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.
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