to 0.35 V resulted in marked

(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.

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 100mV in bias voltage and 5 nA in feedback current.

Figure 6. In situ constant-current STM images of sulfuradatoms on Ru(0001) at 0.35 (a) and 0.4 V (b). These imageswere recorded at 50 mV in bias voltage and 5 nA in feedbackcurrent.


changes of this (2 × x3)rect adlattice to generate localdomain walls, which are pointed out by arrows in theSTM image of Figure 6a. The inset of Figure 6a shows aclose-up view of a domain wall, sandwiched between two(2 × x3)rect domains. This domain wall graduallydeveloped into local (x7 × x7)R19.1° at the expense of (2× x3)rect, leading to the formation of patches of theseordered adlattices or a “glass phase”, as seen in Figure 6b.Muller et al. also observed this structure in a vacuum.5The latter structure eventually predominated at 0.4 V,despite the fact that the range of ordering was usuallyless than 100 Å. At potentials positive of 0.5 V, disorderingoccurred and predominated potentials equal to or morepositive than 0.6 V. We have not observed SA clusteredto form well-defined S8-type features such as those seenin Au(111). Instead, in situ STM revealed randomnucleation of protruding islands on Ru(0001) at even morepositive potentials, which could be associated with thecommencement of sulfur bulk deposition.

To illustrate that the potential-induced phase transitiondescribed above was reversible, real-time in situ STM wasused to follow structural changes as the potential wasstepped from 0.6 to 0.3 V. The results are presented inFigure 7, which shows the time-dependent phase transi-tion of SA. The identical surface morphology seen in theseimages ensures that the tip was scanned in the same area.

These images are presented in a z-offset mode to enablea clear view of atomic structures on all terraces. Therelative corrugations of terraces are unfortunately lost inthis mode. In reality the height of terraces follows thesequence I > II > III, and they are separated by monatomicsteps marked by dotted lines in Figure 7a. The first imagewas recorded at 0.6 V, where the sulfur adlayer was mostlydisordered, with a local (x7 × x7)R19.1° structureremaining at the upper right half of the image. This STMimage also reveals the presence of many pits (P) and kinks(K). Since the coverage of sulfur adatoms increases withpotential, these phase transitions could be ascribed to thepotential-induced changes in coverage.

Figure 7b, obtained 3 min after applying the potentialstep, reveals the formation of ordered (2 × x3)rect,particularly at the upper two terraces, I and II. Locallyordered (2 × x3)rect domains pointed out by arrows alsoemerged on terrace III. It is clear that ordered sulfuradlattices nucleated preferentially near the defects of stepsand pits, followed by growing outwardly into disordereddomains on the terraces. Longer range ordering was finallyobserved in Figure 7c and d, acquired 6 and 31 min,respectively, after the potential step was applied. Theseresults indicate that steps influence the growth of the (2×x3)rect structure, which might act to pin the orientationof imminent (2 × x3)rect domains. This interaction

Figure 7. Time-dependent in situ STM images of sulfur adatoms on Ru(0001). The first image was acquired at 0.6 V, while theimages 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 were100 mV and 5 nA.


betweenstepsandorderedstructurescould leadtounequalweights of the three rotational domains of (2 × x3)rect.Nucleation of ordered structures occurred not only at theupper end but also at the lower end of step ledges, asexemplified by the region highlighted by a dotted circlein Figure 7c. These concurrent events inevitably led tothe 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 Adlayeron Ru(0001). BT was allowed to adsorb from a 0.1 MHClO4 solution saturated with BT. Similarly to the casefor sulfur, the potential exerted marked influence on thestructure of the BT adlayer. Only between 0.1 and 0.3 Vwas BT arranged in an ordered structure, which becamepermanently disordered once the potential was raisedabove 0.4 V. BT was bonded to the Ru substrate, mainlythrough the S end after the S-H bond was cleaved. Onthe other hand, it is difficult to determine whether BTunderwent further decomposition upon its adsorption onRu(0001). Since the STM results obtained (see below) differfrom those of sulfur, it is unlikely that BT decomposed toproduce SA on the Ru(0001) electrode under the presentexperimental 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 aRu(0001) electrode at 0.2 V in 0.1 M HClO4 saturatedwith BT. The 250 × 250 Å2 scan in Figure 8a reveals thatthe BT adlayer was well ordered. The internal atomicarrangement is seen in the higher resolution scan of Figure8b. This structure is identified as (2 × x3)rect, with twospots per unit cell. With the given definition of the (2 ×x3)rect unit cell in Figure 8b, the internal spot appearslower in height than the corner spots by 0.03 Å. TheseSTM results bear strong resemblance to those of sulfuratoms seen in Figure 4, which suggests that BT is attachedto Ru(0001) mainly through the sulfur headgroup, leavingthe phenyl group pendant in solution. This bindingconfiguration has been reported for BT on Pt, Rh, and Auat high saturation.15-18 On the other hand, close exami-nation of Figures 4b and 8b shows that the (2 × x3)rectstructures of sulfur and BT are actually different, in thatthe protrusions inside the unit cells are located at differentlocations. The real-space model depicted in Figure 8cillustrates this difference, where the empty solid circlesrepresent S headgroups of BT molecules. All BT moleculesare believed to reside at hcp threefold hollow sites, the

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 recordedat 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 differentfrom that of sulfur, in that the middle adsorbates are located at different sites. The solid and dotted circles denote the sites of abenzenethiol molecule and a sulfur adatom, respectively. The model in part d illustrates the arrangement of the phenyl groupsof BT and the possible π-π stacking attractive interactions between BT admolecules.


most favorable registries identified for SA.1-5 This is atvariance with that observed for SA (Figure 4c), where theprotrusion inside the cell resides at an fcc threefold hollowssite, 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 themolecular resolution STM image. Although a BT moleculeappears as a single protrusion on Au(111), as reported byWan et al.,16 there is no reason it should give an identicalimage on Ru(0001). In the analogous system of iodoben-zene on Cu(111), the admolecule gives rise to an ellipsoidalprotrusion or two spots, the former being associated withthe sulfur headgroup while the latter is associated withthe phenyl group.37 Consequently, it is possible that notevery spot in the STM image of Figure 8b corresponds toa BT molecule. Knowledge of the coverage is certainlyhelpful in this regard, but this information is not available.We then considered the nearest neighbor spacing of BTadsorbates. In the case of BT/Ru(0001) it is 2.7 Å, ascompared 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 abnormallystrong attractive force between BT molecules to pull themclose to each other. One can adapt an idea of a “π-πstacking” interaction, which has been shown to accountfor the conformation of biological and porphyrin mol-ecules.38,39 This aromatic interaction was also noted forchemical systems at interfaces. For example, a SAM ofBT on Au(111) results in the formation of protrudingmesas,40 rather than pits, which are commonly seen forSAMs of alkanethiol molecules. To put this π-π stackingintermolecular interaction in perspective, a tentativemodel is depicted and presented in Figure 8d, where onlythe phenyl groups of BT admolecules are shown for clarity.One can see that BT admolecules are arranged in a zigzagpattern along the [121] or the [110] direction, and each

molecule can have the π-σ attraction with its two nearestneighbors, as reported by Hunter et al.39 However, it isdifficult to judge if this interaction is strong enough topull BT admolecules to such a close spacing as 2.7 Å.Structural investigations using other techniques, such asglancing angle X-ray scattering techniques, would behelpful.

Conclusion

Results of in situ STM and electrochemical measure-ments show that it is possible to prepare ordered andelectrochemically clean Ru(0001) electrodes by mechanicalpolishing combined with electrochemical reduction inacidic environment. High-quality STM results allowedidentification of a (2 × x3)rect structure for both SA andBT in the potential range between 0.1 and 0.3 V, suggestingthat BT molecules are adsorbed through their S head-groups and that the Ru-S interaction predominates themolecular adsorption. The arrangement of sulfur is notidentical to that of BT in (2 × x3)rect, as the spacingsbetween two nearest neighbors are 3.2 and 2.7 Å,respectively. It is not clear whether all protrusionsobserved in the molecular resolution STM image of (2 ×x3)rect are associated with BT molecules. Potentialmodulation resulted in a reversible phase transition forSA, whereas irreversible changes were observed for BT.The coverages of SA and BT increase with increasingpositive potential. The former produces (2 × x3)rect,domain wall, (x7 × x7)R19.1°, and disordered adlatticesbetween 0.3 and 0.6 V, but only one ordered structure, (2× x3)rect, forms for BT.

Acknowledgment. S.L.Y. acknowledges the financialsupport from the National Science Council, Taiwan (NSC93-2113-M-008-009). This work was partially supportedby the Ministry of Education, Culture, Sport, Science andTechnology, with a Grant-in-Aid for the COE project GiantMolecules and Complex Systems, 2003. The authors thankDr. Y. Okinaka for his help in writing this manuscript.

LA036379V

(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.


scanning tunneling microscopy of sulfur and benzenethiol chemisorbed on ru(0001) in 0.1 m hclo 4

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