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Spontaneous Formation of Au Cluster Anions on ZnO/Cu(111) Bilayer Films Ho Viet Thang, and Gianfranco Pacchioni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03479 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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The Journal of Physical Chemistry

Spontaneous Formation of Au Cluster Anions on ZnO/Cu(111) Bilayer Films Ho Viet Thang, Gianfranco Pacchioni* Departimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, 20125 Milano, Italy Version 9.05.2018

Abstract Recently, bilayer ZnO films grown on Cu, Ag, and Au supports have been reported as new materials with potentially interesting properties. In this work we have investigated the structure and stability of small Aun clusters (n = 2, 3, 4) and of a larger gold aggregate, Au20, deposited on ZnO/Cu(111) ultrathin films using a spin-polarized DFT+U/D2’ approach which includes dispersion corrections. The properties of Au clusters on ZnO/Cu(111) are completely different from those of the corresponding gas-phase units, but also from the same Au clusters deposited on an unsupported ZnO bilayer. On ZnO/Cu(111) small Aun clusters (n = 3, 4) are linear and Au20 is two-dimensional (2D), at variance with the free counterparts. The stability of Au clusters on ZnO/Cu(111) is governed by a spontaneous charge transfer from the underlying Cu(111) substrate via electron tunneling through the thin ZnO film, resulting in negatively charged Au clusters. A Bader charge analysis shows that the excess electrons are localized on the edges of the Au clusters to minimize Coulomb interactions.

*

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

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1. Introduction Metal/oxide interfaces play a crucial role in various fields, including catalysis by supported metals,1,2 sensor, 3 solar cell, 4 microelectronic devices, and so on. Heterogeneous catalysts usually consist of precious metal particles or clusters deposited on a metal oxide surface. Among active metals, gold has been widely studied in the last 20 years because of its particular role in important reactions, such as CO oxidation, water-gas-shift reaction, dehydrogenation of methanol, etc.5 The special properties of supported gold are clearly connected to the small dimensions of the particles: while gold nanoparticles or even clusters supported on oxide surfaces exhibit interesting catalytic properties, large bulk-like gold particles are generally inactive.6,7 The nature of supported gold particles can be modified by the interaction with the support. In particular, defects (e.g. oxygen vacancies) can donate electronic charge to the Au aggregates. 8 , 9 Gold nanostructures are used in combination with semiconducting oxides such as TiO2 where they act as electron scavengers in photoexcited processes, facilitating electron-hole pairs separation.10 Another way to generate negatively charged gold clusters has been discovered some years ago by studying gold deposition on ultrathin MgO films grown on a metal (Ag or Mo) using a combination of DFT calculations and STM images.11,12,13,14,15,16,17,18 Here, a spontaneous electron transfer occurs from the MgO/Ag or MgO/Mo interface to the supported gold nanoparticle, via electron tunneling through the 2-3 layers thick MgO insulating film. The first effect that has been observed is a change in the geometry of the Au clusters deposited on the thin film compared to thicker MgO films or to the bulk MgO surface. There are various consequences of this charge transfer. One is that large gold nanoparticles remain 2D up to room temperature and above when grown on MgO/Ag(100) 3-layer films, why they assume 3D shapes on thicker films where the charge transfer is suppressed. Also the chemistry of gold clusters changes as a function of the occurrence of the charge transfer. In particular, negatively charged Au clusters on MgO/Ag films can promote various reaction, such as CO2 activation,19 CO oxidation,20,21 water splitting.22 Therefore, the growth of Au clusters on ultrathin films can play an important role in catalysis as well as other related fields. So far, these phenomena have been reported for MgO/Ag(100),11,16 MgO/Mo(100),12,13,14 and h-BN/Rh(111).23 Besides the systems mentioned above, zinc oxide (ZnO), is a material used in industrial catalytic processes, in particular in combination with copper, for methanol synthesis.24, 25 Recently it has been shown that thin films of ZnO can grow on Cu particles during the catalytic process, forming what could be classified as an inverse catalyst. 24,26,27 This has attracted attention from both theory and experiment.28,29 Differently from bulk ZnO, whose (0001) surface is polar,30,31 planar or graphitic-like ZnO thin films have been grown on coinage metals and characterized experimentally and 2 ACS Paragon Plus Environment

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theoretically.26,32,33,34,35,36 These films tend to exhibit interesting electronic properties. The occurrence of a charge transfer from the metal support (Cu, Au, Ag) to molecular and atomic adsorbates has been recently reported based on DFT calculations.37 In particular, it has been predicted that the adsorption of a Au atom or of NO2 and O2 molecules will result in negatively charged adsorbates, in close analogy with what has been observed for MgO/Ag or MgO/Mo films.38,39,40,41 It should be noted, however, that while MgO/metal films are model catalysts, ZnO/Cu ultrathin films can form under realistic conditions, and the charge transfer could have an important role in various catalytic processes. For instance, Calaza et al.19 have combined DFT calculations and IR experiments to study CO2 activation on Au clusters supported on MgO/Ag(001) to form C2O42-; this is an elementary step towards the conversion of CO2 into useful chemicals. The occurrence of an electron transfer to the supported Au clusters can represent an important prerequisite for several catalytic reactions, favoring the initial activation of some molecular species on ZnO/Cu films. In this study we have investigated the properties of Au clusters of various size deposited on ZnO/Cu(111) bilayer films. The final aim is to identify possible new supports able to activate CO2.25,28 We studied the adsorption properties of small Au clusters containing 2 to 4 atoms, and of a cluster consisting of 20 Au atoms. To this end, we performed spin-polarized DFT+U calculations including dispersion correction. We will show that indeed small gold clusters deposited on ZnO/Cu(111) films stimulate an electron flow from the support to the adsorbed gold species, leading to the formation of negatively charged gold clusters on the surface of ZnO bilayers.

2. Computational method Spin-polarized periodic DFT calculations were carried out with the VASP-5.3 code. We used the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional42 and a plane wave basis set with 400 eV cutoff.43,44 The lack of self-interaction in PBE functional has been partly compensated by applying an Hubbard-U45,46 correction, with U = 4.7 eV for the Zn cation, as suggested in Ref. 47. Dispersion corrections have been included based on the D2 Grimme scheme48 modified as suggested by Tosoni and Sauer, referred to as D2’.49 Core electron were represented by the PAW method;50,51 the Zn (4p, 3d), O (2s, 2p), Cu (4s, 3d) and Au (5d, 6s) orbitals have been treated explicitly in the valence. Given the relatively large supercell, calculations have been done at the Γ point. To avoid the interaction between Au clusters in different cells, the ZnO/Cu(111) model was simulated using a large supercell. In particular, we adopt a (6x6) bilayer of ZnO on (8x8) four atomic 3 ACS Paragon Plus Environment


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Cu layers, consisting of 400 atoms (Zn72O72Cu256). This super-cell has a double size compared to the real supercell consisting of a (3 × 3) ZnO superlattice supported on a (4 × 4) supercell of Cu(111). This result has been proven experimentally in a recent study.52 The structure and properties of this model are described in detail in Ref. 35. Several possible geometries were considered for each Au cluster adsorbed on ZnO/Cu(111). In the optimization, the two bottom layers of the Cu support were frozen as in the bulk, while the remaining atoms were fully relaxed until the forces are 2D_h > 2D_r); the difference in stability between 2D and 3D Au20 structures on ZnO bilayer is smaller than in gas-phase (0.7 eV vs. 2.3 eV), Table 4 and Table 5, due to the interaction with the oxide support, but this is not sufficient to reverse the order of stabilities. (2) Differently from Au20 on ZnO/Cu(111), virtually no charge transfer is found for the same clusters supported on bilayer ZnO, Table 5, showing that this effect is entirely due to the supporting Cu metal. (3) The absence of charge transfer results in much smaller binding energies of Au20 to ZnO bilayer; these are about 3 eV smaller than on ZnO/Cu(111), Table 5. Not surprisingly, this weaker bond is accompanied by a longer distance between Au20 and the ZnO surface.

Table 5. The properties of Au20 adsorbed on ZnO bilayer Au20 configuration

∆Ea (eV)

∆D2b (eV)

µc

Qd |e|

r(Au-ZnO)e (Å)

Erelf (eV)

3D pyramidal

-3.62

-2.12

0.00

-0.11

2.31

0.00

2D_r rhombic

-2.92 -3.34

-2.84

0.00

-0.29

2.39

0.70

-2.90

0.00

-0.32

2.21

0.28

2D_h hexagonal

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a

Adsorption energy, bdispersion contribution, c µ is the difference of α and β spin densities, µ =

ρα - ρβ,dtotal Bader charge of Au20, ethe distance of Au20 from the ZnO surface, fthe relative energy relative to 3D

4. Conclusions ZnO/Cu(111) bilayer structures are new interesting materials that can be prepared on purpose or can form spontaneously in the course of catalytic reactions on Cu/ZnO supported catalysts.24,26, 27,36 We recently showed, based on DFT calculations, that the adsorption of atomic (Au) and molecular (NO, NO2, O2) species on the surface of the ZnO/Cu(111) graphitic-like film results in the formation of anionic species due to an electron transfer from the Cu support to the adsorbates,37 provided that these have sufficiently large electron affinity. In this work we have extended the investigation towards a new class of adsorbates, consisting of gold clusters containing from 2 to 20 atoms. The investigation has been performed at the PBE+U/D2’ level of theory. We found that very small Aun clusters (n=2-4) preferentially arrange in linear structures on ZnO/Cu(111) while the larger Au20 cluster prefers to adopt a flat 2D structure which wets the ZnO surface. The structures observed on the ZnO/Cu(111) support are completely different from the gasphase counterparts; the most clear example is that of Au20 that in the free state assumes a regular tetrahedral 3D structure, at variance with the same cluster supported on ZnO/Cu(111) which is planar. The driving force causing the change in adsorption geometry is the charge transfer from Cu to the Aun clusters. The excess of charge, of one or two electrons depending on the system, tends to localize on the terminal Au atoms (linear structures) or on the edge Au atoms (planar 2D structures) in order to minimize the Coulomb repulsion. These presence of the extra charge also enhances the adsorption and stability of Aun cluster on ZnO/Cu(111) compared to unsupported, free-standing ZnO bilayers. Finally, the formation of anionic gold is accompanied by a substantial polaronic relaxation of the oxide thin film. These results are pure theoretical predictions, and need an experimental confirmation. In this respect, however, it should be mentioned that the picture emerging from the adsorption of Au clusters on ZnO/Cu(111) ultrathin films closely resembles that of MgO/Ag(100) two- and three-layer films. Also in this case in fact it has been predicted a 3D to 2D transition for Au20 and the formation of planar or linear structures for the smaller gold clusters.13,14 In the case of MgO/Ag(100), very accurate experimental measurements, mostly based on physical vapor deposition (PVD), high-resolution STM14,17,18 and IR spectroscopy of adsorbed CO molecules 61,62 have clearly shown that the picture 17 ACS Paragon Plus Environment


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emerging from the DFT calculations is perfectly consistent with the observations. We hope that in a similar way experiments based on PVD will provide a confirmation of the results presented in this paper. This could be of particular interest because, while the MgO/Ag(100) films have been prepared and studied as model systems of real heterogeneous catalysts, ZnO/Cu(111) thin layers can effectively form under working conditions on Cu/ZnO catalysts.24,26,27 The possible occurrence of electron transfers at the metal/oxide interface could be important to rationalize reaction mechanisms and possibly to design new active catalysts.

Supporting Information The DOS profiles projected on Au clusters of the most stable configuration a) tilt Au2, b) bent Au3, and c) linear Au4 configuration Relative energy (Erel, eV) and adsorption energy (Ead, eV) of Au20 cluster in gas phase and adsorbed on bilayer ZnO/Cu(111) using PBE+U method with and without dispersion correction

Acknowledgments We thank Dr. Sergio Tosoni for several useful discussions. This work has been supported by Italian MIUR through the PRIN Project 2015K7FZLH SMARTNESS. CINECA-LISA Awards are also acknowledged for the availability of high-performance computing resources.

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Cu(111

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