Surface chemistry of H2S-sensitive tungsten oxide films

ELSEVIER

Sensorsand Actuator;B 31 (1996) 167-174

Surface chemistry of H2S-,;ensitive tungsten oxide films B . F r t i h b e r g e r ~,1, M . G r u n z e b, D . J . D w y e r ~ "Department of Chemistry and laboratory for Surface Science and Technology. University of Maine. 5764 Sawyer Research Center. Orono. ME 04469-5764° USA b Lehrstuhlfiir Angewandte Physikalische Chemieo lnstitutfiir Physilmlische Chemie. Universitiit Heidelberg. INF 253. 69120 Heidelberg, Germany

Received31 January 1995;in revisedform29 July 1995;accepted2 August1995

Abstract

We have applied X-ray photoelectron spectroscopy (XPS) arid ultraviolet photoelectron spectroscopy ( UPS ) ~ocharacterize t'lin tungsten oxide films and to investigate their interaction with hydrogen sulfide in view of their use as very sensitive hydrogen sulfide gas sensors. W 4f core-level spectra indicate a partial reduction of W~+ after the ~eaction.No evidence for band bending after H2S dosing could be found in the valence-band spectra. The results suggest that the primary sensing mechanism involves the formation of oxygen vacancies on the surface in the presence of hydrogen sulfide. Alternative mechanisms, such as the formation of a tungsten sulfide or a hydrogen tungsten bronze on the surface, are judged to be unlikely. Keywords: Hydrogensulfidesensors;Tungstenoxide

1. Introduction Thin films of tungsten oxide have received con~+iderable attention as gas-sensing elements [ 1-4 ], in particular for their application in surface acoustic wave (SAW) t~+evices as highly sensitive gas sensors for H2S in ambient air [ 5-8 ]. Xu et al. have studied the electrical properties of both intrinsic and gold-doped tungsten oxide films prepared by r.f. sputtering [6]. In their study, the authors were able to establish that the primary change in the films upon exposme to H2S is a rise in electrical conductivity as a result of the creation of additional negative charge carriers. Gold doping decreases the response time and increases the sensitivity of the films to H2S. Annealing treatments of 'as-depositec~' films in air considerably enhance the long-term stability and reproducibility of the films with respect to their response to H2S. Smith et al. [7] have shown that properly treated gold-doped tungsten oxide films are able to detect H2S in the ppb range at about 475 K with very high selectivity. The interface chemistry responsible for the observed conductivity changes is still very ltale understood. Several authors have suggested that a conducting surface layer of WS2 can be formed on WO3 upon exposure to H2S [3,9]. Dwyer [4] has studied the interaction of H2S with ultra-thin ( < 20/~) WO3 films, prepared by evaporating a tungsten t Present address: Corporate F~.esearchLaboratories. Exxon Research Researchand EngineeringCompany,Annandale.NJ 08801, USA. 0925-40051961515.00© 19% ElsevierScienceS.A. All rightsreserved SSD! 0925 -4005 ( 95 ) 01809- X

filament onto a gold substrate in a background of oxygen. In that study, a partial reduction of WO3 was fouad after exposure to the gas. Formation of a hydrogen tungsten bronze upon decomposition of H2S is also a possible interpretation. Improvement of the critical sensor properties cannot simply rely on empirical observations, but requires a more fundamcntal understanding of the interface phenomena inv,~lved. As has been demonstrated, surface analytical techniques can be used to investigate the sensing mechanism in gas sensors and to assist efforts to construct faster and more reliable devices [8,10]. In the present study, we have used X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) in an attempt to understand further the surface chemistry responsible for the macroscopic response of the H+S-sensitive tungsten oxide films.

2. Experimental The tungsten oxide films were prepared using r.f. and d.c. sputtering techniques as described by Xu et al. [4]. The investigated films were about 500 ,~, thick, deposited onto 1 cm e tungsten foil substrates. After sputter deposition, the samples were introduced into an ultra-high vacuum (UHV) system (base pressure 1 × 1 0 - ' ° torr) for photoemission experiments. The UHV chamber was equipped with a sepa-

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rate high-pressure (up to 10 bar) reaction chamber. The pressure in this reaction chamber was determined with a capacitance manometer, and the chamber was fitted with a circulatingpump to eliminatecomplicationsarising from diffusion effects in the gas phase. The samples were mounted on a stainless-steel block on a transfer arm, which allowed translation from the reaction chamber into the UHV chamber for XPS or UPS analysis without exposure to the laboratory ambient. The sample mounting block could be heated by an integrated heating element. The temperature was measured with a type-K thermocouple spot welded to the sample mounting plate. The experimental set-up for XPS consisted of a Leybold dual anode Mg/Ai X-ray source and a Leybold EAI I hem'~spherical electron energy analyser. All spectra were taken with the Mg Ka radiation at 360 W power ( 12 kV, 30 mA). The pass energy of the analyser in the A E = constant mode was 100 eV for S 2p spectra and 20 eV for O 1s and W 4f spectra. Binding energies were referenced to the Au 4f7/2 line at 84.0 eV. For the S 2p spectra, signal averaging times of up to 15 s per data point were necessary to obtain reasonable signal-to-noiseratios (especially at low coverages of S). UPS spectra were acquired with excitation from a differentially pumped He discharge lamp for ultraviolet radiation (UVS 10/35, Leybold-Heraeus GmbH); the electron analyser was operated at 5 eV pass energy. Care was taken not to exceed the iinearity limit of the electron detector at the rather high intensities found in the UPS spectra, which otherwise would strongly modify the spectra. No charging effects were found in the spectra, e.g., alteration of the spectral features with incident photon flux, broadening of spectral features, etc. The position of the sample Fermi levels could thus be directly determined from a sputter-cleaned Au foil. Gases used were 99.5% liquid-phase purity H2S and research-grade 02 (both obtained from Matheson).

3. Results

3.1. Characterization of sputter-deposi,red tungsten oxide films The srauctural characterization of the: WO3 sputter-deposited films remains to be investigated in detail [ 11 ]. The XPS used in this stuc2yallows a compositionalanalysis of the films. The W 4f core-level binding energy displays a rather strong dependence on the oxidation state of the W atom. For the oxides, measured values for the W 4t"7/2line of the spin--orbit doublet range from about 31 eV binding energy for W° in metallic tungsten to about 36 eV forW 6. in bulk WO3 (XPS studies of tungsten oxides, sulfides and related compounds can be found in Refs. [4,12-18]). Thus XPS can supply important information on the surface composition of the r.f. sputter-deposited films before and after exposure to H2S. It should be noted that given the rather high energies of the available X-ray excitation ( 1253.6 eV), the surface sensitivity for W 4f photoelectrons is not very high. Assuming an

•

|f7/2

after heat treatment

, 42

40

38 36 Binding Energy [eV]

~

as deposited

34

32

after heat treatment

as deposited , 536

L 534

t I 532 530 Binding Energy [eV]

,

I 528

Fig. I. (a) W 4 f and (b) 0 Is XPS spectra of an as-deposited tungsten oxide film, and of the same film after heat treatment in ambient air.

electron attenuation length of 20/~ for the W 4f photoelectrons, tungsten atoms located at a depth of 45/~ from the sample surface still account for 10% of the total signal intensity in normal emission. The films on tungsten foil were analysed before and after heating in ambient air at 675 K for 1 h. The heat treatment has been shown to lead to a phase transition of the films from an amorphous to polycrystalline state [ 19-21 ] and to a loss of water [20,21 ]. Before heating, the 'as-deposited' films were light blue in colour. During the heating procedure the colour changed to pale yellow. Corresponding XPS spectra of the W 4f and O Is lines are shown in Fig. 1. The total width of the W 4f spin-orbit doublet decreases slightly with the heat treatment, and the valley between the W 4t"5/2and W 4f~/2 lines becomes more pronounced. The binding energy for the W 4t"7/2 line of 35.6 eV agrees well with literature values for W6+. The O Is line mainly displays a change in symmetry, becoming more symmetric during heating. The line shapes and relative intensities of the W 4f and O Is spectra after heating were indistinguishable from spectra of WO3 powder pressed into an indium foil (Fig. 2). If the WO3 powder is considered to be stoichiometric, it can be assumed

B. Briihberger et al. / Sensors and Actuators B 31 (1996 ~ 167-174

169

W

g

, to

,

,

8

6

, 4

, 2

~

,

o=E,;

Binding Energy levi F i g . 3. H e l U P S s p e c t r u m L~_I 43

42

, I , ! , : 41

40

39

1~ 38

I , I , I 37

36

35

powder 34

33

32

Binding Energy [eV] Fig.2. Comparisonof the W4f XPSsignalsof a heat-treatedtungstenoxide film,and of WO3powderpressedintoindiumfoil. that the deposited films are fully oxidized after heat treatment with a O/W ratio very close to three. Using the following equation [ 22]:

IA In

NAG(EA)CrAAA NnG(EB)o'nAB

to derive a relative composition for elements A and B with atomic densities NA and Nn, a value of 3.05 + 0.05 was found for the O/W ratio from the integrated intensities of the XPS lines after a Shirley background subtraction [23]. Scofield cross sections [ 24 ] were used for cr, and the energy dependence of G (analyser transmission) and A (electron attenuation length) were taken as G(E)=coast. E -°73 [25] and A(E) =const. ~o.5 [26]. 'As-deposited' films before heat treatment were always found to be sub-stoichiometric with varying O/W ratios. The changes in line shape of the W 4f spectra can thus be attributed to an oxidation of W atoms in lower oxidation states to W6+ in stoichiometric WO3. The change in line shape of the O I s feature could indicate the presence of a variety of chemical states of oxygen atoms in the sub-stoichiometric film before the heat treatment (OH-, adsorbed H20, WOx ( x < 3 ) ) , whereas after heat treatment mainly one chemical state remains. Alternatively, the loss of asymmetry in the oxygen line shape could be caused by the disappearance of electronic states within the band gap of the material upon full oxidation. Sub-stoichiometric tungsten oxides are known to exhibit a finite density of states in the band gap [ 16]. It should be noted, however, that although the XPS analysis appears to indicate the films to be stoichiometric WO3 after the heat treatment, this does not rule out the possibility of a low concentration of oxygen vacancies at the surface, since the surface sensitivity of XPS in the kinetic-energy range of the W 4f transition is rather low. In UPS with HeI radiation, the electron attenuation length is only about 5 A, making the surface sensitivity of the tech-

of a heat-treated

tungsten

oxide film.

nique very high. The HeI UPS spectrum of a heat-treated film is shown in Fig. 3. The two prominentfeatures in the specffum are readily identified. The valence band in WO3 has W 5d or O 2p character [27,28]. The peak at =4.25 eV binding energy is thus mainly due to photoemission from O 2p states; the peak at = 6.3 eV is dominated by photoemission from W 5d levels. In addition to these two spectral features, there is clearly a finite density of states in the band gap around = 2 eV binding energy, which has been attributed in the literature to oxygen vacancies at the surface [29,30]. Impurity levels (mainly carbon) on the heat-treated films were small, even though the films were usually allowed to cool to room temperature before introduction into the vacuum system. Attempts to reduce impurity levels further by Ar + ion sputtering irreversibly produced a highly reduced surface by preferential removal of oxygen, thus enriching the surface in metal. Preferential removal of oxygen by Ar + sputtering has also been observed by Salje et at. [ 14] and by Bringans et at. [29] in single-crystal studies of WO3. In the laRer reference it was also established that WO3 is unstable under electron irradiation (LEED, Auger). Using UPS, the authors found an increase in the density of states around 2 eV below the Fermi level under both Ar + ion bombardment and electron irradiation, which was attributed to loss of oxygen. It was found, however, that as an alternative cleaning procedure, heat a eatments (850 K, 600 s) of the films in high pressures ( 1500 torr) of pure oxygen substantially reduced the carbon concentration on the surfaces. Since the films were already fully oxidized after heating in ambient air, further heating in pure oxygen did not alter the O/W ratio of the films. Oxygen heat treatments were therefore used to rid the film surfaces of impurities. The WO3 films are only moderately stable with respect to heating in vacuum. XPS spectra of the W 4fand O Is core levels showed that heating in vacuum above approximately 600 K also leads to a loss of surface oxygen.

3.2. Interaction with H~ Fig. 4 shows XPS spectra of a WO3 film after exposure to H2S and then 02 at a sample temperature of 475 K (the

170

B. Bri.ihberger et aL / Sensors and Actuators B 31 (1996) 167-174

(a)

W4f

_=_--

44

42

40

38

36

~g

536

32

534

532

530

528

Binding Energy [eV]

Binding Energy [eV]

(C)

S2p

e

d

b

175

170

165

160

155

Binding Energy [eV]

Fig. 4. (a) w 41',(b) O l s and (c) S 2p XPS spectraafterexposureto H:S and subsequentlyto Oz. Sampletemperature was 475 K for all indicatedtreatments. a, clean; b, I x 10-4 torr H2S,200 s; c, 2 x 10-4 tort H~S,300 s; d, 760 torr H2S,30 s; e, 760 tort O2, 30 s; f, 760 tort O2, 2 h. temperature at which the conductivity of the films is most sensitive to H2S exposure). The W 4f spectra gradually broaden and the valley between the spin-orbit doublet becomes less exposed with increasing exposure to H2S. These changes in peak shape are completely reversed upon exposure to 02. W 4f spectra of clean samples were indistinguishable from spectra taken of samples that had been exposed to initially large amounts of H2S (exposures as high as 2 bar, 30 min at 475 K were used) and subsequently extended exposures to 02 under comparable pressure and temperature conditions. During such experiments, the sample colour changed from pale yellow to a dark blue with exposure to H2S and returned to pale yellow after exposure to 02. The total peak

area of the W 4f spectral region remained constant within the experimental error for the spectra in Fig. 4. The O ls spectra displayed an increase in asymmetry on the high-binding-energy side of the spectra and a continuous decrease in intensity with increasing exposure to H2S. After exposure to 02, the intensity of the O I s feature of the clean sample was regained. The peak shape, however, remained slightly less symmetric. Two species of adsorbed sulfur were found after an initially clean sample was exposed to H2S. This can be seen in the S 2p spectra of Fig. 4. Initial exposure to the gas resulted in intensity at 168.8 eV binding energy, which saturated with increasing exposure. After higher exposures, a second feature

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B. Briihberger et aL / Sensors and Actnators B 31 (1996) 167-174

appeared at 162.0 eV binding er,e,"gy. The formation of the higher-binding-energy feature is apparently favoured at initial exposures. The surface is rather unreactive, as may be judged from the small absolute intensities in the sulfur signal even after exposures to atmospheric pressures of H2S. Upon exposure to 02, the sulfur feature at low binding energy was completely removed. On the other hand, the feature at 168.8 eV remained nearly constant in intensity. It was impossible to remove completely the high-binding-energy species to regain a clean surface, which was attempted by heating in vacuum (up to 700 K) and in atmospheric pressures of oxygen. Heating in vacuum to ---700 K did, however, lead to a reduction in spectral intensity. Analysis of the W 4f spectra of Fig. 4 indicates that a reversible change with respect to the W core-level spectra takes place when the film is subjected to H2S and subsequently 02. On comparing the S 2p spectral changes with the changes in the W 4f spectra, it becomes apparent that the formation of the high-binding-energy sulfur species has a negligible effect on the W 4f spectra, although there clearly remained spectral intensity at 168.8 eV in the S 2p spectra after exposure to O2. The meas~ared binding-energy values suggest that the spectral feature ~Lthigh binding energy, represents an SO4 surface sulfate species, and that the feature at low bindmg energy results from sulfur in the form of sulfide [31 ]. The presence of the surface sulfate species might explain the asymmetry observed in the O Is spectra in Fig. 4 after exposure to H2S (which is re,tained even after exposure to O2), in that the spectra contain a contribution from O Is photoelectrons from sulfate oxygen at higher binding energy. The occurrence of such a species upon exposure to 1-12Sis somewhat surprising, and the available data would only allow speculations on its formation, in particular since little information on the exact surface termination is available. Formation of a surface sulfide upon exposure to H2S has been reported for a number of transition-metal oxides [32-35]. For TIP2, an anion exchange has been proposed, which leads to formation of a TiS2 surface layer [33]. It seems well established that supported MOO3, used as a hydro-desulfurization catalyst, forms MoS2 surface phases upon sulfurization [36] and a bulk transformation to WS2 occurs for WO3 powder when allowed to react with H2S at atmospheric pressure and elevated temperatures (i.e., g25 K) [341. The quantitative analysis of XPS data requires assumptions on elemental distribution in the near-surface region. This makes it difficult to decide here whether it is an exchange of S 2- for 0 2 - anions that leads to the sulfide S 2p XPS signal on the WO3 films, or whether a redox reaction, such as 3WO 3 + 7H2S ~

3WS2 + SO2 + 7H20

takes place on the surface, involving a reduction of W ~+ atoms to W 44.. The changes in the W4fspectraupon exposure to H2S suggest the creation of other oxidation states of W atoms. An initial state binding-energy shift could be expected for a simple anion exchange (without reduction of W 6+,

lO (a)

e 6 4 0=6 BindingEnergy [eV]

3 (c)

(b)

BindingEnergy leVI

2 I o =E, BindingEnergy[eV]

Fig. 5. (a) Hel UPS spectra of a WO~ film after increasing exposures to H2S and reoxid~ion with 02. Sample temperature was 475 K for all indicted

treatments.(b ) Band-gapregionsampledwithhigberresolution.(c ) Spectra beforeand aftersulfurizationdisplayedwithoutoffset (see text for details). a, clean;b. I × 10-4 tow H2S,20 s; c, I × 10 - 4 t o r f n 2 s , 200 s; d. 2 X !0-4 ton H2S,300 s; e, 760 tort H2S,30 s; f, 760 torr(3.,tow Oz, 30 s. given the lower electronegativity of S compared to O), but also if surface oxygen loss occurred, which would lead to oxygen vacancies and W 5 + centres. For the interpretation of the XPS results it is significant to note that both the W 4fand O ! s XPS signals show continuous changes with increasing exposure to H2S, whereas no continuous increase in the S 2p signal intensity for the sulfide species was observed. A clear S 2p signal at = 162 eV binding energy was not seen except for very large exposures to H2S. Given these facts, it appears reasonable to contribute the initial changes in the W 4f and O Is XPS signals to the formation of oxygen vacancies, perhaps via reaction of S and/ or H (after decomposition of H2S) with lattice oxygen to form desorbing SO2 and/or H20. The appearance of the sulfide S 2p signal after extremely high exposures then indicates the onset of the formation of a bulk sulfide as observed in Ref. [ 3 4 ] . In addition to XPS spectra, HeI UPS spectra in normal emission were recorded during the above experiment. The spectra obtained are shown in Fig. 5(a). Fig. 5(b) displays a second set of spectra of the band-gap region, measured with higher resolution in the same experiment. The most obvious change in the spectra upon initial exposure to H2S is a decrease in intensity of the O 2p-derived feature with respect to the mainly W 5d-derived feature (this seems true also when variations in the inelastic background are taken into account). A rather drastic change occurred in the spectra after exposure to high pressures of H2S (760 torr, 30 s), suggestive of the formation of a new surface compound. There appears to be a shift of the O 2p-derived structure to higher binding energy

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B. Briihberger et al. /Sensors and Actuators B 31 (I 996) 167-174

with increasing exposure to HaS, possibly indicating band bending in the surface region of the substrate. Band bending is a result of electrostatic interactions between electrons in the near-surface region and a partially charged adsorbate layer, which leads to changes in the electron energy levels. If this was the case, the binding energies of the W 4f and O ls XPS core-level peaks should also shift during the experiment, which was not observed, even for the highest exposure to HaS. In addition, a closer look at Fig. 5(b) reveals that there is no shift in the valence-band cut-off at ~ 2.7 eV binding energy. Band bending, therefore, does not appear to be significant upon exposure to HaS, and the observed changes in the valence-band structure are more likely to be caused by chemical changes, such as replacement of oxygen atoms with sulfur or the creation of oxygen vacancies. Inspection of the spectral changes in the gap region (Fig. 5(b)) shows a reduction in the intensity between 1.5 and 2.0 eV binding energy. Since this intensity is attributable to oxygen defects at the surface [29,30], it would appear that the initial reaction of HaS with the surface removes these vacancies. Alternatively,it could be argued that there is an increase in intensity at -- 2.5 eV binding energy, which gradually fills the valley between the oxygen-defect-derived feature and the valence-band edge with increasing exposure. An overlay of the spectra seems to support this latter argument, as exemplified in Fig. 5(c), where spectra a (clean surface) and d ( after exposure to 2 × 10- 4 tort HaS for 300 s ) of Fig. 5 (b) are displayed without offset. The intensity at = 2.5 eV binding energy could be due to S 3p emission, which for sulfurized TiOa occurs at ~ 3.5 eV binding energy [35], or it could be caused by the formation of additional oxygen vacancies. The latter interpretation appears favourable, since it is consistent with the XPS results. Inspection of spectrum f in Fig. 5(b) shows that, upon exposure to O2, the spectral features of a clean surface are regained. The possible formation of a hydrogen tungsten bronze, HxWO3, upon exposure to HaS was suggested in Section I. The presented UPS spectra seem to rule out the formation of such a compound. UPS spectra of tungsten bronzes (even for small x) show a clear peak near the Fermi energy [37,38]. Although there are possibly slight modifications in the density of states near the Fermi energy in Fig. 5 (b), the spectra never resembled spectra for the HxWO3 band-gap region given in the literature. No evidence for HxWO3 formation was thus found, even after extensive exposure to HaS. These findings are further supported by the results of Bringans et al. [37], who did not find any indication of HxWO3 formation after exposing a WO3 (001) single-crystal surface to H atoms or H + ions.

4. Discussion The presented experimental results suggest that additional oxygen vacancies are produced when the films are exposed to HaS. W 4f core-level spectra indicate a partial reduction

of W6+ after the reaction and the density of states in the band gap of the material is changed by the introduction of additional electronic states above the valence band edge. The created oxygen vacancies can be reoxidized fully reversibly in 02. An onset of sulfide formation on the surface was not observed until atmospheric pressures of pure H2S were used for sulfurization. We propose that the change in conductivity used in gassensing applications of WO3 films is caused by the formation of additional surface oxygen vacancies. These vacancies are created by the reaction of H2S decomposition products (S and H) with lattice oxygen and the subsequent desorption of reaction products (probably SO2 and H20). If a film is exposed to a certain gas mixture of H2S and 02 under flow conditions, a steady-state concentration of vacancies should develop, determined by the competition between the formation of the oxygen vacancies by interaction with H2S and the reoxidation by O: A difference i,a activation energy for these two competing processes could explain the observation of an optimal operating temperature for the gas sensor at which the response displays a maximum [4]. The steady-state oxygenvacancy concentration necessary to produce the observed changes in conductivity could be very small. The experimental data of Smith et al. [7] show that = 3 × 1012 charge carriers per cm3 were created during exposure to 10 ppm HaS in ambient air at 475 K (which leads to a 30-fold increase in conductivity). For a 500 & thick film, this carrier concentration amounts to as few as 1 × 107 in a 1 cm × 1 cm × (5 × 10 -6) cm volume, compared to roughly 10Is surface atoms per cma. Although little is known about the exact mechanism of the carrier creation, these numbers demonstrate that the extent of surface changes during actual sensor operation may well lie below the detection limit of standard surface-science analytical techniques (about a few percent of a monolayer).

5. Conclusions The compositional characterization of r.f. sputter-deposited WO~ films employed in gas sensing showed that asdeposited films were sub-stoichiometric, with various O/W ratios. Heat treatment in ambient air leads to nearly fully stoichiometric films with small numbers of surface oxygen vacancies. Heat-treated films contain only small amounts of surface impurities (mainly carbon). For surface studies, impurity concentrations are best reduced by heating in pure 02, since Ar + ion sputtering or heating in vacuum results in loss of surface oxygen. The interaction with H2S was studied at 475 K, where the sensitivity of the fihn to the H2S gas is highest. When the films are exposed to H2S additional oxygen vacancies are produced. W 4f core-level spectra indicate a partial reduction ofW 6+ after the reaction and the density of states in the band gap of the material is changed by the introduction of additional electronic states above the valence-band edge. The created oxygen vacancies can be reoxidized reversibly in 02.

R Brahberger et aL I Sensors and Actuators B 31 (i 996) 167-174 T h e o n s e t o f sulfide formation on the surface was not o b s e r v e d until a t m o s p h e r i c p r e s s u r e s o f pure H2S were used for sulfurization. T h e g a s s e n s o r s ' c h a n g e in conductivity is m o s t likely c a u s e d by the formation o f a steady-state concentration o f surface o x y g e n v a c a n c i e s w h e n the s e n s o r is e x p o s e d to a g i v e n partial pressure o f H2S in air.

Acknowledgements W e gratefully a c k n o w l e d g e financial support by the National Science F o u n d a t i o n , grant no. ECS-9019551, and travel support f r o m the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t a n d the M a x B u c h n e r Stiftung. W e thank Professor John Vetelino a n d h i s s e n s o r g r o u p at the University o f M a i n e for providing the t u n g s t e n oxide films u s e d in this study a n d for m a n y helpful s u g g e s t i o n s a n d discussions. W e w o u l d also like to t h a n k D r D a v i d Frankel for his help.

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Biographies B e r n d Friihberger received his Diploma degree (1989) in chemistry from the University of Heidelberg (Germany) for research conducted at the Fritz-Haber-Institutder Max Planck Gesellschaft, Berlin (Germany). He received his Ph.D. degree (1994) in physical chemistry from the University of Heidelberg (Germany), for which the majority of the experimental work was carried out at the Laboratory for Surface Science and Technology, University of Maine (USA). Currently, he is a post-doctoral fellow at Exxon's corporate research laboratory. His primary research interests include chemical reactions on solid surfaces. M i c h a e l G r u n z e (1947) is professor of physical chemistry and director of the Institute of Physical Chemistry at the University of Heidelberg. He studied chemistry and physics at the Freie Universit~t Berlin and at Knox College, Galesburg, IL (Fulbright fellowship). He received his Diploma degree in 1972 and his Ph.D. in physical chemistry in 1974. After p0st-doctoral research at the University of Munich and

research collaborations at the University of London, he had a tenured position as a scientist at the Fritz-Haber-Institutof the Max-Planck-GesellschafiBerlin. In 1984, he became professor of physics at the Laboratory for Surface Science and Technology (LASST), University of Maine at Orono, then director of the same laboratory. Since 1987 he has been professor of physical chemistry at the Lehrstuhl fiir Angewandte Physikalisch Heidelberg, Germany and since 1989 director of the Institute of Physical Chemistry at the University of Heidelberg. Daniel J. D w y e r received his B.S. degree (1972) in chemistry from the State University of New York at Oswego. He received his M.S. degree (1974) and his Ph.D. degree (1976) from Lehigh University, Bethlehem, PA. He rose to the rank of senior research chemist at Exxon's corporate research laboratory before joining the University of Maine in 1988. He is presently a professor of chemistry and the director of the Laboratory for Surface Science and Technology. His research interests are primarily in the area of gas-surface interactions.