Au Adsorption on Regular and Defected Thin MgO(100) Films

Using density functional theory we studied systematically a Au atom adsorption on a Mo-supported regular and defected ultrathin MgO film with 1 to 5 M...
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J. Phys. Chem. C 2007, 111, 4319-4327

4319

Au Adsorption on Regular and Defected Thin MgO(100) Films Supported by Mo Karoliina Honkala* and Hannu Ha1 kkinen Nanoscience Center, Departments of Physics and Chemistry, UniVersity of JyVa¨skyla¨, P.O. Box 35, JyVa¨skyla¨ FIN-40014, Finland ReceiVed: October 17, 2006; In Final Form: December 21, 2006

Using density functional theory we studied systematically a Au atom adsorption on a Mo-supported regular and defected ultrathin MgO film with 1 to 5 ML thickness. On Mo(100) grown regular MgO Au prefers to adsorb at a hollow site instead of an O site found for single-crystal MgO. The metal support also enhances Au adsorption energy in agreement with the earlier theoretical results. Adsorption energy decreases with increasing film thickness being, however, even in the case of 5 ML thick MgO much higher than that on single-crystal MgO. The Bader analysis was performed to estimate the spatial distribution of charge in different cases. The results show charge transfer (0.7-0.8 e) to the Au atom, fairly independent of the oxide thickness, and density difference plots display a rich polarization pattern on the oxide upon Au adsorption. The Au atom bound to an oxygen vacancy forms a chemical bond with the defect. In this case, the Au adatom is charged by about 1.2 e, and the charge transfer depends on neither the MgO film thickness nor the presence of the Mo support.

I. Introduction For a number of years metal-oxide interfaces have received intensive, both experimental and theoretical, attention due to their role in applications for catalysis, protective coatings, and thermal and electric insulators.1,2 In particular, size-controlled, oxide-supported metal clusters and nanoparticles have a wide unexplored range of physical and chemical properties and a high potential for chemical, photochemical, and catalytic applications. Understanding the rich interplay between diverse chemical properties of different-sized particles, variations in the particlesupport interactions, ensemble effects, and the possible direct activity of the support is a challenging task but in principle it allows tuning the various factors in order to optimize a “nanocatalytic” process regarding selectivity, sensitivity, and turnover for the desired product channel. In contrast to bulk-Au,3 nanosized gold particles on various oxide materials have been observed to exhibit an enhanced catalytic activity for several reactions,4-6 such as propylene epoxidation,7,8 hydrogenation of unsaturated hydrocarbons,9 water-gas shift reaction,10 and CO oxidation.11-14 Recently it was found that supported ultrathin Au films have high activity toward CO oxidation, comparable to activity for supported Au clusters.15 The most reactive Au nanoparticles have the diameter of a couple of nanometers16 but also tiny Au clusters containing only a few Au atoms show catalytic activity toward CO oxidation.17 Catalytic properties of Au clusters over oxide surfaces are believed to depend on several different factors like charge transfer,18,19 structural effects,15 and support effects,20 just to mention a few. We chose MgO as a support oxide for several reasons. It is structurally simple, which makes it easy to handle computationally. Nanosize21 and tiny17 Au clusters on MgO show activity * Address correspondence to this author. Phone: +358-14-2602609. Fax: +358-14-2604756. E-mail: [email protected].

toward CO oxidation and thin (1-5 ML) MgO films, grown on Mo(100)22 and Ag(100)23 surfaces, are commonly used to make templates to study the properties of the smallest Au clusters. Recently, small Au clusters on single-crystal MgO surfaces have been studied extensively by using density functional theory calculations.24-26 With increasing computer power it has become possible to construct a model system for a Au atom or cluster on a thin MgO film supported by transition metal, which mimics the situation in experiments where thin films are used.27-29 By means of density functional theory calculations Pacchioni and co-workers have shown that Au binding on a regular ultrathin Mo-supported MgO film is stronger than that on single-crystal MgO. They suggest that the increased binding is due to enhanced charging of gold by direct tunneling of electrons from Mo to Au through the ultrathin oxide film.27 This charge transfer to Au is also predicted to take place on a Ag grown MgO film.30 A Au atom binding and charge state have been recently explored experimentally and theoretically also on Cu-supported insulating ultrathin NaCl films.31 Oxide films and surfaces seldom have perfect ordering. Depending on preparation conditions various defects may appear: steps, kinks, adatoms, and vacancies.32 Recent experimental and theoretical studies show that Au atoms and clusters bind strongly to an oxygen vacancy,17-19,22,33 which acts as a nucleation site for a cluster growth. An oxygen vacancy can be present at terraces, steps, and corners and it can exist in three different charge states: neutral, singly, or doubly occupied.22,34 STM images show that oxygen vacancies are mainly located at the edges and corners of MgO.22 Carrasco et al. have investigated bulk and surface oxygen vacancy formation in MgO.35,36 They have also studied oxygen vacancy formation and diffusion on Ag-supported ultrathin, 2-3 layers thick, MgO film. Also Lopez and Valerie have performed DFT calculations for the oxygen vacancy formation on Ag-supported MgO films.37 They studied vacancies both on the surface of the MgO and at the

10.1021/jp066822l CCC: $37.00 © 2007 American Chemical Society Published on Web 02/27/2007

4320 J. Phys. Chem. C, Vol. 111, No. 11, 2007 MgO-Ag interface and found the interface vacancy energetically more favorable. In this work we investigated systematically the adsorption of a Au atom on regular and defected (O-vacancy) MgO(100) films supported mainly by Mo but in one particular case by Ag. In agreement with the findings of Pacchioni and co-workers we found that the Mo support plays an important role for defining the characteristics of adsorption of the Au adatom on 1 to 5 ML thick regular MgO films. Despite the previous intensive work on this subject we found a new low-energy adsorption site for Au: a hollow site. Compared to calculations without the Mo support (mimicking a situation with a thick MgO film or single-crystal MgO(100) surface), the optimal adsorption site changes and the adsorption energy increases by 0.9 to 1.6 eV by the Mo support, depending the oxide film thickness. A Ag support has weaker effects on the Au adatom adsorption. To our knowledge no DFT calculations on oxygen vacancy formation on Mo supported MgO film, Au adsorption on the vacancy, and the role of the buried vacancy to Au adsorption exist. Therefore, we first focus on vacancy formation energetics on the surface of the MgO/Mo system and then at the MgOMo interface as a function of MgO film thickness. The interaction of the Au adatom with a surface O-vacancy on MgO(100)/Mo is strong (about 3 eV) for 1-5 ML films. Similar to the reported results for the MgO-Ag interface37 we found that O-vacancies at the metal/metal-oxide interface are thermodynamically more favorable than surface vacancies for MgO films with a thickness of 2 ML or more. However, these buried vacancies do not play any role in the Au adsorption. We further discuss the role of charging and polarization effects for the Au adsorption and compare our results to the related recent literature. II. Computational Details The density functional theory (DFT) calculations were performed with the DACAPO code,38 where Kohn-Sham equations are solved in a plane wave basis restricted by a kinetic energy cutoff of 25 Ry. We employed the RPBE39 generalized gradient correction self-consistently, and the core electrons of all the atoms were treated with Vanderbilt ultrasoft pseudopotentials.40 A spin-unpolarized approach was used for the calculations. We checked by a spin-polarized calculation that the adsorption energy and the charging of Au on MgO(3ML)/ Mo is not significantly affected by this choice. We used a 3-layer-thick Mo(100) slab support on which a thin MgO film was deposited. The thickness of the film varied from one to five layers for regular films and from two to five layers for defected films. Oxide anions (O2-) were aligned with Mo atoms at the Mo/oxide interface, since this is the optimal geometry found in the FP-LMTO calculations.44 The optimized lattice constant for bulk MgO is 4.30 Å, and that for Mo is 3.150 Å. In all calculations Mo was kept at its theoretical equilibrium lattice constant, which introduces 4.7% tensile strain to MgO to match the Mo lattice. In the calculations where unsupported MgO films were studied we used the optimized MgO lattice constant. The bottom Mo layer was frozen at the bulk geometry, while all other atoms were free to relax. We modeled the surface with a (3 × 3) unit cell and the sampling of (2 × 2 × 1) Monkhorst-Pack k-points was used. The vacuum region was at least 10 Å between adjacent slabs. We used dipole correction in all calculations which solves the problem related to the formation of the surface dipoles.45 For comparison, some calculations were repeated with a Ag(100) support. The Ag

Honkala and Ha¨kkinen support was modeled with a 3-layer slab with a (3 × 3) unit cell and optimized lattice constant of 4.14 Å. All the calculations for an oxygen vacancy and a Au adatom diffusion presented in this work were carried out with use of the nudged elastic band (NEB) method,41-43 where as an initial guess for the reaction path, linear interpolation between the initial and final states was used. For analyzing the charge state of atoms we applied the Bader approach46 to partition electrons to atoms. The Bader approach differs from other schemes (e.g., Mulliken analysis) in that it focuses on the density of a system and not on orbitals. Recently developed fast implementation of the Bader approach has made it a suitable tool for large systems calculated with DFT and plane waves.47 For a test, we performed the Bader analysis for the ionic bulk MgO. An average charge transfer from Mg to O was found to be 1.71 e, which agrees very well with the value 1.73 e calculated by Henkelman and co-workers.48 III. Results and Discussion The results will be presented as follows. We start with Au adsorption on single-crystal MgO and continue with the investigations on the properties of a Mo-oxide interface. This is followed by the studies of Au adsorption on the regular Mo(100) grown MgO film together the one special case where a comparison between Mo and Ag supports is presented. Finally Au adsorption on defected MgO over the Mo support is discussed. A. Au Atom on Single-Crystal MgO(100). First we consider Au adsorption on a 3-layer-thick MgO(100) slab that is used to model single-crystal MgO and thick supported films. On the regular surface the best adsorption site for a Au atom is at the top of the oxygen with adsorption energy of -0.66 eV (negative sign indicates exothermic reaction) in agreement with the earlier DFT calculations.25 The adsorption energy is somewhat smaller than given in refs 30, 49, and 50, but in those works PW91 GGA functional was applied and it is known to bind adsorbates more strongly than the RPBE functional used here. At hollow and Mg sites adsorption energy is -0.5 eV. An oxygen vacancy is created when a neutral O atom leaves the MgO surface but two electrons are left to a vacancy. Placing the Au atom on such a vacancy increases adsorption energy to -2.5 eV, which is about 0.5 (0.7) eV smaller than that found with the PW91 (BP86) functional.49,51 The fact that the binding energy is larger on an O-vacancy than on a regular surface shows that if the vacancy is present it acts as a nucleation center for the growth and trapping of Au clusters.17,33 If one brings a metal and an insulator into contact, thermal equilibrium requires that the Fermi energy is the same on both sides of the interface. The alignment of Fermi energies involves a charge redistribution and creates an interfacial dipole layer between the materials in contact.52 By means of the Bader method the charge transfer between Au and MgO(100) was evaluated. We obtained a minor charge transfer, 0.29 e, to Au at an O-top site. Integration of the local density of states (LDOS) projected on a Au atom supports the result of the Bader analysis giving a charge transfer of 0.37 e to the adsorbed Au atom. A larger charge transfer, 1.20 e, is found with the Bader method for Au at the O-vacancy. B. MgO/Mo Interface. Next, we analyze geometric and electronic properties of the MgO/Mo interface. Two different cases are considered, namely, regular and defected ultrathin MgO films on Mo; in the latter case both surface and oxide/ metal interface O-vacancies were investigated. The comparison

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Figure 1. Projected local density of states for (a) 1 ML of MgO over Mo(100), (b) 2 ML of MgO, (c) 3 ML of MgO, and (d) 4 ML of MgO. In parts a-c the red lines stand for d electrons in Mo and the blue line is for s and p electrons in MgO. In part d the green line stands for d electrons in interfacial Mo atoms, the red line for d electrons for bulk Mo atoms, the black line is for interfacial O atoms, and the blue line is for bulk O atoms. Thick solid lines in part c mark the edges of the MgO band gap. In this figure and Figures 2, 4, and 9 the Fermi level is set to zero.

of the oxide/metal interfacial distance shows that a single oxide layer behaves somewhat differently compared to the bilayer and thicker MgO films. For the single oxide layer the interfacial distances Mo-Mg and Mo-O are 2.04 and 2.25 Å, respectively, giving the rumpling of 0.2 Å. For bilayer and thicker films the Mg-Mo distance varies between 2.13 and 2.17 Å and the O-Mo distance between 2.23 and 2.25 Å. The rumpling is on the average ∼0.1 Å, which is half of that for the single oxide layer. In line with the previous study53 we found the average oxide-metal distance shorter at the MgO(3ML)/Mo system than at the MgO(3ML)/Ag(001) (∼0.6 Å). The longer MgO-metal distance in the case of silver reflects the increased repulsion due to the filled 4d shell. The MgO-Mo interaction results mainly from the polarization of metal due to the electrostatic field of MgO and charge transfer at the interface is negligible53,54 giving a weak adhesion of oxide to metal. This is also seen for the metal-oxide counterpart55 and for the MgO/Ag interaction.37 In this work we found adhesion energy per MgO unit to be -0.23 eV for the 3 ML thick oxide film. The Bader analysis gives charge transfer of less than 0.1 e from oxide to Mo and a density difference plot (not presented here) shows polarization at the MgO-Mo interface. Upon Au adsorption adhesion energy increases to -0.28 eV according to our calculations. The local density of states analysis for electronic structure was performed for the MgO film thicknesses ranging from 1 to 4 ML and shown in Figure 1. Three features are seen: first the formation of a band gap with increasing oxide thickness, the presence of metal induced gap states, and hybridization between the orbitals of interfacial Mo and O. In agreement with the earlier calculations for Ag-supported MgO23 the formation of MgO band gap at an oxide film thickness of 2-3 ML takes place. For the four-layers-thick MgO film the band gap is roughly 3.5 eV. This is 2.5 eV too small compared to the

measured value for ultrathin supported MgO film23 but it is wellknown that DFT underestimates the band gaps of wide-gap insulators. The d states of Mo fill the band gap and define the Fermi level. The LDOS plots show that the main part of the MgO band is below the Mo band. We present the detailed LDOS for the 4-layers-thick Mo supported MgO film shown in Figure 1d, where LDOS for interface O and Mo atoms is plotted separately but Mg atoms are not shown and thus the upper edge of the band gap is missing. The plot displays some hybridization between interfacial Mo and O atoms. We find interfacial oxygen states in the MgO band gap and interfacial Mo states below the bulk Mo states. Comparing the LDOS plots we also see that the broadening of Mo d-band increases with the increasing oxide film thickness, which is due to increasing overlap between O and Mo wave functions at the interface. Next we focus on the energetics of neutral oxygen vacancy on the MgO/Mo surface. Previously oxygen vacancies on Agsupported MgO ultrathin films have been investigated computationally in refs 35 and 36, but this is a first study on vacancy formation and diffusion on Mo-supported MgO films. Two different O-vacancy geometries were investigated in detail: a vacancy at a MgO surface and one at a MgO-Mo interface. In these calculations the MgO film thickness was always at least two layers. For both surface and interface vacancies the average interfacial distances and rumpling are very similar to those calculated for regular films. Surprisingly, the vertical relaxation of the cations next to a vacancy differs only slightly from that of other cations. Also the extraction of the Mo atom just below the interfacial O-vacancy is minor, only 0.06 Å, for all MgO film thicknesses. This is in contrast to Ag-supported MgO films, where the extraction of 0.2-0.3 Å has been calculated.37 The Bader analysis for a surface vacancy does not give any charge

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Honkala and Ha¨kkinen

Figure 2. Projected local density of states for (a) a surface oxygen vacancy and (b) an interface oxygen vacancy on 3 ML thick MgO(100) over Mo(100). The solid line stands for the Mo d electron and the dashed line for s and p electrons of MgO.

transfer between oxide and metal whereas for an interface vacancy the charge transfer from the oxide to the metal is 1.32 e. The calculation of the vacancy formation energies (Ev) (where the reference energy for an oxygen atom is taken to be half of the total energy of an oxygen molecule in the gas phase) shows that Ev depends on the position of the vacancy. For an unsupported 3 ML thick MgO film Ev is 4.96 eV and it is 0.29 eV smaller than for the Mo-supported MgO film. The corresponding energy difference for the Ag-supported 2 ML MgO film is 0.07 eV.36 The comparison of the O-vacancy formation energies (Ev) for surface and interfacical vacancies with a different number of MgO layers shows that formation energy saturates already with the three oxide layers and that Ev is always smaller for the interface (4.54 eV) than for the surface vacancy (5.25 eV) and the difference in stability is 0.7 eV for all numbers of MgO layers studied here. For MgO/Ag the difference is somewhat larger being 1.12 eV, but also in this case the interface formation energy is smaller.37 For a three-layer-thick MgO film it is possible to calculate the formation energy of the subsurface vacancy (the vacancy is in the second layer). The vacancies in the order of decreasing stability (in parenthesis Ev with respect to the subsurface Ev) are the interface (-1.26 eV), surface (-0.53 eV), and subsurface. Higher subsurface vacancy formation energy agrees with the cluster calculations of Pacchioni et al.56 The smallest formation energy at the interface makes it thermodynamically most favorable as seen on MgO/Ag.37 The interfacial vacancy might not be attainable via diffusion if the vacancy diffusion barrier is too high. For the 2 ML MgO/Mo system we calculated a barrier from the surface to the interface to be 1.23 eV (vice versa 1.93 eV), which is considerably lower than the bulk value of 3.5 eV estimated from experiments57 but identical with the calculated barrier for 2 ML MgO/Ag.36 We expect that with increasing oxide thickness the diffusion barrier increases because the oxide structure becomes more rigid. An interfacial vacancy may also form during an oxide growth process and therefore it is important to know if the vacancy acts as a hidden nucleation center. Figure 2 compares the electronic structures of surface and interface vacancies over 3 ML thick MgO(100) on Mo. From the LDOS plots and Figure 1 we see that the surface vacancy forms an extra peak in the band gap but the presence of the vacancy does not lead to the reduction of the band gap. The formation of an extra peak due to an oxygen vacancy is also found on MgO/Ag.58 In the LDOS plot of the interface vacancy the extra peak is smaller but broader indicating an interaction with Mo d electrons. C. Au Atom Adsorption on MgO/Mo. Table 1 and Figure 3 display Au binding energies at three different adsorption sites as a function of the number of MgO layers on the Mo(100)

TABLE 1: Au Adsorption Energies (in eV) on Three Different Adsorption Sites on 1-5 ML Layers Thick Mo-Supported MgO Film no. of ML

Mg

O

hollow

1 2 3 4 5

-2.19 -2.25 -1.94 -1.66 -1.58

-1.87 -2.02 -1.65 -1.43 -1.28

-2.27 -2.30 -1.99 -1.73 -1.52

substrate. They show a general trend that Au binding decreases with increasing film thickness. A notable exception to this trend is the 2 ML of MgO which binds Au more strongly than 1 ML of MgO. Even with 5 MgO layers on Mo(100) Au binding energy at the O site is twice and at the Mg site 2.3 times that on single-crystal MgO(100). Obtained adsorption energies are close to values reported by Pacchioni and co-workers27 in the cases where the comparison can be done. However, we found a new low-energy Au adsorption site: a 4-fold hollow, see Figure 3. In contrast to NO2 adsorption over BaO- and Pt-supported BaO59 where the adsorption site stays unchanged upon supporting the oxide, the preferred adsorption site for a Au atom changes from unsupported to supported MgO. Up to the 4 MgO layers the 4-fold hollow site is slightly more stable compared to the Mg site, whereas an O-top site is always 0.2-0.3 eV less stable than the Mg site. This is in contrast to the DFT results for Au adsorption on MgO(100) given in this work and published earlier.24,25

Figure 3. Au adsorption energy at O-top, hollow, and Mg-top sites with film thickness from 1 to 5 layers. In the inset figures Au atoms are yellow, Mo blue, O red, and Mg gray. Solid lines are a guide to the eye.

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Figure 4. Projected local density of states for Au at a hollow site on MgO/Mo (left) and MgO/Ag (right). The blue line stands for d electrons in Mo or Ag, the black line for MgO, the red line for Au d, and the green line for Au s electrons.

The Mo support below the oxide stretches average Au-MgO distances from 2% to 5% compared to the distances on the unsupported MgO film. The LDOS plot on the left in Figure 4 shows that s and d states of Au and part of the Mo d states lie in a MgO band gap. This raises the question if the increased Au binding to MgO is a due to a direct interaction between Au and Mo such that Mo electron density penetrates through the thin oxide film and interacts with the Au electron cloud. This was tested for Au on the 3 layers thick MgO film over Mo. We removed oxide between the metals, and kept all atoms at frozen positions in an adsorption energy calculation. In this case the Au-Mo distance is around 9 Å and adsorption energy is -0.2 eV. This shows that direct Au-Mo interaction is not possible due to the long physical distance and thus another mechanism is behind the increased Au adsorption energy on MgO(100)/ Mo(100). The bond enhancement cannot be due to the structural straining of oxide on the Mo(100). This was verified by calculating Au binding to the O site on single-crystal MgO with the increased lattice constant that corresponds to the one MgO has on the Mo support. Adsorption energy increases less than 0.1 eV compared to the value calculated with the optimized lattice constant. Thus structural straining of oxide cannot account for increased Au adsorption energy. Pacchioni and co workers27 suggested that the mechanism behind the increased Au adsorption on MgO/Mo is electron tunneling from Mo to Au but unfortunately no numbers about the amount of charge transfer were presented. Tunneling would naturally explain why Au binding decreases with the increasing MgO film thickness. A good measure for decreasing tunneling with increasing oxide film thickness is charge transfer to the Au atom and therefore we performed the Bader analysis for Au at a hollow site over MgO/Mo. The Bader analysis can also help us to understand where the charge comes from. Analyzing Mo and MgO separately we see how much their charge states change. Table 2 summarizes the obtained results. The most striking feature is that the amount of charge transferred to Au is fairly independent of oxide film thickness. According to the Bader analysis the Au atom obtains 0.71 e upon adsorption on a Mo(100) surface and 0.7-0.8 e on MgO(100)/Mo(100). Separate charge analysis for MgO and Mo unravels that oxide donates more charge than Mo in all cases except with 5 ML of MgO. In a more detailed analysis we can consider MgO layers separately. For Au on 3 ML MgO/Mo the largest charge transfer, 0.31 e, is from the oxide layer in a direct contact with the Mo surface, while the middle layer stays neutral and the top-most MgO layer donates 0.11 e. The large charge transfer on the first oxide layer at the MgO-Mo interface is induced by the presence

TABLE 2: Atom Resolved Electronic Charges of the Au Atom (from the Bader Analysis) at a Hollow Site over Mo-Supported Ultrathin MgO Filma no. of ML

Au

MgO

Mo

0 1 2 3 4 5

+0.71 +0.78 +0.82 +0.82 +0.77 +0.69

0 -0.56 -0.47 -0.42 -0.51 -0.28

-0.71 -0.22 -0.35 -0.39 -0.26 -0.41

a The film thickness ranges from 0 to 5 ML/s. The plus sign indicates the gain and the minus sign the loss of electrons.

of the Au atom since as shown earlier in this paper no charge transfer is seen at the Mo-MgO interface without Au. The fact that we found the charge transfer to Au on MgO/ Mo to be fairly independent (0.7 to 0.8 e) of the MgO film thickness contradicts very recent theoretical findings for an Au8 cluster adsorbed on MgO/Mo.29 There, charge transfer from a substrate to a cluster was decreased significantly upon increasing the film thickness, varying from about 0.45 to 0.05 e for 2 to 7 ML of MgO. However, the way the charge transfer analysis was conducted in ref 29 is drastically different from ours, which might explain the observed discrepancies. Therefore, we repeated the charge analysis using an analogous method to that used in ref 29. Specifically, we performed a layer-wise integration of the density change, ∆F, induced by the adsorption of Au on MgO/Mo.29 The upper integration limit was placed in the vacuum about 5 Å from Au. We found that the integrated charge transfer to the Au adatom depends sensitively on the position of the lower integration limit. If the limit is placed halfway between Au and the first MgO layer, we obtain the average charge transfer of 0.22 e to Au, independent of the oxide film thickness as in the Bader analysis. If the lower limit is placed between the first and the second MgO layer, again almost constant charge transfer is seen but it is considerably decreased (∼0.1 e), which results from the opposite polarization of the top-most MgO layer compared to the Au atom. This is seen in Figure 5, which presents density difference plots, projected on the plane that includes the Au adatom, displaying a rich polarization pattern for 1 and 5 ML thick MgO films all the way from the MgO/Mo interface throughout the oxide. Additionally, substantial charge accumulation around Au is seen as well as charge depletion at the surface. We conclude that our results indicate that the increased Au binding to ultrathin oxide film compared to single-crystal MgO results from (i) the alignment of the Fermi level throughout the system and (ii) from the associated charge-transfer to Au, which in turn induces (iii)

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Honkala and Ha¨kkinen TABLE 3: The Comparison of the Properties of Au at a Hollow Site on MgO/Mo and MgO/Ag Supportsa support

Eads (eV)

dMg-Au (Å)

dO-Au (Å)

∆zMg (Å)

dmetal-Mg (Å)

dmetal-O (Å)

MgO/Mo MgO/Ag

-1.99 -1.21

2.85 2.85

3.19 3.29

0.24 0.39

2.18 2.71

2.21 2.62

a ∆zMg measures the extraction of the Mg next to Au compared to other surface Mg atoms.

TABLE 4: Atom Resolved Charge Transfer of the Hollow Site Adsorbed Au on MgO(100)/Mo(100) and MgO(100)/ Ag(100) Obtained from the Bader Analysis and Integrated LDOSa Bader analysis support

Au ∆e

MgO ∆e

metal ∆e

integrated LDOS Au ∆e

MgO/Mo MgO/Ag

+0.82 +0.81

-0.42 -0.71

-0.40 -0.1

+0.83 +0.88

a

Figure 5. Density difference plots from Au at a hollow site on MgO/ Mo with 1 (top) and 5 (bottom) layers of oxide. Blue (red) regions indicate charge depletion (accumulation). Yellow, red, blue, and black balls mark Au, O, Mg, and Mo atoms, respectively. The MgO-Mo interface is marked with the white line. The scale is (0.003 e/bohr3.

polarization throughout the oxide film. We note that the spatially varying polarization pattern makes strong spatial variations in the effective dielectric constant of the film, consequently, models made for macroscopic films and metal/oxide interfaces, utilizing macroscopic dielectric properties and materials constants,29,60 should be applied with care for ultrathin oxide films supported on metals. D. The Role of the Support Metal: Mo versus Ag. Ag is commonly used as a support to grow MgO films because the lattice mismatch is only ∼3%. To test how the type of support

The plus (minus) sign stands for the gain (loss) of electrons.

metal affects the binding strength of the Au atom we replaced Mo with Ag. Au adsorption energies were calculated on hollow and O sites over 3 ML thick MgO on Ag. Compared to the Mo support case Au binding drops dramatically, being 1.00 eV for an O-top site and 1.21 eV for a hollow site. Similar metal support dependent behavior in adsorption energies is observed for Cu, Pd, and Pt on supported Al2O3.61 Pacchioni and coworkers report Au adsorption on 2 ML thick MgO film supported by Ag. They obtained 0.46 eV larger binding energy to an O-top site than we did but the large difference is mainly due to our 1 ML thicker MgO film since binding decreases with increasing film thickness. Table 3 summarizes the details of the Au geometry and energetics on Mo- and Ag-supported MgO and Figure 4 compares their electronic structures. The main differences in the LDOS plots (Figure 4) are the lower gap state density and the narrower MgO bandwidth with the Ag support. The calculated support metal-oxide interface distance is longer for Ag than for Mo. The Mg atoms next to Au relax strongly toward Au leading to equal Au-Mg distance on both surfaces. This indicates similar charges on Au which were confirmed with the charge analysis, see Table 4. Compared to our result Giordano et al.30 obtained ∼0.2 e smaller charge on Au at MgO/ Ag than we did but without knowing the details of how the analysis was conducted in ref 30 it is not possible to say where the difference comes from. Table 4 presents the results of the Bader analysis and integrated LDOS. Note that both approaches give the same charge transfer although they employ diverse partitioning strategies of the charge density. The Au charge state on both surfaces is the same but the origin of the charge is different. In the Mo case both metal and oxide donate the same amount of charge, whereas in the case of the Ag support charge comes mainly from oxide and the Ag contribution is minor. The detailed analysis shows that the main charge transfer from the oxide takes place from the oxide layer in direct contact with Mo or Ag. E. Au/FC/MgO/Mo and Au/MgO/FC/Mo. Finally, we discuss Au adsorption on a defected ultrathin MgO film on Mo(100). Both surface and interfacial neutral oxygen vacancies are studied, see Figure 6 for structures. Earlier results49,50 and those presented here show that the Au atom binds more strongly on an oxygen vacancy than on a regular O site at the MgO(100) single crystal. Recent experiments show a direct correlation between oxygen vacancy concentration on MgO and the

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Figure 6. (a) Au at a surface O-vacancy and (b) Au at the O-top site above the interfacial O-vacancy. The color coding is as in Figure 3.

Figure 7. Au binding energy to a surface O-vacancy as a function of MgO film thickness. The dashed line is a guide to the eye.

catalytic activity of deposited Au, which indicates the important role of the vacancies to the activity of the Au/MgO catalyst.62 Figure 7 gives Au adsorption energies on an O-vacancy as a function of MgO film thickness. As on the single-crystal MgO(100), Au binds strongest at the O-vacancy site. For one and two MgO layers Au binding energy differs from that of thicker films. The reason for this is very simple: films with one and two layers are too thin to be “real” oxide especially when one oxygen is removed. In the case of one MgO layer, Au adsorbed at the vacancy is in direct contact with a Mo atom below. With two layers of MgO, Au sits on the top of an interfacial Mg atom. Not until with the three layers of MgO does vacancy adsorbed Au have both Mg and O atoms beneath it and thus has an environment that resembles “real” oxide. From three to five layers the binding energy is more or less saturated and it is on average 0.35 eV larger than that on singlecrystal MgO(100). From the comparison of adsorption energies between a hollow site and surface vacancy we conclude that the influence of the Mo support is much smaller for Au on the oxygen vacancy than on the hollow site. On both the unsupported and supported MgO(100) films, however, the Au atom prefers to adsorb on an oxygen vacancy, if such a site is available, and forms a chemical bond to the substrate. The vacancy acts thus as a nucleation center for cluster growth. We note that our calculations predict a high mobility of Au atoms on Mo-supported MgO since the calculated diffusion barrier is only 0.1 eV. A somewhat higher barrier, 0.24 eV, has been reported for Au diffusion on an unsupported MgO film.50 According to the Bader analysis Au gets approximately 1.2 electrons from the vacancy and the charge transfer depends neither on MgO film thickness nor the presence of the Mo

Figure 8. A density difference plot from a Au atom at an oxygen vacancy on MgO/Mo. Blue (red) regions correspond to charge depletion (accumulation) upon Au adsorption. Yellow, red, blue, and black balls are Au, O, Mg, and Mo atoms respectively. The MgO-Mo interface is marked with the white line. The scale is (0.003 e/bohr3.

support. This indicates that the electronic properties of vacancies are localized in nature. The density difference plot in Figure 8 supports this picture showing strong charge transfer in the vicinity of Au but a much weaker polarization pattern in MgO and the MgO-Mo interface than in Figure 5. Figure 9a displays a LDOS plot of the Au atom on the surface vacancy. Two main features are seen when compared to the LDOS plot for the Au atom on a hollow site given in Figure 4, namely, the peak corresponding to Au d electrons drops from -1.8 to -5 eV and no clear peak from Au s electrons is seen for a vacancy, indicating strong interaction between Au and vacancy seen also as a large adsorption energy of Au. Additionally, the vacancy state observed in the MgO gap on the clean MgO(100)/Mo(100) surface disappears, see Figure 2. Previously we showed that the formation of an interfacial O-vacancy is thermodynamically more favorable than the formation of the surface vacancy. Under experimental conditions, one could then expect a certain population of these “hidden” defects to be present, which raises the question whether they can act as nucleation centers for Au cluster growth. We studied the effect of the interface vacancy on Au adsorption by calculating the adsorption energy on the three layers thick oxide film. Two different adsorption sites were tested: Au at the O-top

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Figure 9. Projected local density of states plot for (a) an Au atom at a surface oxygen vacancy and (b) at a hollow site next to the interfacial oxygen vacancy. The blue line stands for Mo d electrons, the black line for MgO s and p electrons, the red line for Au d electrons, and the green line for Au s electrons.

site aligned vertically with the vacancy (see Figure 6) and at the adjacent hollow site. In both cases Au adsorption energy is close to the value calculated on the regular Mo grown MgO(100) film. The LDOS plot of the Au atom at the hollow site in the vicinity of interfacial oxygen vacancy, Figure 9b, differs only slightly from the plot of Au at the regular hollow site and thus it is not surprising that Au adsorption energies are also identical in these systems. From this we can infer that since interfacial O-vacancies do not influence adsorption energies they do not act as a hidden nucleation site for Au. IV. Summary and Conclusions We performed systematic DFT calculations on Au adsorption at regular and O-vacancy sites over Mo grown ultrathin MgO film with the film thicknesses ranging from 1 to 5 layers. The role of the metal support is 2-fold: it introduces a switch in the preferred adsorption site and enhances adsorption energy. Adsorption energy decreases with the increasing oxide film thickness being, however, with 5 oxide layers, still 0.9 eV larger than on the single-crystal MgO surface. We demonstrated that the increased Au binding to ultrathin oxide film compared to single-crystal MgO is not due to the support metal induced stretch in the oxide film nor due to the direct interactions (tunneling) between support metal and Au. Our results indicate that the increased Au binding results from (i) the alignment of the Fermi level throughout the system and (ii) from the associated charge transfer to Au, which in turn induces (iii) polarization throughout the oxide film. Ag support has a clear but a weaker effect on the Au atom adsorption. We investigated the stability of vacancies at the MgO surface and at the support/metal-oxide interface. We found the latter one to be thermodynamically more stable. Surface vacancies bind Au atoms strongly and thus stop their diffusion and facilitate cluster growth but the role of the Mo support is minor for trilayer and thicker films. Although interface oxygen vacancy is thermodynamically more stable and can exist, for example, as a consequence of the film growth process it does not act as a buried nucleation site for Au adsorption since it does not increase Au binding to the sites in the vicinity of the vacancy. Acknowledgment. The work was financially supported by the Academy of Finland. K.H. acknowledges the computer resources from the Finnish IT Center for Science. The support from L. B. Hansen and V. Apaja with the DACAPO code is acknowledged, as well as fruitful discussions with N. Lopez, H. Gro¨nbeck, and M. Walter.

References and Notes (1) Heinrich, V.; Cox, P. The surface science of metal oxides; Cambridge University Press: Cambridge, UK, 1994. (2) Woodruff, D., Ed. The chemical physcis of solid surfaces; Oxide Surfaces, Vol. 9; Elsevier: New York, 2001. (3) Hammer, B.; Nørskov, J. K. Nature 1995, 376, 238. (4) Bond, C. G.; Thompson, D. T. Catal. ReV. Sci. Eng. 1999, 41, 319. (5) See articles in the Special Issue on catalysis by Gold: Hutchings, G. J.; Haruta, M., Eds. Appl. Catal. A 2005, 291, 1-261. (6) Thompson, D. T. Top. Catal. 2006, 38, 231. (7) Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566. (8) Sinha, A. K.; Seelan, S.; Tsubota, S.; Haruta, M. Top. Catal. 2004, 29, 95. (9) Haruta, M. CATTECH 2002, 6, 102 and references therein. (10) Andreeva, D. Gold Bull. 2002, 35, 82. (11) Haruta, M. Catal. Today 1997, 36, 153. (12) Choudhary, T. V.; Goodman, D. W. Top. Catal. 2002, 21, 25. (13) Bond, G. C.; Thompson, D. T. Gold Bull. 2000, 33, 41. (14) Haruta, M. Gold Bull. 2004, 37, 27. (15) Chen, M.; Goodman, D. Science 2004, 306, 252. (16) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (17) Sanchez, A.; Abbet, S.; Heiz, U.; Schneider, W.; Ha¨kkinen, H.; Barnett, R.; Landman, U. J. Phys. Chem. A 1999, 103, 9573. (18) Ha¨kkinen, H.; Abbet, S.; Sanchez, A.; Heiz, U.; Landman, U. Angew. Chem., Int. Ed. 2003, 42, 1297. (19) Yoon, B.; Ha¨kkinen, H.; Landman, U.; Worz, A.; Antonietti, J.; Abbet, S.; Judai, K.; Heiz, U. Science 2005, 307, 403. (20) Arrii, S.; Morfin, F.; Renouprez, A.; Rousset, J. J. Am. Chem. Soc. 2004, 126, 1199. (21) Grisel, R.; Nieuwenhuys, B. J. Catal. 2001, 199, 48. (22) Sterrer, M.; Fischbach, E.; Risse, T.; Freund, H. Phys. ReV. Lett. 2005, 94, 186101. (23) Schintke, S.; Messerli, S.; Pivetta, M.; Patthey, F.; Liboulle, L.; Stengel, M.; Vita, A. D.; Schneider, W. Phys. ReV. Lett. 2001, 87, 276801. (24) Molina, L.; Hammer, B. Phys. ReV. Lett. 2003, 90, 206102. (25) Molina, L.; Hammer, B. Phys. ReV. B 2004, 69, 155424. (26) Molina, L.; Hammer, B. J. Chem. Phys. 2005, 123, 161104. (27) Pacchioni, G.; Giordano, L.; Baistrocchi, M. Phys. ReV. Lett. 2005, 94, 226103. (28) Giordano, L.; Baistrocchi, M.; Pacchioni, G. Phys. ReV. B 2005, 72, 115403. (29) Ricci, D.; Bongiorno, A.; Pacchioni, G.; Landman, U. Phys. ReV. Lett. 2006, 97, 036106. (30) Giordano, L.; Pacchioni, G. Phys. Chem. Chem. Phys. 2006, 8, 3335. (31) Repp, J.; Meyer, G.; Olsson, F. E.; Persson, M. Science 2004, 305, 493. (32) Noguera, C. Physics and chemistry at oxide surfaces; Cambridge University Press: Cambridge, UK, 1996). (33) Wahlstro¨m, E.; Lopez, N.; Schaub, R.; Thostrup, P.; Rønnau, A.; Africh, C.; Lægsgaard, E.; Nørskov, J. K.; Besenbacher, F. Phys. ReV. Lett. 2003, 90, 026101. (34) Sterrer, M.; Fischbach, E.; Heyde, M.; Nilius, N.; Rust, H. P.; Risse, T.; Freund, H. J. J. Phys. Chem. B 2006, 110, 8665. (35) Carrasco, J.; Lopez, N.; Illas, F. Phys. ReV. Lett. 2004, 93, 225502. (36) Carrasco, J.; Lopez, N.; Illas, F.; Freund, H.-J. J. Chem. Phys. 2006, 125, 074711. (37) Lopez, N.; Valeri, S. Phys. ReV. B 2004, 70, 125428. (38) Http://dcwww.fysik.dtu.dk/software/. (39) Hammer, B.; Hansen, L. B.; Nørskov, J. K. Phys. ReV. B 1999, 59, 7413.

Au Adsorption on Thin MgO(100) Films (40) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (41) Millis, G.; Jo´nsson, H. Phys. ReV. Lett. 1994, 72, 1124. (42) Millis, G.; Jo´nsson, H.; Schenter, G. Surf. Sci. 1995, 324, 305. (43) Jo´nsson, H.; Millis, G.; Jacobsen, K. W. In Classical ans Quantum Dynamics in Condensed Phase Simultaions; Berne, B. J., Ciccotti, G., Coker, D. F., Eds.; World Scientific: Singapore, 1998. (44) Goniakowski, J. Phys. ReV. B 1999, 59, 11047. (45) Bengtsson, L. Phys. ReV. B 1999, 59, 12301. (46) Bader, R. Atoms in Molecules; Oxford University Press: Oxford, UK, 1990. (47) Henkelman, G.; Arnaldsson, A.; Jo´nsson, H. Comput. Mater. Sci. 2006, 36, 354. (48) Henkelman, G.; Uberuaga, B.; Harris, D.; Harding, J.; Allan, N. Phys. ReV. B 2005, 72, 115437. (49) Bracaro, G.; Fortunelli, A. J. Chem. Theory Comput. 2005, 1, 972. (50) Vitto, A. D.; Pacchioni, G.; Delbecq, F.; Sautet, P. J. Phys. Chem. B 2005, 109, 8040. (51) Neyman, K. M.; Inntam, C.; Matveev, A. V.; Nasluzov, V. A.; Ro¨sch, N. J. Am. Chem. Soc. 2005, 127, 11652.

J. Phys. Chem. C, Vol. 111, No. 11, 2007 4327 (52) Lu¨th, H. Solid Surfaces, Interfaces and Thin Films; Springer: New York, 2001. (53) Giordano, L.; Cinquini, F.; Pacchioni, G. Phys. ReV. B 2005, 73, 045414. (54) Goniakowski, J.; Noguera, C. Interface Sci. 2004, 12, 93. (55) Yudanov, I.; Pacchioni, G.; Neyman, K.; Ro¨sch, N. J. Phys. Chem. B 1997, 101, 2786. (56) Pacchioni, G.; Pescarmona, P. Surf. Sci. 1998, 412/413, 657. (57) Popov, A. I.; Monge, M. A.; Gonzalez, R.; Chen, Y.; Kotomin, A. E. Solid State Commun. 2001, 118, 163. (58) Sterrer, M.; Heyde, M.; Novicki, M.; Nilius, N.; Risse, T.; Rust, H. P.; Pacchioni, G.; Freund, H. J. J. Phys. Chem. B 2006, 110, 46. (59) Broqvist, P.; Gro¨nbeck, H. Surf. Sci. 2006, 600, L214. (60) Stoneham, A. M.; Tasker, P. W. Phil. Mag. B 1987, 55, 237. (61) Jennison, D. R.; Bogicevic, A. Surf. Sci. 2000, 464, 108. (62) Yan, Z.; Chinta, S.; Mohamed, A. A.; Fackler, J. P.; Goodman, D. W. J. Am. Chem. Soc. 2005, 127, 1604.