The adsorption of water on tungsten bronze (001) surfaces: a study by HREELS and photoemission

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The adsorption (001) surfaces: photoemission

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of water on tungsten bronze a study by HREELS and

D G Aitken, P A Cox, R G Egdell’, M 0 Hill and I Sach, inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK

The adsorption of water on Na,,, WO, (001) has been studied by HREELS and photoemission. Efficient screening of lattice vibrations by the conduction electrons leads to a low intensity of phonon losses in HREELS and enhanced sensitivity to details of surface structure as compared with non-metallic oxides. Water does not adsorb on the crystal surface at room temperature, but binds to specific surface cation sites at 150 K. This contrests with adsorption on metal surfaces where islands of ice are found even at submonolayer coverage.

lntroductlon

High-resolution electron-energy-loss spectroscopy (HREELS) is now well established as a means of measuring vibrational spectra of adsorbates on metal surfaces’. The success of the technique depends on the absence of strong substrate vibrational excitations for low index metal faces’. Despite current interest in metal oxide surface chemistry3, HREELS has proved to be less popular in studying adsorbates on the surfaces of dielectric oxide crystals owing to the strong background of substrate surface phonon lossesc6. The Fuchs-Kliewer surface modes’ decay into the bulk with a long characteristic penetration depth and thus involve a large dynamic dipole normal to the surface. In the present paper we present results which demonstrate efficient screening of the Fuchs-Kliewer modes by the conduction electrons of a prototype metallic oxide Na,,,WO,. EELS thus probes genuine microscopic surface excitations of the substrate and is sensitive to details of surface structure. In particular vibrational spectra of adsorbates can be measured much more easily than with insulating oxide substrates. Experimental Crystals of nominal composition Na,,7W0, were grown in evacuated sealed quartz tubes using the method of McNeil and Conroy’. The crystal selected for spectroscopic study was established by Laue back-reflection to have a large (-5 mm x 5 mm) (001) growth face which was subsequently polished to an optically smooth finish with progressively finer diamond pastes down to 0.1 pm. Electron spectra were measured in an ESCALAB 5 spectrol

Author to whom correspondence should be addressed.

meter (VG Scientific, East Grinstead, UK) with facilities for X-ray and ultraviolet photoelectron spectroscopy(XPS and UPS) and low energy electron diflraction (LEED) as well as an electron monochromator for HREELS. The crystal was mounted on a platinum stub and cleaned in the spectrometer preparation chamber (base pressure lo-” mbar) by annealing at 800 K for several hours. Note that the cleaning procedure simulates the conditions of crystal growth and unlike cleaving or filing gives a surface in equilibrium with the bulk. Following transfer to the main chamber (base pressure 7 x 10‘” mbar) the XPS was free of signals due to carbon or other contaminants and HREELS revealed no loss features associated with unwanted adsorbates. Moreover the Nals/W4f intensity ratio in XPS and the surface plasmon frequency in HREELS’ were consistent with the nominal bulk composition. He(I) and (II) photoelectron spectra were similar to those reported by Hiichst et Cal”, although we found a much lower intensity of emission between valence and conduction bands than in their spectra. We agree with Hollinger et al’ ’ that this emission is probably due to surface oxygen vacancies. The dominant LEED pattern at room temperature showed the fourfold symmetry expected from termination of the bulk structure, with weak spots indicative of (2 x 1) superlattice orderingt2. EEL spectra were recorded at beam energies between 2 eV and 100 eV. The anglebetween the electron beam and the analyser was fixed at 90” but the angular dependence of the loss features could be studied by tilting the crystal. Spectra reported here were all taken in the specular mode with a 5 eV beam incident at 45” to the sample surface. Losses peaked strongly in the specular direction. Water was dosed onto the crystal by flooding the main chamber to lo-’ mbar (uncorrected ion gauge reading). Because water produces memory effects by adsorption on the chamber walls quoted exposures are necessarily approximate. 753

D G Aitken et al: The adsorption of water on tungsten bronze (001) surfaces: a study by HREELS and photoemiasion

Discussion

2

The ideal cubic structure of Na,WO, belongs to the space group 0: (Pm3m). It is based on comer sharing WO, octahedra with the xNa ions per cell randomly distributed in 12coordinate sites between the octahedra (Figure 1). At the zone centre the little

0

=w

@ t

I-

x

xNa

Figure 2. Schematic representation of infrared active lattice modes for the perovskite structure, with atoms projected onto the (010) plane. Only the components of the threefold degenerate modes with dynamic dipole along the [OOl] direction are shown. The oxygen atoms fa above the basal plane are not shown and motion of heavy W ions has been ignored.

0 =

0

Figure 1. Unit cell oftheperovskite tungsten bronzestructure, extended to show the octahedra1 coordination of the tungsten ions.

P(w) = e2/vhw Zm(e(w) group is Oh (m3m) and the 15 lattice modes span irreducible representations rvls as follows: rvle=4qU+

(I)

T2u.

The translation vectors belong to the T,, irreducible representation so one is left with just three infrared active modes (T, ,) ofnonzero frequency at k=O. From a detailed study13 of the lattice dynamics of the related perovskite material SrTiO, it appears moreover that the ionic displacements are such that the normal modes may be usefully classified in decreasing order of energy along the following lines: (1) WO, stretch mode; (2) W06 bend mode; (3) Na displacement (‘rattling’) mode. These normal modes are shown schematically in Figure 2. Note that the classification scheme is not rigorous and there must be symmetry allowed mixing between the idealized modes of Figure 2. In an insulating

perovskite

crystal associated with each bulk

phonon mode there is a Fuchs-Kliewer surface mode with a dynamic dipole normal lo the surface and a frequency just below that of bulk longitudinal optical phonons. The surface modes decay into the bulk of the crystal with penetration depth i that increases with increasing beam velocity r. Thus one has jl=v/w

(2)

where hw is the energy of the surface phonon. The angle integrated loss function P(w) is given by the following expression : 754

- 1)/(&(w) + 1).

(3)

Here E(O) is the complex dielectric function of the substrate. Equation (3) predicts strong loss features in EELS whose intensity decreases with increasing beam energy as 1/E1’2, where E is the beam energy. The energy loss spectrum of NaO.,WO, shows the three principal features expected on symmetry grounds (Figure 3), although the loss intensity is much lower than for typical ionic crystals. To emphasize this point we also show in Figure 3 the loss spectrum of the insulating oxide WO,. This material has a crystal structure closely related to that ofNa,WO, and the two principal loss features correspond to unscreened Fuchs-Kliewer surface phonons associated with W06 stretch and bend modes, as indicated in the assignment in Table 1. The weakness of the loss features in the HREELS of Na,,,WO, thus implies there there is efficient screening of the Fuchs-Kliewer modes by the conduction electrons of this metallic material. Noting the short Thomas-Fermi screening length for Na,.,WOJ (i.,,= 1.05 A) we propose that the losses are of microscopic surface origin and relate to an unscreened surface dielectric layer involving only ions in the topmost unit cell of the crystal. This model has been developed in some detail by Ibach and Mills to deal with loss spectra of adsorbates on metal surfaces”. The probability of exciting microscopic surface losses decreases with increasing beam energy more rapidly than for Fuchs-Kliewer surface excitations: at sufficiently high beam energy we find that the loss intensities for Na,,,WO, vary roughly as 1/E15, as expected for microscopic excitations. In view of the surface sensitivity of HREELS in the present

D G Aitken et al: The adsorption of water on tungsten bronze (001)

surfaces: a study by HREELS and photoemission

ations. Experiments with higher resolution are needed to explore the possible splitting of the other phonon peaks. No change in the HREELS was found at room temperature following exposure of the crystal to up to lo-* mbar-s ,H,O at lo-’ mbar. However, exposure at 150 K led to pronounced changes. For low exposures three principal new loss features were apparent (Figure 4): the loss energies are summarized in Table 1. To interpret these spectra we assume that Hz0 binds nondissociatively to surface cations with the molecular C2 axis normal to the crystal surface. There are then three dipole allowed vibrational excitations corresponding to the symmetric stretch

Nao.7w03

Clean

Na0.7W03

+ Hz0

Energy loss I eV

Figure 3. Electron-energy-loss spectra of Na,,,WOJ (upper panel) and WO, (lower panel), both taken at room temperature. The inset shows the loss spectrum of Na,.,WO, close lo the elastic peak recorded at an instrumental resolution of -8 meV. Note that the energy loss feature at 15 meV has its energy gain counterpart, indicating thermal population of the corresponding phonon mode. 0

Table I. Vibrational frequencies for WO,, NaO.,WO,,

0.2

04

0.6

0.8

1.0

Energy loss I eV

NaO,,WOI+ Hz0 and Hz0

Figure 4. Electron-energy-loss spectra of NaO.,WO, recorded at 150 K WO,

380 940

Frequency (cm-‘) NaO.,WOJ NaO.,WOJ +H,O 120 420

U 440

740(s)

U

990

1010 600 1740 3590

Assignment Hz0 Na+ raMin mode WO, bend mode

1595 3657

WO, stretch mode W-H20 stretch mode H-O-H bend mode H-O-H symmetric stretch mode

u: unresolved, s: shoulder context it is necessary to consider the surface structure in further detail. Following Benbow er 01’~ we note that the Na,WOa (001) surface may terminate either in an Na,O plane or in a WO, plane. The (2 x 1)superstructure in LEED has been attributed to sodium ordering in depleted Na,.,O domains’z. Surface tungsten ions in the WO, planes are five coordinate, so that along the [OOl] axis one has an atomic sequence O-W-O, where 0 represents a vacancy. By contrast W ions below the Na,O planes are sixcoordinate with an atomic arrangement O-W-O. The incipient splitting of the octahedral stretch peak apparent in Figure 3 may be associated with these two differing tungsten surface coordin-

before (upper) and after (lower) exposure lo 1 x 10e6 mbar-s H,O. The adsorbate induced loss features are indicated by arrows in the lower spectrum.

scissor bend modes of the free molecules and to motion of the molecule normal to the crystal surface (Figure 5). It will be seen from Table I that the two highest loss frequencies correspond closely to those for molecular H,O. Two points deserve comment here. Firstly the O-H stretch frequency (3590 cm- ‘) is much higher than values of around 3400 cm-’ found following adsorption of water on Pt(OOl)“, Pt(ll1)” and R~(Cl01)‘~. In these systems there is apparently no specific interaction between H,O dipoles and the crystal surface and the mobility of Hz0 on the crystal is sufficient to allow the formation of islands of ice even at the lowest coverages. In our system, broadening of the O-H stretch band and the shift to lower frequency characteristic of H-O---H bonding was found only for exposures beyond -4 I..; presumably by this stage surface cation sites are fully occupied. Secondly, observation of the bend mode at 1740 cm-’ establishes that the adsorption is non-dissociative. It is well known from studies of vibrational spectra of coordination complexeszO that the bending mode of the hydroxyl (OH) group is at very much lower frequency, typically below 1200 cm- ‘. Bending frequencies for surface -OH groups have been found at 820 cm- ’ on Si( 100) 2 x 12* and at 1015 cm-’ on Pt(111)22. and

755

D G Aitken

et al: The adsorption

of water on tungsten

bronze (001) surfaces: a study by HREELS and photoemiaaion

\ H/ “\,/ t -M-

;rH b

\L/ I -M-

Fiie 5. Dipole-allowed vibrations for H,O bound to a surface cation on the Na,.,WO, surface, assuming local C,,. symmetry. The modes are arranged in order of decreasing frequency.

Further guidance as to the mode of adsorption comes from consideration of the M(surface) -OH, stretch frequency. Of course we must recognize that there can in principle be strong coupling between adsorbate and substrate phonon modes when both are of similar frequency. However, in the present system the small shifts in substrate loss frequencies suggest rather weak coupling and one can reasonably retain a description in terms local M-OH, displacements. From studies of M(H,O)“+ aquo complexes it is well established that the M-OH, stretch frequency increases with increasing charge on the cation Mz3. For Ml+ species frequencies fall in the range 3 10 cm- ’ -405 cm- 1 and for M3+ species in the range 488 cm-‘-540 cm-‘. By contrast, for M+ species frequencies fall below 300 cm- I. Note also that a stretch frequency 240 cm- ’ was calculated for the Na+-OH, complex usingab inicio molecular orbital methods24. It thus seems that the observed M(surface)-OH, stretch frequency of 600 cm-’ cannot arise from a Na+-OH, surface bond but must be due to binding of H,O to surface tungsten cations. The formal charge on tungsten is 5.3, although metallic tungsten bronzes are known to be highly covalent and the true effective charge of a surface cation must be somewhat lower. Unfortunately, introduction of water into our spectrometer degraded the resolution of our instrument from an optimum value of 8 meV to around 12 meV. It was thus difficult to verify that there was no shift in the frequency of the Na’ ‘rattling’ mode at 15 meV upon adsorption of H,O which might be expected if H,O were attached to a surface Na+ ion. Further work is needed to explore the possibility of specific binding to surface Na’ ions for exposures beyond that required to saturate surface tungsten sites but before onset of hydrogen bonding leading to buildup Of an ice multilayer. The proposed mode of adsorption finds confirmation in He(H) photoelectron spectra (not shown) which reveal the presence of

756

three new peaks corresponding to the b,, 0, and b2 molecular orbitals of the free molecule following exposure at 150 K. in conclusion wenote that the observation ofspecific binding of H20 to cations on the surface of a metallic oxide has the implication that the double layer at a metal oxide/electrolyte interface is more highly structured than with conventional metal electrodes. Thus reactions such as oxygen reduction which must involve displacement of surface bound H,O will have higher activation energies on metal oxide electrodes than on noble metal electrodes. We believe that it will be of great interest to use HREELS to study any peculiarities in the adsorption of water on metal oxides such as Pb2Ru20,_p and Bi,Ru,O, which are as efficient as the noble metals in electrolytic reduction of oxygen : it has been speculated that this is due to disruption of order in the double layer due to a predominance of surface cations with lonepair electrons that cannot act as surface Lewis-acid centres25.

Acknowledgements The tungsten bronze crystal was mounted by Mr G Read of the

Clarendon Laboratory, Oxford. We thank Mr F Peplinski and Mr A F Orchard for a number of stimulating discussions. One of us (RGE) would like to thank Shell Research Ltd for the award of a Fellowship.

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