Surface Science xxx (2015) xxx–xxx

SUSC-20450; No of Pages 8 March 21, 2015; Model: Gulliver 5

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Surface Science

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Chemical properties of two-dimensional oxide systems: Adsorption of(WO3)3 clusters on CuWO4

Liying Ma a, Martin Denk a, David Kuhness a, Svetlozar Surnev a, Venu Mankad a, Giovanni Barcaro b,Alessandro Fortunelli b, Falko P. Netzer a,⁎a Surface and Interface Physics, Institute of Physics, Karl-Franzens University, A-8010 Graz, Austriab CNR-ICCOM & IPCF, Consiglio Nazionale delle Ricerche, via G. Moruzzi 1, I-56124 Pisa, Italy

⁎ Corresponding author at: Institute of Physics, Karl-FrGraz, Austria.

http://dx.doi.org/10.1016/j.susc.2015.03.0060039-6028/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: L. Ma, et al., Chemic(2015), http://dx.doi.org/10.1016/j.susc.2015

a b s t r a c t

a r t i c l e i n f o

Available online xxxx

Keywords:Two-dimensional oxideCu-tungstateTungsten trioxideOxide clusterScanning tunneling microscopyDensity functional theory

A two-dimensional ternary oxide layer, Cu-tungstate (CuWO4) on Cu(110), has been tested as a substrate for theadsorption of (WO3)3 clustermolecules from the gas phase. Scanning tunnelingmicroscopy/spectroscopy and X-ray photoelectron spectroscopy have been used to characterize the adsorbed species, while density functionaltheory has scanned the energy landscape for stable adsorptionmodes. The (WO3)3 clusters adsorb in preferentialadsorption sites at low temperature (85 K) as stable, only slightly perturbed units, as established both experi-mentally and theoretically. The preferred sites are the Cu surface rows in the [100] substrate direction,which fea-ture elastic flexibility as a result of ultrasoft phonon modes and are a characteristic of this two-dimensionalternary oxide structure.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The search for two-dimensional (2-D) materials is an increasinglyactive area of research in advanced materials science and in the designof newmaterials for nanotechnology. Indeed, the discovery of graphenein 2004 has prompted a veritable hype in the study of 2-D materialssuch as graphene and materials beyond graphene [1], the latter com-prising, for example, the transition metal dichalcogenides or boron ni-tride. Most 2-D materials presented so far are based on so called vander Waals solids, that are layered solids characterized by strong intra-layer but weak inter-layer bonding. If deposited on a substrate, suchvan der Waals-type layers feature typically weak overlayer–substrateinteractions, since the 2-D layers are covalently bond saturated withinthemselves. Ultrathin film oxides and (quasi-) 2-D oxide systems arealso actively studied since the past decade [2]. In contrast to van derWaals layered materials, the interaction between oxide overlayers andmetal support surfaces is however often strong, in particular if transi-tionmetal oxides are involved. This strong oxide–metal interaction cre-ates a new combined system, an oxide–metal hybrid system, withproperties that are not shared by the individual constituents [3,4].Recently, we have reported the formation of a novel 2-D ternary oxidesystem in the form of a Cu-tungstate single-polyhedron thick layer ona Cu(110) substrate and have described its structural and electronicproperties [5]. In the present work, we address the surface chemical

anzens University Graz A-8010

al properties of two-dimensio.03.006

aspects of this CuWO4/Cu(110) hybrid system, using the adsorption of(WO3)3 clusters as probe molecules.

The cyclic (WO3)3 may be regarded either as a small oxide cluster oras an oxide molecule. It has been investigated in its pristine state in thegas phase by photoelectron and vibrational spectroscopies and its cyclicstructure has been established by density functional theory (DFT) calcu-lations [6–8]. (WO3)3 molecules feature an interesting chemical bond-ing state, which has been described as d-level aromaticity [7]. In theirview as ultra-small oxide clusters, (WO3)3 supported on oxide surfaceshas been used as a platform for model catalyst studies [9]. (WO3)3 sup-ported on TiO2 (110) shows excellent catalytic activity for the dehydra-tion and dehydrogenation of alcohols [9,10] and for the polymerizationof formaldehyde [11,12]. In contrast, (WO3)3 supported on a 2-DFeO(111) bilayer on Pt(111) failed to display catalytic activity in thepartial oxidation of alcohols [13]. This inactivity was interpreted asdue to the dissociation of (WO3)3 as a consequence of the strong clus-ter–surface interaction, resulting in WO3 surface monomers, whichlack the necessary coordination sites for the dissociative adsorption ofalcohols. The study of the adsorption of (WO3)3 on nanostructuredCu–O surfaces at low temperature revealed that on the clean Cu(110)patches the molecules accommodate in an undistorted fashion, butthat on the Cu–O(2 × 1) surface oxide reconstruction both moleculesand the Cu–O substrate become significantly distorted due to the strongaffinity of W atoms to the surface oxygen [14]. Theoretical DFT analysishas shown that adsorption is accompanied by a significant mixing ofadsorbate-substrate electronic states and an appreciable charge flowfrom the support to the oxide molecules [14]. The charge state and the

nal oxide systems: Adsorption of (WO3)3 clusters on CuWO4, Surf. Sci.

2 L. Ma et al. / Surface Science xxx (2015) xxx–xxx

interaction of (WO3)3 with MgO/Ag(100) films have also been investi-gated by first principle molecular dynamics calculations by Zhu et al.as a function of the thickness of the MgO film [15]. Charge transferfrom theAg substrate through the ultrathinMgO overlayers leads to un-distorted geometries of the charged (WO3)3 anions, whereas on thickMgO films the electron tunneling from the substrate is inhibited andcovalent bond formation with concomitant strong distortions of themolecular (WO3)3 units has been predicted [15]. In this context, theCuWO4/Cu(110) phase seems an interesting substrate to consider.This phase shares with the MgO/Ag(100) and FeO/Pt(111) systems a2-D character, but exhibits a higher work function than MgO/Ag(100)and is structurally more robust than FeO/Pt(111), at the same timedisplaying a strongly anisotropic arrangement which should be favor-able in terms of anchoring the ligand species.

Here we thus use the adsorption of (WO3)3 clusters as probes, bothto gauge the electronic and chemical properties of the 2-D Cu-tungstatelayer on Cu(110) and to investigate its functioning as a substrate inheterogeneous catalysis. We have employed scanning tunnelingmicroscopy (STM) and spectroscopy (STS) to study adsorptiongeometry and adsorbate electronic states and X-ray photoelectronspectroscopy (XPS) to determine the chemical state of the adsorbed(WO3)3 units; DFT calculations allow us to project the experimentalresults onto the electronic properties and surface chemical behavior ofthe Cu-tungstate surface. For the latter, the comparison of the presentresults with corresponding data on the Cu–O(2 × 1) surface oxide isuseful.

We find that the (WO3)3 clusters adsorb as individual entities on theCuWO4/Cu(110) phase. The experimentally preferred adsorption sitespresent two brighter STM spots aligned along the [100] directions ofthe substrate, at two different distances from the characteristically elon-gated STM rows of the substrate, with one distance slightly favored. STScharacterization suggests a weak cluster/substrate electronic interac-tion, while XPS indicates that the tungsten atoms in the clusters havea slightly higher oxidation state with respect to the 2-D CuWO4 phase.The CuWO4/Cu(110) phase seems to be an excellent substrate forsupporting catalytically active nanoscopic species in heterogeneous ca-talysis, with an interaction sufficiently strong to immobilize the (WO3)3clusters but sufficiently weak not to perturb their basic chemical andstructural characteristics.

The article is organized as follows. In Section 2 we provide experi-mental and computational details. Results are presented and discussedin Section 3, while Section 4 summarizes our main conclusions.

2. Experimental and theoretical details

The STM/STS experiments have been performed in a low-temperature STM system (Createc, Germany) in ultrahigh vacuum witha base pressure of ~5 × 10-11 mbar [14], operated for the measurementspresented here with liquid N2 in the cryostat stage, yielding a sampletemperature of ~85 K. Sample surface cleaning and preparation and insitu STM tip treatments were performed in the preparation chamber,which is equipped with a sample transfer manipulator, a LEED optics, amass spectrometer, and the usual provisions for surface cleaning andthinfilm evaporation. Electrochemically etchedW tips have been treatedin situ by electron bombardment heating and by field emission via volt-age pulses. STS spectroscopy (differential conductance dI/dV versusV)was performedwith a lock-in techniquewith a 1.175 kHzmodulationfrequency and a typical 50mVmodulation amplitude. The voltages givenare sample potentials with respect to the tip. XPS spectra were recordedwith the use of synchrotron radiation at beamline I311 in the Swedishsynchrotron radiation laboratory MaxLab.

The clean Cu(110) surfacewas prepared by cycles of Ar ion sputteringat 700 eV and annealing in UHV at 770–820 K. After oxidation at 520–570 K for 3 min in 7.5 × 10−9 mbar O2, the surface shows a sharp(2 × 1) LEED pattern of the Cu–O surface oxide reconstruction.The well-ordered Cu-tungstate (CuWO4) surface phase was prepared

Please cite this article as: L. Ma, et al., Chemical properties of two-dimensio(2015), http://dx.doi.org/10.1016/j.susc.2015.03.006

by deposition of ~1 monolayer (ML) of (WO3)3 clusters onto theCu–O(2 × 1) surface at room temperature followed by annealing at570 K. The (WO3)3 cluster molecules have been generated by vacuumsublimation of WO3 powder at ~1300 K in a thermal evaporator [16–18]andhave been directed in amolecular beamonto the Cu–O(2×1) surface(note that 1 ML (WO3)3 is defined here somewhat loosely as a denselypacked layer of (WO3)3 clusters, estimated from molecular dimensionsto contain ~2.5 × 1014 clusters/cm2). The subsequent annealing step in-duces a 2-D surface chemical reaction to yield the ordered Cu-tungstatemonolayer (CuO + WO3 → CuWO4). For the adsorption studies, the(WO3)3 clusters have been deposited at 85 K onto the sample surface onthe cooled manipulator and transferred cold into the STM cryostat.

Fig. 1 specifies the CuWO4 surface layer: panel (a) shows a LEED pat-tern and panels (b,c) STM images, while in (d) theDFT derived structuremodel is presented [5]. The LEED pattern displays sharp reflectionscharacteristic of a well ordered surface structure; the (2 × 1) unit cellof the Cu–O substrate and the rhombic unit cell of the CuWO4 are indi-cated. The CuWO4 layer is strictly speaking incommensurate to the Cu–O substrate, but can be approximatedwith only aminor distortion of thelattice parameters by a (5 × 2) overlayer structure, which simplifies thetheoretical analysis [5]. The large scale STM image (Fig. 1b) shows thatthe Cu-tungstate forms a perfectwetting layer on the Cu surface,where-as the high resolution STM image of Fig. 1(c) gives the atomic details ofthe structure. The DFT calculated model structure of the 2-D Cu-tungstate (Fig. 1d) reveals that the structure consists of a trilayer slab,with anO layer at the interface to the Cu substrate, aW layer in themid-dle and amixed Cu–O layer forming the outer surface [5]. It is crucial forthe discussion of adsorption results in the next section to recall thatsome of the Cu atoms in the topmost layer aremoreflexible than others.In the (5 × 2) structural model, this happens specifically in one Cu rowevery four aligned along the [100] direction (highlighted by red dottedlines in Fig. 1d) and is due to the difference in the epitaxial relationshipswith the underlying Cu(110) surface; it is reverberated in the STMimages as a characteristic elongation of bright spots along the [100]direction. Clearly, since the (5 × 2) structural model representsonly an approximation to the real situation, in the experimentalSTM the periodicity of these features occasionally deviates from the1:4 ratio.

All calculations are performed at the density-functional theory(DFT) level using the plane-wave Quantum Espresso (QE) package[19]. The Perdew–Burke–Ernzerhof (PBE) exchange-correlation func-tional [20] is used together with PAW (Projector Augmented Wave)pseudopotentials (PP) [21], taken from the QE website with the excep-tion of theWone, developed for the investigation of Ref. [5]. Energy cut-offs of 40 Ry and 240 Ry are used for the selection of the planewaves fordescribing the wave functions and electronic density, respectively.Structural optimizations are carried out in a spin-restricted formalismby applying a Gaussian smearing of 0.002 Ry. The superlattice for the(5 × 2) overlayer is constructed by employing a rectangular cell (withdimensions 12.9, 7.3 and 15.5 Å, respectively, in the three Cartesian di-rections) containing an asymmetric Cu(110) slab composed of two fro-zen bottom Cu layers plus two active ones, in addition to the topmostCuWO4 layer. The Brillouin-zone integration is performed using aMonkhorst-Pack kpoint sampling with a 2 × 4 × 1 k-point mesh. Thestructural optimizations were carried out in a spin-restricted formalismuntil the forces on atoms resulted smaller than 0.01 eV Å-1.

In order to simulate the adsorption of (WO3)3 units, the original(5 × 2) cell has been doubled along its shorter direction to give a(5 × 4) cell (thus extending in size to 12.9 × 14.6 Å2 in the xy plane).An empty space region of about 15 Å has been inserted in order toavoid interaction between replicated units. As in Ref. [5], core-electronenergies of the W atoms were evaluated as in Ref. [22] by consideringexplicitly only the valence electrons and creating a core-hole in the 4flevels. As a scalar relativistic approachwas applied in the present inves-tigations, the theoretical values of core-level energies should be com-pared with the spin-orbit average of the 4f7/2 and 4f5/2 experimental

nal oxide systems: Adsorption of (WO3)3 clusters on CuWO4, Surf. Sci.

(a) (c)(b)

Cu-Tungstate

(d)

[1 0]1[ ]001

[110]

[001]

[110]

[001]

Fig. 1. The2-D CuWO4phase: (a) LEEDpattern (Ep=73eV); the Cu–O(2×1) (yellow) and rhombic CuWO4 (red) unit cells are indicated. (b,c) Large scale and high-resolution STM imagesof the CuWO4 surface (tunneling conditions: (b) 800 × 800 Å2; −0.95 V; 0.085 nA. (c) 100 × 100 Å2; −0.97 V; 0.085 nA). (d) DFT derived structure model, viewed from two differentdirections, in which the ‘flexible’ Cu rows aligned along [100] directions of the substrate are highlighted with red dotted lines. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

3L. Ma et al. / Surface Science xxx (2015) xxx–xxx

peaks. STM imageswere simulated via a Tersoff–Hamann approach [23]as constant-height projected-density-of-states plots at 2 Å above theheight of the topmost surface atoms.

3. Results and discussion

The adsorption of (WO3)3molecules on the CuWO4 surface has beeninvestigated at 85 K. Fig. 2 shows STM images of a low coverage of(WO3)3 (~0.01 ML), recorded with different tunneling conditions. The(WO3)3 adspecies appear as triangular units, but the detailed shapes de-pend on the tunneling conditions. In Fig. 2(a), recorded with a positivesample bias of +0.59 V, the atomic geometry of the substrate is clearlyapparent and the (WO3)3 display two bright protrusions at the upperpart of the triangles and, somewhat separated by a horizontal line, asmaller, more diffuse protrusion at the lower apex (note that the scan-ning direction in this image is from top to bottom). This indicates thatthe admolecules move under the influence of the STM tip while beingimaged. In Fig. 2(b), the two (WO3)3 units at the top of the imagehave a more symmetric triangular shape, while the admolecule at thebottom right obviously has become unstable under the tip. Higher pos-itive bias voltages at similar tunneling currents generated more stableimages suggesting that the tip influence is a mechanical effect: themol-ecules are pushed away under the scanning tip suggesting a weak ad-sorbate–substrate interaction. At negative biases, the tip effects weregenerally more pronounced and high-resolution images were difficultto obtain (not shown in the figures). Fig. 2(c) gives a magnified viewof two (WO3)3 molecules, showing that the two lobes at the upperright hand side have a somewhat brighter contrast than the lower leftlobe. This is confirmed by the line scan in Fig. 2(d), taken along thedashed line in image (c). The STM size of the (WO3)3 admolecule asmeasured from Fig. 2(d) is ~5 Å between themaximawith an apparentheight of ~1 Å. This is compatible with the (WO3)3 molecules stayingintact after adsorption, as measured previously [17,14,13].

Please cite this article as: L. Ma, et al., Chemical properties of two-dimensio(2015), http://dx.doi.org/10.1016/j.susc.2015.03.006

Fig. 3 contains two STM images, which show a larger field of view oftwo areas of the CuWO4 surface covered with individual (WO3)3 mole-cules. It appears that the (WO3)3 species adsorb along preferred lines ofthe substrate structure, as indicated by the dashed lines, suggestingpreferential adsorption sites. These lines can be identified with the“flexible” Cu lines of the CuWO4 surface structure, as discussed below.Fig. 4 shows a statistical evaluation of the adsorption positions of the(WO3)3 clusters in the form of a bar diagram (~200 cluster positionsevaluated): positions A are on the abovementioned Cu lines, positionsB are on other sites (see STM insert). There is a clear preference for ad-sorption on sites along the A lines (73% of clusters are on A sites), al-though such a difference in population at 85 K corresponds to anestimated difference in adsorption energy of the order of only 7 meV.

A thorough scan of possible adsorptionmodes was performed at thetheoretical level by positioning the clusters in various sites of the unitcell, considering various interaction modes with the substrate andperforming local geometry relaxations. It is useful to catalogue the var-ious possibilities resulting from this scanby defining: (a) the orientationand positionwith respect to the unit cell, and (b) the adsorption config-uration of the (WO3)3 clusters. This is schematically depicted inFig. 5(a) and (b), respectively. The clusters typically adsorb with threebottom apex oxygens interactingwith topmost Cu atoms on the surface.In terms of orientation and position with respect to the unit cell, thesetriangles of oxygen atoms can have one side perpendicular, slightly ro-tated (intermediate) or parallel to the [100] direction, and can be locat-ed in two non-equivalent positions of the unit cell, either closer orfarther from the flexible Cu lines, thus giving rise to 6 possibilities —as depicted in the three columns of Fig. 5(a). In terms of cluster struc-ture, the cluster can adsorb in three configurations: (i) with the planeof W atoms parallel to the substrate surface and practically identical tothe gas-phase geometry — ‘tripod’ mode, (ii) with one of the bottomapex O atoms rotated (tilted) and pointing parallel to the surface sothat the W atom bound to it can interact with one of the O atomsfrom the substrate — ‘tilted’ mode, and (iii) with the bottom apex O

nal oxide systems: Adsorption of (WO3)3 clusters on CuWO4, Surf. Sci.

(a)

(d) (c)

(b)

Z[Å

]

X[nm]

1

0.8

0.6

0.4

0.2

0

10.80.60.40.20 1.2 1.4 1.6

[110]

[001]

[110]

[001]

[110]

[001]

Fig. 2. STM images of (WO3)3 cluster molecules (0.01 ML) adsorbed on CuWO4 at 85 K: (a) 88 × 88 Å2; +0.59 V; 0.085 nA. (b) 50 × 50 Å2; +1.5 V; 0.085 nA. (c) 40 × x 40 Å2; +0.90 V;0.082 nA. (d) Line scan along dashed line in (c).

4 L. Ma et al. / Surface Science xxx (2015) xxx–xxx

atom further rotated and the top apexO atom also rotating into a bridgeposition— ‘collapsed’mode. These 3 adsorption configurations are illus-trated in the three columns of Fig. 5(b). It is important to underline inconnection with the following discussion that in configuration (iii) thestructural rearrangement involves also one of the Cu atoms from the

(a)

[110]

[001]

Fig. 3.Adsorption of 0.01ML (WO3)3 on CuWO4 at 85 K, revealing preferential adsorption sites:indicate the “flexible” Cu rows.

Please cite this article as: L. Ma, et al., Chemical properties of two-dimensio(2015), http://dx.doi.org/10.1016/j.susc.2015.03.006

surface underneath the (WO3)3 cluster, which moves from the topmostlayer to the CuWO4/Cu(110) interface— see the right-hand-most columnof Fig. 5(c). This is possible and energetically allowed because in our DFTapproach the energy difference between locating Cu atoms in the top-most layer or at the CuWO4/Cu(110) interface is not large: we can recall,

(b)

[110]

[001]

(a) 200 × 200 Å2;+0.1 V; 0.085 nA. (b) 200 × 200 Å2;+0.5 V; 0.085 nA. The dashed lines

nal oxide systems: Adsorption of (WO3)3 clusters on CuWO4, Surf. Sci.

0

20

40

60

80

A

Clu

ster

dist

ribut

ion

(%)

B

A

B

[110]

[001]

Fig. 4. Histogram of (WO3)3 adsorption sites, on A lines (flexible Cu rows) and elsewhere(B). Insert: STM image specifying A and B: (200 × 200 Å2; +0.5 V; 0.085 nA).

Fig. 5. (a) Schematic depiction of the three possible orientations and positions of the (WO3)3 clutriangles joining the bottom three apex O atoms; (b) atomistic configurations of the (WO3)3 clustmiddle column, (iii) collapsed— right-hand-most column (color coding: W atoms in gray; O atotypical isomers, with tripod, tilted and collapsed modes in the same columns as in (b) — simula+0.59 V bias of the configurations illustrated in (c)— the dotted red lines highlight theflexible Cuand blue (oxygen) for clarity. (For interpretation of the references to color in this figure legend,

5L. Ma et al. / Surface Science xxx (2015) xxx–xxx

Please cite this article as: L. Ma, et al., Chemical properties of two-dimensio(2015), http://dx.doi.org/10.1016/j.susc.2015.03.006

for example, that in an isomer of the bare CuWO4/Cu(110) phase, inwhich one of the Cu atoms is ‘misplaced’ at the interface, lies at only0.33 eV above the global minimum fully symmetrical structure —

see Fig. S5(b) in the Supporting Information of Ref. [5]. We have simulat-ed the STMof the CuWO4defect coming fromsuch a ‘misplaced’Cu atom:it corresponds to a dark depression in theposition of themissing Cu atom.As this kind of defect is never experimentally observed on the bare sur-face, we can affirm that DFT underestimates its formation energy(0.33 eV), and as a consequence, we can also affirm that DFT underesti-mates the formation energy of the collapsed adsorption mode (iii).

The major conclusion of this structural scan is that the energy land-scape resulting from the DFT scan is somewhat flat. First, for each givenorientation and adsorption configuration the energy difference betweentwo different positions in the unit cell (farther or closer to the flexiblelines) is minor, always ranging below 0.03 eV, i.e., below the intrinsic ac-curacy of DFT. This is in tune with experiment, in which the difference inpopulations corresponds to an estimated energy difference below0.01 eV.Second, adsorptionenergies of thedifferentmodes are in the order: tripod~0.5 eV b tilted ~1.0 eV b collapsed ~1.5 eV, where the adsorption energyis defined as the energy difference between the fragments (substrate andtungsten oxide cluster) and composite system, all in their relaxed (fullyoptimized) geometries. These adsorption energy values are appreciably

sters in the (5 × 4) unit cell of the CuWO4/Cu(110) phase: the clusters are schematized asers in the three possible adsorptionmodes: (i) tripod— left-hand-most column, (ii) tilted—

ms in red, Cu atoms in orange); (c) side views of complete atomistic configurations of threeted W 4f core level energies are also indicated; (d) top views and simulated STM images atlines along [100] directionswhile the atoms in the clusters are colored in yellow (tungsten)the reader is referred to the web version of this article.)

nal oxide systems: Adsorption of (WO3)3 clusters on CuWO4, Surf. Sci.

6 L. Ma et al. / Surface Science xxx (2015) xxx–xxx

smaller than those on the Cu(110)-O(2 × 1) substrate, in which they cantypically exceed 3 eV. This explainswhy the clustersmaymove under theinfluence of the STM tip, when the tip is close to the surface, while suchbehavior was not observed on the Cu–O(2 × 1) substrate [14].

The cluster/surface interaction is thus weak, with the tripod andtilted modes basically resembling analogous adsorption configurationsfound for the bare Cu(110) surface, see Fig. 6(c) in Ref. [14], in whichthe cluster/substrate interaction was also weak. The collapsed moderepresents an exception, because in this adsorptionmode the structuralrearrangement is more pronounced and the adsorption energy is some-what larger. However, if the energy difference between the global min-imum of the bare CuWO4 phase and the isomer configurationwith a Cuatom moved from the topmost layer to the interface is underestimatedby our DFT approach, as argued above, and considering the similaritybetween the collapsed mode and this isomer configuration of the barephase, we can expect that also the stability of the collapsed mode isoverestimated by our DFT approach. It is thus possible that the adsorp-tion energy of the tilted mode (lower in energy than the tripod modebut without any large atomistic restructuring) is actually larger thanthat of the collapsed mode, and the tilted mode indeed corresponds tothe lowest-energy physical situation.

Three representative configurations with full atomistic details aredepicted in side views in Fig. 5(c), and correspond to: tripod — left-

A

B

(WO ) cluster3 3

(a)

(c)

[110]

[001]

[110]

[001]

Fig. 6. STS spectra of bare CuWO4 (2 locationsA and B) and of (WO3)3, in the bias ranges (b)−1the spectra have been taken. a: (50 × 50 Å2; −0.917 V; 0.085 nA); c: (40 × 40 Å2; +0.9 V; 0.0

Please cite this article as: L. Ma, et al., Chemical properties of two-dimensio(2015), http://dx.doi.org/10.1016/j.susc.2015.03.006

most column; tilted —middle column; and collapsed — right-most col-umn. The corresponding top views of the atomistic structures togetherwith simulated STM images are shown in Fig. 5(d). The STM imagescan be easily interpreted by noting that the bright STM spots are in-variably associated with top apex O atoms of the (WO3)3 cluster [14],so that the resulting STM images are determined by the height ofthese O atoms along the axis perpendicular to the surface. The Oatoms result brighter for the following reasons: at positive biases,STM samples the system's empty states, which are mainly localizedon the oxidized W atoms; nevertheless, empty states are also foundon the O atoms due to an incomplete reduction of them. Further-more, as the tails of the STM currents are spatially sampled, alsothe relative height of the atoms is important to detect signals; asthe O atoms are higher with respect to the W atoms of about 1.6 Å,the tip finds a strong signal from the tails of the empty states of theO atoms in a region where the tails of the W atoms have already dis-appeared. Thus, in the tripod configuration one finds three spots ofapproximately identical size, whereas in the tilted and collapsedconfigurations one finds two brighter spots and a dimmer one. It isnoted parenthetically that in simulations, STM images are presentedat constant height rather than at constant current as in the experi-ment, which amplifies the intensity of bright spots and leads to un-derestimate the intensity of the other ones.

(d)

(b)

(WO ) cluster3 3

AB

(WO ) cluster3 3

AB

to+1V and (d)−2 to+2V. The STM images (a) and (c) specify the positions, fromwhere82 nA).

nal oxide systems: Adsorption of (WO3)3 clusters on CuWO4, Surf. Sci.

7L. Ma et al. / Surface Science xxx (2015) xxx–xxx

The electronic energy level structure of the adsorbed (WO3)3 clustermolecules has been probed by differential conductance STS (dI/dV vs.V) spectra. Fig. 6 contains STS spectra of the bare CuWO4 surface, takenat two different locations as marked in (a), and on top of the (WO3)3unit (c). STS spectra (b) cover the bias range from−1V to+1V,whereasspectra (d) show an extended range from−2 V to+2 V. The STS curvesfrom the clean CuWO4 surface display moderately pronounced spectralstructure in the range (−1 to +1) V (Fig. 6b), but on the two differentlocations A and B the spectral structure is not too different, apart fromsome variations in intensity. In the extended −(-2 to +2) V range(Fig. 6d), the darker spot B has a peak in the filled states at -−1.46 V,which is not apparent on the brighter A lines. The STS from the (WO3)3cluster displays a number of features in the (−1 to +1) V range, whichare similar to the ones on the bare surface, albeit with higher intensity(Fig. 6b). In the extended (−2 to +2) V range, two additional pro-nounced structures are observed at −1.75 V and +1.70 V, which arevery similar to what has been seen in STS of the individual clusters onthe Cu(110) surface [14] and which have been identified as HOMO–LUMO of the intact and slightly negatively charged (WO3)3 molecules(~0.3 electrons). Theoretical analysis confirms this picture: we noticethat for both tripod and tilted adsorption modes the partial density ofstates of the interacting system is very similar to the sumof the separatedfragments, in line with the low adsorption energies of the two configura-tions. According to our analysis, there are no new states appearing in theHOMO–LUMO gap. This is consistent with STS features mostly due to thebare surface, except the two additional STS peaks at −1.75 V and+1.70 V characteristic of HOMO–LUMO of the individual clusters.

Fig. 7 compares the W 4f core level spectra of the clean CuWO4 sur-face, of ~0.4 ML (WO3)3 adsorbed on the CuWO4 surface, of ~0.4 ML(WO3)3 adsorbed on the Cu–O(2 × 1) surface oxide, and of a thick con-densed layer (13 ML) of (WO3)3 (from bottom to top). The W 4f7/2core level emission of the condensed film of (WO3)3 clusters (topcurve) is measured at a binding energy of 35.9 eV, which is typical forthe 6+ oxidation state of the W atoms in the cluster [24,25], whereasin the CuWO4 phase, the 4f7/2 binding energy is reduced to 34.5 eV as aresult of its particular 2-D character (bottomcurve) [5]. The (WO3)3mol-ecules adsorbed on the CuWO4 surface feature a W 4f7/2 binding energyof 35.4 eV — see Fig. 7, second spectrum from the bottom, which hasbeen decomposed into the spectral components of CuWO4 and (WO3)3

42 41 40 39 38 37 36 35 34 33 32

0.4ML (WO3)3

on Cu-O(2×1)×3

×0.1

×1.5

Inte

nsity

[a.u

.]

Binding energy [eV]

0.4ML (WO3)3

on CuWO4

clean CuWO4

13ML condensed (WO3)3

Fig. 7.W4f core level spectra (photon energy hν=140 eV) of the clean CuWO4 surface, of0.4 ML (WO3)3 adsorbed on CuWO4, of 0.4 ML (WO3)3 adsorbed on Cu–O(2 × 1), and of athick film (13 ML) of condensed (WO3)3 (from bottom to top). (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of thisarticle.)

Please cite this article as: L. Ma, et al., Chemical properties of two-dimensio(2015), http://dx.doi.org/10.1016/j.susc.2015.03.006

(the latter is the red curve). The W 4f7/2 binding energy of (WO3)3adsorbed on the Cu–O(2 × 1) surface is found at 34.7 eV (Fig. 7, secondspectrum from top). Assuming that the final state screening effects inXPS are similar on the CuWO4 and the Cu–O(2 × 1) surfaces – both are2-D surface oxides on a Cumetal substrate – the significantW4f bindingenergy difference of 0.7 eV of adsorbed (WO3)3 indicates a quite differentbonding and charge transfer situation on the two substrates. It appearsthat the (WO3)3 on CuWO4 retains largely its molecular character andoxidation state.

XPS core level energies were theoretically predicted for several con-figurations: in Fig. 5 those for the three representative adsorptionmodes are explicitly reported. We recall that we use a scalar relativisticHamiltonian, so that the predicted value for the tungsten core level en-ergies should be compared with a weighted average of 4f7/2 and 4f5/2experimental peaks. For the bare CuWO4 surface we predict a value of35.5 eV [5], which is in very good agreement with the experimental re-sult. For the adsorbed clusters we find significant differences among thedifferent adsorption modes. For the tripod the XPS energies practicallycoincide with the gas-phase values. This is not surprising, given that inthis mode the (WO3)3 cluster is essentially undistorted with respect tothe free molecule. For the tilted configuration, the W atom bonded tothe tilted O presents an increased energy of 36.0 eV. This is easily ex-plained by noting that this tungsten atom is coordinated to 5 O atoms(two bridge, two apex and one from the substrate) rather than 4 Oatoms (two bridge and two apex) as in the gas phase. The increase incore level binding energy by 0.5 eV with respect to the W atoms in thesubstrate, from 35.5 to 36.0 eV, is consistent with the experimentaltrend, although reduced by roughly a factor of 2. It can be noted thatthis shift is not associated with an increased oxidation state (the Lowdincharges are pretty uniformasnoted above) but rather to afinal state effectdue to the coordinated oxygen neighbors. Finally, in the collapsed config-uration all the three W atoms exhibit a value of XPS energy very close tothat of the CuWO4 phase. In this configuration, the O-coordination of theW atom bound to a tilted oxygen decreases due to the O atom further ro-tating into a bridge binding mode. This reduced value is in disagreementwith experiment, further casting doubts on the accuracy of our DFT ap-proach in estimating the relative stability of the configurations involvinga Cu atom at the interface. Our hypothesis is therefore that the experi-mentally observed (WO3)3 clusters adopt a tilted configuration, possiblyin the same orientation and position as the collapsed ones but withoutthe restructuring observed in the DFT relaxation.

4. Conclusions

The adsorption of (WO3)3 cluster molecules on a 2-D ternary oxideCuWO4 phase, grown on a Cu(110) surface, has been studied experi-mentally by STM, STS and XPS, and theoretically by DFT calculations.At low temperature (85 K), the (WO3)3 clusters adsorb on preferentialsites on the CuWO4 surface, which have been identified by STM andDFT simulations as Cu surface rows, displaying a “flexible” elastic behav-ior via ultrasoft phononmodes along the [100] direction [5]. The (WO3)3clusters at low coverage adsorb as individual, only slightly perturbedunits, as evidenced by STM, STS and XPS, and this is fully confirmed byDFT modeling. The energy landscape derived from the computationalstudy shows a variety of adsorption modes, with weak to moderate in-teraction energy (up to 1.5 eV) and energetically preferred adsorptionsites, whose positions and simulated STM images are approximately intunewith experimental STMmeasurements. Although the (WO3)3 clus-ters rearrange somewhat upon adsorption, they can still be recognizedas individual entities and essentially preserve their original electronicfeatures. The comparison between experimental and predicted XPScore level energies suggests that the theoretically most stable adsorp-tion mode, in which a more pronounced restructuring of both the(WO3)3 cluster and one of the Cu atoms in the substrate takes place,may be an artifact of the DFT modeling.

nal oxide systems: Adsorption of (WO3)3 clusters on CuWO4, Surf. Sci.

8 L. Ma et al. / Surface Science xxx (2015) xxx–xxx

The CuWO4/Cu(110) 2-D phase seems to be a promising, roughlyinert substrate for the deposition of oxide clusters. The (WO3)3 clustersremain essentially intact units, thus preserving their tungstyl W_Ogroups, which have been advocated as important for catalytic oxidationactivity.

Acknowledgments

Thiswork has been supported by the ERCAdvancedGrant SEPON, bythe FWF project P26633-N20 and by the EU COST ActionCM1104. FPNacknowledges the excellent hospitality of Zdenek Dohnalek during hisresearch stay at the Pacific Northwest National Laboratory, Richland,WA, USA.

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