Site Specific Molecular Chemisorption of O$_{2}$ on TiO$_{2}$(110): A Scanning Tunneling Microscopy Study

The investigation of O$_{2}$ adsorption on TiO$_{2}$ is critical since it can help us to better understand the photooxidation mechanism of TiO$_{2}$. In our work, molecularly chemisorbed O$_{2}$ were directly imaged on reduced TiO$_{2}$(110) at 50 K with scanning tunneling microscopy (STM). Two O$_{2}$ adsorption channels, one at bridging oxygen vacancies (V$_{O})$ and another at five-fold coordinated Ti atoms (Ti$_{5c})$, have been identified. While O$_{2}$ at Ti$_{5c}$ appears as a single protrusion, the O$_{2}$ at V$_{O}$ manifests itself by a disappearance of the V$_{O}$ feature. It is found that STM tip can easily dissociate O$_{2}$ and the dissociation details strongly depend on the tunneling conditions and the type of the O$_{2}$ adsorption site. The chemisorbed O$_{2}$ at these two distinctive sites are the most likely precursors for the two previously established O$_{2}$ dissociation channels, observed at temperatures above 150 and 230 K at the V$_{O}$ and Ti$_{5c}$ sites, respectively