Water Dissociation and Further Hydroxylation of Perfect and Defective

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Water Dissociation and Further Hydroxylation of Perfect and Defective Polar ZnO Model-Surfaces Mathilde Iachella, Jérémy Cure, Mehdi Djafari Rouhani, Yves J. Chabal, Carole Rossi, and Alain Esteve J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04967 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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The Journal of Physical Chemistry

Water Dissociation and Further Hydroxylation of Perfect and Defective Polar ZnO Model-Surfaces Mathilde Iachella,1 Jérémy Cure,2 Mehdi Djafari Rouhani,1 Yves Chabal,2 Carole Rossi,1 Alain Estève1,* 1

University of Toulouse, LAAS-CNRS, 7 avenue du colonel Roche, 31031 Toulouse, France

2

Department of Materials Science & Engineering, The University of Texas at Dallas,

Richardson, Texas 75080, United Sates * [email protected]

Abstract ZnO is a high-band gap semiconductor material important for microelectronic and catalytic applications, such as water splitting among others. While the non-polar face of ZnO has been well studied, its polar faces (Zn- and O-terminated) are less studied because of intrinsic difficulties to model. We combine here DFT calculations and analytical modelling to determine the thermodynamics of water molecule interaction with perfect ZnO polar model surfaces, (0001) and (0001) surfaces (noted p-Zn and p-O). Defects (oxygen vacancies, pits and missing oxygen rows) are also investigated. Adsorption, dissociation, surface migration and agglomeration are considered. We find that H2O preferentially adsorbs and dissociates on Zn atoms on p-Zn and at defects on p-O. At room temperature, water is found to spontaneously dissociate, except for p-O for which dissociation is endothermic. After dissociation, the resulting protons either bind to surface oxygen atoms, or to zinc atoms to 1 ACS Paragon Plus Environment

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form hydrides. Migration of H and OH is limited on p-Zn with moderate barriers and absent on p-O. Interestingly, further agglomeration or islanding of OH species is inhibited by repulsive OH-OH electrostatic forces. Consequently, although polar surfaces are highly reactive with water, they cannot sustain high OH coverages, unless highly defective. This limitation is one obstacle to ZnO catalytic activity, pointing to the need to tune temperature and pressure conditions.

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Introduction Metal oxides constitute an essential material for a wide range of technologies in microelectronics,1-2 medicine3-4 and chemistry.5 In particular, zinc oxide (ZnO) is a leading technological material due to its unique optical and physico-chemical properties:6-7 high thermal stability, high cohesive energy, high piezoelectric constant, and an n-type semiconductor with a relatively large band gap (~3.3 eV). In addition, ZnO can be engineered in many different ways, with controlled shapes at the nanoscale.8 Consequently, ZnO is widely used in mainstream microelectronics and catalysis applications, such as optical devices, converters, water gas shift reaction, methanol synthesis.8-9 Furthermore, it is used for applications such as photo-assisted water splitting for hydrogen production.10-11 In this process, water adsorption and dissociation lead to neutral or charged intermediates generating the release of H2 or O2 species. All these applications require atomic scale control of the surface/interface, i.e. an understanding of ZnO surface structure and interaction with water, including the identification of defects and surface states. This paper focuses on the evolution of ZnO surfaces when exposed to water, as ZnO is naturally covered with two to three monolayers (noted ML) of water molecules at ambient conditions. It models water behavior on these surfaces, including adsorption, dissociation, induced surface modifications and reactivity, which are all important to control surface chemistry. Despite decades of active research on water/ZnO interactions, both experimentally,12-15 and theoretically,11,

16-20

deriving realistic model systems for

studying catalysis or interfacial chemistry remains difficult and of keen interest. ZnO surface reactivity depends on both surface orientation and presence of defects. Typical thin films, deposited by Chemical Vapor Deposition (CVD) or Atomic Layer Deposition (ALD), are characterized by a wurtzite structure with three possible surfaces: polar Zn-terminated (p-Zn) (0001), polar O-terminated (p-O) (0001) and apolar mixed (1010) 3 ACS Paragon Plus Environment


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surfaces, which have been well characterized over the past 30 years.6-7 In order to describe their interaction with water, both experimental characterization techniques and theoretical calculations11-14, 16, 18-19, 21-26 have explored molecular and dissociative adsorption processes. Most work has so far been performed for non-polar surfaces and has concluded that water organizes upon the surface, with partial dissociation within this adsorbed layer, exhibiting well-organized patterns with half of the ZnO surface sites occupied by OH groups.27-29 The role of local defects (steps, adatoms, Zn-O dimer vacancies …) was analyzed,

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with

calculations from single water molecular exposure27 up to full water coverage.20, 30-32 In contrast, polar surfaces models are much more challenging because they require defects or other stabilizing chemical species to compensate for the charge accumulation on both O- and Zn-terminated surfaces.15,

18, 33-35

This variety of starting surfaces has led to

apparently conflicting results regarding both type of reconstruction and water interaction. On the experimental side, some early findings, based on ultraviolet photoemission spectroscopy and temperature-programmed desorption, concluded that water adsorption was nondissociative. More recent scanning tunneling spectroscopy imaging and photoemission spectroscopy studies have uncovered specific dissociative sites, namely pit and cavities, obtained upon 300 K annealing in ultra-high vacuum.14 The authors also evaluated the binding energy of hydroxyl species on the oxygen vacancies at 200 K to be 1.35 eV from thermal desorption spectroscopy. On the polar O-terminated ZnO (0001) surface, H2O was found by X-ray photoelectron spectroscopy experiments to spontaneously dissociate on the missing-oxygen rows of a (1×3) reconstructed surface, even at low temperatures (200 K).12 To get more insight into water interaction with polar ZnO surfaces, only few theoretical calculations have been performed, using Density Functional Theory (DFT),11, clusters19 or periodic calculations.11, 16-18


16-19

with

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In the case of polar surfaces, a variety of model surfaces have been considered, focusing mostly on clean p-Zn termination with addition of defects identified experimentally,17, 19, 26 without the presence of chemical species (i.e. interaction with gas phase species). The problem is that, while all the experimental work report pit defects (ref.

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also identifies

double Zn rows on flat terraces), they do not show equivalent pit morphologies (truncated pits at vertex locations,35 stabilization of adatoms at pit border15), and report a wide range of size and spatial distributions. With such experimental input, theoretical calculations considered a number of reconstructed model surfaces satisfying charge compensation of the constructed polar slabs, leading to apparent discrepancies.15, 18, 34 Only few theoretical contributions have focused on water interaction with polar surfaces, each using specific model-surfaces, either perfect Zn-terminated or pit-defective structures, leading again to discrepancies, such as the calculations of two widely different adsorption energies, -1.15 eV17 and -0.28 eV19 for water on the p-Zn surface. It is therefore difficult to derive a predictive model of water reaction on the polar faces of ZnO from these case-by-case studies, particularly since the p-O surface has not been investigated theoretically at all. Importantly, an experimental study demonstrated that the surface structure is drastically modified upon H2O exposure, with a marked decrease of pit size and concentration, leading to the stabilization of flat surfaces. These findings justify the consideration of flat surfaces as starting structures for water adsorption studies, in addition to consideration of defects, since there is clearly a close relationship and synergy between surface chemical interactions and overall surface reconstruction.14 Furthermore, the determination of the spatial distribution of OH and H species, their migration and potential recombination on various ZnO surfaces is absent in state-of-the-art calculations that have mostly focused on surface modification upon water heterolytic dissociation. It is therefore important to perform theoretical calculations of water reaction and product behavior on a variety of starting model surfaces, both flat and defective.



This paper focuses therefore on the interaction of water with a selection of perfect and defective polar ZnO model surfaces, introducing new surface configurations in modeling as needed by these complex surfaces. The goal is to derive a broader quantitative and qualitative understanding of the reactivity on these surfaces. In a second stage, we discuss the ability of these model surfaces to accommodate adsorbed OH species. The Methods section describes the theoretical methods developed to deal with all the model surfaces considered here. The Results section describes the surface configurations of the initial adsorption and the subsequent dissociation, leading to the thermodynamics of the system. The energy pathways for water dissociation are also calculated from the most stable configurations, and the hydroxyl distribution (location, aggregation) on different ZnO surfaces are discussed. A comparison of our findings with the literature is finally presented with goal to address the more controversial points.

Methods Computational details All our calculations have been performed using the CP2K-2.7 package,36 within the GGAPBE functional.37 Nuclei and core electrons have been depicted by GTH pseudo-potentials and valence electrons have been represented by molecularly optimized DZVP basis sets,38-39 a 500 Ry energy cut-off, a 60 Ry relative energy cut-off and five grid levels. For slab calculations, a wavelet Poisson solver has been used in order to complete 2D periodic boundary conditions.40 In this method, the potential is expressed as a Fourier series in directions parallel to the surface, where Periodic Boundary conditions apply, but is explicitly solved in the direction normal to the surface, where periodicity no longer applies. The result is that the electric potential remains constant in the whole vacuum, as shown in Figure S1 of the Supporting Information. Climbing-image Nudged Elastic Band (NEB) method has been used,


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with 6 images between the initial and final steps, in order to determine reaction pathways and activation barriers. ZnO has three different allotropic forms: blende, wurtzite and rocksalt.6-7,

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The current

control during the crystallization process leads to the formation of wurtzite, which is the most widespread form found in industry.41 As mentioned earlier, a wide variety of model surfaces and technical approaches have been proposed in the literature. In our work, we consider a wide set of model surfaces, which may be not always be optimum for charge compensation and stability without adsorbates, but offer systematic and more comprehensive models to examine adsorption in different chemical environments. Nonetheless, we have made two important methodological choices to correct polar effects: (i) we use state-of-the-art dipolar corrections for all calculations, and (ii) we eliminate the infinite replication of any residual dipole or dipole induced-interaction by using a 2D periodic scheme instead of the typical 3D periodicity used to date for treating polar ZnO surfaces via plane wave codes. The main assumption is that the basic and local chemistry of water interaction with our model surfaces is only marginally affected by imperfect dipole treatment on the top surface, i.e. that chemical effects dominate.

Model surfaces Figures 1 and 2 show the perfect wurtzite ZnO-model (0001) and (0001) surfaces and the defective surfaces considered in this work, respectively. The surfaces are built from ZnO slabs containing six full ZnO layers with a 19.8 × 17.1 Å2 surface area and composed of either 36 zinc or 36 oxygen atoms depending on the particular cleavage plane. The two bottom layers are kept frozen in their bulk positions to mimic the strain of a perfect substrate and the four topmost layers are allowed to relax completely during geometry optimization. In order to perform calculations at the Γ-point, we use for all cases the sufficiently large c(6×3) supercell.



We include 20 Å of vacuum on the top of the ZnO slabs, in order to obtain full energy convergence (

Water Dissociation and Further Hydroxylation of Perfect and Defective

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