Electron- and Hole-Mediated Reactions in UV-Irradiated O2 Adsorbed on Reduced Rutile TiO2(110)

The ultraviolet (UV) photon-stimulated reactions in oxygen adsorbed on reduced TiO2(110) at low temperatures (<100 K) are studied. When a single O-2 is chemisorbed in each bridging oxygen vacancy, only similar to 14% of the O-2 desorbs after prolonged UV irradiation. For the remaining O-2 on the surface after irradiation, about one-half dissociates, and the other one-half is left in a nondissociated state that is inactive for hole-mediated photodesorption. For the maximum coverage of chemisorbed oxygen, the fraction of O-2 that photodesorbs increases substantially, but is still only similar to 40%. However, when physisorbed oxygen is also present, similar to 70% of the initially chemisorbed O-2 photodesorbs. On the basis of the experimental results, we propose that both hole- and electron-mediated reactions with O-2 chemisorbed on TiO2(110) are important. Hole-mediated reactions lead to O-2 photodesorption, while electron-mediated reactions lead to O-2 dissociation. The electron-mediated reactions explain the low total photodesorption yield when no physisorbed O-2 is present. For a fixed amount of chemisorbed O-18(2), its PSD yield increases substantially if O-16(2) is subsequently chemisorbed, indicating that the hole-mediated O-2 photodesorption probability depends on the charge state of the chemisorbed O-2; it decreases for more negatively charged O-2. Because the charge state of the chemisorbed O-2 depends on the total oxygen coverage, the coverage influences the photodesorption process. A simple model based on the oxygen coverage and the charge of the chemisorbed oxygen, which accounts for the observations, is presented. In the model, O-2 chemisorbs as either O-2(-) or O-2(2-) depending on the oxygen coverage. O-2(-) (O-2(2-)) reacting with a hole leads to O-2(0) desorption with a high (low) probability. O-2(2-) plus an electron typically leads to O-2 dissociation, while O-2(-) + e(-) does not lead to dissociation.