First-principles study of potassium adsorption on TiO₂ 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 TiO₂ (100) surface when a 1/2 monolayer of potassium atoms is adsorbed. The K atoms are reduced to K⁺ ions as charge transfer occurs from the K_3s orbital to localized Ti_3d orbitals at the surface of the substrate. The occupied Ti_3d states lie about 1 eV above the O_2p valence band maximum in agreement with recent photoemission studies. The Ti_3d states are spin polarized and are similar to those found at oxygen deficient TiO₂ 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.