INTERACTION OF WATER, OXYGEN, AND HYDROGEN WITH TIO2(110) SURFACES HAVING DIFFERENT DEFECT DENSITIES

We have studied the stoichiometric (annealed in oxygen), the slightly oxygen-deficient (annealed in vacuum), and the highly defective (sputtered with Ar+) TiO2 (110) surfaces and their reactivities to molecular oxygen, molecular water, and 50-eV hydrogen ions using x-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LEIS). The use of isotopically labeled O-18 enables us to distinguish adsorbed oxygen from lattice oxygen, and the concentration of surface oxygen vacancies is titrated by O-18(2) adsorption. LEIS (1-keV He+) is used to analyze the chemical composition of the outermost surface layer before and after O-18(2) and (H2O)-O-18 exposure. Water adsorbs on both stoichiometric and slightly O-deficient surfaces [with oxygen vacancies approximately 0 and 0.08 monolayer (ML), respectively] at room temperature. There is little or no dependence of saturation water coverage (lower limit of approximately 0.07 ML for both surfaces) on the concentration of surface oxygen vacancies. On the highly defective surfaces, the saturation water coverage increases to a lower limit of 0. 15 ML and the saturation O-18 coverage increases to 0.4 ML. The interaction of hydrogen with the stoichiometric surface creates defect states that can be observed by XPS and by subsequent adsorption of O-18.