Direct Observation of Site-Specific Molecular Chemisorption of O2 on TiO2(110)

Abstract

Molecularly chemisorbed O2 species were directly imaged on reduced TiO2(110) at 50 K with high-resolution scanning tunneling microscopy (STM). Two different O2 adsorption channels, one at bridging oxygen vacancies (VO) and another at 5-fold coordinated terminal titanium atoms (Ti5c), have been identified. While O2 species at the Ti5c site appears as a single protrusion centered on the Ti5c row, the O2 at VO manifests itself by a disappearance of the VO feature. It is found that the STM tip can easily dissociate O2 species, unless extremely low magnitude of the tunneling parameters are used. The O2 molecules chemisorbed at low temperatures at these two distinct sites are the most likely precursors for the two O2 dissociation channels, observed at temperatures above 150 and 230 K at the VO and Ti5c sites, respectively.