A PHOTOEMISSION STUDY OF THE ELECTRONIC STRUCTURE INDUCED BY POTASSIUM ADSORPTION ON TiO2(110)

Abstract

Electronic structure effects induced by potassium adsorption up to one monolayer (ML) on a nearly stoichiometric TiO 2(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 on K/TiO 2(100) [P.J. Hardman et al., Surf. Sci.269/270, 677 (1992)], potassium adsorption at room temperature leads—due to K-to-substrate charge transfer—to the reduction of surface Ti ions (to nominally Ti 3+ ions), evidenced by lowered Ti 2p core-level binding energy (ΔBE=–1.6 eV ) and occupation of Ti 3d-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 [Souda et 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) Ti 2p core-levels lowered by 3.2 eV in BE (nominally “ Ti 2+” ions). This dramatic reduction of the surface is healed out with complete desorption of potassium. A discussion in terms of desorption of KO x species and oxygen diffusion from the bulk to the surface is given.