A PHOTOEMISSION STUDY OF THE ELECTRONIC STRUCTURE INDUCED BY POTASSIUM ADSORPTION ONTiO2(110)

Electronic structure effects induced by potassium adsorption up to one monolayer (ML) on a nearly stoichiometricTiO2(110)surface has been studied by means of angle-resolved photoemission spectroscopy (ARUPS and ARXPS) from valence states and core levels. In agreement with the observations onK/TiO2(100)[P.J. Hardmanet al.,Surf. Sci.269/270, 677 (1992)], potassium adsorption at room temperature leads—due to K-to-substrate charge transfer—to the reduction of surfaceTiions (to nominallyTi3+ions), evidenced by loweredTi2pcore-level binding energy(ΔBE=–1.6eV)and occupation ofTi3d-like band-gap states centered at 0.9 eV BE. The gap-state intensity exhibits a pronounced maximum at 0.37 ML coverage, where the work function has a weak minimum. This behavior is in agreement with a ionic-to-neutral transition of the K-substrate bonding with increasing K coverage, as suggested recently [Soudaet al.,Surf. Sci.285, 265 (1993)]. Annealing of a surface precovered with 0.27 ML potassium up to 1000 K results in metallization of the surface, evidenced by (i) the occupation of a second gap-state centered at 0.4 BE and with a considerable state-density at the Fermi energy, and (ii)Ti2pcore-levels lowered by 3.2 eV in BE (nominally “Ti2+” ions). This dramatic reduction of the surface is healed out with complete desorption of potassium. A discussion in terms of desorption ofKOxspecies and oxygen diffusion from the bulk to the surface is given.