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J. Phys. Chem. C 2009, 113, 19578–19584
LEED and DFT Study of the Quasihexagonal TiO2 Structure on Cu(001) Andrea Atrei,*,† Anna Maria Ferrari,‡ Paola Finetti,§ Alessandra Beni,§,| and Gianfranco Rovida§ Dipartimento di Chimica, UniVersita` degli Studi di Siena, Via A. De Gasperi 2, 53100 Siena, Italy, Dipartimento di Chimica IFM, UniVersita` di Torino, Via P. Giuria 5, 10125 Torino, Italy, and Dipartimento di Chimica, UniVersita` di Firenze, Via della Lastruccia 3, 50129 Sesto Fiorentino (FI), Italy ReceiVed: July 15, 2009; ReVised Manuscript ReceiVed: September 18, 2009
Reactive deposition of Ti on the Cu(001) surface saturated with chemisorbed oxygen results in the formation of an ordered ultrathin film with a slightly distorted hexagonal (quasihexagonal) unit cell. We find that, under appropriate substrate temperature, evaporation rates, and O2 pressure, it is possible to cover the whole substrate surface with the TiO2 quasihexagonal phase. An O-Ti-O trilayer model for the (quasi) hexagonal structure is consistent with angle scanned XPD data relative to the Ti 2p signal and with the LEED I-V experimental intensity curves. From a careful inspection of the LEED pattern, we derive a distortion of the hexagonal unit cell corresponding to a p(2 × 7) coincidence mesh with the substrate. The p(2 × 7) coincidence was used to model XPD data. For the analysis of LEED-IV data and for DFT calculation, the p(2 × 7) was approximated by a c(2 × 6) coincidence mesh. The c(2 × 6) approximation is very close to a p(2 × 7) in terms of lattice parameters but contains a reduced number of nonequivalent atoms. From LEED-IV analysis and DFT calculations, we derive the registry between the c(2 × 6) titanium oxide mesh and the substrate. We find that interfacial O atoms located at the corners and at the center of the unit mesh sit on top of the Cu atoms at the interface. The comparison of the DFT calculations performed for the free-standing O-Ti-O trilayer with a regular hexagonal unit cell and for the trilayer on Cu with a c(2 × 6) coincidence mesh indicates that the strain energy is more than compensated by the interaction with the substrate. The values of the interlayer spacing and the registry of the oxide film derived from the DFT calculations are in good agreement with the experimental results. The DFT calculations indicate that the bonding at the interface is characterized by a direct Cu-O interaction with overlap between the O 2p and the Cu 4sp states. The calculations indicate that, because of an upshift of the O 2p bands and a downshift of the Ti 4s states, the TiO2 film loses its insulator character and the electron states close to the Fermi level are dominated by the contribution from the Ti 3d orbitals. 1. Introduction The epitaxial growth of ultrathin films of titanium oxides on single crystal metal surfaces is an interesting method to investigate the effect of low dimensionality and of the interaction with the substrate on the geometric and electronic structure of these oxides.1 It is well-known in fact that epitaxy can stabilize titanium oxide structures that are not observed in bulk compounds. From a general point of view, deposition of oxide thin films on a metal support greatly reduces charging effects that may interfere with experimental techniques aimed at the characterization of the oxide itself. The role of the metal interface on the reactivity of oxides is a subject currently under investigation2 because of its relevance in areas such as heterogeneous catalysis and metal corrosion inhibition. The reactivity of titanium oxides is tightly related to its geometric and, in turn, electronic structure (e.g., presence of defects or oxygen vacancies).3 Therefore, an accurate determination of the atomic structure of the titanium oxide thin film geometry, including * Corresponding author. E-mail:
[email protected]. Tel: +39 055 4573123. Fax: +39 055 4573121. † Universita` degli Studi di Siena. ‡ Universita` di Torino. § Universita` di Firenze. | Present address: Empa, Swiss Federal Laboratories for Materials Testing and Research, 8600 Du¨bendorf, Switzerland.
its registry with the substrate, is a key point to understand its potential properties. The study of the growth of titanium oxide on Cu(001) is motivated by several reasons. Copper is a relevant metal substrate in electronics. The growth of titanium oxide on metal substrates with d10 electronic configuration has recently attracted attention and studies were reported for Au(111)4 and Ag(100).5 The choice of the Cu(001) is particularly interesting from the point of view of the metal oxide interaction. The Cu(001) surface, in fact, interacts rather strongly with atomic oxygen, as demonstrated by previous studies concerning the O chemisorption process. The O-Cu(001) interaction results in a rearrangement of the substrate, consisting of a missing row reconstruction that gives rise to a (2 × 22)R45° surface structure.6-8 In a previous study it was found that titanium oxide films on the Cu(001) surface form an ordered phase with a slightly distorted hexagonal (“quasihexagonal”) structure.9 This titanium oxide phase, which can be prepared either by reactive deposition of Ti (i.e., Ti evaporation in O2 atmosphere) or by reaction of Ti with O chemisorbed on the Cu(001) surface, has a TiO2 stoichiometry.9 An O-Ti-O trilayer consisting of a (nearly) hexagonal titanium layer between two closed packed oxygen planes was proposed as the structural model for this oxide.10 This structure, which does not have any equivalent in the atomic arrangement of a crystallographic plane of the various stable
10.1021/jp9066923 CCC: $40.75 2009 American Chemical Society Published on Web 10/19/2009
Quasihexagonal TiO2 Structure on Cu(001) bulk phases of TiO23 seems to exist only as a two-dimensional phase. The compact O-Ti-O hexagonal trilayer structure was originally proposed by Atrei et al. for the growth of titania on the Ni96Ti4(110) alloy.11 Recently the same structure was proposed for the growth of TiO2 on pure Ni(110) surfaces.12,13 In this work we present the results of a combined experimental and theoretical study aiming at determining the crystallographic, and electronic structure as well as the energetics, and electronic structure of the quasihexagonal TiO2 phase on the Cu(001) surface. The experimental structural investigation was carried out by means of angle scanned X-ray photoelectron diffraction (XPD) and by low-energy electron diffraction (LEED) intensity analysis. The crystal structure, the stability, and the electronic structure of the TiO2 quasihexagonal layer were studied by density functional theory (DFT) calculations. 2. Materials and Methods 2.1. Experimental Details. The experiments were carried out in an ultrahigh vacuum (UHV) apparatus with a base pressure in the low 10-10 mbar range. The chamber was equipped with the facilities for X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), XPD, and LEED measurements. Additional details about the experimental set up are reported elsewhere.9 The XPD data were acquired by making use of the same X-ray source and electron energy analyzer employed for the XPS and LEIS measurements. The sample was a plate of 10 mm × 10 mm × 2 mm cut and polished along the (001) surface with an accuracy of (0.1°. The surface was prepared by cycles of argon ion sputtering (600 eV) and annealing (700 K for 30 min) until no contamination was detectable by means of XPS and LEIS and a sharp (1 × 1) LEED was visible. The sample was mounted on a manipulator that allows polar rotation (rotation axis parallel to the sample surface) and azimuthal rotation (rotation axis normal to the sample surface). Ti (99.999% purity) was evaporated using an electron beam evaporator. The amount of the deposited titanium was estimated by STM and XPS. The procedure to calibrate the amount of evaporated titanium is explained in detail in ref 9. Titanium oxide films were prepared by reaction of Ti evaporated in UHV and oxygen previously chemisorbed on the Cu(001) surface up to saturation. When the amount of deposited Ti was such that the full oxidation process would require a larger amount of oxygen than the one made available by the saturated chemisorption layer, the film was subject to a postoxidation process. Oxygen chemisorbed on Cu(001) exhibits a (2 × 22)R45° LEED pattern. The substrate temperature during deposition and postoxidation (when required) was 300 °C. Once the desired amount of TiO2 was deposited onto the Cu(100) surface, the film was then flash annealed at 500 °C. The stoichiometry of the film was determined by XPS. The XPD curves were acquired by monitoring the intensity of the Ti 2p3/2 and Cu 2p3/2 peaks as a function of the polar and azimuthal emission angles. The LEED intensity versus accelerating voltage (I-V) curves of the diffracted beams was collected by means of a video LEED system. The LEED I-V curves were measured at normal incidence of the electron beam in the energy range 50-350 eV for two sets of nonequivalent diffraction beams. Only the firstorder diffraction beams due to the titanium oxide (one set coincident with the (10) spots of the substrate) were intense enough so that their I-V curves could be measured in a reliable way. The I-V curves of symmetric beams were averaged to compensate for minor differences due to small deviations from
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19579 normal incidence. The I-V curves were background subtracted and normalized to constant incident electron current. The LEED intensities were collected at room temperature. The intensity of the diffracted beams as a function of the primary electron beam energy was calculated using the tensor LEED (TLEED)14 method. The TLEED calculations were performed by means of the Barbieri/Van Hove symmetrized automated tensor LEED package (SATLEED).15 The Barbieri/Van Hove program package was used for the calculation of the phase shifts.15 Ten phase shifts (lmax ) 9) were used in the LEED calculations. The phase shifts for O and Ti were evaluated from the muffin-tin potential calculated for rutile TiO2. The Cu phase shifts were calculated from the muffin-tin potential of bulk copper. The LEED calculations were performed between 40 and 350 eV. The imaginary part of inner potential was set to 5 eV, whereas the real part was set to 10 eV and optimized in the search procedure. The Debye temperatures of O, Ti, and Cu were set equal to 500, 400, and 350 K, respectively. The intensities of the appropriate beams were averaged prior to the comparison with the experimental I-V curves to take into account the existence of symmetry related domains of the oxide overlayer. The Pendry reliability factor (Rp) factor was used in the analysis to judge the agreement between experimental and calculated I-V curves.16 The calculation of the XPD intensities were performed using the single scattering cluster method,17 which turned out to be adequate to describe the XPD data for ultrathin films at relatively high kinetic energies of the photoelectrons. 2.2. DFT Calculations. The calibration of the computational model for a complex system such as the one considered in this paper is far from trivial, and it needs to satisfy both the requirement of accuracy and feasibility. All the present calculations were performed by means of the CRYSTAL0618 periodic code, by adopting a periodic slab model, a gradient corrected (GGA) density functional Hamiltonian, and by using a localized basis set of Gaussian type functions (GTF). The Perdew-Wang (PW) exchange correlation functional19 was used, and it was found to satisfactorily reproduce the properties of both metal and oxide.20-24 An all-electron 8-411G basis set was used for O ion, whereas Hay-Wadt small core pseudopotentials25 were employed, together with 4sp/2d and 3sp/2d contracted Gaussian type functions GTF,26 for Cu atoms and Ti cations, respectively. A smearing procedure was used that consists of assigning a fractional occupancy to one-electron states in a proximity of the Fermi level Ef, owing to a Fermi-Dirac finite temperature distribution with T ) /kb (kB is the Boltzmann constant, and β is the smearing parameter used measured in hartree). A value of β ) 0.005 hartree was employed for the present calculations.24,27,28 The performances of the selected computational approach was tested by considering some relevant properties of bulk Cu and rutile TiO2. For instance, the Cu lattice parameter, a0 ) 3.59 Å, and its cohesive energy, Ecoh ) 3.16 eV, are in good agreement with the corresponding experimental values, a0 ) 3.61 Å and Ecoh ) 3.39 eV.29-31 Similarly, the computed lattice constants of rutile TiO2, a0 ) 4.65 and c0 ) 2.97 Å and the cohesive energy, Ecoh ) 22.0 eV, are satisfactorily close to the experimental values, a0 ) 4.58 and c0 ) 2.95 Å, Ecoh ) 19.9.32,33 The TiO2 band gap is underestimated, Eg ) 1.87 eV, with respect to the experimental value, Eg ) 3.06 eV.34,35 However, such theoretical underestimation of the band gap is well documented in the literature, and it is known to occur whenever pure GGA functional are employed.36-38 A three-layer Cu slab was used to model the metal substrate. Despite its thinness, a three-layer metal slab proved to yield a
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Figure 1. LEED pattern observed for the Cu(001) surface covered by the TiO2 film with the quasihexagonal structure. Energy: 55 eV.
satisfactory description of the metal substrate in many cases of metal-oxide interaction.27,28,36 The geometry of the slab dictates the 2-D unit cell of the overlayer to be considered. In order to reduce computational efforts, a coincidence c(2 × 6) cell was taken as an approximation of the p(2 × 7) periodicity derived from the LEED pattern. In all cases the same TiO2 overlayer was made to interact with both faces of the slab; this means that the central Cu layer is always a symmetry plane. The atom geometry optimization was carried out by means of analytical gradients with respect to atomic coordinates. The coordinates of oxygen and titanium ions in the adlayers were allowed to relax whereas the positions of copper atoms were kept fixed at the bulk values. The basis set superposition error (BSSE) was evaluated by applying the standard counterpoise method.39 3. Results and Discussion 3.1. XPD Results. Titanium oxide films prepared using the procedure reported above exhibit the LEED pattern shown in Figure 1. The oxide films prepared using this procedure cover almost completely the substrate surface as indicated by LEIS and XPS measurements. As discussed in a previous work, this structure has a slightly distorted hexagonal (quasihexagonal) unit mesh.9 The most plausible model for this oxide film consists of a slab of one layer of titanium atoms between two layers of oxygen atoms with a (quasi) hexagonal closed packed structure.9 The XPD measurements were performed in order to confirm the proposed structure, particularly the O-Ti-O sandwich arrangement and to estimate the spacing between the Ti and O outermost layer prior to the LEED intensity analysis. The XPD curves of a complete oxide layer of quasi-hexagonal structure are shown in Figure 2a. These data consist of a set of azimuthal scans relative to the Ti 2p photoelectron peak acquired in the polar angle region between 60° and 72° measured from the surface normal. Given the kinetic energy of this core level, once excited by means of Al KR radiation (Ti 2p kinetic energy ≈ 1020 eV), the Ti 2p XPD data can be qualitatively explained in terms of forward scattering regime. Polar scans acquired over a wider angular range (data not shown) clearly indicate that the XPD structure is confined to the angular region shown in Figure 2a. O 1s XPD data were also acquired. However, since the O 1s peak is located at the onset of the Cu LVV Auger peaks, spurious features due to the scattering of these Auger electrons prevent a reliable analysis of these data. From a purely qualitative point of view the Ti 2p XPD data provide direct evidence for the validity of the trilayer structure
Atrei et al. of the hexagonal TiO2 phase on Cu(100) proposed in ref 9. The Ti 2p XPD data in fact show (quasi) equivalent maxima with an azimuthal angular spacing of ca. 30°. This XPD geometry is consistent with an arrangement of four symmetry-related domains of equivalent scatterers on top of the titanium overlayer. Given the stoichiometry of the film, this topmost layer can be identified with a set of oxygen atoms. Each azimuthal XPD feature shows broad maxima in the polar direction centered at about 65° and 71°. In order to derive an estimation of the outermost interlayer distance, we performed single scattering model calculations for the O-Ti-O trilayer structure with a p(2 × 7) coincidence mesh of the oxide layer (see also section 3.2 for more details about this structure). The XPD calculations were performed for a range of values of the spacing between the outermost oxygen layer and the titanium layer underneath. The calculated XPD curves corresponding to the best agreement with experimental data are shown in Figure 2b. The first O-Ti interlayer spacing derived from the analysis turns out to be 0.9 ( 0.1 Å. The XPS calculations show that the two different polar maxima we observe are related to scattering by the first and second nearest neighbor topmost layer atoms. 3.2. LEED Results. In order to achieve a more detailed characterization of the titanium oxide film, we carried out a LEED intensity analysis. Although the extra beams corresponding to a coincidence mesh are not observed in the LEED pattern, we assumed a commensurate structure. The hypothesis of a commensurate overlayer is needed for the LEED (as well as for the DFT) calculations. This assumption is reasonable because the matching of the oxide periodicity with that of the substrate appears to be the driving force for the distortion from a regular hexagonal unit cell. Along one of the 〈110〉 directions of Cu(001) (for instance the [11j0] direction), the distance between the rows of the oxide coincides with the distance between the rows of the substrate. Then, in the [110] direction, a small distortion from the regular hexagonal unit cell allows the oxide structure to match the periodicity of the substrate. The mesh of the oxide film derived from an accurate inspection of the LEED pattern corresponds to a p(2 × 7) coincidence mesh. Adopting such a mesh implies that in the [110] there is a coincidence between six spacing of the oxide films and seven of the substrate. Considering the quasihexagonal O-Ti-O trilayer, the p(2 × 7) coincidence cell contains 36 atoms (Figure 3, top part). As an approximation of the oxide film with the p(2 × 7) coincidence mesh, we use a smaller c(2 × 6) coincidence cell to reduce the number of atoms in the coincidence cell. Despite the different coincidence unit cell, the lattice parameters of the unit cells of the oxide with the p(2 × 7) and (c2 × 6) periodicity are very similar: The sides of the unit cells are 2.97 Å and 2.95 Å, and the angles between the sides of the unit cell are 118.1° and 119.5° for the c(2 × 6) and p(2 × 7) periodicity, respectively. However, the c(2 × 6) coincidence mesh contains only 15 atoms (Figure 3, middle part). Therefore, the LEED calculations were performed for the c(2 × 6) cell as an approximation of the actual coincidence mesh. It will be shown below that this approximation is justified. Even for this smaller cell, the number of structural parameters is extremely large considering also the reduced experimental data set, which is limited to two nonequivalent beams. In the fitting procedure, only the first three interlayer spacings were optimized whereas the in-plane atomic coordinates were kept fixed. Each titanium atom was located in the center of the triangle corresponding to the three nearest neighbor oxygen atoms of the layer underneath. We considered two different high symmetry registries of the titanium oxide film with respect to
Quasihexagonal TiO2 Structure on Cu(001)
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19581
Figure 2. (a) Experimental XPD data measured on the Ti 2p peak represented as contour plot on orthogonal axes. The experimental data were symmetrized around the 0° angle that coincides with the 〈110〉 azimuth. (b) Ti 2p XPD data calculated for the quasihexagonal O-Ti-O trilayer (see text for details about the model structure). The 0-100 values of the color scale (see inset of Figure 2b) correspond to the minimum and maximum value of each of the contour plot reported in a and b.
Figure 3. Schematic structural model of the O-Ti-O trilayer with a quasihexagonal unit cell. Top part: p(2 × 7) coincidence unit cell. Oxygen atoms at the corners of the cell occupy the “on-top” positions with respect to the copper atoms at the interface. Middle part: c(2 × 6) coincidence unit cell. Oxygen atoms at the corners and at the center of the coincidence cell occupy the “on-top” positions with respect to the copper atoms at the interface (registry A). Bottom part: c(2 × 6) coincidence unit cell. Oxygen atoms at the corners and at the center of the coincidence cell are on the 4-fold hollow sites on the copper atoms of the substrate (registry B).
the substrate. In registry A, oxygen atoms at the corners and at the center of the c(2 × 6) unit cell are on-top of the copper atoms in the first layer of the substrate (Figure 3, middle part). In the registry B, oxygen atoms at the corners and at the center of the cell occupy 4-fold hollow sites on the Cu(001) surface (Figure 3, bottom part). The LEED intensities of the reference structures were calculated for a grid of values of the three
interlayer distances, and the real value of the inner potential was optimized in the search of the best agreement with the experimental data. The calculations performed for the registry B over the explored range of structural parameters give a low level of agreement with the experimental data (Rp higher than 0.5). On the other hand, the I-V curves calculated for the registry A (i.e., oxygen atoms at the corners and center of the unit cell on top of the copper atom of the topmost layer) produce a significantly better agreement with the experimental data (Rp 0.29). The I-V curves calculated for the registry A corresponding to the best agreement with the experimental data are shown in Figure 4. In this figure the I-V curve calculated for the registry B, with the same interlayer distances as in registry A, are also shown. These results indicate that the LEED intensities are rather sensitive to the registry of the titanium oxide film. The values of the interlayer spacings corresponding to the best fit are reported in Table 1. Some test calculations were performed for the same structural model but with a p(2 × 7) coincidence mesh. Because of the large number of atoms in the coincidence mesh, we performed the calculations only for the interlayer spacings corresponding to the best fit obtained for the c(2 × 6) periodicity. The difference between the R factors obtained for the p(2 × 7) and c(2 × 6) coincidence mesh is within the limit of accuracy of the present analysis (which is limited because of small experimental data set). This result (i.e., the scarce sensitivity of the LEED intensity to the coincidence mesh) is expected since in most oxygen atoms have the same local structure with respect to the copper atoms in the c(2 × 6) and p(2 × 7) coincidence cells. We investigated a possible buckling in the sublayers of the O-Ti-O trilayer. In particular, for the registry B oxygen atoms at the interface occupying 4-fold hollow sites may be located at a smaller height compared to that of the oxygen atoms in other positions. The results of some test calculations indicate that no significant improvement of the agreement between calculated and experimental I-V curves is obtained by intro-
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Atrei et al.
Figure 4. Comparison between calculated and experimental I-V curves. The I-V curves are calculated for the O-Ti-O trilayer with a c(2 × 6) coincidence cell. The I-V curves (A) are calculated for the A registry and correspond to the best fit with the experimental data. For the B registry (B), the I-V curves are calculated for the same set of interlayer distances as for the A registry. For each calculated I-V curve the corresponding Rp value is reported.
TABLE 1: Structural Parameters Derived from LEED-IV Analysisa model
dO-Ti (Å)
dTi-O (Å)
dO-Cu (Å)
Rp
c(2 × 6), reg. A
0.90 ( 0.05
1.1 ( 0.1
1.9 ( 0.1
0.29
a dO-Ti is the interlayer spacing between the outermost O layer and the Ti layer. dTi-O is the interlayer spacing between the Ti layer and the O layer at the Cu-oxide interface. dO-Cu is the interlayer distance between the inner oxygen layer and the outermost Cu layer. The uncertainties of the structural parameters are derived from the variance of Rp.16
ducing a buckling of the individual sublayers, either for registry A or registry B. This finding is also consistent with the DFT calculations (see below), indicating that the individual sublayers of the O-Ti-O trilayers are not significantly buckled. 3.3. DFT Results. The optimized structure of a single O-Ti-O trilayer corresponds to a perfect hexagonal arrangement of the Ti and O atoms that is described by a twodimensional hexagonal cell with a lattice parameter b0 ) 2.987 Å (see Figure 5). This two-dimensional cell has been derived by considering that an O-Ti-O trilayer structure with a local hexagonal symmetry appears to be the 〈111〉 planes of a TiO2 cubic phase. Cubic TiO2 belonging to the Fm3m space group (the same as fluorite CaF2) has been suggested by high-pressure experiments (at more than 60 GPa), but the structure has not been yet determined. The computed structure of the cubic phase corresponds to a lattice parameter a0 ) 4.840 Å, in line with previous40 calculations. A single O-Ti-O trilayer cut from the fluorite-like phase along the 〈111〉 crystallographic planes would exhibit a hexagonal symmetry with a lattice constant b0 ) 3.421 Å. Upon relaxation, the geometry of the trilayer undergoes significant variations with respect to the bulk counterpart. In fact the optimized lattice parameter lowers to b0 ) 2.987 Å (see Figure 5). Despite the puzzling character of this phase, the corresponding Ti-O average bond length, dTi-O ) 1.980 Å, is quite close to the computed values for rutile and anatase TiO2 (2.013 Å and 2.002 Å, respectively). As far as the electronic structure of the trilayer is concerned, it turns out that it is quite similar to the one of bulk rutile (or
Figure 5. Schematic view of the TiO2 trilayer: (a) top view and (b) side view of the unsupported film; red circles are O atoms, blue circles are Ti atoms, and gray circles are Cu atoms; b0 )2.987 Å is the lattice constant of the regular hexagonal cell; b1 ) 5.177 and b2 )14.935 Å are the lattice parameters of a c(2 × 6) cell for a regular hexagonal cell of the unsupported trilayer; (c) the Cu-supported TiO2 film; b1 ) 5.105 and b2 ) 15.316 Å are the lattice parameters of the c(2 × 6) coincidence cell.
anatase) TiO2: the Mulliken net charges qTi ) 2.2 and qO ) -1.1 au (the same values, qTi ) 2.2 and qO ) -1.1 au, are computed for rutile) indicate a picture of the Ti-O bond corresponding to a significant variation from a purely ionic interaction [i.e., Ti(IV) and O(-II)] with, on the contrary, a mixed ionic covalent character.41-43 The larger band gap, Eg ) 2.77 eV with respect to that computed for rutile, is nonetheless of the same type: O 2p states at the top of the valence bands and Ti 3d states at the bottom of the conduction band. See the projected density of states PDOS of Figure 6. Let us consider now the supported phase. The creation of the metal-oxide interface requires the trilayer to match the translational symmetry of the metal substrate. A coincidence c(2 × 6) cell with the periodicity of the support is characterized by lattice parameters, b1 ) 5.105 Å and b2 ) 15.316 Å, whereas a c(2 × 6) cell with the periodicity of the ideal hexagonal phase shows the lattice constants b1 ) 5.177 Å and b2 ) 14.935 Å, see Figure 5. Thus, the perfect hexagonal trilayer is to be moderately deformed in order to fit the substrate; the percentage of mismatch is only 1.4% and -2.4% for b1 and b2, respectively. The strain energy to fit the Cu substrate is 0.13 eV per TiO2 unit. Both registry A and B have been considered for the supported film (see section 3.2). However, registry B does not correspond to a stationary point in the potential energy surface and spontaneously moves to the structure of registry A. The Cu/TiO2 interface adhesion energy, ∆Eadh ) 0.33 eV per TiO2 unit, computed with respect to an unsupported film with the same geometry as when supported, compensates for the strain energy necessary to match the periodicity of the substrate. The strain-adhesion energy balance indicates that the quasihexagonal overlayer is thermodynamically more stable than the unsupported hexagonal film by 0.2 eV per TiO2 unit. Similar
Quasihexagonal TiO2 Structure on Cu(001)
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19583 TABLE 2: Main Computed Properties of Cu-Supported and Unsupported TiO2 Trilayera int
int
int
z O 1 , zO 2 , zO 3 ZO1, ZO2, ZO3 ZTi1, ZTi2, ZTi3 dO-Ti dTi-O dO-Cu qOi)1,2,3int qOi)1,2,3 qTii)1,2,3 Ef ∆Eadh, per TiO2 unit
Cu/TiO2
TiO2
1.991, 2.014, 2.023 4.026, 4.031, 4.031 3.089, 3.100, 3.114 0.927 1.096 2.007 -1.3 -1.1 2.1 -5.17 0.33
0.972 0.972 -1.1 -1.1 2.2 -
a zX is the equilibrium height between the Cu surface plane and the atom X of the overlayer; dO-Ti is the interlayer spacing between the outermost O layer and the Ti layer; dTi-O is the interlayer spacing between the Ti layer and the O layer at the Cu-oxide interface; dO-Cu is the interlayer distance between the inner oxygen layer and the outermost Cu layer. ∆Eadh is the BSSE corrected adhesion energy computed as ∆Eadh ) E(TiO2/Cuslab) - E(TiO2) E(Cuslab) with the TiO2 trilayer at the same geometry as when supported. Ef is the Fermi level. All distances in Å and energy in eV. Mulliken net atomic charges qX in au. See Figure 5 for atomic labeling. In the unsupported film all the oxygen atoms are symmetry equivalent.
Figure 6. Projected density of states of the unsupported (top panel) and Cu-supported (bottom panel) TiO2 trilayer. The zero of the energy scale corresponds to the vacuum level.
values have been computed for Pd- and Ag-supported NiO and for Ag-supported MgO.22,23,27,38 The present results were derived considering the c(2 × 6) coincidence cell. However, it can be noticed that the distortion from the regular hexagonal unit cell is larger for the c(2 × 6) than for the p(2 × 7) coincidence cell (i.e., the periodicity derived from the LEED pattern). This means that the strain energy is expected to be lower in the latter than in the former case. On the other hand, the local structure of the atoms at the substrate-film interface is very similar for the two coincidence cells. Therefore, the conclusion that the strain energy of the O-Ti-O is more than compensated by the interaction with the substrate derived from the calculations carried out for the c(2 × 6) coincidence cell can be extended to the real p(2 × 7) coincidence mesh. We move now the to the analysis of the main interface properties. The interface geometry, dO-Ti ) 0.972 Å, dTi-O ) 1.096 Å, dO-Cu ) 2.007 Å, is in excellent agreement with the LEED observations (compare Table 1 and Table 2). The bonding at the interface is characterized by a direct Cu-O interaction with overlap between the O 2p and the Cu 4sp states. The bonding is accompanied by a charge transfer from the 4sp states of the surface Cu in favor of the O 2p orbitals and to a minor extent of the Ti 3d and 4s ones: this is monitored by the variations in the net Mulliken charges with respect to the unsupported film, Table 2, and by the downshift of the Fermi (Ef ) -5.17 eV) level with respect to the Cu bare substrate (Ef ) -4.36 eV); the PDOS of supported and unsupported TiO2 trilayers reported in Figure 6 are consistent with these findings: inspection of the figure shows an upshift of the O 2p and occupied Ti 3d bands together with a tiny downshift of the Ti virtual 3d states; as a consequence, the band gap of the TiO2 film is considerably reduced and metal induced gap states (predominantly Ti 3d derived states) appear close to the Fermi level. Such states, which are close to the conduction band of the oxide, show an acceptor like character in agreement with
TABLE 3: Surface Energy ES (J/m2) and Lattice Parameter b0 (Å) of the TiO2 Hexagonal Film as a Function of the Thickness Measured in Number n of Trilayersa n
ES
b0
1 2 3 4
-0.834 -1.724 -2.616 -3.500
2.987 2.986 2.986 2.986
a ES ) (E(TiO2)slab - nE(TiO2)bulk)/2A, where A is the area of the surface unit cell. All multilayered structures were allowed to relax.
the flow of charge from the metal to the oxide. The large charge transfer (0.3 au) appears also to be the driving mechanism for the large upshift in the work function φ (φ ) -Ef) of the metal-oxide system with respect to the bare substrate (for a detailed discussion, see ref 38). Finally we consider briefly the subject of the growth of the hexagonal phase. We start by analyzing unsupported films of increasing thickness. The case of a number n of trilayers with n ) 1, 2, 3, and 4 were considered. The films were allowed to relax (both in the parallel and perpendicular direction relative to the layer), and the surface energy was used as indicator of the stability of the resulting structures (see Table 3). A single unsupported trilayer is characterized by a negative value of the surface energy, Es(1)) -0.83 J/m2 (see Table 3). The negative energy value means that the film is more stable than the bulk, and, in turn, this is due to the fact that the cubic TiO2 phase is not stable under normal pressure conditions. The scarce propensity of TiO2 trilayers to bind together and form threedimensional structures is evident by considering the surface energy Es(n) and the lattice parameter b0(n) values as a function of the thickness measured in number n of trilayers. From the values reported in Table 3, one can see that ES scales exactly with n, i.e., ES(n) ) nES(1). Moreover, the lattice parameter b0(n) is always close to the value computed for n ) 1. Such a trend in the surface energy indicates that with increasing n, one obtains a piling of separate trilayers rather than the formation of a proper three-dimensional structure. Moving now to supported films, we find that these interact very weakly with the
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substrate as indicated by the small changes in the electronic structure induced by the support itself. As a result, also the surface energy values are expected to be very similar to those calculated for the unsupported case. All these findings point to the conclusion that the thickness of the quasihexagonal phase cannot exceed a single trilayer and that the formation of a thicker films necessarily requires a large reconstruction of the overlayer. 4. Conclusions By combining experimental and theoretical results, we characterized the crystallographic, energetic, and electronic structure as well as the energetics of the TiO2 ultrathin films with a quasihexagonal unit cell. As far as the structural aspects are concerned, the XPD and LEED I-V results confirm the O-Ti-O nearly hexagonal model. The LEED results provide a clear indication about the registry of the titanium oxide film with respect to the Cu(001) substrate. The experimental results of the crystallographic structure of the oxide film and of the oxide-substrate interface are in good agreement with those derived from the DFT calculations. The calculations provide information about the electronic structure of the oxide film, indicating a direct Cu-O interaction with overlap between the O 2p and the Cu 4sp states and that the titanium oxide film on Cu is not an insulator because of the upshift of the O 2p bands and a downshift of the Ti 4s states compared to the case of the unsupported film. The DFT calculations clearly indicate that the O-Ti-O trilayers cannot bind together to form threedimensional structures. This is consistent with the fact that the cubic TiO2 phase with the CaF2 structure is not stable at normal pressure conditions. Acknowledgment. This work was partially supported by MIUR. A.B. and P.F. are grateful to KME spa for the financial support. References and Notes (1) Freund, J. H.; Kuhlenbeck, H.; Staemmler, V. Rep. Prog. Phys. 1996, 59, 283. (2) Schoiswohl, J.; Tzvetkov, G.; Pfuner, F.; Ramsey, M. G.; Surnev, S.; Netzer, F. P. Phys. Chem. Chem, Phys. 2006, 8, 1614. (3) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (4) Biener, J.; Farfan-Arribas, E.; Biener, M.; Friend, C. M.; Madix, R. J. J. Chem. Phys. 2005, 123, 094705. (5) Kaneko, H.; Ono, M.; Ozawa, K.; Edamoto, K. Solid State Commun. 2007, 142, 32. (6) Zeng, H. C.; McFarlane, R. A.; Mitchell, K. A. R. Surf. Sci. 1989, 207, L7. (7) Leibsle, F. M. Surf. Sci. 1995, 33, 51. (8) Fujita, T.; Okawa, Y.; Matsumoto, Y.; Tanaka, K. Phys. ReV. B 1996, 54, 2167. (9) Finetti, P.; Caffio, M.; Cortigiani, B.; Atrei, A.; Rovida, G. Surf. Sci. 2008, 602, 1101.
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