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The adsorption of water on NaxWO3(100) surfaces: a study by photoemission
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Surface Science 297 (1993) 286-292 North-Holland
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The adsorption of water on Na,WO,( 100) surfaces: a study by photoemission F.H. Potter
and R.G. Egdell
Inorganic Chemistry Laboratory, South Pa&v Road, Oxford OX1 3QR, UK
Received 24 May 1993; accepted for publication 22 July 1993
The adsorption of water on the WIO) surfaces of sodium tungsten bronze single crystals has been studied by photoemission at even on highly defective ion-bombarded surfaces. However, at 140 K, molecular adsorption occurs with preferential stabilisation of the molecular 3a, valence level.
hv = 40.8 eV. No adsorption is found at room temperature,
1. Introduction There has been a longstanding interest in the adsorption of water on oxide surfaces. Valence region photoemission has played an important part in helping to clarify the mode of adsorption [l]. Molecular H,O has three orbitals designated lb,, 3a, and lb, in the binding energy region below 20 eV [2]. Thus, non-dissociative adsorption is characterised by the appearance of three new peaks in difference photoemission spectra, two of which usually lie to high binding energy of the oxide 0 2p valence band [3]. By contrast OHhas only two corresponding electronic levels of u and r symmetry and dissociative adsorption may be fingerprinted by the appearance of a single peak below the 0 2p valence band, associated with the u bonding orbital [3-61. Note, however, that if the surface hydroxyl group is tilted relative to the surface normal, the degeneracy of the r levels may be lifted so that three peaks are again observed in photoemission [7] and the distinction between dissociative and non-dissociative adsorption requires detailed consideration of the binding energies. Non-dissociative adsorption on truly defect-free, low-index oxide surfaces is usually observed only at low temperatures, but this lowtemperature adsorption can be distinguished from simple condensation onto the surface by virtue of 0039-6028/93/$06.00
a pattern of molecular energy levels different to that of free H,O and of H,O ice. Specifically there is preferential stabilisation of the 3a, molecular level so that the separation A(3a,-lb,) is bigger than for free H,O, and the separation A(lb,-3a,) is less. The 3a, molecular orbital is directed along the C, rotation axis of H,O and the preferential stabilisation is consistent with a model where the molecule bonds to a surface cation via the oxygen atom, with resulting stabilisation of the highly directional 3a, level. This situation pertains for low-temperature adsorption on non-metallic oxide surfaces such as SrTiO,(100) [3,4,6], TiO,(lOO) [7] and TiO,(llO) [81. However, in a recent study of the metallic oxide Bi,Sr,CaCu,O,(OOl), which of course becomes superconducting at low temperatures 191, it was found that there was no specific stabilisation of the 3a, level [lo]. This was attributed to the termination of this layered material in a BiO plane within which the Bi ions carry a localised 6s-6p hybrid lone pair of electrons. These prevent the surface Bi3+ acting as a Lewis acid centre. In order to explore the validity of this idea we present here a photoemission study of water adsorption on Na,WO,(lOO). The tungsten bronzes Na,WO, (x > 0.3) are prototype cubic metallic oxides [ll], but in contrast to Bi,Sr, CaCu,O, there are no “lone pair” cations within
0 1993 - Elsevier Science Publishers B.V. All rights reserved
F.H. Potter, R.G. Egdell /Adsorption of water on Na,W03(100)
the structure and we are therefore able to explore in a more direct way the relationship between adsorption on this metallic perovskite phase and the non-metallic perovskite SrTiO,. In particular we are able to compare the relative stabilisation of the molecular levels at the two surfaces and to explore the possibility that specific interactions between the 3a, level and surface cations may be screened out by the itinerant conduction electrons.
2. Experimental The Na,WO,(lOO) single crystals used in the present study were grown by electrolytic reduction of molten mixtures of WO, and Na,WO, [12]. They presented square growth faces up to 1 cm x 1 cm whose (100) orientation was confirmed by Laue back-reflection. Crystals were polished with successively finer diamond pastes down to 0.25 pm and washed in propan-2-01 and distilled water to remove any trace of polishing material. The data presented here all relates to adsorption on a purple crystal with composition Na,,sWO,. For convenience we refer to this as Na,,WO,. However, similar results were obtained on an orange crystal with x = 0.67. Electron spectra were measured in an ESCALAB 5 Mark I spectrometer (VG Scientific Ltd., East Grinstead) equipped with facilities for XPS, UPS and LEED and a sample preparation chamber. The base pressure in the spectrometer main chamber was 5 x lo-” mbar. The crystals were mounted on platinum stubs and held in position with platinum wires. Atomically clean surfaces free of XPS signals due to carbon or other contaminants were produced by annealing in UHV at 680°C for several hours in the spectrometer preparation chamber (base pressure low9 mbar) with the aid of a water-cooled copper workcoil coupled to a 1.5 kW radiofrequency generator. The stub temperature was calibrated in a separate series of experiments in which a chromel-alumel thermocouple was spot-welded to the stub. Defective surfaces were produced by bombardment with 2 keV argon ions from a Penning-type ion gun. The sharp Fermi energy cut-off
287
in the conduction band in He(I) photoemission spectra provides a well-defined energy reference point, even though there is attenuation of substrate intensity with adsorption. In this respect adsorption studies are easier on metallic oxide substrates, such as that used here, than on nonmetallic oxides where adsorbate-induced band bending may make it difficult to align spectra prior to taking differences [13]. For location of adsorbate peaks, He(I1) radiation (hv = 40.8 eV) was preferred to He(I) radiation (hv = 21.2 eV> because the primary photoemission structure is less strongly influenced by secondary electron emission and the lower electron pathlength ensures greater surface sensitivity. All spectra were stripped of structure due to satellite radiation. The analyser resolution was set at 100 meV for He(I) photoemission measurements and at 400 meV for He(I1) measurements. Sample cooling in the main chamber was effected by pumping liquid nitrogen through the sample manipulator. A thermocouple monitorred the probe temperature to be 105 K, but the true sample surface temperature was considerably higher than this. Fitting a Fermi-Dirac distribution function to the photoemission cut-off for the cooled sample suggested a true surface temperature of about 140 K. Water was dosed onto the crystal surface through the ion gun, which provides a crudely collimated beam. The background pressure of H,O was monitorred by an ion gauge, but the background pressure was obviously much lower than the pressure at the sample surface. Surface fractional coverage in the submonolayer regime was therefore estimated by comparing the intensity of the adsorbate-induced molecular levels with the intensity of the main valence band. We assumed that ionisation cross sections for the adsorbate levels and for valence band states are roughly the same, which should be reasonable given that both are of dominant 02p atomic character. A value of 5 A was taken for the inelastic mean free path at hu = 40.8 eV. It was further assumed that below monolayer coverage water adsorbs only on surface cation sites, so that monolayer coverage corresponds to 6.8 x 1014 molecules/cm2. The effective exposure (in langmuirs) was then estimated through the usual
288
F.H. Potter,R.G. Egdell /Adsorption of wateron Na,WO,(lOO)
Knudsen equation [14] assuming unit sticking coefficient for water at low temperatures. The higher exposures were then derived by scaling in proportion to the integrated ion gauge reading.
3. Results and discussion The cleaned sample surfaces gave square LEED patterns (fig. 1) at room temperature with spots at Cm + l/2, n) and Cm, II + l/2) positions, but with streaking through both integral- and half-order spots and some diffuse intensity at (m + l/2, IZ+ l/2) (h ere n and m are integers). This suggests dominance of orthogonal (2 x 1) and (1 x 2) domains, in agreement with the experiments of Lange11 and Bernasek [l&16]. However, on cooling to 140 K, the patterns became much sharper and the intensity at Cm + l/2, n + l/2) increased, demonstrating the emergence of a well-defined (2 x 2) superstructure. The (2 x 2) superstructure was also observed at room temperature on several occasions after prolonged sample cleaning. It is clear from the recent work of Peacor and Hibma [17] that the interplay between (2 x 1) and (2 X 2) superstructures depends intimately on the sample pretreatment. The Na,WO,(lOO) surface may terminate in WO, or Na,O planes, or in domains of both. The superstructures were originally attributed to ordering of Na ions at the surface [15,161, but the more recent work suggests that tilting of WO, octahedra at the surface plays a crucial role in defining the surface reconstruction [17]. He(I) photoemission spectra such as that shown in fig. 2 were obtained from the annealed surfaces. These show a weak but well-defined conduction band peak extending from the Fermi energy down to about 1 eV. There is then a gap before a sharp valence band onset 2.8 eV below the Fermi energy. The valence band itself shows two sharp shoulders at 3.2 and 4.4 eV binding energy: these are resolved as distinct peaks in spectra of samples with somewhat higher x values [18,19]. The valence band contains in addition a strong broad peak with maximum intensity at 6.1 eV. Argon-ion bombardment with 2 keV ions at a flux of 3.5 kA/cm’ leads to major changes
Fig. 1. LEED patterns for Na,,,WO,(lOO) measured at 60 eV beam energy slightly away from normal incidence. (a) Clean surface at room temperature showing pattern characteristic of orthogonal (2 x 1) and (1 x 2) domains. (b) Clean surface at 140 K. New spots at (l/2, l/2) and related positions are characteristic of (2 x 2) superstructure.
in the valence spectra (fig. 2). In particular there is a large increase in intensity of emission at the Fermi energy and the well-defined gap between valence and conduction bands is filled in with new states. The valence band edge shifts downward relative to the Fermi energy by about 1 eV. These changes are consistent with preferential sputtering of oxygen and population of new states of dominant W 5d atomic character. The elec-
F.H. Potter, R.G. Egdell /Adsorption
1
(a)
00 I
0
/
I
I
-5
5 10 Binding Energy / eV
I
I
15
20
Fig. 2. He(I) photoemission spectra of Nao,6WOs(100) (a) after annealing in UHV to produce ordered (‘2X 2) surface, (b) after argon-ion bombardment with 2 keV ions at 3.5 PA sample current for 10 min. Structure due to satellite radiation has been subtracted from the spectra.
289
of water on iVaxWO~~~lOO~
states extending down to the valence band edge appears after ion bombardment [20,21]. He(I1) photoemission spectra of annealed Na*.~WO~(lOO) are similar to He(I) spectra (fig. 3), but the bottom of the valence band is reached before onset of the strong secondary electron structure that is charactersistic of He(I) spectra and He(H) photoemission is therefore preferred in the study of ad~rbate-produced features below the main valence band. Exposure of the annealed surface at room temperature for exposures up to 800 L led to only minor changes in
1
I
!
1
I
trons associated with the oxygen vacancies partially occupy conduction band states, but in addition there is clearly a large density of midgap states extending down to the valence band edge. The photoemission structure for ion-bombarded Na,,WO, is somewhat reminiscent of that found after bombardment of WO,(lOO). WO, is a 5d0 oxide, with no conduction band emission when stoichiometric. However, a large density of defect
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I
-5
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5
10
15
Binding Energy / eV 0
-5
/
/
/
0
5
10
!
15
Binding Energy / eV Fig. 3. He(H) photoemission spectra of Naa,,WOs(lOO) at room temperature (a) after annealing in UHV, (b) after exposure to 800 L water vapour.
Fig. 4. He(H) photoemission spectra of Na,,,WO,(lOO). (a) Room temperature. (b) After cooling to 140 K. There is some indication of adsorption of molecular H,O from the residual vacuum. (c) At 140 K following exposure to 0.8 L H,O. The surface coverage is estimated as 3.9X 1014 molecules/cm*. Cd) Following 1.6 L exposure to H,O at 140 K (e) Following 16 L exposure to H,O at 140 K. (f) Following 160 L exposure to H,O at 140 K.
290
F.H. Potter, R.G. Egdell /Adsorption of water on Na,WO,(lOO)
He(I1) photoemission spectra: difference spectra showed no well-defined molecular (H,O) or hydroxyl (OH-) adorbate peaks. By contrast, exposure of defect-free surfaces to H,O at 140 K leads to progressive changes in the spectra (fig. 4) and at the largest exposure (160 L), the spectra are essentially those of ice [22,23]. No new superstructures appear in LEED and the only pattern to be observed is (2 x 2) although of course this pattern is progressively extinguished as the ice layer builds up. Difference photoemission spectra (fig. 5) reveal that down to the lowest exposures adsorption is characterised by the appearance of three new peaks, corresponding to the molecular orbitals of H,O. However, as on SrTiO, it is found that at low coverages the 3a, level is preferentially stabilised relative to the lb, and lb, levels [3,4]. This is emphasised in fig. 6 which shows the position of the molecular adsorbate levels relative to that of the lb, level. In
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Relative Binding Energy I eV Fig. 6. Energies of lb,, 3a, and lb, molecular peaks relative to the lb, peak in (a) free H,O (ref. [2]), (b) water on Bi,Sr,CaCu,O,(OOl) at 1 L exposure (ref. [lo]), (c) water on Na,~,WO,(lOO) at 1.6 L exposure (present work), (d) water on SrTiO,(lOO) at 2 L exposure (ref. [3]), (e) ice (present work).
*..
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-5
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I
0 5 10 Binding Energy I eV
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20
Fig. 5. He(U) difference photoemission spectra relating to spectra in fig. 3: (a) b-a: weak adsorbate features are due to water adsorption from the residual vacuum, (b) c-b, corresponding to 0.8 L exposure, (c) d-b, corresponding to 1.6 L exposure, (d) e - b, corresponding to 16 L exposure.
terms of a donor-acceptor model for the interaction between water and the oxide surface, the specific stabilisation of the 3a, level implies an interaction between the filled 3a, donor level and an empty level on surface cation, probably the e&a *) level of W. This implies net electron transfer out of the 3a, level into the surface. In a Walsh diagram approach to the bonding in H,O [24], occupation of the 3a, level is largely responsible for the non-linear geometry of the H,O molecule. Complete removal of a 3a, electron to give HzO+ in the ‘Ai state leads to both an increase in the HOH bond angle and a decrease in the HOH bending frequency from 1595 to 975 cm-’ [2]. It is somewhat surprising then that an earlier HREELS study of water adsorption on Na,WO,(lOO) surfaces [25] revealed an increase in the HOH bending frequency to the exceptionally high value of 1740 cm-‘. However, the earlier HREELS work supports the present conclu-
F.H. Potter, R. G. Egdeli / Adsorption of water on Ma, W03(100f
sion that low-temperature adsorption differs from simple condensation to give an ice layer. Specifically it was found that at low coverages, a sharp OH asymmetric stretch mode characteristic of non-H-bonded H,O is observed at much higher frequencies than the broad stretch characteristic of hydrogen-bonded molecules. Returning to fig. 6, it is clear that the inert cleavage surface of Bi~Sr~~Cu~Os(OOl) is the exception amongst the three oxides in not showing a specific stabilisation of the 3a, level. This reinforces the conclusion that the behaviour of the latter material is related to the unique fea-
(e)
291
tures of the inert BiO cleavage plane and is not a simple consequence of the metallic nature of the oxide [lo]. Turning next to argon-ion-bombarded surfaces, fig. 7 shows the effects of exposure of a highly defective ion-bombarded surface to water. No adsorbate-induced features appear in difference spectra. This is a surprising result because in previous work it has been found that rapid dissociative adsorption occurs on ion-bombarded surfaces of TiO, [3] and SrTiO, [3,5,61, with the emergence of characteristic hydroxyl adsorbate peaks. It is perhaps tempting to believe that water may be reduced to molecular hydrogen on the highly reducing bombarded surface and this accounts for the failure to observe H,O or OH adsorbate peaks. However, this would in turn require oxidation of the surface and depopulation of some of the W 5d states in the conduction band or in the bandgap. There is no indication of changes of this sort in difference spectra so one is forced to conclude that the bombarded surface shows an unusually inert behaviour. The reasons for this are not clear to us at present. In summary then the adsorption of water on defect-free annealed surfaces of Na~,~WO~(lOO) follows a pattern very similar to that for annealed planar surfaces of SrTiO,(lOO), i.e. no adsorption is found at room temperature, but at low temperatures, molecular adsorption occurs with preferential stabilisation of the 3a, molecular level. Defective, ion-bombarded surfaces differ from those of other oxides in being inert to adsorption at room temperature.
Acknowledgement -5
0
5
10
1.5
Binding Energy I eV Fig. 7. He(E) photoemission spectra measured at room temperature. (a) Defect-free annealed surface. (b) After ion bombardment with 2 keV argon ions. (c) After exposure of the bombarded surface to lo5 L water vapour. (d) Difference spectrum b-a, showing ~pulation of conduction band and bandgap states as a result of bombardment, as well as attenuation of the strong valence band feature at 4.4 eV binding energy. (e) Difference spectrum c-b, showing that water exposure leads to no attenuation of conduction band or defect state intensity and that no molecular adsorbate peaks appear.
We are grateful to Dr. K. Kang for growth of the tungsten bronze crystals used in the present work.
References [ll V.E. Henrich and P.A. Cox, The Surface Science of Metal Oxides (Cambridge University Press, Cambridge, 1993).
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F.H. Potter, R.G. Egdell /Adsorption
[2] D.W. Turner, C. Baker, A.D. Baker and CR. Brundle, Molecular Photoelectron Spectroscopy (Wiley, London, 1970). [3] S. Eriksen, P.D. Naylor and R.G. Egdell, Spectrochim. Acta 43 A (1987) 1535. [4] I.R. Owen, N.B. Brookes, C.H. Richardson, D.R. Warburton, F.M. Quinn, D. Norman and G. Thornton, Surf. Sci. 178 (1986) 897. [5] R.G. Egdell and P.D. Naylor, Chem. Phys. Lett. 91 (1982) 200. [6] P.A. Cox, R.G. Egdell and P.D. Naylor, J. Electron Spectrosc. Relat. Phenom. 29 (1983) 247. [7] C.A. Muryn, G. Tixvengadum, J.J. Crouch, D.R. Warburton, G. Thornton and D.S-L. Law, J. Phys. Condensed Matter 1 (1989) SB127. [8] R.L. Kurtz, R. Stockbauer, T.E. Madey, E. Roman and J.L. de Segovia, Surf. Sci. 218 (1989) 178. [9] P.A. Cox, Transition Metal Oxides: An Introduction to Their Electronic Structure and Properties (Clarendon, Oxford, 1992). [lo] W.R. Flavell, J.H. Laverty, D.S.L. Law, R. Lindsay, C.A. Muryn, C.F.J. Flipse, G.N. Raiker, P.L. Wincott and G. Thornton, Phys. Rev. B 41 (1990) 11623. [ll] J.P. Doumerc, M. Pouchard and P. Hagenmuller, in: The Metallic and Non-Metallic States of Matter, Eds. P.P. Edwards and C.N.R. Rao (Taylor and Francis, London, 1985) p. 287.
of water on Na,WO,(lOO) [12] A. Wold, W. Kunnmann, R.J. Arnott and A. Ferretti, Inorg. Chem. 3 (1964) 545. [13] V.E. Henrich, Surf. Sci. 284 (1993) 200. [14] G.A. Somorjai, Chemistry in Two Dimensions: Surfaces (Cornell University Press, Ithaca, 1981). [15] M.A. Lange11 and S.L. Bernasek, Surf. Sci. 69 (1977) 727. [16] M.A. Lange11 and S.L. Bernasek, J. Vat. Sci. Technol. 17 (1980) 1287. [17] S.D. Peacor and T. Hibma, Surf. Sci. 287/288 (1993) 403. [18] H. Hochst, R.D. Bringans and H.R. Shanks, Phys. Rev. B 26 (1982) 1702. [19] F.H. Potter and R.G. Egdell, Surf. Sci. 287/288 (1993) 649. [20] R.D. Bringans, H. Hochst and H.R. Shanks, Vacuum 31 (1981) 473. [21] R.D. Bringans, H. Hiichst and H.R. Shanks, Phys. Rev. B 24 (1981) 3481. [22] M.J. Campbell, J. Liesegang, J.D. Riley, R.C.G. Leckey, J.G. Jenkin and R.T. Poole, J. Electron Spectrosc. Relat. Phenom. 15 (1979) 83. [23] D. Schmeisser, F.J. Himpsel, G. Hollinger, B. Reihl and K. Jacobi, Phys. Rev. B 27 (1983) 3279. [24] J.K. Burdett, Molecular Shapes (Wiley, London, 1980). [25] D.G. Aitken, P.A. Cox, R.G. Egdell, M.D. Hill and I. Sach, Vacuum 33 (1983) 753.
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Surface Science 297 (1993) 286-292 North-Holland
.r.....
kurface science ........... ;;.;.;;:. .. ....... .............~,.~.:::::::~3~~:~~:~.~ I.~~:~:~:~~:~:~::~.:.:.:,,:::“:___: ,...‘.’ _,,,,,,, ._
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The adsorption of water on Na,WO,( 100) surfaces: a study by photoemission F.H. Potter
and R.G. Egdell
Inorganic Chemistry Laboratory, South Pa&v Road, Oxford OX1 3QR, UK
Received 24 May 1993; accepted for publication 22 July 1993
The adsorption of water on the WIO) surfaces of sodium tungsten bronze single crystals has been studied by photoemission at even on highly defective ion-bombarded surfaces. However, at 140 K, molecular adsorption occurs with preferential stabilisation of the molecular 3a, valence level.
hv = 40.8 eV. No adsorption is found at room temperature,
1. Introduction There has been a longstanding interest in the adsorption of water on oxide surfaces. Valence region photoemission has played an important part in helping to clarify the mode of adsorption [l]. Molecular H,O has three orbitals designated lb,, 3a, and lb, in the binding energy region below 20 eV [2]. Thus, non-dissociative adsorption is characterised by the appearance of three new peaks in difference photoemission spectra, two of which usually lie to high binding energy of the oxide 0 2p valence band [3]. By contrast OHhas only two corresponding electronic levels of u and r symmetry and dissociative adsorption may be fingerprinted by the appearance of a single peak below the 0 2p valence band, associated with the u bonding orbital [3-61. Note, however, that if the surface hydroxyl group is tilted relative to the surface normal, the degeneracy of the r levels may be lifted so that three peaks are again observed in photoemission [7] and the distinction between dissociative and non-dissociative adsorption requires detailed consideration of the binding energies. Non-dissociative adsorption on truly defect-free, low-index oxide surfaces is usually observed only at low temperatures, but this lowtemperature adsorption can be distinguished from simple condensation onto the surface by virtue of 0039-6028/93/$06.00
a pattern of molecular energy levels different to that of free H,O and of H,O ice. Specifically there is preferential stabilisation of the 3a, molecular level so that the separation A(3a,-lb,) is bigger than for free H,O, and the separation A(lb,-3a,) is less. The 3a, molecular orbital is directed along the C, rotation axis of H,O and the preferential stabilisation is consistent with a model where the molecule bonds to a surface cation via the oxygen atom, with resulting stabilisation of the highly directional 3a, level. This situation pertains for low-temperature adsorption on non-metallic oxide surfaces such as SrTiO,(100) [3,4,6], TiO,(lOO) [7] and TiO,(llO) [81. However, in a recent study of the metallic oxide Bi,Sr,CaCu,O,(OOl), which of course becomes superconducting at low temperatures 191, it was found that there was no specific stabilisation of the 3a, level [lo]. This was attributed to the termination of this layered material in a BiO plane within which the Bi ions carry a localised 6s-6p hybrid lone pair of electrons. These prevent the surface Bi3+ acting as a Lewis acid centre. In order to explore the validity of this idea we present here a photoemission study of water adsorption on Na,WO,(lOO). The tungsten bronzes Na,WO, (x > 0.3) are prototype cubic metallic oxides [ll], but in contrast to Bi,Sr, CaCu,O, there are no “lone pair” cations within
0 1993 - Elsevier Science Publishers B.V. All rights reserved
F.H. Potter, R.G. Egdell /Adsorption of water on Na,W03(100)
the structure and we are therefore able to explore in a more direct way the relationship between adsorption on this metallic perovskite phase and the non-metallic perovskite SrTiO,. In particular we are able to compare the relative stabilisation of the molecular levels at the two surfaces and to explore the possibility that specific interactions between the 3a, level and surface cations may be screened out by the itinerant conduction electrons.
2. Experimental The Na,WO,(lOO) single crystals used in the present study were grown by electrolytic reduction of molten mixtures of WO, and Na,WO, [12]. They presented square growth faces up to 1 cm x 1 cm whose (100) orientation was confirmed by Laue back-reflection. Crystals were polished with successively finer diamond pastes down to 0.25 pm and washed in propan-2-01 and distilled water to remove any trace of polishing material. The data presented here all relates to adsorption on a purple crystal with composition Na,,sWO,. For convenience we refer to this as Na,,WO,. However, similar results were obtained on an orange crystal with x = 0.67. Electron spectra were measured in an ESCALAB 5 Mark I spectrometer (VG Scientific Ltd., East Grinstead) equipped with facilities for XPS, UPS and LEED and a sample preparation chamber. The base pressure in the spectrometer main chamber was 5 x lo-” mbar. The crystals were mounted on platinum stubs and held in position with platinum wires. Atomically clean surfaces free of XPS signals due to carbon or other contaminants were produced by annealing in UHV at 680°C for several hours in the spectrometer preparation chamber (base pressure low9 mbar) with the aid of a water-cooled copper workcoil coupled to a 1.5 kW radiofrequency generator. The stub temperature was calibrated in a separate series of experiments in which a chromel-alumel thermocouple was spot-welded to the stub. Defective surfaces were produced by bombardment with 2 keV argon ions from a Penning-type ion gun. The sharp Fermi energy cut-off
287
in the conduction band in He(I) photoemission spectra provides a well-defined energy reference point, even though there is attenuation of substrate intensity with adsorption. In this respect adsorption studies are easier on metallic oxide substrates, such as that used here, than on nonmetallic oxides where adsorbate-induced band bending may make it difficult to align spectra prior to taking differences [13]. For location of adsorbate peaks, He(I1) radiation (hv = 40.8 eV) was preferred to He(I) radiation (hv = 21.2 eV> because the primary photoemission structure is less strongly influenced by secondary electron emission and the lower electron pathlength ensures greater surface sensitivity. All spectra were stripped of structure due to satellite radiation. The analyser resolution was set at 100 meV for He(I) photoemission measurements and at 400 meV for He(I1) measurements. Sample cooling in the main chamber was effected by pumping liquid nitrogen through the sample manipulator. A thermocouple monitorred the probe temperature to be 105 K, but the true sample surface temperature was considerably higher than this. Fitting a Fermi-Dirac distribution function to the photoemission cut-off for the cooled sample suggested a true surface temperature of about 140 K. Water was dosed onto the crystal surface through the ion gun, which provides a crudely collimated beam. The background pressure of H,O was monitorred by an ion gauge, but the background pressure was obviously much lower than the pressure at the sample surface. Surface fractional coverage in the submonolayer regime was therefore estimated by comparing the intensity of the adsorbate-induced molecular levels with the intensity of the main valence band. We assumed that ionisation cross sections for the adsorbate levels and for valence band states are roughly the same, which should be reasonable given that both are of dominant 02p atomic character. A value of 5 A was taken for the inelastic mean free path at hu = 40.8 eV. It was further assumed that below monolayer coverage water adsorbs only on surface cation sites, so that monolayer coverage corresponds to 6.8 x 1014 molecules/cm2. The effective exposure (in langmuirs) was then estimated through the usual
288
F.H. Potter,R.G. Egdell /Adsorption of wateron Na,WO,(lOO)
Knudsen equation [14] assuming unit sticking coefficient for water at low temperatures. The higher exposures were then derived by scaling in proportion to the integrated ion gauge reading.
3. Results and discussion The cleaned sample surfaces gave square LEED patterns (fig. 1) at room temperature with spots at Cm + l/2, n) and Cm, II + l/2) positions, but with streaking through both integral- and half-order spots and some diffuse intensity at (m + l/2, IZ+ l/2) (h ere n and m are integers). This suggests dominance of orthogonal (2 x 1) and (1 x 2) domains, in agreement with the experiments of Lange11 and Bernasek [l&16]. However, on cooling to 140 K, the patterns became much sharper and the intensity at Cm + l/2, n + l/2) increased, demonstrating the emergence of a well-defined (2 x 2) superstructure. The (2 x 2) superstructure was also observed at room temperature on several occasions after prolonged sample cleaning. It is clear from the recent work of Peacor and Hibma [17] that the interplay between (2 x 1) and (2 X 2) superstructures depends intimately on the sample pretreatment. The Na,WO,(lOO) surface may terminate in WO, or Na,O planes, or in domains of both. The superstructures were originally attributed to ordering of Na ions at the surface [15,161, but the more recent work suggests that tilting of WO, octahedra at the surface plays a crucial role in defining the surface reconstruction [17]. He(I) photoemission spectra such as that shown in fig. 2 were obtained from the annealed surfaces. These show a weak but well-defined conduction band peak extending from the Fermi energy down to about 1 eV. There is then a gap before a sharp valence band onset 2.8 eV below the Fermi energy. The valence band itself shows two sharp shoulders at 3.2 and 4.4 eV binding energy: these are resolved as distinct peaks in spectra of samples with somewhat higher x values [18,19]. The valence band contains in addition a strong broad peak with maximum intensity at 6.1 eV. Argon-ion bombardment with 2 keV ions at a flux of 3.5 kA/cm’ leads to major changes
Fig. 1. LEED patterns for Na,,,WO,(lOO) measured at 60 eV beam energy slightly away from normal incidence. (a) Clean surface at room temperature showing pattern characteristic of orthogonal (2 x 1) and (1 x 2) domains. (b) Clean surface at 140 K. New spots at (l/2, l/2) and related positions are characteristic of (2 x 2) superstructure.
in the valence spectra (fig. 2). In particular there is a large increase in intensity of emission at the Fermi energy and the well-defined gap between valence and conduction bands is filled in with new states. The valence band edge shifts downward relative to the Fermi energy by about 1 eV. These changes are consistent with preferential sputtering of oxygen and population of new states of dominant W 5d atomic character. The elec-
F.H. Potter, R.G. Egdell /Adsorption
1
(a)
00 I
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/
I
I
-5
5 10 Binding Energy / eV
I
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Fig. 2. He(I) photoemission spectra of Nao,6WOs(100) (a) after annealing in UHV to produce ordered (‘2X 2) surface, (b) after argon-ion bombardment with 2 keV ions at 3.5 PA sample current for 10 min. Structure due to satellite radiation has been subtracted from the spectra.
289
of water on iVaxWO~~~lOO~
states extending down to the valence band edge appears after ion bombardment [20,21]. He(I1) photoemission spectra of annealed Na*.~WO~(lOO) are similar to He(I) spectra (fig. 3), but the bottom of the valence band is reached before onset of the strong secondary electron structure that is charactersistic of He(I) spectra and He(H) photoemission is therefore preferred in the study of ad~rbate-produced features below the main valence band. Exposure of the annealed surface at room temperature for exposures up to 800 L led to only minor changes in
1
I
!
1
I
trons associated with the oxygen vacancies partially occupy conduction band states, but in addition there is clearly a large density of midgap states extending down to the valence band edge. The photoemission structure for ion-bombarded Na,,WO, is somewhat reminiscent of that found after bombardment of WO,(lOO). WO, is a 5d0 oxide, with no conduction band emission when stoichiometric. However, a large density of defect
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10
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Binding Energy / eV 0
-5
/
/
/
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5
10
!
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Binding Energy / eV Fig. 3. He(H) photoemission spectra of Naa,,WOs(lOO) at room temperature (a) after annealing in UHV, (b) after exposure to 800 L water vapour.
Fig. 4. He(H) photoemission spectra of Na,,,WO,(lOO). (a) Room temperature. (b) After cooling to 140 K. There is some indication of adsorption of molecular H,O from the residual vacuum. (c) At 140 K following exposure to 0.8 L H,O. The surface coverage is estimated as 3.9X 1014 molecules/cm*. Cd) Following 1.6 L exposure to H,O at 140 K (e) Following 16 L exposure to H,O at 140 K. (f) Following 160 L exposure to H,O at 140 K.
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F.H. Potter, R.G. Egdell /Adsorption of water on Na,WO,(lOO)
He(I1) photoemission spectra: difference spectra showed no well-defined molecular (H,O) or hydroxyl (OH-) adorbate peaks. By contrast, exposure of defect-free surfaces to H,O at 140 K leads to progressive changes in the spectra (fig. 4) and at the largest exposure (160 L), the spectra are essentially those of ice [22,23]. No new superstructures appear in LEED and the only pattern to be observed is (2 x 2) although of course this pattern is progressively extinguished as the ice layer builds up. Difference photoemission spectra (fig. 5) reveal that down to the lowest exposures adsorption is characterised by the appearance of three new peaks, corresponding to the molecular orbitals of H,O. However, as on SrTiO, it is found that at low coverages the 3a, level is preferentially stabilised relative to the lb, and lb, levels [3,4]. This is emphasised in fig. 6 which shows the position of the molecular adsorbate levels relative to that of the lb, level. In
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2
3
4
5
6
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Relative Binding Energy I eV Fig. 6. Energies of lb,, 3a, and lb, molecular peaks relative to the lb, peak in (a) free H,O (ref. [2]), (b) water on Bi,Sr,CaCu,O,(OOl) at 1 L exposure (ref. [lo]), (c) water on Na,~,WO,(lOO) at 1.6 L exposure (present work), (d) water on SrTiO,(lOO) at 2 L exposure (ref. [3]), (e) ice (present work).
*..
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20
Fig. 5. He(U) difference photoemission spectra relating to spectra in fig. 3: (a) b-a: weak adsorbate features are due to water adsorption from the residual vacuum, (b) c-b, corresponding to 0.8 L exposure, (c) d-b, corresponding to 1.6 L exposure, (d) e - b, corresponding to 16 L exposure.
terms of a donor-acceptor model for the interaction between water and the oxide surface, the specific stabilisation of the 3a, level implies an interaction between the filled 3a, donor level and an empty level on surface cation, probably the e&a *) level of W. This implies net electron transfer out of the 3a, level into the surface. In a Walsh diagram approach to the bonding in H,O [24], occupation of the 3a, level is largely responsible for the non-linear geometry of the H,O molecule. Complete removal of a 3a, electron to give HzO+ in the ‘Ai state leads to both an increase in the HOH bond angle and a decrease in the HOH bending frequency from 1595 to 975 cm-’ [2]. It is somewhat surprising then that an earlier HREELS study of water adsorption on Na,WO,(lOO) surfaces [25] revealed an increase in the HOH bending frequency to the exceptionally high value of 1740 cm-‘. However, the earlier HREELS work supports the present conclu-
F.H. Potter, R. G. Egdeli / Adsorption of water on Ma, W03(100f
sion that low-temperature adsorption differs from simple condensation to give an ice layer. Specifically it was found that at low coverages, a sharp OH asymmetric stretch mode characteristic of non-H-bonded H,O is observed at much higher frequencies than the broad stretch characteristic of hydrogen-bonded molecules. Returning to fig. 6, it is clear that the inert cleavage surface of Bi~Sr~~Cu~Os(OOl) is the exception amongst the three oxides in not showing a specific stabilisation of the 3a, level. This reinforces the conclusion that the behaviour of the latter material is related to the unique fea-
(e)
291
tures of the inert BiO cleavage plane and is not a simple consequence of the metallic nature of the oxide [lo]. Turning next to argon-ion-bombarded surfaces, fig. 7 shows the effects of exposure of a highly defective ion-bombarded surface to water. No adsorbate-induced features appear in difference spectra. This is a surprising result because in previous work it has been found that rapid dissociative adsorption occurs on ion-bombarded surfaces of TiO, [3] and SrTiO, [3,5,61, with the emergence of characteristic hydroxyl adsorbate peaks. It is perhaps tempting to believe that water may be reduced to molecular hydrogen on the highly reducing bombarded surface and this accounts for the failure to observe H,O or OH adsorbate peaks. However, this would in turn require oxidation of the surface and depopulation of some of the W 5d states in the conduction band or in the bandgap. There is no indication of changes of this sort in difference spectra so one is forced to conclude that the bombarded surface shows an unusually inert behaviour. The reasons for this are not clear to us at present. In summary then the adsorption of water on defect-free annealed surfaces of Na~,~WO~(lOO) follows a pattern very similar to that for annealed planar surfaces of SrTiO,(lOO), i.e. no adsorption is found at room temperature, but at low temperatures, molecular adsorption occurs with preferential stabilisation of the 3a, molecular level. Defective, ion-bombarded surfaces differ from those of other oxides in being inert to adsorption at room temperature.
Acknowledgement -5
0
5
10
1.5
Binding Energy I eV Fig. 7. He(E) photoemission spectra measured at room temperature. (a) Defect-free annealed surface. (b) After ion bombardment with 2 keV argon ions. (c) After exposure of the bombarded surface to lo5 L water vapour. (d) Difference spectrum b-a, showing ~pulation of conduction band and bandgap states as a result of bombardment, as well as attenuation of the strong valence band feature at 4.4 eV binding energy. (e) Difference spectrum c-b, showing that water exposure leads to no attenuation of conduction band or defect state intensity and that no molecular adsorbate peaks appear.
We are grateful to Dr. K. Kang for growth of the tungsten bronze crystals used in the present work.
References [ll V.E. Henrich and P.A. Cox, The Surface Science of Metal Oxides (Cambridge University Press, Cambridge, 1993).
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[2] D.W. Turner, C. Baker, A.D. Baker and CR. Brundle, Molecular Photoelectron Spectroscopy (Wiley, London, 1970). [3] S. Eriksen, P.D. Naylor and R.G. Egdell, Spectrochim. Acta 43 A (1987) 1535. [4] I.R. Owen, N.B. Brookes, C.H. Richardson, D.R. Warburton, F.M. Quinn, D. Norman and G. Thornton, Surf. Sci. 178 (1986) 897. [5] R.G. Egdell and P.D. Naylor, Chem. Phys. Lett. 91 (1982) 200. [6] P.A. Cox, R.G. Egdell and P.D. Naylor, J. Electron Spectrosc. Relat. Phenom. 29 (1983) 247. [7] C.A. Muryn, G. Tixvengadum, J.J. Crouch, D.R. Warburton, G. Thornton and D.S-L. Law, J. Phys. Condensed Matter 1 (1989) SB127. [8] R.L. Kurtz, R. Stockbauer, T.E. Madey, E. Roman and J.L. de Segovia, Surf. Sci. 218 (1989) 178. [9] P.A. Cox, Transition Metal Oxides: An Introduction to Their Electronic Structure and Properties (Clarendon, Oxford, 1992). [lo] W.R. Flavell, J.H. Laverty, D.S.L. Law, R. Lindsay, C.A. Muryn, C.F.J. Flipse, G.N. Raiker, P.L. Wincott and G. Thornton, Phys. Rev. B 41 (1990) 11623. [ll] J.P. Doumerc, M. Pouchard and P. Hagenmuller, in: The Metallic and Non-Metallic States of Matter, Eds. P.P. Edwards and C.N.R. Rao (Taylor and Francis, London, 1985) p. 287.
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