Adsorption of Li, Na, K, and Mg Atoms on Amorphous and Crystalline

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Adsorption of Li, Na, K and Mg Atoms on Amorphous and Crystalline Silica Bilayers on Ru(0001): A DFT Study Philomena Schlexer, Livia Giordano, and Gianfranco Pacchioni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp504746c • Publication Date (Web): 26 Jun 2014 Downloaded from http://pubs.acs.org on July 1, 2014

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The Journal of Physical Chemistry

Adsorption of Li, Na, K and Mg Atoms on Amorphous and Crystalline Silica Bilayers on Ru(0001): A DFT Study Philomena Schlexer, Livia Giordano, and Gianfranco Pacchioni∗ Dipartimento di Scienza dei Materiali, Universit` a degli Studi Milano-Bicocca, Via Cozzi 55 - 20125 Milano, Italy E-mail: [email protected]

Keywords: Silica, Bilayer, Zeolites, Alkali Metals, Amorphous Abstract The interaction of Li, Na, K and Mg atoms with crystalline and amorphous silica bilayers grown on Ru(0001) supports has been studied by performing DFT calculations. (Si4 O6 H4 )n clusters have been chosen to represent different ring sizes (n=4-8) of amorphous silica bilayers. We found that on unsupported silica films the alkali metal atoms adsorb as neutral entities with weak adsorption energies, whereas the diffusion into the cage is not thermodynamically favorable. Interaction of Na with Al-doped silica cages has also been investigated. The presence of this Al-dopant enhances the strength of the metal-framework interaction changing completely the bonding mechanism which is dominated by charge transfer contributions. Na and Mg interacting with a crystalline silica bilayer on a 3O(2 × 2)/Ru(0001) surface adsorb preferentially at the interface between the support and the silica film with high adsorption energies and transfer their valence electrons to the Ru metal. ∗

To whom correspondence should be addressed

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Introduction Oxide thin films are of great technological importance as insulating layers in integrated circuits or as supports for metal nanoparticles in sensors and catalysis. 1–4 Ultra-thin oxide films exhibit special features, which emerge with decreasing film thickness. 5 The surface structure can be modified in a controlled manner by the proper choice of the supporting material. 3,6 The support can influence the growth direction of the oxide thin film and can interact with adsorbates, which are deposited on the thin film, and so influence their chemistry. 7 Among crystalline ultra-thin films, silicon dioxide has been intensively investigated in the recent past. 4,8–10 Silica mono- and bilayers are also denoted as silicatene, 11 the silica analogs of graphene. Silica monolayers consist of SiO4 tetrahedra, which are bound together by 3 of 4 oxygen corners. 12 A freestanding monolayer would exhibit an unpaired electron at each of the residual oxygen atoms. Therefore, binding to the support can be realized by a polar chemical bond (chemisorption), as in the silica monolayer on Mo(112). 8,13 On the contrary, silica bilayers (double-layer silicatene) exhibit closed shell structures. 14 Here, the two layers of SiO4 tetrahedra share the “residual” oxygen atom, so that a mirror plane emerges. Bilayers therefore bind to supporting materials by dispersion forces. 1 Ultra-thin silica films can be grown on different metal supports. On Molybdenum, silica forms a monolayer. 13,15 On Ruthenium, 14 Palladium 16 and Platinum 6 crystalline and amorphous bilayers of silica can be formed. As supporting material, 3O(2 × 2)/Ru(0001) was found by Wlodarczyk et al. 1 to be of particular interest. The formation of an oxygen superstructure is favorable under common experimental conditions. 1 The 3O(2 × 2) oxygen superstructure consists of 6 oxygen atoms in an 2 × 2 Ru surface supercell. Amorphous silica bilayers exhibit a variety of pore sizes, whereas crystalline silica only shows rings and pores of the same size. 12,14,17,18 The recently discovered silica bilayer films represent two-dimensional models of porous materials and in particular of zeolites, 19–21 opening the opportunity to study with surface science methods the adsorption properties of these systems. 22 A special characteristic of 2


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zeolites is the capability to incorporate in the nanopores atoms and molecules, and to adsorb large amounts of species from the gas-phase or the liquid-phase. Zeolites also contain metal cations that act as compensating charges for the presence of the Al atoms in the framework. 20,22 For this reason zeolites are largely used for ion-exchange applications. Incorporation and stabilization of metal atoms and metal cations in the cages of the supported bilayer silica films is therefore of special interest for the comparison of these new materials with bulk porous structures. The purpose of this study is to verify if alkali or alkaline-earth metal atoms or ions can be stabilized in the cages of the silica bilayer that represent two-dimensional models of zeolites. In this study we have considered the adsorption properties of Li, Na, K and Mg atoms on unsupported and supported silica bilayers. The role of Si substitution by Al will also be considered. We have used simplified, yet efficient methods to represent the different pore sizes of the amorphous phase, and we have compared the nature of the bare silica films with that of the silica film deposited on oxygen covered 3O(2 × 2)/Ru(0001).

Computational details Spin polarized periodic Density Functional Theory (DFT) calculations have been performed using the Vienna ab Initio Simulation Package (VASP). 23–26 Generalized gradient approximations (GGA) for the exchange-correlation functional were applied within the PBE (Perdew, Burke, Ernzerhof) formulation. 27,28 To describe electron-ion interactions, the projector augmented wave (PAW) method was used. 29,30 Wave functions were expanded in the plane wave basis up to a kinetic energy of 400 eV. Calculations included DFT-D2 Van-der-Waals energy correction, as developed by Grimme et al. 31,32 and we used the blocked Davidson iteration scheme. 33,34 Selected ions were allowed to relax until ionic forces are smaller than |0.01| eV/˚ A. Unit cell parameters were not allowed to change during structure optimizations. To model the rings and cages of silica bilayers, we used finite (Si4 O6 H4 )n clusters, where

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n=4-8. In the following we will refer to these clusters as n-member rings. Atomic positions of terminating groups (H2 Si−O−SiH2 ) were held fixed during structure optimizations. As starting structure, we used fully optimized silica cages. Gamma-point calculations were done ˚3 , where a = 20.00 ˚ with a unit cell of (a × a × 17.85) A A for n=4-6, a = 24.00 ˚ A for n=7 and a = 28.00 ˚ A for n=8. The crystalline system SiO2 /3O(2 × 2)/Ru(0001) was chosen to represent supported silica bilayers. 1 Metals (Na, Mg) interacting with supported bilayers were investigated. The Ru(0001) slab was represented by five Ru layers, which reproduce well the band structure of bulk Ru, 35 at the optimized bulk Ru lattice parameters (a = 2.70 ˚ A and c = 4.26 ˚ A). During structure optimizations of SiO2 /3O(2 × 2)/Ru(0001), ionic positions of the two lowest Ru layers were held fixed. The slabs were separated by a distance of more than 12 ˚ A of vacuum and a dipole correction has been added in order to eliminate the interaction between the slabs. For the adsorption of metal atoms, a (2 × 2) SiO2 supercell was employed, corresponding to a (4 × 4) Ru(0001) supercell. A (3 × 3 × 1) k-point mesh set was set for structure optimizations and a (5 × 5 × 1) k-point set for further electronic structure investigations. A diffusion barrier was estimated for Na on SiO2 /3O(2 × 2)/Ru(0001) (see below). Adsorption energies Eads with respect to adsorbate atoms in the gas phase have been computed. Negative values correspond to exothermic adsorption processes. Atomic charges have been estimated with the Bader decomposition scheme. 36–38 Furthermore, work function changes (∆Φ) were calculated as ∆Φ = Φjoint − Φref , where ∆Φ is the work function change of the joint system with respect to the reference system, whereby the joint system is M/SiO2 /3O(2 × 2)/Ru(0001) and the reference is SiO2 /3O(2 × 2)/Ru(0001).

Results and discussion In the first part of this study, adsorption of metal atoms (Li, Na, K and Mg) and ions (Li+ and Na+ ) on free standing silica rings is considered. For Na, also rings with an Al

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impurity 20 are treated. In the second part, we present the adsorption properties of Na and Mg on SiO2 /3O(2 × 2)/Ru(0001).

Metal atoms adsorption on silica cages To represent the rings of amorphous silica bilayers, a cluster approach was used. Fig. 1 shows the case of Na interacting with silica rings of different sizes. The starting positions for the optimization of metal atoms were above the silica surface or inside the cage. The atomic arrangements of Li, K or Mg interacting with the silica cages are similar as for Na. Adsorption energies Eads are reported in Table 1 together with metal-oxygen (M-O) distances and the coordination number (CN), which is defined as the number of the closest oxygen atoms with a similar distance to the metal center. In general, the metals keep a central position both on the surface and inside the cages (interstitial positions). However, for larger ring sizes, Li and Na move to one side of the silica cages, as shown for Na above the 7-member ring in Fig. 1. For Li, this displacement occurs already for n=6 and 7. For K and Mg no displacement occurs. A similar behavior is observed for metals positioned inside the cages. Here, the metals remain at the same vertical height of the oxygen layer that connects the two silica monolayers. Spin density plots of adsorbed metals and the density of states (DOS) indicate that no electron transfer occurs from adsorbed metals to silica. Bader charges of the metal centers are positive, about +0.5 |e|. This is not in contradiction with the previous statement since the ns electron cloud is strongly polarized away from the surface. The Bader charge in this case does not reflect the real electronic structure of the adsorbed or incorporated species, which remains atomic-like, with a single electron occupying the outer ns level of Li, Na or K. This effect has been discussed in detail for the case of alkali metal atoms adsorbed on the MgO surface. 39,40 The direct comparison of measured and computed hyperfine coupling constants clearly shows the limits of population analysis when adsorbed atoms are strongly polarized. Also Mg stays atomic-like. The bonding arises therefore from polarization and 5



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dispersion contributions. For all metals considered, adsorption outside the cages is clearly preferred compared to interstitial sites, independent of the size of the rings, Table 1. On the surface of a 6-member ring, the adsorption energies are -0.45 eV (Li), -0.37 eV (Na) and -0.53 eV (K), Table 1. The interaction is dominated by dispersion forces. This is demonstrated for instance by the Na case where the interaction is repulsive by +0.2 eV without VdW forces and becomes -0.37 eV after inclusion of VdW interactions. Mg, adsorption energies are very small, going from -0.08 eV for the 5-member ring to -0.10 eV for the 7-member ring, Table 1. In several cases the inclusion of the metal atom in the silica cage results in positive adsorption energies (indicating an unbound complex). The highest value is observed for Mg with an adsorption energy of +1.62 eV. This is clearly due to the repulsive effects originating when the atom is forced into the small cavity of the 6-member ring. In fact, as the size of the rings increases also the stability of the atoms inside the cages increases (in absolute value) and the adsorption becomes exothermic. But even for the largest cages considered, n= 7, 8, adsorption on the surface is preferred for all metals. By increasing the size of the ring the adsorption energy tends to increase because the metal atom can reduce the distance to the surface oxygen atoms and so achieve a better coordination environment. As a result, Na stays well above the surface on a 4-member silica ring, while it is in the surface plane for a 7-member ring, Fig. 1. The adsorption energy does not follow a regular trend going from Li, to Na and to K. In fact, it is largest for K, and smallest for Na, with Li in intermediate position, Table 1. This is the result of two different contributions to the bonding. On one side we have the atomic polarizability which is largest for heavier atoms (K > Na > Li); on the other side we have the atomic dimensions that favor a closer interaction and a stronger bond for the lighter atoms (Li > Na > K). The result is that while Li and K adsorb with energies close to half an eV, the highest adsorption energy for Na is 0.37 eV (on the 6-member ring). When we consider adsorption inside the cages, the largest value is found for Li in the

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7-member ring, with an adsorption energy of -0.33 eV. As we mentioned above, the tendency of the metal atom is to move towards the walls of the cage, when there is enough space, in order to maximize the interaction with the O atoms of the cage. So far we have considered the adsorption of the metal atoms inside or outside the cages, and we have found that no charge transfer occurs. This is not surprising since silica has a wide band gap and there are no low-lying acceptor states below the conduction band (which is very high in energy, close to the vacuum level) unless defects are present in the structure. In order to study this possibility, we have considered a model of doped silica where a Si atom is replaced by an Al atom. The position of the aluminum defect with respect to Na is indicated by the black arrow in Fig. 1 for n=4. We chose to place the Al dopant far from the silica ring in order to avoid a direct interaction with the alkali metal. For larger cages, the aluminum position is analogous. The presence of Al, trivalent, in the silica structure creates a hole in the silica O 2p valence band. Electron transfer from the alkali metal atom to this acceptor state is very favorable and, therefore, a completely different interaction occurs in this case. This interaction has been studied in detail only for Na which has adsorption properties intermediate between those of Li and K (see above). The interaction of Na with the Al-doped silica is one order of magnitude stronger than in the undoped structure, Table 1. The adsorption energy goes from -0.3/-0.4 eV (defect-free silica) to nearly -4 eV (Al-doped silica). This is due to the fact that on Al-doped silica Na becomes Na+ (Bader charge +0.9 |e|) and a strong electrostatic interaction takes place between the metal cation and the negatively charged silica support. However, this is not sufficient to change the order of stability of the adsorption sites, surface and interstitial. A stable configuration is found for Na+ trapped inside a 5-member ring cage where the adsorption energy, -3.39 eV, is substantial. However, this is still 0.4 eV less stable than adsorption on the surface, Table 1, indicating that steric repulsion prevails over electrostatic interaction. Na+ inside 6- and 7-member rings diffuses out of the cage to approach the Al

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dopant in the second layer of the film, indicating that spontaneous diffusion through these large rings is possible and barrier-less.

Figure 1: Sodium atom interacting with n-member rings, where n= 4 to 7, of a silica bilayer. Table 1 also contains information about M-O distances. For Li, an increasing M-O distance can be observed with increasing ring size, until adsorption in an asymmetric position occurs. This tendency is verified both inside the cage and on the surface. After displacement towards the O atoms of the silica ring or of the cage, the distance stays more or less constant. Not surprisingly, the trend in M-O distances follows the atomic dimensions (K > Na > Li) for all ring sizes. Considering Na or K above the silica rings, the distance to oxygen atoms stays approximately constant by increasing the ring size. The interaction of alkali ions (Li+ , Na+ ) with silica rings (n = 5-7) has also been investigated using a uniform background of charge to compare the results with those obtained with an Al impurity, see above. As a reference for the adsorption energy, we calculated the energy of M+ atoms in an empty unit cell. In general, it is confirmed that the adsorption energies of the cations are much bigger than for the neutral atoms. For Li+ the adsorption energy goes from -3.0 eV (7-member ring) to -3.8 eV (5-member ring); for Na+ the corresponding values are -2.7 eV and -3.8 eV, respectively. Adsorption above the silica rings (in central position) is preferred compared 8


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Table 1: Adsorption Energies, average metal-oxygen distances, coordination numbers (CN) and Bader charges of Li, Na, K and Mg interacting with silica rings of different size n and composition. Al-(Si4 O6 H4 )n denotes rings with Al-impurity. (Si4 O6 H4 )n Metal n surface 4 5 6 7 cavity 4 5 6 7 8

Eads eV

Li dLi-O ˚ A

Eads eV

Na dNa-O ˚ A

Eads eV

Al-(Si4 O6 H4 )n K dK-O ˚ A

Eads eV

Mg dMg-O ˚ A

Eads eV

Na dNa-O ˚ A 2.48 2.48 2.62 2.62

-0.43 -0.48 -0.45 -0.45

2.27 2.31 2.54 2.53

(4) (5) (5) (4)

-0.20 -0.30 -0.37 -0.34

2.75 2.71 2.71 2.79

(4) (5) (6) (4)

-0.25 -0.36 -0.53 -0.58

3.15 3.01 2.99 3.11

(4) (5) (6) (7)

— -0.08 -0.09 -0.10

— 3.84 (5) 3.84 (6) 3.91 (7)

-3.42 -3.79 -4.05 -3.91

(4) (5) (6) (4)

0.11 0.06 -0.18 -0.33 —

2.06 (4) 2.42 (5) 2.46 (4) 2.41 (4) —

— 0.51 0.04 -0.13 -0.19

— 2.53 (5) 2.99 (6) 2.89 (2) 3.85 (6)

— 1.64 0.13 -0.14 —

— 2.66 (5) 2.98 (6) 3.52 (7) —

— 1.62 0.31 -0.08 —

— 2.73 (5) 3.07 (6) 3.50 (7) —

-1.74 2.25 (4) -3.39 2.49 (5) —(a)— —(a)— — —

(a) In these cases, the Na atom diffuses spontaneously outside the silica cage to the direction, where the Al-defect is (see Fig. 1).

to the adsorption inside the cage. For Li+ , adsorption on the 5-member rings is preferred, while for Na+ the 6-member ring is the most stable one. The optimal structures are similar to those obtained for neutral atoms, while the M-O distances are slightly shorter in the case of the M+ ions (not shown for brevity). To summarize this part, we have found that Li, Na, K and Mg adsorb as neutral species on the surface of silica bilayers, with a bonding mechanism which is dominated by polarization and dispersion contributions. Adsorption inside the pores of the film is always unfavorable compared to adsorption on the surface. In a second set of calculations an Al defect is introduced in the structure which results in the spontaneous formation of Na+ from the adsorption of neutral Na. Still, the Na+ ion prefers to bind on the surface of the silica film, and not inside the cages. In the following section, the interaction of Na and Mg with supported silica bilayers is considered.

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Supported silica bilayers Here we consider the interaction of Na and Mg atoms with SiO2 /3O(2 × 2)/Ru(0001) films. Both amorphous and crystalline double-layer silicatene has been obtained in the experiments, 8,14 but here we consider only the crystalline phase which consists of silica 6-member rings. Three initial adsorption positions have been considered: one with the metal above the silica bilayer (surface), the second with the metal inside the bilayer (interstitial) and the third one with the metal atom at the interface between the silica bilayer and the Ruthenium support with adsorbed O atoms. The interstitial position is unstable for Na, which spontaneously diffuses to the interface. This is the same behavior found for the unsupported films in the presence of Al-dopants. Adsorption inside the bilayer is not favorable. For Mg, only the surface and interface positions were considered, since the position inside the unsupported silica cages is highly unfavorable (see above) and Na is unstable in the interstitial side of the supported bilayer. Optimized structures are shown Fig. 2.

Figure 2: Na and Mg interacting with SiO2 /3O(2 × 2)/Ru(0001). a)-b) Na at the surface of the silica bilayer, c)-d) Na at the interface position between support and silica bilayer. e)-f) Mg at the surface of the silica bilayer, g)-h) Mg at the interface position.

The adsorption energy of Na on the surface of SiO2 /3O(2 × 2)/Ru(0001) is -2.66 eV, Table 2. This is much larger than the adsorption energy of Na on the unsupported 6member silica ring (about -0.4 eV, Table 1, Fig. 3) indicating a completely different bonding 10


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mechanism. Even larger is the adsorption energy at the interface, -4.16 eV, which indicates that stabilization of the adsorbed Na atom below the silica layer is expected, provided that there is enough thermal energy in the system to overcome the barrier for diffusion from the surface to the cage and then to the interface. The large amount of energy released by the adsorption of a single Na atom will be dissipated mostly via excitation of phonons in the Ru metal. The barrier for penetration has been estimated by varying the vertical position of the Na atom above the ring center and by re-optimizing the SiO2 /3O(2 × 2)/Ru(0001) support for each fixed z(Na) position. In this way a section of the potential energy profile has been obtained, Fig. 3. The local minimum for adsorption on the surface is taken as reference of zero energy. The energy increases when Na goes through the 6-member ring of the first silica layer, but the energy maximum is found when Na is about 0.6 ˚ A below the plane of the Si atoms. In the center of the cavity, at the same height of the O atoms connecting to two silica layers, the energy is about 0.3 eV higher than on the surface. From this position, the potential energy curve starts to decrease very rapidly, indicating that the Na atom feels the presence of the Ru surface and its attractive potential. The attraction is strong enough that there is no barrier for diffusion through the 6member ring of the second silica layer. The potential then becomes very deep, being the bonding at the interface of more than 4 eV. The computed barrier, about 0.4 eV, does not correspond to a real transition state but provides a reasonable estimation of the energy required for diffusion through the film. It can be overcome at relatively low temperatures. For Na adsorbed on larger rings of the amorphous silica film this barrier is expected to disappear completely. A diffusion to the interface between silica thin film and metal support was also observed for Li on SiO2 /Mo(112) 41 and for Pd on SiO2 /Ru(0001). 42

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Figure 3: Potential energy curve for the diffusion of a Na atom into the 6-member rings of the SiO2 /3O(2 × 2)/Ru(0001) films. The zero of energy corresponds to Na adsorbed on the surface of the double-layer silicatene film. The vertical bars indicate the position of the O atoms (red) and of the Si atoms (blue) in the silica bilayer (see also Fig. 2). Differently from the case of unsupported defect-free silica bilayers, here an electron transfer is possible from the Na or Mg 3s states to the Fermi level of the support. Indeed, for Na adsorbed on the surface or at the interface, we find a net charge on Na of + 0.9 |e|, indicating full ionization. The occurrence of a charge transfer is further demonstrated by the projected DOS, Fig. 4. Considering both Na positions (surface/interface), Na related states are above the Fermi level indicating a full ionization of Na to Na+ . For Mg at the interface the Bader charge is + 1.7 |e|, which indicates a transfer of both valence electrons. For Mg above the silica bilayer (surface adsorption), the Bader charge is smaller (+ 0.7 |e|), indicating that only one electron is transferred to the support. This means that the formation of an adsorbed Mg2+ ion is unfavorable. The formation of a doubly charged species, in fact, can only occur when this is stabilized by a strong electrostatic (Madelung) potential or by the screening provided by the electrons of the metal support (interface bonding). On the surface of the silica film the electrostatic potential is small and 12


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the tails of the Ru wave function cannot efficiently screen the double charge entity. For this reason, only one electron is donated, and the charge transfer is partial. For Mg at the interface position the 3s valence states are above the Fermi level indicating a full ionization. On the contrary, in the surface case, there is a significant contribution from the Mg 3s valence orbital at the Fermi level. This is consistent with a partial ionization. The final system is spin polarized and the spin density plot (see inset in Fig. 4e) shows that one unpaired electron is located at the Mg+ ion. However, a non-spin polarized solution is only 0.07 eV higher in energy, suggesting strong coupling of the Mg 3s1 valence state with the SiO2 /3O(2 × 2)/Ru(0001) system. Notice that van-der-Waals contributions to adsorption energies are similar for all considered systems and are minor compared to electrostatic and polarization contributions. Table 2: Adsorption Energies (Eads ), work-function changes (∆Φ), metal-oxygen distances (dM-O ), coordination numbers (CN) and Bader charges (q(M)) of Na and Mg interacting with the supported silica bilayers. Eads eV

∆Φ eV

dSilica M-O (CN) ˚ A

dSupport (CN) M-O ˚ A

q(M) e

Na Surface Interface

-2.66 -4.16

-3.26 -1.33

2.66 (6) 2.71 (6)

— 2.34 (2)

0.9 0.9

Mg Surface Interface

-0.56 -3.94

-2.15 -1.11

2.97 (6) 2.25 (2)

— 2.00 (2)

0.7 1.7

In the surface adsorption site, Na stays approximately in the central position, equidistant from the O atoms of the ring and at the same height of the surface atoms (see Fig. 2). A slight displacement of Na/Mg from this central position is found for adsorption at the SiO2 /Ru(0001) interface. This displacement is even larger in the Mg case. The neighboring oxygens move slightly towards the metal ion to screen its positive charge. However, the silica film and the Ru support do not undergo major geometrical changes. The distance of the 13



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bilayer and the support is around 3 ˚ A and does not change significantly, when Na or Mg are adsorbed on the surface or at the interface.

Figure 4: Projected density of states. In all cases, Ru states are scaled with a factor of 0.4. a) Na adsorbed at SiO2 /3O(2 × 2)/Ru(0001) the interface and b) on the surface. c) PDOS for Mg at the interface. d) PDOS for Mg the surface; here, Mg states are scaled with a factor 10.0. e) Spin density plot of Mg on the film surface including 99.4 % of the spin density. Of course, the formation of a positive ad-atom has direct consequences on the work function of the system, as shown in Table 2. As expected for a positive ion adsorbed on the surface, the work function (Φ) decreases compared to the reference system. The change in work function is substantial and is directly correlated to the ion position and charge. A positive charge above the surface of a conductor located in (0, 0, z) results in a polarization response and in the formation of an image charge below the conductor surface. The larger is z, the larger the charge, the larger is the surface dipole resulting from atomic adsorption. When Na or Mg are adsorbed on the surface of the silica film (less stable adsorption side), the distance from the Ru metal surface is larger than for adsorption at the interface (more stable) and the corresponding surface dipole and work function change are also larger. This is fully consistent with the occurrence of a net charge transfer from Na and Mg 14


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to Ru. When Na and Mg atoms are adsorbed at the more stable interface positions ∆Φ is still quite substantial, being of the order of 1 eV or more, Table 1. A reduction of the work function of this amount can result in important modifications of the adsorption properties and can turn the inert silicatene film into an active surface. This effect has been observed for SiO2 /Mo(112) monolayers with adsorbed Li atoms. 43,44 The Li atoms have the right size to penetrate into the pores and diffuse at the interface of the SiO2 /Mo(112) film forming adsorbed Li+ . The consequent reduction of work function has been used to promote the nucleation and growth of small Au clusters on the surface of the SiO2 /Mo(112) films. The Au clusters became negatively charged due to the reduction of the SiO2 /Mo(112) work function by Li doping. Without doping with alkali metals, no stabilization of Au clusters is possible. In principle, a similar effect could be obtained with the SiO2 /3O(2x2)/Ru(0001) bilayers once Na or Mg ions have been added and stabilized at the metal/oxide interface.

Conclusions We have investigated the interaction of Li, Na, K and Mg atoms with unsupported and supported silica bilayers. These systems have attracted a considerable interest as twodimensional models of porous materials and in particular of zeolites. 21 Since zeolites are widely used for ion exchange due to their ability to incorporate large amounts of metal cations in their cavities, it is interesting to study the possibility to adsorb and stabilize alkali metals in the cages of silica bilayers. In the first part of the study we have considered simplified models of the complex structure of amorphous silica bilayers. These vitreous films exhibit the presence of silica rings and cages of various size. Here, we have modeled rings containing from four to eight members using finite cluster models terminated by H atoms. The results show that, in absence of point defects, the silica films are rather inert and the interaction of Li, Na, K and Mg is

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dominated by dispersion and polarization contributions. The atoms keep their configuration with one electron (or two for Mg) in the outer ns orbital. The adsorption on the surface of the silica layer is always preferred compared to incorporation into the cages. This is due to the fact that steric repulsion destabilizes this adsorption configuration. In a second set of calculations low-lying acceptor levels have been introduced in the unsupported silica film by doping it with Al. The adsorption of Na results in a completely different bonding mechanism since Na becomes ionized and transfers its valence electron to the Al defect. The interaction energies increase by an order of magnitude but still adsorption of Na on the surface of the silica bilayer is preferred with respect to incorporation into the cages. The interaction of alkali metals with an unsupported Al-doped silica film is similar to that with the supported, defect free system. Here we have considered a realistic model of SiO2 /3O(2 × 2)/Ru(0001), the system used in experiments. Only the crystalline phase has been considered, where 6-member rings are present. Even in absence of defects, the presence of the Ru metal support introduces new states in the band gap of silica which can act as acceptor levels for the valence electrons of the alkali metal atoms. Indeed, Na adsorption on SiO2 /3O(2 × 2)/Ru(0001) results in the formation of a Na+ cation. This adsorbs on the surface of the silica bilayer with a strong bond (2.7 eV). On the contrary, Mg adsorbs on the surface with a rather small bond strength (around 0.6 eV), forming an Mg+ ion. In this surface configuration, atoms are adsorbed near a central position of the silica ring, at close contact with the O atoms. This is the position assumed by metal cations adsorbed inside zeolite frameworks. 45 However, this is not the absolute minimum. In the case of Na, a small barrier of 0.4 eV separates this from the global minimum which is at the interface between the silica bilayer and the Ru surface. Smaller barrier or no barrier at all are expected for rings with more than 6 members. No stabilization of the Na+ or Mgn+ ion inside the cage is found. At the interface, both Na and Mg donate their valence electrons to the Ru metal and bind very strongly, by about 4 eV. Therefore, it is

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expected that upon deposition of Na or Mg atoms from the gas-phase, ions will form and diffuse on the surface but with a given probability to penetrate into the rings and diffuse below the silica layer. This behavior is reminiscent of that of other atoms with appropriate size like Li and Pd deposited on SiO2 /Mo(112) monolayers and SiO2 /3O(2 × 2)/Ru(0001) bilayers. 42,46,47 The silica bilayer acts therefore as an “inert” nanoporous membrane that allows a selective diffusion of atomic species to the surface of the metal support.

Acknowledgement We thank Prof. H. J. Freund, Dr. M. Heyde and Dr. S. Shaikhutdinov for several useful discussions. Financial support from the European Marie Curie Project CATSENSE and from the Italian MIUR (FIRB Project RBAP115AYN “Oxides at the nanoscale: multifunctionality and applications”) is gratefully acknowledged. We also thank the COST Action CM1104 “Reducible oxide chemistry, structure and functions” and the CINECA grant under the LISA initiative for the availability of high performance computing resources and support.

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Graphical TOC Entry

Interaction of Li, Na, K and Mg with supported and unsupported silica bilayers.

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Adsorption of Li, Na, K, and Mg Atoms on Amorphous and Crystalline

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