Water adsorption and dissociation on ${\text{SrTiO}}_3\left(001\right)$ revisited: A density functional theory study

We present a comprehensive density functional theory study addressing the adsorption, dissociation, and successive diffusion of water molecules on the two regular terminations of ${\text{SrTiO}}_3\left(001\right)$. Combining the obtained supercell-geometry converged energetics within a first-principles thermodynamics framework we are able to reproduce the experimentally observed hydroxylation of the SrO-termination already at lowest background humidity, whereas the ${\text{TiO}}_2$-termination stays free of water molecules in the regime of low water partial pressures. This different behavior is traced back to the effortless formation of energetically very favorable hydroxyl-pairs on the prior termination. Contrary to the prevalent understanding our calculations indicate that at low coverages also the less water-affine ${\text{TiO}}_2$-termination can readily decompose water, with the often described molecular state only stabilized toward higher coverages.

Figure 1

(Color online) Schematic illustration of the ${\text{SrTiO}}_3$ perovskite bulk structure and the two regular terminations of the (001) surface. Big red (dark) spheres: strontium, big green (bright) spheres: titanium, small blue (bright) spheres: oxygen.

Figure 2

(Color online) Perspective view of the most favorable adsorption geometry A1 at the SrO termination, in which the water dissociates into an adjacent hydroxyl pair. Atoms discussed in the text and Table are labeled. Coloring here and in all consecutive figures follows the one of Fig. 1, with small black spheres denoting hydrogen atoms.

Figure 3

(Color online) Binding energy, internal $\text{O}1\text{-H}2$ bond length and adsorption induced change in the Hirshfeld charges as a function of the vertical height $z$ of the center of mass of an approaching water molecule above the SrO termination. The different atom labels follow the definition given in Fig. 2. The zero reference for the vertical height corresponds to the equilibrium height in the adsorbed state A1. The presented data have been calculated in geometry optimizations, in which the $z$ value of center of mass of the water molecule was constraint at different heights.

Figure 4

(Color online) Energy profile for surface diffusion of the protruding $\text{O}1\text{-H}1$ hydroxyl group over the SrO-terminated surface. The initial state A1 corresponds to the hydroxyl pair after the immediate water dissociation process shown in Fig. 3. The transition from state A1 to A2 breaks the hydroxyl pair and is accompanied by a substantial energetic barrier. Configurations A2 and A3 differ essentially in the mutual orientation of the two hydroxyl groups, whereas state A4 represents the maximum possible distance of the diffusing species to its original position in a periodically continued $\left(3\times 3\right)$ surface model.

Figure 5

(Color online) Perspective view of the molecular B1 (left) and dissociated B2 (right) adsorption geometry at the ${\text{TiO}}_2$ termination. Atoms discussed in the text and Table are labeled. In a second dissociated state (not shown), which is energetically equivalent to the state B2, the ${\text{O}}_1{\text{-H}}_1$ group is rotated by $180°$ degrees and thus points in the same direction as the tilted $\text{O}2\text{-H}2$ hydroxyl.

Figure 6

(Color online) Binding energy, internal $\text{O}1\text{-H}2$ bond length and change in the Hirshfeld charges along the reaction pathway from the molecular bound state B1 to the dissociated state B2 at the ${\text{TiO}}_2$ termination. The different atom labels follow the definition given in Fig. 2. The zero reference for the Hirshfeld charges corresponds to gas-phase water and the clean surface.

Figure 7

(Color online) Energy profile for surface diffusion of the split-off ${\text{H}}_2$ atom over the ${\text{TiO}}_2$-terminated surface. The initial states B1 and B2 correspond to the molecular precursor and dissociated hydroxyl pair shown in Fig. 5. Further disintegration takes place via a hop of the surface ${\text{H}}_2$ atom to an adjacent lattice O anion (B3) with a barrier $\Delta E_{\text{B}23}=0.51\text{eV}$, followed by a flip of the orientation of the newly formed hydroxyl group toward the neighboring surface unit cell (B4) with a barrier $\Delta E_{\text{B}34}=0.38\text{eV}$. With the ensuing equivalent hop exhibiting virtually the same energy profile and suggesting thereby only small longer-range lateral interactions between the hydroxyl groups, the diffusing hydrogen atom $\text{H}2$ has reached in B5 the maximum distance to its original position in B1 that is possible in a periodically continued $\left(3\times 3\right)$-surface model.

Figure 8

(Color online) Surface free energies, cf. Eq. (2), of the most stable water adsorption geometries at the different coverages at the SrO-termination (left) and ${\text{TiO}}_2$ termination (right). In the top $x$ axis, the dependence on the water chemical potential is converted into pressure scales at 300 and 500 K. The plain (blue) background boxes mark the region of gas-phase conditions probed by Iwahori et al. in their FFM experiments (Ref. 10), while the dotted (gray) boxes indicate the region above the ${\text{H}}_2\text{O}$-rich limit, i.e., where the present approach assuming equilibrium with water vapor is no longer strictly applicable.