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Surface Core Level Shift: High Sensitive Probe to Oxygen-Induced Reconstruction of Rh(100) Laura Bianchettin,† Alessandro Baraldi,*,† Stefano de Gironcoli,‡ Erik Vesselli,† Silvano Lizzit,§ Giovanni Comelli,† and Renzo Rosei† Physics Department and Center of Excellence for Nanostructured Materials, UniVersity of Trieste, Via A. Valerio 2, 34127, Trieste, Italy and Laboratorio Nazionale TASC INFM-CNR, in AREA Science Park, S.S. 14 Km 163.5, 34012 Trieste, Italy, Scuola Internazionale Superiore di Studi AVanzati and INFM-CNR DEMOCRITOS National Simulation Center Via Beirut 2-4, 34014, Grignano (Trieste), Italy, and Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, 34012 Trieste, Italy ReceiVed: February 10, 2009; ReVised Manuscript ReceiVed: May 12, 2009
Oxygen-induced Rh3d5/2 surface core level shifts were used to probe the local electronic structure of first layer Rh atoms in the (2 × 2)pg reconstructed phase formed upon oxygen adsorption. By comparison of the computed shifts for differently reconstructed geometries with the experimental shifts obtained from highenergy resolution photoemission measurements, we confirm that the reconstructed phase is formed by Rh atoms single- and double-bonded to oxygen, yielding a shift from the bulk component of -185 and +140 meV, respectively. We find that the core level shifts are affected by the local rhomboidal distortion of the surface lattice. Moreover, we show that, on Rh(100), the oxygen-induced surface core level shifts are dominated by initial state effects and proportional to the calculated shift of the d-band center. The latter is therefore correlated to the local surface chemical reactivity, as already found for other adsorbate systems on transition metals. 1. Introduction The measurement of surface core level shifts (SCLS) by means of synchrotron radiation high-resolution X-ray photoelectron spectroscopy (XPS) represents a powerful tool, which is extremely sensitive to the atom- or molecule-surface interactions1,2 and capable of providing, for specific systems, direct information about the trends in chemical reactivity of transition metal surfaces.3 Because of their localized nature, core electrons are influenced by the electronic charge redistribution (charge transfer, polarization, hybridization, etc.) resulting from the creation of adsorbate-substrate bonds. This property has been exploited in a large variety of surface structures, such as oxide thin films,4 surface defects,5 and hydrogen chemisorbed phases.6-9 It is therefore expected that also the strain/stress induced on surfaces by atomic and molecular adsorption should induce changes in the core electron binding energies of similar magnitude as that observed in the case of under-coordinated transition metal atoms, where a strong interplay between the coordination number and the local bond strain was found.10,11 A peculiar class of surface structures where the potential of the surface core level shift approach can be employed is represented by the reconstructed surfaces with a reduced symmetry, where chemisorption determines a large distortion of lattice parameters. Here, the local character of the information derived from SCLS measurements and calculations can be used to unveil how the combined changes in surface morphology and electronic structure affect the local chemical reactivity of transition metal surfaces. * Author to whom the correspondence should be addressed: baraldi@ elettra.trieste.it. † University of Trieste and Laboratorio Nazionale TASC INFM-CNR. ‡ Scuola Internazionale Superiore di Studi Avanzati and INFM-CNR DEMOCRITOS National Simulation Center. § Sincrotrone Trieste.
A specific case belonging to this class is the clock reconstruction of Rh(100). The latter structure, induced by 0.5 monolayer (ML) of adsorbed oxygen, was initially described as a (2 × 2)p4g12-14 (as for C and N adsorption on Ni(100)) with the oxygen atoms sitting in the 4-fold hollow sites at the center of the Rh squares, rotated alternatively clockwise and counterclockwise. Subsequent theoretical calculations15,16 confirmed the proposed reconstruction but placed the oxygen atoms in 3-fold sites, along the main diagonal of the rhombus created by the substrate rotation. This prediction was later confirmed experimentally17-19 and also reproduced theoretically.20 The local atomic arrangement is peculiar, with a remarkable 11% distortion of the nominal (100) terminated bulk lattice, which can be considered as the onset of oxygen subsurface penetration in the surface oxide formation process.21 In the present paper, we investigate the oxygen-induced reconstruction of Rh(100) by combining synchrotron radiation high-energy resolution XPS and first-principles density functional theory (DFT) within the local density approximation. The SCLS results of the high-coverage reconstructed phase are compared with the p(2 × 2) and c(2 × 2) ordered structures obtained at lower coverage, thus providing new insight on the relationship between geometric structure, electronic properties, and chemical reactivity of the oxygen-Rh chemisorption system. 2. Experimental Results The photoemission measurements have been performed at the SuperESCA beamline of the Elettra synchrotron radiation laboratory.22 A double pass hemispherical electron energy analyzer with 150 mm mean radius and equipped with a multichannel detector was used to collect the spectra.23 The temperature during the measurements was maintained at 80 K
10.1021/jp901223d CCC: $40.75 2009 American Chemical Society Published on Web 07/02/2009
Oxygen-Induced Reconstruction of Rh(100)
Figure 1. Series of Rh3d5/2 core level spectra acquired at normal emission using 380 eV photons during oxygen uptake at T ) 300 K. Bold lines indicate the spectra corresponding to the p(2 × 2), c(2 × 2), and (2 × 2)pg long-range ordered structures.
in order to minimize the phonon-induced line shape broadening. The electron binding energies are referred to the Fermi energy position, measured under the same experimental conditions (photon energies, analyzer setup, and surface temperature). The residual background pressure in the experimental chamber was always in the low 10-10 mbar range. The Rh sample was cleaned by repeated cycles of Ar+ sputtering, oxygen, and hydrogen treatments; absence of contaminants was confirmed by XPS measurements in the C1s, S2p, and O1s core level regions, while surface order was probed by means of low energy electron diffraction. By use of well-established procedures,13,14 the following oxygen adsorption long-range ordered structures were formed on Rh(100): (i) p(2 × 2) at 0.25 ML, (ii) c(2 × 2) between 0.3 and 0.45 ML, and (iii) (2 × 2)pg at 0.5 ML. Rh3d5/2 core level spectra have been measured either in real time24 while dosing oxygen at T ) 300 K in the 10-9-10-7 mbar pressure range or with high-energy resolution (HR) (∆E ) 70 meV) at T ) 80 K using different photon energies to vary surface sensitivity and photoelectron diffraction conditions, thus changing the surface to bulk intensity ratio. The whole uptake series of the Rh3d5/2 core level spectra is reported in Figure 1. Already at a first glance, two main components can be distinguished in the photoemission spectra, originated from bulk (Rhb) and surface atoms (Rh0), respectively. The latter component is centered at -680 meV form the bulk. The photoemission spectra were fitted with a Doniach-Sˇunjic´25 function convoluted with a Gaussian and superimposed to a linear background. As previously reported,3 with increasing oxygen exposure the Rh0 intensity drops, while a new component at about -420 meV from Rhb (originated by Rh atoms bonded to a single oxygen atom in the p(2 × 2) structure) simultaneously grows until it disappears for longer exposures, which correspond to a higher oxygen coverage. At an intermediate coverage (about 0.4 ML, corresponding to the c(2 × 2)
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Figure 2. High-energy resolution Rh3d5/2 core level spectra from the (2 × 2)pg-O structure at 0.5 ML, recorded at T ) 100 K using three different photon energies. Open circles are the experimental data; the superimposed solid lines are the result of the final fits and the full colored blue, light blue, and gray peaks are the Rh2O, Rh1O, and bulk resolved components, respectively. The component in green is due to a residual population of first layer Rh atoms single bonded with oxygen in a local p(2 × 2) configuration, as explained in the text. The individual components were plotted after linear background removal. The residual of the fitting procedure is shown at the bottom of each spectrum. Thin gray curves are the spectra from the clean surface, measured at the same photon energy.
structure), a new component, shifted by -200 meV, appears due to Rh atoms bonded to two oxygen atoms. Upon development of the (2 × 2)pg structure at 0.5 ML, new features, at binding energies very close to the bulk component, can be observed. To distinguish the different peaks appearing in the proximity of the bulk component, measurements at high resolution and low temperature are essential. Figure 2 shows the HR Rh3d5/2 spectra (data points) corresponding to the oxygen saturated surface at three different photon energies (365, 375, and 400 eV). The spectra (gray curves) corresponding to the clean (1 × 1) surface under the same experimental conditions (photon energy and polar/ azimuthal photoelectron emission angles) are superimposed for direct comparison. The filled peaks are the distinct components, resolved by the fitting procedure, of the oxygen induced spectra. The values of the bulk and surface line shape parameters used at the three photon energies are in good agreement with previous investigations.26-28 For each layer, the reported values of the SCLSs were obtained from a parallel analysis of the spectra measured at different photon energies. Clean surface spectra were fitted by using only two components, corresponding to bulk (Rhb) and surface atoms (Rh0), respectively. The resulting SCLS is -680 ( 20 meV, in agreement with previous experimental studies.26-28 The spectra corresponding to the oxygen saturation phase were fitted using two different strategies: first, we used only a bulk and a surface peak. The final residual curve was however structured, unless free line shape
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Figure 3. Top and side views of the structural models for the O/Rh(100) surface: p(2 × 2), c(2 × 2), asymmetric reconstruction (2 × 2)pg-AR, symmetric reconstruction (2 × 2)pg-SR, and the (2 × 2)p4g. Oxygen and rhodium atoms are represented as blue and large circles, respectively. Second layer nonequivalent atoms are labeled with different numbers. Rhodium in-plane radial displacements and second layer relaxations (∆di) are also reported, where the arrows specify the displacement directions.
parameters and a very large, unphysical value of the Gaussian width were used. In a second step, we introduced in the fits, besides the bulk component, two additional components which were best centered at -195 ( 20 meV and +115 ( 20 meV with respect to Rhb, keeping the same line shape parameters found for the clean surface component. Finally, to eliminate a small shoulder present in the residual curve at low binding energy, a low intensity peak at -390 meV (green curve in Figure 2) had to be included in the fitting procedure. To take into account possible subsurface layer contributions, as found in the case of oxygen adsorption on Rh(111)29 and Ru(0001),30 the bulk peak position was allowed to slightly vary with oxygen coverage, shifting at saturation by about -50 meV toward lower binding energy with respect to the clean surface case. We assume that this shift originates from the contribution of second layer atoms. From coordination arguments, and in analogy with previous measurements on Rh surfaces,3 we assign the two main components to nonequivalent Rh surface atoms bonded to one (Rh1O) and two (Rh2O) oxygen atoms. As described in the next section, this interpretation is supported by the results of the ab initio calculations. 3. Theoretical Results 3.1. Surface Structure. DFT calculations were performed within the local density approximation of the exchangecorrelation functional in the Ceperley and Alder form,31 as parametrized by Perdew and Zunger.32 The Kohn-Sham equations are solved self-consistently using a plane-wave basis set restricted by a kinetic energy cutoff of 30 Ry, in an ultrasoft pseudopotential scheme,33 as implemented in the QuantumESPRESSO open-source distribution.34 The Rh(100) surface was modeled by a slab with seven layers and a vacuum region of 10 Å, corresponding to five interlayer spacings, with the first two which were allowed to relax. To deal with the metallic character of the system, we used a Methfessel and Paxton smearing function35 of order 1, with a width σ ) 0.03 Ry and a (12 × 12 × 2) Monkhorst-Pack grid for the (1 × 1) surface unit cell, resulting in 21 special k points in the irreducible
wedge.36 Equivalent k point samplings were used when dealing with larger supercells. Oxygen atoms are adsorbed on one of the two slab surfaces, allowing full relaxation until the total force on each atom is less than 0.001 Ry/bohr. We get a bulk equilibrium lattice constant of 3.78 Å. For the clean Rh(100) surface, we find that the first to second layer distance contracts by ∆12/d0 ) -3.7% when compared to the bulk interlayer distance. Calculations were performed for all the O structures illustrated in Figure 3: p(2 × 2), c(2 × 2) (left panel), and three possible structures at oxygen saturation (right panel). Among the latter ones, the first is the (2 × 2)pg, usually referred to as the asymmetric reconstruction (AR), with oxygen in the 3-fold hollow rhomboidal sites; the second is the (2 × 2)pg symmetric reconstruction (SR), with oxygen in the center of the rhomboidal hollow sites; finally, the last is the (2 × 2)p4g reconstruction (R), with oxygen in the 4-fold hollow sites of the Rh square. Surface atomic structures, electron densities of states, initial state, and full CLSs were calculated for the first four structures only, as the (2 × 2)p4g structure resulted to be locally unstable: starting with the initial configuration, the atoms relax toward the c(2 × 2) structure, in agreement with the results of ref 15. In Figure 3, Rh1O and Rh2O indicate nonequivalent first layer Rh atoms having respectively one or two bonds with O atoms in the unreconstructed geometries; similarly, Rh1O(AR), Rh2O(AR), Rh2O(SR), and Rh2O(R) refer to the differently reconstructed geometries. The nonequivalent second layer atoms are labeled with different numbers from 1 to 3. The results of the equilibrium geometries are summarized in Table 1. The oxygen adsorption height is always around 1 Å, while the O-Rh bond length ranges from 2.12 Å in the p(2 × 2) phase to 2.00 Å in the (2 × 2)pg-AR geometry. These values nicely compare with typical bond lengths calculated for two other low index rhodium surfaces.37,38 In the p(2 × 2) phase the first layer Rh atoms are coplanar and exhibit a radial displacement of 0.01 Å from the oxygen atoms. In contrast, there is a buckling of 0.11 Å in the second layer. In the c(2 × 2) structure the top layer atoms are also
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TABLE 1: Summary of the Minimal Energy Structural Parameters Calculated for the O/Rh(100) Structures Displayed in Figure 3a I layer structure
hO-Rh
dO-Rh
buckling
p(2 × 2)
0.98 Å 2.12 Å
0.00 Å
c(2 × 2)
0.99 Å 2.13 Å
0.00 Å
(2 × 2)pg-SR 1.00 Å dshort ) 2.07 Å dlong ) 2.20 Å (2 × 2)pg-AR 1.08 Å dshort ) 2.00 Å 0.99 Å dlong ) 2.71 Å
0.00 Å 0.09 Å
δxy
II layer
0.01 Å ∆d1 ) 1.90 Å ∆d2 ) 1.87 Å ∆d3 ) 1.79 Å 0.00 Å ∆d1 ) 1.88 Å ∆d2 ) 1.93 Å 0.07 Å ∆d1 ) 1.89 Å ∆d2 ) 1.93 Å 0.21 Å ∆d1O-1 ) 1.99 Å ∆d2O-1 ) 1.90 Å
a The oxygen adsorption height hO-Rh, the oxygen-rhodium bond lenght dO-Rh, the first layer buckling, the in-plane rhodium radial displacement δxy, and first to second layer distance ∆di are reported, where i ) 1, 2, 3 denotes the inequivalent second layer atoms indicated in Figure 3. In the reconstructed geometry we have reported both the O-Rh bond length along the short dshort and long dlong diagonals of the rhombus formed by Rh surface atoms upon oxygen chemisorption.
coplanar and the buckling in the second layer is reduced to 0.05 Å. The average interlayer spacing is 1.91 Å, in good agreement with the +0.5% relaxation found by Alfe` et al.15,16 In the (2 × 2)pg-AR phase the 13° rotation of the empty squares sites that we have found induces a distortion of the surface structure (δRh ) 0.21 Å) and the formation of rhomboidal sites. Oxygen atoms are adsorbed in one of the two 3-fold sites of the rhombus with an in-plane displacement δO ) 0.38 Å from the rhombus center along the diagonal, in agreement with previous theoretical (0.35 Å),15,16 and experimental (0.29 ( 0.15 Å)17 results. As a consequence of this asymmetric displacement, the outermost layer atoms become nonequivalent: Rh1O(AR) and Rh2O(AR), i.e., coordinated with one or two oxygen atoms, respectively. The oxygen adsorption heights are 0.99 and 1.08 Å for the Rh1O(AR) and Rh2O(AR) atoms, respectively, to be compared with the previous results of 0.98 and 1.06 Å.15,16 As a consequence, the uppermost layer is buckled by 0.09 Å, with an outward displacement of the Rh2O(AR) atoms, which was not considered in the previous low-energy electron diffraction (LEED) experimental determination.17 In the less-stable symmetric reconstruction (SR), the oxygen adsorbs in bridge sites in the center of the rhombus, 1.00 Å above the surface layer. The rhodium radial displacement is 0.07 Å, corresponding to a 4% relaxation, in good agreement with previous ab initio calculations.15,16 This value is considerably lower than the 11% distortion found for the asymmetric, more favorable configuration. Figure 4 shows the local electronic density of states (LDOS) projected onto the px, py, and pz (a, b and c curves) oxygen orbitals and onto d3z2-r2, dxz, dzy, dx2-y2 and dxy (d, e, f, and g curves) rhodium orbitals for the p(2 × 2), c(2 × 2), and (2 × 2)pg-AR structures. The LDOS obtained for the symmetric reconstruction and the c(2 × 2) phase are very similar and are therefore not shown. In the p(2 × 2) and c(2 × 2) structure, the px and py orbitals are equivalent for symmetry reasons, while the reconstruction removes the degeneration. As shown below, by comparison of p- and d-projected LDOS for each structure, it is possible to identify resonant peaks between interacting orbitals, forming bonding and antibonding states below and above the Fermi level, respectively. In the p(2 × 2) structure, the most important features are observed at -6.5 and -6.1 eV in the px O and in the corresponding dxz and dxy Rh surface atoms projections, indica-
Figure 4. Local densities of states projected on to the px (a), py (b), and pz (c) oxygen orbitals and on to the d3z2-r2 (d), dxz (e), dzy (f), dx2-y2 (g), dxy (h) orbitals of the Rh atoms for all the nonequivalent surface atoms present in three of the adsorption structure represented in Figure 3: the p(2 × 2) (full color), c(2 × 2) (dashed line), and asymmetric reconstruction (2 × 2)pg-AR (thin and thick lines).
tive of the dxz-px and dxy-px interactions. In contrast, the dyz component does not contribute to bond formation as it does not have resonance features. When the nearby equivalent Rh surface atom of the same square site is considered, the opposite situation is found, with dyz-py and dxy-py interactions determining the bond while the dxz orbital does not participate as it points toward the empty site. The same considerations hold for the c(2 × 2) structure, with the bonding state located at around -6.7 and -5.9 eV and the antibonding state at +1.2 eV. In the reconstructed phase the observable PDOS features evidence dxz-px and dxy-px interactions for the Rh2O(AR) atom and dyz-py and dxy-py interactions for the Rh1O(AR) atom; the reverse situation takes place for the other two equivalent atoms of the rhombus site. From the above considerations it appears that the oxygen atoms on this Rh surface find their optimal geometry by shortening and strengthening the O-Rh bond, resulting in a lower coordinated adsorption site, where a significant amount of negative charge is transferred. As a consequence, the resulting bond is covalent but with a high degree of ionicity, due to the charge transfer because of the electronegativity difference between Rh (µ ) 2.28) and O (µ ) 3.44) atoms.39 3.2. SCLSs. The formalism adopted for the full SCLS calculations has been described elsewhere.28 Briefly, the initial state shift is defined as the energy difference between the core eigenvalues c of bulk and surface atoms ∆cinit ) cbulk(nc) csurf(nc), where nc denotes the core state occupation number; although in a pseudopotential approach we do not have the core
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TABLE 2: Calculated Values (in meV) for the Rh3d5/2 SCLS, Initial State, and Screening Contribution in the p(2 × 2), c(2 × 2), (2 × 2)pg-SR, and (2 × 2)pg-AR Structures Formed by Oxygen on Rh(100)a structure clean
Rh atom
Rh0 II layer p(2 × 2) Rh1O II layer (1) II layer (2) II layer (3) c(2 × 2) Rh2O II layer (1) II layer (2) (2 × 2)pg-SR Rh2O-SR II layer (1) II layer (2) (2 × 2)pg-AR Rh1O-AR Rh2O-AR II layer (1) II layer (2)
full initial calculations state screening ∆Bd experiment -580 +140 -385 +55 +115 +35 -135 -65 +25 -120 -60 +15 -185 +140 -85 -65
-650 +80 -420 +40 +115 -25 -120 -95 +85 -125 -105 +60 -235 +140 -110 -10
+70 +60 +35 +15 0 +60 -20 +30 -65 +5 +45 -45 +50 0 +25 -50
-580 +100 -390 +90 +10 -5 -165 -35 +45 -155 -35 +40 -245 +10 -50 -40
-420
-200
-195 +115
a The inequivalent Rh first- and second-layer atoms are labeled as in Figure 3. The right end column reports the value (in meV) of ∆Bd, the calculated d-band center shift.
level one electron eigenvalue; the latter quantity can be estimated from the linear variation of the total energy when transforming a Rh atom into an excited one.28 The final state (or screening) contribution, originated from the different response of the bulk and surface valence electrons to the creation of the core hole, was determined as the difference between the calculated initial and full SCLS.40 The estimated numerical error in the SCLS determination is (20 meV. The final SCLS results are listed in Table 2 together with the initial and final state contributions. The discrepancy between experimental (-680 meV) and theoretical (-580 meV) SCLS for the clean surface has been explained in previous studies10 as due to the overlapping contribution of second layer atoms, predicted to be at +140 meV from the bulk and neglected in the curve fitting procedure. The inclusion of a further component in the fit to take into account deeper layer contribution produces a SCLS of -600 meV, considerably improving the agreement with the theoretical value. The calculated Rh1O-induced SCLS is -385 meV, thus supporting the hypothesis assumed in the experimental analysis that the intensity component found at -390 meV originates from atoms coordinated with one oxygen adsorbed in 4-fold geometry. The SCLS calculated values for Rh2O atoms in the c(2 × 2) structure (-136 meV) and in the (2 × 2)pg-SR geometry (-120 meV) are comparable and lower than the ones experimentally found at -200 meV. As shown above for the clean surface case, indeed neglecting the second layer components in the analysis yields an overestimation of the experimental shift. Moreover the small discrepancy between measured and calculated SCLS for the p(2 × 2) and c(2 × 2) structures originates most probably from the presence of antiphase boundaries and nonstoichiometry of the oxygen adlayer, as previously reported in the SPA-LEED measurements.13,14 The similarity of the features observed in the projected density of states of the c(2 × 2) and (2 × 2)pg-SR structures is interpreted as due to the similar geometry of these structures. The symmetric reconstructed surface presents a distortion of only 4% with respect to the regular square c(2 × 2) phase, which does not modify appreciably its electronic structure. In ref 15, it was found that the two structures are energetically equivalent. More precisely the (2 × 2)pg-SR is favored by only 3 meV with respect to the c(2 × 2), while in the case of the asymmetric
reconstruction the surface undergoes a more substantial distortion (around 11%) that definitively stabilizes the system. In the asymmetrical reconstructed (2 × 2)pg phase, the computed SCLS values for the two nonequivalent surface atoms Rh1O(AR) and Rh2O(AR) are -185 and +140 meV, respectively, compatible within the uncertainties with the measured shifts of -195 and +115 meV. Although the calculated SCLSs induced by the Rh1O(AR) atoms of the (2 × 2)pg-AR and by the Rh2O of the (2 × 2)pgSR (or in the c(2 × 2) phase) are similar, it is possible to distinguish between the two structures from the experimental results. Indeed, in order to achieve a good fit of the measured SCLS spectra an additional component has to be included in the analysis. The higher BE of this peak with respect to the bulk peak is not compatible with the symmetric reconstruction and can be related to the Rh2O-AR atoms only. The comparison with the theoretical calculations allowed us to explain also the shift of the bulk component Rhb reported in section 2. Indeed, while for the clean surface the omission of a second layer component (otherwise located at +140 meV) yields a bulk peak at higher BE, in the case of the oxygen saturated surface the two nonequivalent second layer atoms induce a CLS of -85 and -65 meV. Hence it is expected that the center of gravity of the deeper layer contribution would shift toward lower BE with respect to the clean surface case. This negative second layer induced core level shift can be interpreted as due to the second to first layer expansion, resulting in a slightly lower effective coordination number.10 A remarkable finding of our study is that even though Rh1O atoms are bonded to just a single oxygen atom both in the p(2 × 2) and in the (2 × 2)pg-AR structure the corresponding CLSs are very different. This is linked to the fact that, in the reconstructed geometry, the surface Rh atoms loose approximately twice the amount of electron charge, i.e., 0.07 e-. Furthermore, in this geometry the Rh1O-O bond length is slightly shorter. Both contributions affect the core levels, inducing a positive shift. The same arguments account for the different shifts of the Rh2O atoms in the c(2 × 2) and (2 × 2)pg-AR, while in the latter case the Rh surface atoms loose 0.17 e-, in the c(2 × 2) structure the charge decreases by 0.11 e-. Additionally, in the reconstructed surface the Rh2O bond length is only 2 Å, against 2.3 Å found for the SR structure. 4. Core Levels and d-Band Shifts Another relevant parameter characterizing the electronic structure of the investigated systems was calculated, i.e. the d-band energy center position Bd ) µ1/µ0, where µp ) ∫pni()d is the pth momentum of the DOS ni(E). The displacement of the surface atoms d-band center with respect to the bulk atoms is obtained as ∆Bd ) Bdbulk - Bdsurf. This value is reported in the right column of Table 2. In this way changes in d-band center can be closely compared with core level shifts in both initial and finale state. Indeed ab initio calculations give us the possibility of decomposing the full SCLS into initial and final state contributions, thus providing us with information experimentally inaccessible. Within the initial state picture, i.e., neglecting screening effects, the SCLS is due to the d-band narrowing generated by the reduced atomic coordination at the surface with respect to the bulk: indeed, in order to maintain an almost constant d-band filling, when more than half of the d band is full, as in the case of rhodium, its center shifts toward the Fermi level, as reported in the bottom panel of Figure 5, which shows the DOS projected onto the Rh0 and Rhb atoms.
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Figure 5. Calculated densities of states (states/eV) projected onto the 4d orbitals of the Rh atoms for all the nonequivalent surface atoms present in three of the adsorption structure represented in Figure 3: p(2 × 2), c(2 × 2), and asymmetric (2 × 2)pg-AR. The projections onto the 4d orbitals for the clean surface and bulk atoms are also plotted. The thick lines mark Bd, the d-band center position; the dashed line highlights the Fermi energy position.
Conversely, as a consequence of the oxygen adsorption, the d-band undergoes a broadening and thus the d-band center shifts downward. Figure 6a shows the initial state ∆cinit contribution plotted as a function of the calculated d-band shift ∆Bd for all the inequivalent surface and second layer atoms in the p(2 × 2), c(2 × 2), and (2 × 2)pg-AR structures. As explained in previous studies for different adsorption systems,28-30,40-42 the qualitatively linear trend highlights the correlation existing between these physical quantities, both originating from the change of the Kohn and Sham surface potential but sampled in the core and valence band, respectively. As for the oxygen and nitrogen adsorption on Rh surfaces,29,41 the core level shift toward higher binding energy can be explained mainly due to charge transfer from the surface to the electronegative adspecies, leading to a more attractive potential. The same trend holds for the full shift vs ∆Bd, reported in Figure 6b, being the final state contribution always less than 70 meV. This result is significant for understanding the chemical reactivity properties of the reconstructed phase. Indeed the Hammer-Nørskov model43,44 proposes to use the d-band center Bd as an indicator of the local surface chemical reactivity. Our study shows that the SCLS vs ∆Bd linear relationship holds also for the O-induced reconstruction of Rh(100), confirming that SCLS can be used to evaluate the surface chemical reactivity changes. 5. Conclusions In summary, we have demonstrated that SCLS measurements are sensitive to the local geometrical distortion and to the changes in the chemical environment caused by oxygen adsorption on Rh(100). The adsorption geometry drastically changes
Figure 6. Calculated initial CLSs (a) and SCLSs (b) plotted as a function of the d-band center displacement for all the nonequivalent first (circles) and second layers (squares) atoms of the four adsorptions structures reported in the top panel. Different colors correspond to nonequivalent atoms.
with increasing oxygen coverage, until formation of the (2 × 2)pg reconstruction at 0.5 ML. The atomic rearrangement requires a certain energy cost, which is largely compensated by the energy gained in the optimization of the oxygen-rhodium bond, so that the total surface energy decreases. Ab initio calculations make it possible to identify the origin of the different components contributing to the Rh3d5/2 core level spectra measured from the reconstructed phase. In the (2 × 2)pgAR structure, oxygen adsorbs off the rhombus center, thus generating two nonequivalent surface atoms, which are characterized by considerably different core level shifts, dominated by initial state effects. In addition, the theoretical approach allowed us to unravel the existing correlation between the experimentally observed SCLS and the electronic structure properties, in particular the d-band average energy displacement that can be used as a valid indicator of the local chemical reactivity. Acknowledgment. We acknowledge financial support from Sincrotrone Trieste SCpA and from Regione Friuli Venezia Giulia through the project “Catalizzatori nanostrutturati per la produzione di idrogeno e sperimentazione su prototipi di fuelprocessor”. A.B. acknowledges precious technical support from Angst-Pfister.
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