CO adsorption on graphite-like ZnO bilayers supported on Cu(111), Ag(111) and Au(111) surfaces
Version 4.9.2017 Graphitic-like ZnO bilayer films deposited on coinage metals, Cu(111), Ag(111), and Au(111) have been studied by density functional theory calculations including dispersion corrections. The scope is to compare on an equal footing the properties of the three systems and in particular the nature of the metal/oxide interface. To this end we have considered the adsorption of a CO probe molecule and the vibrational shifts induced by adsorption on ZnO/Cu(111), ZnO/Ag(111), and ZnO/Au(111) compared to adsorption on the unsupported ZnO bilayer and on the wurtzite ZnO surface.We find that while the interaction of ZnO with Ag and Au supports is dominated by dispersion interactions with little or no charge transfer at the interface, in the case of Cu a moderate electron transfer occurs towards the ZnO bilayer. As a consequence, while the stretching frequency of CO on ZnO/Au is blue-shifted, that on ZnO/Cu is redshifted compared to free CO. CO on ZnO/Ag is intermediate. In all three cases, however, the ZnO bilayer is almost flat, with a modest rumpling found in the case of Cu as a consequence of the stronger chemical interaction. The results fully explain the CO vibrational shifts of CO on ZnO/Cu(111) [Schott, V. et al. Angew. Chem. Int. Ed. 2013, 52, 1-6] without implying major distortions in the supported film.
Introduction<!>Computational details<!>Graphitic-like ZnO on Cu(111), Ag(111) and Au(111) surfaces<!>CO ZnO wurtzite and unsupported graphitic ZnO bilayer<!>54). Using adsorption isotherms Esser and<!>CO on ZnO/Cu(111), ZnO/Ag(111) and ZnO/Au(111) films<!>Comparison with previous models<!>Conclusions<!>Aknowledgments
<p>The role of the metal/oxide interface in heterogeneous catalysis by supported metals cannot be underestimated. 1 Increasing theoretical and experimental evidence points to the fact that the region of contact between a metal nanoparticle and an oxide support presents various sites where reactivity is enhanced compared to other regions of the catalyst. 2 For instance, it has been shown that oxygen atoms at the metal/oxide boundary can be removed more easily, 3 resulting in a higher reducibility of the oxide without supported metal particles. This concept is also at the basis of the so called inverse catalysts where an oxide nanoparticle is grown on the surface of a metal. 4,5,6 Here two effects contribute to an enhanced reactivity, the presence of low-coordinated ions in the oxide nanostructure, and the contact of the oxide with the metal. This aspect is particularly relevant since the metal support can provide an unlimited source of electronic charge to activate chemical bonds of species adsorbed on the hybrid catalyst.</p><p>A similar concept is at the basis of the field of ultrathin, two-dimensional (2D) oxide films supported on a metal. 7,8 It has been shown, first theoretically then proven experimentally, that 2D oxides can behave very differently from their bulk counterparts.</p><p>For instance, ultrathin films of MgO deposited on Ag or Mo allow the exchange of charge between the adsorbed species and the metal support, 9 thus providing an alternative way to activate adsorbed molecules (e.g. O2) 10 or to charge supported clusters (e.g. Au). 11 2D MgO films are also used to decouple magnetic atomic adsorbates from a metallic support in order to measure their magnetic properties. 12 Most of ultrathin oxide films on metals are produced in very controlled conditions, e.g. in UHV, in order to generate model systems of real catalysts. However, there are cases where these films grow spontaneously under real catalytic conditions. A relevant example is that of ZnO thin films grown on Cu. The interest around this system is not purely academic since Cu/ZnO mixed catalysts are used industrially in methanol synthesis by hydrogenation of CO and CO2. The presence of a thin ZnO layer which forms under reactive conditions on the surface of Cu has been suggested some time ago, 13,14 and later supported by specific experiments. 15,16,17 In particular, Schott et al. 15 have prepared 1-3 monolayers of ZnO on a brass surface and concluded that due to the interaction with the support the ZnO adlayers adopt a distorted structure in between the ideal wurtzite (0001) and the planar modification of ZnO, also known as graphitic ZnO.</p><p>They also found a lower oxidation state of the Zn ions which leads to a different interaction of molecules such as CO and pyridine with the surface of the thin ZnO film compare to wurtzite. 15 The formation of the graphitic-like ZnO overlayer with hexagonal BN structure (h-BN) on Cu has been reported by Lunkenbein et al. 16 as the consequence of strong metal support interaction (SMSI) on real Cu/ZnO/Al2O3 catalysts. To this end, high-resolution electron microscopy images have been reported.</p><p>The graphene-like ZnO layer is a metastable structure that evolves into the classical wurtzite structure as the thickness is increased or under a thermal stimulus provided by the electron beam. 16 ZnO crystallizes in the wurtzite structure under ambient conditions. However, a graphitic or layered phase with layer stacking following that in h-BN was predicted as the most stable phase of ZnO up to 18 monolayers. 18 Indeed, a nonpolar two monolayer thick ZnO(0001) film was grown on Ag(111) and characterized by x-ray diffraction and scanning tunneling microscopy (STM) by Tusche et al. 19 Later studies have shown using field emission resonance spectroscopy that the local work function of Ag(111) slightly decreases for 2 ML thick ZnO but it dramatically drops by around 1.2 eV between 2 and 3 ML suggesting a structural transformation of the ZnO layer. 20 The CO oxidation behavior of ZnO films grown on Ag(111) and Cu(111) has also been investigated and it has been found that on Ag(111) the planar structure of the films is maintained in the course of the reaction. 21 Graphitic-like ZnO films and nanostructures on Ag(111) have then been characterized theoretically in various studies. 22,23 Ultrathin ZnO nanostructures have also been grown on Au(111) using reactive deposition at room temperature and characterized by X-ray photoelectron spectroscopy, STM, and DFT calculations. 24 Also in this case one and two ML ZnO adopt a planar, graphitic-like structure. More recently, ZnO thin films with graphitic structure have been obtained on Au(111) by pulsed layer deposition. 25 The reactivity of the planar ZnO(0001) bilayers supported on Au(111) towards D2O has been studied using temperature programmed desorption, X-ray photoemission, and DFT. 26 Finally, ZnO bilayer formation has been obtained also on other supports, such as Pd(111) 27 or Pt(111). 28 In a comparative theoretical study it has been shown that a ZnO monolayer adopts the preferred h-BN structure on various metals (Ag, Cu, Pd, Pt, Ni, and Rh) while thicker ZnO layers evolve into a wurzite structure. 29 In this theoretical study we investigate in a systematic way the nature of the interaction of a ZnO bilayer with three metal surfaces, Cu(111), Ag(111), and Au(111).</p><p>The purpose of the work is to study using exactly the same computational approach the three interfaces, and in particular to establish the presence of a chemical interaction between the metal support and the extended 2D oxide film. This is relevant because the special catalytic activity of Cu/ZnO hybrid structures has been attributed to a modified oxidation state of the Zn ions and to the occurrence of a given charge transfer at the interface. 15 These conclusions have been largely based on the calculation of the vibrational properties of an adsorbed CO probe molecule. 15 Therefore, we have considered CO adsorption on free-standing ZnO bilayer films and on the same films supported on Cu(111), Ag(111) and Au(111) surfaces. For comparison, also CO adsorption on wurtzite has been considered. It turns out that the theoretical description of CO vibrational properties on ZnO is less simple than one could expect, and that not always this aspect has been carefully considered in previous reports.</p><p>The study of the CO vibrational properties provides a way to identify the extent of the charge transfer at ZnO(0001)/M(111) (M = Cu, Ag, Au) interfaces which can be directly compared to experiment. The results show indeed a variation in the CO vibrational shift as one moves down the series, from Cu to Au, with a negative shift for Cu and a positive shift for Au. This reflects the slightly different nature of the bonding of ZnO with the three coinage metals. Most important, our work shows that the observed red-shift when CO is adsorbed on ZnO/Cu(111) films can be explained without implying a large distortion in the film. 15</p><!><p>Density functional theory (DFT) based calculations were performed with the Vienna Ab-Initio Simulation Package (VASP 5.3), 30,31 with plane waves as basis set with a kinetic energy cutoff of 400 eV. The core electrons are treated with the Projector Augmented Wave approach. 32,33 C and O(2s, 2p), Zn(4p, 3d), Cu(4s, 3d), Ag(4d, 5s)</p><p>and Au(5d, 6s) electrons are considered explicitly. The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, within the Generalized Gradient Approximation (GGA) was applied. 34 The GGA+U approach implemented by Dudarev et al. 35,36 was used to partly correct the self-interaction error intrinsic in GGA functionals. An onsite Coulomb correction, Ueff = U -J, was set to 4.7 eV for the Zn 3d orbitals, as previously suggested by Janotti and Van de Walle. 37 Long-range dispersion forces are included by means of the D2' method 38 on top of the PBE functional; this is a slight modification of the original parametrization of the D2 approach proposed by Grimme 39 where we changed the parameters C6 and R0 of the DFT-D2 approach, as suggested by Tosoni and Sauer. 38 For several cases, the PBE+U+D2' results have been crosschecked versus PBE+D2', opt-B86b-vdW+U, or PBE0+D2' approaches. The comparison of PBE+U+D2' with PBE+D2' provides an indication of how important is the use of the DFT+U approach for the present case. The comparison with PBE0+D2' allows to verify our results against the more accurate hybrid functionals (PBE0 in this case); 40 finally, the role of dispersion has been checked by comparing PBE+U+D2' with opt-B86b-vdW+U calculations. In this latter case the vdW functional proposed by Langreth, Lundqvist et al. is employed. 41,42,43,44 This is a non-local correlation functional that approximately accounts for dispersion interactions. For the hybrid functional calculations with PBE0 functional, in order to reduce the computational cost, we employed the CRYSTAL14 code, 45 using a localized periodic-optimized-TZVP basis set. 46 The lattice parameters of Cu, Ag, Au and ZnO (both wurtzite and free-standing bilayer) have been relaxed with a kinetic cutoff of 600 eV. For Cu, the PBE+D2' approach gives a lattice constant of 3.57 Å, quite close to the experimental value of 3.61 Å. 47 Also for Ag and Au the lattice constants are close to the experiment (4.15 Å compared to 4.09 Å 47 for Ag and 4.12 Å compared to 4.08 Å 47 for Au). In the case of wurtzite (hexagonal lattice), X-ray diffraction experiments quantify the lattice parameters as a,b = 3.25 Å and c = 5.21 Å, 48 while the PBE+U+D2' scheme yields a,b = 3.22 Å and c = 5.20 Å. For the ZnO bilayer, finally, we obtain with PBE+U+D2' a lattice constant of the 2D hexagonal lattice of 3.29 Å. Since no comparison to the experiment is possible, we performed a test calculation with the same plane-wave approach using the higher-quality screened hybrid functional HSE06, 49 obtaining a very similar value (3.32 Å).</p><p>Simulation of CO adsorption on the stable, non-polar surface of wurtzite is performed on supercells of growing size spanning from 1×1 (CO coverage of 1ML), to 2×1 (CO coverage of 0.5 ML), and 3×2 (CO coverage of 0.167 ML). In all cases, a six-atomic layers slab model is adopted, where the two-bottom layers are frozen to the bulk lattice positions while the top four layers and the adsorbate are free to relax.</p><p>The structures of metal-supported ZnO bilayer (BL) consist of four layers of metal where only the bottom layer is frozen, the rest being fully relaxed. The strain due to the ZnO-metal lattice mismatch is, in all cases, released on ZnO. Proper interfaces are built in order to reduce the strain as much as possible by recurring to larger supercells. For ZnO/Cu, a 3×3 ZnO(BL) superlattice is supported on a (4×4) supercell of Cu(111), releasing a strain of -2.32% on ZnO. A negative strain means that the oxide film has to be slightly expanded to adapt it to the Cu(111) support (tensile strain). This differs from other calculations reported in the literature where a small compressive strain (of about 1%) was found in the supported ZnO film on Cu(111) by superimposing a 4x4 ZnO supercell to a (5x5) metal substrate. 29 With the presently adopted PBE+U+D2' approach, the (4x4) ZnO on (5x5) Cu superlattice would lead to a compressive strain as large as 4.2%. We therefore rely on the minimal strain structure, even if this is different from the one previously adopted in the literature. We think that this discrepancy with the previous reports in the identification of the superstructure corresponding to the minimal strain is due to the use of a DFT+U approach for ZnO. The different strain in the models used here and in previous studies, tensile versus compressive, may be the origin of some differences found in the rumpling of the supported films (see below).</p><p>For both ZnO/Ag 18 and ZnO/Au, 24 a superlattice of ZnO BL (7×7) on Ag/Au (8×8) has been experimentally reported. We adopted these superstructures in the present calculations, releasing on the ZnO BL the residual strain of -1.86% and -1.17% for Ag and Au, respectively. The geometry optimizations were carried out with a 2×2×1 centered K mesh for the ZnO/Cu interface and at the Γ-point only for the other cases, up to ionic forces smaller than |0.05| eV/Å and a threshold of 10 -5 eV for the electronic self-consistent cycles.</p><p>Before to discuss the CO adsorption properties, we report the characteristic features of the free CO molecule as obtained with the present methods. At the PBE+U+D2' level free CO exhibits a bond distance of 1.144 Å and a harmonic frequency (CO) = 2125 cm -1 ; very similar values are obtained at the opt-B86b-vdW level, 1.143 Å and 2128 cm -1 . The experimental values are 1.128 Å and 2143 cm -1</p><p>(anharmonic) and 2170 cm -1 (harmonic). 50 More than the absolute values, what counts here are the shifts related to the interaction of CO with the solid surface.</p><!><p>In this section we describe the nature of the ZnO bilayer (BL) on the three metal surfaces. When studying metal/oxide interfaces, a very important aspect is to control and minimize the strain between the two components. Here we have been able to find some combinations of ZnO and metal surfaces that limit the strain to around -2%, Fig. 1 and Table 1, a value which should avoid unphysical reconstructions or rumpling. For ZnO/Cu(111), we found a very small rumpling for the two layers, slightly more pronounced for the bottom ZnO layer at direct contact with the metal, Table 1.</p><p>The rumpling of a given ZnO layer L is calculated according to:</p><p>Where NL is the number of ions included in the layer, Zi is the z-coordinate of the i-th ion and Zav the z-coordinate of the layer's center of mass. In the case of ZnO/Cu(111), on the bottom layer B an average rumpling of 0.15 Å is found, while on the topmost layer T the average rumpling is smaller, 0.09 Å. As one can see in Figure 1, both O and Zn ions contribute to the rumpling.  20 The absence of charge transfer or covalent bonding and of rumpling in the supported ZnO bilayer film on Ag(111) is in agreement with previous experimental and theoretical studies. 18,21,53 Finally, very similar results are obtained for ZnO/Au(111). Also in this case the film is flat, with virtually no rumpling, Table 1. The interface distance, 2.85 Å, is slightly longer than in case of Ag and the adhesion energy slightly smaller, 37.3 meV/Å 2 . Also here the charge transfer is very small, but of opposite sign compared to Ag (the Bader charge per ZnO unit is +0.01 e) and there is a small reduction of the metal work function, by 0.17 eV. The general characteristics of the ZnO/Au(111)</p><p>bilayer studied here confirm those reported in previous studies (bonding dominated by dispersion forces, absence or very limited rumpling). 24,25 In summary, we found that the graphitic-like ZnO bilayer binds to Cu(111), Ag(111), and Au(111) with similar characteristics, but also some relevant difference. In all cases the oxide film remains graphitic-like, flat or almost flat. Some deviation is found for the case of Cu, although this rumpling is much smaller than that reported in ref. 15 where a pronounced distortion has been observed. Also the bonding has a partly different character. For ZnO on Cu(111) we found a small, but non negligible charge transfer from the Cu metal to the oxide film, which also reflects in a substantial increase of the work function. None of these effects is seen for ZnO on Ag(111) or Au(111)</p><p>supports. Here the films are atomically flat, the interface distance is longer, the charge transfer is absent, and the work function is slightly reduced compared to the free metal.</p><!><p>To calibrate our approach, we have considered the adsorption of CO on the surface of ZnO wurtzite. In Table 2 we report the values calculated for CO adsorbed on top of a Zn cation of the wurtzite surface, Fig. 2a, as obtained with different methods and for different coverages. CO binds with a Zn-C distance of 2.13 Å and a tilt angle from the surface normal of 37°; the CO binding energy is of about -0.5 eV, Table 2.</p><p>Various values have been reported in the literature (for a discussion on the difficulty to measure the binding of CO to ZnO see ref.</p><!><p>Göpel estimated an adsorption energy of -0.52 eV, 54,55 similar to various theoretical estimates. 56,57 FTIR experiments on ZnO powders show that the CO frequency shifts with the coverage from 2190 cm −1 (θ ≈ 0, singleton) to 2168 cm −1 (θ = 1), as a consequence of through-space interactions among the oscillators, 58 an effect recently confirmed by IRRAS measurements on ZnO single crystals (the frequency shifts from 2185 cm -1 at θ = 0.5 to 2169 cm -1 for θ = 1). 59 Similar values have been reported in other studies on wurtzite ZnO powders (2187 cm -1 ) 60 or single crystals (2192 cm -1 ). 61 This corresponds to a shift  = +47±3 cm -1 in the limit of zero coverage with respect to the free molecule. It turns out that the positive shift measured experimentally and due to the electrostatic interaction with the local electric field generated by the exposed Zn cation is qualitatively reproduced by the calculations, but not quantitatively. In fact, in the limit of very low coverage a  = +9 cm -1 is computed at the PBE+U+D2' level, Table 2. This shift is five times smaller than the experimental one. In order to find the origin of this underestimation, we have recomputed the same property at two different computational levels. First we used the opt-B86b-vdW+U approach to check that the problem does not reside in the method used to estimate dispersion forces. The calculations show a positive shift of +3 cm -1 and similar adsorption properties, see Table 2. We conclude that the problem is not due to the adopted dispersion corrections.</p><p>Indeed, it is well known that the CO molecule is not properly described with standard functionals since the 2π MO of CO lies too low in energy (too small HOMO-LUMO gap) which can be the origin of the low frequencies computed here (a low 2π empty state on CO will result in a too large back donation from Zn to CO and in a red-shift of the CO vibration). Therefore, we have considered the same system by using a hybrid functional, PBE0, and the same dispersion correction, PBE0+D2'. Hybrid functionals correct the position of the Kohn-Sham energy levels, and give higher HOMO-LUMO gaps in closer agreement with experiment. With this method, the free CO molecule has a bond distance of 1.128 Å and a harmonic frequency, (CO) = 2220 cm -1 . When the molecule is adsorbed on ZnO wurtzite, the bonding exhibits a similar strength, 0.47 eV, a similar Zn-CO distance, 2.18 Å, and a positive shift of +13 cm -1 , Table 2. The shift is a bit larger than with the DFT+U approach, but still almost 4 times smaller than the experimentally measured shift.</p><p>This seems to be a general problem for theory, and quite surprisingly it has not been addressed in detail in the literature so far (to the best of our knowledge). Clearly more work is needed to understand the origin of this discrepancy and of the too low blue shift of CO adsorbed on wurtzite ZnO. For the moment we have to consider this error as intrinsic in our calculations, but this discussion is very important for the final conclusions of this work and for the interpretation of the experiments on CO adsorbed on ZnO/Cu(111) films.</p><p>Having established that in the limit of zero coverage the CO stretching frequency is blue-shifted by 9 cm -1 , we have considered the coverage effects and we found the same trend observed experimentally of a decreasing CO frequency as the coverage increases. In the experiment going from θ = 0, to θ = 1 there is a shift of -22 cm -1 ; in our calculations going from θ = 1/6 to θ = 1 the shift is -25 cm -1 , very similar.</p><p>However, while in the experiment even at full coverage the final shift remains positive, +25 cm -1 , in our case the too low initial value of (C-O) leads to a red-shift for full coverage of CO on ZnO, with a  = -17 cm -1 , Table 2. Thus, the coverage effect is correctly described, at variance with the absolute value of the CO frequency shift.</p><p>We consider now CO adsorption on the unsupported ZnO bilayer (Figure 2b).</p><p>Here, of course, no comparison with experiment is possible. On the other hand, the idea is to compare, using precisely the same computational approach, CO adsorbed on graphitic ZnO and on the same film supported on Cu(111), Ag(111) and Au(111)</p><p>surfaces. Here, the lattice parameters of the unsupported ZnO bilayer have been fully</p><p>optimized for each method used, PBE+U+D2', opt-B86b-vdW+U, PBE0+D2', Table 3.</p><p>The results are very similar, going from 3.28 to 3.32 Å.  Only very low coverage, θ = 1/9, has been considered (notice that the CO-CO lateral distances are slightly different because of the different lattice constants of the ZnO bilayer computed with different approaches, see above). The results are rather similar independently from the adopted method, and can be summarized as follow: the binding of CO to the ZnO bilayer is much weaker than on the wurtzite surface, e.g. -0.18 versus -0.53 eV at PBE+U+D2' level. The distance of CO from the surface is dependent on the method and on the tilt angle of the CO molecule, and goes from 2.28 to 2.48 Å, Table 3. Notice that at the PBE0+D2' level (hybrid functional and Crystal code) the molecule forms an angle of 43°, probably because of the basis set superposition error present in calculations based on localized basis functions. On the contrary, virtually no effect is observed on the C-O distance, which remains very close to that computed for the gas-phase molecule. Finally, the vibrational shift with respect to free CO. This is close to zero (+3 cm -1 , PBE+U+D2') or slightly negative (-12 cm -1</p><p>with the other two approaches).</p><p>In conclusion, on the unsupported ZnO bilayer CO is more weakly bound than on wurtzite, and it has small red shift or no shift of the CO vibrational frequency. On wurtzite the bonding is stronger, and the vibrational frequency is blue-shifted, although the absolute shift is severely underestimated compared to the experiment.</p><!><p>In this section we consider the adsorption of a CO molecule on the top layer of the ZnO ultrathin films deposited on the three metal surfaces. We start considering CO adsorbed on-top of the Zn cations of the ZnO/Cu(111) films. Here CO binds with De = -0.18 eV, a bond length of 1.145 Å, a tilt angle of 10°, and exhibits a stretching frequency of 2102 cm -1 , with a red shift of -23 cm -1 with respect to free CO. These results have been obtained at the DFT+U+D2' level. We also analyzed the role of dispersion forces by repeating the calculations at the opt-B86b-vdW+U level, but the results are very similar:</p><p>the bonding is a bit stronger, -0.26 eV, and the red-shift a bit smaller, -19 cm -1 . Thus, only the DFT+U+D2' method has been adopted for the other metals. The results for CO on ZnO/Cu(111) should be compared with those obtained at the same level of theory on graphitic-like ZnO bilayer, Table 3. The CO adsorption energies have remained almost unchanged going from the unsupported to the supported layer. A small change is found at the level of CO stretching frequency, and frequency shift. In fact, we notice a clear red shift at the DFT+U+D2' level (from +3 to -23 cm -1 ). This shift is smaller but still present at the opt-B86b-vdW+U level (from -12 to -19 cm -1 ). The presence of a negative shift is consistent with the data of the Bader analysis of charge and of the work function change for ZnO/Cu indicating a small charge flow from Cu towards ZnO. shift with respect to the gas phase CO, . a From ref. 15 We consider now CO adsorption on ZnO/Ag(111). We notice only very minor changes going from the unsupported to the supported oxide film, see Table 3 and 4. The most relevant one is that the CO stretching frequency is a bit higher than on the free standing ZnO bilayer (+3 cm -1 , Table 4; +6 cm -1 compared to free CO). This is different from CO on ZnO/Cu(111) where there is a red-shift, Table 4, showing that the interface with Ag induces very little electronic effects on the thin ZnO film.</p><p>The last case is that of CO adsorption on ZnO/Au(111). Here we notice a small reinforcement of the bonding (from -0.18 eV on unsupported ZnO to -0.27 eV on ZnO/Au), a slightly shorter the C-O distance (from 1.142 to 1.140 Å) and a consequent blue-shift of the adsorbed CO molecule, by +25 cm -1 if measured with respect to free CO and by +22 cm -1 considering the unsupported ZnO bilayer. This can be attributed to the positive electrostatic potential resulting from the small but non-zero charge flowing from the ZnO film towards Au, Table 1. These results show that, despite the relatively long interface distance, 2.85 Å, some modifications are induced in the ZnO film by the Au support and that as a result, CO binds differently than on the free ZnO bilayer.</p><!><p>It is important to compare our results with the recent study of Schott et al. 15 dealing with ZnO/Cu(111) films. In this paper it is found, based on theoretical DFT calculations, that the perfectly planar graphitic ZnO bilayer is unstable when brought into contact with a Cu substrate. 15 As we have seen above, this is not what we found in the present study. We do find a small rumpling, but much smaller than that reported in ref. 15. A possible explanation is the different direction of the small residual strain in the two studies. While in ref. 15 the strain is compressive, in our case the strain is tensile. It is not surprising that a tensile strain results in a more flat film (and viceversa).</p><p>Indeed, we obtain a structure with a strong rumpling (0.32 Å and 0.24 Å average rumpling on bottom and top ZnO layers, respectively) with a compressive strain of 4.2% in a ZnO(4x4)/ Cu(5x5) structure.</p><p>Since the extent of rumpling is not accessible experimentally, the analysis of the CO adsorption becomes crucial, Table 5. The first consequence of the different distortions found in the ZnO film is that the binding of CO to the distorted film, -0.52 eV according to ref. 15, is stronger than that found in our study, about -0.2 eV. This is clearly due to the more exposed nature of the Zn ions if the film is distorted. We come now to the comparison of the CO vibrational shifts.</p><p>Table 5 -Vibrational frequencies  for CO adsorbed on ZnO wurzite and ZnO/Cu(111) thin films, and shifts  referenced to the respective gas-phase CO frequency.</p><p>Exp. 15 DFT, this work DFT 15  (cm -1 )  (cm -1 )  (cm -1 )  (cm -1 )  (cm -1 )  (cm  15 measured a frequency for CO/ZnO/Cu(111) of 2116 cm -1 , with a redshift of -73±3 cm -1 with respect to wurtzite (single crystals or powders). However, another useful reference is the free CO molecule. In the experiment, taking free CO as a reference (2143 cm -1 ), CO on wurzite is blue shifted by +47 cm -1 , while CO on the ZnO film is red-shifted by -27 cm -1 , Table 5. This clearly shows that CO binds with a different mechanism on the surface of wurzite and ZnO/Cu(111) films. In our calculations, CO has a frequency of 2134 cm -1 on ZnO , with a positive shift of +9 cm -1 , and of 2102 cm -1 on ZnO/Cu(111), with a red-shift of -23 cm -1 , compared to free CO (2125 cm -1 ). Thus, the opposite direction of the shifts in the two systems is properly reproduced, and in the case of the thin film the magnitude of the shift, -23 cm -1 is close to the experimental one (-27 cm -1 ), We conclude that the large red-shift in CO stretching frequency going from wurtzite to graphitic-like ZnO bilayer 15 can be explained without implying that the film undergoes an important rumpling.</p><!><p>In this work we have considered the interaction of ZnO bilayers with Cu(111), Ag(111), and Au(111) surfaces using a DFT+U approach including dispersion interactions via a classical additive term. First of all, we have checked the method, PBE+U+D2', against other ways to include dispersion, such as the Lundqvist functional. We also verified the outcomes against hybrid calculations including the PBE0 functional. The results, however, are rather similar, so for the rest of the discussion we refer to the PBE+U+D2' results.</p><p>We first considered the three interfaces, graphitic-like ZnO bilayer on Cu(111), Ag(111), and Au(111). Particular attention has been given to the problem of strain. The used supercells guarantee that the strain on the ZnO film is always below 2.5%. The results show that the ZnO bilayer forms a regular, almost undistorted film on the three metals, with the exception of Cu where a very small rumpling is found (about 0.1 Å).</p><p>The bonding is partly different on Cu compared to Ag and Au. While on Ag and Au it is dominated by dispersion, on Cu there is also some modest but non-zero charge transfer in the direction of the oxide. This results in an increase of the work function of the system by about 0.7 eV. On the contrary, on Ag and Au where the charge transfer is negligible, the film induces a compressive effect of the metal charge density that results in a small reduction of the work function, in full agreement with some recent measurements. 20 Next we considered the CO adsorption properties on unsupported and supported ZnO bilayers. Since we make use of CO as a probe molecule to check the changes in oxidation state or electrostatic potential on the surface of the graphitic-like ZnO bilayer, we first studied the properties of the same molecule adsorbed on the surface of ZnO wurtzite. For this system in fact, clean experimental data exist, 54,55 showing a binding energy of about 0.5 eV, a blue shift of +473 cm -1 of the CO stretching frequency in the low coverage limit, and a decrease of (CO) as the coverage  increases. While the theoretical results correctly reproduce the strength of the bond and the decrease of (CO) with increasing , they fail in reproducing quantitatively the large blue shift. Only a moderate blue-shift is found in the calculations for low coverage of CO, of the order of about +10 cm -1 , with an underestimation of about 40 cm -1 . The origin of this problem, that has not been discussed so far in the literature, remains unclear, and further work is needed to clarify it.</p><p>Then we have considered CO adsorption on the Zn cations of the supported film.</p><p>The results have been compared with those obtained on the unsupported ZnO bilayer, computed in its relaxed structure. Very little changes are found in the bond strength of CO to the ZnO bilayer as a function of the support. This is 0.2 eV on the free-standing ZnO film, and goes from 0.18 to 0.27 eV on the supported films, with very little change.</p><p>Not surprisingly, the bonding is much weaker than on wurtzite (about 0.5 eV), due to the flat nature of the ZnO graphic-like surface, see Fig. 2. Some significant differences emerge when we considered the CO vibrational properties. On ZnO/Cu(111) there is a net red shift of about -20 cm -1 with respect to the unsupported film (and with respect to gas-phase CO) that reflects the small flow of charge from Cu to Zn, with consequent increase of the back donation. On ZnO/Au(111), on the contrary, the frequency is clearly blue-shifted (again with respect to both free CO and to CO/ZnO bilayer) which reflects a change in the electric field at the surface. The ZnO/Ag(111) case is in between.</p><p>For ZnO/Cu(111) films experimental and theoretical data have been reported for CO adsorption. Our results agree qualitatively with those of Schott et al. 15 in these aspects: a small charge transfer occurs from Cu to ZnO, the film is distorted (although only slightly), and the CO stretching frequency is red-shifted compared to free CO (by -27 cm -1 in the experiment 15 and by -23 cm -1 in our calculations). However, we cannot reproduce the strong interaction of Cu with ZnO, nor the large distortion, which also results in a large binding of CO to the bilayer. The large red-shift measured experimentally when CO/ZnO/Cu(111) is compared to CO/ZnO -73 cm -1 , is qualitatively reproduced by the calculations which indicate a corresponding shift of -32 cm -1 ; the difference must be attributed to the underestimate by about 40 cm -1 of the computed CO/ZnO(1010) blue shift. Even more convincing are the shifts for CO/ZnO/Cu(111) with respect to free CO: -27 cm -1 in the experiment, -23 cm -1 in this work, as mentioned above. Thus, we conclude that the red-shift of CO adsorbed on ZnO/Cu(111) can be explained without implying major distortions in the oxide film.</p><p>Some care is necessary since the entire discussion is based on CO frequency shifts, theory has a problem in reproducing the frequency shift of CO on wurzite, and some residual strain is always present in the models used. Thus, this result must be considered as a stimulus to invest more work to unravel the precise structure of the ZnO/Cu(111) films.</p><p>To summarize, ZnO/M(111) films, produced experimentally in various ways in recent years, represent very interesting systems to study the properties of twodimensional oxides. The small but significant changes in the properties of the three interfaces, for instance of their work functions, can be used in principle to tune the properties of adsorbed species. Work is in progress to study the response of atomic and molecular adsorbates deposited on the three systems.</p><!><p>We thank Prof. Christof Wöll, Prof. Karsten Reuter, and Prof. Piero Ugliengo for useful discussions, and Prof. Karsten Reuter for making available some unpublished data. This</p>