Surface defect-promoted interaction of TiO$_2$(110) with molecular oxygen

We present recent results on the local interactions between surface defects on a TiO$_{2}$(110) and their reactivity with oxygen studied by a combination of Scanning Tunneling Microscopy and Spectroscopy. High-resolution STM images, interpreted with first-principles theory, show that the observed one-dimensional strands have partially reduced Ti atoms coordinated at oxygen octahedral sites forming edge-sharing octahedra. When strands are exposed to 5 x 10$^{-7}$ Torr O$_{2}$ at 300 K, oxygen is adsorbed and preferentially nucleated on and along the strands, indicating the presence of exposed Ti ions. The exposure to molecular oxygen also results in oxygen adsorption at Ti$^{4+}$ sites on (1x1)-terminated terraces. The adsorbed oxygen species result in dark ``defective''features in STM, identical to those on the pristine TiO$_{2}$(110), and further diffuse on the surface. The implications of this behavior and specific interaction between defects and gas molecules for catalytic activity are discussed.