First-principles study of potassium adsorption on ${\mathrm{TiO}}_2$ surfaces

We present an ab initio study of alkali metal adsorption on metal oxide surfaces. We have investigated the changes that occur to the structure and electronic properties of the rutile ${\mathrm{TiO}}_2$ (100) surface when a $\frac{1}{2}$ monolayer of potassium atoms is adsorbed. The K atoms are reduced to ${\mathrm{K}}^{+}$ ions as charge transfer occurs from the ${\mathrm{K}}_{3s}$ orbital to localized ${\mathrm{Ti}}_{3d}$ orbitals at the surface of the substrate. The occupied ${\mathrm{Ti}}_{3d}$ states lie about 1 eV above the ${\mathrm{O}}_{2p}$ valence band maximum in agreement with recent photoemission studies. The ${\mathrm{Ti}}_{3d}$ states are spin polarized and are similar to those found at oxygen deficient ${\mathrm{TiO}}_2$ surfaces. The geometric structure also undergoes significant relaxation as the lattice is polarized around the reduced Ti sites. The calculated surface structure is in excellent agreement with x-ray adsorption data.