Chemisorbed phases of O2 on TiO2 and SrTiO3

Abstract

We have investigated the chemisorption of O2 on TiO2 and SrTiO3 surfaces by using ultraviolet photoemission difference spectroscopy. Both nearly perfect surfaces and surfaces having a high density of defects associated with filled Ti(3d) surface states were studied. On all surfaces low O2 exposures produced an initial adsorbed phase, with a sticking coefficient of 0.2–1, that yields a difference spectrum consisting of two peaks separated by 2.5 eV. The location of these peaks corresponds closely to the valance-band photoemission structure of the substrate, suggesting that the chemisorbed phase consists of O2− ions. For surfaces having initially filled Ti(3d) surface states, formation of this phase is accompanied by charge transfer from the substrate. With increasing exposure, a second adsorbed phase, with a sticking coefficient of 10−3 or less, is formed on some surfaces. This phase exhibits a sharp peak about 2 eV below the bottom of the substrate valence band and a broad peak overlapping the valance band.