reduction of vanadium-oxide monolayer...

Reduction of vanadium-oxide monolayer structures

J. Schoiswohl, S. Surnev,* M. Sock, S. Eck, M. G. Ramsey, and F. P. NetzerInstitut für Experimentalphysik, Karl-Franzens-Universität Graz, A-8010 Graz, Austria

G. KresseInstitut für Materialphysik, Universität Wien, A-1090 Wien, Austria

sReceived 17 June 2004; revised manuscript received 29 November 2004; published 29 April 2005d

The reduction of vanadium oxide monolayer structures on Rhs111d has been investigated by variable-temperature scanning tunneling microscopy, low energy electron diffraction, photoelectron spectroscopy of thecore levels, and the valence band, and by probing the phonon spectra of the oxide structures in high-resolutionelectron energy loss spectroscopy. A sequence of oxide phases has been observed following the reduction fromthe highly oxidizedsÎ73Î7dR19.1° V-oxide monolayer:s535d, s533Î3drect, s939d, and “wagon-wheel”oxide structures are formed with decreasing chemical potential of oxygenmO. The structures have beensimulated byab initio density functional theory, and structure models are presented. The various V-oxidestructures are interrelated by common VuO coordination units, and the reduction progresses mainly via theremoval of VvO vanadyl groups. All oxide structures are stable at the appropriatemO only in the two-dimensional V-oxide/Rhs111d phase diagram and are thus stabilized by the metal-oxide interface. The resultsdemonstrate that oxides in ultrathin layer form display modified physical and chemical properties as comparedto the bulk oxides.

DOI: 10.1103/PhysRevB.71.165437 PACS numberssd: 68.47.Gh, 68.55.2a, 68.37.Ef, 71.15.Mb

I. INTRODUCTION

Ultrathin layers of metal oxides of thickness of one to afew unit cells, often designated as oxide nanolayers, displayparticular physical and chemical properties that are notshared by their respective bulk counterparts. These propertiesinclude new structural, stoichiometric, and bonding behavioras well as a significantly modified chemical reactivity. Mostof these new properties are caused by the high surface-to-volume ratio of these systems and are determined by theinteraction at interfaces to the environment or to dissimilarmaterials, which are necessary to support the nanostructuresin thin film form. In this study we have investigated thechemical and structural properties of metal oxide nanolayers,which are supported on a different metal surface, viz. vana-dium oxides on Rh surfaces. Vanadium oxides in bulk phasesare interesting materials because of the possibility of differ-ent oxidation states of the V atoms and the resulting wealthof structural and chemical behavior, which is associated withdifferent oxide phases due to the variable V cation valenciesand the different VuO coordination spheres.1 Vanadium ox-ides are components of important industrial catalysts for oxi-dation reactions,2 and in fact, vanadium is the most importantmetal used in metal oxide catalysis for the manufacture ofhigh-value chemicals and in environmental pollutioncontrol.3

Vanadium oxides in ultrathin film form have been inves-tigated recently on Pds111d and Rhs111d single crystalsurfaces.4–7As shown recently, a peculiar growth behavior asa function of oxide coverage and oxide layer thickness withseveral interface stabilized structures has been found onRhs111d. Here, this work was extended and we report a seriesof vanadium-oxide monolayer structures on a Rhs111d sur-face, which develops in the presence of reducing environ-mental conditions. This structure sequence is significantlydifferent from that observed during growth.

Vanadium-oxide nanostructures deposited by physical va-por deposition onto clean Rhs111d surfaces and subsequentlysubjected to reducing conditions, i.e., to annealing in vacuumor to the exposure of hydrogen, have been characterized byvariable-temperature scanning tunneling microscopysVT-STMd, low energy electron diffractionsLEEDd, photoelec-tron spectroscopy of core and valence states with use of syn-chrotron radiationsXPS, UPSd, and high resolution electronenergy loss spectroscopysHREELSd. The oxide surfacestructures and their development with the variation of thechemical potential of oxygenmO have also been simulatedby ab initio density functional theorysDFTd calculations. Wefind that a sequence of new V-oxide phases evolves from thehighly oxidized as-deposited structures with decreasingmO,which all are stable only in the quasi-2D limit of nanolayerfilms. Although the present work has been conducted as amodel study on a well-characterized idealized system, webelieve that it has more general implications for the practicalpurposes of oxide nanostructure fabrication.

II. EXPERIMENTAL AND COMPUTATIONALPROCEDURES

The experiments have been performed in three differentultrahigh vacuum chambers, with typical base pressuresp=1310−10 mbar. The STM and HREELS measurementshave been carried out in our laboratories in Graz, in twocustom-designed systems. The STM system is equipped witha variable-temperature STMsOxford Instrumentsd, a LEEDoptics, an CMA Auger electron spectrometersAESd, and theusual facilities for crystal cleaning and physical vapordeposition.4 The STM images were recorded in a constantcurrent mode at room temperature or at elevated temperature,with electrochemically etched W tips, which were cleanedinsitu by electron bombardment. The HREELS measurements

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were performed with an ErEELS 31 spectrometer, as de-scribed in Ref. 8. The HREELS spectra were taken at roomtemperature with a primary energy of 5.5 eV in specular re-flection geometry, with a typical resolution of,3.5 meV asmeasured at the FWHM of the reflected primary peak.

High-resolution photoemission measurements with use ofsynchrotron radiation were carried out at beamline I311 inthe Swedish synchrotron radiation laboratory MAX-lab inLund.9 The experimental end-station is equipped with a Sci-enta SES 200 hemispherical electron energy analyzer. Photonenergies of 620 eV and 110 eV have been employed for ex-citing V 2p core level and valence band spectra, respectively,with corresponding energy resolutions of 250 meV and100 meV. Clean Rhs111d surfaces were prepared by 1.5 keVAr+-ion sputtering, followed by annealing to 830 °C for sev-eral minutes, and by heating cycles in O2 followed by a finalflash to 800 °C. The cleanliness of the Rhs111d surfaces waschecked by AES, XPS or HREELS in combination withLEED. LEED was also used to control the preparation of therespective oxide surface structures in the different experi-mental systems. Vanadium-oxide films were prepared by re-active evaporation of V metal in 2310−7 mbar O2 onto theclean Rhs111d surface at 250 °C or 400 °C, and after depo-sition the sample was kept at these conditions for additional5 minutes; the sample was subsequently cooled down toT,100 °C in oxygen atmosphere to prevent the thermal re-duction of the oxide. The vanadium deposition rate wasmonitored by a quartz crystal microbalance and a rate of0.2 monolayer/min was typically employed. The V-oxidecoverage is given in monolayer equivalentssMLEd, where 1MLE contains the same number of V atoms as one mono-layer of Rhs111d atoms. The deposited V-oxide films weresubjected to reducing conditions by annealing in ultrahighvacuum or by exposure to a background pressure of H2 atelevated temperature. In both cases, the structural evolutionof the V-oxide overlayers has been followed directly in theVT-STM.

The DFT calculations were performed using the Viennaab initio simulation packagesVASPd.10 The interaction be-tween the valence electrons and the ionic cores was de-scribed by the projector augmented wave method in theimplementation of Kresse and Joubert,11 and the plane wavecutoff was set to 250 eV. Generalized gradient correctionswere applied throughout this work.12 Generally four layerthick slabs for the substrate andk-point grids correspondingto s838d points in the Brillouin zone of the primitive sur-face cell were used. The vibrational spectra of the consideredsurface oxides were calculated using finite differences. Tothis end, each atom in the oxide was displaced by 0.02 Å ineach direction, and the interatomic force constants were de-termined from the induced forces. The simulated STM im-ages were calculated in the Tersoff Hamann approach.13 Thecharge isosurfaces were evaluated at a value that the bright-est spots are located 4 Å above the core of the topmost atomin the surface oxide. For further calculational details we referto Ref. 7.

III. RESULTS

A. The highly oxidized „Î7ÃÎ7…R19.1° vanadium-oxide layer

The reactive evaporation of vanadium on to the Rhs111dsurface as described in the experimental section generates a

highly oxidized vanadium-oxide overlayer at submonolayercoverages. The STM image of Fig. 1sad shows island struc-tures of thesÎ73Î7dR19.1° V-oxide phase, which partiallycover the Rhs111d surface at a coverage of,0.25 MLE.Figure 1sbd displays a high resolution STM image of thesÎ73Î7dR19.1° structure, which shows a hexagonalhoneycomb-type network of bright protrusions; asÎ73Î7dR19.1° unit cell is indicated on the image. This unitcell is also superimposed on the LEED pattern of this sur-face, shown in Fig. 1scd. The DFT calculations have estab-lished the model of thesÎ73Î7dR19.1° structure,7 that isillustrated in Fig. 1sdd. According to this model theÎ7 struc-ture corresponds to a V3O9 oxide phase and contains identi-cal pyramidal O4VvO building blocks fsquares in Fig.1sddg, which have the V atom in the center, four bridging Oatoms in the basal plane, and a vanadyl-type O atom at theapex. The pyramids are linked together via the four bridgingO atoms in the basal plane as shown in the model, eachÎ7unit cell is formed by three of these formal VO3 units. Notethat these formal VO3 units are not in conflict with the maxi-mal oxidation state of +5 of the V atoms, since the peripheralfour bridging O atoms are shared with the Rh substrate. For-mal electron counting procedures are not applicable to ametal surface, but a simple argument would be as follows:each V atom donates two electrons to the double bondedvanadyl O, and three electrons to the four surrounding Oatoms in the basal plane. To fill the 2p shell of these interfa-cial O atoms, 0.5 electron is missing, that must be providedby the Rh surface.

The structure model of this highly oxidized V-oxide phaseis confirmed by the HREELS phonon spectrumssee bottom

FIG. 1. sColor onlined sad STM image of thesÎ73Î7dR19.1°vanadium oxide on Rhs111d s8003800 Å2; tunneling conditions:sample biasU= +2 V; tunneling currentI =0.05 nAd. sbd High-resolution STM image of thesÎ73Î7dR19.1° V-oxide s15315 Å2; U= +0.75 V,I =0.2 nAd. A unit cell and the Rhs111d sub-strate direction is indicated.scd LEED pattern of the sÎ73Î7dR19.1° V-oxide on Rhs111d selectron energy 50 eVd. sdd DFTderived model of thesÎ73Î7dR19.1° vanadium oxide. Unit celland structural units are indicated.

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curve in Fig. 4d, where the characteristic stretching vibra-tions of the vanadyl VvO group at 128 meV along with thevibrations of the bridging O atoms at 67 meVsRef. 7d areclearly discerned. It is necessary to emphasize that thesÎ73Î7dR19.1° phase is a stable structure in the V-oxide/Rhs111d phase diagram, but that it exists only in the singlelayer limit, where it is stabilized by the Rhs111d interface.

B. Reduction sequence of the„Î7ÃÎ7…R19.1°vanadium-oxide structure

Exposing the as-depositedsÎ73Î7dR19.1° V-oxide layerto H2 at elevated temperature leads to a transformation into asequence of reduced structures. Figure 2 shows a series ofSTM images, starting from the 0.25 MLEsÎ73Î7dR19.1°on Rhs111d surfacefFig. 2sadg, which have been recordedconsecutively at 400 °C in 1310−9 mbar H2; the timeelapsed during the reduction process is indicated on the im-ages. The reduction begins with the introduction of disorderinto the sÎ73Î7dR19.1° structure—see Fig. 2sbd. After4530 s hydrogen exposure a new ordered structure appears,which is characterized by as535d unit cell fFig. 2scdg. Thisstructure is transformed into as533Î3d-rectangular struc-ture after 6250 s reduction timefFig. 2sddg, which in turngives way to as939d structure after 17 350 s of reduction

fFig. 2sedg. The final, most reduced V-oxide phase in thissequence is formed after 22 850 s hydrogen exposurefFig.2sfdg: it is a complicated structure, which is designated as“wagon-wheel” structure as explained below. Note that thedescribed sequence of structures has also been observed byannealing in UHV from 250°C–800 °C. The various struc-tures will be presented and discussed in more detail in Sec.III C.

The V 2p XPS spectra confirm the reduction of theV-oxide phases upon H2 exposure at elevated temperature. InFig. 3sad V 2p3/2 XPS traces of thesÎ73Î7dR19.1°, thes533Î3d-rect, thes939d, and the “wagon-wheel” structuresare compared. The V 2p photoemission lines shift progres-sively from 515.4 eV for thesÎ73Î7dR19.1° structure tolower binding energy, ending up at 513.2 eV for the “wagon-wheel” structure. The V 2p photoemission lines are broadand exhibit complicated shapes due to many-body and con-figuration interaction effects,14,15 we therefore refrain from apeak decomposition analysis of the spectra. Moreover, it isdifficult to associate the absolute core level binding energiesof the cations with the oxidation state in ultrathin oxide lay-ers by comparison with the respective core level binding en-ergies measured on oxide bulk samples, because the influ-ence of the interfacial bonding and the proximity of theunderlying metal surface modifies both the initial and finalstate in the photoemission process. This can lead to errone-ous assignments of the formal oxidation state and of thestoichiometry as discussed in previous work.4,16 However,the shifts of the V 2p core level peaks to lower bindingenergy as apparent in Fig. 3 are clear indications of the pro-gressive reduction of the V-oxide phases.

The valence band spectra of the various V-oxide struc-tures are collected in Fig. 3sbd. The intensity in the spectralregion betweenEF and 4 eV binding energy is due to pho-toemission from the overlapping V 3d and Rh 4d states.However, since the photon energy of 110 eV is at the Cooperminimum of the Rh 4d photoionization cross section,17 the V3d contribution to the valence band should be dominant. Theregion from 4 eV to 8 eV is due to the O 2p valence bands.The V 3d intensity at the Fermi level increases in going fromthe sÎ73Î7dR19.1° structure to the “wagon-wheel” phaseffrom bottom to top in Fig. 3sbdg, with a sharp peak at the

FIG. 2. sColor onlined STM images taken consecutively duringreduction of the sÎ73Î7dR19.1° vanadium-oxide phase onRhs111d at T=400 °C,pH2

=1310−9 mbar. The cumulative reduc-tion time is indicated on each frames8003800 Å2,U= +2 V,I=0.1 nAd.

FIG. 3. sad V 2p3/2 XPS core level spectra of various V-oxidephases on Rhs111d. sbd Corresponding valence band spectra.

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Fermi level in the latter structure. This is due to the increas-ing V 3d density of states as a result of the progressivereduction of the oxide phases. The spectral changes in the O2p valence band region accompany the structural changes,but have less diagnostic value apart from testifying the pres-ence of oxygen at the surface. Thus, the prominent feature at,6 eV in the valence band of the “wagon-wheel” structureindicates that this most reduced structure still contains sig-nificant amounts of oxygen.

The HREELS phonon spectra provide very specific fin-gerprints of the different V-oxide structuressFig. 4d. As men-tioned above thesÎ73Î7dR19.1° structure is characterizedby the vanadyl VvO stretching frequency at 128 meV anda vibrational loss peak at 67 meV due to vibrations involvingthe bridging O atomssdensity functional theory predicts di-pole active modes at 137 and 65 meV,7 in good agreementwith the experimental datad. The s533Î3d-rect HREELSspectrum still contains the vanadyl loss peak, albeit at re-duced intensity and shifted to a somewhat lower loss energyof 124 meV, and more complex loss structures at 52 meVand 72 meV, the latter with a shoulder at,80 meV. Thereduced intensity of the vanadyl loss suggests a reduction ofvanadyl groups in thes533Î3d-rect structure, and this isimportant experimental input for the construction of structuremodels in the DFT simulations. In contrast, thes939d phasedisplays no vanadyl loss peak, but the two broader loss struc-tures at,54 meV and,72 meV. The “wagon-wheel” struc-ture shows a simple vibrational loss spectrum with one singlepeak at 62 meV and perhaps a weak lower-energy feature ataround 40 meV. The HREELS spectra thus indicate that thereduction of thesÎ73Î7dR19.1° V-oxide progresses via theloss of vanadyl VvO units, as discussed in the followingsection.

C. The reduced V-oxide structures:„5Ã5…, „5Ã3Î3…-rect, „9Ã9…, “wagon-wheel”

STM images of thes535d V-oxide phase are displayed inFigs. 5sad and 5sbd. Figure 5sad shows well-ordered islandsof the s535d structure at a nominal surface coverage of 0.15MLE vanadium deposition. Figure 5scd shows the LEED pat-tern of the corresponding surface. The high-resolution imageof Fig. 5sbd reveals that bright maxima are linked together ina hexagonal honeycomb-type arrangement, so that a darkdepression is seen at the corners of the unit cellsindicated onthe imaged. Note that the bright spots in between thes535d islands represent planar V6O12 oxide clusters whichhave been addressed in detail in a previous publication.18

The DFT derived model of thes535d structure in Fig.5sdd discloses the structural details, hexagonal V6O12 unitsscircle on the figured are linked to O4VvO ssquare on thefigured and O3VvO striangle on the figured units yielding aV11O23 stoichiometry pers535d unit cell. The overall oxidestoichiometry is thus reduced from formally VO3 in thesÎ73Î7dR19.1° phase to VO2.09 in thes535d structure. Theinset of Fig. 5sbd shows a simulated STM image as calcu-lated with the model structure, the agreement with the ex-perimental image is excellent and confirms the structuremodel.

The s533Î3d-rect V-oxide structure is illustrated in Fig.6. The STM images of Figs. 6sad and 6sbd show nanometerdimensions533Î3d-rect islands on the Rhs111d surface cor-responding to a nominal coverage of 0.25 MLE and a high-resolution view of this structure, respectively. This V-oxidephase forms well-ordered islands of rectangular shape, whichare oriented along thek11I0l symmetry directions of the Rhsubstrate. The sharp LEED pattern of this surfacefFig. 6scdgconfirms the high structural order. Thes533Î3d-rect unitcell, indicated in Fig. 6sbd, contains a bright maximum in the

FIG. 4. HREELS phonon spectra of various V-oxide phases onRhs111d. Vertical bars indicate density functional theory predicteddipole active modes.

FIG. 5. sColor onlined sad STM image of thes535d V-oxide onRhs111d s100031000d Å2, U= +2 V, I =0.02 nAd. sbd High-resolution STM image of thes535d V-oxide s40340 Å2, U= +2 V, I =0.02 nAd. The inset shows a DFT simulated STM im-age.scd LEED pattern of thes535d V-oxide on Rhs111d selectronenergy 50 eVd. sdd DFT model of thes535d V-oxide. Unit cell andstructural units are indicated.


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center and dark holes at the corners. According to the DFTmodel fFig. 6sddg this structure contains hexagonal V6O12scircle on the figured and pyramidal O4VvO ssquare on thefigured with a unit cell content of V13O21, yielding a formalVO1.6 stoichiometry. The STM simulationfinset of Fig. 6sbdgreveals that the bright maximum in the unit cell is due to thevanadylsVvOd group of the tetragonal pyramidal structureunit. The presence of vanadyl groups is also confirmed bythe vibration at 124 meV in the HREELS spectrumsFig. 4d.Similar to thes535d structure, thes533Î3d-rect structureconstitutes a mixed-valent V-oxide with V atoms in differentoxygen coordination spheres. The DFT calculated dipole ac-tive vibrational frequencies are located at 130, 71, 55, and33 meV, with an intensity distribution which resemblesclosely the experimental ones. Additionally, a weak shoulderis calculated at 84 meVswhich is also visible in the experi-mental datad. These frequencies are in excellent agreementwith the experimental data, which is a further confirmationfor the suggested modelfas for thesÎ73Î7dR19.1° phase,the frequency of the VvO stretch mode is somewhat toohighg.

STM images of thes939d V-oxide layer are presented inFigs. 7sad and 7sbd. Thes939d islands display more roundedboundaries and less well-defined geometrical shapes, how-ever the sharp LEED patternfinset in Fig. 7sadg indicates ahigh structural order within the islands. The high-resolutionSTM imagefFig. 7sbdg shows circles of maxima with a de-pression in the center, which are arranged in a hexagonalfashion. The centers of four hexagons define thes939d unitcell as indicated on the figure. The HREELS spectrum of thes939d structuresFig. 4d reveals that vanadyl groups are ab-sent in this structure. Based on the previous experience withV-oxide structures on Rhs111d a structural model is shown inFig. 7scd. The model involves hexagonal V6O12 units scircleson the figured and no pyramidal structures, and a V36O54

content of thes939d unit cell is derived, which correspondsto a formal V2O3 stoichiometry. The V 2p3/2 core level peakat 514 eV binding energy is compatible with the formal V3+

oxidation state in V-oxide nanolayers.4 The model was re-laxed using DFT calculations, and the final STM simulationfor tunnelling into empty states between 0 and 1 eV is shownin the inset of Fig. 7sbd. The agreement with experiment isexcellent, every single detail is reproduced leaving littledoubt about the correctness of the model.

The phase with the lowest oxidation state in the reductionsequence of V-oxide nanolayers on Rhs111d is examined inFig. 8. The STM imagessad andsbd illustrate the morphologyand the atomic details of the structure, respectively, whichhas been designated as “wagon-wheel” structure. The namehas been chosen because of the resemblance of the brightarray of maxima in the STM imagesfFig. 8sbdg with the huband spokes of a wagon-wheel—a sketch of such a wagon-wheel is drawn on the image. The hubs of the “wagon-wheel” form a hexagonal lattice with a unit cell periodicityof 18.9±0.2 Å fmarkedA on the image 8sbdg, which is ro-

FIG. 6. sColor onlined sad STM image of thes533Î3d-rectV-oxide on Rhs111d s100031000 Å2, U= +2 V, I =0.1 nAd. sbdHigh-resolution STM image of thes533Î3d-rect V-oxide s33333 Å2, U= +2 V, I =0.1 nAd. The inset shows a DFT simulatedSTM image.scd LEED pattern of thes533Î3d-rect V-oxide phaseselectron energy 50 eVd. sdd DFT model of the s533Î3d-rectV-oxide. Unit cell and structural units are indicated.

FIG. 7. sColor onlined sad STM image of thes939d V-oxide onRhs111d s200032000 Å2, U= +2 V, I =0.05 nAd. The inset showsthe LEED pattern of thes939d phaseselectron energy 50 eVd. sbdHigh-resolution STM image of thes939d V-oxide s60360 Å2,U= +2 V, I =0.1 nAd. The inset shows a DFT simulated STM im-age.scd DFT model of thes939d V-oxide. Unit cell and structuralunits are indicated.

FIG. 8. sColor onlined sad STM image of the “wagon-wheel”V-oxide on Rhs111d s100031000 Å2, U= +2 V, I =0.1 nAd. sbdHigh-resolution STM image of the “wagon-wheel” V-oxides80380 Å2, U= +2 V, I =0.1 nAd. A “wagon-wheel” and two unitcellsA,B are drawn on the image.scd LEED pattern of the “wagon-wheel” phaseselectron energy 60 eVd. sdd Autocorrelation diagramof the STM imagesbd. Unit cellsA andB are indicated.


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tated by 21° ±1° with respect to thek11I0l directions of theRh substrate. The periodicities of this structure are recog-nized best in an autocorrelation representation of the STMimage as displayed in Fig. 8sdd. The measured unit cell di-mensions are very close to as737dR21.8° overlayer, sug-gesting a commensurate superlattice. This is compatible withthe observed LEED patternfsee Fig. 8scdg, which is consis-tent with thes737dR21.8° superstructure. The autocorrela-tion image reveals that around each corner maximum of theunit cells A, a smaller hexagon of maxima appears, whichindicates a hexagonal basis lattice of the structure with unitcell distances of 3.1±0.2 Å labelledB in Figs. 8sbd and 8sdd.This sublattice has only moderate long range order as indi-cated by the wavy lines in the autocorrelation image. Thedata suggest that the concept of a Moiré pattern might be atthe root of these experimental observations. Superimposingthe Rh substrate lattice withaRh=2.69 Å and the V-oxidelattice with aVOx=3.1 Å and rotating the latter by 3.5° withrespect to the Rhk11I0l direction yields 18.8 Å and 21.8° forthe periodicity and rotation, respectively, of the Moiré pat-tern. This mirrors very closely the experimental observationsof the “wagon-wheel” lattice. The V 2p3/2 binding energy of513.2 eV together with the significant amount of oxygenpresent at the surface suggests an oxidation state of 2+ forthe V atoms. The HREELS spectrumsFig. 4d shows a singleloss peak at 62 meV and is similar to the loss spectrumfound for the so-called surface-V2O3 phase on Pds111d.4 Thelatter structure has V atoms at the VuPd interface and is Oterminated towards the vacuum side. The V density of the“wagon-wheel” structure, with 0.73 MLE for a full mono-layer, is however higher than that of the surface-V2O3 phases0.5 MLEd. A further experiment provided additional hintsfor the stoichiometry of the “wagon-wheel” phase, the struc-ture has been obtained on the Rhs111d surface after deposi-tion of 0.5 MLE of V atoms onto the O-precoveredRhs111d231-O structure sO coverage 0.5 monolayerd.Taken together, these data suggest that the “wagon-wheel”phase corresponds most likely to a VO stoichiometry, with Vatoms at the VuRh interface and O atoms at the vacuumside.

Overlayers with “wagon-wheel”-type structures seem tobe a more general phenomenon and have also been observedin other systems, e.g., in the bimetallic Cr/Pts111d system19

or in the V-oxide/Pds111d sRef. 6d and Pd-TiO2s110d metal/oxide systems.20 Following the model of Bennettet al. inRef. 20 for the so-called “pinwheel” superstructure of Pd onTiO2s110d swhich is very close to the “wagon-wheel” struc-ture observed hered the geometric model of Fig. 9 has beenderived.d

In Fig. 9sad a superlattice withaOXIDE=3.1 Å, rotated by3.5° with respect tok11I0l, has been superimposed on theRhs111d lattice. The Moiré pattern obtained by emphasizingoverlayer atoms, which are in or close to on-top and bridgepositions, with thicker lines reflects the brightness distribu-tion in the experimental STM image of Fig. 8sbd. This is alsoillustrated in the billiard ball representation of Fig. 9sbd,where the adatoms in on-top and bridge positions on theRhs111d lattice are drawn with the stronger color tone,whereas those in or close to threefold hollow positions are

colored less bright. The “wagon-wheel”-type pattern evolvesin remarkably good agreement with the experimental STMcontrastfFig. 8sbdg. Drawing on previous experimental andtheoretical evidence of V-oxides on Pds111d and Rhs111dsurfaces4,6,7 we presume that the maxima in the STM imagesreflect the positions of the V atoms; these correspond tospheres drawn in Fig. 9sbd. The oxygen atoms required forthe VO model stoichiometry are located on top of the three-fold hollow sites of the V layer, but they are not included inFig. 9sbd for the clarity of the presentation. Our model of theVO “wagon-wheel” structure suggests thus a VOs111d over-layer; this oxide layer is V terminated at the interface to theRh and oxygen terminated at the surface to the vacuum. Theoxygen termination at the outer surface is stable for allV-oxide layers on Pds111d and Rhs111d surfaces, but the Vtermination at the metal-oxide interface has only been foundfor the more reduced oxide structures.4–6,21 The wagon-wheel structure was also simulated using DFT. To estimatethe lattice constant of an ultrathin VuO overlayer on Rh,four and three VO unitsfsÎ33Î3d structureg were placed ontop of a Rhs232d slab. The three VO units, with a VOin-plane lattice constant of 3.14 Å, are found to be energeti-cally preferred over an epitaxial VO overlayer with a latticeconstant matching the Rhs111d surface. For the final model-ling of the wagon-wheel structure, 37 VO units were placedon top of as737dR21.8° Rh substrate, and the model wassubjected to a gentle annealing between 800 and 500 K for3 ps. The final structure and the STM simulation are shownin Figs. 9scd and 9sdd For this particular structure, the agree-ment with experiment is admittedly only modest, which wetend to relate to a slight misplacement of the oxide layer withrespect to the substrate. In fact, the STM image is very sen-sitive to small variations of the surface oxide, STMsimulations of structural models obtained by relaxing snap-shots of the finite temperature molecular dynamics showedsignificant variations in the long range undulations of theSTM simulations, although the surface energies of all modelswere within 5 meV. With the present computational re-

FIG. 9. sColor onlined sad and sbd Billard ball models of the“wagon-wheel” V-oxide structure. A “wagon-wheel” is indicated insbd. scd DFT model of the “wagon-wheel” V-oxide.sdd DFT simu-lated STM image.


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sources a more careful determination of the epitaxtial rela-tionship between the surface oxide and Rh was however notpossible.

Finally, for the four phases considered in the DFTcalculations, the calculated densities of states are collectedin Fig. 10; the O, V, and Rh projected DOS are shownas solid, dashed, and dotted lines, respectively. For thesÎ73Î7dR19.1° structure, the most pronounced feature isthe double peak at 3 and 3.5eV, which corresponds to theexperimental peak at 4.5 eV. Inspection of the ion decom-posed DOS reveals that it is related to the vanadyl group.A similar double peak was also observed in the valenceband of the vanadyl terminated V2O3 surface, both inexperiment22 and in DFT calculations.23 In DFT, the doublepeak is slightly shifted towards lower binding energies,which is a common problem related to the present densityfunctionals. Furthermore, a pronounced peak at higherbinding energiess6 eVd is visible in the calculated DOS,corresponding to the main peak in the experimental valenceband spectrum at 6.5 to 7 eV. It is noted that the vanadiumstates hybridize significantly with the O2p levels, with theonset of the antibonding VuO2p hybrid orbitals locatedwell above the Fermi level for thesÎ73Î7dR19.1° structures+1.5 eVd.

For the s533Î3d-rect structure, the VvO double peakis still present but only as two weak shoulders at 3 and3.5 eV sin the experimental spectrum this peak is not re-solvedd. For thes939d structure, the VvO related doublepeak has vanished, and the main oxygen 2p related

peak has gained in intensity and sharpness and isshifted somewhat towards lower binding energies. The onsetof the V states shifts progressively towards higher bindingenergies in excellent agreement with the experimental obser-vations. Overall, the DFT calculations resemble the mea-sured spectra remarkably well, although the valence bandspectra alone would be insufficient to construct structuralmodels.

IV. CONCLUSIONS

In summary, a sequence of new vanadium-oxidephases has been detected on Rhs111d upon reduction ofthe highly oxidized sÎ73Î7dR19.1° V-oxide overlayer,which is formed up to monolayer coverages on theRhs111d surface after reactive evaporation of V metalin oxygen atmosphere at 250°C–400 °C substratetemperature.7 The structures have been investigated experi-mentally by variable-temperature STM, LEED, photoelec-tron spectroscopy with use of synchrotron radiation,and HREELS phonon spectroscopy, and have been analyzedtheoretically by DFT calculations. The sequence of structuresobserved is sÎ73Î7dR19.1°→ s535d→ s533Î3d-rect→ s939d→ “wagon-wheel” with decreasing chemicalpotential of oxygenmO ssee Fig. 11d.

The formal oxidation state of the V atoms decreasesfrom ,5+ in the sÎ73Î7dR19.1° structure to 2+ in the“wagon-wheel” phase. The structures in this reductionsequence are connected by the evolution of commonVuO coordination building units, with the reductionproceeding mainly by a progressive loss of vanadyl VvOgroups. The here observed phases are stable at the appropri-ate mO only in the two-dimensional V-oxide/Rhs111dphase diagram, they are thus all stabilized by the oxide-Rhs111d interface.

Ultrathin layers of oxides on metal single crystal surfacesconstitute excellent model systems to study oxide materialsin nanometer dimensions at the atomic level, under con-trolled conditions. The results of this work indicate thatmetal oxides in nanolayers exhibit new properties in terms ofstructure and stoichiometry, with new bonding configurationsand particular metal-oxygen coordination spheres. In combi-nation with noble metal surfaces as support materials oxidenanolayers may show new chemical properties, with en-hanced reactivity towards reducing or oxidizing conditions.This is an important aspect for the promotion effect of metaloxides in heterogeneous catalysis. Although the presentstudy has model character, the results may have more generalimplications for the fabrication of nanostructured oxide sys-tems.

FIG. 10. DFT derived atom resolved densities of states of vari-ous V-oxide phases on Rhs111d. The shaded region refers to theVvO related density of states. Solid lines, oxygen; dashed lines,vanadium; dotted lines, rhodium in the topmost surface layer.

FIG. 11. Sequence of vanadium-oxide monolayer structureswith their unit cell stoichiometries forming on a Rhs111d surface asa function of the chemical potential of oxygenmO.


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ACKNOWLEDGMENTS

This work has been supported by the Austrian ScienceFoundation,fDOCg the PhD program of the Austrian Acad-

amy of Sciences and by the EU-TMR Programme underContract No. ERB FMGE CT98 0124. The support of theMAX-Lab staff during the synchrotron radiation experimentsis gratefully acknowledged.

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reduction of vanadium-oxide monolayer...

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