A PHOTOEMISSION-STUDY OF THE ELECTRONIC-STRUCTURE INDUCED BY POTASSIUM ADSORPTION ON TIO2(110)

Electronic structure effects induced by potassium adsorption up to one monolayer (ML) on a nearly stoichiometric TiO2(110) surface has been studied by means of angle-resolved photoemission spectroscopy (ARUPS a nd ARXPS) from valence states and core levels. In agreement with the o bservations on K/TiO2(100) [P. J. Hardman ct al., Surf Scd. 289/270, 6 77 (1992)], potassium adsorption at room temperature leads - due to K- to-substrate charge transfer - to the reduction of surface Ti ions (to nominally Ti3+ ions), evidenced by lowered Ti 2p core-level binding e nergy (Delta BE = -1.6 eV) and occupation of Ti 3d-like band-gap state s 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 minim um. This behavior is in agreement with a ionic-to-neutral transition o f the K-substrate bonding with increasing K coverage, as suggested rec ently [Souda el al., Surf. Sci. 285, 265 (1993)]. Annealing of a surfa ce precovered with 0.27 ML potassium up to 1000 K results in metalliza tion of the surface, evidenced by (i) the occupation of a second gap-s tate centered at 0.4 BE and with a considerable state-density at the F ermi energy, and (ii) Ti 2p core-levels lowered by 3.2 eV in BE (nomin ally ''Ti2+'' ions). This dramatic reduction of the surface is healed out with complete desorption of potassium. A discussion in terms of de sorption of KOx species and oxygen diffusion from the bulk to the surf ace is given.