Interaction of molecular oxygen with the vacuum-annealed TiO2(110) surface: Molecular and dissociative channels

We have examined the interaction of molecular oxygen with the TiO2(110) sur face using temperature-programmed desorption (TPD), isotopic labeling studi es, sticking probability measurements, and electron energy loss spectroscop y (ELS). Molecular oxygen does not adsorb on the TiO:(110) surface in the t emperature range between 100 and j00 K unless surface oxygen vacancy sites are present. These vacancy defects are generated by annealing the crystal a t 850 K, and can be quantified reliably using water TPD. Adsorption of O-2 at 120 K on a TiO2(110) surface with 8% oxygen vacancies (about 4 x 10(13) sites/cm(2)) occurs with an initial sticking probability of 0.5-0.6 that di minishes as the surface is saturated. The saturation coverage at 120 K, as estimated by TPD uptake measurements, is approximately three times the surf ace vacancy population. Coverage-dependent TPD shows little or no Oz desorp tion below a coverage of 4 x 10(13) molecules/cm(2) (the vacancy population ), presumably due to dissociative filling of the vacancy sites in a I:1 rat io. Above a coverage of 4 x 10(13) molecules/cm(2), a first-order O-2 TPD p eak appears at 410 K. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with ether coadsorbed oxygen molecules. Se quential exposures of O-16(2) and O-18(2) at 120 K indicate that each adsor bed O-2 molecule, irrespective of its adsorption sequence, has equivalent p robabilities with respect to its neighbors to follow the two channels (mole cular and dissociative), suggesting that Oz adsorption is not only precurso r-mediated, as the sticking probability measurements indicate, but that all O-2 molecules reside in this precursor state at 120 K;. This precursor sta te may be associated with a weak 145 K O-2 TPD state observed at high Oz ex posures. ELS measurements suggest charge transfer from the surface to the O z molecule based on disappearance of the vacancy loss feature at 0.8 eV, an d the appearance of a 3.8 eV loss that can be assigned to an adsorbed O-2(- ) species based on comparisons with Ti-O-2: inorganic complexes in the lite rature. Utilizing results from recant spin-polarized DFT calculations in th e literature, we propose a model when three O-2 molecules are bound in the vicinity of each vacancy site at 120 K. For adsorption temperatures above 1 50 E;, the dissociation channel completely dominates and the surface adsorb s oxygen in a 1:1 ratio with each vacancy site. ELS measurements indicate t hat the vacancies are filled, and the remaining oxygen adatom which is appa rent in TPD, is transparent in ELS. On the basis of the variety of oxygen a dsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical an d photochemical oxidation processes on TiO2 surfaces.