Polarization-dependent water adsorption on the LiNbO${}_3$(0001) surface

The effect of ferroelectric poling on the adsorption characteristics of water at lithium niobate surfaces is investigated by ab initio calculations. Thereby we model the adsorption of H${}_2$O monomers, small water clusters, and water thin films on the LiNbO${}_3$(0001) surface. The adsorption configuration and energy are determined as a function of the surface coverage on both the positive and negative (0001) surfaces. Confirming the results of temperature programed desorption measurements [Garra, Vohs, and Bonnell, Surf. Sci. 603, 1106 (2009)], polarization-dependent adsorption energies, geometries, and equilibrium coverage are found. Our calculations predict the adsorption to occur mainly nondissociatively, independently of the coverage. The water structures formed at the surface are coverage-dependent, though. The different affinity of water to the two surfaces is explained in terms of electrostatic interactions between the substrate and polar molecules. Water adsorption accentuates the surface relaxation, thus increasing the microscopic surface roughness.

Figure 1

PES for the adsorption of a single H${}_2$O molecule on the positive (upper picture) and negative (lower picture) LN(0001) surface. Li atoms are gray, Nb white, and oxygen red. Adsorption energies are in eV.

Figure 2

Charge redistribution upon adsorption of the H${}_2$O molecule at the positive (a) and negative (b) (0001) surface. Charge isolines in the ($1\overline{1}00$) plane (which is the $y$ plane as defined in 25) are plotted. This plane contains the two bonds formed by the water molecule with the LN surface. In red are regions in which electronic charge is accumulated, in blue are regions of electronic charge depletion.

Figure 3

Adsorption energy (in eV) as a function of the molecule-surface distance. The equilibrium position is the scale zero. (a) Positive surface, (b) negative surface.

Figure 4

Possible water configurations at the positive (0001) surface include (a) two-dimensional regular hexagonal structures and (b) chains. The rhombohedral unit cell is highlighted.

Figure 5

Possible water reconstructions at the negative (0001) surface include two-dimensional (a) hexamer structures and (b) less regular chain structures.

Figure 6

Calculated phase diagram of the positive (a) and negative (b) LiNbO${}_3$(0001) surface as a function of the water availability. The latter is represented by the water chemical potential ${\mu }^{{\mathrm{H}}_2\mathrm{O}}$. Two representative values of ${\mu }^{{\mathrm{H}}_2\mathrm{O}}$, corresponding to water in gas (ideal gas, i.e., noninteracting molecules) and solid (ice $Ih$) states, are indicated. Between the two values, the water availability changes continuously, whereby the water chemical potential is at each point in equilibrium with the LiNbO${}_3$ surface.

Figure 7

Calculated phase diagram of the positive (upper part) and negative (lower part) LiNbO${}_3$(0001) surface as a function of the temperature and pressure. Dotted lines indicate the ambient conditions. The values of the chemical potential variations $\Delta {\mu }^{{\mathrm{H}}_2\mathrm{O}}$ (given with respect to ice $Ih$) at which a particular structure is formed are indicated by solid lines.

Figure 8

Charge density isosurface (0.01 $e$/Å${}^3$) calculated for the clean positive (upper part) and negative (lower part) LN(0001) surface.