Interaction of CO2 with oxygen adatoms on rutile TiO2(110)

Abstract

The interactions of CO₂ with oxygen adatoms (O_a's) on rutile TiO₂(110) surfaces have been studied using scanning tunneling microscopy. At 50 K CO₂ is found to adsorb preferentially on five-coordinated Ti sites (Ti_5c's) next to O_a's rather than on oxygen vacancies (V_O's) (the most stable adsorption sites on reduced TiO₂(110)). Temperature dependent studies show that after annealing to 100–160 K, V_O's become preferentially populated indicating the presence of a kinetic barrier for CO₂ adsorption onto the V_O's. The difference between the CO₂ binding energy on V_O's and Ti_5c sites next to the O_a's is found to be only 0.009–0.025 eV. The barrier for CO₂ diffusion away from O_a's is estimated to be ∼0.17 eV. Crescent-like features of the images of CO₂ adsorbed on Ti_5c's next to O_a's are interpreted as a time average of terminally bound CO₂ molecules switching between the configurations that are tilted towards O_a and/or towards one of the two neighbouring bridging oxygen (O_b) rows. In the presence of V_O defects, the Ti_5c bound CO₂ is found to tilt preferentially away from the V_O containing O_b row. If another CO₂ is present on the neighbouring Ti_5c row, both CO₂ molecules tilt towards the common O_b row that separates them.