Synchrotron radiation studies of H2O adsorption on TiO2(110)

178

Surface Science 218 (1989) 178-200 North-Holland, Amsterdam

SYNCHROTRON ON TiO,(llO)

RADIATION

Richard L. KURTZ,

STUDIES

OF H,O ADSORPTION

Roger STOCKBAUER,

Surface Science Division, National Institute Gaithersburg, MD 20899, USA

of Standards

Theodore E. MADEY and Technology

*,

Elisa ROMAN and Jose L. DE SEGOVIA Fisica de Superficies,

Instituto

Ciencia de Materiales,

Received 6 July 1988; accepted for publication

CSIC,

Madrid, Spain

16 March 1989

Synchrotron radiation photoemission has been used to study the interaction of H,O with defective and nearly-perfect TiO,(llO) surfaces at temperatures between 160 and 300 K. Ti3+ 3d defect sites are implicated in the adsorption process, and by tuning the photon energy to 47 eV we find that a resonant photoemission process gives an enhanced photoemission sensitivity to the 3d defect states. Defects are produced on TiO,(llO) by annealing to 1000 K in UHV; subsequent exposure to lo4 L 0, produces nearly perfect surfaces, based on the suppressed Ti3d emission. Both nearly perfect and defective surfaces give rise to dissociative adsorption of H,O at 300 K. The saturation coverages are near 0.1 ML, independent of the initial defect concentration; however, the rate of dissociative adsorption (sticking probability) is higher on defective surfaces. The enhanced sensitivity to the Ti 3+ defect states has allowed the observation of a surprising effect; the dissociative adsorption of H,O results in increased defect state intensity on the nearly perfect surfaces. This apparent charge-transfer to the substrate implies that a new model for the dissociation process on oxide surfaces is needed. At 160 K H,O adsorbs molecularly on both the nearly-perfect and the defective surfaces. Subsequent annealing experiments allow estimates of the interaction energies involved in the dissociation process.

1. Introduction The electronic structure of the rutile oxide TiO, has been the subject of a number of experimental as well as theoretical studies [l-13]. Interest in this material was prompted by the realization that it is of importance in photolysis of water, dissociating the water to produce hydrogen and oxygen [2]. For this reason, the interaction of water with TiO,(llO) surfaces has been studied by a number of groups using X-ray and ultraviolet photoelectron spectroscopies (XPS, UPS) [3,10-131. The nature of defects on TiO, surfaces has also been * Formerly, the National Bureau of Standards.

0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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et al. /

H,O

adsorption

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TiO,(I IO)

179

the subject of continued interest and has been studied using a number of other experimental techniques including those mentioned above as well as recent high-resolution electron energy-loss spectroscopy (EELS) studies [6,14-181. In this work, we have studied the interaction of water with TiO,(llO) by applying synchrotron radiation techniques. Using resonant photoemission, we can improve the sensitivity of the ultraviolet photoelectron spectra to Ti’+ surface defects by a factor of - 5 with respect to photoelectron spectra obtained using He I radiation. This enhanced sensitivity is shown to be critical in obtaining a more complete picture of the interaction of H,O with surface defects. In addition to the enhanced sensitivity to defect states resulting from the use of synchrotron radiation, we have extended the temperature range of previous H,O adsorption studies to surfaces cooled to - 160 K. This is the first photoemission study of H,O adsorption on a cooled TiO, surface. Cooling is found to be extremely useful in obtaining a better understanding of the interaction of H,O and the thermally-driven conversion from adsorbed molecular species to dissociated fragments. These studies imply a new model for the interaction of these dissociated fragments with lattice ligands and defect states. The experiments described here were performed on a TiO,(llO) surface with two different defect densities, related to the extent of surface O-deficiency. Section 2 gives a brief description of the experimental configuration and sample preparation techniques. In section 3 we present the results of these measurements, beginning with a study of the behavior of surface Ti3+ defects upon exposure to 0,. The water adsorption studies were performed on both low and high defect-density surfaces at room temperature and at low temperature. In section 4 we present a discussion of these results and a new model for the interaction of H,O with TiO,(llO) surfaces.

2. Experimental The experiments described below were performed in an ultra-high vacuum (UHV) surface analysis chamber on beam-line 1 of the National Institute of Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility, SURF-II. [19]. Monochromatized photons were obtained from a laminar profile toroidal grating monochromator (TGM), whose superior flux and improved rejection of higher-order light has been described previously [20]. Photoelectron spectra were obtained using a commercial double-pass cylindrical-mirror analyzer. The analyzer was operated in a constant pass energy mode and combined electron and photon energy resolution was better than 0.3 eV. The energies quoted here are referenced to a binding energy scale with E Fermi = 0; the location of E, was determined by photoemission from a clean

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Au foil in electrical contact with the (semiconducting) TiO, crystal. The work function, C#B, was measured by monitoring the energy separation between E, and the cutoff in the secondary electron background, Es,,: Q = hv - (E, - Esb). Surface cleanliness was monitored using Auger electron spectroscopy (AES). During these experiments a monotonic decrease in photoemission intensity versus time was observed (- 20% change over the course of a day). To correct for this, the photoelectron spectra reported here are normalized to the same intensity at a binding energy of 17 eV. This binding energy corresponds to an energy where the sole intensity contribution is due to inelastic secondaries scattered from the valence band emission. No adsorbate features were observed to appear at this energy. This procedure does not affect the conclusions of the data; comparisons of intensities are all relative to the intensity of the 02p valence band. Three photon energies were used for obtaining the photoelectron spectra: hv = 35,47, and 57 eV. These energies correspond to below-resonant, resonant and above-resonant photon energies, respectively. These energies demonstrate the different sensitivities to Ti 3+ (3d) defect states that are located at a binding energy of - 0.8 eV. The resonant enhancement is produced by an interference of Ti 3d and Ti 3p excitations and results in additional channel for emission of Ti 3d electrons, as has been discussed previously [22]. If the enhancement in these states is compared for different photon energies by producing the ratio, jTi 3d/jO 2p, at resonance, hv = 47 eV, the intensity of the defect states is enhanced by a factor of 3 relative to that at hi = 35 eV, and by a factor of 5 relative to that at hv = 25 eV. The TiO,(llO) surface was prepared on a 4 mm X 5 mm X 2 cm rod that was cut and polished to within 0.5O. The sample mounting configuration has been described previously [21]; the sample used in those measurements is the same one that was used here. Radiant and electron-bombardment annealing was accomplished with a W filament placed behind the crystal and temperatures were measured with a W/Re thermocouple pressed between the sample front face and a Ta support tab. It is difficult to assess the accuracy of the temperature measured in this fashion. At low temperatures, thermal gradients across the crystal may be significant and the point-contact of the thermocouple with the sample may contribute a systematic error in the measurement. The sample surface was cleaned by sputtering with 500 eV Ar+; this also depletes the surface of 0 [l]. The surface 0 stoichiometry was restored by annealing to 1000 K in UHV and subsequently exposing to 0,. This technique of heating a TiO, crystal in vacuum has long been used to induce enough bulk 0 vacancies (- 10’8-1019 cmP3) to promote electrical conductivity; this treatment produces samples that are opaque and a metallic slate-gray color [16]. This procedure prevents the surface from charging under electron or photon impact, an effect that can be clearly observed in photoemission in non-annealed samples. The oxygen that is evolved in annealing can be

RL.. Kurtz et al. / Hz0 ahorption

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replaced by annealing in a few Torr of 4. Under typical experimental conditions, annealing in vacuum will cause 0 to diffuse to the surface [l]. Depending on sample history, this can result in a nearly stoichiometric TiO, surface with a small, and perhaps undetectable density of Ti3+ surface defects. After numerous annealing treatments in UHV, the sample eventually can become sufficiently oxygen deficient in the bulk that a small but observable defect density remains on the surface. For the experiments in ref. [21], annealing to 1000 K produced surfaces with Ti‘3+ defect-state intensities that were approximately 0.6% of the 02p intensity at hu = 47 eV. At lower photon energies where the resonant enhancement does not occur, 25-35 eV, these defect states were not observed above the noise in the data. These surfaces were considered to be nearly perfect (110) planes. After many additional cycles of annealing, the crystal began to show an increase in the intensity of these defect states in photoemission. It was found to be necessary to prepare these surfaces by annealing and subsequently exposing them to a partial pressure of 02; defect states were substantially reduced by these treatments. Although some intensity was observed in the characteristic Ti3+ region, the lack of additional interaction of the surface with 0, indicates that this intensity was due to defects, perhaps subsurface defects, not chemically active, and yet still detected in the photoelectron experiment. In the present H,O adsorption studies, the surface was prepared in two ways, one that resulted in nearly perfect surfaces with a low density of Ti3+ surface defects and one that produced a higher defect-density surface. The higher defect-density surface was prepared by annealing to 1000 K in UHV and the lower defect-density surface was prepared by exposing the 1000 K annealed surface to - lo4 L (1 langmuir = 10e6 Torr . s) O2 at 400 K. During the short time interval between annealing the surface and beginning the experimental measurements, the sample was maintained at 400 K to prevent adsorption of H,O from the background gas. Experiments were subsequently carried out at 300 K or after cooling using liquid N2; the lowest temperatures attained were near 160 K as indicated by the thermocouple readings.

3. Results 3. I. 0, aakorption As a preliminary investigation into the nature of the defects produced upon annealing the TiOz crystal in UHV, we conducted a series of Oz adsorption studies. In fig. 1 we present the results of a series of Oz exposures, in doses from 0.01 to lo4 L (1 L = 1 X 10m6 Torr . s - 3.6 X 1014 (cm - s)-l) at 400 K. In this series of curves, the experimental data are offset by constant vertical increments for clarity. These UPS data were obtained with hv = 47 eV, which

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is the photon energy where the Ti3d intensity is most enhanced. The Ti3+ defect states are located in the band-gap, 0.7-0.9 eV below E,. The (primarily) 02p valence band is located between 3 and 10 eV below E, and the 02s is observed at 23 eV. As the surface is exposed to increasing amounts of 0, the Ti3d states are depopulated, the 02p band bends up toward decreasing binding energy, and the work function changes. These changes, as well as the change in the gap between the Fermi level and the top of the valence band, A( E, - EF), are plotted in fig. 2 versus the log of the 0, exposure. The work function rises by 0.5 eV for exposures 2 lo2 L. The Ti 3d electron feature decreases in intensity with 0, exposure; at lo2 L it is 0.33 that of the 1000 K annealed surface and by lo4 L it is down to 0.29. Larger exposures do not produce any additional reduction in the intensity of this feature; it is believed that the residual intensity arises from subsurface defect states that are not sensitive to room temperature 0, exposure. Presumably, these states would be depopulated by exposure to 0, at elevated temperatures (T = 1000 K). The photoemission spectra indicate that oxygen is adsorbing dissociatively on this surface at 400 K; molecular 0, adsorption, likely to occur as a condensation, would only be observed at temperatures much lower than those obtained in these experiments.

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The photoemission signal from the Ti 3p state (not shown in fig. l), located at a binding energy of 38.0 eV for the annealed surface, shifts to 37.7 eV after a lo4 L exposure of 0,. The binding energy observed here is significantly larger than the 36.5 eV binding energy observed by Gbpel et al. [6]; this may be due in part to the different zero of energy used in that XPS measurement. As well, our observed binding energy shifts in the opposite direction upon 0, exposure from that expected by the simple depopulation of lower binding energy defect states observed in that work [6]. On the other hand, the magnitude and direction of the Ti 3p shift observed here is consistent with the bending of the 02p valence band. The intensities of the “bonding” and “non-bonding” components of the valence band vary dramatically with photon energy so that a direct and meaningful comparison with the X-ray induced valence band spectra observed in ref. [6] is not possible. These results are consistent with a previous study in which Henrich et al. observed three distinct phases of defect states on TiO1(llO) [16]. In that work, region I corresponded to a stoichiometric TiO,(llO) surface with geometric order that ranged from nearly perfect to disordered, based on the presence or absence of LEED patterns. Region II was characterized by a loss of oxygen relative to a perfect surface, resulting in interacting Ti3+ pairs; this is believed

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to be due to the loss of bridging 02- species. The third region, III, was characterized by the growth of ordered patches of a reduced oxide, Ti,O,. From the change in the work function, A+, and the shift in the location of the top of the valence band (see fig. 2), our 1000 K annealed surface is on the boundary between region II and III, described in ref. [16]. 0, exposure of our annealed surface transforms it from phase II to phase I; the border between these two phases is near the 10 L O2 exposure, determined from the A+ and A( E, - EF) data. In phase I, Henrich observed that there was little difference in composition compared to that of the perfect surface, however there was a change in the quality of the (1 x 1) geometric order of the surface as determined by LEED [16]. In the work reported here, the best TiO,(llO) surfaces were produced by 0, exposure of the annealed surface, which already gave good (1 x 1) LEED patterns; these patterns are dominated by electron scattering from Ti cations and are not likely to be substantially different after 0, exposure. We believe that our lo4 L exposure produces a well-ordered and nearly perfect surface. The preparation scheme used in the present work allows us to study the interaction of H,O with nearly perfect TiO,(llO) surfaces as well as ones with the distinct Ti3+-pair defect sites [16]. 3.2. H,O adsorption

at 300 K

When the 1000 K annealed, lo4 L O,-exposed (nearly-perfect) surface is exposed to increasing amounts of H,O, the photoelectron spectra shown in fig. 3 are obtained. Fig. 3a shows the n(E) data for the nearly-perfect surface and the lo3 L exposure. The data for the two extremes, clean and water dosed, are shown; data for intermediate exposures progress monotonically between these two curves. Inset in fig. 3a is a plot of the band-bending induced by the H,O exposure; these data were obtained by monitoring A(&, - Er), the shift of the upper edge of the 02p valence band, at - 4.25 eV on the clean surface. Interaction with water is observed to increase the separation of the Fermi level, E,, from the top of the valence band, E,, or bend the bands down at the surface. By the highest dose, lo3 L, the bands have been bent by 165 meV. The direction of this band motion is consistent with the presence of a negatively charged surface species. Difference spectra can be obtained from these data by subtracting the clean surface spectrum from those obtained after exposing to H,O. These difference curves (fig. 3b) show, in more detail, the changes induced by the adsorbate and, in some cases, can be correlated with the molecular orbital structure of the adsorbate. The band-bending shifts that occur with increasing adsorbate coverage can be taken into account by aligning the upper edge of the clean surface spectrum with that of the H,O dosed surface prior to taking the difference. This is a valid procedure if molecular orbital features due to the

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adsorbate do not appear at energies corresponding to the top of the valence band. The difference spectra displayed in fig. 3b are obtained using this procedure. The data shown there are for a range of four orders-of-magnitude increase in the overall dose, from 0.01 to lo3 L. There are several noticeable features in these spectra. Centered near 11 eV, a new feature rises with H,O dose. A broader and asymmetric feature also rises between 8.5 and 4.5 eV and there is a corresponding decrease in intensity between 4.5 and 3 eV. Slight changes in the shape of the upper edge of the 02p valence band are believed to be responsible for the derivative-like structure between 3 and 5 eV. An additional peak, located at 0.8 eV is also observed to rise monotonically with increasing H,O exposure; this feature is associated with the defect Ti3d electron states observed previously. In previous H,O adsorption studies, this feature was observed only to become depopulated under all the experimental conditions in which it was seen to change in amplitude [1,3,11]. As well, this peak (originally located at - 0.8 eV on the nearly perfect surface) shifts to - 0.95 eV at the highest H,O exposure. Also shown in fig. 3b are markers indicating the location of the 3a and the llr levels observed for the OH- species of solid NaOH [23], these markers are shifted by 1.5 eV to higher binding energy relative to the bulk hydroxide spectrum. When the 1000 K annealed surface, containing Ti3+ point defects, is exposed to H,O at 300 K, the photoemission spectra shown in fig. 4 are obtained. The changes induced by the water are similar to those seen on the nearly perfect surface although saturation coverage is reached by 0.1 L. The Ti 3d electron intensity varies little upon H,O exposure and the upper edge of the valence band shifts to higher binding energy by - 0.14 eV. Two features are observed in the difference spectra (fig. 4b)) located at 7.6 and 10.8 eV which suggest the presence of OH- species. Based on a comparison of the intensity 10.8 eV OH- 3a feature in fig. 4b with the intensity of the 13 eV lb, level of 1 monolayer (ML) of molecular water discussed below, the total coverage of OH- is - 0.1 ML. Although the saturation coverage of - 0.1 ML is observed for both the annealed and for the annealed plus O-exposed surfaces, they have dramatically different initial sticking coefficients. Preliminary measurements on highdefect-density sputtered surfaces indicate that saturation coverages are nearly the same, - 0.1 ML, for exposures up to 100 L. 3.3. H,O adsorption at 160 K The interaction of H,O with the nearly perfect and the point defect-containing surfaces is significantly different at low temperature. The lowest temperature reached in this study with the present configuration was 160 K; at this temperature, a single monolayer (or less) of molecular H,O is stable but

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the surface is not cold enough to allow the formation of multilayer ice under LJHV conditions [24]. In fig. 5a the pi(E) photoemission data for H,O adsorbed on the nearlyperfect TiO,(llO) surface at 160 K are shown for hv = 47 eV. Two new

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R. L. Kurtz et al. / H,O aakorption on TiO,(I IO)

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R.L Kurtz et al. / H,O ahorption on Ti02(I IO)

189

features located below the 02p valence band at 10.3 and 13.2 eV rise in intensity with increasing dose. In addition, the valence band changes shape but the band bending induced by the adsorbate is small; by 0.3 L the upper edge has shifted down by - 80 meV. Fig. 5b shows the difference spectra obtained from these data. The solid curves were obtained by subtracting the clean surface spectrum from the data obtained after H,O exposure; no corrections for band-bending have been made. Several features emerge from these difference spectra. The features in fig. 5b located at 10.3 and 13.2 eV are quite distinct from the 3a and the In features that were observed near 8.5 and 11 eV on the room-temperature surface (cf. fig. 3b). A proper description of the changes induced in the valence band region is more difficult to obtain, however. As a test of the sensitivity of the structure in these spectra to the method of producing the differences, an additional difference curve is obtained and plotted in fig. 5b with the solid squares. In this method we assume that the overlayer attenuates the substrate emission; the nearly perfect spectrum is scaled down by a factor of 0.65 before producing the difference curve. In this case, we observe the same features at 10.3 and 13.2 eV, however, the shape of the curve between 5 and 8 eV changes substantially. In this region we are unable to produce reliable difference structures. It is believed that the adsorbed H,O contributes to the enhanced intensity in this region and a subtle redistribution in the emission intensity of 02p features (e.g. depopulation near 7 eV combined with enhancement near 5.5 eV) may result in the lack of a single distinct feature. The reproducibility of the other two features, however, is clear. These features are identified as the lb, and 3a, molecular orbitals of adsorbed molecular H,O. From these data the lb, is located between 5 and 9 eV, most probably at - 8 eV. The gas phase photoelectron spectrum is also plotted in fig. 5b; the lb, molecular orbital is aligned with the peak at 13.2 eV in the difference spectrum. Comparison of the relative positions of these peaks indicates that in the adsorbed H,O layer, the 3a, level is stabilized relative to the lb,. An important point to note is that identical features are observed in the difference spectra at the lowest exposures as are observed at saturation, implying that at 160 K H,O adsorbs molecularly even at the lowest exposures. Fig. 6a shows the photoemission spectra obtained when the 10 L H,O dosed surface in fig. 5 is heated. These data were obtained by flashing to the indicated temperature and cooling back down to 160 K, where the spectra were taken. The dashed curve in fig. 6a is the same spectrum as the dashed curve of fig. 5a. As can be seen from these data, essentially nothing happens when the surface is heated to 165 K but at temperatures between 170 and 190 K the peaks at 13.2 and 10.3 eV decrease in intensity. In the same temperature range, the 0 2p valence band undergoes the greatest change in shape. The changes in these data are easier to see when one produces the difference spectra, plotted in fig. 6b. These data are obtained by subtracting

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R.L. Kuriz et al. / H,O adrorption on TiO,(llO)

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the clean surface spectrum (dashed curve) from each of the spectra obtained after annealing. This shows the signatures of the electronic structure of the species adsorbed on the surface. Also indicated in this figure are the locations of the 3% and 1s levels of OH on defective (Ar+-bombarded, O-deficient) Ti,O,(1012) [26] on Ti(OOO1) [19] and on the nearly-perfect TiO,(llO) surface at 300 K (fig. 3b). The positions of gas-phase H,O are indicated at the bottom of fig. 6b. The difference spectra curves show that by 190 K, nearly all of the molecular H,O has desorbed and between 190 and 240 K new features appear at different energies. One, between 11 and 12 eV is low in intensity and a second between 8 and 9 eV appears to increase in intensity. Both peaks appear to shift to higher binding energy with increasing annealing temperature. It is important to note that the difference spectra show essentially no molecular H,O by 200 K, as evidenced by the lack of intensity in the region of the lb, orbital at 13 eV. Another way to display these data is by plotting the sequential difference spectra, shown in fig. 6c. The curves plotted there were obtained by subtracting the higher temperature spectrum from the lower temperature spectrum, giving an indication of the species that have left the surface upon annealing. Here, we see that substantial amounts of molecular H,O desorb between 170 and 200 K but even at annealing temperatures up to 220 K we see that a small amount of molecular H,O may be removed from the surface. This form of difference spectra is more sensitive to small changes than those in fig. 6b, and

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indicates that a small amount of molecular H,O may be present at that temperature. The negative dip at 7 eV for annealing temperatures between 170 and 190 K indicate that the 02p valence band is regaining intensity in this region. This overlaps the region of the H,O lb, feature which is contributing some positive intensity in the region between 7 and 9 eV. At the highest annealing temperature, 240 K, it appears that OH is also removed from the surface; the slight negative dip near 1 eV indicates that charge is being transferred back to Ti 3d states at this point. Note that the markers for the OH species on various Ti and Ti oxides are in much better agreement with these sequential difference data (Fig. 6c) than with the usual difference curves (fig. 6b). This may be due to a multiplicity of adsorbate configurations that are sequentially desorbed as the sample is heated. These different states may give rise to molecular orbital features that overlap slightly; in the normal difference curves these features are viewed summed together while the sequential difference curves resolve them more closely on a state-by-state basis. Heating the H,O-dosed, partially defective surface produces similar adsorbed species, as indicated by the photoelectron spectra in fig. 7. In this case, a 1000 K annealed (110) surface is dosed with 100 L H,O at 160 K. This dose would produce a multilayer ice at lower temperatures, however, the 160 K substrate temperature only allows a monolayer of molecular H,O to be adsorbed. Heating this layer to 200 K produces a dramatic drop in intensity of the lb, and the 3a, orbital features located at - 13.5 and 10.5 eV. The 02p valence band changes shape and the spectrum after annealing to 320 K is very similar to that of the clean, 1000 K annealed surface. Very little happens in the Ti3d electron region at - 0.9 eV; its intensity drop after dosing is consistent with simple attenuation by the adsorbed H,O monolayer. Note that the data in this figure were obtained with hv = 35 eV. This has the effect of enhancing our sensitivity to the adsorbate molecular orbital features at the cost of reducing our sensitivity to the Ti 3d feature. The changes in these spectra are better observed in the difference spectra plotted in fig. 7b. At 160 K the difference spectra indicate that the adsorbed species are molecular but the intensity of the lb, orbital is observed to be dependent on the method used to obtain the difference. If the monolayer H,O is assumed to attenuate the substrate to 0.65 of its original intensity, then the difference spectrum plotted in the bottom of fig. 7b is obtained. This shows an intense feature near 8 eV and no depletion of the 02p band in the region between 5 and 6 eV and is in very good agreement with the location of the molecular orbital features of gas-phase H,O. The difference spectrum obtained by subtraction of the unscaled clean surface is plotted above this curve. Although a comparison of these two curves shows that the structure in the region of the 0 2p valence band depends on the details of the method used to produce the differences, the features at higher binding energies are essentially unaffected. It is evident that by 200 K, the molecular H,O is removed from

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0

Binding Energy f&l) Fig. 7. (a) UPS spectrafromdefective TiO,(llO)

exposed to 100 L H,U at ‘I60 K and heated to successively higher temperatures; (b) difference spectra obtained from the data in (a): the bottom curve is obtained by scaling the intensity of the clean surface spectrum in (a) by 0.65. The cupves above were produced by subtracting the clean surfam spectrum and the dashed curve (top) was obtained by subtracting the 320 K spectrum from the 200 K spectrum.

the swfwc and there is evidence for a new feature at a binding energy of - 21 eV. There is also evidence for another peak at - 8 eV that appears to shift to higher binding energy with increasing annealing temperature but it appears at

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binding energies overlapping the valence band; slight changes in the position of the 02p band may produce this apparent shift. The conversion of the remaining molecular H,O to dissociated OH fragments at - 200 K corresponds to an activation energy of 12.6 kcal/mol. Despite the enhanced adsorbate sensitivity from the higher 3a and In cross-sections at hv = 35 eV, the difference curves of fig. 7b, when compared with those of fig. 3b, show what appears to be only a small signal from OH in the temperature range from 200 to 320 K. It must be noted that the scaling of those two panels differ; the normalized full-scale intensity range of fig. 7b is - 22.5 times that of fig. 3b. If we compare the difference spectrum in fig. 7b obtained by subtracting the 320 K from the 200 K spectrum (and plotted with the dashed curve) we see that by 320 K, a significant amount of OH is removed from the surface. If we compare the intensity of that 11 eV 3a peak with the 13 eV lb, feature in fig. 7b and assume that the 160 K difference curve represents a monolayer of adsorbed H,O, we can estimate that - 0.25 ML of OH is removed from the surface when it is heated from 200 to 320 K. For comparison, markers at the top of fig. 7b indicate the locations of OH species observed previously on defective (sputtered) Ti,O, [26] and on Ti(OOO1) [19]. The features observed in these difference spectra are located at energies that are in very good agreement with those previous measurements.

4. Discussion The information obtained here gives a clearer picture of the interaction of H,O with perfect and defect surfaces of TiO,(llO). These results are summarized in table 1 where the intensities and behaviors of the Ti 3d states are indicated qualitatively and the estimated saturation coverage is indicated. At 300 K, water is seen to adsorb dissociatively, regardless of the degree to which Table 1 Summary of results for Ha0 exposure of TiO,(llO) Surface preparation

Adsorption temperature

3d Intensity

3d Behavior

Saturation coverage

(K)

Adsorbate

(ML)

Annealed Annealed + 0,

3ocl 300

Strong Weak

Unchanged Increases

0.1 0.1

Annealed Annealed + 0,

160 160

Strong Weak

Slight decrease Slight decrease

1 1

OHOHH2O H2O

behavior of the intensities of the Ti 3d-electron features are given qualitatively and an order-of-magnitude estimate of the saturation coverage is given. The annealed surfaces have been annealed to loo0 K in UHV and the annealed + 0, surfaces have been annealed to 1000 K in UHV followed by dosing with lo4 L O2 at 400 K. The

R.L. Kurtz et

51.

/

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adsorption on TiO,(IIO)

195

the surface is free of Ti3+ point defects. The most perfect surface that we have been able to produce, as well as the surfa= containing point defects produced by annealing, exhibit the characteristic two-peaked difference spectra indicative of adsorbed OH radicals. Saturation coverage is low, estimated to be about 0.1 ML on both the perfect and defect surfaces. The initial sticking coefficient on the defect surface is much larger than on the perfect surface indicating that the Ti*3+ defects affect the kinetics of the dissociation process. This similar saturation coverages on the two surfaces is surprising considering the lack of appreciable numbers of Ti3+ defect sites on the nearly-perfect surface. Even more surprising, however, is the increase in intensity of the 3d-electron states on the surface with H,O exposure. These results and their ~te~retation differ substantially from those of Lo et al., even considering that a different surface, the (lOO), was studied in that work [3]. From UPS data that are not dissimilar from those presented here, they concluded that H,O was adsorbed molecularly at room temperature. At issue is the binding energies of the molecular orbitals of the adsorbates, OHand H,O. From the data in figs. 5 and 7 for the condensed H,O monolayer, it is clear that the binding energy for the lb, level is greater than 13 eV and that the 3a, is stabilized relative to the lb,, when compared with the gas phase spectrum. This relative shift of the “lone-pair” 3a, level is not surprising since it is the orbital expected to interact most strongly in bonding to the surface. This has been observed previously in molecular H,O chemisorption on Ti,O,(lOi2) 1261. On the other hand, the location of the OH- features near 10.8 and 7.6 eV in figs. 3 and 4 is in good agreement with the locations of features observed by Lo et al. in ref. [3]. Caution must be exercised when interpreting additional features that arise in difference spectra located on the edges of intense bands, here, the 02p valence band. Small differences in band alignment used to account for band bending or in normalization of spectra can substantially affect the resulting difference spectra (see fig. 5b). The essential difference between the two surface preparations studied here is the degree to which the surface contains its full complement of oxygen ligands. From the oxygen adsorption study, it is apparent that after annealing in UHV, the surface is O-deficient and presumably the absence of significant numbers of bridging oxygen is the form that this deficiency takes. Upon exposure to O,, O-deficient sites are replenished and a nearly perfect surface results. On this surface, the bridging-O sites are likely to be nearly filled although there may be a low density of O-vacancy point defects. We can estimate the upper limit of the densities of such bridging-O defects for the two surface preparations on the basis of a simple model. We can approximate the relative cross-sections of the Ti3d and the 02p levels by comparing the intensities observed in resonant photoemission from Ti,O,(lOi2) at hv = 48 eV [27]; this material consists of cations purely in Ti3+ state. If we assume that the Ti3+ states in the TiOZ(llO) surfaces studied here

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are all concentrated in the top Ti layer of the surface (- 3.25 s;> then we can estimate the fraction of surface Ti cations in the 3 + state. In this approximation, we use attenuation lengths of 10 and 12 w for the Ti3d and the 02p features, respectively. For the annealed surface these assumptions imply that - 0.3 of the top layer Ti is in the 3 + configuration; for the lo4 L O,-dosed surface, 0.03 of the top layer Ti is in the 3 -I- state. If 2 Ti3+ cations are formed for every bridging-O removed, this implies that nearly 1 in 6 bridging 0 are missing on the annealed surface while approximately 1 in 60 are missing on the O,-exposed surface. These observations suggest the following model for the adsorption of H,O on TiO,(llO). The proposed model is indicated schematic~ly in figs. 8a and 8b, which shows the mechanism for the dissociation on both the perfect and defect surfaces, respectively. For adsorption on the nearly-perfect surface, I-I,0 initially adsorbs on an accessible Ti site; an in-plane 5-fold coordinated site may accommodate this. Dissociation, then, involves an interaction with a neighboring bridging 02- ligand. The water molecule then dissociates and the H that is released in this dissociation combines with the lattice 02- and forms another OH radical. The kinetic barrier in the process may be the interaction which requires substantial molecular tilt and displacement from an a-top Ti site. When the H,O dissociates, the OH radical that is formed may contain only a partial negative charge. If this is also the case with the OH radical that forms from the H + 02- reaction, there is a net charge-transfer back to the Ti cation. This may be the source of the additional Ti3d electron states that are observed to become populated when the H,O dissociates. On the defect surface, however, the increase in d-electron intensity is not nearly as strong although the same adsorbed species is formed. This may be accounted for, as shown in fig. 8b, by preferential adsorption at another site, the bridging-O vacancy Ti3+ site. There dissociation results in the formation of two OHspecies. In this case, attenuation of the photoelectron signal by the adsorbed species may account for the lack of the observed rise in d-electron intensity that was observed from the nearly perfect surface. These defects may act as “feeder” sites, producing dissociated species which diffuse to other sites where adsorption occurs. These conclusions are in agreement with those based on el~troche~cal measurements where is has been proposed that two different forms of OHare produced upon H,O exposure of the TiO,(llO) surface [28]. As well, the kinetics involved in the dissociation of HI,0 are thought to be different for these two different sites 1281. The observation of charge transfer resulting in Ti3+ sites on the nearly-perfect (110) surface is also in agreement with electrochemical notions [29]. Two other interesting points concern the differing adsorption rates (or sticking coefficients) and the similar saturation coverages for the dissociative adsorption of H,O on these two surfaces. From the band-bending versus

Rt

Ktutz ef al. / f&O adwption on TiO,fIIOf

Initial AdsorptIon

Oxygen Vacancies

Btldglng Oxyaen

lnltial Adsow~tion

197

Dlssoclatlon Products

Dissociation

Products

Fig. 8. Schematic diagram of the proposed dissociation mechanism. Ti cations are the solid circles and the O-ligands are the larger, shaded circles. (a) 00 the nearly-perfect surface, H,O adsorbs on a a-fold O-coordinated Ti site and interacts with bridging 0 ligaad. Dissociation OCCUTS producing an adsorbed ON with the free H interacting with a bridging 0 resulting in the conversion to OH. (b) On the defective surface, the initial adsorption site is a bridging-O vacancy and dissociation results in the formation of two OH species.

198

R.L.. Kurtz et al. / H,O aakorption on TiO,(llO)

exposure insert and the slow rise of intensity in the OH features of fig. 3, we can see that the sticking coefficient for H,O on the nearly-perfect surface is coverage dependent, and that saturation is not reached until an exposure of - lo3 L. On the other hand, the surface containing appreciable numbers of defects shows a saturation coverage of 0.1 ML after an exposure of 0.1 L (fig. 4) meaning that the sticking coefficient is essentially 1 until 0.1 ML coverage, and many orders of magnitude smaller after that coverage is reached. These observations imply that H,O dissociates more readily on the surface containing appreciable amounts of O-vacancies. According to the model proposed above, the kinetic barrier for dissociation at a bridging-O vacancy Ti3+ site (“initial adsorption site” in fig. 8b) must be lower than that for dissociation on a S-fold O-coordinated Ti site such as that in fig. 8a. The low value of the saturation coverage, observed to be - 0.1 ML for both surfaces, is not well understood. This seems to imply that the occupation of one adsorption site by dissociated OH fragments quenches the activity of neighboring sites with respect to additional H,O adsorption and dissociation. Such an effect may result from OH dipole-dipole interactions combined with subtle changes induced in the surface electronic structure by the adsorbate. The destruction of the activity of neighboring adsorption sites may occur if these effects are not screened out by the substrate’s relaxation and electronic polarization response. Another explanation may account for the rise in the Ti3d intensity with H,O exposure. That is, the H that is freed upon dissociation may simply migrate to another Ti cation and form a hydride. A hydrogen-induced surface state has been observed for H/Ti(OOOl) as well as for H,O on Ti(0001) at a binding energy of 1.3 eV. This is a significantly different energy from the Ti3+ defect state, however. Moreover, preliminary experiments on the TiO,(Oll) surface (nearly the same surface geometry as Ti,0,(1012) [15,21]) indicate that atomic H can produce surface OH radicals and this interaction also produces additional Ti3+ states [30]. For this reason, we believe it unlikely that the rise in d-electron intensity is due to solely hydride formation, but rather, is associated with adsorbed hydroxyl species. In summary, we have studied the adsorption of Hz0 at room temperature and at 160 K on two TiO,(llO) surfaces: one being nearly perfect and the other well-ordered but O-deficient. OZadsorption studies have allowed us to correlate our surface preparation with a previous report detailing the different phases of O-deficiency-related defect states on TiO,(llO). Water dissociates at room temperature on both nearly-perfect and defect surfaces at exposures up to lo4 L and produces an increase in the number of Ti3+ 3d-electron states at the surface. At 160 K, the H,O adsorbs molecularly on both surfaces but does not influence substantially the number of Ti3+ states. Heating these 160 K H,O-dosed surfaces results in the desorption of most of the adsorbed molecules; between 200 and 240 K, photoemission difference spectra give evidence for the presence of dissociated OH fragments. Higher annealing temperatures

R. L. Kurtz ei al. / H,O adsorption on TiO,(I IO)

199

(> 320 K) result in the desorption of these species, as well. These results suggest a mechanism for the dissociative adsorption that involves the interaction of adsorbed molecular species with bridging 02- ligands that result in the formation of adsorbed hydroxyl species.

Acknowledgement This research has been supported by the United States - Spain Joint Committee for Scientific and Technical Cooperation under grant number CCA-8510/051. RLK, RLS and TEM would also like to acknowledge the partial support of the United States Office of Naval Research. We gratefully acknowledge the support of the staff of the NIST SURF-II storage ring.

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[22] E. Bertel, T.E. Madey and R. Stockbauer, Surface Sci. 141 (1984) 355; E. Bertel, R. Stockbauer, R.L. Kurtz, T.E. Madey and D.E. Ramaker, Surface Sci. 152/153 (1985) 776. [23] J.A. Connor, M. Considine, I.H. Hillier and D. Brigs, J. Electron Spectrosc. Related Phenomena 12 (1977) 143. [24] P.A. Thiel and T.E. Madey, Surface Sci. Rept. 7 (1987) 211. [25] D.W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, New York, 1970) p. 113. [26] R.L. Kurtz and V.E. Henrich, Phys. Rev. B 26 (1982) 6682. [27] J.M. McKay, M.H. Mohamed and V.E. Hemich, Phys. Rev. B 35 (1987) 4304. [28] J. Augustynski, in: Solid Materials, Structure and Bonding, Vol. 69, Eds. M.J. Clarke et al. (Springer-Verlag, New York, 1988) pp. l-62. [29] P. Salvador, New J. Chem. 12 (1988) 35. [30] R.L. Kurtz, H.H. Chen, R.L. Stockbauer and T.E. Madey, unpublished.