J. Phys. Chem. C 2007, 111, 6095-6102
6095
Structure of a TiOx Zigzag-Like Monolayer on Pt(111) Giovanni Barcaro,† Francesco Sedona,‡ Alessandro Fortunelli,† and Gaetano Granozzi*,‡ Istituto per i Processi Chimico-Fisici (IPCF) del CNR, Via GioVanni Moruzzi 1, I-56124 Pisa, Italy, and Dipartimento di Scienze Chimiche and Unita` di Ricerca CNR-INFM, UniVersita` di PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy ReceiVed: January 31, 2007; In Final Form: March 5, 2007
The structure of a monolayer phase of TiOx on Pt(111) has been investigated by low-energy electron diffraction (LEED), atomic resolved scanning tunneling microscopy (STM), and density functional (DF) calculations. According to LEED, the rectangular unit cell (6.8 × 8.6 Å2) is incommensurate with respect to the Pt(111) substrate unit cell. The STM data show a clear zigzag-like motif and dimensions in perfect agreement with the LEED data. A structural model, which is in tune with the whole set of experimental data, has been obtained by a DF geometry optimization starting from a guessed structure proposed on the basis of chemical considerations and the comparison with literature data. The stoichiometry of the monolayer is Ti6O8 and the Ti atoms are formally in the +2.7 oxidation state, in agreement with previously reported photoemission data. However, two different types of Ti atoms have been found, that is, Ti atoms coordinated by four oxygen atoms, which give rise to the brighter bumps in the zigzag-like STM motif, and Ti atoms coordinated by only three oxygen atoms, which appear darker in the STM images. Analogously, two different types of oxygen atoms can be distinguished, with those lying in the throughs of the STM images (“bridge” oxygens) being less coordinated and in a lower oxidation state. The energetics of the interaction of the oxide monolayer with the Pt substrate has been computationally evaluated. Even if the oxide/metal interaction is important in determining the high stability and the structure of the oxide film, it conveys that this interaction is only weakly directional, thus, justifying the incommensurate nature of the film.
1. Introduction Titanium oxide (TiO2, usually referred as titania) unquestionably represents one of the most strategic material for advanced applications in many fields: in heterogeneous catalysis, as a photocatalyst, in solar cells for the production of hydrogen and energy, as gas-sensing, as a pigment in cosmetics and paints, as an optical coating, and in electric devices. Many of its properties are dictated by its peculiar semiconducting electronic structure and are typically surface-dependent. This is the reason why titania is one of the best characterized oxide systems in surface science.1 While the number of investigations on bulk surfaces is really remarkable, studies on ultrathin films are much less frequent. This fact is in contrast with the current interest toward titania based devices associated with size and shape on a nanometer scale (e.g., nanoparticles, nanosheets, and nanotubes) which obviously implies the possible existence and relevance of spatially confined phases (hereafter named nanophases), whose structure and properties could be significantly different from the ones pertinent to the standard bulk-like phases. In particular, understoichiometric and defective phases (TiOx, where x usually ranges from 1 to 2) could be stabilized either by the spatial confinement or by the interaction with the substrate, and they could actually control the properties of the titania based nanodevices. Structural and chemical information on TiOx nanophases can be conveniently derived by studying ultrathin films grown in * Corresponding author. E-mail:
[email protected]. † Istituto per i Processi Chimico-Fisici. ‡ Universita ` di Padova.
ultrahigh-vacuum (UHV) conditions on metal single crystals: in this context, the sophisticated structural tools provided by modern surface science can be very effective in obtaining the requested information. X-ray photoemission spectroscopy (XPS), Auger spectroscopy (AES), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM) have been used in literature to study TiOx ultrathin films grown on Pt(100),2,3 Pt(110),4 Pt(111),5 Ru(0001),6 W(100),7 Ni(110),8and Mo(112)9,10 surfaces. Recently, some of us have devoted much effort in the preparation and characterization of ultrathin films of TiOx on Pt(111).11-13 By a careful choice of the preparation conditions, several different nanophases were obtained having a rather different topography and degree of long-range ordered defectivity. In particular, it was shown that, by varying the growth parameters, different one monolayer (ML) thick understoichiometric TiOx films (x < 2) can be obtained, each having a distinct LEED pattern. Photoemission and photoelectron diffraction data demonstrated that the monolayers with x < 2 have a Pt-Ti interface and that the oxygen represent the topmost layer.11,13 Atomically resolved STM images have shown that they have a rather different and peculiar habitus, for example, either a wagon-wheel-like, a kagome´-like, or a zigzag-like contrast.11,12 In the present paper, we report a complete structural characterization of a well-defined incommensurate rectangular TiOx phase, having a zigzag-like structure, obtained by combined LEED, STM, and computational investigations. Further experimental evidence on the preparation of such a phase, labeled as z-TiOx in accordance with the nomenclature previously adopted,11 are herein reported which indicate the high stability of such a
10.1021/jp070820z CCC: $37.00 © 2007 American Chemical Society Published on Web 04/04/2007
6096 J. Phys. Chem. C, Vol. 111, No. 16, 2007
Barcaro et al.
TABLE 1: Results of DF Calculations on Intermediate and Large Cellsa metal layers
empty space
Ecutwfc
Ecutrho
kmesh
Eadh per Ti atom
Eatom of isolated oxide per basic cell
〈∆z(Pt-Ti)〉
〈∆z(Ti-O)〉
7.85 7.88 7.86 7.87 7.94
2.30 2.38 2.39 2.39 2.39
0.66 0.63 0.63 0.63 0.63
1 2 3 4 6
10 8 10 7.5 15
30 30 30 30 50
150 150 150 150 200
Intermediate cell (substrate strained) 241 3.00 241 2.45 241 2.57 241 2.44 481 2.50
2
8
30
150
221
Large cell (substrate strained) 2.68
7.87
2.32
0.64
221
Large cell (overlayer strained) 2.72
7.87
2.28
0.62
2
8
30
150
a
See section 3.2 for the definition. The number of metal layers, the empty space between replicated cells in Å, the energy cutoffs on the wave function (Eadhwfc) and the electronic density (Eadhrho) in Ryd, the number of points in the first Brillouin zone (kmesh), the adhesion energy (Eadh) per titanium atom in contact with the support in eV, the atomization energy (Eatom) of the isolated oxide films per basic cell in Ryd, the average value 〈∆z(Pt-Ti)〉 of the Pt-support/Ti-layer vertical distance in Å, and the average vertical separation 〈∆z(Ti-O)〉 between the Ti and O layers in Å are reported.
zigzag-like atomic arrangement. Similar atomic arrangements have been reported in literature for related systems, that is, TiOx/ Pd(111)/TiO214 and VOx/Pd(111),15 which suggests that the zigzag-like structure is of general relevance. However, the models reported in literature for these related systems do not fit with our experimental data of z-TiOx. Starting from the proposed models and through an accurate analysis of our experimental data, we were able to suggest a new model. In the present contribution, we show that this model is fully substantiated by computational simulations. 2. Experimental and Theoretical Methodology 2.1. Experimental Procedure. The preparation and the structural characterization for the different TiOx films on Pt(111) have been reported in detail elsewhere.11 In the following, we will concentrate on a particular phase, which, because of the peculiar zigzag-like STM appearance, we call the z-TiOx phase. The initial Ti coverage of approximately 0.8 MLE (1 MLE corresponds to 1.5 × 1015 atoms cm-2, as determined with a quartz microbalance) is obtained at room temperature (RT) by Ti reactive deposition in the presence of oxygen (pO2 ) 10-4 Pa). The sample is then annealed at 823 K in a 10-5 partial pressure of oxygen in order to obtain a well-ordered z-TiOx phase, as checked by the LEED pattern. However, this phase can be obtained in a broad interval of temperature and annealing conditions. Actually, it is very stable and easy to obtain. As a consequence, z-TiOx is almost always present when, not adopting a specific strategy to obtain a single phase, the coexistence of different phases is found. The STM images were taken at RT in an Omicron VT-STM system operating at a base pressure of 5 × 10-9 Pa. The system is equipped with a four-grid LEED optics. Pt-Ir tips were used in all of the experiments. Tunneling voltages are given with respect to the sample. The tunneling parameters are reported in the corresponding captions of the reported STM images. The scanner was calibrated in the z direction with respect to the step edge of the clean Pt(111) surface. For the lateral calibration, a (2 × 1) reconstructed Pt(110) surface has been used. 2.2. Theoretical Method. Density functional (DF) calculations were performed using the PWscf (plane-wave selfconsistent field) computational code,16 employing ultrasoft pseudopotentials.17 A total of 6, 12, and 10 electrons are explicitly considered for O, Ti, and Pt atoms, respectively. The PW91 exchange-correlation functional,18 which is a gradientcorrected functional, is used. The titanium oxide film grown
on the Pt(111) support is described by means of a supercell approach. Simulated STM images are obtained using the Tersoff-Hamann approach,19 at a height corresponding to the average value of the Ti ions. The parameters of the calculations (empty space between replicated slabs, energy cutoffs for the selection of plane waves for the description of the wave function and of the electronic density, number of k points in the first Brillouin zone, and number of platinum (111) face centered cubic (fcc) layers) are chosen on the basis of the results presented in Table 1. The quantities reported in Table 1 are (i) the adhesion energy between the metal support and the oxide layer, Eadh, which is calculated as the difference between the total energy of the metal/oxide system and the sum of the energy of the isolated metal support and of the isolated oxide film, the latter frozen in the interacting geometry; (ii) the atomization energy of the isolated oxide film per basic unit cell; (iii) the average vertical equilibrium distance between the metal support and the layer formed by the titanium atoms; and (iv) the average vertical distance between the layer formed by the titanium atoms and by the oxygen atoms. The results on the intermediate cell (see below, section 3.2 for its definition) obtained using 15 Å of empty space between replicated cells, 50 Ryd as cutoff on the wave function, 200 Ryd as cutoff on the electronic density, 6 layers of Pt to describe the metal support, and a (4,8,1) kmesh can be considered as a benchmark to validate calculations employing less accurate numerical parameters. From an inspection of Table 1, it can be concluded that a value of 30 Ryd for the cutoff on the wave function, a value of 150 Ryd for the cutoff on the electronic density, about 8-10 Å of empty space between replicated cells, 2 layers of Pt to describe the support, and a (2,4,1) kmesh is sufficiently accurate. Because of the doubling of the unit cell, this also implies that a (2,2,1) kmesh is sufficient to obtain the same level of accuracy for the large oblique cell (see below, section 3.2 for its definition). 3. Results and Discussion 3.1. Preparation and LEED and STM Data. The LEED pattern of the z-TiOx phase obtained with the optimized procedure described in Experimental and Theoretical Methodology is reported in Figure 1a: it has been assigned to an incommensurate superstructure with a rectangular unit cell of (6.8 ( 0.1) × (8.6 ( 0.1) Å2 as sketched by the arrows in Figure 0.0 2.11 The corresponding cell matrix notation is [2.5 1.8 3.6].
Structure of a TiOx Monolayer on Pt(111)
J. Phys. Chem. C, Vol. 111, No. 16, 2007 6097
Figure 1. (a) Experimental LEED pattern of the z-TiOx phase obtained by the optimized recipe discussed in ref 11; (b) experimental LEED pattern of z-TiOx phase obtained by high temperature annealing of the w-TiOx phase: an azimuthal spot splitting due to the presence of two domains rotated by 15° with respect to each other is present; (c) experimental LEED pattern of z-TiOx obtained on a substrate having a large number of step edges, where only one domain is visible, as evidenced by the rectangle; and (d) high-resolution constant-current STM images of the z-TiOx phase at low positive bias (60 × 60 Å2; V ) 0.1 V, I ) 1.5 nA) and (e) at negative bias (170 × 170 Å2, V ) -0.3 V, I ) 1.0 nA).
Figure 2. Real space schematic drawing of the lattice of the z-TiOx phase: the arrows indicate the rectangular unit cell which is incommensurate to the Pt(111) lattice. Two bigger cells, intermediate and large, are also shown which have been adopted in the theoretical calculations (see text).
The LEED pattern reported in Figure 1b is obtained when the z-TiOx phase is prepared in a different way with respect to the recipe already reported: in this case, the film was obtained from the higher coverage (1.2 MLE) w-TiOx phase11 by a severe annealing at high-temperature (900 K). As a consequence of the temperature-induced Ti diffusion into the substrate, a lower coverage phase is obtained from a higher coverage one. Since the w-TiOx phase presents two different domains rotated by 15° with respect to each other,11,12 the z-TiOx phase obtained along this way also presents the two domains rotated by 15° (see Figure 1b). Indeed, z-TiOx is an incommensurate phase, and this presumably implies a weak interaction between the film and the substrate: for this reason, the orientation of the phase is most probably induced not only by the symmetry of the substrate but also by kinetic factors. This is also in tune with
further experimental LEED data on the z-TiOx phase. In Figure 1c, we report an LEED pattern of the z-TiOx phase where it is well evident that only one of the three possible domains is present. This type of LEED is observed when the growth is carried out under nonideal conditions of the substrate (i.e., a sample with small terraces and a large number of step edges): most probably, this is to be associated with directional growth along the step edges. A similar not hexagonal LEED pattern was reported by Boffa et al.5 and tentatively assigned to a phase with a Ti4O7 stoichiometry. As already reported,11 STM images demonstrate that the z-TiOx phase forms a flat and continuous film which completely wets the Pt(111) substrate. Therefore, the apparent height of the z-TiOx phase with respect to the Pt(111) surface could not be determined. An atomically resolved STM image obtained at a low positive bias is reported in Figure 1d: it clearly reveals straight dark troughs with a periodicity of 8.6 Å perpendicular to their direction, a zigzag-like motif, and a rectangular unit cell whose dimensions perfectly match the LEED values. The LEED data confirm that the troughs are aligned along the 〈11h0〉 substrate directions as sketched in Figure 2 and reported in Figure 1d. The STM image obtained with a negative bias is reported in Figure 1e: even though the atomic resolution is much worse with respect to the positive bias one, it is clearly discernible that the dark troughs do not show a significant bias dependence, whereas the blurred bright spots are presumably to be associated to states prevalently localized on oxygen atoms (see section 3.2.1 below). As already discussed in our original report,11 a striking dimensional similarity exists between our z-TiOx phase and two different zigzag-like phases observed when a TiOx overlayer is formed by a thermally activated encapsulation of Pd(111) islands either on the TiO2(110) rutile surface14 or in VOx ultrathin film on Pd(111).15 In order to propose a model for our z-TiOx phase, we have thoroughly analyzed the structure derived on the basis
6098 J. Phys. Chem. C, Vol. 111, No. 16, 2007
Barcaro et al.
Figure 3. Front view of the proposed zigzag-like model for z-TiOx/ Pt(111) system. This structure has been derived from the VOx/Pd(111) model (ref 15), by eliminating the titanyl oxygen atoms in the top most layer and reversing the stacking to O/Ti/Pt (see text). Ti atoms in light blue, O atoms in red.
of DF calculations for the zigzag-like VOx/Pd(111) phase.15 This structure has been assigned to an OsVsO/Pd stacking: a layer of oxygen atoms at the interface with the substrate and, in the topmost layer, vanadyl type oxygens (VdO) forming the zigzaglike motif. The presence of vanadyl oxygen species was confirmed by the HREELS spectra and by the fact that the tipinduced loss of surface vanadyl oxygen destroys the order in the surface layer when scanning repeatedly over the same area. The stoichiometry associated with this model is V6O14; that is, the vanadium atoms are formally in the +4.7 oxidation state. However, even if the identical dimensions of the two structures would suggest to directly translate this model to our system, a series of evidence suggests that this is not actually the case. The first is associated with the analysis of the photoemission11,13 and photoelectron diffraction11 data, which strongly indicates that the correct stacking sequence for all of the reduced TiOx phases is PtsTisO, that is, a double layer with Ti at the interface with the Pt. Moreover, chemical intuition is adverse to the hypothesis of the existence of both a Ti oxidation state of +4.7 (perfectly compatible for V) and a group of titanyl TidO, which are very rarely observed in the chemical literature (in contrast with several examples of vanadyl oxygens). For this reason, we have been looking for a new model for the z-TiOx phase starting from the V6O14 structure: simply excluding the presence of titanyl oxygen atoms and reversing the stacking, we end up with the model reported in Figure 3. The stoichiometry becomes Ti6O8, and the Ti atoms are formally in the +2.7 oxidation state, in agreement with our photoemission results.11,13 Moreover, the presence of Ti at the interface with the Pt substrate is also consistent with the higher Pt-Ti versus Pt-O interface stability predicted by Jennison et al.20 In Figure 3, it is also shown how the Ti atoms could give rise to the zigzag-like motif: according to this model, we would have Ti atoms coordinated with four oxygen atoms (labeled as Ti4) and Ti atoms coordinated with only three oxygen atoms (labeled as Ti3). The relative abundance between the two types of Ti atoms, expressed as n(Ti4)/n(Ti3), would be equal to 2. Moreover, with this model, the black troughs would be formed by bridging oxygens that connect two Ti4 and Ti3 atoms. This structural hypothesis, based on qualitative considerations, has been quantitatively tested by using DF computations which are reported in the following subsection. In these calculations, we started from a proposed configuration corresponding to the
Figure 4. Relaxed structure (a) of the intermediate cell (right-hand side) and the corresponding simulated STM image at +1.00 V (lefthand side): the STM image is obtained by using four Pt layers to describe the metal support; the brighter four-coordinated Ti atoms (denoted as Ti4 in the figure) draw a zigzag-like motif (underlined) along the 〈11h0〉 direction of the support, whereas the three-coordinated Ti atoms (denoted as Ti3 in the figure) are hardly visible. Relaxed structure (b) of the large oblique cell (right-hand side) and the corresponding simulated STM image at +1.00 V (left-hand side): the image is obtained by using two Pt layers to describe the metal support (note the qualitative similarity with the image reported in (a) but the weaker intensity due to the limited number of Pt layers); as in the case of the intermediate cell, the brighter Ti atoms draw a zigzag-like motif along the 〈11h0〉 direction of the support. Only the outermost Pt layer is reported in the figure for clarity. Ti atoms in light blue, O atoms in red, and Pt atoms in gray.
one shown in Figure 3, and we locally minimized the energy, always finding stable local minima shown in Figure 4. 3.2. Computational Results. When incommensurate oxideon-metal systems (such as the present one) are described via computational simulations by means of a supercell approach, one has to face the problem of matching the unit cell of the oxide phase with that of the metal support. This is usually enforced20,24 by considering larger unit cells for both systems and by stretching the unit cell of one of the two systems to properly match the other one. Usually, the metal substrate is strained, as it is more important to have a better description of the oxide, and in the following, unless otherwise specified, it is intended that the platinum support has been strained to match the experimental values of the oxide lattice. In our case, the mismatch of the metal support with the minimal oxide unit cell is rather large (see Figure 2): an expansion by approximately 18% (with respect to the overlayer unit cell) along the 〈11h0〉 direction and a compression along the orthogonal direction by approximately 11% are needed to bring the Pt into registry with the (6.8 × 8.6) Å minimal oxide unit cell. This mismatch can be reduced by doubling the cell in the 〈11h0〉 direction, thus
Structure of a TiOx Monolayer on Pt(111) considering a [ 52 04] cell, which only needs a compression by approximately 2% in the 〈11h0〉 direction to be in register with the oxide, but the compression by 11% in the orthogonal direction would still remain. This cell will be indicated as intermediate as a short notation in the following (see Figure 2). If one takes a larger oblique [ 56 07] cell (see Figure 2), the Pt mesh now requires a very small deformation (ca. 2% in both directions, one in compression and one in expansion) to be in registry with the oxide phase. This cell will be called large as a short notation in the following. We thus chose to perform calculations on two systems: (a) the intermediate rectangular cell shown in Figures 2 and 4a, used for test calculations because of its reduced computational effort; and (b) the large oblique cell, shown in Figures 2 and 4b, computationally more demanding but with an optimal description of the metal substrate. The intermediate cell measures 13.6 × 8.6 Å and contains 20 Pt atoms per metallic layer and 28 atoms in the oxide film (12 titanium atoms + 16 oxygen atoms). The large unit cell measures 13.6 × 18.5 Å, θ ) 68.43°, and contains 35 Pt atoms per metallic layer and 56 atoms in the oxide film (24 titanium atoms + 32 oxygen atoms). Because of the presence of a plane of symmetry in the minimal unit cell, only five titanium and six oxygen atoms are nonequivalent: this symmetry is present and has been exploited in the intermediate cell but is absent in the large cell. 3.2.1. Calculations on the Intermediate Cell. On the basis of the results reported in Table 1, it can be seen that the adhesion energy presents an oscillating behavior as a function of the number of Pt layers, whereas the geometrical parameters characterizing the oxide film are much less sensitive to the accuracy used to describe the metal support. Nevertheless, by using only two Pt layers, we can obtain a rather accurate description of the system, from both a structural and an energetic point of view. On the contrary, care has to be used in the simulation of the STM images: in fact, the density of empty states close to the Fermi level is not accurately reproduced when using a reduced number of metallic layers. As a consequence, to obtain realistic STM images at low biases to be compared with the experimental ones, one has to improve the description of the metal support by increasing the number of Pt layers in the unit cell. In Figure 4a, we show a simulated STM image obtained by employing an intermediate unit cell in which the metal support is formed by four Pt layers and the applied bias is +1.00 V. Calculations with up to eight Pt layers have been performed (not reported), showing that the simulated STM images with four Pt layers are essentially converged with respect to the number of substrate layers. The image obtained at a voltage of +0.1 V (not shown) is characterized by the same features, but the intensity of the signal is much weaker, probably because of an underestimation of the density of low-energy empty states at the DF level. First of all, it is interesting to observe that the most intense Ti spots show a fine structure which identifies them as due to empty states with an unequivocal dxy orbital character. Naturally, fluctuations of various origins make these fine features blurred out in the experimental STM images. The brightest spots are due to the Ti4-family titanium atoms of the basic unit cell: by replicating the unit cell in the (x,y) plane, we see that the sequence of the brightest titanium atoms forms a zigzag-like motif oriented along the 〈11h0〉 direction of the underlying Pt support. The other two (Ti3-family) titanium atoms give rise to a weaker STM signal. These results are in perfect agreement with the experimental STM images of this phase obtained at high resolution and fully confirm the proposed
J. Phys. Chem. C, Vol. 111, No. 16, 2007 6099 model. STM images at negative bias have also been simulated. The contrast is rather faint, so that they are not reproduced here, as they would be of little help in interpreting the barely resolved experimental images of Figure 1e. However, they strongly suggest that only oxygen atoms are imaged at negative biases and that the oxygen atoms lying in the throughs are not visible. 3.2.2. Calculations on the Large Cell. For this cell, the reduction of the metal strain improves the interaction between the support and the oxide film: from the data reported in Table 1, we observe an increase of the adhesion energy and a corresponding reduction of the average distance between the metal surface and the Ti layer. We have also performed calculations on a cell in which the metal support is not strained, whereas the oxide film is strained: in this case, the total energy of the system is lowered by only 0.19 eV per basic cell (note that the energy of the Pt support also gives a small contribution to the energy balance), confirming that straining either the support or the oxide film gives very similar results, with the adhesion energy only slightly enhanced and the average Pt-Ti distance correspondingly reduced. In Figure 4b, we show the STM simulated image for the large cell at an applied bias of +1.00 V: this image is qualitatively similar to the one in Figure 4a, with the titanium atoms of the Ti4 family forming the zigzaglike motif along the 〈11h0〉 direction of the support; however, the intensity of the signal is reduced with respect to Figure 4a because here the metal support is described by only two Pt layers, and thus the density of empty states close to the Fermi level is smaller. To rationalize these STM results, we need to understand the difference between the two groups of Ti atoms: on the one hand, the titanium atoms from the Ti3 family that give weak signals, and, on the other hand, the titanium atoms from the Ti4 family that give brighter signals. By inspecting the equilibrium height of the Ti atoms above the metal support (2.332.35 Å for the Ti3 family; 2.34-2.43 Å for the Ti4 family), we can rule out that the brightness of the STM features is due to a topographic feature. The only possible explanation is thus an electronic effect. In fact, by inspecting the structure of the oxide film reported in Figures 2 or 4, it can be noted that the Ti3 atoms are more isolated than the Ti4 atoms: each Ti atom belonging to the zigzag-like motif has two Ti neighbors at about 2.90-2.95 Å and is surrounded by four oxygen atoms, whereas the minimum distance between the Ti3 atoms and atoms of the zigzag-like motif is 3.3 Å, and they are surrounded by only three oxygen atoms. A higher oxygen coordination can be correlated to a higher oxidation state, that is, a higher positive charge, and thus to a higher density of low-energy empty states above the Fermi level. This is the reason why the atoms of the zigzag-like motif give a brighter STM signal at low bias. To corroborate this hypothesis, we have performed an accurate analysis of the PDOS (projected density of states) of the metal/oxide system, focusing on the regions around the 3s level of the Ti atoms and around the 2s level of the O atoms: as a pseudopotential approach is used for the core levels, these are the lowest-energy orbitals that are explicitly included in the calculations. Let us start with the discussion concerning the Ti atoms. In the large cell, we have a total of 24 Ti: 8 atoms give a weak signal (Ti3 family), whereas the other 16 (Ti4 family) give a stronger signal. In Figure 5a, we show a decomposition of the DOS around -52 eV (which corresponds to the Ti 3s states) in terms of contributions due to the different Ti atoms: the function can be divided into two main peaks, one on the left-hand side at higher binding energy (BE) and one on the right-hand at lower BE. The peak on the right is generated by
6100 J. Phys. Chem. C, Vol. 111, No. 16, 2007
Barcaro et al.
Figure 6. DOS for the isolated Ti6O8 oxide film (in red), the isolated Pt(111) bilayer (in blue), and the full oxide-on-metal system (in black). The Fermi energies for the three systems are shown as straight vertical lines. Energies in eV, DOS in arbitrary units, smearing parameter equal to 0.15 eV. The DOS are shifted so as to align the vacuum level.
Figure 5. (a) DOS of the oxide/metal system around -52 eV: contribution arising from the Ti 3s orbitals. The low BE peak is generated by the group of Ti atoms belonging to the Ti3 family (in red); while the high BE peak is generated by the Ti atoms belonging to the Ti4 family (in blue). (b) DOS of the oxide/metal system around -13 eV: contribution arising from the O 2s orbitals; the contributions due to the bridge oxygen are also shown in blue. The smearing parameter is always set to 0.03 eV.
the group of Ti atoms belonging to the Ti3 family (red colored) and has an area about one-half of that of the peak on the left, generated by the Ti atoms belonging to the Ti4 family (blue colored): this is a clear indication that the atoms in the zigzaglike motif, those giving a strong STM signal, are characterized by a reduced electron density, which means a higher oxidation state, in agreement with the above discussion. Interesting information can also be derived from the analysis of the DOS around -13 eV, shown in Figure 5b, which roughly corresponds to the 2s states of the O atoms. In this figure, we have explicitly shown only the contributions due to the “bridge” oxygen 2s orbitals (blue colored), as the contribution from the other oxygens is spread among the higher BE peaks. It can be seen that the 2s orbitals of the bridge oxygens (i.e., the oxygens which link the Ti stripes and are low coordinated, being bound to only two first-neighbor Ti atoms) present a sharp 2s signal at lower BE. For these atoms, eight in the large cell, the 2s state is poorly hybridized and is destabilized at lower BE because of a larger negative charging, again in agreement with the above discussion. It is also instructive to compare the DOS of the separate Pt substrate and the Ti6O8 overlayer with that of the whole system (Figure 6). Note that the DOS are shifted so as to align the vacuum level; the initial values of the “raw” Fermi energies calculated on the same large cell are 1.90, -1.28, and 5.31 eV for the separate Pt substrate, the separate Ti6O8 overlayer, and
the combined system, respectively. Two main points can be drawn from such a comparison. First, the Fermi energy of the Ti6O8 overlayer, frozen in its interacting configuration, is -1.3 eV and thus lies within the conduction band of the oxide. Metallization of polar oxide surfaces is a common phenomenon21 which, however, is not particularly pronounced in this case and possibly is overestimated by our choice of a gradientcorrected DF xc-functional.18 Moreover, the oxide conduction band hybridizes with the metal conduction band once the oxide layer is deposited on to the Pt(111) surface. Second, despite the fact that the metal Fermi energy moves to higher values after oxide deposition (from 1.9 to 5.3 eV), due to a compensation between Fermi energy and asymptotic value of the electrostatic potential in the vacuum, the work function of the system does not undergo a similar shift upon deposition, changing only from 5.54 eV (isolated metal support) to 5.43 eV (combined oxide-on-metal system). We do not thus expect dramatic changes induced in the electronic structure of the metal surface upon deposition of the oxide films,22 which have been shown to occur and concur to be the causes of interesting electronic effects, such as charging of metal atoms absorbed on oxide-on-metal systems.23 This is consistent with the fact that Pt(111) is a “harder” metal surface than, for example, Ag(100). 3.2.3. Total Energy Considerations. Following the lines of previous literature,4,20,24 it is interesting to perform calculations on the pure unsupported oxide phase to better understand the factors governing the structure of the film. To this aim, as a first attempt, we optimized the structure of the z-TiOx film in the gas-phase, fixing the unit cell parameters to 6.8 × 8.6 Å. However, we found that the unsupported film undergoes a dramatic reconstruction into a final structure in which the Ti and O atoms intermix into a somewhat buckled single layer. The instability of the unsupported structure is due to its low density and to the tendency to reduce its dipole moment when not stabilized by the image charge at the metal surface. In these conditions, it is not possible to define “intrinsic” oxide lattice parameters. To overcome this problem, we performed a breathing of the oxide film and relaxed its structure by retaining the underlying metal support; then we froze the oxide film into the obtained structure and finally evaluated its energy via a singlepoint DF calculation. These calculations have been performed on the large cell, with its lattice parameters shrunk or expanded by 5%. The results are shown in Figure 7: it can be seen that
Structure of a TiOx Monolayer on Pt(111)
J. Phys. Chem. C, Vol. 111, No. 16, 2007 6101 4. Concluding Remarks
Figure 7. Relative energy (∆E, in eV) of the oxide film subjected to a shrinkage or an expansion of the lattice parameters of the large cell. The zero of the energy corresponds to the experimental lattice parameters.
the equilibrium structure of the oxide film grown on the metal support is close to the experimental one, as shrinking or enlarging the film corresponds to an energy destabilization. This fact, together with the analysis of the structure shown in Figure 4 (which shows that there is apparently no preferential absorption site for the Ti atom on the Pt(111) metal support, thus pointing to a rather weakly directional Ti-Pt interaction) rationalizes the recurrence of incommensurate phases for TiOx/ Pt(111) systems.11 This interpretation has been supported by a selected calculation on the large cell, where we shifted the oxide layer by 1 Å in both the 〈11h0〉 and the orthogonal directions; we froze the in-plane coordinates of one of the Ti atoms and locally relaxed the coordinates of all of the other atoms in the oxide film. The result is an oxide-on-metal system where the oxide layer is shifted with respect to the one shown in Figure 4b. The energy of such a configuration is essentially identical to the original one, thus further supporting the notion that the oxide/metal interaction is only weakly directional, though at the same time important to determine the oxide structure and give it its peculiar stability. The driving force of the zigzag-like motif results from an interplay of different factors. First of all, the Ti ions prefer to be at the interface with the Pt support, as shown by Jennison et al.20 Due to their weak directional interaction with the support, they tend to arrange in a close-packed fcc (111) layer, with an overlayer of oxygen ions in an approximately close-packed arrangement. However, at the low titanium dosing at which the z phase and the similar z′ phase11 are found experimentally, there is not enough titanium to form a complete (111) layer, and the Ti atoms prefer to arrange themselves into close-packed stripes separated either by bridging oxygen atoms (as in the present incommensurate z phase) or by void throughs (as probably occurs in the analogous commensurate z′ phase).25 The Ti atoms in such stripes are not equivalent in terms of both oxygen coordination and location with respect to the underlying Pt support, and this causes them to exhibit different STM intensities. However, kinetic factors could also play a role: trapping due to the initial formation of honeycomb structures typical of the k-TiOx phase at very low Ti dosing11 might induce the formation of close-packed stripes and thus contribute to the driving force of the zigzag-like motif (this point is under investigation in our laboratories).25
In this study, we have outlined that three different systems, namely, TiOx/Pt(111), TiOx/Pd(111)/TiO2, and VOx/Pd(111), although composed of different metal/oxide combinations, show very similar incommensurate rectangular unit cells with similar zigzag-like structures. In the first two cases, the similarity appears obvious: we have the same overlayer forming an incommensurate structure which is not very sensitive to the different substrates, Pt(111) versus Pd(111). On the other hand, the similarity between the zigzag-like TiOx/Pt(111) and VOx/ Pd(111) phases is more intriguing because both the substrate and the metal-oxide are different. We have demonstrated that the two systems, showing almost the same dimensions and the same STM habitus, can be explained by two very different models (namely, Ti6O8 vs V6O14): the V6O14 model proposed by Surnev et al.15 implies there are vanadyl oxygen atoms and oxygen at the interface with the substrate, but both of these two points are in contrast with the general knowledge on the titanium oxides systems and with our stacking evidence.11 The Ti6O8 model proposed in this study unravels this apparently contrasting situation: we have demonstrated by DF calculations that the same geometric M6O8 backbone of the V(Ti) and O atoms holds in the two systems; however, in the case of Ti6O8, the Ti atoms are at the interface with the substrate, and no titanyl double bonded oxygen atoms are present. This certainly reflects the different chemistry of titanium versus vanadium and the different oxygen affinity of the substrate (Pd-O is greater than Pt-O). Finally, the incommensurate nature of such monolayers, together with the energy considerations reported in section 3.2.3, points to a strong stiffness of the discussed zigzag-like M6O8 framework, thus suggesting its possible use as a template for the growth of metal clusters. Acknowledgment. This work has been funded by European Community through two STRP projects (NanoChemSens, Nanostructures for Chemical Sensors, and GSOMEN, Growth and Supra-organization of Transition and Noble Metal Nanoclusters), by the Italian CNR through the project SSATMN within the framework of the ESF EUROCORES SONS, and by the Italian Ministry of Instruction, University and Research (MIUR) through the fund “Programs of national relevance” (PRIN-2003, PRIN-2005). Calculations have been performed at Cineca within an agreement with Italian INSTM. References and Notes (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53 (2) Matsumoto, T.; Batzill, M.; Hsieh, S.; Koel, B. Surf. Sci. 2004, 572, 127. (3) Matsumoto, T.; Batzill, M.; Hsieh, S.; Koel, B. Surf. Sci. 2004, 572, 146. (4) Orzali, T.; Casarin, M.; Granozzi, G.; Sambi, M.; Vittadini, A. Phys. Rev. Lett. 2006, 97, 156101. (5) 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. (6) Ma¨nnig, A.; Zhao, Z.; Rosenthal, D.; Christmann, K.; Hoster, H.; Rauscher, H.; Behm, R. J. Surf. Sci. 2005, 576, 29. (7) Bennett, R. A.; McCavish, R. D. Top. Catal. 2005, 36, 11. (8) Ashworth, T. V.; Muryn, C. A.; Thornton, G. Nanotechnology 2005, 16, 3041. (9) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (10) 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. (11) 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. (12) Sedona, F.; Agnoli, S.; Granozzi, G. J. Phys. Chem. B 2006, 110, 15359.
6102 J. Phys. Chem. C, Vol. 111, No. 16, 2007 (13) 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. (14) Bennett, R. A.; Pang, C. L.; Perkins, N.; Smith, R. D.; Morrall, P.; Kvon, R. I.; Bowker, M. J. Phys. Chem. B 2002, 106, 4688. (15) Surnev, S.; Sock, M.; Kresse, G.; Andersen, J. N.; Ramsey, M. G.; Netzer, F. P. J. Phys. Chem. B 2003, 107, 4777. (16) Baroni, S.; Dal Corso, A.; de Gironcoli, S.; Giannozzi, P. http:// www.pwscf.org. (17) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (18) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671.
Barcaro et al. (19) Tersoff, J.; Hamann, D. R. Phys. ReV. Lett. 1983, 50, 1998. (20) Jennison, D. R.; Dulub, O.; Hebenstreit, W.; Diebold, U. Surf. Sci. 2001, 492, L677. (21) Noguera, C. J. Phys.: Condens. Matter 2000, 12, R367. (22) Giordano, L.; Cinquini, F.; Pacchioni, G. Phys. ReV. B 2005, 73, 45414. (23) Pacchioni, G.; Giordano, L.; Baistrocchi, M. Phys. ReV. Lett., 2005, 94, 226104. (24) Surnev, S.; Ramsey, M. G.; Netzer, F. P. Prog. Surf. Sci. 2003, 73, 117. (25) Preliminary results of a work in progress.
