A PHOTOEMISSION INVESTIGATION OF THE ADSORPTION OF POTASSIUM ON PERFECT AND DEFECTIVE TIO2(110) SURFACES

Potassium adsorption on stoichiometric and defective rutile TiO2(110) surfaces at room temperature has been studied by means of angle-resolved photoemission spectroscopy (ARUPS and ARXPS) from the valence states and core levels. Potassium adsorption on the stoichiometric surface creates reduced Ti ions (''Ti3+'') with lowered Ti2p core-level binding energy and 3d-like band-gap states centered at 0.9 eV binding energy, arising from K-to-substrate charge transfer. The Ti3+ 3d intensity versus coverage, which is a measure of the transferred charge, shows a behavior similar to that of the work function, with a very pronounced maximum at the coverage of the weak work-function minimum. In contrast, the Ti3+ 2p signal, which is a measure of the number of reduced Ti ions, increases continuously with K coverage. The results are discussed in terms of a recently proposed ''ionic-to-neutral transition'' of the K-substrate bond. On the defective surface, K adsorption results in a decrease of the initial concentration of ''Ti3+-V-o'' surface defects, probably caused by K-induced oxygen diffusion from the bulk to the surface.