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J. Phys. Chem. C 2009, 113, 2896–2902
Water Adsorption on Nonpolar ZnO(101j0) Surface: A Microscopic Understanding Arrigo Calzolari*,† and Alessandra Catellani‡ CNR-INFM-S3, National Center on nanoStructures and bioSystems at Surfaces, I-41100 Modena, Italy, and CNR-IMEM, Parco Area delle Scienze, 37A, I-43100 Parma, Italy ReceiVed: October 1, 2008; ReVised Manuscript ReceiVed: December 15, 2008
We report on first-principles density functional calculations about the adsorption of water molecules on the nonpolar ZnO(101j0) surface, as a function of the molecular coverage. Our results allow us to unravel the reaction mechanisms that drive the partial dissociation of water molecules at saturation coverage: Although not a favored event, concurrent adsorption/dissociation may occur as a compromise between steric repulsion and covalent and hydrogen bond formation with both the substrate and the impinging molecules. The scenario is altered by the presence of defects. We discuss the role of the most common point and extended defects at the outermost layer: these systems exhibit remarkably different electronic properties leading to peculiar and unexpected characteristics for the wet defective surface. Enhanced reactivity of edged nanostructures is predicted, while the catalytic role of oxygen vacancies is questioned. The effects of metallicity induced by hydrogen adsorption on the interaction with water are finally analyzed. We suggest also experimental probes to identify the various adsorption geometries and fully characterize the water vapor/oxide interface. Introduction The reactivity of inorganic surfaces to water exposure is a subject of continuous interest. In particular, the interaction of water with metal oxides is largely studied both experimentally and theoretically, with the oxide hydrous phases being crucial to physical and chemical phenomena ranging from geochemistry to corrosion, electrochemistry, and electronic devices: although it is accepted that most oxide surfaces react readily in humid ambient conditions, and become partially covered with molecular water, the understanding of the role of the defects and edges is still under debate,1-4 and a clear characterization of the oxide surface in ambient conditions is still to be provided. This is particularly critical in view of the increased number of applications of oxide-based nanostructures, where the surface and particle edges evidently play a dominant role.5-7 Among the many relevant metal oxide compounds, we focus on ZnO,8 a material of paramount importance in a wide range of applications, such as photocatalysis, solar cells, gas sensors, and bio- and optic devices.9 While great effort has been directed experimentally10,11 and theoretically12,13 in order to characterize the morphology and the stability of the wet ZnO surfaces, the electronic properties of the water/ZnO interface have been poorly investigated up to now, albeit being the driving force in many relevant processes such as catalysis and photovoltaics. Here, we extensively investigate the properties of water molecules adsorbed on clean and defective nonpolar ZnO(101j0) surfaces from first principles, in terms of charge transfer and electrostatic interactions. The (101j0) surface is very stable and well-ordered, with a small amount of surface defects, mainly vacancies of oxygen atoms or Zn-O dimers,14 while the absence of surface Zn ions is experimentally rarely observed.15 Furthermore, the nonpolar surfaces are nowadays of great interest in the active fields of nanotechnology, since the most common * E-mail:
[email protected]. † CNR-INFM-S3. ‡ CNR-IMEM.
ZnO nanostructures grow along the more reactive polar axis and expose nonpolar lateral faces.16 Our simulations clearly describe the adsorption patterns at the interface, which is dominated by the formation of a dative bond at the surface zinc site. The analysis of the complex interplay between the steric packaging of molecules and the surface periodicity, along with the balance of the electrostatic interactions, furnish a rationale to explain, from the electronic point of view, the possible water dissociation mechanisms that can take place as a function of coverage and of the local properties of the substrate. In particular, the comparative investigation of several ZnO(101j0) substrates, including clean, defective, and hydrogenated surfaces, emphasize how the morphological analysis of the final adlayer is not sufficient to fully characterize the system, since adsorption and dissociation of water molecules can occur following different processes, driven by different electronic environments. The presence of electronic-active binding sites and the local charge distribution, modified by defects, becomes the fundamental issue for the following adsorption/dissociation of water molecules. Method We perform first-principles total-energy-and-force calculations, based on density functional theory (DFT), as implemented in the PWscf code.17 PBE18 generalized gradient approximation is applied to the exchange-correlation functional. The atomic potentials are described by ab initio ultrasoft pseudopotentials in the Vanderbilt’s formulation.19 For a more accurate description of the atomic interactions, the semicore 3d electrons of zinc are explicitly included in the valence shell. The electronic wave functions (charge density) are expanded in a plane-wave basis with an energy cutoff of 28 Ry (280 Ry). The metal oxide surfaces are simulated by periodic supercells, where we included a thick vacuum layer (∼15 Å) in the directions perpendicular to the surface. Each slab contains six bilayers of ZnO(101j0), while the number of water molecules and the lateral periodicity of the unit cell vary, depending on the particular system we studied. Water molecules are sym-
10.1021/jp808704d CCC: $40.75 2009 American Chemical Society Published on Web 01/26/2009
Water Adsorption on Nonpolar ZnO(101j0) Surface
J. Phys. Chem. C, Vol. 113, No. 7, 2009 2897
TABLE 1: Structural and Electronic Parameters for Clean and Functionalized ZnO(101j0) Surfacesa d(Zn-Ow)
d(Ow-Os)
∆Eads
∆Eg
I
clean single monolayer
2.09 2.06b 1.96c
Ideal 2.54 2.69b 2.73c
-0.95 -1.16
0.77 0.80 1.07
5.83 5.23 5.67
Ovac Ovac+W dimer dimer+W
2.22 1.96
Defects 3.14
-0.47d -2.38d
0.83 0.76 0.42 0.76
5.29 4.80 5.30 5.38
Hyd Hyd+W
2.26
Hydrogenated 3.26
-0.29d
0.00 0.00
3.32 2.49
a Distances d are expressed in Å; adsorption energy ∆Eads (∆Eads Wat/ZnO ZnO Wat ) 1/n[Etot -Etot -nEtot ]) is expressed in eV/molecule; energy bandgap ∆Eg, and ionization potential I are expressed in eV; subscript labels refer to oxygen atoms as in Figure 1. b Undissociated molecule. c Dissociated molecule. d Calculated with respect to the corresponding relaxed defective surface.
metrically adsorbed on both sides of the slab in order to avoid spurious electrostatic interactions between adjacent replicas. For Brillouin zone (BZ) integration, we use 8 special k points in the irreducible wedge of the 2D BZ of the surface cells. All structures are relaxed until forces on all atoms are lower than 0.03 eV/Å. We checked the accuracy of our results with respect to the approximations adopted in our DFT approach. First, we considered the effects of on-site electron-electron correlation beyond the mean-field level: we tested that the inclusion of ad hoc Hubbard potential on zinc atoms corrects the bandgap, in agreement with previous calculations;20 however, it does not modify the main physical properties of the clean surface or the water/ZnO interface. We also verified that inclusion of spin contribution, within the local spin density approach, does not change the properties of the water vapor/metal oxide interface.21 Results Ideal Substrate. Our simulation of the clean ZnO(101j0) surface reproduces well the existing results:8,22,23 the outermost threefold-coordinated atoms assemble in ordered rows of Zn-O “dimers” along the [12j10] direction. The Zn-O dimers exhibit a vertical relaxation with a 0.34 Å relative displacement between the outward O and the inward Zn atoms. The surface is semiconducting and nonmagnetic with a net charge accumulation around the oxygen atoms. The calculated structural and electronic properties of the surface are summarized in Table 1. The presence of dimer rows of acidic metal and basic oxygen atoms together with buckling relaxation offers a natural template for the deposition of water molecules. As a first step in our investigation, we study the structural and electronic properties of a single water molecule adsorbed on the clean ZnO(101j0) surface, with (2 × 2) lateral periodicity (inset of Figure 1). This configuration corresponds to a coverage of 1/4 of monolayer (ML); however, the distance between neighboring H2O molecules is large enough to consider the water molecule effectively isolated. In the following, we refer to this system as single molecule configuration, and we label Ow and Os the oxygen atoms of the water molecule and of the surface facing the molecule, respectively (Figure 1). After relaxation, the adsorbed water molecule is undissociated and bridge-bonded to two Zn-O dimers belonging to neighbor-
Figure 1. Side view representation of optimized H2O/ZnO(101j0) interfaces: (a) the undissociated molecule, corresponding to low coverage conditions, (b) the half-dissociated monolayer. Labels Ow and Os are the oxygen atoms of pristine water molecule and metal oxide surface, respectively. The inset describes the (2 × 2) lateral periodicity adopted in the simulations. Large (small) white (black) circles identify first (second) layer zinc (oxygen) atoms, respectively.
ing rows along the polar [0001] axis: Ow is oriented toward a Zn atom, restoring the fourfold bulk coordination, while a hydrogen is facing the surface in order to form an H-bond with the closest Os atom. The surface does not exhibit structural distortions, except for the adsorption site where the substrate de-relaxes, removing the buckling of the dimer involved in the Zn-Ow bonding process (Figure 1a). If we increase the number of H2O molecules in the cell until the saturation of every surface Zn-Os unit (one-monolayer configuration), we observe the formation of an ordered (2 × 1) superstructure, resulting from the partial dissociation of the water molecules, as shown in Figure 1b. The optimized structure exhibits alternating rows of undissociated and dissociated water molecules along the [12j10] direction. As in the previous case, the Zn-O dimers involved in water bonding de-relax, restoring the bulk-like fourfold coordination. From the analysis of the bonding lengths (Table 1), we note that undissociated molecules maintain the same configuration of the single molecule case, while the dissociated OwH hydroxyl groups become closely bonded to the Zn atoms beneath. The molecular adlayer rearranges in order to optimize the H-bond path both with the surface (OwH · Os and Ow · HOs) and among molecules (OwH · Ow), as confirmed also by the enhancement of the adsorption energy per molecule (Table 1). Notably, the calculated adsorption energy for the saturated surface agrees well with the experimental value (1.02 eV) obtained through He thermal desorption spectroscopy techniques.12 The partial-dissociated configuration that we obtain perfectly agrees with previous experimental10,11 and theoretical12,13 structural analysis. A detailed study of the electronic structure of the interface is, however, required to elucidate the otherwise unresolved dissociation mechanism. The electronic properties of the water/surface complex can be best understood via the analysis of the individual bonding orbitals as represented by the maximally localized Wannier functions (MLWFs).24 In fact, because of their localized character, MLWFs are particularly useful to describe the nature of the chemical bond.25 We calculate MLWFs by using the WanT code26,27 on the basis of the ground-state DFT electronic structure, described above. A few selected maximally localized Wannier functions for the monolayer configuration are reported in Figure 2. In particular, panel (a) accounts for water chemisorption: it shows the σ-like Zn-Ow bonding state, which derives from the hybridization of Zn s orbital and the Ow p lone pair. Similar Zn-Ow states are observed both for the dissociated molecule and in the undissociated single molecule configuration: they are a marker of
2898 J. Phys. Chem. C, Vol. 113, No. 7, 2009
Figure 2. (a-e) Isosurface plots of selected MLWS’s for 1 ML configuration, relative to molecule-substrate (a-d, side view) and molecule-molecule (e, top view) interactions. (f,g) Top view scheme diagrams of bond formation upon water adsorption at (f) submonolayer and (g) monolayer coverage. Labels follow Figure 1; dZn and dOO are the minimum Zn-Zn and Ow-Ow distances along the [12j10] direction.
the charge polarization upon adsorption. The formation of the Zn-Ow bond induces a local charge redistribution, which is responsible for the removal of the clean dimer relaxation, described above (Figure 1). The saturation of every surface dangling bond at 1 ML coverage is responsible also for the increase of the energy bandgap ∆Eg (Table 1). In a similar way, Figure 2b describes the formation of the σ-like Os-H bond, upon water dissociation. The adsorption and the partial dissociation of water molecules break the ×2 periodicity of the substrate. From the energetic point of view, the formation of different chemical bonds corresponds to the appearance of novel features in the lowenergy region of the density of states (DOS) that are reflected in a redistribution of the semicore oxygen peaks. We observe a chemical shift of the substrate oxygen that is moved out in the ionicity ZnO gap; oxygen atoms linked to reacted hydroxyl groups give rise, instead, to peaks that move upward with respect to the unreacted water (see Figure SI-1 of Supporting Information). These features should be directly detectable in photoemission spectroscopies. The charge redistribution induced by water adsorption is also reflected in the change of the ionization potential (Table 1): we observe a general decrease in the ZnO ionization potential for all regimes,28 as experimentally observed for ZnO22 and other metal oxide surfaces upon water exposure.29 From the comparison between the two structures, we conclude that the formation of Zn-Ow bonds, associated to an electron transfer to the substrate, induces a reduction of the ionization potential. On the contrary, the formation of OsH bonds, associated to a hole transfer (H+) to the surface, increases the ionization potential. For monolayer configuration, the final ionization potential (greater than the single molecule case) is the result of two opposite trends, due to the formation of different kinds of bonds at the interface. Figure 2c,d represents the Ow-H-Os hydrogen bonds between water and the ZnO surface, in the case of the undissociated (c) and dissociated (d) molecule, which stem from the electrostatic polarization of p-like orbitals of Os and Ow
Calzolari and Catellani atoms, respectively. Similar states occur also between pairs of water molecules, representing the intermolecular (OwH · Ow) bonds, as shown in Figure 2e. The presence of these water-water hydrogen bonds is the key issue that drives the stabilization of the adlayer and the dissociation process that occurs at high coverages. The final configuration depends on the relationship between the minimum Zn-Zn distance (dZn ) 3.3 Å), and the Ow-Ow distance (dOO), at which the formation of H-bonds is optimized. In particular, the periodicity of the substrate imposes a specific template for the formation of the (OwH · Ow) bonds along the [12j10] direction, so that molecules dimerize forming one H-bond every two H2O molecules. The lattice parameter of the ZnO surface compares fairly well to the structural parameters of liquid water, which ref ) 2.7 Å is the realizes a very efficient H-bond network (dOO first peak in the oxygen-oxygen radial distribution function),30,31 favoring a robust intermolecular coupling. The formation of strong intermolecular hydrogen bonds compares well with the large red shift of the H2O stretching mode, with respect to that of non-H-bonded OH groups, measured in HREELS experiments.11 We can summarize the adsorption and the dissociation processes as a function of coverage as follows: as water is easily involved in dative bond formation at a Zn site, the separation between the adsorbed molecules is dictated by the Zn spatial distribution at surface. At low coverage (
