Structural Evolution of Water on ZnO(101‾ 0): From Isolated Monomers via Anisotropic H‐Bonded 2D and 3D Structures to Isotropic Multilayers

Abstract The surface chemistry of water on zinc oxides is an important topic in catalysis and photocatalysis. Interaction of D2O with anisotropic ZnO(101‾ 0) surfaces was studied by IR reflection absorption spectroscopy using s‐ and p‐polarized light incident along different directions. Interpretation of the experimental data is aided using isotopologues and DFT calculations. The presence of numerous species is revealed: intact monomers, a mixed 2D D2O/OD adlayer, an anisotropic bilayer, and H‐bonded 3D structures. The isolated water monomers are identified unambiguously at low temperatures. The thermally induced diffusion of water monomers occurs at elevated temperatures, forming dimers that undergo autocatalytic dissociation via proton transfer. Polarization‐ and azimuth‐resolved IR data provide information on the orientation and strength of H‐bonds within the 2D and 3D structures. Ab initio molecular dynamics simulations reveal strong anharmonic couplings within the H‐bond network.


IRRAS and XPS experiments. The in situ IRRAS experiments were performed in an advanced
UHV apparatus, which combines a state-of-the-art FTIR spectrometer (Bruker Vertex 80v) with several other surface-sensitive techniques (XPS and LEED). 1 The innovative design not only allows us to record grazing-incidence IRRAS data for macroscopic oxide single crystal surfaces, but also enables transmission IR experiments on oxide powders supported on an inert metal mesh. This apparatus has been optimized for sensitivity and allows to reliably detect absorbances as low as 1×10 -5 , a prerequisite for detecting the vibrational signatures of adsorbates on oxide substrates. For a detailed description of the spectrometer see Ref. 1 and Ref. 2 .
The ZnO(101 ̅ 0) surface (10×10 mm 2 ) was cleaned by a sequence of Ar + sputtering (0.5 kV, 3 mA, 1×10 -6 mbar, 5 min) and annealing (850 K, 5 min) cycles until a high-quality (1×1) LEED pattern was observed. The cleanliness and oxidation states of the sample were monitored by grazing incidence XPS equipped with a VG Scienta R4000 electron energy analyzer. The IRRA spectra were recorded with both p-and s-polarized light at a fixed grazing incidence angle of 80° along the [0001] and [12 ̅ 10] crystallographic directions. Unlike metals, where the so-called surface selection rule states that only vibrational modes with a transition dipole moment (TDM) oriented perpendicular to the surface can be observed, 3 for dielectric substrates also vibrations with a TDM oriented parallel to the substrate can be observed (see Figure S1b).
In other words, IR signals can be detected not only for p-polarized but also for s-polarized light.
Note, that both, the p-(tangential, Ep,t) and s-polarized (Es) components of the incident light, couple to vibrational modes of adsorbed molecules with a TDM component orientated parallel to the surface. [4][5][6][7] In addition, in contrast to metals, IRRAS data can show variations when changing the azimuth.
Exposure to H2O or D2O was carried out by backfilling the IR chamber through a leak-valve based directional doser connected to a tube (2 mm in diameter) that terminated 3 cm from the sample surface. Less than 1% of other isotopologues were contained in the D2O. Additional purification of H2O and D2O was achieved by repeated cycles of freezing, pumping and thawing.
The base pressure during acquisition of IRRAS data was below 1×10 −10 mbar. Prior to each exposure, a spectrum of the clean ZnO(101 ̅ 0) surface was recorded as a background reference.
All IRRAS data shown here are difference spectra obtained by subtracting the reference.
Computational details. The density-functional theory (DFT) calculations for the normal mode vibrational frequencies were carried out with the periodic plane-wave code PWscf of the Quantum Espresso software package, 8 using the Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional, 9 Vanderbilt ultrasoft pseudopotentials, 10 and a plane wave kinetic energy cutoff of 30 Ry. The surface structures were represented by periodically repeated slabs with a thickness of four ZnO double layers and a lateral extension of (2×1) surface unit cells in case of a full water monolayer and bilayer and a (4×2) cell for the single isolated water molecule.
The supercells without water thus contained 32 and 128 atoms, respectively. For the k-point sampling a (4,2,1) Monkhorst-Pack mesh was used for the small cell and a (2,1,1) mesh for the large supercell.
All atomic configurations were relaxed with a very tight convergence criterion of 0.001 eV/Å for the residual atomic forces. The atoms in the bottom half of the slab were kept fixed at their bulk positions and only the atoms in the upper half together with the adsorbed water molecules were allowed to move in the geometry optimization. The normal mode vibrational frequencies were calculated in harmonic approximation by a finite difference scheme. The atoms of the adsorbed water molecules and the upper half of the slab were displaced by 0.01 Å in all three Cartesian directions (forward and backward) and the dynamical matrix was determined from the resulting atomic forces, taking the average of the forces from forward and backward displacement. For comparison with experiment, all calculated frequencies were scaled to adjust for the DFT error and to account for deviations due to anharmonic effects. The scaling factors were determined as ratio between the experimental and computed frequencies of the gas phase water molecule, see Table S1. Table S1. Vibrational frequencies (in cm -1 ) of the gas phase water molecule. S is the ration between the experimental 11  The ab initio molecular dynamics simulations (AIMD) were carried out with the CPMD software package. 12 The same functional, pseudopotentials and plane-wave kinetic energy cutoff as in the normal mode calculations were used. All simulations were done with the (4×2) supercell. As in the geometry optimizations, the atoms in the bottom half of the slab were kept fixed at their bulk positions. k-point sampling was restricted to the Γ-point.
All The final spectrum was obtained by averaging the spectra from the three production runs.
Simulations were performed for three water coverages: a single monolayer (1 ML), a bilayer (2 ML) and a thick water film consisting of 80 water molecules in the (4×2) supercell. So altogether, we performed about 1 ns of AIMD simulations (three 100 ps trajectories for each of the three structures plus equilibration).   Figure S1a). 13 After water adsorption at 250 K, a broad, asymmetric O 1s peak was observed. It should be noted that for exposure to water at 250 K the formation of water multilayers can be definitively ruled out since upon water adsorption the total intensity of O 1s as well as Zn 2p

Grazing-emission XPS results
shows no substantial changes ( Figure S1a and S1b).     azimuth at 120 K.