Characterization of the RuS2 (100) Surface by Scanning Tunneling

Characterization of the RuS2(100) Surface by Scanning Tunneling Microscopy, Atomic Force Microscopy, and Near-Edge X-ray Absorption Fine Structure ...
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J. Phys. Chem. B 1999, 103, 4649-4655

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Characterization of the RuS2(100) Surface by Scanning Tunneling Microscopy, Atomic Force Microscopy, and Near-Edge X-ray Absorption Fine Structure Measurements and Electronic Band Structure Calculations S. P. Kelty* and J. Li Department of Chemistry, Seton Hall UniVersity, South Orange, New Jersey 07079-2694

J. G. Chen Department of Material Science and Engineering, UniVersity of Delaware, Newark, Delaware, 19716

R. R. Chianelli Department of Chemistry, UniVersity of Texas, El Paso, El Paso, Texas 79968-0513

J. Ren and M.-H. Whangbo* Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed: October 29, 1998; In Final Form: March 13, 1999

The (100) surface of single-crystal RuS2 was characterized using atomic force microscopy (AFM), scanning tunneling microscopy (STM), near-ddge X-ray absorption fine structure (NEXAFS), and electronic band structure calculations. High-resolution STM and AFM images show that the as-cleaved RuS2(100) surface has a square lattice with the same cell parameters as does bulk RuS2, but the unit cell features are distinctly differently from those expected for the unreconstructed surface. Tunneling spectra reveal that significant surface states are present within the band gap of bulk RuS2. Comparison of the electron and fluorescence yield NEXAFS analysis of the RuS2(100) surface indicates that the surface Ru atoms are bonded only to S atoms and exist in a bulklike valence state. Some of the surface S atoms are passivated with bound oxygen atoms. On the basis of electronic band structure calculations, we propose a model for the reconstructed RuS2(100) surface consistent with the experimental observations.

Introduction RuS2 is a unique material among the transition metal dichalcogenides, particularly regarding its catalytic1-4 and photoelectrochemical5-8 properties. For example, RuS2 is wellknown as one of the most active hydrodesulfurization catalysts and exhibits very high activity in many other hydrotreating reactions. Furthermore, RuS2 has wide absorption in the visible spectral region, is resistant to corrosion, and has a comparatively narrow band gap,7,9 making it attractive in photoelectrochemical applications such as solar energy conversion and alternative fuel production. Previous theoretical studies have suggested how the bulk electronic structure of RuS2 is related to those of other high activity and commercial catalysts and how knowledge of the electronic structure of bulk RuS2 might allow one to mimic its catalytic activity with less costly materials. Numerous theoretical and experimental reports have appeared in the literature aimed at establishing the bulk electronic and structural properties of RuS2.2,3,10 RuS2 crystallizes in the Pa3h space group with Ru2+ ions forming an fcc lattice (a ) 5.6106 Å).11,12 The sulfur atoms lie at fractional coordinates: ((0.5 r, 0.5 - r, 0.5 -r), ((r, -r, 0.5 - r), ((0.5 - r, r, -r), and ((-r, 0.5 - r, r) with r ) 0.1120. The sulfur atoms exist as S22- dimers occupying octahedral sites with the S-S bonds oriented along the [111] and [1h11] crystallographic directions. Bulk RuS2 is a semiconductor with an indirect bulk band gap (at 300 K) of 1.33 (.03 eV.10 Electronic band structure

calculations and experimental studies have shown that the valence and conduction bands are composed of the t2g and eg states of the (distorted) octahedrally coordinated Ru2+ (d6) ions.10,13,14 Electronic structure calculations for the RuS2(100) and RuS2(111) surfaces by Frechard and Sautet15 suggested that these surfaces differ significantly in electronic and atomic structure and in their corresponding catalytic activity. These studies predict that the (100) face formed from a homolytic cleavage of Ru-S bonds self-passivates without significant reconstruction, whereas the (111) surface is significantly more reactive and is responsible for the unique catalytic properties of RuS2. However, this study did not consider the possibility that surfaces cleaved in air (as is commonly done during catalysis pretreatment) may be oxygen-passivated, and to our knowledge, there has not been any definitive experimental study that clearly identifies which face, if any, has dominant chemical reactivity. Surface-sensitive investigations including low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) of RuS2 have recently indicated that the (100) surface appears to be essentially unreconstructed.16 Experimental surface studies of single-crystal RuS2 have been primarily concerned with exploring its resistance to photocorrosion and its novel photoelectronic properties.5-7,9,16,17 Recent STS investigations in humidity-controlled environments show a direct proportionality between photoinduced tunneling current density and humidity

10.1021/jp9842594 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/14/1999

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levels,8 suggesting the presence of surface states resulting from an adlayer of water under ambient (>50% relative humidity (RH)) humidity conditions. A thorough characterization of the surface structural and chemical properties is necessary to fully understand the catalytic properties of RuS2. In the present work, we report a detailed investigation of the as-cleaved air-exposed surface of RuS2 using STM and atomic force microscopy (AFM) for local surface structure and near-edge X-ray absorption fine structure (NEXAFS) for surface chemical composition. These results are modeled using extended Hu¨ckel tight binding (EHTB) calculations. Experimental Details RuS2 single crystals were supplied by Prof. Y.-S. Huang (National Taiwan Institute of Technology, Taipei University) and by Phoenicon, Berlin. The preparation of single-crystal RuS2 was reported elsewhere.18,19 Samples used in this study were typically 1-2 mm on a side. As-grown surfaces were found to be unsuitable for scanning probe microscopy (SPM) imaging most likely because of adsorbed transfer salts that remain after preparation. Only cleaved crystal faces gave reproducible results. Air-cleaved surfaces were stable over many months as evidenced by the fact that they give reproducible STM and AFM images. All STM and AFM studies were carried out using a commercial ambient scanning probe microscope (Park Scientific Inst., Sunnyvale, CA) employing mechanically sharpened Pt-Ir (80:20) tips for STM and Au-coated Si3N4 cantilevers with sharpened (20 nm nominal tip radius) pyramidal tips for AFM. Atomic-resolution images were recorded in the height and current imaging modes for STM and in the height and force imaging modes for AFM. The SPM sample chamber was modified with gas introduction ports to allow purging with dry nitrogen as needed. The images and spectra shown in the present study are typical of several hundred recorded over the course of a year using numerous tips and scan conditions. The NEXAFS experiments were carried out at the U1 beam line of the National Synchrotron Light Source, Brookhaven National Laboratory. Details concerning the optics on the beam line, as well as the ultrahigh vacuum (UHV) chamber with facilities for high-pressure reactions have been described previously.20,21 The NEXAFS spectra in the present study were recorded as a function of the incident X-ray photon energy in the vicinity of the sulfur L-edge (160-190 eV), ruthenium M-edge (450-500 eV), and oxygen K-edge (510-590 eV). All NEXAFS results reported here were obtained by measuring the electron yield and fluorescence yield simultaneously, with the incident photon beam at a normal incidence angle with respect to the RuS2(100) sample surface. The electron yield intensity was recorded by a Channeltron electron multiplier located near the sample holder. The fluorescence yield intensity was measured by using a differentially pumped UHV-compatible proportional counter filled with 200 Torr of P-90 gas (90% methane, 10% argon) as countergas, as described previously.20,21 Powder samples of RuS2, Ru, and RuO2 were pressed into stainless steel sample holders of about 12 mm in diameter and 1 mm in depth. A single-crystal RuS2 sample (3 mm × 4 mm) was mounted on a 1.5 cm × 1.5 cm HOPG graphite plate with the (100) face oriented normal to the incident beam. Results and Discussion Characterization of the Surface Structure. In describing the (100) surface of RuS2, it is convenient to view the crystal structure of bulk RuS2 in terms of stacking layers of composition (Ru2S4), namely, ‚‚‚(Ru2S4)(Ru2S4)(Ru2S4)‚‚‚. Each (Ru2S4) layer

Figure 1. (a) Schematic representation of the crystal structure of RuS2. Dark and light spheres represent Ru and S atoms, respectively. (b) Schematic representation of unreconstructed Ru(100) surface obtained by homolytic cleavage of Ru-S bonds. The zigzag pattern depicts the topmost sulfur atoms.

has two Ru2+ and two S22- ions per unit cell, which are arranged as in NaCl (Figure 1a). Thus, each Ru2+ ion of a (Ru2S4) layer is located at a square-planar coordination site made up of four S22- ions, and its axial positions are occupied by the S22- ions of the adjacent (Ru2S4) layers. The (100) surface of RuS2 is prepared from homolytic cleavage of all Ru-S bonds between adjacent layers (Figure 1a). The structure of such a surface is represented by that of the topmost (Ru2S4) layer of (Ru2S4)(Ru2S4)(Ru2S4)‚‚‚. Normal to the surface (Ru2S4) layer, the topmost atoms are sulfur atoms, which form zigzag chains running along one crystallographic direction (Figure 1b). The Ru atoms lie in a plane 0.629 Å below the top S atom layer.11,12 Lying 0.629 Å below the Ru layer is another sulfur atom layer. Therefore, the unreconstructed (100) surface plane of RuS2 should yield (at minimum surface energy) a square, centered Ru atom lattice with a zigzag pattern of topmost sulfur atoms lying at the bulk fractional xy coordinates11,12 (0.11210, 0.3879) and (0.3879, 0.8879) within the surface unit cell. Although the predominant growth face appears to be (111), our experience shows that cleavage predominantly results in the exposure of the (100) face, and so this face was selected for study. Figure 2 shows a representative contact-mode AFM height image of RuS2. To a first approximation, a contact-mode AFM image of a sample surface corresponds to the total electron density plot of the sample surface22,23 so that the bright spots in the image are identified as atomic positions. The observed surface unit cell (indicated in white in Figure 2) is characterized as having square symmetry with a ) b ) 5.6 ( 0.3 Å, consistent with that of the bulk unit cell. Each square unit cell has pronounced atomic features in the corners with less pronounced

Characterization of RuS2(100)

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Figure 4. Tunneling spectra of RuS2 surface: I vs V (×) and (d ln I/ d ln V) vs V (b) curves. Figure 2. Representative (1/f filtered) 43.8 Å × 43.8 Å AFM image of RuS2 obtained with the contact imaging mode (tip load ) 6.0 nN). The white graphic indicates a 5.6 Å × 5.6 Å repeat unit.

Figure 3. Representative (unfiltered) 64 Å × 64 Å STM image of RuS2 obtained in constant current imaging mode (Vbias ) -0.7 V, Iset ) 3.0 nA). The white graphic indicates a 5.6 Å × 5.6 Å repeat unit.

features between. Although the surface unit cell has the same period and symmetry as the bulk unit cell (a ) b ) 5.61 Å, γ ) 90°), we find that the repeat pattern is different. We do not observe the zigzag pattern expected for the unreconstructed (100) surface (Figure 1). This result has been consistently observed within a large sample set particularly whenever highresolution images have been obtained. A typical STM constant current mode image recorded for negative Vbias (sample-to-tip tunneling) is shown in Figure 3. Stable STM images could be obtained for both positive and negative Vbias throughout a wide bias voltage range (-1500 to 2000 mV). The atomic features in Figure 3 exhibit a square pattern with period of 5.6 ( 0.2 Å, similar to that observed in the AFM images. The STM images obtained for positive Vbias (tip-to-sample tunneling) are similar in structural detail. These patterns differ considerably from the unreconstructed surface structure (Figure 1) and suggest a relaxation of the (100) surface. Recently, tip-sample interactions have been implicated in pressure-induced local structure changes, especially in layered materials.24 Under conditions of low gap resistance, the tipsample separation decreases, resulting in increased pressure on the sample surface and partial compression of the surface layers.

This compression has been shown to result in anomalous image artifacts. Our experiments did not indicate any variation in the observed surface structure as a function of gap resistance in the STM data or as a function of tip loading force in the AFM data. Thus, the observed surface structure in our STM and AFM images is not caused by tip-sample interactions and is characteristic of the material surface. It has recently been suggested that under humid conditions (room temperature, relative humidity of >55%) the RuS2(100) surface is covered with adsorbed water, as evidenced by an increase in tunneling photocurrent under illumination.8 In our laboratory, atomic resolution STM and AFM images obtained under dry nitrogen (RH < 10%) were identical to those obtained at ambient RH. Thus, we find that either the adsorbed surface water does not form an ordered layer or does not affect the surface local density of states (LDOS) in the absence of illumination. The electronic states near the Fermi level were probed using scanning tunneling spectroscopy (STS). Normalized conductance curves (∂ ln I/∂ ln V)s are approximately proportional to the density of states (DOS) of the surface.25 Figure 4 shows a typical I-V characteristic (tunneling current versus bias voltage, I vs V) and normalized conductance curve obtained for air-cleaved RuS2(100). The normalized conductance of an intrinsic semiconductor is expected to approach unity in the gap region. As shown in Figure 4, conductance is observed throughout the gap region, indicating a high density of surface states within the band gap region of bulk RuS2. Thus, the surface is highly conductive. In fact, stable STM images can be obtained at bias voltages as low as (300 mV. To obtain information about the relative spatial distribution of filled and empty state density in the surface unit cell, simultaneous images were collected at positive and negative bias voltages. Such images are obtained by alternating the bias voltage in adjacent line scans during image collection. Shown in Figure 5 is a superimposed image of filled (blue) and empty (red) state density. The gray scale of the filled state density was normalized to that of the empty state density prior to superimposing. In this image, blue (red) spots indicate higher filled (empty) electron state density while magenta indicates spatial overlap of filled and empty electron state density. Figure 5 clearly indicates that the filled and empty state densities are spatially coincident. Surface Chemical Composition. To obtain a detailed understanding of the surface chemical composition of RuS2, we

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Figure 5. Superimposed 90 Å × 90 Å STM images obtained simultaneously for Vbias ) -0.40 eV (blue) and Vbias ) +0.60 eV (red), Iset ) 2.5 nA. The images were slightly filtered using a low-frequency band-pass.

Figure 6. NEXAFS spectra of oxygen K-edge features of RuS2 single crystal and powder, RuO2 and FeSO4. The data were collected by measuring the electron yield.

undertook NEXAFS studies using both electron yield and fluorescence-yield methods. As discussed earlier,20,21 the electronyield method is primarily sensitive to the topmost surface layers (10-20 Å) and the fluorescence-yield method to bulk (several thousand angstroms). Figure 6 shows a comparison of the oxygen K-edge features of RuS2(100) with those of several other model compounds. The oxygen K-edge features of both single-crystal and powder RuS2 samples are characterized by an intense peak at 537.5 eV and a low-energy shoulder at 532.5 eV. In NEXAFS measurements, the peak positions of the oxygen K-edge features are determined by the nature of the O-X bond, with X being either a metal or other surface species. For example, when oxygen atoms are bonded to sulfur, as in the case of FeSO4, the oxygen K-edge is characterized by a NEXAFS feature centered at 537.5 eV. When oxygen atoms are bonded directly to Ru, as in RuO2, the oxygen K-edge features are characterized by a set of three features at 530.5, 533.0, and 543.5 eV. A molecular orbital

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Figure 7. NEXAFS spectra of Ru M-edge features of RuS2, Ru and RuO2. The data were collected by measuring the electron yield. The bulk compositions of Ru and RuS2, from the fluorescence yield (FY) measurements, are also included.

model for transition metal oxides21,26 shows that these features are related to the electronic transition of the oxygen 1s electron to the t2g, eg and a1g orbitals of RuO2, respectively. In addition, the 537.5 eV feature for FeSO4 can be assigned to the transition of the O 1s electron to the unoccupied p-states of sulfate. The comparison of the oxygen K-edge features of RuS2 with those of RuO2 and FeSO4 clearly indicates that the oxygen species on the ambient RuS2(100) surface are primarily bonded to sulfur instead of ruthenium. However, the observation of the relatively weak shoulder at 532.5 eV also suggests that a small fraction of the surface oxygen is also directly bonded to Ru possibly at defects or step edges. More details about the NEXAFS assignment of oxygen K-edge features can be found elsewhere.21 The observation that oxygen is present but not directly bonded to Ru is also supported in the NEXAFS measurement of the ruthenium M-edge features. Figure 7 shows a comparison of the ruthenium M-edge features of RuS2 and RuO2. The fluorescence-yield measurements of Ru and RuS2 powders are also included in Figure 7. As expected, the peak position of the ruthenium MIII feature increases from 462.5 eV for Ru0 to 465.0 eV for Ru4+ in RuO2. The ruthenium MIII feature of the (100) surface of RuS2 appears at 463.5 eV, which is similar to the electron-yield and fluorescence-yield measurements of RuS2 powder materials. If a linear relationship were assumed between the NEXAFS peak position and the oxidation state of Ru,21 the peak position at 463.5 eV would indicate that the oxidation state of Ru on the (100) surface of RuS2 is approximately +1.5. If the Ru atoms on the (100) surface were directly bonded to oxygen, one would expect the oxidation state of Ru to be substantially higher than +2.0. Thus, the Ru M-edge NEXAFS spectra indicate that the surface Ru atoms exist in a bulklike oxidation state close to +2.0. Finally, NEXAFS measurements of the sulfur L-edge features are shown in Figure 8. For both single-crystal and powder RuS2 samples, the sulfur L-edge features are characterized by two sets of doublet features (at 164.0 and 165.3 eV and at 169.3 and 170.5 eV). As described elsewhere,21 we assign these doublets as transitions from the S 2p3/2 and 2p1/2 states to the empty Ru 4d states, respectively. The observation of two sets of doublets arises from the presence of the two low-lying empty Ru eg and σ* levels.

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Figure 8. NEXAFS spectra of sulfur L-edge features of RuS2 single crystal and powder materials. The data were collected by measuring the electron yield.

Electronic Structure Calculations. To gain insight into the nature of the (100) surface of RuS2, we calculate the electronic structures of model unreconstructed and reconstructed (100) surfaces using the extended Hu¨ckel tight-binding electronic band structure method. The EHTB method27 is well suited for simulating STM and AFM images.22,23 For a number of layered transition metal dichalcogenides, the partial and total density plots calculated by the EHTB method are in excellent agreement with the observed STM and AFM images.28-32 For our EHTB calculations of the unreconstructed (100) surface, we employ a slab of three consecutive (Ru2S4) layers (Figure 1). One side of this slab is capped with S22- ions in order that the Ru2+ ions of that side (Ru2S4) layer have the octahedral coordination as in bulk RuS2. Thus, the uncapped side of the resulting slab, (Ru2S4)(Ru2S4)(Ru2S4)(2S22-), in which all the atomic positions are taken as in bulk RuS2, represents the unreconstructed (100) surface of RuS2. This three-layer slab will be referred to as the unreconstructed slab. Figure 9a shows the F(r0) plot, corresponding to an AFM image, calculated for the (100) surface of the unreconstructed slab. Figure 9b shows the F(r0,ef) plot, corresponding to an STM image, calculated for positive Vbias, and Figure 9c shows that for negative Vbias. In the F(r0) plot the high electron density (HED) spots are associated with the most protruding S atoms of the (100) surface forming zigzag patterns. The same is observed in the two F(r0,ef) plots. Thus, the F(r0) and F(r0,ef) plots of the unreconstructed (100) surface are not consistent with the brightness patterns of the observed AFM and STM images, respectively. To explain the AFM and STM data as well as the NEXAFS spectra of the RuS2(100) surface, it is necessary to consider its appropriate reconstruction. The latter must accommodate the observations that the (100) surface has S-O bonds but no Ru-O bonds and that the oxidation state of the surface Ru is about +1.5. These observations can be satisfied if about one-fourth of the (S-S)2- anions on the surface (Ru2S4) layer is converted into neutral species S-S-O. Disulfur monoxide S2O is a known species, albeit an unstable one (S-S ) 1.88 Å, S-O ) 1.46 Å, ∠S-S-O ) 118°).33 We now examine if the presence of S-S-O units on the (100) plane leads to total and partial density plots consistent with the observed AFM and STM images, respectively. For simplicity, we devise the reconstructed slab (Ru2S2S2O)(Ru2S4)(Ru2S4)(2S22-) from the unreconstructed slab

Figure 9. Electron density plots calculated for the unreconstructed slab using the tip-sample distance r0 ) 0.5 Å from the most protruding atoms of the (100) surface: (a) F(r0) plot, where the variation of the brightness contrast covers the 0.00-0.20 electrons/au3 range; (b) F(r0,ef) plot calculated for the sample-to-tip tunneling, where the variation of the brightness contrast covers the 0.00-0.05 electrons/au3 range; (c) F(r0,ef) plot calculated for the tip-to-sample tunneling, where the variation of the brightness contrast covers the (0.00-0.14) × 10-2 electrons/au3 range. A unit cell is indicated by a square, and the large and small circles represent the S and Ru atoms, respectively. Only the most protruding S atoms are shown.

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Figure 10. Schematic projection view of the (Ru2S2S2O) layer of the model slab (Ru2S2S2O)(Ru2S4)(Ru2S4)(2S22-) used to represent the reconstructed (100) surface of RuS2. The small black circles refer to the Ru atoms. The shaded and unshaded large circles represent the higher- and lower-lying S atoms, respectively. The medium hatched circles refer to the oxygen atoms. Only S-S and S-O bonds are shown. The square box represents a surface unit cell.

Figure 11. Projected density of states calculated for the surface Ru, S, and O atoms in the energy region of the band gap of bulk RuS2 using the model slab (Ru2S2S2O)(Ru2S4)(Ru2S4)(2S22-). The vertical dashed line refers to the Fermi level, i.e., the highest occupied level of the reconstructed slab. The atoms are defined with respect to the surface unit cell of Figure 10, and the higher-lying sulfur atoms of the S2 and S2O units are denoted by S(S2) and S(S2O), respectively. The plots are normalized to an atom per unit cell. The legends are as follows. The solid is for the corner Ru atom, the dashed line for the center Ru atom, the dotted line for the O atom, the dash-dot line for the S(S2) atom, and the dash-dash-dot-dot line for the S(S2O) atom.

Figure 12. Electron density plots calculated for the reconstructed slab using the tip-sample distance r0 ) 0.5 Å from the most protruding atoms of the (100) surface: (a) F(r0,ef) plot for the sample-to-tip tunneling, where the variation of the brightness contrast covers the (0.00-0.92) × 10-2 electrons/au3 range; (b) F(r0,ef) plot for the tipto-sample tunneling, where the variation of the brightness contrast covers the (0.00-0.13) × 10-1 electrons/au3 range. A unit cell is indicated by a square, and the large and small circles indicate the S and Ru atoms, respectively. Only the most protruding S atoms are shown.

(Ru2S4)(Ru2S4)(Ru2S4)(2S22-) by modifying its topmost (Ru2S4) layer such that every second S2 unit is replaced with S2O as depicted in Figure 10. In making each S-S-O unit, the O atom was attached to the lower-lying S atom of a chosen S-S unit, with O-S ) 1.46 Å and ∠S-S-O ) 120°, and the molecular plane of S-S-O is made perpendicular to the (100) surface. The S-S bond length of each S-S-O unit was kept at the value found for the (S-S)2- unit (i.e., 2.17 Å). Using the reconstructed slab, we calculated the projected density of states (PDOS) for the surface Ru, S, and O atoms in the energy region of the band gap of bulk RuS2 (calculated to occur between -12.6 and -9.4 eV from our calculations). As summarized Figure 11, the surface Ru, S, and O atoms contribute to the band gap states of the reconstructed

RuS2(100) surface. Around the Fermi level of the reconstructed slab, the corner Ru atoms and the higher-lying S atoms of the S2O units contribute more than do the center Ru atoms, the higher-lying S atoms of the (S-S)2- units, and the O atoms of the S2O units. In addition, there is no abrupt change in the contributions of these atoms around the Fermi level. The F(r0) plot calculated for the (100) surface of the reconstructed slab is similar to that of the unreconstructed slab shown in Figure 9a (hence not shown). This is not surprising because, in the model reconstructed slab described above, the higher-lying S atom of the S-S-O unit has the same height as does that of the (S-S)2- unit. The S-S bond of S2O is shorter than that of the (S-S)2- (i.e., 1.88 versus 2.17 Å). A shortening of the S-S bond can be introduced into the S-S-O unit by

Characterization of RuS2(100) keeping intact the position of its lower-lying S atom. Then on the reconstructed slab, the higher-lying S atom of the (S-S)2unit lies higher than that of the S-S-O unit. Consequently, in the resulting F(r0) plot, the electron density becomes larger on the higher-lying S atom of the (S-S)2- unit (not shown), thereby forming a square lattice of bright spots, in better agreement with the observed AFM image. The F(r0,ef) plots of the reconstructed slab calculated for the negative and positive Vbias are shown in parts a and b of Figure 12, respectively. In both plots, the electron density is high on the corner Ru atoms and the higher-lying S atoms of the S2O units, and low on the center Ru atoms and the higher-lying S atoms of the S2 units. This is expected because the PDOS plots of Figure 11 show that the electron density around the Fermi level is higher on the corner Ru atoms and the higher-lying S atoms of the S2O units. Note that the high electron density spots form almost straight-chain patterns, and so do the low-density spots. These observations are entirely consistent with the observed STM images at positive and negative bias voltages. Furthermore, patterns of the high electron density spots in Figure 12 are essentially coincident, in good agreement with observation (Figure 4). Finally, it should be noted that if the S-S bond length of the S-S-O unit is slightly shortened as described above, the density on the higher-lying S atom of the S-S-O unit is reduced somewhat. It is possible that the ordered oxygen layer may exhibit domain structures. However, we have not observed any such structures; the partial oxidation of surface sulfur atoms appears to be uniform. Concluding Remarks The present work clearly indicates the presence of surface structure and chemical composition distinct from that expected from an unreconstructed surface. The observation of gap states in the tunneling spectra and the ability to obtain STM images at low bias (Ef ( 0.3 eV) both indicate the presence of surface states on the RuS2(100) surface. NEXAFS measurements indicate that the surface Ru atoms are bonded only to S atoms as in the bulk but that surface S atoms are also bonded to O atoms. The essential aspects of all these observations are explained in terms of the structural model proposed for the reconstructed (100) surface, i.e., (Ru2S2S2O)(Ru2S4)(Ru2S4)‚‚‚. Although it remains unclear as to the relative catalytic reactivity of the RuS2(111) and RuS2(100) surfaces, future investigations of these surfaces will doubtless benefit from a more accurate understanding of the surface composition described here. Acknowledgment. Work at North Carolina State University was supported by the Office of Basic Energy Sciences, Division of Materials Sciences, U.S. Department of Energy, under Grant DE-FG05-86ER45259. We gratefully acknowledge Prof. Y.-S.

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