Resonant ion stimulated desorption of O+ and H+: A probe of interactions of oxygen and hydrogen with TiO2(110)

Abstract

The mechanism of noncollisional H+ and O+ sputtering from reduced, hydrogenated and oxygenated TiO2(110) surfaces has been investigated by low-energy noble-gas ions and electron bombardment. The noncollisional sputtering is found to be initiated by formation of the O 2s core hole via the quasiresonant charge exchange. Desorption of oxygen and hydrogen occurs from core-excited oxygen and hydroxyl group, respectively, which have an antibonding character with a long lifetime. To survive efficient reneutralization, ionization of oxygen should occur after bond breakage via the intraatomic Auger decay of the O 2s hole while hydrogen can be ionized in the course of the O–H bond breakage at the surface via the interatomic Auger decay of the O 2s hole. On the basis of these findings, the interaction of oxygen and hydrogen with the reduced TiO2(110) surface is investigated using resonant ion stimulated desorption (RISD) by He+. At the reduced surface, the chemisorbed oxygen atom either fills a vacancy site of bridging oxygen atoms or chemisorbs at a fivefold-coordinated Ti4+ site as an adatom. In the RISD experiment, the oxygen adatom is detected much more efficiently than the bridging oxygen. Such oxygen adatoms are found to be unstable on the TiO2(110) surface; they diffuse into the bulk at the oxygenated surface or segregate from the bulk at the reduced surface. Hydrogen, bonded to Ti at the reduced surface, tends to be reorganized to form hydroxyl group upon oxygenation.