Aligning electronic energy levels at the ${\text{TiO}}_2/{\text{H}}_2\text{O}$ interface

The electronic energy levels of a model titanium dioxide water interface computed using the Perdew-Burke-Ernzerhof (PBE) density functional are positioned relative to the normal hydrogen electrode and vacuum. As energy reference we use the solvation free energy of the ${\text{H}}^{+}$ ion computed by reversible insertion of a proton in the aqueous part of the model system. The interaction with water raises the energy levels bringing the conduction-band edge in fair alignment with experiment. The error in the PBE band gap must therefore be largely attributed to underestimation of the ionization potential.

Figure 1

(Color online) Model ${\text{TiO}}_2/{\text{H}}_2\text{O}$ interface with (a) an excess electron in the solid slab and proton in the water layer in between slabs and (b) a hole and a compensating ${\text{OH}}^{-}$ in solution. Red (dark gray) balls are O, white balls H, and small yellow (light gray) balls Ti. ${\text{H}}_2\text{O}$ molecules with added or removed H are highlighted. The spin density is visualized by isosurfaces in green (gray) (density is $1.5\times {10}^{-3}$).

Figure 2

Computed conduction- and valence-band edges of the ${\text{TiO}}_2/{\text{H}}_2\text{O}$ interface compared to the experiment (Ref. 2), the clean surface in vacuum and a surface with 1 ML of ${\text{H}}_2\text{O}$.