Ag(001) from first principles calculations

SUSC-20488; No of Pages 8

April 30, 2015;

Model: Gulliver 5

Surface Science xxx (2015) xxx–xxx

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Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations J. Nevalaita a, H. Häkkinen a,b, K. Honkala b,⁎ a b

Department of Physics, Nanoscience Center, University of Jyväskylä, P.O. Box 35, FIN-40014 Jyväskylä, Finland Department of Chemistry, Nanoscience Center, University of Jyväskylä, P.O. Box 35, FIN-40014 Jyväskylä, Finland

a r t i c l e

i n f o

Available online xxxx Keywords: DFT calculations Ag-supported MgO thin films Au adatoms Adsorption and dissociation of water

a b s t r a c t The molecular and dissociative adsorption of water on a Ag-supported 1 ML, 2 ML and 3 ML-a six atomic layer-thick MgO films with a single Au adatom is investigated using density functional theory calculations. The obtained results are compared to a bulk MgO(001) surface with an Au atom. On thin films the negatively charged Au strengthens the binding of the polar water molecule due to the attractive Au–H interaction. The adsorption energy trends of OH and H with respect to the film thickness depend on an adsorption site. In the case OH or H binds atop Au on MgO/Ag(001), the adsorption becomes more exothermic with the increasing film thickness, while the reverse trend is seen when the adsorption takes place on bare MgO/Ag(001). This behavior can be explained by different bonding mechanisms identified with the Bader analysis. Interestingly, we find that the rumpling of the MgO film and the MgO–Ag interface distance correlate with the charge transfer over the thin film and the interface charge, respectively. Moreover, we employ a modified Born–Haber-cycle to analyze the effect of film thickness to the adsorption energy of isolated Au and OH species on MgO/Ag(001). The analysis shows that the attractive Coulomb interaction between the negatively charged adsorbate and the positive MgO–Ag-interface does not completely account for the weaker binding with increasing film thickness. The redox energy associated with the charge transfer from the interface to the adsorbate is more exothermic with the increasing film thickness and partly compensates the decrease in the attractive Coulomb interaction. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ultra-thin or 2D oxides are materials with unique and highly tunable functionalities [1,2] and their applications range from gas sensing and protective coatings to electrodes in fuel cells, heterogeneous catalysis, and bio-compatible and solar energy materials [3]. Metal supported thin oxide films are grown by vaporizing a cation metal in an oxygen background on a substrate metal, and this facilitates the controlling of the film thickness in atomic precision [4]. Typically, the metal grown oxide films have properties distinct from their bulk oxide counterparts, and the characteristics can be tailored by varying the support metal or the film thickness [5–7]. The great advance of the thin film arrangement is that originally insulating materials, such as MgO, can be locally probed using STM techniques [8]. Among the simple oxides, MgO thin films on Ag(001) or Mo(001) surfaces are prototypical model systems due to a small lattice mismatch between the oxide and the metal. Density functional theory calculations have been applied to explore the impact of the finite film thickness on adsorption characteristics of electronegative species like Au atoms and clusters, and O2 molecules showing that their behavior on thin film is quite different compared to ⁎ Corresponding author. E-mail address: karoliina.honkala@jyu.fi (K. Honkala).

a bulk oxide surface. Specifically, calculations demonstrate that the adsorption of molecular oxygen on a MgO(001)/Ag(001) surface leads to the enhanced binding and activation of the molecule compared to bulk MgO(001) owing to a charge transfer from the support [9,10]. Moreover, calculations predict that isolated Au atoms bind stronger and are negatively charged on MgO/Ag(001) [11] and on MgO/ Mo(001) [12,13], which is opposite to what is seen on bulk MgO(001), where the interaction is weak and Au remains neutral. From the interplay between STM experiments and DFT calculations, it is shown that adsorbed Au-clusters form 2D islands on MgO/Ag(001) [11,14,15], while 3D shapes are preferred on bulk MgO. The calculations suggest that supported Au-clusters are negatively charged with the excess charge concentrated at the edges of the 2D islands. These edge sites can, according to DFT calculations, readily adsorb and activate multiple oxygen molecules under ambient conditions [16]. Similar calculations indicate that also 1D Au chains on MgO/Mo(001) can activate oxygen [10]. Since many oxides are hydroxylated in ambient conditions [17] and this may impact on their chemical properties [18,19], an understanding of dissociative adsorption of water is important. Hydroxylation typically occurs because water reacts strongly with defect sites on oxide. For example, calculations predict that step sites on bulk MgO(001) and Ag-supported ultra-thin MgO(001) dissociate water spontaneously

http://dx.doi.org/10.1016/j.susc.2015.04.013 0039-6028/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: J. Nevalaita, et al., Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations, Surf. Sci. (2015), http://dx.doi.org/10.1016/j.susc.2015.04.013

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[20], while bulk MgO [21] and terraces of oxide thin films interact weakly with water and the dissociative adsorption of water is exothermic only for a 1 monolayer (ML) thick film [22]. Barrierless dissociation can also occur on a MgO terrace when assisted by another water molecule [23]. If the MgO–Ag interface is modified by introducing interfacial vacancies [24–26] or dopants, it may impact on water dissociation [27, 28]. Two different dissociation pathways have been identified on ideal MgO(001)/Ag(001) from STM experiments [29]. Water dissociation can be either induced by using the vibrational states of the molecule or by an electronic excitation to the LUMO-state. From low-energy electron diffraction experiments, it is known that higher water coverages on bulk MgO lead to the formation of a (3 × 2) superstructure [30]. Interestingly, calculations predict that in the presence of an Au cluster a water molecule prefers to sit on the oxide at the vicinity of the Au cluster [31]. However, the thorough understanding of interaction between Au and water on the metal-supported MgO(001) is still lacking, despite its vital importance to understand catalytic processes such as a water– gas shift reaction. Herein, we present a density functional theory study to address the interaction between a molecular and dissociated water and an Au atom over MgO films with varying thickness supported on Ag(001). In particular, we consider energetics by studying multiple adsorbate configurations. Since earlier calculations illustrate that the bonding of dissociated H2O is different on MgO/Ag(100) and MgO(100) [20], we also explore the same adsorption structures and systems on bulk MgO. 2. Computational methods The density functional theory (DFT) calculations are performed in the real-space grid implementation of the projector augmented wave (PAW) method as implemented in the GPAW-code [32,33]. The Perdew–Burke–Ernzerhof (PBE) approximation [34] for the exchange and correlation functional is applied self-consistently in all calculations. The scalar relativistic corrections are used for the frozen core and projection operators of Au and Ag. All systems are treated as spin polarized with a 0.20 Å grid spacing. The previously calculated lattice constants for bulk MgO and Ag are 4.27 and 4.17 Å, respectively [20]. The single crystal MgO(001) surface is represented with a six atomic layer thick slab with the bottom layer frozen into bulk positions. The Ag support is modeled with a three atomic layer-thick slab of a (001) orientation, where two bottom layers are frozen into bulk positions. Structures are relaxed until residual forces are below 0.05 eV/Å. The O-anions in the MgO thin films are aligned with the Ag-atoms, which is known to be the preferred structure [35]. A (2 × 2) unit cell and a (5 × 5 × 1) Monkhorst–Pack k-point sampling [36,37] is used to model all surfaces. The considered film thicknesses were 1 ML, 2 ML and 3 ML. The adsorption energy Eads is calculated as Eads ¼ EX=surface −ðEsurface þ EX Þ;

ð1Þ

where EX/surface is the energy of a surface with X adsorbed, Esurface is the energy of a pristine surface and EX is the energy of adsorbate in vacuum. The charge states are analyzed with the Bader approach [38,39] where the total density is separated onto the atoms in the system with zero flux surfaces. 3. Results 3.1. Au and water adsorption We start by addressing the computational results for an isolated Auatom and a single water molecule on MgO/Ag(001), and after that consider the coadsorption of water and Au. From previous calculations, it is known that Au prefers to bind on the O-top site on bulk MgO(001) [40] and on the hollow site on MgO/Ag(001) [13]. While the Au-atom stays neutral on bulk MgO, as indicated by its Bader charge (− 0.3 |e|)

and magnetic moment (0.6 μB), on MgO thin films the Au gains an electron from the support and the charge transfer is accompanied with a polaronic distortion of the oxide [12,13]. The MgO-films are strongly ionic and the structure is distorted in the presence of charged adsorbates. In the case of the Au atom, Mg2+ cations move toward the negatively charged Au, and O2− anions move away from the adsorbate. In line with the previous calculation [13], we find the adsorption energy of an Au become less exothermic with increasing film thickness. The calculated energies are −1.7 eV, − 1.5 eV, and −1.3 eV for 1 ML, 2 ML and 3 ML, respectively, while the Bader charge of Au stays constant being − 0.8 |e|. A water molecule adsorbs on atop of a Mg cation on both unsupported MgO [41] and MgO thin film [20,22] systems. The calculated structural parameters of adsorbed water are very similar to those reported earlier. On MgO(100), the H atoms coordinate in the direction of surface anions, whereas on MgO/Ag(001) the structure is asymmetric with one shorter H-lattice oxygen, HOH–Os, distance. Water binds weakly on both surfaces with similar adsorption energy [22,20] and the adsorption proceeds without any marked charge transfer between the adsorbate and the surface. When water and Au are placed into the same surface cell, we consider the adsorption configuration, where both species are in their preferred adsorption sites and in the next nearest position to each other. As can be seen from Fig. 1 column Au + H2O, water adsorbs on the Mg cation such that one H points towards the Au atom. The second H atom faces away from the surface in the case of the supported thin film, while it is more parallel to the surface on bulk MgO. The adsorption energy of water on the Au/MgO(001) is − 0.4 eV, which is only slightly less exothermic than the previously calculated value for the MgO(001) surface [20,22], and indicates that the H2O–Au interaction is nearly zero. The presence of Au on MgO/Ag(001) enhances the binding of water by ∼ 0.4 eV compared to MgO/Ag(001) [20,22] and adsorption energy becomes − 0.9 eV exothermic. The increased exothermicity is due to an attractive electrostatic interaction between the negatively charged Au atom and the neutral but polar water molecule (see Table 1). Since Au stays neutral on unsupported MgO(001), the stabilization effect is missing and the adsorption energy of water is not affected by the presence of Au. Similar attractive interaction with a comparable Au–H distance of 2.2 Å has been reported for a water molecule and a MgO/Ag(001)-supported Au8-cluster [31]. 3.2. OH adsorption As a next step, the adsorption of water dissociation products (OH and H) is investigated. We start with the adsorption of OH for which three different adsorption geometries with the varying Au–OH distance are considered. The studied geometries include 1) OH far from Au on a bridge site between two Mg2 + cations, 2) OH close to Au, again on a bridge site, and 3) OH adsorbed atop Au. These geometries are shown in columns Au + OH, Au ⋯ OH and AuOH in Fig. 1, respectively. We find that the Au ⋯ OH geometry is not stable but transforms to the AuOH geometry on bulk MgO. Similarly, on a 1 ML-thick film Au ⋯ OH-geometry transforms to the Au + OH-geometry. Fig. 2 shows OH adsorption energies, which range from 2 eV endothermic to − 0.5 eV exothermic calculated with respect to gas-phase H2O and H2. In general, the OH atop an Au configuration is preferred on all surfaces except on the 1 ML-thick film, where the Au + OH and AuOH geometries are nearly isoenergetic. We notice that the film thickness has the inverse effect on the adsorption energies for Au + OH and AuOH structures. In the case of Au + OH, the adsorption becomes less exothermic with the increasing film thickness while for AuOH the opposite trend holds and adsorption strength increases with the increasing film thickness. Different trends can be understood by analyzing charges of studied configurations collected into Table 2, which illustrates adsorption configuration dependent charge transfer. In the case, where Au and OH are far from each other, that is the Au + OH geometry,

Please cite this article as: J. Nevalaita, et al., Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations, Surf. Sci. (2015), http://dx.doi.org/10.1016/j.susc.2015.04.013

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Fig. 1. Optimized geometries for studied adsorbates and surfaces. The column gives an adsorbate configuration. The label Au + X (where X = H2O, OH or H) stands for a configuration, where Au and X are adsorbed on the surface but far away from each other. The Au ⋯ X means that both species are adsorbed on the surface close to each other. The AuX label indicates that the X is sitting on atop of Au. The boxes correspond to different surfaces and for each surface both side and top views are given. The atomic color code is Ag gray, Au yellow, H white, Mg green and O red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

a charge transfer takes place from the Ag surface to the adsorbates and adsorption becomes less exothermic with the increasing film thickness. The Bader charge analysis renders both adsorbates negatively charged and they have small magnetic moments. This indicates that altogether two electrons are transferred from the underlying metal so that both species get one electron. We note that on the 3 ML-thick film OH gains the electron partially from the Ag support and the Au atom, which is seen as a non-zero magnetic moment of Au. Moreover, on bare MgO(100), Au becomes cationic giving an electron to the OH

Table 1 The Bader charges q [|e|] and magnetic moments μ [μB] of the adsorbed Au and H2O species and the average O–Ag distance [Å] at the metal–oxide interface.

1ML 2ML 3ML Bulk

q Au

q H2 O

q Ag

μAu

d Au − H

d O − Ag

−0.74 −0.77 −0.77 −0.29

−0.09 −0.10 −0.10 −0.07

−0.04 0.08 0.09

0.0 0.0 0.0 0.54

2.19 2.17 2.15 2.21

2.53 2.53 2.51

group, which is supported by the zero magnetic moments and the negative Bader charge of the OH-group. When the direction of a charge transfer is from the adsorbate to the surface, as it is for the AuOH complex, adsorption becomes more exothermic with the increasing film thickness. Herein, a covalent bond forms between Au and OH as indicated by small Bader charges and zero magnetic moments, and the reverse charge transfer takes place from Au to the Ag support. In the third geometry considered, Au ⋯ OH, which stands for a situation where species are close to each other but bound both to the surface and to each other, we find that the adsorbates have non-zero magnetic moments and less negative Bader charges compared to the Au + OH geometry. This implies that no further charge transfer takes place from the underlying support to the OH group. OH forms an elongated covalent bond with the negatively charged Au atom, which transfers charge to OH. 3.3. H adsorption Before addressing the dissociative adsorption of water, we investigate H adsorption on MgO/Ag(001) in the presence of an Au atom.

Please cite this article as: J. Nevalaita, et al., Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations, Surf. Sci. (2015), http://dx.doi.org/10.1016/j.susc.2015.04.013

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Fig. 2. Site specific adsorption energies for OH on Au/MgO/Ag and Au/MgO surfaces. The structures are shown for the surface with most exothermic adsorption energy. Colors as in Fig. 1. The zero level reference is H2O(g)-0.5 × H2 (g), where g denotes gas-phase. Dashed lines are a guide to the eye. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Site specific adsorption energies for H on Au/MgO/Ag and Au/MgO surfaces. The structures are shown for the surface with most exothermic adsorption energy. Colors as in Fig. 1. The zero level reference is 0.5 × H2 (g), where g denotes gas-phase. Dashed lines are a guide to the eye. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Again, three different adsorption geometries are considered with a varying Au–H distance. The studied adsorption configurations include 1) H adsorption on atop a surface anion far away from the Au atom, 2) H atop a surface anion close to the Au atom, and 3) H atop the Au atom. These geometries are labeled as Au + H, Au ⋯ H, and AuH, respectively and displayed in Fig. 1. In the first two configurations hydrogen adsorption leads to a formation of an Os H− group, where Os is a surface anion, while in the third case forms an AuH complex. The adsorption energies range from slightly endothermic, ∼ 0.3 eV to the clearly exothermic, ∼ − 1.5 eV, and they are displayed in Fig. 3. On MgO/Ag(001), the most favorable configuration is AuH and adsorption becomes more exothermic with the increasing film thickness. Interestingly, calculations predict that binding strength increases with a decreasing Au–H distance from the Au + H to the Au ⋯ H configuration. Bader charges, given in Table 3, demonstrate that this is due to the attractive electrostatic interaction between the negatively charged Au and the polar OsH. However, H adsorption on Au/MgO(001) is clearly the most favorable for all studied geometries. This is opposite to what is seen for H adsorption without an Au adatom, since in this case bare MgO is by far the least favorable. To rationalize the obtained results, we carefully analyzed Bader charges and magnetic moments and considered the direction of a charge transfer. We are able to identify four different charge transfer processes for H adsorption. In the first case, an electron transfers from the OsH to the support. This occurs on a MgO/Ag(001) surface for the Au + H and AuH geometries, where the Au atom retains its negative charge and the Os anion loses one electron. In the second case, the charge is transferred from the OsH-group to the Au atom. This happens on bulk MgO

for Au + H and Au ⋯ H geometries. Both adsorbates have no magnetic moment and the Au (H) has a negative (positive) Bader charge. In the last case, that is AuH on MgO/Ag(001), charge transfer takes place from the Au atom to the support, and the Bader charge of Au changes from −0.8|e| to −0.2|e| upon H adsorption. Finally, no charge transfer is seen for the AuH configuration on the bulk MgO, where the neutral Au atom forms a covalent bond with the H-adsorbate.

Table 2 The Bader charges q [|e|] and magnetic moments μ [μB] of the adsorbed Au and OH species with the distance [Å] between Au and O in the OH-group and the average O–Ag distance [Å] at the metal–oxide interface.

Au + OH: 1 ML 2ML 3ML Bulk Au ⋯ OH: 2 ML 3ML AuOH: 1 ML 2ML 3ML Bulk

q Au

q OH

−0.73 −0.77 −0.48 0.05 −0.34 −0.24 0.13 0.16 0.17 0.15

−0.86 −0.87 −0.87 −0.75 −0.71 −0.66 −0.45 −0.46 −0.46 −0.46

q Ag 0.46 0.59 0.26 0.20 0.06 −0.46 −0.37 −0.43

μAu

μOH

d Au − O

d O − Ag

0.0 0.0 −0.2 0.0 0.5 −0.3 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.2 −0.1 0.0 0.0 0.0 0.0

4.56 5.88 4.59 4.46 2.37 2.26 1.99 1.98 1.98 1.99

2.47 2.42 2.47 2.53 2.52 2.66 2.69 2.67

3.4. Dissociative adsorption of water Dissociative adsorption of water is also considered for three configurations, which differ by adsorption sites of the dissociation products. The relaxed configurations are shown in Fig. 1 and they are 1) AuOH and H on atop nearest surface anions, 2) AuH and OH on atop a surface anion and on a bridging configuration between two Mg2 + cations, respectively, and 3) OH and H on a bridging configuration between two Mg2+ cations and atop a surface anion, respectively. These configurations are classified as AuOH + H, AuH + OH and Au + H + OH and displayed in Fig. 1. In line with the previous calculations on MgO(2 ML)/Ag(001) [20,22], our calculations predict that water dissociation on MgO(001) and MgO/Ag(001) surfaces is always endothermic compared to the adsorption of molecular H2O. We do find a strong geometry dependence in adsorption energies, which range from endothermic for the AuOH + H configuration to exothermic for the AuH + OH configuration on MgO/Ag(001), while the adsorption on bulk MgO(001) is the least favorable for all configurations. Table 3 The Bader charges q [|e|] of the adsorbed Au and H species together with the distance [Å] between Au and H and the metal–oxide interface distance.

Au + H: 1 ML 2ML 3ML Bulk Au ⋯ H: 1 ML 2ML 3ML Bulk AuH: 1 ML 2ML 3ML Bulk

q Au

q Os H

q Ag

d Au − H

d O − Ag

−0.79 −0.83 −0.83 −0.82 −0.72 −0.71 −0.70 −0.73 −0.18 −0.17 −0.16 −0.16

−0.94 −0.92 −0.92 −0.93 −1.01 −1.05 −1.06 −1.04

−0.50 −0.45 −0.44

4.58 4.46 4.45 5.22 2.09 1.96 1.95 2.00 1.58 1.58 1.58 1.58

2.67 2.63 2.62

−0.46 −0.48 −0.42 −0.48 −0.38 −0.43

2.68 2.64 2.63 2.67 2.69 2.66

Please cite this article as: J. Nevalaita, et al., Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations, Surf. Sci. (2015), http://dx.doi.org/10.1016/j.susc.2015.04.013

J. Nevalaita et al. / Surface Science xxx (2015) xxx–xxx

For comparison, dissociation energies are also considered with respect to gas-phase water. Previous calculations demonstrate that for MgO/Ag(001) dissociative adsorption is only exothermic on the 1 MLthick MgO-film [22]. With an Au adatom on the surface dissociative adsorption is exothermic also for the 2 ML and 3 ML thick films in the AuH + OH configuration. However, when both dissociation products bond directly to the surface, the adsorption energy is endothermic for 2 and 3 ML thick films as it is without the adatom [22]. The computed adsorption energies are almost identical in both cases within the limits of DFT accuracy. To rationalize the energetics, we analyzed the charge states of the studied configurations. In the AuOH + H configuration the MgO/ Ag(001)interface takes up two electrons: one from the OsH and the other one from Au, which forms a neutral complex with OH. On unsupported MgO(001), however, AuOH becomes negatively charged as indicated by the elongated Au–O distance given in Table 4 and the more negative Bader charge reported in Table 5. In the case of the AuH + OH configuration, Au gives its electron to the OH group and, thus, the adsorption proceeds without charge transfer between the adsorbates and the support, since the Ag surface has an excess charge of 0.15|e| at most. This nicely correlates with the more exothermic adsorption energy of the AuH + OH compared to that of AuOH + H as seen in Fig. 4, because in the first configuration Au donates its electron to the OH group, while in the latter configuration both adsorbates give away an electron to the MgO–Ag interface [42]. On MgO/Ag(001), Au stays negatively charged in the Au + H + OH configuration, while a charge transfer is from OsH to the OH group. Again, only minor changes in the charge state of Ag are seen. On bulk MgO(001), further charge transfer from OsH to the adatom takes place. The fact, that the adsorption of Au + H + OH is less exothermic than AuH + OH, is due to weaker binding of H, since H prefers to form an AuH complex as demonstrated in Fig. 3. Fig. 5 displays that the molecular adsorption of water is energetically favored for all film thicknesses and the dissociative adsorption of water becomes more endothermic with the increasing film thickness. Molecular adsorption remains energetically favored for all film thicknesses as seen in Fig. 5. The preferred adsorption geometry for dissociative adsorption is the AuH + OH geometry for all cases. Dissociation becomes more endothermic with the increasing film thickness as shown by the red curve. 4. Discussion 4.1. A link between a charge transfer and an atomic structure The detailed computational analysis of the MgO–metal interface attributed rumpling to a structural response to a interfacial charge transfer and formation of an interfacial dipole [43], where rumpling, r, is defined as a separation between anion and cation atomic layers. Note that a negative rumpling corresponds to the oxygen atoms moving closer to the support metal. A further computational study found correlation between rumpling and the charge state of an adsorbate on a metal supported oxide thin film [44]. Now, we are interested in to find a similar correlation between structural changes in MgO and a charge transfer between the interface and an adsorbate for systems displayed

Table 4 Distances for adsorbed Au, H and OH species. Distances between Au and O in OH-group, Au and H, and the average O–Ag distance at the metal–oxide interface in [Å]. AuOH + H

1ML 2ML 3ML Bulk

AuH + OH

Au + H + OH

dAu–O

dO–Ag

dAu–H

dO–Ag

dO–Ag

1.99 1.99 1.98 2.08

2.77 2.83 2.77

1.58 1.58 1.58 1.58

2.57 2.58 2.52

2.60 2.56 2.51

5

in Fig. 1. The atomic structure of the system responses to a charge transfer from an adsorbate such that O anions move further away from the metal surface while Mg cations move closer to the surface. This leads to a distortion of the film and increases rumpling. Fig. 6a) shows that we find the rumpling of the film to linearly correlate with the charge difference qads − qint, where qads is the Bader charges of the adsorbates and qint stands for an interface charge, which is estimated as the charge state of the Ag support i.e., qint = qAg. Since a zero charge difference should correspond to zero rumpling, a charge difference over a thin film can be estimated with a linear fit qads −qint ¼ a  r;

ð2Þ

where a is a fit parameter. The solid line in Fig. 6a) is the fit and demonstrates the existence of a correlation. The second important structural parameter is the interfacial MgO–Ag distance, d, which is closely linked to the interface charge. The interfacial distance d increases (decreases) upon a charge transfer to (from) the interface. We find that d and qint correlate linearly and they can be estimated via the following simple equation qint ¼ b  d þ c;

ð3Þ

where b and c are fit parameters. This fit is shown in insert b) of Fig. 6. By combining Eqs. (2) and (3), the total charge of the adsorbates, qads, can be estimated with the help of structural parameters r and d as follows qads ðr; dÞ ¼ a  r þ b  d þ c:

ð4Þ

The comparison between Eq. (4) and calculated Bader charges is given in Fig. 6. All of these correlations clearly suggest that the structural response of the surface is a suitable indicator of an adsorbate charge state and thus the structural data can be used to estimate the adsorbate charge state without explicitly considering the electron density. However, our calculations on a larger (3 × 3) computational cell indicate that correlations depend on adsorbate coverage. This is seen as changes in the slopes of the fits in Fig. 6a) and b) and can be explained as follows: Oxide ions further away from an adsorbate are less affected and therefore the amount of rumpling corresponding to a charge difference is reduced. The changes in the total interface charge lead to smaller changes in the charge density at the interface for the larger cell and thus the structural response to an equal change in the total interface charge is smaller for the bigger cell. 4.2. Correlation between H and OH adsorption energies Recent DFT calculations show that on metal surfaces adsorption energies of simple molecules can be estimated with the help of adsorption energies of atoms [45]. Herein, we propose a similar correlation, which allows us to estimate the adsorption energy of an OH group for films with varying thickness using the adsorption energy of a H atom. Specifically, we find that the adsorption energies of a H atom and an OH group depend linearly on each other in the Au-top configurations. The formation of the AuOH configuration leads a neutral, internally covalently bonded AuOH complex. This requires reverse charge transfer from Au to the MgO–Ag interface. Moreover, we need to break down the OH adsorption process into two energy contributions. The first contribution is the reverse charge transfer process: Au −/MgO/Ag + → Au/MgO/Ag. The energy change related to this term is called − ΔECT and it depends on film thickness. The second contribution describes the formation of a AuOH complex as OH + Au/MgO/Ag → AuOH/MgO/Ag. The energy change associated with this process is nearly independent of the support surface, since it mainly depends on the local interaction between Au and OH. Therefore, this energy change can be approximated with OH adsorption energy on Au/MgO(001), ΔEOH bulk. Thus, OH adsorption

Please cite this article as: J. Nevalaita, et al., Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations, Surf. Sci. (2015), http://dx.doi.org/10.1016/j.susc.2015.04.013

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Table 5 The Bader charges q [|e|] of the adsorbed Au, H, and OH species. AuOH + H

1ML 2ML 3ML Bulk

AuH + OH

Au + H + OH

q AuOH

q Os H

q Ag

q AuH

q OH

q Ag

q Au

q Os H

q OH

q Ag

−0.31 −0.30 −0.28 −0.76

−0.95 −0.92 −1.12 −1.05

−0.96 −0.94 −0.97

−0.24 −0.23 −0.23 −0.24

−0.86 −0.87 −0.87 −0.50

0.05 0.15 0.08

−0.75 −0.66 −0.67 −0.55

−0.96 −1.08 −1.08 −1.18

−0.81 −0.86 −0.86 −0.59

0.02 0.10 0.07

energy as a function of a number of MgO-layers, n, can then be written as OH ΔEOH ads ðnÞ ¼ ΔEbulk −ΔECT ðnÞ:

ð5Þ

In a similar fashion, H adsorption energy in an AuH configuration can be split into two contributions, which are the energy change associated with reverse charge transfer − ΔECT and the local binding energy between the neutral Au and H atoms. − ΔECT can be assumed to be adsorbate independent for H and OH, since both AuOH and AuH complexes are neutral and the local binding energy can be approximated with H adsorption energy on Au/MgO(001) ΔEH bulk. The adsorption energy of H is then H ΔEH ads ðnÞ ¼ ΔEbulk −ΔECT ðnÞ:

ð6Þ

Combining Eqs. (5) and (6) gives a relation H OH H ΔEOH ads ðnÞ ¼ ΔEads ðnÞ þ ΔE bulk −ΔEbulk ;

ð7Þ

which is plotted in Fig. 7 and shows the correlation between adsorption energies of H and OH. The correlation stems from the local nature of the Au–adsorbate interaction, which enables to break adsorption energy down into two contributions, where adsorbate dependent part does not depend on the film thickness and vice versa. The correlation allows us to estimate the adsorption energy of the one species when the adsorption energy of the another one is known. The approach is potentially expandable to other adsorbates, which also form a neutral, covalently bonded species with Au-adatom. 4.3. Born–Haber scheme for Au and OH adsorption

and OH species on MgO/Ag(001) as a function of the film thickness. Here, the adsorbates act as acid accepting charge and the metal surface behaves as a base donating charge. The adsorption energy can be analyzed in terms of a Born–Haber cycle [46], and divided into three contributions as follows ΔEads ¼ ΔEicov þ ΔEredox þ ΔEcoul :

ð8Þ

The cycle is schematically shown for Au in Fig. 8a). The first contribution is the local iono-covalent interaction between the adsorbate and the oxide thin film and involves their direct orbital overlapping. The energy change related to this is called ΔEicov and it becomes more endothermic with the increasing film thickness, since the thicker films are less reactive. The second contribution arises from charge transfer from the Ag-support to the adsorbate and the corresponding energy change is termed ΔEredox. For Au, it is calculated by subtracting the energies of the neutral systems Au/MgO and MgO/Ag from the charged ones Au −/MgO and MgO/Ag +. It becomes more exothermic with the increasing film thickness for the studied thickness range. The last term, ΔECoul, stands for the electrostatic interaction between the negatively charged adsorbate and the positively charged Ag-support. It is calculated by subtracting the energies of the charged systems Au −/MgO and MgO/Ag + from the neutral ones MgO and Au −/MgO/Ag +. ΔECoul becomes weaker as a function of film thickness and demonstrates roughly 1/r behavior, where r is the adsorbate–support distance, providing further support to that the energy change, ΔE Coul , describes electrostatic interaction. Fig. 8b) illustrates that adsorption of both OH and Au species weakens with the increasing film thickness. This cannot be fully explained with the decrease in the strength of the electrostatic interaction, ΔECoul, which points to a considerably larger change in adsorption

Finally, we employ the acid–base interaction concept [42] to rationalize the adsorption strength of isolated and negatively charged Au

Fig. 4. Molecular and site specific dissociative adsorption energies for water on Au/MgO/ Ag and Au/MgO surfaces. The structures are shown for the surface with most exothermic adsorption energy. The atomic color code as in Fig. 1. The zero level reference is a gasphase H2O. Dashed lines are a guide to the eye. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. Adsorption energy of water and the most exothermic dissociative adsorption energy for different film thicknesses in the presence of the Au adatom. The Eads (H2O) is the molecular adsorption energy and Eads (H + OH) is the most exothermic dissociative adsorption energy of water. The dissociation energy is given by a red curve. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: J. Nevalaita, et al., Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations, Surf. Sci. (2015), http://dx.doi.org/10.1016/j.susc.2015.04.013

J. Nevalaita et al. / Surface Science xxx (2015) xxx–xxx

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Fig. 6. The total charge of adsorbates on the surface obtained from positions of atoms and Bader analysis. Insert a): Correlation between rumpling and charge difference over oxide film. Insert b): Correlation between Ag–charge and O–Ag distance.

energy than calculated. We find that the decreased Coulomb interaction is partially compensated by an increased energy gain from the redox process. While for the 1 ML film the energy contribution from the Coulomb interaction is more exothermic than the redox energy, the reverse behavior holds for the 2 ML film, where the redox term is now more exothermic compared to the Coulomb interaction. This is in line with the findings for the work function, which is 0.3 eV larger for the 1 ML film than for the 2 ML film [47] and make charge extraction from the surface with the 1 ML thick film more endothermic than from the 2 ML film. Fig. 8b) highlights that the dominant contributions to ΔEads are different for Au and OH. While for OH the energy gain is associated with the redox and Coulomb terms, for Au the leading terms are the iono-covalent and Coulomb interaction for the 1 ML film and the ionocovalent and redox terms for 2 and 3 ML films. Interestingly, ΔEcoul and ΔEredoc display similar trends for both adsorbates but the energies are shifted ≈ 0.5 eV. Since the charge states of Au and OH are nearly identical with OH having on average 0.1 [|e|] more negative Bader charge, the more exothermic values for OH could be due to shorter OH–MgO/Ag(001) distances since the O atom in the OH group is on average 0.7 Å closer to both MgO–Ag interface and the two closest Mg2+ surface cations.

Fig. 7. Scaling relation between H and OH adsorption energies in AuH- and AuOHgeometry.

Fig. 8. (a): Schematic representation of the modified Born–Haber cycle for Au adsorption. (b): Calculated values of ΔEicov, ΔEredox and ΔECoul for adsorption of Au and OH as a function of the film thickness.

5. Conclusions In this work, we investigated the effects of MgO film thickness and interaction with an Au adatom and OH, H and H2O species using DFT calculations. We find that molecular adsorption of water on MgO/Ag(001) is more exothermic in the presence of the Au atom due to an attractive electrostatic interaction between the negatively charged Au atom and the polar water molecule. On bulk MgO(001), the Au atom is neutral and the electrostatic interaction is missing. The adsorption of dissociation products of water leads to formation of neutral AuH and AuOH species. This requires a charge transfer from the Au-adatom to the support. The adsorption of OH and H in the separate computational cells becomes more exothermic with the increasing film thickness, whereas the reverse trend is seen for Au adsorption. Moreover, since the energy associated with a charge transfer between the Au atom and the support is approximately independent of the adsorbate, the adsorption energies of H and OH are correlated for AuH and AuOH complexes. The distortions of a MgO–film and the MgO–Ag-interface distance are shown to correlate with Bader charges of the adsorbates and the MgO–Ag interface. The rumpling of the film correlates with the charge difference over the film while the interface distance correlates with the interface charge. These correlations were used to estimate the total charge of adsorbates using only the atomic positions. To unravel the adsorption characteristics Au and OH, we employed the modified Born–Haber cycle to analyze the role of different energy contributions to adsorption energies as a function of the film thickness. The decrease in attractive Coulomb interaction between the adsorbate and the

Please cite this article as: J. Nevalaita, et al., Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations, Surf. Sci. (2015), http://dx.doi.org/10.1016/j.susc.2015.04.013

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support is compensated by a more exothermic redox energy associated with a charge transfer. Acknowledgments This work was financially supported by the Emil Aaltonen Foundation through Finnish Academy of Science and Letter. The computational resources were provided by the Nanoscience Center University of Jyväskylä and the Finnish IT Center for Science (CSC) Espoo.

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Please cite this article as: J. Nevalaita, et al., Dissociative adsorption of water on Au/MgO/Ag(001) from first principles calculations, Surf. Sci. (2015), http://dx.doi.org/10.1016/j.susc.2015.04.013