Evidence of charge transfer to atomic and molecular adsorbates on

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Evidence of charge transfer to atomic and molecular adsorbates on ZnO/ X(111) (X = Cu, Ag, Au) ultrathin films. Relevance for Cu/ZnO catalysts. Ho Viet Thang, Sergio Tosoni, and Gianfranco Pacchioni ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03896 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Evidence of charge transfer to atomic and molecular adsorbates on ZnO/X(111) (X = Cu, Ag, Au) ultrathin films. Relevance for Cu/ZnO catalysts. Ho Viet Thang, Sergio Tosoni, Gianfranco Pacchioni* Departimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, 20125 Milano, Italy

ABSTRACT ZnO bilayers grown on Cu(111), Ag(111), and Au(111) surfaces belong to the class of twodimensional materials. We show, by means of density functional theory (DFT) calculations including dispersion, that Au, NO2, and O2 species adsorbed on ZnO/Cu(111) induce a spontaneous net charge transfer (CT) via electron tunneling from the Cu support through the insulating ZnO film, resulting in the formation of negatively charged atomic, Au−, or molecular, NO2− and O2−, adsorbates. We show for the case of gold that the CT is found also for ZnO/Ag(111) and ZnO/Au(111) interfaces. The stabilization of the anionic species is accompanied by a polaronic distortion of the ZnO lattice. Other molecules with low electron affinity such as NO and CO2, on the contrary, do not induce the CT. However, charge transfer and activation of two CO2 molecules to form an oxalate species, [C2O4]2-, is promoted by Au1 or Au2 supported anions, and similar catalytic activity is expected for the negatively charged Au clusters. By comparing the properties of the metal supported ZnO films with those of the freestanding ZnO bilayer, we demonstrate the key role of the metal/oxide interface. These results are relevant in the field of methanol synthesis based on ZnO/Cu catalysts where ultrathin layers of ZnO supported on Cu are formed under reaction conditions.

KEYWORDS: charge transfer, ZnO thin films, adsorption, density functional theory, metal/oxide interface

*

Corresponding author: [email protected] 1 ACS Paragon Plus Environment

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1. INTRODUCTION Thin films of insulating materials deposited on a metal support exhibit important properties and find application in electronic devices, 1 in catalysis, 2 sensors, 3 and as adsorption systems with peculiar properties.4 This is largely due to the effect of support. Under special circumstances, despite the insulating nature of the thin layer, an electronic communication is established between the metal support and atomic, metallic or molecular species adsorbed on the top layer of the thin insulating film.5 This mechanism has been originally predicted for the adsorption of Au atoms on MgO/Mo(100) films;6,7 later, the occurrence of a spontaneous charge transfer has been proven experimentally by low-temperature STM measurements8 and extended to gold particles of various size.9,10,11,12 Also molecular species such as O213,14 and NO215,16 can induce an electron transfer from the underlying metal support when deposited on MgO thin films. In other cases, the charge transfer can occur in the opposite direction, from an adsorbed species towards the support, as shown for the case of Au atoms deposited on FeO/Pt(111) ultrathin films which become positively charged. 17 The charge flow occurs via electron tunneling from (or to) the metal support through the thin insulating layer.18 The properties of the films depend on the film thickness which can be modified to tune the nature of the supported species. The observed electron transfer processes require the thickness to remain small, typically below 1 nm or 3-4 atomic layers. Beyond this dimension, the tunneling probability goes to zero and the film behaves as a normal insulator, preventing any flow of charge to or from the metal support. Stimulated by this important finding, several oxides have been prepared in form of ultrathin films (two-dimensional oxides) in the last decade, including SiO2,19 ZrO2,20,21,22 ZnO,23,24,25 and others. 26 However, until now the formation of negatively charged adsorbates on oxide ultrathin films is restricted to the case of MgO films. Zinc oxide (ZnO) is a material widely used in many applications, including catalysis and solar cells. 27 For this reason, the structure and properties of ZnO have been investigated intensively. Freestanding ZnO28 thin films or ZnO bilayers deposited on different metal supports, Pd, 29 Cu, 30 Ag 31 and Au 32 , have been synthesized and characterized theoretically and experimentally. Differently from the wurtzite polymorph which exhibits a polar surface,33,34 the ZnO thin films supported on metals assume an almost planar, structure, classified by some authors as graphitic-like. The role of the substrate is essential for the growth and the stabilization of the ZnO thin films. 35 The interest for these systems stems from the fact that in catalytic 2 ACS Paragon Plus Environment

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reactions such as methanol synthesis, based on Cu/ZnO catalysts, the active phase under reaction conditions could be a thin ZnO layer that covers the Cu metal, according to recent experimental studies.23-25In this respect, the properties of atomic or molecular species adsorbed on ZnO/X(111) films (X = Cu, Ag, Au) become of great interest to rationalize the catalytic activity of real catalysts, assuming that a thin oxide layer forms under reactive conditions. One could imagine, in fact, that the initial activation of some of the reactive species occurs specifically via electron transfer from the ZnO/Cu support. The understanding of the nature and the structure of ZnO/X(111) films becomes thus crucial. The topic has been studied in various works, and discussed in a comparative study of the three interfaces, ZnO/Cu(111), ZnO/Ag(111), ZnO/Au(111) in a recent study from our group.35 What emerges from this study and from the analysis of the existing literature, is that the nature of the ZnO/metal interaction is similar for the three metals, as confirmed by the comparison of computed and measured stretching frequencies of adsorbed CO. Actually, a small but non negligible difference is observed for the three interfaces, with ZnO/Cu(111) characterized by a modest charge transfer from Cu to ZnO, while the opposite is found for ZnO on Au(111). However, the interaction of the ZnO bilayer with the three metals remains dominated by dispersion forces. In this study we consider the properties of various adsorbates: Au, NO2, O2, NO and CO2. These species are chosen because some of them, in particular, Au, NO2, and O2, have been demonstrated to induce a spontaneous charge transfer when deposited on MgO/Ag(100) or MgO/Mo(100) films.5,6,8,13,15 NO and CO2 are, on the contrary, more difficult to activate, but they are nevertheless relevant for catalytic reactions such as CO2 to methanol conversion or environmentally relevant processes such as the degradation of NOx to N2. For the Au atom all three interfaces have been considered, ZnO/Cu(111), ZnO/Ag(111), and ZnO/Au(111). For the molecular species only ZnO/Cu(111) has been studied. We show, based on a robust computational approach, that Au, NO2, and O2 deposited on ZnO/Cu(111) behave exactly as on MgO ultra-thin films, i.e. from neutral species they convert into full anions. We also show that the stabilization of the anionic species is accompanied by a polaronic distortion of the ZnO lattice. This is the first report of this kind of interaction for ZnO thin layers, and there is a fundamental difference with respect to the previous report of an analogous phenomenon on MgO/Ag and MgO/Mo thin films. In fact, while these materials have been prepared as model 3 ACS Paragon Plus Environment

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systems of heterogeneous catalysts, ZnO/Cu(111) layers films form in industrially relevant real catalysts. This observation can thus be of critical importance to better understand the working principles of the Cu/ZnO catalysts. Furthermore, since the discovery of the charge transfer from MgO/Ag films to supported Au, no other systems with a similar behavior have been reported in the literature. The fact that this can occur also for ZnO/Cu, and other ZnO/metal interfaces, generalizes the finding and makes it of wider relevance for heterogeneous catalysis, in particular when supported metal particles encapsulated in an oxide layer may form under working conditions. NO and CO2 on the other hand, do not induce the spontaneous charge transfer and their behavior is similar on free-standing and supported ZnO/Cu(111) films.

2. COMPUTATIONAL METHOD Spin-polarized DFT calculations were carried out with the Vienna Ab-Initio Simulation Package (VASP)36,37 using a plane wave basis set with cutoff energy of 400 eV. The interaction of core electrons was treated with the Projector Augmented Wave method. 38 , 39 The Perdew-BurkeEnzerhof (PBE) exchange-correlation functional 40 was used and the self–interaction error inherent in the PBE method was partly corrected using a DFT+U approach by setting the Hubbard-U41,42 parameter for Zn 3d levels at 4.7 eV.43 Dispersion interactions are described by the D2’ approach, a modification of the semi-empirical DFT-D2 scheme.44 For selected cases, the hybrid functional HSE06 45 together with the D2’ correction has been used to check the reliability and accuracy of the PBE+U/D2’ method. In general, the use of the DFT+U approach to study phenomena at metal/oxide interfaces and on supported thin films has been carefully checked by a series of studies performed in our group where the results have been compared in a systematic way with those from hybrid functional calculations.46, 47 The results have shown that, despite the limitations of DFT+U in describing the band gap of insulating and semiconducting oxides, the band alignment with metal supports and adsorbed species is reproduced with sufficient accuracy to guarantee that the obtained results are physically meaningful. Bader charges were calculated using the scheme proposed by Henkelman et al.48,49,50,51. The reaction pathway and transition states were identified by the climbing image nudged elastic band (cNEB) method, sampling five images along the reaction path.52 All the calculations are done with large supercells at the Γ point. 4 ACS Paragon Plus Environment

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For the ZnO/X(111) models we adopted different supercells, in order to minimize the strain in the supported layer. In particular, for Cu we considered a ZnO (3x3) supercell (10.1 Å × 10.1 Å) on Cu (4x4) consisting of 100 atoms, Zn18O18Cu64; the results have been checked by considering also a ZnO(6x6)/Cu(8x8) supercell (20.2 Å × 20.2 Å Zn72O72Cu256). For Ag and Au we used a ZnO (7x7) on Ag (8x8), Zn98O98Ag256, and Au (8x8) supports, Zn98O98Au256. These models consist of two layers of ZnO oxide and four layers of metal, Fig. 1. For further details see Ref. 35. For geometry optimizations, the two bottom atomic layers in the metal support were frozen as in the bulk materials while the remaining atoms were relaxed until the ionic forces are less than 0.02 eV/Å. In all models a vacuum of 15 Å avoids the interaction between slabs. In both unsupported and Cu-supported ZnO bilayers, Au, NO2, O2, NO and CO2, were placed on various possible sites, on top of Zn, on top of O, and in bridge and hollow positions, Fig.1. Here we discuss only the most stable adsorption sites (for other adsorption sites see Table S1 and Table S2 in Supplementary Information).

Figure 1. The top view and side view of (a) free-standing ZnO, (b) ZnO/Cu(111). Adsorption sites of adsorbates are also shown. Zn and O, metal are depicted as blue, red and grey spheres, respectively.

3. RESULTS AND DISCUSSION 5 ACS Paragon Plus Environment

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3.1. Adsorption of Au atom on ZnO/X(111) (X = Cu, Ag, Au) We start by considering a single Au atom deposited on the ZnO bilayer. The Au atom has a high electron affinity, 2.31 eV, 53 and therefore is an excellent candidate to stimulate an electron transfer from the supporting metal. On the free-standing ZnO bilayer, Table S1, four adsorption sites were considered, with adsorption energies that go from -0.65 eV (on-top of Zn) to -0.87 eV for the most stable site, bridging a ZnO bond, Fig. 1 and Table 1. The Au atom is bound via hybridization of the Au 6s level with the O 2p states; a non-negligible contribution to the bonding, 0.20 eV, comes from dispersion, Table 1. The most relevant aspect of the interaction is that, as shown in the projected density of states (PDOS) plot, Figure 2a, the Au atom keeps the atomic valence configuration of the gas-phase, 6s1. The presence of an unpaired electron results in a magnetic doublet ground state (see the magnetization in Table 1, defined as the difference between α and β spin electrons, µ = 1.0); also the Bader charge, -0.11 |e|, indicates the absence of a net charge transfer.

Table 1. Properties of a Au atom adsorbed on unsupported ZnO bilayer and on ZnO/X(111), (X = Cu, Ag, Au) films.

1.00

Qc |e| -0.11

r(Au-Zn)d (Å) 2.59

r(Au-O)e (Å) 2.33

-0.15

0.00

-0.51

2.68

3.16

-1.94

-0.19

0.00

-0.47

2.68

3.16

Hollow

-2.10

-0.26

0.00

-0.46

2.67

3.16

Au/ZnO(7x7)/Ag(8x8)

Hollow

-2.20

-0.36

0.00

-0.47

2.67

3.17

Au/ZnO(7x7)/Au(8x8)

Hollow

-1.73

-0.31

0.00

-0.46

2.66

3.18

ZnO-bond

∆E (eV) -0.87

∆D2’a (eV) -0.20

Au/ZnO(3x3)/Cu(4x4)

Hollow

-2.00

Au/ZnO(3x3)/Cu(4x4)f

Hollow

Au/ZnO(6x6)/Cu(8x8)

System Au/ZnO(3x3)

a

Au site

µb

dispersion energy; bµ = magnetization of Au; cBader charge of Au atom, dshortest Au-Zn

distance, eshortest Au-O distance, fThe calculation has been repeated with a dense 4x4x1 k-point grid.

Things change completely when we consider the Au adsorbate on ZnO/X(111) films (X = Cu, Ag, Au). First of all, the preferred adsorption site changes, and becomes the hollow site, 6 ACS Paragon Plus Environment

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Table 1. The other sites do not even represent local minima on the potential energy surface. But also the adsorption energy changes dramatically and becomes -2 eV or more, i.e. more than twice that found for the unsupported ZnO bilayer. Only a small difference is found in the adsorption energy of Au on ZnO/Cu(111), ZnO/Ag(111), and ZnO(Au(111), Table 1 (at most 0.2 eV). Other important differences emerge from the analysis of the magnetization and Bader charge, Table1. In particular, on supported ZnO the magnetization is completely quenched and the net charge is close to half an electron, Table 1. These data are clearly indicating the negatively charged nature of the Au atom on the supported ZnO bilayer. To better understand the origin of the different adsorption properties, we analyze the spin density and the PDOS of an Au atom adsorbed on unsupported and supported ZnO films, Fig. 2. For Au on the free-standing ZnO film, only one component of the 6s orbital is occupied, the other is a virtual state located above the Fermi level; this results in a spin density localized on the Au atom, Fig. 2(a). For Au on ZnO/Cu(111), on the contrary, both α and β 6s levels of Au are occupied, below the Fermi level, and no spin density is observed, Fig. 2(b). Very similar curves are obtained for a gold atom on ZnO/Ag(111) and ZnO/Au(111), see Fig. S1.

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Figure 2. Structure (top and side view), spin density, and PDOS profiles of Au adsorption on (a) free-standing ZnO bylayer and (b) ZnO/Cu(111). Cu, Zn, O, and Au are depicted as grey, blue, red and yellow spheres, respectively.

It has been shown in previous studies, that the stabilization of the charged adsorbate is closely related to the occurrence of a polaronic distortion in the oxide layer.54 The geometrical displacement of the oxide cations helps screen the negative charge of the adsorbed species, resulting in a substantial local distortion of the lattice. This effect can be clearly seen on the ZnO supported films, Fig. 2(b). The Zn ions nearest neighbor to Au move outward by 0.417 Å, while the nearest O ions move inward by 0.396 Å. Also in the free-standing ZnO film the adsorption of a neutral Au species leads to remarkable lattice distortion (Zn outward displacement of 0.426 Å and inward relaxation of O of 0.161 Å). However, this is restricted mostly to the first layer of the ZnO film, where on the supported film there is an important relaxation also in the bottom ZnO layer. The electrons of the supporting metal are easily polarizable and favor the distortion of the oxide film. However, there is no special involvement of interface or surface states in the charge transfer, as this is simply related to the position of the metal Fermi level. Furthermore, no major distortion is observed on the metal surface. In order to demonstrate the importance of the polaronic distortion, we have repeated the calculation on ZnO/Cu(111) by freezing the ZnO atomic positions and relaxing only Au. Quite interestingly, in this case the charge transfer does not occur, showing the key role that the structural flexibility of the oxide film has in the charge transfer mechanism. Before closing this section, we further corroborate our finding with two sets of calculations. First we considered a larger supercell, by doubling the size, ZnO(6x6)/Cu(8x8) instead of ZnO(3x3)/Cu(4x4), see Table 1. Not only the charge transfer is confirmed, but the adsorption properties are very similar. There is modest increase in the adsorption energy, from 2.00 on the smaller supercell to -2.10 eV on the larger one. This shows that there is only a small repulsion among the negatively charged Au anions on the smaller supercell. This is due to the very efficient screening of the negative charge provided by the polaronic distortion which, however, is very local and restricted to a region about 5 Å from the adsorbate. Also the


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geometrical parameters and the other properties are well converged with respect to supercell size, Table 1. We also checked the reliability of the DFT+U approach. A delicate aspect of the charge transfer mechanism is the alignment of the levels of the three components of our system: the Fermi level of the Cu(111) metal, the valence and conduction bands of the ZnO bilayer, and the frontier orbitals of the Au atom adsorbate. An incorrect positioning of these levels can produce unphysical results (stated differently, the charge transfer could be the consequence of an incorrect alignment of the levels in DFT+U). To this end, we have performed a single point calculation on the optimal DFT+U geometry using the hybrid functional HSE06.45 At the HSE06/D2’ level of theory we confirm the occurrence of the charge transfer (see Table S3). This is in line with similar comparisons done for MgO/Ag(100)46 and FeO/Pt(111)47 films where the use of an hybrid functional gave the same physical picture obtained with the DFT+U approach. Notice however that differences exists in the absolute values of the adsorption energies when hybrid functionals and DFT+U approaches are compared. Based on these results, the investigation of molecular adsorbates on supported ZnO bilayer films was carried out with the smaller ZnO(3x3)/Cu(4x4) and the DFT+U/D2’ level. For the molecular adsorbates we did not consider explicitly ZnO/Ag(111) and ZnO/Au(111) films, but given the similar properties found for adsorbed Au atoms, we expect that the conclusions reported below for ZnO/Cu(111) are valid also for the other systems.

3.2 Adsorption of NO2 on ZnO/Cu(111) The NO2 molecule has a doublet 2A1 electronic ground state and a bent geometry. Its electron affinity, 2.27 eV,55 is close to that of the Au atom and thus NO2 is expected to behave in a similar way. Indeed, a charge transfer to NO2 has been predicted for MgO/Ag(100) ultra-thin films15 and proven experimentally. We have considered several configuration of NO2 on freestanding ZnO and found two stable minima. In the bridge configuration, NO2 is adsorbed with the two O atoms bound directly to two nearest Zn cations, while N points towards the vacuum, Figure 3(a); the adsorption energy is ∆E= -0.27 eV. In the planar configuration, the oxygen atoms are bound to Zn and the N atom is bound to a surface O ion; ∆E = -0.16 eV, Table S1. In both cases the adsorption is rather weak, and dominated by dispersion, Table 2.


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Table 2. Properties of the most stable configuration of NO2 adsorbed on unsupported ZnO and ZnO/Cu(111) films. System

NO2 site

∆D2’a (eV) --

1.00

QNO2c |e| --

rN-Od (Å) 1.21

rNO2-Zne ∠ONOf (Å) deg. -134

µb

NO2 gas

--

∆E (eV) --

ZnO

Bridge

-0.27

-0.19

0.85

-0.33

1.18

2.21

125

ZnO/Cu(111) Bridge

-1.65

-0.12

0.00

-0.69

1.28

2.04

118

a

dispersion energy; bµ = magnetization of NO2; cBader charge of NO2, dN-O bond length in

NO2 molecule, e shortest distance between NO2 and ZnO, f NO2 bond angle.

On ZnO/Cu(111) we found only one adsorption configuration, the bridge one, see Table 2 and Fig. 3(b). Here NO2 is strongly bound, by -1.65 eV, Table 2, and the structure of the NO2 adsorption complex has changed. The N-O bond length is 0.07 Å longer, the ONO bond angle is reduced by 6°, and the distance from the ZnO layer is shortened by about 0.17 Å, Table 2. The bridge configuration was found to be the most favorable one also in previous DFT-based calculation for supported MgO film.15


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Figure 3. Structure (top and side view), spin density, and PDOS profiles of NO2 adsorption on (a) free-standing ZnO, and (b) ZnO/Cu(111). Cu, Zn, O and N are depicted as grey, blue, red and purple spheres, respectively.

The difference in NO2 adsorption on ZnO and ZnO/Cu(111) can be explained by analyzing the electronic structure. First of all, while on ZnO the NO2 molecule has a net magnetization due to the presence of one unpaired electron, as for the free molecule, on ZnO/Cu(111) the ground state is diamagnetic. In the PDOS curve of NO2 on ZnO, Fig. 3(a), the lowest unoccupied molecule orbital (LUMO) is crossed by the Fermi level, indicating that it is half occupied; on ZnO/Cu(111), on the contrary, this state is well below the Fermi level, Fig. 3(b), indicating double occupancy. This is also consistent with the Bader charges which show a much larger negative charge on NO2 when this is adsorbed on the ZnO/Cu(111) bilayer, -0.69 |e| versus -0.33 |e| on the free-standing film, Table 2. All these data clearly indicate the occurrence of a net electron transfer from the support to NO2 which thus transforms into a nitrite NO2− 11 ACS Paragon Plus Environment

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anion. This is accompanied by a distortion of the surface structure, with the Zn ions that move outwards by 0.450 Å and the O ions moving downward by 0.191 Å, Fig. 3(b).

3.3 Adsorption of O2 on ZnO/Cu(111) The O2 molecule, with an electron affinity of 0.45 eV56 and a 3Σg− (2π*)2 ground state, plays a very important role in oxidation processes. The molecule can be activated by adding one electron to the 2π* orbitals leading to a superoxide O2− (2Π). O2− is a key intermediate in many chemical processes57 and can be detected by electron spin resonance (ESR). It has been shown that when O2 is adsorbed on MgO/Ag(100) 2-3 layer films, it induces an electron transfer with formation of the superoxide anion, O2−.13 Notice that this occurs for a perfect, defect free MgO film, at variance with the MgO surface where the formation of superoxo ions by O2 adsorption occurs only in correspondence of trapped electrons at specific surface defects. 58 It is therefore interesting to study the behavior of O2 also on ZnO bilayer films. On unsupported ZnO layers, we found several O2 adsorption complexes (see Table S1); in all cases only physisorption occurs, with adsorption energies of less than 0.1 eV entirely due to dispersion, Table 3. In the most stable configuration the molecule bridges two Zn ions. The molecule keeps the (2π*)2 valence configuration (see Fig. 4(a) where spin density and PDOS curves are reported). Table 3. Properties of the most stable structure of a O2 molecule adsorbed on unsupported ZnO bilayer and on ZnO/Cu(111). System

O2 site

∆E

∆D2’a

(eV)

(eV)

µb

QO2c|e|

rO-Od

rO2-Zne

O2 gas

-

-

-

2.00

-

1.23

-

ZnO

Bridge

-0.09

-0.09

2.00

-0.01

1.23

3.14

ZnO/Cu(111)

Bridge

-0.66

-0.07

-1.00

-0.69

1.35

2.06

a

dispersion energy;

b

µ = magnetization of O2; cBader charge of O2 molecule,

bond length of O2 molecule, e shortest distance between O2 and ZnO surface


d

O-O

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When O2 is adsorbed on ZnO/Cu(111), things change completely. O2 forms only two stable complexes, a bridge and a tilt configuration, see Table S2. The bridge complex is the most stable one, as found for MgO ultra-thin films. A substantial binding is now found for O2 adsorbed on ZnO/Cu(111), -0.66 eV, about 7 times stronger than on freestanding ZnO, Table 3. The O-O bond length is elongated by about 0.12 Å and the distance from the ZnO surface is reduced by 1.08 Å compared to the unsupported ZnO bilayer. The origin of the changes is, once more, the charge transfer from the underlying Cu support to O2. The strongest proof comes from the analysis of the magnetization, as now there is only one unpaired electron on the adsorbed O2 molecule, consistent with the formation of a O2− superoxide anion. This is further shown by the PDOS profiles, Fig. 4(b), and by the spin density of O2/ZnO/Cu(111). The occurrence of a charge transfer also reflects in the Bader charge which is almost zero for O2/ZnO, and -0.69 |e| for O2/ZnO/Cu(111), see Table 3. As for the Au case, the charge transfer is accompanied by a polaronic distortion of the ZnO film: the Zn ions move upward by about 0.447 Å, and the O ions move downward by about 0.243 Å, Fig. 4. No significant distortion is observed for the unsupported ZnO film, Fig. 4.


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Figure 4. Structure (top and side view), spin density, and PDOS profiles of O2 adsorption on (a) freestanding ZnO, and (b) ZnO/Cu(111). Cu, Zn, and O are depicted as grey, blue, and red spheres, respectively.

3.4 Adsorption of NO on ZnO/Cu(111) Compared to the other species considered, NO is different due to the very small electron affinity, 0.026 eV, as measured by laser photo-detachment experiments. 59 NO can adsorb on freestanding ZnO in several configurations, reported in Table S1. If the molecule is initially placed N-down on top of a Zn ion, a tilted relaxed structure is obtained (Table 4 and Table S1), with a weak adsorption energy of -0.22 eV. If NO is put O-down on top of a Zn lattice site, a less stable tilted structure (∆E= -0.10 eV) is found. NO on a hollow site is even less stable (∆E= -0.07 eV). Again, the interaction is entirely due to dispersion forces. Various adsorption modes of NO on ZnO/Cu(111) have been tried, Table S2. As for unsupported ZnO, however, a N-down Zn-top configuration is preferred (Table 4), with a


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slightly larger adsorption energy (-0.40 eV). This is accompanied by a moderate increase in the N-O bond length, 0.02 Å, and decrease of the Zn-N distance, 0.18 Å, Table 4. This result can be explained by a modest effect of the underlying support Cu which results in a small electron transfer to NO, as shown by the Bader charge of -0.28 |e|, Table 4. In addition, the comparison of PDOS profiles for NO adsorbed on ZnO and ZnO/Cu(111) reveals an interesting feature: while on the free-standing ZnO bilayer the unoccupied 2π* states of NO are well above the Fermi energy, Fig. 5, in ZnO/Cu the lowest-lying 2π* state is crossed by the Fermi level, thus allowing for a moderate charge transfer from the support to the adsorbed NO molecule. However, no quenching of the NO magnetic moment is observed, Table 4. This shows unambiguously that no net charged transfer is observed in case of NO on ZnO/Cu(111), as this would result in a diamagnetic adsorbate. There is a reinforcement of the covalent part of the interaction of NO with ZnO, with partial delocalization of the surface electrons on the NO adsorbate; this is probably due to the weak overlap of the NO states with the tails of the Cu metal charge distribution, which is absent on the unsupported ZnO bilayer.

Table 4. Properties of the most stable configuration of NO molecule adsorbed on unsupported ZnO and ZnO/Cu(111) System

a

NO site

∆E

∆D2’a

(eV)

(eV)

µb

QNOc

rN-Od

rNO-Zne

|e|

(Å)

(Å)

NO gas

-

-

-

1.00

-

1.17

-

ZnO

Zn(tilt)

-0.22

-0.11

1.00

-0.07

1.18

2.34

ZnO/Cu(111)

Zn(tilt)

-0.40

-0.07

1.39

-0.28

1.20

2.12

dispersion energy; b µ = magnetization of NO; cBader charge of NO, dthe bond length of N-O,

e

the shortest distance Between NO and surface of ZnO


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Figure 5. Structure (top and side view), spin density, and PDOS profiles of NO adsorption on (a) freestanding ZnO, and (b) ZnO/Cu(111). Cu, Zn, O and N are depicted as grey, blue, red and purple spheres, respectively.

3.5 Adsorption and activation of CO2 on ZnO/Cu(111) CO2 activation on oxide surfaces is a topic of great interest in photo-electrochemical conversion of CO2 into methanol, methane and other species. On basic oxide surfaces, such as MgO60,61 or ZrO2,62 adsorption of CO2 leads to the formation of a surface carbonate. On more acidic surfaces such as TiO2, CO2 is only weakly physisorbed and maintains its linear structure.62 Under particular conditions, it is possible however that CO2 captures one electron from the substrate and forms the much more reactive paramagnetic carboxylate complex, CO2−.63 The formation of carboxylate species on electron-rich polycrystalline MgO samples has been reported long time ago, 64 and the problem has been extensively studied with a combination of ESR and DFT results.63,65 Activation of CO2 via formation of carboxylate and then oxalate species has been reported experimentally on MgO/Ag(100) model systems by electron transfer stimulated by the 16 ACS Paragon Plus Environment

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presence of small Au particles. 66 The Au adsorbates act as electron scavengers from the metal/oxide support and then transfer the electron to adsorbed CO2 molecules with formation of CO2−. In a recent theoretical study,67 we have shown that carboxylate CO2− species can form on the bare MgO/Mo(100) one-layer films even in absence of gold, but this competes with the formation of carbonates. We have considered the adsorption of CO2 on both unsupported and Cu(111) supported ZnO bilayers. Several possible CO2 configurations were considered, starting form variously deformed structures. However, only a linear O-C-O configuration is observed in both freestanding and supported ZnO, Fig. S2. The bond is weak, dominated by dispersion, and no activation is found, Table S4. Thus, at variance with NO2 and O2, and, to a much smaller extent, NO, no direct activation of CO2 is observed on the ZnO/Cu(111) films. This does not rule out the possibility that CO2 can be activated on ZnO/Cu(111) films. In fact, a quite different reactivity is expected if steps or other defects are present in the oxide film. For instance, a recent study on ZnO/Au(111) films has shown that partial oxidation of methanol to formaldehyde can occur only when the methanol molecule is adsorbed at bilayer–trilayer step sites of the film.68 Then, in analogy with what experimentally observed for MgO/Ag(100) films,66 we have considered CO2 activation mediated by a supported Au atom or dimer. The choice is dictated by the fact that for this system the role of the charge transfer in the reaction has been proven experimentally. Here we want to check if the same can occur on the surface of relevant catalytic material, ZnO/Cu, with the hope that this can be verified in future experiments stimulated by this paper. To this end, a CO2 molecule has been adsorbed near a supported Au anion or a Au2− species. This results in a direct bond, with partial charge transfer from Au to CO2. Various orientations have been considered, but for the Au anion the most stable one corresponds to a CO2 molecule lying flat on the ZnO surface, Fig. 6(a) (see also S5). In this optimal configuration the molecule is bent, α(OCO) = 127°, and the bonding is of -0.36 eV.


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Figure 6. (a) Structure (top and side view), of CO2 adsorbed on Au/ZnO/Cu(111); (b) Structure (top and side view), of [C2O4]2- formed on Au/ZnO/Cu(111) Cu, Zn, O, Au and C are depicted as grey, blue, red, yellow and brown spheres, respectively.

Based on this promising result we have adsorbed a second CO2 molecule, but on Au1/ZnO/Cu the charge on gold is not enough to activate also this second CO2 molecule, which remains linear and only physisorbed (by -0.08 eV) on the surface complex, Fig. 7(a).


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(a)

(b)

Figure 7. Reaction profile for the activation of CO2 and formation of an oxalate species, [C2O4]2on Au1/ZnO/Cu(111). Cu, Zn, O, Au and C are depicted as grey, blue, red, yellow and brown spheres, respectively.

From this local minimum a search for a transition state has been performed, and a path have been found that leads, by overcoming a substantial energy barrier of 1.61 eV, to a [C2O4]2complex coordinated to the gold atom, Fig. 6(b). This species is more stable than two CO2 molecules by -0.54 eV, Fig. 7(b). An oxalate formed on ZnO/Cu(111) by this reaction is a complex which, in absence of the Au atom, is bound by -0.04 eV with respect to two gas-phase CO2 molecules. The process is even more favorable on the supported Au dimer. Here the presence of the extra charge on Au2 leads to a very stable activated CO2 species (bound by 0.80 eV) and to the formation of a stable oxalate complex. These results do not prove that one can easily form oxalate complexes by this mechanism, but provide enough evidence that the supported gold species can promote the activation of the adsorbed CO2. We expect that the presence of larger Au clusters where more negative charge is accumulated can result in the simultaneous activation of two adjacent CO2 molecules, thus favoring their combination to form an oxalate species.

4. CONCLUSIONS We have studied the adsorption properties of ZnO bilayers. These systems have attracted considerable attention in the last decade because they belong to the general class of two-


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dimensional materials with structure and properties very different from the bulk counterparts. This diversity is confirmed here. The special properties of ZnO bilayers, however, are not intrinsic to the low dimensionality of the ZnO film, but rather to the interface with the metal support. In fact, we have compared the adsorption of Au atoms, NO2, O2, NO, and CO2 molecules on the surface of ZnO/Cu(111) films, using a DFT+U approach with inclusion of dispersion. We found that the adsorption of species with high electron affinity such as Au, O2 and NO2 leads to completely different bonding mechanisms on free-standing ZnO, where the bonding is either of covalent nature or dominated by dispersion, or on ZnO/Cu(111) films where a net charge transfer occurs. This is in full analogy with what observed experimentally and theoretically on metal supported MgO thin films.5-15 However, while MgO/Ag and MgO/Mo are model systems of real catalysts, the phenomenon is reported here for ZnO/Cu, an active catalysts for methanol synthesis. The charge transfer occurs via electron tunneling through the ultrathin ZnO insulating layer. In the case of Au atom, the effect has been found also for ZnO/Ag(111) and ZnO/Au(111) films, and we expect that the same applies also to the other molecules. An important effect that is associated to the charge transfer mechanism is the occurrence of a distortion of the oxide film in correspondence of the formation of the charged adsorbate (polaron formation). Not surprisingly, molecules with very low or even negative electron affinities, such as NO and CO2, do not exhibit the same behavior. However, CO2 can be adsorbed and activated by a process mediated by a supported Au atom or an Au dimer with formation of oxalate species, [C2O4]2-. Lower barriers for the formation of oxalate are expected on the electron-rich Au clusters. These results show that the occurrence of a charge transfer on ultrathin films is not restricted to MgO. To the best of our knowledge this is the first report of another existing oxide film that behaves exactly in the same way. To some extent this result is even more important because there is direct evidence that 2-3 layer ZnO films can form on Cu during catalytic reactions.23-25 The occurrence of a charge transfer would represent an important information to rationalize existing catalysts and possibly design new ones based on these effects.


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Supporting Information Adsorption properties of Au, NO2, O2 and NO on unsupported bilayer ZnO and on ZnO/Cu(111) are reported in Table S1 and Table S2, respectively. Comparison with HSE06/D2’ calculations for Au on ZnO/Cu(111) is given in Table S3. The properties of CO2 adsorbed on bilayer ZnO and on ZnO/Cu(111) are given in Table S4. Other configurations of activated CO2 on Au/ZnO/Cu(111) are reported in Table S5. Figure S1 show the DOS profiles of Au on ZnO/Ag(111) and ZnO/Au(111). All structures of CO2 on ZnO/Cu(111) and on Au/ZnO/Cu(111) are shown in Figure S2 and S3, respectively.

Acknowledgments This work has been supported by Italian MIUR through the PRIN Project 2015K7FZLH SMARTNESS. CINECA-LISA Awards are also acknowledged for the availability of highperformance computing resources.

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