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J. Phys. Chem. C 2008, 112, 19616–19619
Adsorption of 3d Transition Elements on a TiO2(110) Surface Abu Md. Asaduzzaman*,†,‡ and Peter Kru¨ger† Institut Carnot de Bourgogne, UMR 5209, UniVersite´ de Bourgogne-CNRS, 21078 Dijon, France, and Department of Chemistry, UniVersity of Manitoba, Winnipeg, R3T 2N2, Canada ReceiVed: August 07, 2008; ReVised Manuscript ReceiVed: September 26, 2008
A first-principles study on the adsorption of 3d transition metal atoms on a stoichiometric TiO2(110) surface is reported. For all 3d elements except Cu, the most stable on-surface adsorption site is a site where the adatom binds to two twofold and one threefold surface oxygen atoms. For Ti, V, and Cr, however, a subsurface site, where the adatom substitutes a sixfold Ti atom, is more stable. The adatoms are oxidized in all cases. The charge transfer to the substrate is larger for the substitutional site than for the on-surface adsorption sites and decreases with atomic number along the 3d series. The relative stabilities of the adsorption sites are discussed in terms of the charge state of the adatoms, the electronegativity of their neighbors, and the metal-oxygen bond enthalpies of the 3d elements. The results indicate that, at submonolayer coverage, the early 3d elements wet the surface and may diffuse into the substrate, whereas the late 3d elements tend to form large three-dimensional clusters. I. Introduction Heterointerfaces are important components in microstructures of advanced materials such as composites, coatings, and heterogeneous catalysts. Their interfacial properties often determine overall performance and reliability of the materials systems. Among the heterointerfaces, combinations of a metal with a metal oxide have been extensively studied experimentally and theoretically,1,2 in order to obtain detailed knowledge of interaction across the interfaces. However, it is often difficult to describe the interaction of metal and metal oxide simply by a particular chemical bonding state such as ionic or covalent, because the two constituent materials have different electronic structures. Moreover, differences in crystal structure and lattice parameter may also affect the interfacial bonding state. Therefore, it is important to unravel the general bonding mechanisms that take place at metal/oxide interfaces but also the finer details which are due to the particular choice of the constituents. Over the past few years, several complementary approaches have been used to gain further insight into the microscopic structure of these interfaces. Experimentally, either thin oxide films are grown on a metallic support3 or, vice versa, metal atoms are deposited on an oxide surface where they aggregate to clusters or form continuous films.4 In the latter case of a metal deposited on an oxide, a few experimental studies showed that the adsorbate atoms often diffuse into the bulk at elevated temperature.5 Adsorption, diffusion, and cluster growth are governed by the microscopic interactions at the interface, i.e., the chemical bonding between metal adsorbate and the oxide surface. These interactions are influenced by a variety of parameters, such as the oxygen affinity and the number of valence electrons of the deposited metal, the presence of defects and impurities at the surface, or the existence of mixed oxide phases. The oxygen affinity of the metal adsorbate is the most important parameter in the case of a stoichiometric surface. With increasing oxygen * Corresponding author. E-mail:
[email protected]. † Universite´ de Bourgogne. ‡ University of Manitoba.
affinity a transition from a three- to a two-dimensional growth mode can be expected together with an enhanced tendency for subsurface diffusion. Indeed, when the oxygen affinity of the metal goes up, the metal/oxide interface energy goes down. When the interface free energy is below a critical value, surface wetting and two-dimensional growth becomes thermodynamically favorable.6 The second parameter is the number of valence electrons, which is related but not equivalent to the oxygen affinity. The number of valence electrons is directly linked to the possible oxidation states and the possible number of metal-oxygen bonds, both of which strongly determine the structure and energetics of the adsorbed atom. For a deeper understanding of this dependence, comparative studies for adsorbates from a whole series of elements are required. Such a systematic study is presented here for 3d transition elements adsorbed on a TiO2(110) substrate. When going up in the 3d series from Sc to Cu, the number of 3d valence electrons increases from 1 to 9 and the oxygen affinity, measured by the metal-oxygen bond enthalpy, decreases from 681 to 269 kJ/ mol.7 The rutile TiO2(110) surface is considered as a model system because its structural and chemical properties are much better known than those of most other oxide surfaces.6 Many works on metal adsorption and cluster growth on TiO2(110) have been reported,8-10 but comparative studies for different adsorbate elements are lacking. Experimentally it was found that certain transition metals diffuse into the substrate upon adsorption on TiO2(110).11-13 In our earlier theoretical studies,14,15 we have confirmed this for Mo and V adsorbates, by showing that there is a subsurface site which is more stable than any on-surface site; the adsorbed atom can reach the subsurface site if it overcomes a rather low potential barrier. The aim of the present paper is to investigate the effect of the metal atom valency on adsorption and subsurface diffusion. To this end we have carried out a systematic first-principles study on single atoms on TiO2(110) for the whole 3d transition metal series. We consider three different sites for the metal adatom. Two on-surface adsorption sites (labeled 1 and 2, see Figure 1) and one substitutional site (labeled S, see Figure 2b).
10.1021/jp807060x CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
Adsorption of 3d Transition Elements on TiO2(110)
Figure 1. Top view of the TiO2(110) surface showing the (2 × 4) supercell used in the calculations. Red and blue balls represent oxygen and titanium atoms, respectively. O2, O3, Ti5, and Ti6 stand for bridging and in-plane oxygen and fivefold and sixfold titanium, respectively. The two most stable on-surface adsorption sites are indicated with labels 1 and 2.
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19617
Figure 3. Displacement of sixfold Ti atom upon adsorption of 3d elements at site 1.
TABLE 1: Relative Energies ∆E (in eV) of Adsorbed 3d Atoms ∆E
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
site 1 -3.65 -2.57 -0.97 0.47 1.18 1.21 1.81 1.59 1.06 site 2 -2.77 -1.48 0.69 1.97 1.98 1.93 2.01 1.64 0.98 site S -3.60 -3.58 -1.63 0.11 1.16 1.67 2.62 3.31
Figure 2. Perspective view (approximately along [001]) of the metal/ TiO2(110) system. Black and gray balls represent oxygen and titanium atoms, respectively. The adsorption site 1 (a) and the subsurface substitutional site (b) are shown. The adsorbed atom is indicated as “M” with an arrow and the interstitial Ti atom with “I”. The substitutional site “S” means adsorbed atom at position M and surface Ti atom at position I. Note that both the adsorbed atom and the displaced/interstitial Ti atom share the same crystallographic (001) plane.
In the case of site S, the adatom substitutes a sixfold Ti atom in the first surface layer while the replaced Ti atom moves to an interstitial site under the surface. We have limited ourselves to these three sites, because they have been found to be the two most stable on-surface sites (1 and 2) or substitutional sites (S) in our previous studies on V and Mo.14,15 Indeed, all other high-symmetry on-surface adsorption sites are at least 1.7 eV higher in energy than sites 1 and 2 (both for Mo and V) and they are not local minima (as are sites 1 and 2) but maxima or saddle points of the potential energy surface. The paper is organized as follows. The computational method is briefly described in section II. In the result section III, we first investigate adsorption of a single 3d metal atom at the onsurface sites 1 and 2. We discuss the structural and electronic differences between the two sites and their relative stability as a function of the 3d element. Then we investigate the subsurface site S in the same manner. In the final section IV, we draw some general conclusions. II. Computational Procedure All calculations were performed using the plane-wave code VASP (Vienna ab initio simulation package)16 within density functional theory and the generalized gradient approximation
“Perdew-Wang 91”. We used ultrasoft pseudopotentials that are provided in the package.17 A cutoff energy of 400 eV was chosen for the plane-wave basis. The systems were modeled in a supercell approach. A (2 × 4) supercell and a slab of nine atomic layers that is separated from its periodic images by vacuum space of 10 Å was used. Atoms from the two lowest atomic layers (the most far away from the adatom) were fixed in their bulk position, and others were allowed to relaxed. The Brillouin zone was sampled using a 2 × 2 × 1 Monkhorst-Pack mesh. All calculations were done without spin polarization. In earlier studies on Mo and V adsorption on TiO2,14,15 we have found that spin polarization has only a very small effect on atomic positions and structural energies and could therefore be neglected for this type of adsorption studies. III. Results and Discussion The on-surface adsorption sites 1 and 2 are shown in Figure 1. Figure 2 provides side views of site 1 (Figure 2a) and of the subsurface substitutional site S (Figure 2b). The calculated energies are listed in Table 1. As in our previous paper on V/TiO215 we give relative energies ∆E ) E - E0, where the reference energy E0 is the sum of the energy of the clean substrate and the energy per atom of the bulk metal.18 ∆E is the difference between the cohesive energy of the bulk metal and the adsorption energy (assuming that the latter two energies are defined as absolute values). The data of Table 1 are shown graphically in Figure 4 along with the metal-oxygen bond enthalpies taken from ref 7. At a glance it can be seen from Figure 4 that ∆E essentially increases as a function of atomic number along the 3d series (except Cu) for all considered adsorption sites 1, 2, and S. The sharpest rise of ∆E occurs between the elements Ti-Cr. Both features are also seen in the metal-oxygen bond enthalpy curve E(MO), and so ∆E and E(MO) have roughly the same dependence along the 3d series (excluding Cu). This observation confirms our expectation that the metal-oxygen bond enthalpy is one of the most important parameters that control the adsorption energetics. We now come to the differences between the adsorption sites and shall first compare the sites 1 and 2. As seen from Table 1 and Figure 4, site 1 is more stable than site 2 for all 3d elements
19618 J. Phys. Chem. C, Vol. 112, No. 49, 2008
Asaduzzaman and Kru¨ger TABLE 2: Charge Transfer ∆Q (in e) of 3d Atoms upon Adsorption at Sites 1 and S
Figure 4. Relative energies, ∆E, for the adsorption sites 1, 2, and S (data of Table 1) along with the (negative) metal-oxygen bond enthalpy E(MO) (taken from ref 7).
except Cu. The energy difference is substantial (0.7-1.7 eV) for Sc-Fe but very small for Co-Cu. The sites 1 and 2 have a similar environment, namely, three oxygen atoms and one titanium atom as neighbors, where we consider as neighbors all atoms within a distance of 3 Å. However, the neighboring atoms themselves have different environments in the two cases. The Ti neighbor of site 1 is of Ti6 type, but that of site 2 is of Ti5 type. Moreover, site 1 has two O2 neighbors and one O3 neighbor, but it is the other way around for site 2. Since an O2 atom has one Ti-O bond less than O3, O2 is more electronegative than O3. The higher electronegativity of O2 implies a greater capability for accepting electrons from the electropositive metal adatom. Thus, the adatom-O2 bonds are stronger than the adatom-O3 bonds. As site 1 has more O2 neighbors than site 2, the overall adatom-oxygen bonding is stronger for site 1. From this argument we expect adsorption site 1 to have a lower energy than site 2, which is indeed observed for all elements except Cu. At each adsorption site, the substrate atoms relax around the adsorbed atom. The largest relaxation is found for the Ti6 atom closest to the adatom at site 1 (Figure 2a). This Ti atom moves away from the adatom. The displacement vector has components along [1j 1j 0] (into the substrate) and along [1j 1 0]. They are plotted in Figure 3 as a function of the 3d element. The displacement along [1j 1j 0] is 0.30-0.35 Å for Sc-Mn adsorption, about 0.20 Å for Fe-Ni, and 0.06 Å for Cu. Along [1j 1 0] we found 0.30-0.50 Å for Sc-Ni and 0.12 Å for Cu. The substrate relaxation is larger for the early transition metals Sc-Mn than for the late 3d elements Fe-Ni, and it is by far smallest for Cu. The metal adatom forms short bonds (∼1.8 ( 0.2 Å) with the three neighboring O anions but keeps a larger distance (2.2-2.7 Å) from the Ti cation as can be expected from metal-oxygen and metal-metal bond lengths in ionic compounds. In order to make short bonds with O and a longer bond with Ti6, the metal adatom moves toward the center of the triangle formed by its three O neighbors and pushes the Ti6 ahead of it (roughly along the direction [1j 1 0]). If the metal-Ti6 pair moves further in the same direction, the system eventually reaches the situation shown in Figure 2b, where the adatom has substituted the Ti6 atom. The latter has then been pushed out of its original O-octahedron and occupies an interstitial site, which also has an octahedral O-coordination. In refs 14 and 15 we have shown that this is the main mechanism for subsurface diffusion of Mo and V on TiO2(110). It provides an explanation
∆Q
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
site 1 site S
2.06 2.29
1.95 2.40
1.77 2.24
1.55 1.99
1.30 1.70
1.16 1.48
1.01 1.34
0.95 1.18
0.75
for the experimental observation of substitutional Mo19 and subsurface diffusion of V/TiO2.11,12 Such a diffusion process from site 1 to site S can obviously only take place if it is energetically favorable, i.e., if the substitutional site S is more stable then on-surface site 1. As seen from Table 1 and Figure 4 site S is indeed more stable than site 1 for Ti, V, and Cr, but site 1 is more stable for the late 3d elements Fe-Cu. For Sc and Mn adatoms, the two sites are practically degenerate. A Cu adatom is completely unstable at site S. It is interesting to note that among the 3d elements, only Ti, V, Cr, and Mn form stable dioxides. This immediately suggests that a Ti ion in TiO2 can more easily be substituted by a Ti, V, Cr, or Mn atom than by any other 3d element. This nicely correlates with our finding that the substitutional site is the most stable site precisely for the elements Ti-Mn (even though in the case of Mn, the energy difference with respect to site 1 is tiny). Looking at Figure 4 one can see that the energy of the substitutional site ∆E(S) keeps increasing rapidly from Mn to Ni, in contrast to both the energy of the on-surface sites ∆E(1, 2) and the metal-oxygen bond enthalpy E(MO), which all stay roughly constant from Mn to Ni. This indicates that the energetics of the substitutional site involves different factors from those of the on-surface sites and that the metal-oxygen enthalpy is not sufficient to explain the 3d element dependence of the substitutional site energy. A possible explanation comes from fact that the 3d adatom is more oxidized at site S than at the on-surface sites. Table 2 lists the calculated Bader charge transfer from the 3d atoms to the substrate upon adsorption at sites 1 and S. In all cases, the adatoms are strongly oxidized with charges ranging from 0.75 to 2.4 e. For a given adsorption site, the charge decreases in a roughly linear way along the 3d series (except for Sc at site S). Assuming purely ionic bonding and fixed atomic positions, the adsorption energy of an ion is proportional to its charge. From this simple model we expect a roughly linear decrease of the adsorption energy along the 3d series and thus a linear increase of ∆E which is indeed essentially observed for site S, see Figure 4. The more the adatoms are oxidized and the less the atomic structure varies over the 3d series, the better this simple model should work. It is easy to see that the model applies better to site S than to site 1. First for each element the adatom has a larger charge at site S than at site 1 (by 0.35 ( 0.12 e), so the assumption of purely ionic bonding applies better to site S than to site 1. Second, for site S the atomic structure does not vary much along the 3d series, but it does more so for site 1. At site S both the adatom and the displaced Ti atom occupy the center of an oxygen octahedra which is a very stable position for a cation. Consequently, at site S the atomic structure is very much constant over the 3d series. At site 1, however, the atomic structure variations are larger. Indeed the substrate atoms relax more strongly for the early 3d elements than for the late ones. These arguments provide an explanation for the observation that, at site S, ∆E scales in a roughly linear way with ∆Q, which in turn gives rise to linear dependence of ∆E(S) with atomic number (except for Sc, where both ∆Q and ∆E deviate from the linear dependence). At site 1 the dependence of ∆E on
Adsorption of 3d Transition Elements on TiO2(110) atomic number is more complex because the simple ionic model applies less well than at site S. Finally some information can be extracted from the sign of the relative energies ∆E. When ∆E > 0, the cohesive energy is larger than the adsorption energy, and the system can lower its total energy if the adatom is removed from the surface and added to a large (bulklike) cluster. From purely energetic considerations, i.e., disregarding any kinetic effects, this means that the adatoms tend to coalesce and form big clusters, even at lowest coverage. Conversely, when ∆E < 0, such clustering is energetically unstable, at least for the very low coverage below about 0.1 monolayers to which the present results apply. Since ∆E(1) < 0 for Sc-V, these elements will wet the surface, and subsurface diffusion may occur for Ti-Cr where ∆E(S) < ∆E(1). For the late 3d elements, however, ∆E is positive for all adsorption sites, and so a wetted surface is unstable against formation of large three-dimensional metal clusters. IV. Conclusions We have carried out a first-principles study on the adsorption of 3d transition elements on a stoichiometric TiO2(110) surface. We have found that most 3d elements (except Cu) are more stable at adsorption site 1 than at site 2, which can be understood from the differences in the electronegativity of the oxygen neighbors. The relative adsorption energies follow the same trend as the metal-oxygen bond enthalpies as a function of atomic number along the 3d series. At adsorption site 1, the sixfolded Ti atom closest to the adatom displays a strong displacement into the substrate, especially for the early 3d elements. Upon adsorption, the 3d atoms become strongly oxidized in all cases. The charge transfer to the substrate is larger for the substitutional site than for the on-surface sites, and it decreases continuously along the 3d series. For the elements Ti-Cr the Ti6 substitutional site is more stable than site 1, and it is the opposite for the elements Fe-Cu. Our results show that for coverages in the 0.1 monolayer range, the early 3d
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19619 elements (Sc-V) wet the surface and may diffuse into the substrate (Ti-Cr). For the late 3d elements, on the contrary, a wetted surface is, from a purely energetic point of view, unstable with respect to the formation of large metallic clusters. Acknowledgment. We thank the Centre de Ressources Informatiques of the Universite´ de Bourgogne for technical assistance and computing time. References and Notes (1) Finnis, M. W. J. Phys.: Condens. Matter 1996, 8, 5811. (2) Ernst, F. Mater. Sci. Eng., R 1995, 14, 97. (3) Meinel, K.; Schindler, K.-M.; Neddermeyer, H. Surf. Sci. 2003, 420, 532–535. (4) Freund, H.-J. Surf. Sci. 2002, 500, 271. (5) Centi, G.; Pinelli, D.; Trifiro´, F. J. Mol. Catal. 1990, 59, 221. (6) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (7) CRC Handbook of Chemistry and Physics 1999 2000, 81st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2000. (8) Pillay, D.; Hwang, G. S. Phys. ReV. B 2005, 72, 205422. (9) Vijay, A.; Mills, G.; Metiu, H. J. Chem. Phys. 2003, 118, 6536. (10) Albaret, T.; Finocchi, F.; Noguera, C.; DeVita, A. Phys. ReV. B 2001, 65, 035402. (11) Agnoli, S.; Castellarin-Cudia, C.; Sambi, M.; Surnev, S.; Ramsey, M. G.; Granozzi, G.; Netzer, F. P. Surf. Sci. 2003, 546, 117. (12) Sambi, M.; Della Negra, M.; Granozzi, G. Thin Solid Films 2001, 400, 26. (13) Chang, Z.; Piligkos, S.; Møller, P. J. Phys. ReV. B 2001, 64, 165410. (14) Asaduzzaman, A. Md.; Kru¨ger, P. Phys. ReV. B 2007, 76, 115412. (15) Asaduzzaman, A. Md.; Kru¨ger, P. J. Phys. Chem. C 2008, 112, 4622. (16) (a) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558; 1994, 49, 14251. (b) Kresse, G.; Hafner, J. Comput. Mater. Sci. 1996, 6, 15. (c) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (17) Kresse, G.; Hafner, J. J. Phys.: Condens. Matter 1994, 6, 8245. (18) The 3d bulk energies were calculated for the non-spin-polarized state and experimental lattice constants. The values are (in eV): Sc-6.133, Ti-7.728, V-8.897, Cr-9.419, Mn-8.687, Fe-7.650, Co-6.769, Ni5.417, Cu-3.696. (19) Domenichini, B.; Rizzi, G. A.; Kru¨ger, P.; Negra, M. D.; Li, Z.; Petukhov, M.; Granozzi, G.; Møller, P. J.; Bourgeois, S. Phys. ReV. B 2006, 73, 245433.
JP807060X
