M(111

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Oxygen Vacancy in Wurtzite ZnO and Metal Supported ZnO/M(111) Bilayer Films (M = Cu, Ag and Au) Ho Viet Thang, and Gianfranco Pacchioni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06474 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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The Journal of Physical Chemistry

Oxygen Vacancy in Wurtzite ZnO and Metal Supported ZnO/M(111) Bilayer Films (M = Cu, Ag and Au)

Ho Viet Thang, Gianfranco Pacchioni* Departimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, 20125 Milano, Italy Abstract Using spin-polarized DFT+U calculations we have studied the nature of the O vacancy in graphiticlike ZnO bilayer films supported on Cu, Ag, and Au (111) surfaces and compared it with the same defect center formed on free-standing ZnO bilayers and on the ZnO wurtzite 1010 surface. The formation energy of the oxygen vacancy is similar in bulk ZnO wurtzite and in free-standing ZnO bilayers, about 4.3 eV, while it is about 1 eV smaller on the wurtzite 1010 surface. The analysis of the density of states, electron density, and charge distribution, shows that the two excess electrons associated to the vacancy are localized at the vacancy site in all these systems. The situation is more complex on the bilayer films on metal. Removing oxygen from the top layer of ZnO/Cu(111) and ZnO/Ag(111) results in charge delocalization over the entire ZnO film, no charge transfer to the support, and the formation energy remains high, as for the unsupported layer, about 4.2 eV. In the case of ZnO/Au(111), however, due to the higher work function of Au, electrons are transferred from the oxide top layer to the metal, and the cost to remove oxygen is strongly reduced by 1.7 eV. For all ZnO/metal supported films, the formation energy of the vacancy is reduced at the metal/oxide interface, showing the important role that metal/oxide interfaces have in determining the reducibility of an oxide. Beside electronic effects, also the local structural distortions of the ZnO thin films and the metal support also contribute to reduce the oxygen vacancy’s formation energy.

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

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1. Introduction Due to its optical, electronic, chemical and magnetic properties, zinc oxide (ZnO) finds several uses in various technologies and applications, including as food additive, pigment, UV absorber, in sensors, heterogeneous catalysis, photoelectronic devices, etc. 1 , 2 , 3 , 4 The relevance of ZnO as material of practical use motivates the fundamental interest on its properties and electronic structure, and their dependence on the form of the material, bulk, nanoparticles, thin films, nanowires, and so on.5,6,7 In particular, thin and ultrathin ZnO films have been studied intensively in the last decade either in free-standing form6 or grown on various metal supports such as Pd8, Cu9, Ag10, Au11,12. While some surfaces of bulk ZnO are polar, e.g. the (0001), ZnO in free-standing or supported films assumes a graphitic-like structure, with little or no corrugation. Actually, a very small corrugation is found for ZnO/Cu(111) films, while this is practically absent on ZnO/Ag(111) and ZnO/Au(111) layers.13 The stability of the ZnO bilayer film on metal supports mainly derives from dispersion which contributes for more than 70% to total adhesion energy.13 While crystalline ultrathin films of oxides such as MgO,14 ZrO2,15,16 or SiO2,17 have been grown on metal supports with the aim to prepare model catalysts, ZnO/metal films are not only simplified versions of the actual catalysts. In fact, it has been recently found that under working conditions ZnO/Cu real catalysts used in methanol synthesis or in CO2 hydrogenation can exhibit the formation of a thin ZnO layer encapsulating the Cu metal particles.18,19,20,21 The properties of ultrathin ZnO films on metals have been the subject of detailed studies both at experimental, 22 and theoretical levels.13 In this context it has been proposed that the chemical properties of ZnO thin films supported on Cu, Ag, and Au metals can differ substantially from those of the corresponding wurtzite surface:23,24 in fact, the adsorption of molecules, NO2 or O2, or metal clusters with high electron affinity, Aun, stimulates a spontaneous charge transfer from the underlying metal to the adsorbed species that become negatively charged. The phenomenon is due to an electron tunneling mechanism through the semiconducting oxide layer and is accompanied by a local polaronic distortion of films.25,26,27 As a consequence, Au clusters which form three-dimensional structures on unsupported ZnO films, prefer to form flat, two-dimensional islands on the supported ZnO/metal bilayer films. ZnO nanopowders with supported metal particles also exhibit interesting catalytic properties in methanol28 or CO oxidation,29 following a Mars-van Krevelen mechanism. In these reactions, the lattice O anions of ZnO directly participate in the reaction with formation of an oxygen vacancy 2 ACS Paragon Plus Environment

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which is then refilled by reaction with gas-phase oxygen molecules. A better understanding of the properties of O vacancies formation in ZnO bulk and thin films can be of interest for the design of new catalytic materials for oxidation reactions. So far, O vacancies in ZnO have been studied in the bulk and on a monolayer ZnO film.30,31,32,33,34,35 The results showed an enhancement activity for photocatalysis and adsorption compared to the pristine material. This has been connected to a reduction of the band gap resulting in an increased absorption of visible light. On the contrary, the nature of O vacancies on metal supported ZnO bilayer films has not been considered so far. In this respect, it is interesting to investigate the cost of reducing the ZnO material when interfaced with a metal since this can contribute to an higher reactivity of the interface oxygen ions with important consequences on reactions based on the Mars-van Krevelen mechanism.36 In this work, we have applied the DFT+U approach to study the nature of O vacancies in ZnO/M(111) (M = Cu, Ag, Au) films, and to compare this with the same defect in the bulk of ZnO wurtzite, in the non-polar ZnO wurtzite 1010 surface, and on free-standing ZnO bilayers. Several O sites were considered. O vacancies at the interface between ZnO bilayer and M(111) are easier to remove than the O atoms on the surface of ZnO/M(111) or on free-standing ZnO bilayers and bulk ZnO. The O vacancy formation energy on wurtzite 1010 surface is smaller than in the bulk, and increases going from surface to subsurface. Excess electrons are localized in the void space formed by removing O in ZnO bulk, free-standing ZnO bilayer, and on the wurtzite 1010 surface. On the contrary, a different charge distribution has been found for O vacancies on ZnO/metal films, which depends on both the position of the vacancy (top or interface ZnO layer) and on the supporting metal (Cu and Ag, or Au).

2. Computational Methods Spin-polarized density functional theory (DFT) calculations have been performed with the Vienna Ab-initio Simulation Package (VASP). 37 , 38 We used the Generalized Gradient Approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. The selfinteraction error in the PBE method was partly compensated by using an on-site Coulomb correction in the frame of the DFT+U approach. An effective, Ueff = U – J = 4.7 eV was applied to the Zn 3d states.39 With this U value the lattice parameters of ZnO wurtzite were reproduced with good accuracy.13 Dispersion correction where introduced using a modified Grimme scheme; this is 3 ACS Paragon Plus Environment


referred to as D2’; the C6 and R0 values from the original D2 approach 40 have been modified as suggested by Tosoni and Sauer.41 The core electrons were described using the Projector Augmented Wave method;42,43 the following valence electrons are treated explicitly: Cu(4s, 3d), Ag (5s, 4d), Au (6s, 5d), Zn (4p, 3d), and O (2s, 2p). A plane wave basis set with cutoff of 400 eV was used. To investigate the oxygen vacancy in bulk ZnO wurtzite, we used a (3 x 3 x 2) supercell (Zn36O36), in order to reduce the interaction between defects 44 . For free-standing ZnO bilayer models, two supercell structures were used, (3 x 3) and (6 x 6) supercells, containing 36 (Zn18O18) and 144 atoms, (Zn72O72) atoms, respectively. For the non-polar wurtzite 1010 surface we used two supercells, (3 x 2) (Zn36O36), and (6 x 4) (Zn144O144), including six atomic layers. Finally, for metal-supported ZnO bilayers the following supercells were used: (6 x 6) ZnO on (8 x 8) Cu(111); (7 x 7) ZnO on (8 x 8) Ag(111); (7 x 7) ZnO on (8 x 8) Au(111). Further details can be found in our previous study.13 All structures were optimized until the forces are less than 0.01 eV/Å; the metal atoms of the two bottom layers in ZnO/M(111) are frozen as in the bulk, while all remaining atoms are free to relax. A 2 x 2 x1 k-point mesh was applied for bulk ZnO, for the small supercell of the ZnO bilayer, and for the non-polar ZnO 1010 surface. For the larger supercells and for ZnO bilayers on metal only Γ-point was considered. The 6 x 6 x 1 and 2 x 2 x 1 k-point mesh were used to characterize the density of states for the small and large unit cells, respectively. The oxygen vacancy formation energy, Ef(VO), is calculated as the difference between the total energy of the defective structure plus the energy of ½O2 molecule in gas phase, and the total energy of the perfect structure. The effective charge of ZnO bilayer was calculated by applying the Bader partition scheme.45,46,47,48

3. Results and Discussion surface of ZnO wurtzite 3.1. Bulk and The O vacancy in the bulk ZnO wurtzite was considered by removing one lattice oxygen atom from the (3 x 3 x 2) supercell; this corresponds to a vacancy concentration of 2.78 %, defined as the ratio of VO centers with respect to the O atoms in supercell. The formation energy of this defect is 4.39 eV, Table 1. This value is lying in the range of 3.7 - 5.5 eV which were reported in previous studies by using different methods.30,31,32 Compared to the non-defective ZnO structure, only very small distortions are induced by the O removal. This is not so surprising if one considers that the two 4 ACS Paragon Plus Environment

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excess electrons associated to the neutral oxygen vacancy are localized in the vacancy, with a moderate perturbation of the Madelung potential of the crystal. The electron localization is confirmed by an analysis of the electron density and by the appearance of an impurity state located about 0.90 eV above the top of valence band, see PDOS profile, Figure 1. This finding is reminiscent of the nature of O vacancy in more ionic oxides, such as bulk MgO49 or ZrO2.50 Table 1. Formation energy, Ef(VO), and concentration, %VO, of an oxygen vacancy in ZnO, bulk, ZnO 1010 wurtzite surface, and unsuppoted ZnO bilayer. System

Vacancy

ZnO bulk wurtzite

VO

2.78

4.39

ZnO 1010 wurtzite surface

VO_surface1

2.78

3.47

0.69

3.40

2.78

3.95

0.69

3.89

2.78

4.14

0.69

4.11

(3 x 3)

5.55

4.04

(6 x 6) fixed

1.39

4.25

(6 x 6) relaxed

1.39

4.22

VO_surface2 VO_subsurface ZnO unsupported bilayer

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%VO Ef(VO), eV


Figure 1. Side view and top view, and PDOS profiles of: (a) bulk ZnO wurtzite; (b) defective bulk ZnO (oxygen vacancy). In yellow is the electron density associated to the oxygen vacancy (0.005 |e|/Å3). Zn and O are blue and red spheres, respectively. The non-polar wurtzite 1010 surface is the most stable structure, compared to another non-polar 1120 surface and to the polar zinc- 0001 or oxygen-terminated 0001 surfaces.51 Therefore, we investigated the oxygen defect only on this surface. On the 1010 surface we considered two different O sites on the top layer (denoted surface1 and surface2) and one in subsurface (denoted subsurface), Figure 2. The oxygen vacancy formation energy increases when going from surface to subsurface, from 3.5 eV for the most favorable surface site to 4.1 eV for the subsurface site, Table 1. Both values are smaller than the bulk formation energy computed for the same vacancy concentration, 4.4 eV (see above). Thus, the formation energy correlates with the coordination of the oxygen atoms, and shows an easier reducibility of the surface of ZnO compared to the bulk. This result is in agreement with a previous study using the PW91 method where the oxygen formation energy on surface was found to be 0.8 eV smaller than in the bulk ZnO wurtzite52 (0.9 eV in present work). We also analyzed the dependency of the oxygen vacancy formation energy on the concentration of the point defects, Table 1. In general, we see a moderate dependence of this property (

M(111

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