Ultrathin TiO2 Films on (1Ã2)-Pt(110): a LEED, Photoemission, STM

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J. Phys. Chem. C 2008, 112, 20038–20049

Ultrathin TiO2 Films on (1×2)-Pt(110): a LEED, Photoemission, STM, and Theoretical Investigation Stefano Agnoli,† Tommaso Orzali,† Mauro Sambi,† Andrea Vittadini,‡ Maurizio Casarin,† and Gaetano Granozzi*,† Dipartimento di Scienze Chimiche, Consorzio INSTM and Unita` di Ricerca INFM-CNR, UniVersita` di PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy, and ISTM-CNR, Via Marzolo 1, I-35131 PadoVa, Italy ReceiVed: August 29, 2008; ReVised Manuscript ReceiVed: October 9, 2008

The preparation and characterization of fully oxidized TiO2 ultrathin films obtained by reactive deposition of Ti in an O2 background on the (1×2)-Pt(110) reconstructed surface is described in details. The structure, the electronic properties, and the morphology of the epitaxial films giving rise to a (14×4) coincidence superstructure are discussed on the basis of low-energy electron diffraction, photoemission (both core and valence band), angle-scanned X-ray photoelectron diffraction, scanning tunneling microscopy data, and density functional theory calculations. We show that the oxide overlayer is a stoichiometric lepidocrocite-like singledomain nanosheet. This can be thought of as originating from a (100) oriented anatase bilayer which spontaneously restructure by a uniaxial relative sliding of one single layer with respect to the other by half a unit cell. According to the results of theoretical calculations, the process is self-driven by the spatial confinement, whereas a minor role is played by the interaction with the substrate. The occurrence of the (14×4) coincidence between the overlayer and the substrate is fully rationalized on the basis of the reported data. 1. Introduction Matter at nanometer dimensions forms phases and structures which may have no corresponding bulk counterpart. It is well recognized that many innovative properties of nanosized systems are a direct consequence of the prevailing effects of the interfaces. Studying ultrathin (nanometric) films supported on single crystals by means of the detailed methods offered by surface science can then highly contribute to understanding the relative role of inherent spatial confinement and of interfaces on the relevant properties of nanostructures. Nanostructured titania (TiO2) and related titanates phases, e.g., nanosheets, nanotubes, nanowires, and nanoclusters,1,2 have recently gained much attention for their relevance in important areas like photocatalysis,3 photovoltaic cells,4,5 and sensors.6 Correspondingly, the surface science of titania has been greatly developed.7 An important contribution to the field has been given by the studies carried out on well-ordered TiOx ultrathin films (where x can vary between 1 and 2) grown on metal singlecrystal surfaces,8-19 which offer the opportunity of analyzing different structures, stoichiometries, topographies, and degree of long-range ordered defectivity. According to the different preparation procedures, either fully oxidized stoichiometric titania or reduced understoichiometric TiOx ultrathin films can be obtained. If one considers fully oxidized stoichiometric titania, one point of major interest is to relate the structure of the ultrathin films with that of the known bulk titania polymorphs. Among them, rutile, anatase, and brookite are the most common bulk phases. Upon heating, interconversion between these phases can be observed, viz., anatase to brookite to rutile, brookite to anatase * To whom correspondence should be addressed. E-mail: gaetano.granozzi@ unipd.it. † Universita` di Padova. ‡ ISTM-CNR.

to rutile, anatase to rutile, and brookite to rutile.1 These transformation sequences imply very closely balanced energetics as a function of particle size.20 The surface enthalpies of the three polymorphs are sufficiently different that crossover in thermodynamic stability can occur under conditions that preclude coarsening, with anatase and/or brookite stable at small particle sizes. However, anomalous behaviors and inconsistent results are occasionally reported.1 Studying the structural properties of nanometric ultrathin films of titania can also contribute to clarify the structural properties of titania nanosheets (NSs) (presumably representing the building blocks of titania nanotubes),2,21 which can be obtained by delamination of layered bulk titanates.22 In particular, some NSs obtained from lepidocrocite-like23 3D lattices, have been characterized by X-ray diffraction24 (XRD), transmission electron microscopy25 (TEM), and X-ray absorption fine structure (XAFS)26 methods. We have recently demonstrated that a bottom-up route toward the synthesis of titania lepidocrocite-like NSs, alternative to the top-down exfoliation of layered titanates, is possible where the NSs are assembled from the constituent elements and epitaxially matched to single crystal Pt(111) and (1×2)-Pt(110) surfaces.27,28 In a preliminary report we have shown that, on the highly anisotropic (1×2)-Pt(110) surface, oriented single-domain lepidocrocite-like NSs can be formed.28 In the present paper, we describe in detail the preparation and characterization of such fully oxidized TiO2 ultrathin films obtained by reactive deposition of Ti in an O2 background on the (1×2)-Pt(110) reconstructed surface (Figure 1). The structure, the electronic properties, and the morphology of the films are discussed on the basis of low-energy electron diffraction (LEED), photoemission (both core and valence band, XPS and UPS), angle-scanned X-ray photoelectron diffraction (XPD), scanning tunneling microscopy (STM) data, and theoretical simulations of both the XPD and STM patterns. We show here

10.1021/jp807694r CCC: $40.75  2008 American Chemical Society Published on Web 11/19/2008

Ultrathin TiO2 Films on (1×2)-Pt(110)

Figure 1. Structure of the (1×2)-Pt(110) missing row reconstruction (top); its LEED pattern (Ekin ) 134 eV, bottom left) and mesoscopic STM morphology 67 × 65 nm; V ) 0.15 V, I ) 1.5 nA; bottom right).

that the stoichiometric lepidocrocite-like ultrathin film can be thought of as originating from a (001) oriented anatase bilayer which spontaneously restructure by a uniaxial relative sliding of one single layer with respect to the other by half a unit cell. The process also implies a uniaxial in-plane contraction of the lattice parameter, leading from the square anatase to the rectangular lepidocrocite unit cell of the oxide NS. According to the results of theoretical calculations, the process is selfdriven by the spatial confinement, whereas a minor role is played by the interaction with the substrate. 2. Experimental Section TiO2 films have been grown in an UHV preparation chamber at a base pressure of 5 × 10-9 Pa. The substrate was a Pt(110) single crystal prepared by repeated cycles of argon ion sputtering (KE ) 2 keV) and annealing at T ) 970 K with subsequent cooling down in oxygen (p(O2) ) 5 × 10-5 Pa) up to 700 K. The cleaning cycles were repeated until a clean and well-ordered 1×2 reconstructed surface was obtained as judged by the LEED pattern (see Figure 1) and by XPS. The mesoscopic morphology of (1×2)-Pt(110) surface consists of islands elongated in the [11j0] direction and characterized by varying degrees of the typical fish-scale reconstruction29 (which is usually more abundant close to the single-crystal edges, where slight misalignments with respect to the nominal cut direction are most probable). TiO2 layers were grown by depositing Ti at room temperature (RT) by means of an electron beam evaporator in an oxygen partial pressure of 1 × 10-4 Pa. A post-annealing treatment at 700 K and cooling down in oxygen pressure (p(O2) ) 1 × 10-4 Pa) improve the long-range order of the deposited layer, allowing the complete oxidation of Ti. The deposition rate of TiO2 resulting from this treatment was estimated to be ≈0.075 nm/min of Ti, as determined by quartz microbalance calibrations. Coverage values expressed in monolayer equivalents (MLeq) are calculated assuming the growth of anatase TiO2 (001) (see below), and assuming an interlayer distance between adjacent planes in TiO2 of 0.24 nm (1/4 of the unit-cell dimension in the [001] direction, equal to 0.951 nm). In the present work, coverages up to 2 MLeq were investigated. UPS, XPS, and XPD data were collected using a modified VG ESCALAB MKII photoelectron spectrometer. The sample

J. Phys. Chem. C, Vol. 112, No. 50, 2008 20039 was mounted on a two-axis goniometer which allowed the sweeping of the electron emission direction with an angular resolution of (1° both in polar angle (θ, defined with respect to the surface normal) and in azimuthal angle (φ, defined with respect to the [11j0] direction on the surface). The Al KR source with a pass energy (PE) of 20 eV was used throughout the XPS experiments while the acceptance half angle of the electron analyzer was estimated to be 3.5°. The UPS measurements were performed by means of a He discharge lamp (HIS 13 VUV source, Omicron) using the He I (21.2 eV) and He II (40.8 eV) emission lines. All the spectra were acquired at normal emission (Γ point of the first Brillouin zone), with the maximum angular acceptance of the electron analyzer (∼9°). XPD scans for structural analysis were obtained for Ti 2p and O 1s photoelectron peaks for different overlayer coverages. Line intensities are reported after a simple linear background subtraction. Non-monochromated Al KR radiation was used for the XPD experiments. Theoretical simulations of the XPD patterns for the emitted Ti 2p (Ekin ) 1027 eV) and O 1s (Ekin ) 956 eV) photoelectrons has been accomplished with the aid of single scattering cluster (SSC) simulations within a spherical wave (SW) formalism.30-32 by using the Rehr-Albers procedure33 in the approximation of single scattering cluster-spherical waves (SSC-SW). The 862 atoms cluster used to simulate the Ti 2p and O 1s XPD plots was a lepidocrocite (see below) bilayer supported by a three atomic layers thick Pt slab. Its dimensions were checked to be sufficient to achieve convergence. The inelastic attenuation length was set to 15 Å; an inner potential of 10 V was used. Scattering phase shifts were calculated in the framework of the partial wave method within a muffin-tin model using the MUFPOT program.34 Angular broadening of the photoelectron emission direction to match the spectrometer finite acceptance angle (∼ 6°) was also included in the simulations. STM images were obtained by means of an Omicron variable temperature STM, operating in a constant current mode at RT using electrochemically etched Pt-Ir tips. The scanner was calibrated in the z-direction with respect to the step edge of a clean Pt(111) surface. For the lateral calibration the (1×2)Pt(110) reconstructed surface has been used. Density functional theory (DFT) calculations were performed using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional35 and Vanderbilt ultrasoft pseudopotentials.36 The smooth part of the wave function was expanded in plane waves, with a KE cutoff of 25 Ry, while the cutoff for the augmented electron density was 200 Ry. This setup was tested for titania bulk and surface systems.37 All slab models were built by assuming the theoretical (a ) 0.3957 nm) Pt lattice constant. Only the upper part of the slab was used to simulate the surface, while Pt atoms belonging to the two bottom layers were kept fixed in their bulk positions The structure and the surface energies of the clean (1×1)- and (1×2)-reconstructed Pt(110) surfaces were found to be in good agreement with previous DFT calculations.38,39 Calculations were run both on a 14×1 and on a large 14×4 supercell (see below). For the smaller cell, we used the PWSCF code, where the one-electron Kohn-Sham equations are self-consistently solved, and a 1×4 MonkhorstPack40 sampling of the surface Brillouin zone (SBZ). These calculations were also used to obtain density of states (DOS) and to simulate STM images through the Tersoff-Hamann approach.41 For the larger 14×4 supercell we used the Car-Parrinello (CP)42 code and a Γ-only sampling of the SBZ. In this case, only structural optimizations were performed using a damped molecular dynamics algorithm. To minimize com-

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Figure 2. LEED patterns of TiO2 ultrathin films on (1×2)-Pt(110) as a function of Ti coverage: (a) 0.3 MLeq, Ekin ) 80 eV eV; (b) 1 MLeq Ekin ) 87 eV; (c) 2 MLeq Ekin ) 87 eV. The unit cells of the substrate and of the different superstructures have been outlined.

Figure 3. Ti 2p (left) and O 1s (right) XPS (Al KR) photoemission lines of 1 MLeq and 2 MLeq TiO2 ultrathin films on (1×2)-Pt(110), at θ ) 25° from the surface normal.

putational efforts, only 4 Pt atomic layers were included (vs the 5 atomic layers used for the smaller supercell), and a 4-electron pseudopotential was used for Ti (in place of the 12electron pseudopotential used for the smaller supercell). 3. Experimental Results 3.1. LEED Data. LEED images were obtained on TiO2 ultrathin films grown on (1×2)-Pt(110) as a function of Ti coverage: in Figure 2 we report the LEED patterns obtained for (a) 0.3 MLeq, (b) 1 MLeq, and (c) 2 MLeq ultrathin films. In the sub-MLeq coverage range (Figure 2a), two distinct superstructures can be identified: integer spots exhibit a ×14 splitting along the [11j0] direction, while the fractional spots show, along the same direction, a ×11 periodicity. This latter superstructure, namely a 11×2, can be attributed to the oxygen chemisorption phase on (1×2)-Pt(110), recently characterized by Helveg et al.,43 which is formed in the deposition conditions. The ×14 spots instead can be associated to the formation in nuce of the TiO2 layer, where only the most intense spots (i.e., first satellites around the substrate reflections) are visible because of the limited coherent ordering of the oxide islands. When the coverage is increased (the 1 MLeq film in Figure 2b) and the oxide layer begins to wet a more significant fraction of the surface (though it is by no means fully wetting, see UPS and STM data reported below) the LEED pattern turns into a

superposition of the 1×2 reconstruction of the clean Pt (see for comparison Figure 1) and of a well-developed ×14 splitting of the integer spots, while the 11×2 oxygen superstructure disappears. At closer look it can be seen that the intensity of the ×14 spots largely extends following parallel lines along the [001] direction, though a precise periodicity cannot be determined. When the coverage is further increased (the 2 MLeq film in Figure 2c) and the oxide layer almost completely wets the substrate, the clean 1×2 reconstructed substrate pattern disappears and a 14×4 supercell becomes evident. If we further consider the spots shape, we observe that most of the times they are streaked along the [001] azimuth, which indicates that that the oxide islands grow preferentially along the [11j0] direction, being characterized by rather narrow domains in the perpendicular direction, as also confirmed by STM images (see section 3.5); however, the ×4 periodicity is not well developed, probably because of the presence of frequent defects in the translational symmetry (see STM images in section 3.5 where the spacing between rows is frequently ×3 or ×5). 3.2. Core Level XPS Data. Figure 3 shows the Ti 2p (left) and O 1s (right) photoemission lines from TiO2 deposited films on (1×2)-Pt(110) at a coverage of 1 and 2 MLeq, taken at near normal emission (θ ) 25°). The following observations can be made:


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Figure 4. He I (left) and He II (right) VB photoemission data of 1 MLeq and 2 MLeq TiO2 ultrathin films on (1×2)-Pt(110). All the spectra were acquired in normal emission (Γ point of the first Brillouin zone). For comparison, the spectra of the clean (1×2)-Pt(110) are also reported.

• The Ti 2p spectra do not show any evidence of Ti components in oxidation states other than Ti4+:Ti 2p3/2 peak has a binding energy (BE) of ≈ 459 eV, that is the value reported for bulk TiO2. • In the O 1s region a rather broad peak centered at BE ≈ 530.6 eV and asymmetric toward higher BE values is detected, which suggests the presence of oxygen in several minority chemical states besides the oxidic component bound to Ti; however, a slight asymmetry is also expected because of the metallic screening provided by the Pt substrate.44 • No substantial oxidation of the substrate is apparent from data recorded as the deposition proceeds, at least with the sensitivity warranted by the non-monochromated Al KR source at grazing emission: the fwhm of both Pt 4f doublet components (Pt 4f7/2 and 4f5/2 found at 71.6 and 74.8 eV, respectively) do not change with Ti exposure. 3.3. Valence Band UPS Data. Figure 4 shows the valence band UPS data for the 1 MLeq and 2 MLeq films taken with He I (21.2 eV) and He II (40.8 eV) excitation lines at normal emission (Γ point of the first Brillouin zone). For comparison, the spectra of the clean (1×2)-Pt(110) surface measured in the same conditions have been also reported. It is evident from the analysis of the intensity close to the Fermi edge that in the 1 MLeq ultrathin film there is a large portion of the substrate which is left uncovered. The most evident change induced by the presence of the oxide overlayer is the formation of several extra peaks (Figure 4) in the 5-9 eV region: the features at 5.0, 6.3, and 8.2 eV are clearly visible in the He I spectrum of the 1 MLeq ultrathin film. The lower photoemission cross section of Ti and O with respect to Pt at the He II energy makes these peaks less resolved in the He II data. Nevertheless, a He I/He II comparison shows that they are not shifted in energy, at variance with the behavior expected for true surface states. Interestingly, increasing the coverage from 1 to 2 MLeq does not add new main features but it rather determines the increase of the intensity of the overlayer peaks with respect to the substrate-related ones, and the formation of further resolved low intensity features (e.g., a peak at 6.9 eV in the He I spectrum).

Figure 5. Ti 2p (left, KE ≈ 1027 eV) and O 1s (right, KE ≈ 956 eV) 2π plots from the 1 MLeq TiO2 film on (1×2)-Pt(110). The polar angle range is 28° e θ e 66° (defined with respect to the sample normal) The [11j0] substrate main azimuth corresponds to the vertical radius pointing to the top.

From a comparison of the spectra of the clean (1×2)-Pt(110) substrate with the data from ultrathin films, a small modification of the spectral fingerprint in the region between EF and 2 eV becomes apparent, where states of Pt 5d character are expected. This is particularly evident in the case of the 1 MLeq film and could be partially due to the presence of Pt-overlayer hybridization. In the case of the 1 MLeq film, which partially covers the substrate surface, the presence of some (11×2)-O reconstructed Pt(110) regions45 also possibly contributes to the observed changes close to the Fermi edge. 3.4. XPD Data. Figure 5 reports Ti 2p and O 1s 2π XPD diagrams in the polar angle range 28° e θ e 66° (defined with respect to the surface normal) for the 1 MLeq TiO2 ultrathin film on (1×2)-Pt(110). In such 2π XPD plots, the center corresponds to the surface normal, a radial section represents a polar scan, a circular section is an azimuthal scan, and the photoelectron intensity is given by the color scale. Intense forward scattering (FS) phenomena are detectable along the [001] substrate direction, while weaker ones are visible along the [11j0] azimuth. Peaks are found at the following polar angles, θ ) 36° for O 1s photoelectrons and θ ) 30° for Ti 2p along [11j0], and θ ) 15°-18° along [001], together with a lowintensity feature at θ ) 45°. Other intermediate FS events are detectable at the azimuthal angles of (40°, (140° with respect


Figure 6. Ti 2p (left, KE ≈ 1027 eV) and O 1s (right, KE ≈ 956 eV) 2π plots from the 2 MLeq TiO2 film on (1×2)-Pt(110). The polar angle range is 28° e θ e 66° (defined with respect to the sample normal) The [11j0] substrate main azimuth corresponds to the vertical radius pointing to the top.

to the [11j0] direction and at polar angles of θ ) 18° for O 1s and θ ) 25° for Ti 2p photoelectrons. Figure 6 reports Ti 2p and O 1s 2π XPD plots for the 2 MLeq ultrathin TiO2 film: the two diagrams are very similar to those of the 1 MLeq thick film, except for a few small differences. Ti 2p FS events along the [001] directions become richer in fine structure, suggesting the development of a longer range order, not yet fully defined in the 1 MLeq film. Along the [11j0] direction the FS event visible at θ ) 45°-50° increases its intensity. There are no significant differences between O 1s 2π diagrams relative to the two different coverages. The independence of the diffraction features on coverage and the development of strong FS features already at the 1 MLeq coverage strongly suggest that the oxide overlayer grows through the lateral enlargement of at least two atomic layers thick islands. 3.5. STM Data. Figures 7 and 8 report the STM images obtained in the constant current mode at RT for two different coverages (i.e., ca. 1 and 2 MLeq) of TiO2 on (1×2)-Pt(110). In the large area image of the lower coverage overlayer (Figure 7a), rectangular islands are most abundant, whose long sides are aligned along the ridges of the 1×2 reconstructed substrate. The island’s apparent height is almost constant (ca. 0.15 ( 0.02 nm): due to the profoundly different charge density distribution on the oxide overlayer viz. on the metal substrate, it is well-known that the apparent STM height is not directly related to the geometrical height difference, which is calculated to be 0.64 nm from DFT calculations (see Discussion). This in accordance with a large amount of data on similar systems reported in the literature.16,17,19 Brighter patches can be seen in some parts of the image: they are still monolayer islands that grow on Pt(110) adislands. In fact, the Pt(110) topography is characterized by a high density of steps along the [001] surface direction (see Figure 1, bottom right), and the step height between different oxide adislands is consistent with the step height of the underlying substrate. Within the islands, some dark stripes parallel to the [001] substrate direction, whose contrast is largely bias-independent, are clearly visible (Figure 7a). The apparent stripe depth is 0.06 ( 0.01 nm with respect to the adjacent brighter areas. The average stripe separation amounts to 3.9 nm along the [11j0] substrate close-packed direction, corresponding to 14 substrate unit cell parameters (we remind the reader that the [11j0] unit vector length for Pt(110) is a ) 0.2775 nm). The higher resolution images (where the 1×2 substrate reconstruction is evident beside to the oxide island, Figure 7c) also evidence a less pronounced, regular corrugation of the overlayer along the [001] substrate direction with a periodicity of 1.6 nm, corresponding to 4 substrate unit cell parameters (the [001] unit vector length for unreconstructed Pt(110) is b ) 0.392 nm). Occasional deviations from the ×4

Agnoli et al. periodicity are observed and appear as ×3 or ×5 “defects”. The corrugation of the ×4 superstructure along [001] is biasdependent and hence likely due to mixture of electronic and topographic effects (compare Figure 7b, and 7c, measured at different bias values). Oxide islands do not seem to preferentially decorate step edges, despite the particularly high step density on the (1×2)Pt(110) surface. They rather develop with comparable probability on flat terraces away from step edges as well close to them. In addition, the 1×2 reconstructed substrate surface areas surrounding the oxide islands are intactsno substantial longrange mass transport in the substrate triggered by the formation of the coincidence superstructure is observed in STM images. In the large area image of the higher coverage (2 MLeq) overlayer (Figure 8), an almost completely covered surface is observed where all adjacent islands have merged to fully wet the substrate. Now a regular array of dark stripes parallel to the [001] substrate direction is also observed, which is the development of the few stripes already seen within the isolated islands in the lower coverage overlayer. The STM image as a whole resembles a Moire´-like pattern well ordered in the long-range, although local deviations are seen, which are responsible for the slightly wavy appearance of the black stripes. In the atomically resolved image reported in the inset of Figure 8 the 14×4 superstructure is clearly visible (see the large dark rectangle in the inset of Figure 8): rows of bright protrusions are seen at a bias of 0.28 V, which turn to dark stripes at higher bias (see Figure 7c). In the same image the unit cell (0.30 × 0.39) nm of the lepidocrocite-like NS is highlighted (see the small dark rectangle in the inset of Figure 8). Y-shaped bright defects are seen to decorate preferentially the ridges of the ×4 reconstruction in the atomically resolved STM map. 4. Discussion As suggested by the core level XPS measurements (Figure 3), at the temperature and oxygen pressure conditions adopted for the deposition of the overlayers, fully oxidized TiO2 films on (1×2)-Pt(110) substrate can be obtained. According to the XPS Ti 2p data there is no evidence of Ti3+ defect states within the film. However, it is difficult to assess the actual level of defectivity on the film surface using VB-UPS because in the energy window where such defective states usually appear (0-2 eV) also Pt substrate states are present (Figure 4). The core level photoemission data are in tune with the picture emerging from the XPD and STM analysis, i.e., the two different coverages (1 and 2 MLeq films) correspond to oxide islands with identical thickness but different lateral sizes, where the high coverage is representative of an almost completely wetting layer. The combined LEED (Figure 2) and STM (Figures 7 and 8) data give important complementary information on the structure and the growth morphology of the TiO2 ultrathin films. At low coverage (1 MLeq films) the LEED data show the splitting of 1×1 Pt spots, to give a ×14 periodicity along the [11j0] direction. In some of the LEED experiments, a pronounced spot streaking along the [001] direction was evident, pointing out that the overlayer grows preferentially along the [11j0] direction and, in agreement with the STM image, the islands are characterized by a rectangular shape where the long side is along the [11j0] azimuth. In other words, the anisotropic diffusion coefficients on the (1×2)-Pt(110) surface is somewhat templating a preferential growth along the atomically flat substrate ridges, while limiting the islands width along the highly corrugated [001] substrate azimuth. When the higher coverage is explored, the oxide overlayer islands become wider and begin to percolate in a completely



Figure 7. STM images of 1 MLeq TiO2 ultrathin films on (1×2)-Pt(110): (a) 110 × 110 nm, V ) 1.9 V, I ) 1.5 nA; (b) 23 × 23 nm, V ) 1.9 V, I ) 2.3 nA; (c) 23 × 23 nm, V ) 0.8 V, I ) 1.5 nA.

Figure 8. STM images of 2 MLeq TiO2 ultrathin films on (1×2)Pt(110): 110 × 110 nm, V ) 0.34 V, I ) 1.46 nA. Inset: 11.7 × 11.8 nm, V ) 0.28 V, I ) 1.65 nA. The unit cell of the superstructure and of the lepidocrocite overlayer have been outlined.

wetting 2D layer, albeit characterized by a high density of steps, mirroring those of the substrate. At the higher coverage, LEED displays a 14×4 coincidence pattern and the STM image (see the 2 MLeq image in Figure 8) concurrently shows clear ×4 modulations along the [001] direction, i.e., perpendicularly to the ridges of the (1×2)-Pt(110) reconstruction. Such a modulation is also visible for overlayer islands obtained at low coverages and it is also strongly bias dependent (see Figure 7b,c): depending on the applied bias, the ×4 modulation appears as bright or dark stripes running along the [11j0] substrate direction. This modulation is subject to a certain degree of defectivity, since also ×3 and ×5 reconstructions can be found in the STM images. This defectivity and the reduced width of the oxide adislands along the [001] substrate direction at coverages less than 2 MLeq can motivate the appearance of the 14×4 LEED pattern only in the higher coverage (2 MLeq) data, where the overlayer islands width is larger. If we consider that the unreconstructed substrate unit cell has dimensions of 0.277 × 0.392 nm, the overlayer coincidence lattice would have the following dimensions: 0.392 × 4 nm ) 1.568 nm along the [001] azimuth and 0.277 × 14 nm ) 3.878 nm along the [11j0] direction. STM measurements reported in section 3.5 show that 13 overlayer unit cell parameters are

comprised in a 3.88 nm length interval along the ridges of the substrate 1×2 reconstruction. The overlayer lattice parameter along the [11j0] substrate direction therefore is 3.88 nm/13 ) 0.30 nm. Along the [001] direction the distance between atomic bumps is 0.39 nm, i.e., the same, within the experimental uncertainty, as the distance between nearest-neighbor (NN) Pt atoms along the [001] azimuth, which gives an overall TiO2 unit cell of 0.30 × 0.39 nm. This rectangular unit cell is very close to the in-plane one found in the individual lamellae of stacked titanates, whose orthorhombic 3D unit cell is of the lepidocrocite-like type (0.30 × 0.38 nm, more on this below).24 The overlayer appears to be slightly strained along the Pt [001] direction (+3.1%) with respect to the unsupported NS, in order to match the substrate lattice parameter. It should be outlined that the long side of the rectangular cell is not far from the dimensions of the square unit cell of the anatase TiO2(001) surface 0.378 × 0.378 nm. This analogy suggested us that there might be a structural relationship between the oxide overlayer and the anatase bulk phase (see also the following discussion based on DFT results). In order to develop a definite model of the overlayer structure, XPD data taken in the FS regime have also added further structural information: they furnish a direct fingerprint of the local atomic arrangement surrounding each atom, since main intensity maxima correspond to interatomic directions and give information regarding the atomic stacking in three dimensions.46 The similarity of Ti 2p and O 1s 2π XPD plots reported in Figures 5 and 6 and the ×4 periodicity revealed by LEED prompted us to consider the possibility that the structure and the orientation of the ultrathin film be reminiscent of the (001) surface of bulk anatase. As a matter of fact, similar XPD diffraction patterns point to similar structural environments of the photoelectron-emitting species: if one considers the aVerage structural environments of anions and cations in an anatase (001) bilayer, one readily finds that these are comparable for both species. On the other hand, according to LEED and STM measurements47 a 1×4 reconstruction of the anatase (001) surface has been documented, whose nature has been ultimately explained by the “ad-molecule” (ADM) model developed by Lazzeri et al.48 which consists in a ×4 superstructure originated from the substitution of one every four bridging oxygen rows terminating the 1×1 anatase (001) surface with a chain of TiO3 rows. Apparently, the ×4 superstructure resembles the recon-


Agnoli et al.

Figure 9. Comparison between selected azimuthal and polar SSC-SW (full lines) and experimental (marked lines) XPD scans for Ti 2p emission from a single lepidocrocite NS on (1×2)-Pt(110). Experimental and theoretical curves are reported with their native anisotropy.

struction we observe along the [001] substrate direction, whose lattice constant (0.392 nm) is not far from the anatase (001) lattice parameter (0.378 nm). However, preliminary XPD simulations (not discussed here) of the Ti 2p and O 1s 2π XPD plots based on the ADM model indeed would give a satisfactory fit of the Ti 2p 2π plot (less so for the O 1s one) if the TiO3 rows run along the [001] rather than the [11j0] substrate direction, which is in contradiction with LEED and STM data. There are, however, additional reasons for such a model being in conflict with our STM and LEED observations: (a) Bulk-terminated anatase (001) is characterized by a square unit cell 0.378 × 3.78 nm whose structural fingerprint is retained as a subcell, some distortions notwithstanding, in STM images of the 1×4 superstructure,24 while our observation of the 14×4 coincidence postulate (and our STM data directly demonstrate) the presence of a rectangular unit cell 0.30 × 3.92 nm in our oxide overlayer. If a moderate expansion (from 0.378 to 0.392 nm, i.e., by 3.7%) of the overlayer lattice parameter along one direction to match the substrate lattice constant along the highly corrugated [001] substrate azimuth would be reasonable, a drastic contraction (from 0.378 to 0.30 nm, i. e., by -20.6%) along the atomically flat [11j0] substrate azimuth would be unrealistic, all the more so for a direction along which a longrange coincidence superstructure (pointing to weak overlayersubstrate interactions) is found.49 (b) A second, equally strong objection to the ADM model arises from the nature of the ×4 reconstruction itself: added TiO3 rows protrude from the anatase surface giving rise to substantial and largely bias-independent corrugation in STM images, at odds with the weak and strongly bias-dependent corrugation of the ×4 features along [001] observed in our case. Incidentally, we note that rutile TiO2 (100) is characterized by a rectangular unit cell50 which is reminiscent of the one we observe in our films. However, in this case the unit cell dimensions 0.30 × 0.45 nm are only partially in accordance with our observation (a substantial contraction would now be

required to match the [001] substrate unit vector). In addition, two domains would be required to explain the 2-fold symmetry of the XPD 2π plots, due to the intrinsically lower symmetry of the rutile TiO2 (100) structure with respect to anatase (001). XPD simulations of the Ti 2p and O 1s 2π XPD plots (not discussed here) definitely confirm that the rutile model has to be discarded. At the time we first reported the preliminary data on the titania NSs on Pt(110),28 we observed that both the so-called rect-VO2 phase on Pd (111)51 and the rect-TiO2 phase on Pt(111)16,27 are characterized by a rectangular unit cell whose dimensions match almost quantitatively those observed in the present work. Subsequently, further titania overlayers grown on a different substrate (i.e., Ni(110))52 were interpreted adopting a similar model. We noted that the structure of the mentioned rect phases is analogous to that of the so-called lepidocrocite NSs, which are generally obtained by exfoliation of layered 3D titanates through soft-chemical methods.22,24-26 All techniques so far adopted consistently yield the picture of a 2D crystal which basically is a double layer of octahedrally coordinated Ti cations arranged periodically parallel to the sheets, with a rectangular unit cell of 0.38 × 0.30 nm (see Figure 11, right). Density functional theory (DFT) calculations on the single lamellae confirm the experimentally derived structure.53 If we assume the lepidocrocite single NS to explain our experimental observations, all of them fall into place to give a consistent picture. The STM rectangular unit cell of the overlayer has almost the same lattice parameters of the lepidocrocite NS, with a ∼3% expansion (from 0.38 to 0.39 nm) of the longer side to match the Pt(110) lattice vector along its highly corrugated [001] direction. The shorter side length of the rectangle is unaltered from its bulklike value, in accordance with the atomic smoothness of the Pt ridges along the [11j0] azimuth. This particular length (0.30 nm) also explains the ×14 coincidence along the same direction detected by LEED and STM.



Figure 10. Comparison between selected azimuthal and polar SSC-SW (full line) and experimental (marked line) XPD scans for O 1s emission from a single lepidocrocite NS on (1×2)-Pt(110). In the upper panel experimental and theoretical curves are displayed with their native anisotropy, while in the lower one both sets are normalized to the same intensity.

Figure 11. Transformation of a TiO2(001) anatase bilayer (left) to of a lepidocrocite titania NS (right) as emerging from our DFT calculations (see text) (oxygen, dark gray/red; titanium, light gray/blue). Relevant DFT-optimized geometrical parameters are reported (pm).


Agnoli et al.

Figure 12. A [001] view of the DFT equilibrium structure of the 14×1 slab model of the lepidocrocite titania NS. The coincidence/anticoincidence regions between the bottom bridging oxygens and the surface Pt atoms are marked by “C” and “A”, respectively.

To check this model against our XPD data, XPD simulations based on the lepidocrocite NS model, optimized through DFT calculations (see below), were done: the excellent fit is put into evidence by the azimuthal and polar cuts of the Ti 2p and O 1s 2π XPD plots displayed in Figures 9 and 10, though a notable difference should be evidenced between the Ti 2p and the O 1s curves. We remind the reader that the XPD simulations have been run at a single scattering (SS) level, which is an adequate description of electron scattering at high photoelectron kinetic energies and for an overlayer whose thickness is limited to a couple of Ti-O bilayers, thus giving rise to FS paths comprising at most one atom apart from the photoelectron emitter (except for along-surface FS events, which are not included in our polar angle range). This amounts to saying that, if the overlayer has a low concentration of structural defects, at least on the shortrange probed by XPD, SS calculations should reproduce fairly well not just the qualitative appearance of the intensity oscillations, but also the quantitative anisotropy χ of the XPD oscillations, defined as

χ)

Imax - Imin Imax

(1)

with Imax (Imin) being the maximum (minimum) intensity in each curve. This is indeed observed in the case of Ti 2p emission (Figure 9), where XPD modulations are reproduced down to the fine structure features, and where the anisotropy of experimental and theoretical curves is comparable (especially in the case of azimuthal cuts, whose constant background makes the comparison easier than for angle-dependent-background polar cuts). The situation is different for O 1s emission (Figure 10), where the simulations, though fitting very well the experimental data, have a sensibly higher anisotropy. For this reason, the comparison between experiment and theory is repeated in the same Figure 10 (bottom) after a renormalization of the curves in the same intensity range. The lower experimental anisotropy can be explained by the presence of a fraction of O atoms which contribute to the overall intensity of the XPS O 1s peak while producing small or no diffraction modulations. Such atoms could either form an ordered phase terminating the surface, i.e., with no scatterers above their plane able to produce relevant FS events, or a randomly scattered set of defect-related O emitters. It is actually possible that both phenomena contribute to the anisotropy reduction observed for O 1s XPD: residual areas of oxygencovered Pt (characterized by the 11×2 superstructure, hardly discernible by both LEED and STM among the 14×4 oxide islands), can coexist with hydroxyl defect on the overlayer surface, which we tentatively identify with the bright Y-shaped

features evident in the high-resolution STM image reported in the inset of Figure 8, and whose XPS fingerprint could be the high BE component partly responsible of the asymmetrical shape of the O 1s XPS peaks reported in Figure 3. Let us now enter into more details of the proposed model by examining some interesting DFT-derived data. Whereas the lepidocrocite titania phase was well described in the literature, in our preliminary report28 we suggested for the first time that this structure is closely related with anatase. In fact, a selfstanding TiO2(001) anatase bilayer appears to be unstable with respect to a transformation to a lepidocrocite NS (see Figure 11):28,54,55 the (barrierless) interconversion may be described as a reciprocal slipping of the TiO2 monolayers in the direction of the surface Ti-O-Ti chains, accompanied by a deformation of the (theoretical) unit cell from 0.379 × 0.379 nm to 0.303 × 0.373 nm, and by a 0.16 eV/cell energy gain. The local coordination of the cations changes from a deformed squarepyramidal shape to an octahedral one, whereas the bridging oxygens maintain their 2-fold coordination, even though the Ti-O-Ti angles are drastically reduced from 143.4° to 111.6°. Concerning the interaction between the Pt(110) surface and the titania NS, we have first studied it with a 14×1 supercell model. In the starting configuration, 13 unit cells of lepidocrocite-TiO2 were placed on the support, adjusting the lattice constant in order to fit the dimensions of the overlayer to those of the substrate. The coordinates of the NS and of the upper part of the substrate were then left free to relax. In the equilibrium structure, a notable structural change was found where the bottom layer O atoms and the topmost layer Pt atoms are not in coincidence (see Figure 12). In fact, moving form the coincidence (C, defined as x ) 0 and 1 in a fractional coordinate framework) to the anticoincidence (A, defined as x ) 0.5) site, the distances between the NN bottom bridging oxygens along the substrate [11j0] direction are (in nm) 0.295, 0.293, 0.291, 0.291, 0.290, 0.318, and 0.366, to be compared with the 0.303 nm value of the unsupported NS. Thus, the coordination of the bottom layer Ti cations remains practically unperturbed as in the unsupported NS, except at the A site, where Ti switches from an octahedral to an almost tetrahedral configuration. Also note that, even if the interfacial NS oxygens essentially relax along [11j0], i.e., the ridge direction, the main effect at the surface appears as the formation of a [001]-oriented ditch whose depth, measured as the difference in height between the vacuumfacing bridging oxygens at the C and at the A sites, is 0.059 nm, in excellent agreement with the STM data. A comparison of the Tersoff-Hamann simulation of the STM image (obtained for a +1.0 V bias) with the experiment is reported in Figure 13. The dark stripes observed experimentally (see Figure 8) are clearly associated to the periodic anticoin-



Figure 15. DFT-computed DOS of the 14×1 slab model of the lepidocrocite titania NS: the different atomic contributions (i.e., Pt, Ti, O) are also reported (energy in eV).

Figure 13. Left panel: top view of the 14×1 slab model of the lepidocrocite titania NS. Central panel: Tersoff-Hamann simulation of the STM image. Right panel: experimental STM image (V ) +1 V).

Figure 14. Close-up of the region around the anticoincidence site of the 14×1 slab model of the lepidocrocite titania NS, showing a map of the difference electron density. The displayed (001) cut plane passes through the first layer Pt atoms. Note the large electron density buildup between the A-Pt atom and the upstanding Ti cation.

cidence points, implying the bending of the NS toward the substrate, resulting in the physical corrugation of ca. 0. 06 nm. It is now interesting to try to understand what is the driving force for the deformation of the lepidocrocite NS. It is difficult to explain the large difference in the along-ridge relaxation of the interface bridging oxygen atoms if we interpret the interaction on the basis of the sole formation of Pt-O bonds. An explanation to this finding can be obtained by analyzing the difference electron density (∆F) map reported in Figure 14. This was obtained by subtracting from the electron density of the

slab model the electron densities of the separated support and NS moieties, each kept in the equilibrium structure of the complete interface system. Interestingly, whereas the charge rearrangement in the Pt-O regions is rather weak, a much larger change appears in the region between the A-Pt atom and the upstanding Ti ion. The extent and the shape of the charge buildup suggest the formation of a dative bond between Ti and Pt.56 This could be perhaps interpreted as a mechanism through which the overlayer gets rid of the electron density otherwise placed in the high-energy NS conduction band states by the O-Pt interaction. In Figure 15 we report the total DOS of the 14×1 DFT model. In the same figure the partial DOS of the different atomic types (i.e., Pt, Ti, O) is also outlined. Note that the Fermi level is quite close to the bottom of the NS conduction band. Though interesting, this result should not, however, be emphasized, due to the well-known inaccuracy of DFT in predicting the energies of the unoccupied states. Let us now examine how the reported theoretical DOS fit with the VB photoemission data reported in Figure 4. In order to compare the experimental He I excited VB spectra of Figure 4 with the electronic structure of the theoretical model, we have computed a weighted density of states (WDOS) which takes into account the experimental cross section at the used photon energy (Figure 16). The WDOS curves were computed by applying a Gaussian broadening of 0.27 eV to the monoelectronic levels, after weighting them by the photoionization cross sections obtained from tabulated values.57 The intensity of the Pt states has been reduced in order to take into account the attenuation due to the overlayer. First of all, we notice that a general good agreement is obtained between the computed WDOS and the experimental data. As already observed in a previous application of such method to DFT-computed DOS,27 both the relative intensity of the different spectral features and the general spacing between them is rather well reproduced, even if some shrinking of the theoretical overall bandwidth with respect to the experimental one is generally observed.58 The WDOS data are characterized by a region between 0 and 3 eV, which is almost exclusively due to the Pt states, followed by an abrupt intensity raise starting at 3-4 eV and producing an envelope of broad and intense peaks which extend up to 8 eV. This region is mostly due to the TiO2 O 2p levels, with some minor contribution from Pt and Ti states. In Figure 16 we have also attempted a one-byone assignment between the experimental spectral features and the WDOS, mainly based on the empirical observation discussed above.58


Figure 16. Comparison between the DFT-computed WDOS (see text) of the 14×1 model of the lepidocrocite titania NS and the He I excited VB data of the 2 MLeq TiO2 ultrathin films on (1×2)Pt(110).

So far we have modeled the whole substrate-NS system with a simple 14×1 slab. However, the true experimental structure displays a 14×4 reconstruction. The occurrence of ×4 reconstructions is well-known in the case of clean Pt(110) surfaces59 and has been explained by a weak pairing of the Pt ridges of the (1×2) reconstruction. A 14×4 model slab of the Pt(110) surface fully covered by a lepidocrocite overlayer is a very large system to be studied theoretically. To reduce as much as possible the computational costs, we removed one layer of Pt atoms, replaced the 12-electron Ti pseudopotential by a 4-electron one and performed the cell optimization by means of the CP algorithm:24 though in general slower than conventional quasiNewton techniques, it has the considerable advantage of considering only the occupied one-electron states to evaluate the atomic forces. This is obviously very convenient when dealing with a large number of atoms. We started our CP calculations from a 14×2 model slab. We find that introducing the 1×2 reconstruction of the Pt(110) surface has a significant effect on the distances between the substrate-facing O ions. The computed values are, for the oxygen atoms between (upon) the Pt ridges, and moving from the C to the A region (in nm): 0.295 (0.294), 0.298 (0.293), 0.298 (0.290), 0.301 (0.293), 0.307 (0.309), 0.321 (0.360). Thus, the configuration of the pseudotetrahedral Ti ions not above the ridges is back-transformed to the octahedral one of the unsupported NS. In spite of this, the coordination of the Ti cations upstanding the ridge is not substantially changed from that computed in the 14×1 model slab. Furthermore, in agreement with experimental STM images, negligible differences occur in the vacuum-facing part of the layer with respect to the 14×1 model slab. We turn now to examine the results of the calculations on the full 14×4 model slab. The starting configuration for these calculations was obtained by doubling the 14×2 slab. The outcome of the CP relaxation is quite interesting: whereas the expected pairing of the Pt ridges is rather weak, a much stronger effect (larger than 0.01 nm) is observed within the NS (see Figure 17), in particular when we consider the Ti cations upstanding the Pt ridges.

Agnoli et al.

Figure 17. Fractional distances along [001] from the 14×4 slab model of the lepidocrocite titania NS, defined as the difference ∆y between the [001] fractional coordinates. The unit length for computing fractional coordinates along [001] is 4 × b ) 4 × 0.392 nm ) 1.568 nm. Hence, y ) 0.5 (y ) 1) corresponds to the distance between two NN (NNN) Pt ridges in the unperturbed 1×2 substrate reconstruction. Circles represent Pt-Pt distances between the two ridges computed at equivalent Pt sites, squares distances between the (approximately) upstanding Ti ions (viz., the cations at the NS substrate-facing side). The coincidence site C along [11j0] is at x ) 0 and 1, the anticoincidence site A at x ) 0.5. Deviations from 0.5 in the ordinate indicate the occurrence of a 14×4 reconstruction, i.e., a pairing effect.

We emphasize that these are extremely demanding calculations. Even with the adopted simplifications, we were forced to stop the run when the atomic forces were lower than 10-2 au. However, when we monitored the structure variations occurring over 800 CP steps, we noted that the trend was toward an increase of the pairing effects, in particular at the A site (x ) 0.5 in Figure 17). Thus, we are confident that our CP results, though poorly converged, give a fair evidence that the ×4 reconstruction is most likely due to a pairing mainly occurring within the overlayer, and not at the substrate surface. It is not easy to explain this effect, which could perhaps be triggered by the weak charge transfer from the substrate. 5. Conclusions A detailed structural, morphological, and electronic investigation of titania NSs epitaxially matched to the (1×2)-Pt(110) surface was carried out by combining several surface-sensitive techniques and DFT calculations. Photoemission data are in accordance with a TiO2 stoichiometry of the overlayer, while LEED experiments reveal a 14×4 coincidence superstructure. This is confirmed by STM, which also gives information on the overlayer growth mode trough the formation of two-atomic layers thick oxide islands elongated along the substrate [11j0] direction, which enlarge laterally and finally coalesce as a function of coverage. XPD 2π plots, complemented by STM and LEED information, have revealed that the structure of the NSs is lepidocrocite-like. The relationship between such structural habit of the NS and the structure of an anatase (001) bilayer has been evidenced and explained on the basis of energyminimization arguments for a 2D spatially confined nanostructure through DTF calculations. A considerable computational effort has also provided a detailed description of the fine-grained interfacial atomic displacements which give rise to the 14×4 coincidence superstructure arising from the overlayer-substrate epitaxial matching, as observed in LEED and STM measurements. Acknowledgment. This work has been funded by the European Community through the STRP project NanoChemSens within the Sixth Framework Programme (contract no. STRP 505895-1), by the Italian Ministry of Instruction, University and

Ultrathin TiO2 Films on (1×2)-Pt(110) Research (MIUR) through the fund PRIN-2005, project title “Novel electronic and chemical properties of metal oxides by doping and nanostructuring”, and by the University of Padova, through the grants CPDA038285 and CPDA071781. We acknowledge a computer time grant from CINECA (Bologna, Italy) and INSTM (Firenze, Italy) under the “Super-Progetti di calcolo” program. All the DFT calculations were done with PWSCF, a code included in the Quantum-ESPRESSO package.60 Graphics were generated by XcrySDen.61 References and Notes (1) Chen, X.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (2) Bavykin, D. V.; Friedrich, J. M.; Walsh, F. C. AdV. Mater. 2006, 18, 28. (3) O Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (4) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weisso¨rtel, F.; Salbeck, J.; Spreitzer, H.; Gra¨tzel, M. Nature 1998, 395, 583. (5) He, J.-A.; Mosurkal, R.; Samuelson, L. A.; Li, L.; Kumar, J. Langmuir 2003, 19, 2169. (6) Mor, G. K.; Carvalho, M. A.; Varghese, O. K.; Pishko, M. V.; Grimes, C. A. J. Mater. Res. 2004, 19, 628. (7) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (8) Matsumoto, T.; Batzill, M.; Hsieh, S.; Koel, B. Surf. Sci. 2004, 572, 127. (9) Matsumoto, T.; Batzill, M.; Hsieh, S.; Koel, B. Surf. Sci. 2004, 572, 146. (10) Boffa, A. B.; Galloway, H. C.; Jakobs, P. W.; Benitez, J. J.; Batteas, J. D.; Salmeron, M.; Bell, A. T.; Somorjai, G. A. Surf. Sci. 1995, 326, 80. (11) Ma¨nnig, A.; Zhao, Z.; Rosenthal, D.; Christmann, K.; Hoster, H.; Rauscher, H.; Behm, R. J. Surf. Sci. 2005, 576, 29. (12) R Bennett, R. A.; McCavish, R. D. Top. Catal. 2005, 36, 11. (13) Ashworth, T. V.; Muryn, C. A.; Thornton, G. Nanotechnology 2005, 16, 3041. (14) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (15) Chen, M. S.; Wallace, W. T.; Kumar, D.; Zhen, Y.; Gath, K. K.; Cai, Y.; Kuroda, Y.; Goodman, D. W. Surf. Sci. 2005, 581, L115. (16) Sedona, F.; Rizzi, G. A.; Agnoli, S.; Llabre´s i Xamena, F. X.; Papageorgiou, A.; Ostermann, D.; Sambi, M.; Finetti, P.; Schierbaum, K.; Granozzi, G. J. Phys. Chem. B 2005, 109, 24411. (17) Sedona, F.; Agnoli, S.; Granozzi, G. J. Phys. Chem. B 2006, 110, 15359. (18) Finetti, P.; Sedona, F.; Rizzi, G. A.; Mick, U.; Sutara, F.; Svec, M.; Matolin, V.; Schierbaum, K.; Granozzi, G. J. Phys. Chem. C 2007, 111, 869. (19) Barcaro, G.; Sedona, F.; Fortunelli, A.; Granozzi, G. J. Phys. Chem. C 2007, 111, 6095. (20) Zhang, Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2000, 104, 3481. (21) R Ma, R.; Bando, Y.; Sasaki, T. J. Phys. Chem. B 2004, 108, 2115. (22) Sasaki, T.; Watanabe, M.; Hashizume, H.; Yamada, H.; Nakazawa, H. J. Am. Chem. Soc. 1996, 118, 8329. (23) Grey, I. E.; Li, C.; Madsen, I. C.; Watts, J. A. J. Solid State Chem. 1987, 66, 7. (24) Sasaki, T.; Ebina, Y.; Kitami, Y.; Watanabe, M.; Oikawa, T. J. Phys. Chem. B 2001, 105, 6116. (25) Sasaki, T.; Watanabe, M. J. Phys. Chem. B 1997, 101, 10159. (26) Fukuda, K.; Nakai, I.; Oishi, C.; Nomura, M.; Harada, M.; Ebina, Y.; Sasaki, T. J. Phys. Chem. B 2004, 108, 13088. (27) Zhang, Y.; Giordano, L.; Pacchioni, G.; Vittadini, A.; Sedona, F.; Finetti, P.; Granozzi, G. Surf. Sci. 2007, 601, 3488. (28) Orzali, T.; Casarin, M.; Granozzi, G.; Sambi, M.; Vittadini, A. Phys. ReV. Lett. 2006, 97, 156101. (29) Hanesch, P.; Bertel, E. Phys. ReV. Lett. 1997, 79, 1523. (30) Fadley, C. S. Prog. Surf. Sci. 1984, 16, 275.

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Ultrathin TiO2 Films on (1Ã2)-Pt(110): a LEED, Photoemission, STM

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Ultrathin TiO2 Films on (1Ã2)-Pt(110): a LEED, Photoemission, STM