Ab initio molecular dynamics calculations of ${\text{H}}_2\text{O}$ on BaO(001)

Density functional theory calculations are used to explore water adsorption on BaO(001). The stable configuration is found to be a novel hydroxide pair. A detailed analysis demonstrates that the electrostatic repulsion between the ${\text{OH}}^{-}$ species is screened by ${\text{Ba}}^{2+}$ cations and that the net interaction is provided by hydrogen bonding. Ab initio molecular dynamics at low coverage reveals that the pair is stable at elevated temperatures. The large structural flexibility of the hydoxylated surface may, however, lead to pair dissociation at high coverage. Simulations of a mixed $\left({\text{H}}_2\text{O}+\text{OH}\right)$ overlayer uncover proton transfer between ${\text{H}}_2\text{O}$ and OH groups.

Figure 1

(Color online) Evolution of Kohn–Sham potential energy (top) and temperature (bottom) during a constant energy molecular dynamics trajectory. The first 4 ps are shown from a 15 ps simulation. The snapshots at 0, 0.7, and 2.5 ps are reported to the right. Atomic color code: Ba (green/gray), oxygen (red/dark), and H (white).

Figure 2

(Color online) Left: Evolution of interatomic distances during ${\text{H}}_2\text{O}$ dissociation on BaO(001). Right: $xy$-projections of the oxygen coordinates in ${\text{O}}_s{\text{H}}_{\text{ads}}$ and ${\text{OH}}_{\text{ads}}$. The green/gray filled circles mark the equilibrium positions of neighboring Ba cations.

Figure 3

(Color online) Optimized structures for ${\text{H}}_2\text{O}$ (a) and OH (b) adsorbed on BaO(001) and F-BaO(001), respectively. (c) and (d) show the corresponding electrostatic potentials $\left[-5:5\right]\text{eV}$ ([red/gray:blue/dark gray]) displayed at the position of the O atom in the adsorbed OH group. (e) shows a slice $\left[0.12:1.2\right]\text{e}/{\text{au}}^3$ through the charge density of the ${\text{OH}}^{-}$ pair in (a). The atomic color code as in Fig. 1 with fluorine (blue/dark gray).

Figure 4

(Color online) Distances between the blue (dark) hydrogen atom and two oxygen atoms that originate from adsorbed ${\text{H}}_2\text{O}$. The snapshots at 0.43, 1.11, 1.43, and 2.61 ps are shown. The atomic color code as in Fig. 1.

Figure 5

(Color online) Left upper panel: Relaxation (solid lines) and rumpling (dashed lines) for the first (black) and second (red/gray) surface layer, respectively. Left lower panel: Vibrational density of states. The black and red/gray lines show results for ${\text{OD}}_{\text{ads}}$ and ${\text{O}}_s{\text{D}}_{\text{ads}}$ groups, respectively. The two snapshots are shown to the right. The upper two models correspond to two views of the structure at 0.3 ps, whereas the lower model represents the structure at 6.9 ps. The atomic color code as in Fig. 1.