Core and Valence Band Photoemission Spectroscopy of Well-Ordered

Nov 18, 2006 - Dipartimento di Scienze Chimiche, INSTM and Laboratorio Regionale Luxor, INFM-CNR, UniVersita` di. PadoVa, Via Marzolo, I-35131 PadoVa ...
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J. Phys. Chem. C 2007, 111, 869-876

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Core and Valence Band Photoemission Spectroscopy of Well-Ordered Ultrathin TiOx Films on Pt(111) Paola Finetti,† Francesco Sedona,† Gian Andrea Rizzi,† Uwe Mick,‡ Frantisek Sutara,§ Martin Svec,| Vladimir Matolin,§ Klaus Schierbaum,‡ and Gaetano Granozzi*,† Dipartimento di Scienze Chimiche, INSTM and Laboratorio Regionale Luxor, INFM-CNR, UniVersita` di PadoVa, Via Marzolo, I-35131 PadoVa, Italy, Institut fu¨r Physik der Kondensierten Materie, Heinrich-Heine UniVersita¨t Du¨sseldorf, UniVersita¨tsstrasse 1, D-40225 Du¨sseldorf, Germany, Department of Electronics and Vacuum Physics, Faculty of Mathematics and Physics, Charles UniVersity, V HolesoVicka´ ch 2, 18000 Prague 8, Czech Republic, and Institute of Physics, Czech Academy of Science, CukroVarnicka´ 10, 16200 Prague, Czech Republic ReceiVed: July 31, 2006; In Final Form: October 8, 2006

Well-ordered ultrathin TiOx layers on Pt(111) surface, prepared by reactive evaporation of Ti in oxygen, were characterized by means of Ti 2p and O 1s core level and by valence band photoelectron spectroscopy. Depending on the details of the preparation procedures, a total of six well-ordered structures, each of them characterized by a well-defined low energy electron diffraction pattern, were obtained. The core level data show that this wide range of structures can be rationalized in two main groups, i.e., a group of three stoichiometric (labeled as k′, rect, and rect′) and a group of a three substoichiometric (labeled as z, z′, and w) ordered films. The valence band data are rather consistent with this basic distinction. In fact, valence band spectra relative to stoichiometric or substoichiometric films share common features and are quite different from spectra relative to the other group. On the other hand, the valence band data appear to be more sensitive to the details of the film structure by also displaying electronic features that are particular to each individual film. The valence band data are discussed with the aid of theoretical and experimental results for bulk surfaces and compounds available in the literature. It turns out that mixing with Pt states plays a major role in determining the electronic structure of the reduced substoichiometric films, whose spectral data are also consistent with a stoichiometry close to TiO and with the presence of a Ti-Pt interface. This finding is in agreement with previously reported photoelectron diffraction data. The stoichiometric films show a valence band structure that is strongly reminiscent of the one measured on the stoichiometric bulk TiO2 surface. Deviations from the bulk band structure appear in the form of a narrowing of the band and in a shift toward lower binding energy. The band narrowing effect is attributed to the spatial confinement of the TiO2-like films, while the shift is attributed to mixing of film and Pt substrate derived states. Finally, the rect structure shows a (film thickness dependent) anomalous spectral shape that is tentatively attributed to its peculiar geometric structure.

1. Introduction Metal oxide nanostructures are very interesting from a fundamental point of view and also for potential applications in the area of advanced technology.1,2 Dimensional confinement may in fact give rise to physical and chemical properties that are quite different from those of the corresponding bulk compounds.3 Metal oxides nanostructures, in the form of well-ordered ultrathin oxide films grown at single-crystal surfaces, are model systems to study the interplay between low dimensionality and electronic structure and the role played by other physical properties, such as defect density or lattice strain. In turn the electronic properties are related to the chemical properties. Moreover, ultrathin films have also been proposed for their potential use as templates for the growth of catalytically active * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +39-0498275158. Fax: +390498275161. † Universita ` di Padova. ‡ Heinrich-Heine Universita ¨ t Du¨sseldorf. § Charles University. | Czech Academy of Science.

nanoparticles.4 Recently, metal oxide nanostructures have been recognized as a key component for the development of novel chemical sensors.5 Titania is certainly one of the best-characterized systems in surface science.6 Most of the studies were performed on singlecrystal bulk surfaces, and only a few reports address the issue of ultrathin films. The large interest in ultrathin TiOx films grown on Pt single crystals has been stimulated by the promotion properties of Pt in photocatalysis2 and by the fact that TiOx/Pt is the prototypical system showing the strong metal support interaction (SMSI) effect.7 In this paper we use the more generic TiOx notation (in place of TiO2) to indicate that some of the films we discuss are substoichiometric (x < 2). The first study of TiOx ultrathin films grown on the Pt(111) surface was reported some years ago by Boffa et al.8 Subsequently, ultrathin films grown on Pt(100) were also investigated.9 More recent experiments, carried out by means of low energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and angle-scanned X-ray photoelectron diffraction (XPD), allowed us to identify a wider range of structures on Pt(111) than

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TABLE 1: Leed Pattern of the Investigated TiOx Ultrathin Films Organized According to the Postannealing Oxygen Pressure (PO2)a

a Tdep and Tann are the substrate temperatures during deposition and postannealing, respectively. The labels “reduced” and “oxidized” appearing to the right refer to the chemical state of the Ti atoms (see XPS section). See the text about the labeling of each structure.

previously reported.10 In fact it was shown that by varying the growth parameters, deposition of about 1 monolayer (ML) of TiOx can yield six different phases, each having a distinct LEED pattern.10 The combined use of LEED and high resolution STM has shown that each of them corresponds to large domains with either hexagonal or rectangular unit cell. The hexagonal type of structures can either be arranged in a so-called wagon-wheellike type of lattice11 or in a kagome-like lattice, where hexagons are sharing their vertices. Some of the rectangular structures are characterized by a peculiar zig-zag appearance. Moreover, both hexagonal and rectangular structures can either be commensurate or incommensurate. Whereas the geometric structure of the ultrathin films has been explored, a description of their electronic structures is still lacking, either from an experimental or a theoretical point of view. In this paper we report an electronic structure investigation of the TiOx/Pt(111) system carried out by valence band photoemission spectroscopy (VB-PES). We also report the results of a Ti 2p and O 1s core level XPS experiment. Both the XPS and the VB-PES experiments were carried out by means of synchrotron radiation. VB-PES has long been established as a valuable technique to detail the reactivity of surfaces as it allows identifying the electronic states involved in the surface chemical processes. Thus a thorough characterization of the electronic structure of such novel surface phases, which are potentially relevant for chemical applications, is clearly a key step in the understanding of their properties. VB-PES studies also have strong fundamental motivation. Studying TiOx films in the single (or just a few) layer regime may allow in fact identifying the role of extreme

2D confinement on the electronic structure. Finally the role played by defects or by the mixing of oxide film and metal substrate electronic states is an extremely important issue to understand the electronic properties of this class of systems. 2. Experimental Section The experiments were carried out on the material science beamline at Elettra in Trieste, Italy.12 This beamline takes light from a bending magnet source and mounts a plane grating monochromator. The experimental end station is provided with a Phoibos 150 mm (SPECS GmbH) multichanneltron analyzer. The XPS data were collected at 650 eV photon energy, while VB-PES data were collected with a 200 eV photon energy and with an overall energy resolution, measured at the Fermi level (Ef), of 0.5 and 0.15 eV, respectively. All the photoemission data were acquired at normal emission. The Pt(111) surface was prepared by repeated cycles of Ar+ bombardment followed by annealing at 873 K. The clean surface was checked by LEED and XPS. The TiOx ultrathin films on the clean Pt(111) were prepared in situ by evaporating Ti from a resistively heated Ti rod in an oxygen background pressure (PO2) of 1 × 10-4 Pa (same oxygen pressure for all samples), followed by annealing in oxygen. In order to obtain a welldefined and ordered TiOx ultrathin film, the amount of deposited Ti, the substrate temperature, and oxygen pressure during the postannealing were optimized. The Pt(111) substrate was kept at room temperature (RT) during Ti deposition, except for the formation of two rectangular structures (rect and rect′ see also Table 1 and the remainder of this section for the labeling of the structures) for which the sample was held at 600 K during the deposition.

Well-Ordered Ultrathin TiOx Films on Pt(111) The different TiOx structures investigated in the present study are reported in Table 1. In the same table we report the corresponding LEED patterns (for more details about the LEED experiments we direct the reader to the previous report).10 All the structures we report are labeled according to the shape of the surface unit cell or to characteristic appearance in their STM images. Thus rect is for a rectangular phase, z is for a zig-zaglike, w for a wagon-wheel-like, and k for a kagome-like habitus, respectively. During the beamtime we have obtained and measured all the structures previously reported10 with the exception of one wagon-wheel-like structure (w′) and of the low coverage-medium oxygen pressure k structure. On the other hand, at about the same Ti coverage of the k structure and by increasing the postannealing oxygen pressure we obtained a new structure, which we label k′, due to the similarity of its LEED pattern to the k pattern. We also report spectroscopy data for another rectangular structure (herein labeled as rect′) that was not included in the previous report but that was subsequently characterized by LEED and STM.13 In Table 1, the different structures are organized according to the oxygen pressure of the postannealing. The ordered structures appear in a limited range of nominal coverage expressed in a conventional unit (equivalent monolayer, MLE), where 1 MLE of Ti corresponds to a Ti metal layer with the same surface density as the Pt(111) surface (i.e., 1.5 × 1015 atoms cm-2). For each of the investigated structures, we gauged the Ti coverage by means of the Ti 2p XPS intensity (peak area) normalized to the average XPS intensity value recorded for a reference structure. To this end we have chosen the wagonwheel-like w structure: actually, according to the previous STM studies,10 this phase forms a completely wetting ML and it has been observed to appear at a Ti coverage of about 1.2 MLE. Integrated O 1s and Ti 2p intensity ratios are also discussed in this paper. All the XPS data used to derive integrated intensities (individual spectra not shown) were acquired by means of a conventional Al KR source, also available at the material science end station. The reason behind this choice is that, although the resolution of Al KR data is worse, their signal-to-noise ratio is higher, thus a lower systematic error is attached to the relative intensity data. The quality of each structure prior to spectroscopy measurements was assessed by LEED. In order to check for the reproducibility of the data and also to further investigate the possible dependence of the electronic properties on the coverage, each well-ordered structure was also prepared at variable amounts of Ti MLE values, whenever this variation did not affect the LEED pattern. Significant dependence of the spectroscopic data on the actual Ti MLE value was observed only in the VB-PES data of the rect structure. This case will be further discussed in section 3.2. Spectroscopy data were also acquired for two higher coverage films, hereafter labeled as “2 MLE on w” and “4 MLE”. These high coverage films do not show a specific LEED pattern. In a previous study we have used STM and XPD to demonstrate that 3D TiO2 clusters grow over a wetting layer with the w structure.14 The 4 MLE and the 2 MLE (on the previously formed w layer) were carried out under heavily oxidizing conditions (PO2 )10-4 Pa). 3. Results and Discussion 3.1. Core Level Photoemission. Figure 1 shows the normal emission Ti 2p and O 1s XPS data (taken with a photon energy of 650 eV) of the TiOx/Pt(111) ordered films reported in Table 1. The corresponding spectra taken with the Al KR standard

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Figure 1. Ti 2p and O1s core level photoemission data of ultrathin TiOx films on Pt(111) taken with a photon energy of 650 eV.

source (not reported here) are identical as to the peak positions, but they show a lower resolution and a higher signal-to-noise ratio. From the point of view of the Ti 2p XPS data, it can be seen that all the TiOx films can be classified in two groups. In fact all the TiOx structures obtained under very low (PO2 ≈ 1 × 10-8 mbar) or medium (PO2 ≈ 1 × 10-5 mbar) oxygen postannealing pressure (see Table 1) share the same Ti 2p peak position (i.e., a Ti 2p3/2 BE value of 456.4 eV) and shape (i.e., an asymmetric, or Doniach-Sunjic type, line shape).15 The Ti 2p3/2 BE value for all the structures obtained under heavily oxidizing postannealing conditions is 458.6 eV, i.e., over 2 eV higher than for the “mildly” oxidized structures and very close to the bulk TiO2 BE value. Also, the Ti 2p shape is narrower and more symmetric. This shift of the Ti 2p levels toward higher BE together with the change in the line shape can be explained on chemical grounds: in fact they are consistent with the formation of more oxidized (ionic) TiO2-like ultrathin layer. Thus, from now on, we will refer to the structures characterized by low and high Ti 2p BE as “reduced” and “oxidized”, respectively. The two Ti 2p components observed in the sample obtained by growing 2 MLE of heavily oxidized titanium on the w-TiOx/Pt(111) system (Figure 1) can be explained by taking into account the morphology of the final film. In fact it was seen by STM that, for coverage higher than 1.2 MLE, 3D stoichiometric TiO2 clusters grow over the w-TiOx wetting layer.14 The peculiar morphology of this film results in a Ti 2p component that is identical to the characteristic w component and in a bulk-like TiO2 component which is further shifted to higher BE up to a value of 459.2 eV. Moreover, the quenching of the low BE component related to the w wetting layer occurs slowly with increasing the TiO2 coverage. For this reason we refer to the actual total 3 MLE coverage of this film as “2MLE on w”. The large shift observed for the Ti 2p core level has no counterpart in the O 1s BE data (see Figure 1). Except for a small shift toward lower BE (by about 0.3 eV) for the z structure and for another small shift of opposite sign for the two highest coverage oxidized films, all the O 1s peaks are placed at 530.2 eV. We think that this result indicate that both the Ti 2p and O 1s BEs are not only determined by initial (chemical) effects

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Figure 2. Top panel: Ti 2p coverage values yielding good quality LEED patterns for each of the investigated structures. The coverage is gauged from Ti 2p XPS intensity normalized to the average value of the Ti 2p intensity relative to the reference structure (w). Bottom panel: corresponding O 1s/Ti 2p XPS intensity ratio. Multiple preparations of structures are numbered to facilitate comparison of top and bottom panels. Preparations labeled as 1* are those whose data are reported in Figures 1 and 3. All the intensity data in this figure are derived from Al Ka excited core level spectra (see also the Experimental Section).

but that also final state effects (extra-atomic relaxation or screening) are important. In fact the lower metallic character of the oxidized film may also result in a less effective corehole screening mechanism by means of valence electrons. The lowered screening in turn would result in an increase of the measured BE for both the Ti 2p and O 1s levels. While the final state effect adds to the chemical shift of the Ti 2p level in the oxide, it may cancel the corresponding O 1s BE decrease that would be expected on the basis of a purely initial state effect. Indeed, as we already pointed out, the line shape of the Ti 2p level changes from a Doniach-Sunjic type for the reduced films, which indicates a nonzero density of states near Ef, to a more symmetric shape in the oxidized ones, which is characteristic of insulating compounds. Finally we note that in the highest coverage films the Ti 2p BE value of the TiO2-like component and O 1s BE undergo a shift of the same sign. As we are dealing with films close to the monolayer regime, it is extremely important to discuss the nature of the interface with the substrate. The fact that all the reduced structures, on one side, and all the oxidized ones, on the other, share the same Ti 2p BE and line-shape, strongly suggest that all structures in each group also share the same type of interface. The low BE and “metallic” Ti 2p shape observed for the reduced structures leads to the proposal of a Ti-Pt interface, whereas the high BE and symmetric line-shape of the oxidized ones is consistent with a O-Pt interface. The interface was actually probed by

means of XPD measurements10 for one reduced (w) and one oxidized structure (rect). The result of the XPD experiment is consistent with the picture emerging from XPS. A direct consequence of the interface energetic is the wetting behavior of the reduced structures (Ti-Pt interface) with respect to the nonwetting 3D (O-Pt interface) growth of the oxidized ones, as observed with STM measurements.10 The analysis of the intensity of the O 1s and Ti 2p XPS peaks can in principle be employed to extract information about the TiOx film stoichiometry. However, this type of determination is to be regarded with suspicion. In fact, structural effects, such as layer stacking and photoelectron diffraction phenomena may hamper the accuracy of the stoichiometry determinations. On the other hand, an analysis of the intensity trend of the O 1s and Ti 2p peaks along the studied series, such as the one reported in the following, can be of value to outline other effects related to the film morphology and structure. The top panel of Figure 2 shows the summary of all the Ti 2p coverage values measured on samples prepared during the present experiment and yielding a good quality LEED pattern (values for the high coverage films are omitted). The horizontal solid line indicates the coverage value (1.2 MLE) yielding the completely wetting w ML. This value was determined by averaging the coverage values of each w-structured film we prepared. The error bar on the Ti coverage values ((10%) was derived from the deviation of these w coverages from their

Well-Ordered Ultrathin TiOx Films on Pt(111) average. It can be seen that all the reduced structures were formed only in a narrow Ti coverage range, while the oxidized rectangular structures appear in a wider range of coverage values. On the other hand, the oxidized hexagonal k′ could only be formed with a coverage well below 1.2 MLE. The bottom panel of Figure 2 shows the corresponding O 1s/Ti 2p intensity ratio. The horizontal solid line indicates the average ratio found on the high coverage 4 MLE sample, whose XPS Ti 2p peak shows only a bulk-like component. Within the present experiments, this ratio (equal to 0.7) is our best reference to a stoichiometric TiO2 sample. All the reduced structures are characterized by a rather well-defined O 1s/Ti 2p ratio (ca. 0.5), that would correspond to a stoichiometry very close to that of Ti2O3. For the oxidized rectangular structures, the large range of coverage values yielding the peculiar LEED pattern finds a counterpart in the apparently irregular distribution of the O 1s/ Ti 2p ratios. These values spread well above our stoichiometric limit. However a more detailed inspection of Figure 2 shows that these over-stoichiometric samples correspond to rectangular oxidized structures formed at a Ti coverage well below the 1.2 MLE value. This observation together with the high O 1s/Ti 2p values observed lead us to the conclusion that the O 1s signal is not entirely related to the oxygen content of the film itself. Actually, STM data10 have shown that the rectangular oxidized films are not wetting the substrate, but they are characterized by a rather rough morphology due to the growth of 3D islands that do not cover the entire Pt(111) surface, actually leaving uncovered portions of the Pt(111) substrate. It is then possible that the excess O 1s signal derives from oxygen interacting with the portion of Pt(111) surface not covered by the overlayer. This assumption is consistent with the observed trend of a decreasing O 1s/Ti 2p ratio with increasing Ti coverage. On the other hand, we do not observe any LEED pattern associated to this oxygen layer. Oxygen chemisorption on Pt(111) yields an ordered p(2 × 2) LEED, although this LEED can only be observed just below RT.16 Anyhow, the presence of a dilute, chemisorbed layer, present only above some parts of the Pt(111), could not entirely account for the intensity increase of the O 1s signal we measure. However, some preliminary analysis of Pt 4f data (not shown here) support the occurrence of an oxidation process of the Pt(111) substrate. Such substrate oxidation may be promoted by the presence of Ti.17 A similar metal-promoted oxidation was also observed on Pd(100) upon Ni alloying.18 Alternatively, to explain the observed O 1s/Ti 2p ratio, it can be proposed that the oxidized substrate patches may lie directly underneath the oxidized TiO2 layer. The convergence toward a stoichiometric O 1s/Ti 2p ratio with increasing rect and rect′ film coverage could be explained with a substrate signal attenuation due to the increasing thickness of the 3D islands. Although the present data alone are not sufficient to elucidate completely the substrate oxidation mechanism, nonetheless they provide strong evidence that under extreme oxidative conditions (giving rise to the rect and rect′ phases), the Pt(111) substrate is itself partially oxidized. 3.2. Valence Band Photoemission. Figure 3a shows the VB data relative to the TiOx structures detailed in Table 1 together with that of a clean Pt(111) surface. All data were collected at 200 eV photon energy, corresponding to the minimum cross section for the Pt 5d levels. It can be seen that, with this choice of the photon energy, the contribution of the substrate is effectively quenched. As an initial remark about the data of Figure 3a we point out that the spectra reveal differences in the electronic structure of

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Figure 3. (a) VB-PES data of TiOx ultrathin films on Pt(111) acquired with a photon energy of 200 eV. All data are normalized to the photon flux. (b) Some of the VB spectra after subtraction of a normalized clean Pt(111) spectrum (see text for the normalization procedure).

the investigated TiOx ultrathin films, such that the simple distinction between oxidized and reduced structures, which emerged from the analysis of the XPS data, appears to be oversimplified. In fact, appreciable spectral differences can be observed when comparing VB data within each of the two categories (we remind the reader that z, z′, and w are classified as reduced; k′, rect, and rect′ are classified as oxidized). A useful guideline for the discussion of the data reported in Figure 3 is provided by the rich database of experimental and theoretical results for bulk TiO2 and TiOx compounds.19-41 In order to make easier the following discussion we here report a brief summary of the literature results. The VB spectra relative to the stoichiometric surfaces of rutile19-27 and anatase TiO228 are all very similar between them. The VB electronic states of TiO2 are primarily O 2p derived and are located in a BE region between 3 and 10 eV. No intrinsic (e.g., due to crystal truncation) surface state located in the band gap region is experimentally observed or predicted by theory.19 Electronic states located in the bulk band gap region are only observed on oxygen deficient surfaces,19 and it has been experimentally shown that these surface states are strongly involved in the reactivity of TiO2. Theoretical calculations based on several computational methods33-38 have shown that the O 2p states are actually mixed with Ti 3d states. Such hybridization causes a splitting of the O 2p derived band that is observed experimentally in the form of two main components (located at about 8 and 5 eV BE and often referred as high and low BE components).20-28,40,41 The Ti-O mixing has been probed by numerous experiments carried out by means of VB resonant photoemission (ResPES).22-25,28 Both theory and experiment agree that the highest Ti 3d content (highest bonding character) is associated to the high BE component. The O 2p part of the VB itself carries information about the content of oxygen on the surface. In fact, the onset of the VB shifts away from Ef by an amount which is proportional to O vacancies concentration.19-21 The most relevant effects of the reduction of TiO2 surfaces is observed in the TiO2 bulk band gap region (0-3 eV) with the appearance of surface states which have a Ti 3d character. The intensity, BE value, and “nature” of this Ti 3d derived peak depend very strongly on the concentration of defects.19-24,26,28,38,39 Considering now substoichiometric oxides, the primarily O 2p derived part of the VB extends over the BE range from 4 to 12 eV in Ti2O3 and from 4 to 9 eV in TiOx (0.93 e x e

874 J. Phys. Chem. C, Vol. 111, No. 2, 2007 1.15).29-31 A prominent feature of the electronic structure of Ti2O3 is the observed emission from surface states having Ti 3d character, which involves the BE range between 1.5 and 4 eV.19,29,30,37 Bulk band emission from Ti 3d states occurs in the region just below the Ef.29,30,37 Finally, in metallic TiOx compounds, Ti 3d emission consists primarily in a peak crossing the Ef.31 Coming back to our experimental data, let us discuss first the most intense part of the spectra (roughly between 3 and 9 eV) corresponding to primarily O 2p derived states. The VB data of the z phase shows only a broad feature centered at about 5.5 eV. Except for the higher intensity near the Ef, the spectrum relative to this structure resembles very closely the one previously reported for the early stages of the oxidation of Ti metal (see the off-resonance spectra in ref 32). The VB spectra of the w and z′ structures are very similar to each other. They show a higher degree of oxidation with respect to the z one: in fact a splitting of about 1.5 eV now appears in the primarily O 2p derived states. According to the O 1s/Ti 2p XPS intensity ratio reported in Figure 2, the stoichiometry of the reduced films is close to that of Ti2O3. On the other hand the O 2p band of bulk Ti2O3 is much wider and rich of structure than the ones observed for w and z′.31 Thus Ti2O3 is not a good candidate for a comparison to our data. The width of the VB in highly reduced TiOx bulk compounds (see ref 31 where 0.93 < x e 1.15) is consistent with the width of the O 2p band of the z′ and w structures, which is about 5 eV. However, it has to be noted that also the z′ and w spectra closely resemble the ones observed in the gradual oxidation process of bulk Ti metal.32 The bulk band of TiO is in fact structureless, whereas the spectra of ref 31 show a gradual splitting of the O 2p band. This difference is probably to be attributed to the difference in the degree of structural order between TiOx (0.93 < x e 1.15) bulk compounds, that are in fact polycristalline, and the oxides obtained in ref 31, where the authors made use of a single-crystal Ti metal substrate. Our thin films are well-ordered too. The spectra of the oxidized structures (rect, rect′, and k′) share common features that are consistent with a highly developed oxidation process: there is now a 3 eV wide splitting into the high and low BE part of the primarily O 2p states, and the onset of the O 2p emission is located at about 3 eV below Ef. Both the observed splitting and onset values are the same as in bulk TiO2. However, there are spectral features of the oxidized ultrathin films that deviate from the bulk-like TiO2 ones: the overall width of the O 2p emission, which is about 1 eV narrower than in TiO2, and the energy position of the high and low BE O 2p components, which is shifted by 1 eV toward lower BE (remember that oxygen deficiency shifts the VB to higher BE). The narrowing of the O 2p band is most probably an effect of the dimensional confinement that was discussed by Sato et al.42 for TiO2 ultrathin layers. According to the authors, bandwidth narrowing is accompanied by a band gap widening. As the band gap widening is an extremely important issue, we have to point out here that there is no evidence of such effect in our VB data. In fact, as a result of the supposed band gap widening, which is indeed experimentally observed for TiO2 ultrathin layers in colloidal suspensions,42 it would be reasonable to expect both the VB top and the conduction band bottom to shift away from Ef. The VB top is measured by the onset of the VB emission. In our case the VB onset value is bulk-like but it is the energy position of the O 2p band, marked by the low and high BE components, that are shifted by 1 eV toward lower BE compared to the bulk. We do not observe an

Finetti et al. analogous shift in the O 1s core level, and this finding seems to rule out a simple charge transfer mechanism to oxygen atoms. We reckon that full quantum mechanical calculations, taking into account also the Pt states, are required to explain these effects. The VB parameters of bulk TiO2 are well reproduced already by the 4 MLE thick film. These data can thus be regarded as a reference for the O 2p part of bulk-like TiO2 electronic structure observed at the photon energy and within the angular resolution (photoelectron momentum or k-sampling) of the present experiment. The peculiar morphology of the 2 MLE on w film (3D TiO2 clusters on a w wetting layer)14 leads to an inhomogeneous film composition, as shown in the XPS section. However the shape of the relative O 2p band is very similar to the 4 ML one, thus indicating that at the more surface-sensitive photon energy used for the VB experiment, the TiO2 clusters contribution to the spectrum is predominant. Let us now discuss the data in the region just below the Fermi edge. In order to single out the contribution of the overlayers in the region between 0 and 3 eV, we performed the subtraction of an attenuated clean Pt(111) spectrum. The attenuation factor used for each TiOx film was determined by measuring the attenuation of Pt 4f photoelectrons extracted with the same kinetic energy as the VB electrons, i.e., with an excitation energy of 270 eV. The results of this subtraction procedure are shown in Figure 3b. The attenuation method we adopt allows canceling spurious instrumental effects (such as variations in the analyzer transmission). On the other hand the validity of this procedure is somewhat hampered by the fact that the angular distribution of VB and Pt 4f photoelectrons are quite different and hence their diffraction effects. Diffraction effects in our case are mitigated by the high ((8°) analyzer angular acceptance we employed. Thus, bearing in mind that the processed data of Figure 3b may not yield the “exact” photoemission intensity contribution of the film, we note however that there is a welldefined trend on going from reduced to oxidized structures, consisting in a strong reduction of the photoemission intensity in the 0-3 eV region. None of the background subtracted spectra show features that can readily be assigned to Ti 3d emission. This result is particularly surprising in the case of the reduced structures that actually appear to be metallic (in TiO Ti 3d emission “peaks” near Ef). It may be that dimensional confinement may induce a spreading of Ti 3d emission, as for the surface Ti 3d emission in Ti2O3. But the overall high photoemission intensity in the 0-3 eV BE region suggests that mixing of Ti-Pt states may play a major role in determining the electronic structure in the class of substoichiometric films. The final part of this section is dedicated to anomalous shape of the VB spectrum relative to the rect structure. The rect structure shows in fact a very high intensity from the low BE component of the O 2p states (at a BE of ca. 4 eV) relative to the higher BE one at. 7 eV (see Figure 3). This peculiar spectral shape has no correspondence in any bulk TiOx compound. Furthermore this peculiar VB spectral shape is strongly coverage dependent. Figure 4 shows spectra acquired on three different rect samples prepared by varying the Ti coverage (see also Figure 2). In all the three cases the LEED patterns were identical and of a good quality. The spectrum relative to 1.2 MLE was previously reported in Figure 3. It can be seen that there is a sharp increase of the photoemission intensity of the low BE component (centered around 4 eV) relative to the high BE one located around 7 eV. The anomalous intensity ratio of these two components is difficult to explain because, as already

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Figure 6. Proposed structural model for rect-TiO2 phase on Pt(111).10

Figure 4. VB spectra of the rect TiO2 phase on Pt(111) as a function of the Ti coverage.

mentioned earlier in this section, in bulk TiO2 the high BE part of the VB is related to O 2p states that are more strongly mixed with Ti 3d states. In order to investigate the character of the electronic states observed in the VB data of the rect structure we acquired photoemission data in the photon energy region of the Ti 3p resonance. The sample under study is the high coverage (1.8 MLE rect film of Figure 4). The corresponding ResPES data are reported in Figure 5. This plot shows a strong resonant enhancement of the 7 eV component. On the contrary, the 4 eV component does not resonate. The intensity of the low BE peak is actually stronger in the off-resonance spectrum. Therefore we rule out contribution of Ti 3d states to the 4 eV electronic feature. We can also easily rule out that the strong emission from the 4 eV component is due to the excess oxygen, directly attached to the substrate, discussed in section 3.1. In fact, it is actually

Figure 5. Solid line: VB spectra of a sample of rect TiO2 (1.8 MLE coverage) on Pt(111) acquired at the photon energy corresponding to the maximum resonant enhancement of the high BE part of the valence band (54 eV) and away from the resonance (84 eV). Dotted line: clean Pt(111) spectra acquired at the same photon energies. Clean Pt(111) data are normalized to the Fermi edge of the rect spectra.

the low coverage sample that shows the highest oxygen content (rect sample number 2 in Figure 2 corresponding to 0.7 MLE in Figure 4) but the corresponding VB spectrum does not show a pronounced emission at 4 eV. We propose to attribute the origin of the sharp component peaked at 4 eV to the peculiar geometric structure of the rectTiO2/Pt(111) film.10 The LEED pattern of the rect structure corresponds to an incommensurate overlayer, having a rectangular unit cell of 3.8 ( 0.1 × 3.0 ( 0.1 Å,2 whose dimensions are very similar to those of the rect-VO2 phase on Pd(111) reported by Surnev et al.43 On this basis, the model obtained by Kresse et al.44 for the rect-VO2 phase by using density functional theory (DFT) calculations has been also proposed for the rect TiO2 phase on Pt(111).10 The schematic drawing of the model is reported in Figure 6. The structure implies a substrate-oxygen interface, with interface oxygen atoms occupying on-top and bridge positions of the substrate, and two layers of octahedrally coordinated Ti atoms. The structure is oxygen terminated on both sides. Such a peculiar structure is identical to the one reported for titania lepidocrocite-like nanosheets (note that rect-TiO2/Pt(111) and the nanosheets have the same lattice parameters),45,46 which can be prepared by wet chemistry methods using a swelling/ exfoliation process of a layered titanate.47 In the layered titanate the nanosheets are intercalated by cations (for instance alkaline) that screen the electrostatic repulsion between the negatively charged layers (charge due to Ti vacancies).48 The key point for the interpretation of the VB data of the rect phase is that the stacking of lepidocrocite-like nanosheets may in principle introduce novel electronic features via either (i) the interaction of O atoms facing each other from adjacent nanosheets or (ii) the presence of intercalating cations. A recent first principle study discussing single and stacked lepidocrocite-like nanosheets by Sato et al.42 seems to rule out the influence of O-O interaction from adjacent layers on the electronic structure: they have demonstrated that the main electronic difference between a single lepidocrocite-like nanosheet and other bulk TiO2 phases is the already mentioned widening of the band gap due to band-narrowing. According to these calculations, stacking of nanosheets does not produce any substantial difference in the relative weight of O 2p and Ti 3d states across the VB compared to the single nanosheet, and they obtain only a very weak interaction between the layers that are separated by 0.8 nm. No intercalating species was included in the calculations. We propose that a different stacking mechanism compared to the one so far reported may yield a shorter distance and thus a stronger interaction between the layers. The different stacking mechanism may originate from the fact that the electrostatic interaction between the nanosheets can be strongly reduced once the layers are supported by a metal substrate. This stacking mechanism could involve intercalation of Pt atoms leaking out from the substrate. If this hypothesis would hold, an alternative explanation of the data reported in Figure 5 could be possible. The increasing intensity of the spectral feature

876 J. Phys. Chem. C, Vol. 111, No. 2, 2007 around 4 eV could be associated to almost atom-like Pt 5d states. Some evidence supporting this hypothesis can be obtained by comparing the relative intensities of the 4 eV feature in the spectra reported in Figure 4 and 5. Actually, in the off-resonance (84 eV) spectrum reported in Figure 5, the relative intensity of the 4 eV feature with respect to the 8 eV component is higher than in the spectrum taken with a photon of 200 eV (Figure 4), i.e. at the minimum cross section of the Pt 5d levels. However, quantum mechanical calculations are in progress to verify such intriguing hypothesis. 4. Conclusions TiOx ultrathin films grown on Pt(111) have been chemically characterized by means of core level photoemission experiments. The analysis of the energy positions and lineshapes of Ti 2p XPS data allow us to rationalize the complex diagram of ordered TiOx structures by splitting them into two main classes: reduced TiOx-like and stoichiometric TiO2-like structures. We have also investigated the electronic structure of the ultrathin films by means of VB-PES. The spectra for the reduced structures are similar to those previously reported for the oxidation of Ti metal. These spectra were interpreted as substoichiometric TiOx (x ∼ 1) species. This evidence is also in tune with preliminary DFT results obtained on the w phase.49 Combined XPS and photoemission results for the reduced type of films are fairly consistent with the presence of a Ti-Pt interface, in agreement with previously reported photoelectron diffraction data.10 The analysis of the region near Ef has also suggested that mixing of Ti-Pt states play a major role in determining the electronic structure in this class of substoichiometric films. The VB structure of the stoichiometric oxidized films has been carefully compared with the results relative to bulk stoichiometric titania surfaces reported in the literature. From this comparison we conclude that there is similarity between the stoichiometric film spectra and the bulk TiO2 ones. However there are two features that are peculiar to the well-ordered ultrathin films: (i) the O 2p bandwidth is narrower (by about 1 eV), and (ii) the energy position of the main components of the O 2p band is shifted by 1 eV toward lower BE. We attribute the first effect to spatial confinement. The second effect is attributed to interaction with the substrate (mixing of film and Pt states). The spectrum relative to a 4 MLE stoichiometric film is entirely consistent with the bulk data. The rect TiO2 phase shows a peculiar coverage dependent enhancement of the photoemission intensity around 4 eV that was not observed in any TiO2 bulk compound. By using resonant photoemission, we have excluded Ti 3d contribution from the 4 eV component. We tentatively associated such experimental evidence to the peculiar structure of the rect phase. Acknowledgment. This work has been funded by European Community through the STRP project with the acronym NanoChemSens (Nanostructures for Chemical Sensors) within the SIXTH FRAMEWORK PROGRAMME (contract no. STRP 505895-1), by the Italian Ministry of Instruction, University and Research (MIUR), through the fund “Programs of national relevance” (PRIN-2003, PRIN-2005), and by the University of Padova, through the grant CPDA038285. We thank Dr. Andrea Vittadini (Padova) for helpful discussion. References and Notes (1) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (2) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33.

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