Cluster and periodic ab-initio calculations on K/TiO2(110)

The interaction of K atoms with the TiO2(110) surface has been studied with cluster model and periodic slab Hartree-Fock calculations. Gradient correc ted 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 an d the dipole moment curves for the vertical displacement of the K adsorbate . Observable consequences of the charge transfer interaction are: (1) the f ormation 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. T his state is spin-polarized and reflects the spin localization following th e charge transfer. The formation of "reduced" Ti3+ ions is associated more with the spin localization than with a real change in the net atomic charge s. The potential energy surface for the motion of the adsorbed K+ ion on th e surface is rather flat, but the structure of the overlayer is largely det ermined 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 d ecreases rapidly with increasing coverage. (C) 1998 Elsevier Science B.V. A ll rights reserved.