Ca Deposition on TiO2(110) Surfaces: Insights from Quantum

Feb 6, 2009 - Michel Posternak , Simon Berner , Alfonso Baldereschi , and Bernard Delley ... Chi Lun Pang , Robert Lindsay , and Geoff Thornton. Chemi...
0 downloads 0 Views 757KB Size
Download PDF
3740

J. Phys. Chem. C 2009, 113, 3740–3745

Ca Deposition on TiO2(110) Surfaces: Insights from Quantum Calculations M. A. San Miguel,* J. Oviedo, and J. F. Sanz Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, UniVersidad de SeVilla, c/ Profesor Garcı´a Gonza´lez, 1. 41012 SeVilla, Spain ReceiVed: NoVember 2, 2008; ReVised Manuscript ReceiVed: December 12, 2008

The deposition process of calcium atoms on the rutile TiO2(110) surface has been investigated using firstprinciples methods based on density functional theory. It is found that calcium atoms adsorb on the surface by forming strong and directional bonds, and a large amount of charge is transferred to the surface. The preferential adsorption site involves binding two bridging oxygens and one in-plane oxygen, forming equivalent bonds. The presence of bridging oxygen vacancies destabilizes this site by 0.5 eV, and the calcium atoms tend to adsorb far away from the vacancies. The effect of increasing the surface coverage has also been investigated. The calculated work function as a function of the coverage indicates that there is a transition at 0.8 ML, and the system becomes metallic. Introduction Alkali and alkaline earth metals act as reaction modifiers on a variety of substrates, including metal oxides. For instance, the interaction between Ca atoms and TiO2 plays a major role in the performance of Ti-containing steel as bone implants. Most of the previous studies have been focused on alkali metal adsorption on the TiO2(110) surface.1 It was established that, upon Na or K adsorption on this surface, there are significant changes in the electronic structure of the system through oxidation of the adsorbed alkali and reduction of the Ti atoms in the substrate.2 Additionally, different models to describe the distribution of alkali atoms on the surface were proposed.3 On the other hand, there have only been a few studies of alkaline earth metal (Ba and Ca) adsorption on the same substrate.4-9 In previous works, we have investigated the adsorption of Ba atoms on the TiO2(110) surface from calculations based on density functional theory (DFT).9 In this study, we intend to extend the analysis to the Ca case. Although Ca deposition on TiO2(110) surfaces was originally studied a decade ago from theoretical and experimental methods6,7 and, more recently, STM and LEED measurements8 have been performed to examine the structure of Ca atoms on the TiO2(110) surface, there is still a lack of clear evidence on either the most favorable adsorption sites or the electronic structure of the adsorption system. In this paper, we report results from DFT calculations of the deposition of Ca atoms on both stoichiometric and defective rutile TiO2(110) surfaces at different surface coverages. To analyze the structural and electronic properties in a wide range of surface coverages, a large supercell (6 × 2) has been used throughout the study. The adsorption sites on both surfaces were explored and compared with Ba. Some results on Ba were reported previously, but some calculations have been extended to obtain adsorption energies at lower coverages. The charge transfer process has been investigated from the analysis of electron density differences, Bader charges, and work function changes after adsorption. Technical Details. In the present work, the implementation of density functional theory (DFT) has been used with plane waves and the PAW (plane-augmented waves) potentials.10-12 * Corresponding author. E-mail: [email protected].

Figure 1. Top view of the TiO2(110) surface. Representative atom types and the five adsorption sites reported in the text.

We use the generalized gradient approximation (GGA) for the exchange-correlation energy, which seems to describe better the formation and creation of bonds on surfaces.13 In particular, we use the GGA attributed to Perdew et al.,14,15 and the calculations were performed with the VASP (Vienna ab initio simulation package) code.16-18 For oxygen, the core only consists of 1s states, whereas for titanium, up to and including the 3p shells are frozen, and the reference state for the potential generation is s1d3. For calcium and barium, 10 electrons, (3s23p64s2) and (5s25p66s2), respectively, are described as valence states. To compute adsorption energies for Ca deposition at low coverages, we have made use of a surface model consisting of four-layer slabs of (6 × 2) unit cells and a vacuum space of 15 Å to separate each slab from its periodic images. Thus, one Ca atom in this supercell represents coverage of 0.083 ML. Similarly, we have used this model to mimic defective surfaces by removing a single bridging oxygen from the supercell. This model represents a defect concentration of 8.3%, which is quite consistent with real surfaces where the concentration is about ∼10%.1 The energetics of the adsorption process has been studied from geometry optimization calculations and molecular dynamics (MD) simulations. An energy of 400 eV was set as the planewave basis set cutoff. Given the large size of the supercell, all the calculations were carried out at the Γ point of the Brillouin zone. The optimized lattice parameters used are a ) 4.616 Å,

10.1021/jp809680h CCC: $40.75  2009 American Chemical Society Published on Web 02/06/2009

Ca Deposition on TiO2(110) Surfaces

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3741

TABLE 1: Adsorption Energies for Both Earth Alkaline Atoms (EA) Investigated and Optimized Geometrical Parameters for the Stable Adsorption Sites on the Stoichiometric TiO2(110) Surfacea EA sites Eads (eV) EA-Obr EA-Oin Oin (∆z) Ti (∆z) Ca Ba

BB OB OI BB OB OI

-4.41 -4.75 -4.35 -4.88 -5.11 -4.78

2.07 2.15 2.07 2.38 2.46 2.36

3.36 2.24 2.30 3.62 2.60 2.69

0.0 +0.20 +0.28 0.0 +0.14 +0.19

-0.40 -0.40 -0.13 -0.35 -0.30 -0.13

charge (e) +1.49 +1.50 +1.50 +1.57 +1.56 +1.57

a Distances and displacements are given in Å. Obr and Oin mean bridging and in-plane oxygen, respectively. Ti is the six-fold-coordinated titanium bound to the bridging oxygen atoms. The z axis is perpendicular to the surface, and displacements are positive when the atoms move outward from the surface.

c ) 2.974 Å, and u ) 0.304, and they were kept fixed during the geometry optimization process in which all the atoms were fully relaxed using the conjugate gradient method until residual forces become smaller than 5 × 10-2 eV/Å. Extended tests on the different choices of parameters have been considered and discussed previously elsewhere.20 Molecular dynamics simulations were performed in selective cases in the canonical ensemble at 300-500 K. The time step was 3 fs, and a typical simulation spanned about 2 ps. Each picosecond run required 72 h CPU time on 32 nodes on a parallel supercomputer. The electronic structure of the adsorption systems and, particularly, the charge transfer process has been investigated from the analysis of density charge differences, the Bader charges, the variation of surface work function induced by the adsorbate, and the projected density of states (DOS) plots. Electron density differences have been calculated as

∆F ) F(EA-substrate) - F(substrate) - F(EA)

(1)

where F(EA-substrate) is the electron density of the adsorption system, and F(substrate) and F(EA) are the densities of the clean substrate and the isolated EA at the optimized adsorption positions; ∆F provides information on the electron redistribution upon the adsorption process. Thus, positive values would correspond to density gain and negative values to density loss. The Bader analysis is based on finding critical points of charge density in order to divide the 3D space into regions assigned to the different atoms, named Bader volumes. The integration of the charge density in the Bader volume leads to the charge of the associated atom.21,22 Variations of work function ∆φ upon adsorption provide significant insights on the charge transfer process. Thus, a decrease/increase of this function is usually associated with positively/negatively charged adsorbates, and the resulting surface dipole is in the opposite/same direction as that of the clean surface. However, this general rule does not always hold, and Michaelides et al.23 demonstrated why a negatively charged N adsorbate on W(100) induces an unexpected work function decrease. They showed that a reduction in the overspill of surface electron density into the vacuum led to the unexpected work function decrease, whereas the adsorbed N still retained a negative charge. Therefore, measurements of the work function change ∆φ provides critical information on the degree of charge redistribution upon adsorption. The work function change ∆φ has been calculated as the difference between the Fermi and the vacuum levels for each adsorption system. A vacuum space of 15 Å separating slabs was required to obtain a converged

Figure 2. Optimized geometries for several cases discussed within the text for the stoichiometric surface. Only a portion of the surface is shown. Calcium is represented by big gray spheres, whereas oxygen and titanium are represented by red and green spheres, respectively: (a) OB, (b) BB, and (c) OI cases.

value of the vacuum level from the planar-averaged electrostatic potential along the z direction. Results and Discussion Adsorption Sites at Low Coverage. There is scarce information about which is the preferred adsorption site for Ca atoms on the rutile TiO2(110) surface. In a previous work on Ca adsorption, several potential sites were proposed (see Figure 1).8 There are two sites on top of bridging oxygen atoms, either on a single bridging atom (TB) or on two of them (BB). There are also two sites where the earth alkaline (EA) atom binds three oxygen atoms: two bridging and one in-plane (OB) or two inplane and one bridging (OI). A final site is on top of a five-fold titanium atom (TiP). Geometry optimizations for all these sites show that only OB, OI, and BB sites were stable, whereas the other two sites were not minima in the potential energy surface. Table 1 reports some calculated properties for the stable sites. The adsorption energies have been computed as

Eads ) EEA-TiO2 - ETiO2 - EEA

(2)

where EEA is the energy corresponding to an EA atom in vacuum. From a thermodynamic point of view, the adsorption energies should be calculated as the difference between the energy of products and the energy of reactants in the most stable

3742 J. Phys. Chem. C, Vol. 113, No. 9, 2009

San Miguel et al.

Figure 3. Electron density difference maps for (a) OB, (b) BB, and (c) OI cases. All atoms are represented by small spheres. Gray areas show gain (top panel) or loss (bottom panel) of electron densities.

Figure 4. Averaged binding energy (BE) per Ca atom adsorbed on the stoichiometric TiO2(110) surface as a function of surface coverage.

forms; therefore, the reactants should be considered in the metal state. However, using the atomic reference allows us to compare the strength of the bonding with other atoms. From the adsorption energy values, the most favorable site is the OB site, which is 0.34 eV for Ca and 0.23 eV for Ba and is more stable than the second preferred site (BB site). In a previous work, similar results were obtained for Ba adsorption using a (4 × 2) surface model. Here, we have recomputed the adsorption energies using the (6 × 2) supercell and have found that Ba atoms adsorb more strongly than Ca atoms do. Figure 2 shows a side view of the three adsorption sites for calcium. It can be seen that the surface atoms associated with

each bond move from the original position. The surface deformation has been quantified by the distance between the EA atom and bridging oxygen or five-fold Ti atoms. In the OB site, the Ca atom binds three oxygen atoms: two bridging and one in-plane atoms. To achieve an equal three-fold coordination, the in-plane oxygen moves outward. A similar bonding situation is found in the OI site, where the three-fold Ca atom binds two upward-shifted in-plane oxygen atoms and one bridging oxygen. Only in the BB site, Ca is two-fold-coordinated and sits on two bridging oxygens, forming two equivalent bonds. Since the barium atomic radius is bigger than the calcium one, the bonding distances are also observed to be longer.

Ca Deposition on TiO2(110) Surfaces

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3743

Figure 5. Averaged charge loss (in e) per Ca atom adsorbed on the stoichiometric TiO2(110) surface as a function of surface coverage.

Figure 6. Work function change (∆φ) upon Ca adsorption on a stoichiometric TiO2(110) surface as a function of surface coverage.

Similar trends in the displacements of the in-plane oxygen and six-fold titanium atoms related to the bonding are found, although they are slightly smaller. Figure 3 shows isosurfaces of electron density differences. The main observation is that there is a significant charge transfer from the Ca atom to the surface oxygen atoms. The electron density accumulates directionally in the Ca-O regions, whereas the major electron depletion is inside the Ca 4s orbital. A Bader analysis of the charge associated with the Ca atom has been carried out (Table 1).19,20 Although absolute values of Bader charge should be taken with caution, the charge transfer from EA to oxygen atoms is quite remarkable, being slightly higher for barium where the bonding is stronger. Despite the differences in adsorption energy among the three sites, Bader charges are practically the same. This fact may reflect that the effectiveness of the bonding is related more to the spacial directionality than to the charge transfer process. Role of Oxygen Vacancies. A defective surface has been considered by removing a bridging oxygen atom from the surface in the (6 × 2) supercell. We have explored different adsorption sites, and the OB is, again, the most favorable,

although it is less stable than in stoichiometric surfaces by ∼0.5 eV. The adsorption energy decrease arises because the oxygen vacancy has some extra charge associated with it that prevents, to some extent, the charge transfer. This result is comparable to what is found for barium and indicates that, upon EA deposition, the most populated sites will be OB sites far from the vacancies. To check the stability of the defective adsorption site, MD simulations at several temperatures were performed, starting from a configuration where the Ca atom was situated on top of the vacancy. At 300 K, after 525 fs, the EA jumps to a contiguous OB site and stays there for the rest of the simulation. Extended simulations up to 500 K failed to observe further diffusion on the surface. Effect of Increasing Surface Coverage. Geometry optimizations were carried out ranging from 1 to 12 EAs on the (6 × 2) supercell. In the starting configurations, the EAs were placed on OB sites as far as possible from each other. (Top views of final configurations are found in the Supporting Information.) To estimate the relative stability of the final adsorbate-substrate

3744 J. Phys. Chem. C, Vol. 113, No. 9, 2009

San Miguel et al.

Figure 7. Density of states (DOS) of Ca atoms adsorbed on TiO2(110) surfaces. (Top panel) DOS as a function of surface coverage expressed as the number of Ca atoms in the (6 × 2) supercell. (Bottom panel) DOS projected on different atomic components for nine Ca atoms adsorbed on the stoichiometric surface.

system, the averaged binding energy per added atom (BE) was computed as

BE ) EnEA-TiO2 - ETiO2 - EnEA ⁄ n

(3)

where each term is known from eq 2 and n is the number of EA adatoms. Figure 4 shows the averaged binding energy (BE) as a function of the surface coverage. At low coverage, a linear increase in BE occurs, indicating that the interaction between adsorbate and substrate becomes less favorable since the adatoms approach each other, and the repulsive electrostatic forces start to be noticeable. The slope is slightly smoother at intermediate coverages and even more and is apparently trending to reach a steady value at ∼0.8 ML. To quantify the amount of charge transferred from the EA to the surface, the charge on each EA upon adsorption has been computed from a Bader analysis. Figure 5 illustrates the charge loss from the adsorbate as a function of the surface coverage. This charge has been normalized by the number of EAs to compare between different coverages. The charge transferred

from each EA up to 0.4 ML is similar and quite significant (1.5 e). Then a rapid decrease occurs up to 0.8 ML when, finally, the slope diminishes more smoothly, and the charge transferred is below 1 e. The charge transferred to the surface does not accumulate in specific atoms but spreads over the surface. Only little increases are found in the bridging oxygen atoms (∼0.09 e) and the closest five-fold titanium atoms (∼0.04 e) associated with the bonding. We have calculated the work function change as a function of the surface coverage (see Figure 6). Initially, a major decrease occurs, which gradually levels off and attains a minimum at coverage of 0.4-0.6 ML. Finally, a slow increase is observed up to a coverage of 0.8 ML when a steady value is reached. The initial rapid decrease is caused by the large EA-surface charge transfer, yielding partially positively charged particles on the surface. As the surface coverage increases, these positive adsorbates are forced closer together, leading to repulsive lateral electrostatic interactions, which, consequently, lead to a reduction in the degree of charge transfer and to a minimum in the curve. The slow increase at 0.8 ML is a transition where some

Ca Deposition on TiO2(110) Surfaces charge originally transferred from the adsorbate to the surface is now returned to the adsorbate. The final plateau corresponds to the situation where the EA atoms behave as fully metallic particles. The top panel in Figure 7 shows the density of states for the stoichiometric surface and for Ca atoms deposited on the surface as a function of the surface coverage. The first noticeable aspect is that the Fermi level is moved from the top of the valence band in the bare stoichiometric surface to the bottom of the conduction band upon the Ba adsorption process, provoking a major change in the electron conduction properties of the support. This effect is similar to what happens when a bridging oxygen atom is removed from the surface, creating a vacancy. As the number of Ca atoms on the surface increases, the lower limit of the conduction band extends. It can be seen from the bottom panel in Figure 7 that this region corresponds mainly to Ca atoms, in particular, to s states partially mixed with p states. Thus, as the surface coverage increases, the conduction band becomes more calcium-like, and, consequently, the system behavior becomes more metallic. The lower region in the conduction band also indicates that the bonding is mainly covalent and formed from the calcium s and p orbitals and the oxygen p orbitals. Conclusions The deposition of calcium atoms on the rutile TiO2(110) surface has been investigated from DFT calculations and compared with that for barium atoms. It has been demonstrated that Ca atoms interact very strongly with the surface, and the charge transferred from the EA atom to the substrate is quite significant. For Ba atoms, the interaction is slightly stronger. The most favorable adsorption site for both EAs is when the EA is bound to two bridging oxygens and one in-plane oxygen forming equivalent bonds. These bonds are mainly covalent in nature and involve calcium s and p orbitals and oxygen p orbitals. In defective surfaces, the OB sites are still the most favorable adsorption sites, although they are destabilized by the bridging oxygen vacancy by 0.5 eV, and, consequently, the EA atoms would move away. The role of surface coverage on the deposition process has been investigated by using a very large supercell, which has allowed the exploration of a wide range of surface coverages. Additionally, with regard to the energetics, the charge transfer process has been studied from work function changes and a Bader analysis. The work function change as a function of the surface coverage reveals that, at low coverage, a rapid decrease occurs, caused by the large EA-surface charge

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3745 transfer and yielding positive charges on the surface. Between 0.4 and 0.8 ML, the repulsive electrostatic interactions between these particles are important, and the work function change curve shows a minimum. Over a 0.8 ML increase in the curve reaching a final plateau indicates that a transition occurs, and the system becomes metal-like. Acknowledgment. This work was funded by the Spanish DGESIC, project MAT2008-04918. Supporting Information Available: Top views of final configurations at 12 different surface coverages. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (2) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1996. (3) San Miguel, M. A.; Calzado, C. J.; Sanz, J. F. J. Phys. Chem. B 2001, 105, 1794, and references therein. (4) Li, Z.; Jørgensen, J. H.; Møller, P. J.; Sambi, M.; Granozzi, G. Appl. Surf. Sci. 1999, 142, 135. (5) Pang, C. L.; Sasahara, A.; Onishi, H. J. Phys. Chem. C 2007, 111, 9221. (6) Zhang, L. P.; Li, M.; Diebold, U. Surf. Sci. 1998, 412/413, 242. (7) Norenberg, H.; Harding, J. H. Appl. Surf. Sci. 1999, 142, 174. (8) Bikondoa, O.; Pang, C. L.; Muryn, C. A.; Daniels, B. G.; Ferrero, S.; Michelangeli, E.; Thornton, G. J. Phys. Chem. B 2004, 108, 16768. (9) (a) San Miguel, M. A.; Oviedo, J.; Sanz, J. F. J. Mol. Struct. (THEOCHEM) 2006, 769, 237–242. (b) San Miguel, M. A.; Oviedo, J.; Sanz, J. F. J. Phys. Chem. B 2006, 110, 19552–19556. (10) Payne, M. C.; Teter, M. P.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. ReV. Mod. Phys. 1992, 64, 1045. (11) Blo¨chl, P. E. Phys. ReV. B 1994, 50, 17953. (12) Kresse, G.; Joubert, J. Phys. ReV. B 1999, 59, 1758. (13) Hu, P.; King, D. A.; Crampin, S.; Lee, M.-H.; Payne, M. C. Chem. Phys. Lett. 1994, 230, 501. (14) Perdew, J. P. In Electronic Structure of Solids; Ziesche, P., Eschrig, H., Eds.; Akademie Verlag: Berlin, 1991. (15) 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. (16) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558. (17) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (18) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (19) One monolayer corresponds to one Ca atom per unit cell, which is indicated in the supercell depicted in Figure 1. (20) Oviedo, J.; San Miguel, M. A.; Sanz, J. F. J. Chem. Phys. 2004, 121, 7427. (21) Henkelman, G.; Arnaldsson, A.; Jo´nsson, H. Comput. Mater. Sci. 2006, 36, 254. (22) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. J. Comput. Chem. 2007, 28, 899. (23) Michaelides, A.; Hu, P.; Lee, M.-H.; Alavi, A.; King, D. A. Phys. ReV. Lett. 2003, 90, 246103.

JP809680H