The Surface Structure of the Metallic Sodium Tungsten Bronze Na0.667WO3(001)

Surface Science 460 (2000) 277291www.elsevier.nl/locate/susc

The surface structure of the metallic sodium tungsten bronzeNa0.667WO3(001)

F.H. Jones a,*, K. Rawlings a, R.A. Dixon a, T.W. Fishlock b, R.G. Egdell aa Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK

b Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK

Received 7 January 2000; accepted for publication 29 April 2000

Abstract

Scanning tunnelling microscopy has been used to identify a number of surface reconstructions on the (001) surfaceof the cubic metallic sodium tungsten bronze, Na0.667WO3. Which is dominant has been found to depend criticallyon sample preparation. As well as a (E2E2)R45 reconstruction that bears a striking similarity to that of theparent material, tungsten trioxide, regions of (21) periodicity are observed that can only be explained in terms ofan Na

yO surface layer. In the current work, we relate the effect of sample preparation on the surface electronic

structure of Na0.667WO3(001) with that on the atomic structure by comparing photoemission spectra with STMimages. Particular interest is focused on band gap defect states in photoemission spectra which, in contrast to similarstates in spectra from WO3, do not appear to correlate with the appearance of localised defects or highly reducedterraces in STM images. The existence of peroxide-like oxygen dimers at the (22) reconstructed surface, on theother hand, is characterised by the appearance of identifiable states in the valence band spectrum. 2000 ElsevierScience B.V. All rights reserved.

Keywords: Low index single crystal surfaces; Scanning tunnelling microscopy; Surface defects; Synchrotron radiation photoelectronspectroscopy; Tungsten oxide; Visible and ultraviolet photoelectron spectroscopy

1. Introduction exposure at elevated temperatures to atomichydrogen [1]. The resulting (21) monohydridesurface is rendered chemically inert by terminationThe close relationship between surface atomic

structure, electronic structure and reactivity is well of all available Si dangling bonds by hydrogenatoms. Defects in the hydrogen layer result inappreciated. In particular, surface defects are

known to play an important role and under opti- highly active dangling-bond sites that controladsorption and overlayer growth (see for examplemal circumstances may be used to tailor the sur-

face, thereby controlling its reactivity. For example Ref. [2]). Scanning tunnelling spectroscopy (STS)has demonstrated the radical difference inthe (100) surface of silicon can be passivated byelectronic structure associated with each type ofsite (unreacted SiMSi dimers, reacted dimers and

* Corresponding author. Present address: Department of dangling bond defects) [3].Biomaterials, The Eastman Dental Institute for Oral Health

For transition metal oxides, much less is under-Care Sciences, University College London, 256, Grays Innstood about the relationship between structure,Road, London WC1X 8LD, UK. Fax: +44 (0)207 915 1246.

E-mail address: [email protected] (F.H. Jones) surface electronic states and resultant reactivity.

0039-6028/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.PII: S0039-6028 ( 00 ) 00573-2

278 F.H. Jones et al. / Surface Science 460 (2000) 277291

Most work has concentrated on TiO2; the low which varies from semiconducting at low x throughto highly metallic for x>>0.25 [6 ]. The bulkenergy (110) surface displays a wealth of surface

chemistry and atomic structure depending on structure of the parent binary oxide has been wellstudied and is based on a framework of cornersample preparation and history [4]. The tungsten

oxides WO3 and NaxWO3 offer an alternative sharing WO6 octahedra. Strictly speaking thestructure is monoclinic at room temperature [7]prototype system of oxides (with bulk electronic

structure ranging from insulating, through n-type (a=7.927 A, b=7.539 A, c=7.688 A and b=90.91) due to slight tilting and distortion of thesemiconducting to highly metallic), whereby sur-

face reactivity as a function of surface structure octahedra. For ease of visualisation, however, itcan be thought of in terms of an idealisedand defect type can be explored. Furthermore,

technological applications of these materials in ReO3-like structure with cubic lattice parametera#3.7 A. The introduction of sodium results in afields such as gas sensing and photochromic dis-

plays will depend critically on the sensitivity of the series of different structures for low doping levels[8]. Once x is greater than ~0.42, the sodiumsurface chemistry and electronic structure to

atomic structure and oxygen vacancy or impurity atoms occupy a fraction of the 12-coordinate inter-stitial sites in the WO3 framework, forming thedefects.

Systematic doping of the 5d0 transition metal cubic perovskite structure (Fig. 1) [9]. Again, rota-tion and distortion of the WO6 octahedra meanoxide WO3 with sodium results in a series of

compounds of variable stoichiometry NaxWO3 that this structure is not perfectly cubic at room

temperature, but is slightly tetragonally distorted(0

279F.H. Jones et al. / Surface Science 460 (2000) 277291

by X-ray diffraction varies linearly with the sodium O 2pW 5d bulk band gap. A number of groupshave observed weak emission in the gap [1722],doping level, obeying the Vegard relationship: a=which has been attributed to a plasmon loss satel-3.7845+0.0821x A [11].lite [21]. However, it has been noted that the bandWhilst X-ray and neutron diffraction studiesgap structure varies immensely with sample prepa-have rigorously determined the bulk structure ofration, as would be expected for states arisingthe sodium tungsten bronzes, elucidation of thefrom surface defects. Several STM studies of tung-true surface structure by diffraction techniques hassten bronze surfaces have already been reportedproven problematic. The major complication arises[2325]. Again, sample preparation is found tofrom the fact that termination of the bulk structureinfluence strongly the observed surface structure.at the surface could be in a plane of stoichiometryUnder certain preparation conditions, aeither WO2 or NayO (where y is not necessarily (E2E2)R45 reconstruction very similar to thatequal to the bulk sodium concentration x). Theof the WO3(001) surface is predominant [24].surface unit cell dimensions would be the same inDespite the similarity of the materials, however,either case. Low energy electron diffractionthe specific surface structures of WO3(001) which(LEED) can be used to determine whether therelate to the band gap electronic states are notsymmetry of the surface structure is the same asobserved on Na0.667WO3 (001); the band gapor reduced from that of the bulk structure.states appear to have different origins in the met-However, it is impossible on the basis of diffractionallic and semiconducting materials. Furthermore,spot patterns alone to resolve unequivocally thethe tungsten bronze surface displays regions ofatomic-scale nature of the terminating layer.(12) and (21) periodicity which have no ana-UHV-STM (ultrahigh vacuum based scanninglogue in the sodium-free binary oxide. This recon-tunnelling microscopy) is an ideal technique withstruction has been rationalised in terms of surfacewhich to examine this problem at a fundamentalperoxide species [25]. In light of this interpretation,level. The technique has already been applied tothe current paper addresses the surface structure

tungsten trioxide [1215]. As in the case of otherof the sodium tungsten bronze (001) surface in

metal oxides, such as TiO2 [4], the observed surface greater detail. We present a combined STM andstructure is found to be highly sensitive to sample photoemission study of Na0.667WO3(001) surface,preparation within the vacuum chamber. The highlighting the relationship between (21) sur-oxygen-annealed surface mainly displays a face geometry and electronic structure.(E2E2)R45 reconstruction (based on the ide-alised ReO3-like cubic unit cell ), while (22) and(11) periodicities are observed as the surface isreduced, for example by annealing in UHV. As 2. Experimentalwell as determining the regular periodic structureof the surface, STM is able to probe the defect A single (001) oriented slice (11 cm2) of astructure; unquestionably of paramount impor- sodium tungsten bronze crystal was used for bothtance in determining surface chemistry and the STM and photoemission experiments. Theelectronic structure. Even optimally prepared (001) crystal was grown by the electrolytic reduction ofsurfaces of oxygen-annealed WO3 contain trough- a molten mixture of WO3 and Na2WO4 [26 ] andlike defects related to oxygen deficiency and its composition (x=0.667) determined from thereduced W(V ) ions. Recent work has been able lattice parameter as measured by X-ray diffraction.to relate specific surface structures observed in the The orientation of the growth face was determinedSTM images of the WO3(001) surface to electronic by Laue back diffraction. Prior to insertion intodefect states observed in photoemission spectra UHV the crystal was mechanically polished using[16 ]. successively finer grades of diamond paste down

Photoemission studies of NaxWO3 have long to 0.25 mm particle size. Traces of polishing mate-

rial were removed by ultrasonic cleaning in highindicated the presence of electronic states in the


purity acetone, isopropanol and finally distilled developed to explain the (E2E2)R45 recon-water. struction on WO3 is based on an analysis of theSTM was carried out in an ion and turbomolec- formal ionic charges of the WO2 and O layers ofular pumped UHV system (base pressure the idealised cubic structure (Fig. 2c). To avoid81011 mbar) equipped with an Omicron STM the infinite surface energy associated with a repeat-microscope stage, Omicron rear view LEED optics ing dipole normal to the surface, the surface con-and a twin anode X-ray gun and Omicron cylindri- sists of a WO2 layer, with half the tungsten ionscal sector analyser for XPS measurements. The carrying an on-top oxygen ion [12]. Atomisticsample was mounted on a Ta plate using Ta clips simulations [14] indicate that the most energeti-and heated radiatively in UHV via a tungsten cally favourable arrangement of the on-top oxygenfilament mounted at the rear of the crystal. Sample ions is a (E2E2)R45 reconstruction (Fig. 2d),temperatures were monitored using a chomel consistent with the STM results. High sodiumalumel thermocouple attached to the sample content tungsten bronzes, such as that imaged inmount. STM tips were fabricated by electrochemi- Fig. 2b, are metallic and strictly speaking cannotcally etching 0.25 mm diameter tungsten wire in be considered in terms of charged ionic planes.NaOH(aq) (2 M) and were degassed at 600C in However, the similarity of the STM images is sovacuo prior to use. striking that it has been proposed that the

Photoemission spectra were recorded on (E2E2)R45 reconstruction of the (001) faceBeamline 4.1 of the SRS at Daresbury, UK. The of Na0.667WO3 has the same structure as that ofspectrometer is housed in a single UHV chamber sodium-free WO3.and is equipped with a Scienta SES200 hemispheri- A serious complication in the interpretation ofcal electron energy analyser with angular accep- STM images of metal oxides is the distinctiontance of ~10. Sample cleaning was effected by between atoms of different elements. It should becycles of argon ion bombardment (1 keV, 810 mA

clarified at this juncture that the bright maxima insample drain current) and annealing in oxygen

the STM images of both materials are attributedpartial pressures (~5106 mbar) via electronto the on-top oxygen ions, rather than the barebeam heating the back of the sample until sharptungsten ions of the uppermost WO2 plane [12,24].(21) LEED patterns were observed. A smallThe argument is based on the premise that WO3amount of potassium contamination that was ini-is significantly covalent, as indicated by the band-tially present at the surface was considerablystructure calculations of Bullett [27,28]. Althoughreduced by this cleaning procedure. At all photonthe empty conduction band states are predomi-energies, potassium accounted for less than 5% ofnantly W 5d-like, there is adequate O 2p characterthe total alkali metal ions detected by photoemis-away from the C point that the electronic contribu-sion, although this value increased to 12% attion to the image is insufficient to outweigh thegrazing emission angles.topographic protrusion of the on-top oxygen ions.Of course, interpretation of STM images on thebasis of bulk band structure calculations shouldbe carried out with caution; such calculations may3. Results and discussionnot provide an accurate picture of the chargedensity associated with the actual surface structure.3.1. Atomic structureIn this case, however, the extent of WMO mixingat the surface is likely to be even greater than inSTM [24] shows that Na0.667WO3(001) surfaces bulk band structure calculations, owing to theprepared by annealing at relatively high temper-removal of inversion symmetry and the reducedatures (~800C) in vacuo display acoordination number for both W and O ions.(E2E2)R45 surface reconstruction that

Despite the similarity of the STM images ofstrongly resembles that observed on WO3 afterannealing in oxygen (Fig. 2a and b). The model Fig. 2, there are a number of striking differences


Fig. 2. (a) 3030 A2 empty states STM image of the WO3(001) surface after cleaning by annealing in oxygen. Sample bias=+2 V,tunnelling current=1 nA. (b) 3030 A2 empty states STM image of the Na0.667WO3(001) surface after cleaning by annealing at~800C. Sample bias=+0.6 V, tunnelling current=1 nA. The similarity to the image from the WO3(001) surface is readily apparent.(c) Schematic illustration showing the stacking of the ionic planes viewed perpendicular to the WO3(001) surface. The surface isterminated in half a monolayer of O ions so that the formal ionic charges may be grouped to give a quadrupolar repeat unit. (d)Schematic illustration showing the arrangement of half a monolayer of O ions on top of a WO2 surface layer to give a(E2E2)R45 reconstruction. Small black circles are W ions in the WO2 plane, open circles indicate oxygen ions in the WO2 planeand grey circles indicate on-top oxygen ions.

between the tungsten bronze surface and that of might be expected, STM imaging of thethe binary trioxide. Slightly oxygen deficient (E2E2)R45 reconstruction was only possibleWO3 is an n-type semiconductor, with the Fermi at positive sample biases (tunnelling into the rela-edge pinned at the bottom of the largely W 5d- tively high density of empty states available);like conduction band by a small density of donor imaging of filled sample states remained unrealisa-

ble for all except the most highly reduced surfacesstates arising from oxygen vacancy defects. As


[15]. Conversely, delocalisation of sodium 3selectrons into the W 5d(t2g) conduction bandmeans that the Fermi edge of metallicNa0.667WO3 is positioned within this band.Electron tunnelling between the tip and sample isthus supported at both positive and negativesample bias. Filled states images are essentiallyidentical to empty states images, although reso-lution of the bare W ions is now also possible[24]. A second difference relates to the defects thatcan be observed on the annealed WO3 surface.These are prominent as dark troughs runningacross the surface in approximately constraineddirections, and correspond to intersection of shearplane structure with the surface. No such featureswere ever observed on the (E2E2)R45 recon-structed tungsten bronze surface.

A further difference between the two materialsis the observation on the sodium tungsten bronze

Fig. 3. 200200 A2 image (0.4 V, 1.0 nA) of the low temper-surface of atomically ordered regions with (12) ature annealed Na0.667WO3(001) surface. Surface contamina-and (21) periodicities [25]; no similar recon- tion is apparent at the centre of the image, but rows of maxima

can clearly be seen running in [100] and [010] directions.structions have ever been observed on tungstentrioxide. In the STM chamber, the (12) and(21) reconstructions were observed when the

the same height. On the upper terrace, the rowsNa0.667WO3(001) surface was cleaned by gentle run parallel to the step edges of the triangular(


Fig. 4. (a) 125125 A2 image (0.4 V, 1 nA). The upper terrace to the left of the image shows rows of maxima running in the [010]direction alone, while the lower terrace (indicated by an arrow) contains rows running in both [100] and [010] directions. Theorthogonal rows meet along the line running from the point of the terrace towards its centre. On this lower terrace, [100] orientedrows border the [100] step edge and [010] oriented rows border the [010] step edge. (b) 100100 A2 image (0.4 V, 1 nA). Anotherillustration of orthogonal rows coexisting within the same terrace. The point where the rows intersect is indicated by an arrow.

Displacement of these ions towards each other In this case, however, this is achieved without lossof surface oxygen. As a pair of O2 ions approach(i.e., perpendicular to the direction of the rows)

by 0.8 A doubles the surface periodicity in this each other, the uppermost antibonding orbital ofthe s2gp4up4gs2u formal valence electron configurationdirection; adjacent maxima are separated by 2.2

and 5.5 A, as shown schematically in Fig. 6b. is pushed up in energy. Movement of this levelabove the Fermi level must result in transfer ofFrom corrugation profiles measured across the

rows (Fig. 5b), the OMO separation within the electron density out of the orbital. In the limit ofcomplete electron transfer, the surface dimersurface dimers is found to be significantly reduced

from twice the ionic radius of the O2 ion approximates to the peroxide speciesO22

(s2gp4up4g ). However, the OMO bondlength of(2rO2=2.8 A [29]) suggesting incipient bondingbetween the ions. Photoemission and electron a true peroxide anion (e.g., 1.49 A in Na2O2) is

considerably shorter than the OMO separationenergy loss studies of the oxidation of alkali metalsadsorbed on metals [30] or oxides [31,32] have measured from the STM image, implying that the

transfer of electron density is incomplete. Thepreviously indicated that peroxide and superoxidespecies can be stabilised on these surfaces. Not OMO dimers are said to be peroxide-like, rather

than true peroxide anions.unreasonably, it has therefore been suggested thatthe dimers observed on Na0.667WO3(001) corre- Of further interest in Fig. 6a is the weak intensity

visible between the dimer rows. This is highlightedspond to peroxide-like species [25]. Under theproposed model, reduction of surface charge is in the corrugation profile of Fig. 5b. In Figs. 5 and

6 this intensity maximises on a direct line betweenonce again the driving force for the reconstruction.


Fig. 5. (a) 100100 A2 image (0.4 V, 1 nA) showing rows of maxima running in a single direction on adjacent terraces. The stepsare one unit cell high and the rows are resolved sufficiently to discern pairs of maxima. The line indicates the direction along whichthe corrugation profile of (b) was measured. (b) Corrugation profile taken along the [010] direction. As well as the step edge, thedouble maxima are resolved. Weak peaks can also be seen between the rows of dimers, as indicated by arrows.

the OMO pairs along [010]; i.e., it maximises suggest that the sodium ions should not be imagedat low sample biases, and imply that other factors,between the positions of the Na+ ions in the model

of Fig. 6b. However, in other images of the recon- such as variations in the local barrier height, maybe responsible for the apparent topography in thestruction (Fig. 7a), maxima in inter-dimer intensity

were observed in positions in between the rows of images. On the other hand, the bandstructure calcu-lations of Christensen and Macintosh for hypotheti-OMO pairs along [010]. This now correlates with

the positions of the Na+ ions in the model. It cal NaWO3 [33] show the onset of empty states ofNa s character at around 1.3 eV above EF, whileshould be noted that the image in Fig. 7a was

recorded using a sample bias of +0.4 eV, compared the onset of Na s-like filled states is much furtheraway at around 3.3 eV below EF. It is tempting towith the 0.4 eV sample bias of Figs. 5 and 6. The

intensity between the rows is highlighted in the suggest tentatively that this may be a contributoryfactor towards the appearance of the protrudingfalse-3D image of Fig. 7b. In these empty states

images, the position of the intensity between the sodium ions in the empty states image of Fig. 7a,whilst rendering them absent from the filled statesrows corresponds to the positions which ordered

Na+ ions would occupy in the surface layer. This images of Figs. 5 and 6. The corrugation along[100] in the filled states images would then be dueobservation is surprising in view of the fact that

bandstructure calculations [33] show the density of to the O bridging the two W ions in the underlyingWO2 plane. It should be borne in mind, however,sodium states near the Fermi level to be very small.

Moreover, oxygen K edge electron energy loss that the two images were not recorded from thesame region of the sample. Regions of the surfacespectroscopy has placed the Na 3s levels about

10 eV above EF [34]. These observations strongly in which the top layer is fully depleted of Na may


Fig. 6. (a) High resolution 2323 A2 image (0.4 V, 1 nA) showing the double rows of maxima comprising the (21) reconstruction.Weak intensity is visible directly between the dimers along the [010] direction. (b) Schematic representation of an Na0.5O surfacelayer, illustrating the lateral relaxation and pairing of the oxygen ions. The ions have been drawn to scale on the basis of the ionicradii tabulated by Shannon and Prewitt [29]. The smaller spheres represent the sodium ions. The positions of these ions between therows of oxygen dimers clearly do not match the weak intensity in the STM image of (a).

also exist. This would give a formal stoichiometry 3.2. Electronic structureof O for the uppermost plane.

In terms of agreement with earlier LEED studies, Photoemission spectra from NaxWO3 surfaces

have previously demonstrated the presence of filledit is clear that the orthogonal (21) and (12)reconstructions do not arise simply from an order- electronic states in the bulk band gap region [17

22]. Theoretical studies by Elliatioglu and Wolframing of Na+ in the surface plane [3537]. Rather, itis the pairing up of the oxygen ions that doubles [39] using a Greens function/LCAO method

showed that intrinsic surface states may exist inthe periodicity along one crystallographic direction.The ambiguity over the appearance or non-appear- the region above the top of the valence band for

insulating transition-metal perovskites (such asance of the Na+ ions in the STM images is insuffi-cient proof of sodium depletion, segregation or SrTiO3) if Coulomb effects are ignored. However,

the mutual Coulomb repulsion associated withordering at the surface. However, it is quite clearthat interpretation of LEED patterns in terms of localised surface state electrons pushes these states

above the Fermi level, so that in self-consistentan unreconstructed bulk termination [38], takinginto account only rotation and deformation of the calculations no occupied gap states were found.

This is in agreement with photoemission measure-WO6 octahedra in the tetragonally distorted roomtemperature structure, is unrealistic. ments on SrTiO3 [40]. It was predicted that oxygen


Fig. 7. (a) High resolution 2323 A2 image again showing the double rows of maxima. This image was recorded at the oppositesample bias to Figs. 5 and 6 (+0.4 V, 1 nA). The intensity between the rows along [100] no longer falls on the line of the rows along[010], but rather matches the positions of the sodium atoms in the model of Fig. 6b. (b) False 3D representation of the image in (a),highlighting the maxima between the rows of dimer pairs.

vacancies near the surface could stabilise such with a broad maximum at a binding energy of~2 eV. This has been assigned to an intrinsic orsurface band gap states. LCAO calculations for

WO3 predicted that no surface states should appear extrinsic surface plasmon loss [21], although thisparticular study focused on a (110) surface. Thewithin the semiconducting gap of the defect free

(100) surface when the tungsten atoms lose no (110) surface of Na2/3WO3 displays two nanofacetstructures and trough-like line defects; the originmore than one coordinating ligand [41].

Hochst et al. [42] observed strong emission in of band gap states may well prove different fromthat on the (001) surface [43]. It has also beentheir angle-resolved He(I ) excited UPS spectra

from vacuum-cleaved Na0.85WO3. This was attrib- noted that poorly annealed (001) surfaces tend toshow much stronger defect intensity than optimallyuted to intrinsic surface states on the basis of

strong dispersion upwards away from C with prepared surfaces, as do surfaces that have beenargon ion bombarded [22]. This has been attrib-photoelectron take-off angle. It was reasoned that

the presence of conduction electrons in metallic uted to the presence of oxygen vacancy defects,similar to those observed on WO3.NaxWO3 screens the Coulomb repulsion, thus sta-

bilising the surface states in the band gap region. Via the application of STM, it has recentlyproved possible to relate the electronic defect statesHowever, it has subsequently been shown that

considerable intensity in the band gap region of on WO3(001) surfaces to specific surface structures[16 ]. These can be formed by reducing the surfaceHe(I ) excited UPS spectra can arise as a result of

excitation by He(I ) b,c satellite radiation [22]. using argon ion bombardment and annealing inUHV. Mild argon ion bombardment and anneal-Careful stripping of satellite-induced intensity

leaves a weak residual feature in the band gap, ing at 580C results in a relatively high density of


defect states deep in the band gap (2.1 eV binding peroxide s-levels. The greater the degree of electrontransfer, the higher the binding energy of the perox-energy), extending to a lower intensity at EF.

Constant initial state (CIS) spectra show the states ide-like feature. Such states have been observedexperimentally in photoemission and electronto have W 5d character. The deep states, towards

the valence band edge, arise from the presence of energy loss studies of the oxidation of alkali metalsadsorbed on metals [30] and oxides [31,32] ands-bonded pairs of W(V ) ions, while the density

of filled states at EF is due to corresponding have been attributed to the presence of surfaceperoxide species. For K oxidised on TiO2 [32], ap-bonding. This effect is similar to that of the

WMW bonding that occurs along the c-axis of the peak in the UPS spectrum at 11 eV below EF wasattributed to peroxide states, while a second peakdistorted rutile structure in the metallic oxide

WO2 [44]. On the basis of tunnelling spectra at around 13 eV below EF was interpreted as beingdue to superoxide states (O

2).(STS), the defect states responsible for the band

gap structure in the photoemission spectra are Fig. 8 shows the photoemission spectrum of theNa0.667WO3(001) surface, recorded using a seriesassociated with the oxygen deficient troughs

observed running across the WO3(001) surface in of photon energies. Although crystal cleaning wasachieved via argon ion bombardment and anneal-STM images.

Heavily reduced surfaces show a different limit- ing in partial pressures of oxygen, LEED patternsfrom the surface displayed the (0, 1/2) and (1/2, 0)ing electronic structure after prolonged annealing

in UHV [16]. Here, the maximum W 5d intensity spots characteristic of the orthogonal (12) and(21) reconstructions identified by STM. A weakis found close to the Fermi energy and the intensity

in the region of the deep peak is significantly but well-defined peak starts at the Fermi energyand extends down to about 1.1 eV, with a maxi-reduced. In this case, the defect states are associ-

ated with large areas of the surface which are mum at ~0.2 eV. This corresponds to emissionfrom the W 5d(t2g) conduction band, and essen-occupied by fully reduced (11) terraces, from

which all the on-top oxygen has been removed. All tially has a free-electron-like shape. There is thena clear gap before the onset of the O 2p valenceof the W(VI) ions in these terraces are reduced to

W(V ), giving rise to a two-dimensional 5d1 metallic band emission at 2.6 eV. The valence band itselfshows a maximum at 4.2 eV below the Fermiband. Some support for this model is provided by

the fact that these (11) regions can be imaged at energy. Bulk bandstructure calculations [28,33]emphasise the covalent nature of the bonding andboth positive and negative sample biases [15].

As mentioned previously, images of the indicate strong W 5dO 2p mixing toward thebottom of the valence band as well as appreciableNa0.667WO3(001) surface show no evidence of

defect troughs corresponding to localised oxygen mixing in the conduction band.Particular attention is drawn to the bottom ofdeficient regions of the surface and incipient WMW

bonding. Nor have terraces with (11) periodicity the valence band, where a shoulder can clearly beseen at ~11 eV in spectra recorded using photonbeen imaged on this surface. The band gap states

in the photoemission spectra appear to have a energies of hn=100 eV and hn=120 eV. This isindicated by an arrow in Fig. 8. At these photondifferent origin to those associated with

WO3(001). Considering the proposed model of the energies, the contribution of the W 5d electrons tothe spectrum is low, suggesting that this feature(21) reconstructed Na

xWO3(001) surface, we

speculate that the presence of peroxide-like species has predominantly O 2p character. The reducedW contribution to the spectrum is seen most clearlywill affect the valence region photoemission

spectrum in two distinct ways. Firstly, the transfer in the significantly weaker intensity of the W 5dband with respect to the O 2p maximum (~4.2 eVof electrons out of antibonding peroxide orbitals

will result in an increase in the density of states binding energy) in the spectrum measured at120 eV photon energy compared with that mea-near the Fermi energy. This will be accompanied

by the splitting-off of an O 2p feature at the bottom sured at lower photon energies. Furthermore, theintensity of states towards the bottom of theof the valence band corresponding to bonding


approaches its maximum for photons with energiesof around this magnitude, indicating that the fea-ture is related to the uppermost atomic layers ofthe crystal surface. We therefore propose that theshoulder at the bottom of the valence band isrelated to the OMO s-bonding orbitals of theperoxide-like dimers that make up the (21)reconstruction of the surface. The position of thepeak is in broad agreement with that due toperoxide states associated with oxidised K onTiO2 [32].

Analysis of the band gap region of the spectrumusing resonant photoemission is less conclusive.Resonant photoemission is commonly used todistinguish the origin of electronic states. Forexample, in a study of ultrathin Fe films on TiO2[45], Diebold et al. used resonant photoemissionto determine between Fe-derived and Ti-derivedstates. More recently, Morris et al. [46 ] have usedthe technique to demonstrate that gap states inV-doped TiO2 relate to electrons trapped ondopant V cations rather than host Ti cations. Theexpanded photoemission spectra are shown inFig. 9a for photon energies ranging from 40 to80 eV. Significant photoelectron emission can beseen in the gap region, maximising at ~2.2 eV.This is very similar to the deep feature observedin the photoemission spectrum of WO3(001). Asthe photon energy is varied, the intensity of thefeature maximises at 57.5 eV relative to the peakin O 2p valence band intensity at ~4 eV. Thismirrors the behaviour of the conduction bandpeak, as highlighted in Fig. 9b. This intensity varia-tion is due to a resonant photoemission processinvolving W 5p electrons. Two competing path-Fig. 8. Valence region photoemission spectra from theways are available for excitation of an electron:Na0.667WO3(001) surface. Spectra have been normalised to the (i) Normal photoionisation:predominantly O 2p-like valence band peak at ~4 eV. At

photon energies of hn=100 eV and hn=120 eV, a shoulder 5p64f 145dn+hn5p64f 145dn1+e;becomes apparent to the high binding energy side of the valence (ii a) W 5p to 5d excitation:band, as indicated by an arrow. This is associated with the O 2p 5p64f 145dn+hn[5p54f 145dn+1]1;bonding orbitals of the peroxide-like species inherent to the

(ii b) Super CosterKronig Auger decay:(21) reconstruction.[5p64f 145dn+1]15p64f 145dn1+e.When the photon energy coincides with the

energy required for the discrete excitation of avalence band, where strong O 2pW 5d mixing isprevalent, can also be seen to be significantly W 5p electron into the 5d shell, this excitation (ii

a) can occur in parallel with the normal ionisationreduced with respect to the O 2p maximum in thehn=100 eV and hn=120 eV spectra. Finally, the process (i). A core excited state is created which

can decay via super CosterKronig Auger emissionsurface sensitivity of photoemission spectroscopy


Fig. 9. Expanded photoemission spectra showing the W 5d(t2g) conduction band and band gap region. Again the spectra have beennormalised to the valence band peak at ~4 eV. Plots of intensity variation with photon energy for conduction band states (opensquares, dotted line) and band gap states (filled circles, solid line). The intensity (relative to that of the valence band peak) maximisesin both cases at hn=57.5 eV. This is a consequence of the resonant photoemission process described in the text and indicates thatboth the conduction band and band gap states have considerable W 5d character.

(ii b), resulting in the same final state as the direct 5p1/2 and 5p3/2. In the case of the spectra of Fig. 9,the resonance behaviour of the band gap statesphotoemission process. Interference between the

different processes gives rise to a characteristic mirrors that of the conduction band states.Unfortunately, CIS measurements were not pos-Fano-type resonance profile.

The variation in intensity of the conduction sible on beamline 4.1 and the deductions madehere are based purely on the variation in energyband states with photon energy as a result of this

resonance is strong, due to the largely W 5d nature distribution curves (EDCs). Even so, it is obviousthat all the states above the top of the valenceof the states [47]. Similar, but weaker, intensity

variation has been seen for the W 5d-like defect band have significant W 5d character. Any featuredue to O 2p-derived peroxide states is masked bystates of WO3 [16 ]; constant initial state (CIS)

measurements showed two maxima at about 45 this effect. In the conduction band region, this isto be expected, since the states are intrinsicallyand 55 eV, due to spinorbit-split core levels


W 5d-like. In the band gap region, however, the WMW bonding or a reduced W(V ) surface layeras have been postulated for WO3(001) in the formorigin of the W character is less clear. If the band

gap states are indeed associated with antibonding of oxygen deficient defect troughs and (11)terraces respectively.peroxide-like states, some degree of mixing with

surface W 5d states may occur. An alternativeexplanation is that the band gap states may beassociated with the (E2E2)R45 reconstruction

Acknowledgementsof the surface. Very faint (1/2, 1/2) order spotswere occasionally observed in LEED patterns at

The STM equipment, F.H.J.s PDRA positionsome beam energies, suggesting that small areasand T.W.F.s studentship were funded by thewith (E2E2)R45 periodicity were present,EPSRC. T.W.F. is also grateful to the Worshipfuldespite a predominance of (21) and (12)Company of Ironmongers for the award of aordering.scholarship. R.A.D. is grateful to JohnsonMatthey PLC for a CASE studentship. Manythanks to Dr. V. Dhanak and Mr. G. Miller fortheir assistance in operating Beamline 4.1.4. Concluding remarks

STM has proven highly successful in solvingthe structure of the Na0.667WO3(001) surface as Referencesprepared by annealing in vacuo. The results are ingood agreement with previous observations of [1] J.J. Boland, Adv. Phys. 42 (1993) 129.(12), (21) and (E2E2)R45 LEED pat- [2] T.-C. Shen, C. Wang, J.R. Tucker, Phys. Rev. Lett. 78

(1997) 1271.terns, but the interpretation is neither that of[3] R.M. Tromp, R.J. Hamers, J.E. Demuth, Phys. Rev. Lett.ordered sodium ions nor tilting and rotation of

55 (1985) 1303.the WO6 octahedra. Instead, the STM images [4] Proceedings of the 114th Faraday Discussion Meeting onindicate that the surface structure is highly depen- Metal Oxide Surfaces, 13 September 1999, Ambleside,dent on the nature of oxygen species at the surface. UK, Royal Society of Chemistry.

[5] G. Hagg, Z. Phys. Chem. B 29 (1935) 192.The high temperature (E2E2)R45 reconstruc-[6 ] P.A. Lightsey, D.A. Lillienfeld, D.F. Holcomb, Phys. Rev.tion, as for WO3(001), arises when the surface B 14 (1976) 4730.

loses half a monolayer of oxygen. (12) and [7] B.A. Loopstra, P. Boldrini, Acta Crystallogr. 21 (1966)(21) reconstructions, on the other hand, are 158.

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Documents

The Surface Structure of the Metallic Sodium Tungsten Bronze Na0.667WO3(001)