Cluster and periodic ab-initio calculations on K/TiO 2(110)

Abstract

The interaction of K atoms with the TiO 2(110) surface has been studied with cluster model and periodic slab Hartree–Fock calculations. Gradient corrected density functional calculations have been performed for selected cases. A charge transfer occurs from the K 4s level to the empty 3d levels of the fivefold-coordinated surface Ti atoms, as shown by the Mulliken charges and the dipole moment curves for the vertical displacement of the K adsorbate. Observable consequences of the charge transfer interaction are: (1) the formation of a surface dipole, which in turn explains the observed reduction of the oxide work function; and (2) the appearance of a gap state about 2 eV above the O 2p valence band due to the occupation of the Ti 3d levels. This state is spin-polarized and reflects the spin localization following the charge transfer. The formation of “reduced” Ti 3+ ions is associated more with the spin localization than with a real change in the net atomic charges. The potential energy surface for the motion of the adsorbed K + ion on the surface is rather flat, but the structure of the overlayer is largely determined by adsorbate–adsorbate repulsive interactions. K prefers to adsorb in the vicinity of the protruding bridging oxygens at low coverage, and in the threefold hollow sites formed by two bridging and one planar oxygen at higher coverage. The adsorption energy of an isolated K atom is high, but decreases rapidly with increasing coverage.