J. Phys. Chem. C 2007, 111, 7437-7445
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Structure, Composition, and Electronic Properties of TiOx/Mo(112) Thin Films Yongfan Zhang,† Livia Giordano, and Gianfranco Pacchioni* Dipartimento di Scienza dei Materiali, UniVersita` di Milano-Bicocca, Via Cozzi, 53 - 20125 Milano, Italy ReceiVed: January 23, 2007; In Final Form: March 23, 2007
Titanium oxide films of various composition grown on a Mo(112) metal substrate have been investigated with the help of first principles density functional theory plane wave calculations using the PW91 functional. We show that at the experimental pressure and temperature conditions used to grow the films a structure with composition TiO3 is preferred. This conclusion is based on the thermodynamic analysis of the film stability as a function of the oxygen partial pressure and is corroborated by the comparison of computed and measured properties like scanning tunneling microscopy images, vibrational modes, and core level binding energies. The electronic properties of the films are discussed in terms of density of states, charge-transfer at metal/ oxide interfaces, and work function changes.
1. Introduction Oxide ultrathin films grown on metal substrates are becoming increasingly interesting because they can exhibit unprecedented properties not shown by thick films or by the corresponding single-crystal surfaces. The main motivation to study oxide films remains their potential use in areas of technological importance such as sensors, electronic devices, spintronic, supports for catalysts, etc.1-3 However, recently it has become clear that in some conditions these films form a new class of materials with largely unexplored properties.4 If we restrict ourselves to the field of metal nanoparticles supported on oxides, it has been shown that by combining a metal cluster with high-electron affinity with a metal/oxide interface with low work function, it is possible to induce spontaneous charging of the supported metal atom or particle, an effect that can substantially modify the chemical and catalytic properties of the nanoparticle.5-7 This effect only occurs when the film is ultrathin and contains only a few atomic layers. In this thickness regime, even the structure of the oxide film can be substantially different from that of the corresponding crystalline or amorphous material. The condition for epitaxial growth leads often to new oxide phases that have no counterpart in the surface of microcrystalline single-crystal oxides. A typical example is that of a newly discovered SiO2 structure grown on a Mo(112) metal substrate.8 For all these reasons, the study of these systems is attracting interest from both theoretical and experimental points of view. It is not surprising that, among the several oxides grown in the form of thin film, special attention has been given to titanium oxide, a material with a wide range of technologically important applications.9 Thin TiOx films have been deposited on various metal surfaces, such as Pt(110),10 Pt(111),11,12 Mo(112),13,14 and W(100).15 One of the problems to face is that the structure and composition of the these titanium oxide films is not easy to determine. Among these various systems, the TiOx/Mo(112) structures have been studied in detail also because this surface can be wetted by Au and can exhibit unprecedented catalytic activity for CO oxidation.13,16-18 The mechanism of activation * To whom correspondence should be addressed. E-mail:
[email protected]. † Permanent address: Department of Chemistry, Fuzhou University, Fuzhou, Fujian, 350002, China.
of gold nanoparticles deposited on oxides of central importance in catalysis is still under debate, and the possibility to study and to understand in detail the interaction of gold with TiOx/ Mo(112) films is an additional reason of interest in these systems. Some theoretical investigations on the structure and properties of TiOx/Mo(112) films have already been reported19-21 but the stoichiometry and atomic structure of the TiOx film are still under debate and have never been investigated in a systematic way. A Ti2O3 stoichiometry was proposed by Hernandez et al.;19 a Ti:O ratio of 1:2 and 1:3 was proposed in other works20,21 but without exploring other possibilities. In this paper, we present a detailed study of the configuration and stoichiometry of TiOx/Mo(112) films. We compare the thermodynamic stability of various phases as a function of the preparation conditions (oxygen partial pressure, temperature, etc.); we investigate the cost of transformation of one structure into the other; we describe the electronic properties of the titania films and the changes induced in the work function of the metal substrate; finally, we determine observable properties, such as scanning tunneling microscopy (STM) images, phonon spectra, and core level binding energies, and we discuss their relation to measured quantities. 2. Computational Details First principles calculations based on density functional theory (DFT) were carried out using the Vienna ab initio simulation package22,23 and the projected augmented wave method.24 The generalized gradient approximation was employed for the exchange-correlation functional in the Perdew-Wang (PW91) form.25 The Mo(112) substrate was modeled by a seven-layer slab in which the bottom three Mo layers were fixed at their bulk positions. The spacing between the adjacent slabs was about 17 Å. The Mo slab well reproduces the bulk properties of the Mo metal.26 The kinetic energy cutoff for the plane-wave expansion was set to 400 eV. Careful tests of the MonkhorstPack k-point meshes were performed, and the results indicated that a (21 × 13 × 1) k mesh for (1 × 1) unit cell and a (21 × 7 × 1) k mesh for (1 × 2) supercell are required to guarantee the convergence of the total energy within 1 meV per atom. Seven models of TiOx films with different stoichiometry and structure have been considered with x varying between 1 and
10.1021/jp070584s CCC: $37.00 © 2007 American Chemical Society Published on Web 04/27/2007
7438 J. Phys. Chem. C, Vol. 111, No. 20, 2007
Zhang et al.
4. The initial configurations of some of the models studied were directly derived from other published works, including the Ti2O2 and Ti2O3 models proposed by Hernandez,19 the (1 × 1) TiO2 by Quek,20 and the Ti2O7 by Chen.17 Special effort has been paid to investigate the possible structure of the film with x ) 3, as our results suggest that this is the thermodynamically preferred composition. Thus, extensive calculations were carried out for the TiO3 film and more than a hundred of possible configurations were explored. In a typical process, an ab initio molecular dynamic (MD) simulation using the Nose´ algorithm27 was performed for an initial structure with a low-energy cutoff (300 eV) and a small k-mesh; the simulation lengths ranged from 1.5 to 2 ps with time step of 1 fs at the temperature of 1500 K. Other possible configurations from this initial geometry were sampled from the results of the MD simulations every fifty steps, and then further structural optimizations were performed to determine the most stable configuration. A similar process was also employed for the Ti2O4 model. The Ti2O5 model, on the contrary, was constructed from the most stable Ti2O6 film by removing an oxygen atom and optimizing the resulting structure. To compare the thermodynamic stability of the various structures, the formation energy, ∆Eform, was used. This is derived from the formation energy of TiOx/Mo(112) starting from a clean Mo(112) surface, from rutile TiO2 and molecular oxygen
Mo(112) + n(TiO2)rutile +
n(x - 2) O2 f (TiOx)n/Mo(112) 2 (1)
where n is the number of TiOx units in a surface unit cell. Thus, ∆Eform is defined as
∆Eform )
[
(
1 (TiOx)n/Mo(112) E - EMo(112) + nE(TiO2)rutile + n n(x - 2) O2 E 2
)]
(2)
where E(TiOx)n/Mo(112), EMo(112), E(TiO2)rutile, and EO2 are the energies of a given TiOx/Mo(112) system, of the clean Mo(112) surface, of bulk TiO2 with rutile phase, and of the oxygen molecule, respectively. Furthermore, the free energy change of the above reaction was also calculated to determine the relative stability of different chemical compositions at the experimental conditions28
1 ∆γ(T,p) ) [∆Eform - n(x - 2)∆µO(T,p)] S
(3)
1 ∆µO (T,p) ) µO - EO2 2
(4)
and
where S is the surface area and µO is the oxygen chemical potential. Details of how the observable properties (density of states, work function changes, STM images, phonon spectra, core level binding energies) have been computed are given below in the course of the discussion. 3. Results and Discussion 3.1. Thermodynamic Stability. Experimentally, well-ordered TiOx thin films on Mo(112) surface can be synthesized by either depositing Ti onto a SiO2/Mo(112) monolayer at 1400 K in 1
× 10-8 Torr of O2 or directly depositing Ti onto an oxygencovered Mo(112) surface followed by subsequent oxidationannealing cycles.14 The TiOx film exhibits a very sharp (8 × 2) low-energy electron diffraction pattern. It is interesting that by depositing Au onto the (8 × 2)-TiOx/Mo(112) surface at room temperature and upon annealing to 900 K, the surface reconstructs to a well-ordered (1 × 1) structure. Because the size of a (8 × 2) supercell is too large to allow a systematic study of the various structures (about 21.8 × 8.9 Å2), we use a (1 × 2) supercell as a model to determine the composition of the TiOx film, as well as the corresponding atomic structure. The main difference between the two supercells is related to the position of the Ti atoms: in the (1 × 2) structure they are all in the same position with respect to the Mo substrate, while in the (8 × 2) the TiOx film is not in register with the substrate and the Ti atoms occupy slightly different positions. Figure 1a-j shows the most stable structures obtained for each TiOx/Mo(112) interface with x going from 1 to 4. In these configurations, the Mo-O bond length varies between 2.02 and 2.30 Å, while the Ti-O bond is shortest (1.78∼1.85 Å) when the oxygen atom occupies the bridging site between two Ti atoms; longer bond lengths (1.83∼2.07 Å) are observed for other Ti-O bonds. Except when the film has a small oxygen content (x < 2.5), the Ti atom is connected to the Mo metal by Ti-O-Mo bonds and forms other Ti-O-Ti linkages. As expected, the coordination number of the Ti atom increases with the increasing value of x (from 1 in Ti2O2 to 6 in Ti2O7). Moreover, increasing the oxygen content also implies a deeper oxidation of the substrate and, consequently, some Mo-Mo bonds may be broken when x is larger than 3. The free energy change (∆γS) for the formation of TiOx film as a function of the oxygen chemical potential ∆µO (oxygen partial pressure) at different temperatures (900 and 1400 K) is displayed in Figure 2. At high oxygen chemical potentials (∆µO > -1.4 eV), the most favorable stoichiometry is TiO4, Figure 1j. The film with x ) 3.5, Figure 1i, becomes stable when the oxygen chemical potential decreases to -1.7 eV. For a wide range of oxygen chemical potentials, -3.2 ∼ -1.7 eV, the film with Ti:O ratio of 1:3, Figure 1g, is energetically most favorable. Notice that we found another (1 × 2) TiO3 configuration, Figure 1h, which is only slightly higher in energy than the structure shown in Figure 1g. The structure in Figure 1h can be obtained by depressing the position of the left Ti atom from the configuration of Figure 1g. Further decreasing the oxygen chemical potential results in the stabilization of TiOx films with x ) 2, Figure 1c,d). At strongly reducing conditions (∆µO < -4.0 eV), the most favored Ti:O stoichiometry is 2:3, Figure 1b. According to these results, a TiO phase will hardly form, Figure 2. Our results show that at experimental conditions, 1400 K and oxygen partial pressure 1 × 10-8 Torr,13 the TiO3 stoichiometry is preferred. However, the free energies of films with x ) 2∼3 differ by less than 0.2 eV, suggesting that films with different compositions in the range TiO2-TiO3 may coexist. The experiments show that the (8 × 2)-TiOx surface is stable when annealed to 900 K in ultrahigh vacuum or in 5 × 10-9 Torr of O2.13 Under these conditions, the films with composition x ) 3 and x ) 3.5 are preferred, as shown in Figure 2. Therefore, even taking into account the above different experimental conditions, it seems that the TiO3 stoichiometry is the most likely. It is worth mentioning that for the TiO3 composition, the reconstructed (1 × 2) structure, Figure 1g, is energetically more favorable than the (1 × 1) one, Figure 1f. A similar stabilization of the reconstructed (1 × 2) supercell is also
TiOx/Mo(112) Thin Films
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Figure 1. Top and side views of the most stable configurations of TiOx/Mo(112) films. The Mo, Ti, and O atoms are denoted by blue-green, gray, and red spheres, respectively. Only three layers of the Mo(112) substrate are shown in the side view; the top view corresponds to the (1 × 2) supercell.
observed for the TiO2 stoichiometry (see Figure 1c,d). Although the energy difference between reconstructed and non-reconstructed TiOx films is small (
