Synchrotron Photoemission Characterization of TiO2 Supported on

Jul 25, 1998 - J. P. Espinós, J. Morales, A. Barranco, A. Caballero, J. P. Holgado, and A. R. González-Elipe. The Journal of Physical Chemistry B 20...
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Langmuir 1998, 14, 4908-4914

Synchrotron Photoemission Characterization of TiO2 Supported on SiO2 J. P. Espino´s, G. Lassaletta, A. Caballero, A. Ferna´ndez, and A. R. Gonza´lez-Elipe* Instituto Ciencia de Materiales de Sevilla, CSIC-Universidad de Sevilla, Avenida Ame´ rico Vespucio s/n. Isla de la Cartuja, E-41092 Sevilla, Spain

A. Stampfl School of Physics, La Trobe University, Victoria 3083, Australia

C. Morant and J. M. Sanz Departamento de Fı´sica Aplicada and Instituto Universitario “Nicolas Cabrera”, Universidad Auto´ noma de Madrid, Cantoblanco, E-28049 Madrid, Spain Received March 9, 1998. In Final Form: May 19, 1998 The system formed by TiO2 deposited on SiO2 has been studied by photoemission and X-ray absorption spectroscopies with synchrotron radiation. TiO2 spreads on the surface of SiO2 where it forms a layer (1-2 ML thick) prior to thickening. Extended X-ray absorption fine structure/X-ray absorption near-edge spectroscopy (EXAFS/XANES) analysis at the TiK edge shows that, whatever the coverage, the TiO2 films are amorphous with titanium ions in a 6-fold coordination of oxygen ions. Photoemission with photons of 140 eV shows that for low coverages, the Ti3p binding energy increases by 0.5 eV with respect to the value in bulk TiO2. Under these conditions, a new feature appears in the O2s peak in the form of an extra shoulder around 24 eV. This new form of oxygen is attributed to oxygen ions at the interface acting as a bridge between TiO2 and SiO2 (i.e., formation of Si-O-Ti cross-linking bonds). The shift in the Ti3p peak is accompanied by a shift to higher binding energies in the valence band edge of the spectra at low TiO2 coverage. This shift would indicate that the band gap of the titanium oxide increases for low coverages of TiO2. A detailed analysis of the valence band region was carried out with photons of 35 < hν < 70 eV. The valence band spectra of TiO2 are narrower (3.6 eV fwhm) and less defined than that of bulk TiO2 (3.9 eV fwhm). Resonance photoemission of the valence band of a thin layer of TiO2 reproduces the pattern reported in the literature for bulk TiO2 characterized by a maximum enhancement of the intensity of the valence band spectrum for hν ) 47 eV.

Introduction Interfacial electronic interactions play a key role in processing and applications of advanced ceramic materials. Interfacial problems govern, for example, adhesion of ceramic coatings, the growth mode of heterostructures, the performance of oxide supported catalysts, etc. An understanding of interface phenomena, such as wetting and reactivity, is therefore necessary to gain the ability to manipulate the microstructure and therefore the properties of such systems.1 Thus, for example, supported oxide catalysts are very often formed by small (bi- or threedimensional) particles or clusters of the active-phase oxide distributed on another oxide that constitutes the carrier support. Typically the dispersion of the active phase on a high surface carrier is intended to increase the dispersion degree of the active oxide, although sometimes, changes in reactivity are produced by the interaction between the two oxide phases. Traditionally, oxide/oxide interfaces have not been studied so extensively as metal/oxide interfaces, apparently because the latter are less complex and experimentally less difficult to be studied.2-5 However, surface investigations of metal oxides6,7 and ultrathin oxide films * Corresponding [email protected].

author.

FAX:

34-5-4460665.

E-mail:

(1) Surfaces and Interfaces of Ceramic Materials; Dufour, L. C., Monty, C.; Petot-Ervas, G., Eds.; Kluwer Academic: Dordrecht, 1989.

supported on oxide substrates8-12 have intensified in the past few years for the purpose of both understanding fundamental properties of interfaces in oxide systems and developing novel layered oxide structures.13 The aim of this paper is to obtain information about the chemical composition and electronic structure of the TiO2/ SiO2 interface. The approach used here is to grow an ultrathin film of TiO2 on amorphous SiO2 via evaporation of Ti in an oxygen atmosphere and then characterize the (2) Sandell, A.; Libuda, J.; Brhhwiler, P.; Andersson, S.; Maxwell, A.; Baumer, M.; Martensson, N.; Freund, H. J. J. Electron. Spectrosc. Relat. Phenon. 1995, 76, 301. (3) Wertheim, G. K.; DiCenzo, S. B. Phys. Rev. B 1988, 37, 844. (4) Espino´s, J. P.; Ferna´ndez, A.; Gonza´lez-Elipe, A. R.; Munuera, G. Surf. Sci. 1991, 251/252, 1012. (5) Mason, M. G. Phys. Rev. B 1983, 27, 748. (6) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University: New York, 1994. (7) Hirschwald, W. In Surface and Near Surface Chemistry of Oxide Materials; Nowotny, J.; Dufour, L.-C., Eds.; Elsevier: Amsterdam, 1988; p 61. (8) Lassaletta, G.; Ferna´ndez, A.; Espino´s, J. P.; Gonza´lez-Elipe, A. R. J. Phys. Chem. 1995, 99, 1484. (9) Mejı´as, J. A.; Jime´nez, V. M.; Lassaletta, G.; Ferna´ndez, A.; Espino´s, J. P.; Gonza´lez-Elipe, A. R. J. Phys. Chem. 1996, 100, 16255. (10) Zhang, Z.; Henrich, V. E. Surf. Sci. 1992, 277, 263. (11) Burke, M. L.; Goodman, D. W. Surf. Sci. 1994, 311, 17. (12) Sambi, M.; Sangiovanni, G.; Granozzi, G.; Parmigiani, P. Phys. Rev. B 1994, 55, 7850. (13) Brun, N.; Colliex, Ch.; Rivory J.; Yu-Zhang, K. Microsc. Microanal. Microstruct. 1998, 7, 1.

S0743-7463(98)00280-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/25/1998

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formed interface by photoemission spectroscopy using synchrotron radiation. A previous characterization by X-ray photoelectron spectroscopy (XPS) in terms of core level binding energies and Auger parameters has been published recently.8,9 We use the Ti3p and Si2p core levels, the O2s, and the valence-band to follow the oxidation state and electronic structure of the overlayer for low coverages of TiO2 on the substrate. Extended X-ray absorption fine structure spectroscopy (EXAFS) is also used to gain information on the local coordination of Ti at the interface. Experimental Section The silica substrate consisted of a 200 Å SiO2 overlayer grown on a Si(111) surface. Prior to the deposition of TiO2, the substrate was cleaned by a mild bombardment with 500 eV O2+ ions. TiO2 was deposited by evaporation of titanium in an oxygen atmosphere at a pressure of 2 × 10-6 Torr. The photoemission experiments were performed in beamline TGM3 at the Berlin synchrotron radiation source (BESSY), equipped with a thoroidal grating monochromator and an ultrahigh vacuum (UHV) station with a base pressure of 10-10 Torr. Photoemission spectra, as a function of the deposited TiO2, were measured using an ARES analyzer at a constant pass energy of 25 eV and different photon energies between 30 and 70 eV and also at 140 eV. The angle of incidence of the p-polarized light was 45° with respect to the sample normal. Most of the spectra were recorded with the analyzer at a normal (90°) collecting angle with respect to surface. For the analysis of coverage, spectra were also collected at a grazing collection geometry (16°). Photon energies of 140 eV were used to measure overview spectra including the Ti3p and Si2p core levels. The ratio between the intensities of the Ti3p and Si2p peaks is used as a quantitative estimation of the TiO2 coverage. The valence-band spectroscopy was performed with photon energies between 30 and 70 eV. Binding energies in our spectra are all referred to the Si2p binding energy of SiO2, (i.e., 103.3 eV). This energy is used to correct any charging effect, thus providing an absolute reference for the binding energy scale of the spectra. All the spectra have been normalized to the photon flux as measured at a gold mirror at the entrance of the chamber. EXAFS spectra were recorded at the DCI storage ring in LURE (current 250 mA) after monochromatization with a Si(111) doublecrystal monochromator. Samples were prepared in a UHV chamber adapted to a conventional EXAFS line in a similar way as that for the photoemission experiments. Details about this experimental setup can be found in ref 14. The spectra were recorded by total electron yield detection, measuring the drain current through the sample. Analysis of the spectra was done according to the usual procedure including preedge and afteredge subtraction, Fourier transformation, and fitting. Bulk rutile was used as a reference sample for the calculations.

Results Core Level Photoemission Spectra (hν ) 140 eV). Photoelectron spectra recorded with hν ) 140 eV for clean SiO2 before and after deposition of increasing amounts of TiO2 were used to determine the areas of the Ti3p and Si2p peaks as a function of the evaporation time. In a previous paper on the TiO2/SiO2 system, we have shown by ion-scattering spectroscopy (ISS) that TiO2 grows on SiO2 according to a layer by layer mechanism.8 In our case, the growth mode could be examined by a quantitative analysis of the Si2p and Ti3p intensities ratio in terms of the overlayer attenuation model.15 Figure 1 shows a plot of the intensity ratios R ) ITiO2ISiO2∞/ISiO2ITiO2∞, as a function of the TiO2 evaporation time. The lines correspond to a partial fitting for an overlayer of TiO2 of thickness “a” according to (14) Caballero, A.; Mun˜oz-Pe´rez, A. In Encyclopedia of Analytical Science; Academic: London, 1995; p 5036. (15) Seah, M. P. In Practical Surface Analysis; Briggs, D.; Seah, M. P., Eds.; John Wiley & Sons: Chichester, 1990.

Figure 1. Plot of the intensity ratio R between the intensity of the Ti3p and Si2p peaks against the evaporation time of TiO2 on SiO2, for normal (9) and grazing (0) angles of collection. The lines correspond to the ratios calculated by assuming a layerby-layer growth mechanism.

R ) θ/

[{

1

[

1 - exp -

]}

a λ cos β

-θ

]

where the usual formalism for thin overlayers is easily recognized.15 We have assumed the same λ for the substrate and the overlayer, and the coverage has been taken as proportional to the evaporation time (i.e., θ ) ptev). By using p ) 0.1 mL/min and a/λ ) 0.5, the lines fit the experimental points rather well up to tev ∼ 7 min for both the grazing and normal data. An estimate of λ according to the Seah and Dench universal curves for oxides15 or the Tanuma et al. values16 leads to values between 1 and 0.7 nm for Ti3p and Si2p, respectively. Thus, the thickness “a” of the TiO2 overlayer, grown up to 7 min, can be calculated as ∼1 or ∼2 monolayers thick, depending on the λ value. The Ti3p photoemission spectra as a function of the evaporation time are shown in Figure 2. All the spectra are very sharp, indicating that only fully oxidized Ti4+ cations are present. In fact, when titanium is evaporated at lower oxygen pressures (e.g., 10-7 Torr), the formation of species in a lower oxidation state (i.e., Ti3+ and Ti2+) leads to very intense shoulders at lower binding energies.17 It is interesting to mention here that the Ti3p spectra of Figure 2 exhibit energy shifts of up to 0.6 eV between the spectrum corresponding to the lowest evaporation time used in this experiment and that of a thick TiO2 film. This shift is more clearly observed in Figure 3, where we have depicted the dependence of the Ti3p binding energy on the evaporation time. The curve is characterized by a sharp decrease to reach a constant value for evaporation times tg7 min when, according to the analysis in Figure 1, a homogeneous overlayer of TiO2 is formed. For the lowest coverage of TiO2 measured here, the Ti3p core level shows a binding energy of 38.4 eV, whereas at coverages well above 1 mL (i.e., evaporation times >14 min), the (16) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1988, 11, 577. (17) Asensio, M. C.; Kerkar, M.; Woodruff, D. P.; de Carvalho, A. V.; Ferna´ndez, A.; Gonza´lez-Elipe, A. R.; Ferna´ndez-Garcia, M.; Conesa, J. C. Surf. Sci. 1992, 273, 31.

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Figure 2. Ti3p photoelectron spectra of TiO2 deposited on SiO2 for increasing evaporation times, as labeled.

Figure 3. Binding energy of the Ti3p core level as a function of the evaporation time.

binding energy is 37.8 eV, which is in good agreement with that measured for bulk TiO2. Similar binding energy shifts have also been observed by XPS in the Ti2p core line for coverages of up to θ ) 1.8,9 These shifts were

Espino´ s et al.

Figure 4. O2s photoelectron spectra for TiO2 deposited on SiO2 for increasing evaporation times.

attributed to both initial and final state effects of the photoemission process caused by the strong interaction between TiO2 and the SiO2 substrate. The existence of significant bonding interactions at the TiO2/SiO2 interface can also be deduced from the analysis of the O2s photoelectron spectra. These spectra are shown in Figure 4 as a function of the evaporation time. The figure clearly shows that as the evaporation time increases, the single O2s spectrum at ∼25 eV, corresponding to the SiO2 substrate, broadens and exhibit new features on the low binding energy side until a single peak at 22.6 eV, characteristic of TiO2, clearly develops for long evaporation times. At evaporation times of ∼7 min, the spectra depict a complex shape, where a maximum of ∼24 eV is clearly distinguished. This feature is likely caused by oxygen ions at the interface interacting with both Si4+ and Ti4+ ions. This assignment is also based on the fact that the feature reaches its maximum intensity after an evaporation time of ∼7-10 min; that is, when the homogeneous overlayer (1-2 mL thick) of TiO2 has been already deposited and the interface completed. This conclusion is sustained by Factor Analysis (FA)18 of the series of experimental O2s spectra. This analysis gives as a result that a minimum of three independent components are needed to reproduce the experimental series of spectra and that one of these components corresponds to a band with a maximum between the O2s bands of pure SiO2 and TiO2. Figure 5 shows some results of this analysis by FA. The left-hand side panel of this figure shows a plot of the three components that by lineal combination reproduce all the spectra of the experimental series. These three components have a different width/shape and are char(18) Gonza´lez-Elipe, A. R.; Holgado, J. P.; Alvarez, R.; Munuera, G. J. Phys. Chem. 1992, 96, 3080.

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Figure 5. (a) Representation of the minimal set of principal components of factor analysis for the O2s photoemission spectra (grazing collection). (b) Plot of the intensity of each component in the series of spectra in Figure 4 for increasing periods of evaporation. Key: (9) SiO2 component; (b) Ti-O-Si component; (2) TiO2 component.

Figure 6. Valence band photoemission spectra recorded with hν ) 140 eV for TiO2 deposited on SiO2 for increasing evaporation times. The inset shows, for increasing coverages, the shift in the valence band position as determined by extrapolation of the band edge.

to ∼7 min, the SiO2 component decreases sharply, whereas the TiO2 component only starts to grow after this time. Meanwhile, the third component, attributed to oxygen ions at the interface, grows sharply to reach its maximum intensity after evaporation for that time, then decreasing slowly up to its complete disappearance. These intensity profiles are consistent with a deposition of a homogeneous overlayer of TiO2 to cover the SiO2 surface after ∼7 min and the growth of massive TiO2 after this first monolayer has been completed. During the formation of this first overlayer, the O2s spectra are dominated by the SiO2 and Ti-O-Si components, whereas for longer deposition times, the TiO2 component prevails. Valence Band Photoemission (hν ) 140 eV). Figure 6 shows valence band spectra in the 0-18 eV bindingenergy range, taken with a photon energy of hν ) 140 eV, for the initial SiO2 substrate and after different evaporation times (i.e., TiO2 coverages). At low TiO2 coverages, the spectra are strongly influenced by the contribution from the SiO2 of the substrate. Incremental TiO2 coverages ultimately result in a band characteristic of TiO2. In fact, the spectrum of the thickest TiO2 layer (i.e., 52.5 min), corresponds to a slightly oxygen-deficient surface, as can be deduced from the observation of the emission band labeled as Ti3+ in the band gap. The valence band photoemission of bulk TiO2 has been widely studied in the literature.19-26 The series of depositions of TiO2 leads to gradual changes throughout the entire valence band, although two general changes are immediately noticeable: the continuous suppression of the substrate emission due to SiO2 and the continuous shift of the valence band edge toward lower binding energies. In any case, it is important to point out here that all the valence band

acterized by maxima at different binding energy (BE) positions at 25.0, 23.1, and 22.7 eV. These maxima can be attributed to oxygen ions pertaining to SiO2, the TiO-Si interface, and TiO2, respectively. The assignment of the second component will be discussed further in connection with the formation of the TiO2/SiO2 interface. Another interesting result of the FA refers to the partition of each of these three components in the experimental spectra of Figure 4. In the right-hand side panel of Figure 5 we show a plot of the normalized intensities of the three components of the O2s spectra obtained for increasing times of deposition of TiO2. For low deposition times up

(19) Henrich, V. E.; Kurtz, R. L. Phys. Rev. B 1981, 23, 6280. (20) Smith, K. E.; Henrich, V. E. Solid State Commun. 1988, 68, 29. (21) Zhang, Z.; Jeng, S.-P.; Henrich, V. E. Phys. Rev. B 1991, 43, 12004. (22) Heise, R.; Courths, R.; Witzel, S. Solid State Commun. 1992, 84, 599. (23) Diebold, U.; Tao, H.-S.; Shinn, N. D.; Madey, T. E. Phys. Rev. B 1994, 50, 14474. (24) Muryn, C. A.; Hardman, P. J.; Crouch, J. J.; Kaiker, G. N.; Thornton, G. Surf. Sci. 1991, 251/252, 747. (25) Hardman, P. J.; Kaikar, G. N.; Muryn, C. A.; van der Laan, G.; Vincott, P. L.; Thornton, G.; Bullett, D. W.; Dale, P. A. D. M. A. Phys. Rev. B 1994, 49, 7170. (26) Sanjine´s, R.; Tang, H.; Berger, H.; Gozzo, F.; Margaritondo, G.; Levy, F. J. Appl. Phys. 1994, 75, 2945.

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Espino´ s et al.

Figure 7. Distance from the edge of the valence band to the origin of the binding energy scale for the valence band spectra of Figure 6 as a function of the evaporation time.

spectra exhibited only one edge during the entire growth process, instead of two as expected for two independent phases (i.e., TiO2 + SiO2). Indeed, we have found that calculated spectra obtained by a weighted sum of pure TiO2 and SiO2 spectra depict a valence band shape with two edges (data not shown). In insulators or wide band gap semiconductors such as TiO2, the determination of the position of the valence band edge by photoemission can be affected by charging effects that may lead to a shift of the whole spectrum. Therefore, to measure the relative positions of the edges as a function of coverage, the binding energy scale of all the spectra have been referenced to that of the Si2p peak in SiO2 at 103.3 eV. The position of the valence band maximum has then been determined by linear extrapolation of the tangent on the low-bindingenergy edge of the valence band to zero intensity, as shown in the inset of Figure 6. The edge for SiO2 corresponds to the highest binding energy, whereas the edge of the thickest TiO2 film appears at lower binding energies. Although, this analysis does not provide a direct measurement of the band gap, it clearly shows that there is an inverse correlation between coverage and the magnitude of the valence band edge, as measured by the position of the top of the valence band. Figure 7 shows a plot of the energy position of the valence band edge as a function of the evaporation time. A similar plot can be obtained from spectra where the SiO2 contribution has been subtracted (vide infra). Interestingly, Figure 7 shows a rapid decrease of the energy position of the valence band edge as the evaporation time is increased, up to an evaporation time of 14 min, when the value corresponding to bulk TiO2 is reached. Coordination of Titanium. An important point to ascertain the existence of interface effects for TiO2 deposited on SiO2 is to figure out the coordination state of the Ti4+ ions at low coverages because the BE changes observed at the Ti3p and O2s levels might be produced

Figure 8. Fourier transform curves obtained from the EXAFS oscillations of the TiK edge absorption spectra for TiO2 deposited on SiO2 for increasing evaporation times.

by a different coordination state of Ti at the interface. Thus, the possibility that Ti4+ may have a tetrahedral coordination at the interface, as has been reported for TiO2 dispersed in a silica network,27 required a direct determination of the titanium coordination at the TiO2/ SiO2 interface. Therefore, we performed a characterization of the interface, as a function of the TiO2 coverage, by X-ray absorption near-edge spectroscopy (XANES). The XANES spectra as a function of coverage (not shown) were all very similar and resembled the spectrum for amorphous TiO2 published in ref 28. Figure 8 shows the Fourier transform (FT) of the EXAFS oscillations at the TiK edge for increasing coverages of TiO2. The key point in that figure is the fact that the intensity of the first peak, associated with the Ti-O coordination, is independent of the coverage. On the contrary, the intensity of the second peak at R ≈ 2.5 Å, clearly observable at low coverages, decreases rapidly as the coverage increases. This behavior suggests that this peak is caused by the second coordination sphere of the titanium ions at the interface with SiO2. Although the quality of the data is not good enough as to perform a detailed fitting that permits an unambiguous determination of the origin of this second peak (i.e., to distinguish between silicon or titanium), the observed behavior with coverage suggests that it is due to a Sis (O)sTi structure at the interface. In fact, fitting of the spectra yields a coordination number close to six for the first Ti-O shell, at a distance characteristic of TiO2 (i.e., (27) Stakheev, A. Y.; Shpiro, E. S.; Apijok, J. J. Phys. Chem. 1993, 97, 5668. (28) Ferna´ndez, A.; Caballero A.; Gonza´lez-Elipe, A. R. Surf. Interface Anal. 1992, 18, 392.

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at 1.94 Å).28 Meanwhile, the second coordination shell for the samples at low coverages (i.e., evaporation for 2.5 and 6 min) can be accounted for by titanium and/or silicon atoms at ∼3.05 Å. From this analysis, two points deserve consideration. First, the fact that whatever the coverage the Ti-O coordination around titanium remains practically constant, and second that at low coverages, the FT curves reveal a well-defined second coordination shell at a distance that is compatible with a Ti-O-Si structure. At high coverages, the increasing disorder of the amorphous layer leads to a self-cancellation of the EXAFS oscillations of the second Ti-O-Ti coordination sphere, so that only the Ti-O peak appears in the spectrum as it occurs for amorphous TiO2.28 Valence Band Photoemission (35 < hν < 70). The observed shift of the valence band edge is not the only change in the electronic structure of TiO2 during its growth on SiO2. In fact, the chemical interactions at the interface and their influence in the electronic structure of the TiO2 can be also observed by valence band photoemission. Previous to the analysis of the valence band of TiO2 at low coverages, the measured valence band spectra were corrected by subtracting the SiO2 contribution from the substrate. To do that, the valence band of SiO2 was also measured at photon energies between 30 and 70 eV under the same experimental conditions as the valence band spectra of the different TiO2 films deposited on SiO2. The corresponding difference valence band spectra in the 0-12 eV binding energy range, recorded at different photon energies, are shown in Figure 9, for samples corresponding to the evaporation of TiO2 for 9 min (i.e., ∼1 mL of TiO2) and 30 min (i.e., ∼3 mL), the latter were included for comparison. The two series of spectra depict an enhancement of the bonding part of the valence band (i.e., the highest BE side) at a photon energy of ∼47 eV. This behavior is typical of bulk TiO2 and results for the resonance enhancement of the photoemission of those levels with a three-dimensional character.21-23 However, the comparison between the two sets of spectra reveals the existence of some differences in shape. Thus, the valence band is clearly narrower at lower coverages, increasing from 3.6 eV for 1 mL TiO2, up to 3.9 eV for 3 mL TiO2. These values have been obtained for the corresponding spectra recorded with hν ) 35 eV and by measuring the full width at the half-maximum (fwhm values). In addition, it is observed that the valence band structure appears more rounded and less defined for the lowest coverage; that is, the bonding and nonbonding regions are not so clearly delineated as in the case of thicker films. This effect has been previously observed when TiO2 was reduced by Ar+ bombardment and it was associated with an increase of the covalence in TiOx (x < 2).21 Nevertheless, the characteristic enhancement of the bonding region of the valence band at photon energies of ∼47 eV is clearly observed in all the cases. Discussion In a previous paper we showed by ISS that the first monolayer of TiO2 completely wets the SiO2 substrate.8,9 This growing mechanism is confirmed here by the behavior shown in Figure 1 of the ratio of the Ti3p to Si2p intensities as a function of the evaporation time. For the analysis of the photoelectron spectra it is important to discard the fact that for the first overlayer of TiO2, the titanium atoms have a coordination state different than for a thicker layer. Thus, for example, a tetrahedral coordination for the titanium ions at the interface would not be unlikely because this coordination

Figure 9. Valence band photoemission spectra at different photon energies between 35 and 65 eV for different TiO2 overlayers on SiO2: (left) 9 min, (right) 30 min evaporation times.

has been found for titanium-doped silica glasses.27 The analysis of the EXAFS spectra has permitted confirmation that whatever the coverage, the Ti first coordination shell remains practically constant, being characterized by a Ti-O distance similar to that existing in rutile and a constant number of oxygen neighbors. Furthermore, at low coverages, the data indicate the appearance of a peak

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associated with the second coordination sphere, that could be compatible with Ti-(O)sSi distances, where Si corresponds to the substrate. This assignment would be in agreement with the formation of well-defined Ti-O-Si cross-linking bonds between the TiO2 overlayer and the SiO2 substrate and would explain the observed wetting of the substrate. The data indicate that although the TiO2 overlayer is amorphous, the Ti-O-Si bonds that anchor the deposited TiO2 to the SiO2 surface are characterized by coherent and well-defined distances. The existence of this well-defined Ti-(O)sSi bonding structure at the interface would be responsible for the main effects observed in the photoemission processes of the O2s and Ti3p core levels. These processes are characterized by shifts in the binding energy of the Ti3p core line and the appearance of a new oxygen species, as indicated by a new feature in the O2s photoemission line just at a binding energy between those corresponding to TiO2 and to SiO2. The magnitude of the shift of the Ti3p binding energy (i.e., ∼0.6 eV) is similar to that previously reported for the Ti2p levels from XPS measurements.8,9 In metals, these shifts are usually explained as a size dependence of the final state relaxation processes and/or of the initial state electronic structure.3-5 At the TiO2/ SiO2 interface, the formation of Ti-O-Si cross-linking bonds would result in an increase in covalence with respect to bulk TiO2. So, although size effects contributions cannot be discarded, the changes in the electronic structure around titanium should be responsible for the observed shifts in Ti3p core level photoemission. It is also expected that the oxygen atoms at the interface notice that change in ionicity. In agreement with previous observations, this change should lead to a shift in the BE of the O2s peak.29 Such a behavior has been in fact observed in our case for the first monolayer of TiO2 characterized by a O2s spectrum with a BE intermediate between those of pure SiO2 and TiO2 (cf., Figures 4 and 5). Recently, Brun et al.13 identified singular bond structures at the interface of TiO2/SiO2 multilayers by spatially resolved electron energy loss spectroscopy (EELS) fine structure.13 The results of this work also indicate that the interaction between TiO2 and SiO2, that leads to modifications in the electronic structure, is limited to the interface layer, a conclusion that is in good agreement with our observations by photoelectron spectroscopy. Core level shifts for the first TiO2 monolayer are not the only effect observed in our case. Redistribution of the electron density at the interface is also noticed by the valence band narrowing and shift in the position of the edge that have been revealed by our analysis of the valence band spectra shown in Figures 6 and 7. However, such valence band changes might be produced by different causes. Thus, although the discussion just presented suggests that the changes observed in photoemission are primarily a result of the interaction with the support due to the formation of Ti-(O)sSi cross-linking bonds, because of the bidimensional character of the TiO2 overlayer, quantum size effects might also be considered. It is well known that the decrease of the particle size of TiO2 colloids leads to an increase in the band gap of this wide gap semiconductor.30 This increase would be in agreement (29) Ranke, W.; Kuhr, H. J. Phys. Rev. B 1989, 39, 1595.

Espino´ s et al.

with the shift observed in this work for the valence band edge. However, the magnitude of the shift (i.e., 1.2 eV; cf. Figure 7) appears too large to be explained just in terms of quantum size effects. In fact, simple cluster models of the TiO2/SiO2 interface and calculations in terms of molecular orbitals have shown that the gap of TiO2 primarily increases because of the influence of the SiO2 substrate and the bonding interactions existing at the interface.9 Our photoemission results also indicate that for low TiO2 coverages, the widening of the band gap occurs simultaneously to a narrowing and a loss of structure in the valence band, when compared with the valence band of bulk TiO2 (cf., Figure 9). This result would be consistent with the amorphous character of the TiO2 layer and the restrictions for overlapping imposed to the Ti and O orbitals by the interaction with the SiO2 substrate. Finally, it must be stressed that the existence, in all the samples, of a resonant enhancement (even of similar magnitude) of the bonding part of the valence band at ∼47 eV (cf. Figure 9), confirms that the O2p-Ti3d hybridization occurs in ultrathin films of TiO2 on SiO2 as it occurs in bulk TiO2. In this respect, the effect of the substrate does not seem to modify to a large extent the hybridization degree between the Ti3d and O2p levels.31 However, a more quantitative analysis of the resonance behavior of the valence band spectra of these and other thin layers of TiO2 would be necessary to ascertain this point. Conclusions We have studied the electronic properties of the TiO2/ SiO2 interface by photoelectron spectroscopy. The chemical interaction between these two oxides was monitored by examining O2p valence band and O2s and Ti3p core levels. Analysis by XAS revealed that the deposited layers have an amorphous structure where the titanium ions present a 6-fold coordination, even at the first monolayer. The TiO2 spreads on the surface of SiO2 where anchoring of the supported phase occurs through cross-linking oxygen ions to form Si-O-Ti bonds. The electron distribution in this structure is different than in the Ti-O-Ti ensembles existing in bulk TiO2. This fact is revealed by shifts in the Ti3p and O2s photoemission spectra taken with hν ) 140 eV. Changes in the electronic structure of the thin layers of deposited TiO2 are also apparent from the photoemission spectra of the valence band levels. Thus, it is possible to detect at low coverage that there is a shift to higher binding energies in the position of the band edge and a narrowing of the bandwidth. These phenomena have been related to size effects and to the interaction of the TiO2 overlayer with the SiO2 support. Acknowledgment. We thank the financial support of the DGICYT (project no. PB93-0240) and CICYT (project no. MAT97-689) of Spain and the HCM program of the EU (CHRX-CT93-0358 and CHGE-CT93-0027). LA980280K (30) Kavan, L.; Stoto, T.; Gra¨tzel, M.; Fitzmaurice, D.; Shklover, V. J. J. Phys. Chem. 1993, 97, 9493. (31) Davis, L. C. J. Appl. Phys. 1986, 59, R25.