Effects of the crystal reduction state on the interaction of oxygen with rutile TiO2(1 1 0)

The interaction of O2 with reduced rutile TiO2(1 1 0)–(1 × 1) has been studied by means of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD) and photoelectron spectroscopy (PES). It is found that the interaction of O2 with TiO2(1 1 0) depends strongly on the reduction state of the TiO2(1 1 0) crystal. High-resolution STM studies revealed that the energy barrier for the non-vacancy-assisted, 2nd O2 dissociation channel decreases with increasing crystal reduction. Additionally, it is found in the STM studies that the Ti interstitial diffusion is slightly more facile in high-reduced TiO2(1 1 0) crystals compared to low-reduced ones. Accompanying TPD studies revealed that the line shape of the O2-TPD peak occurring between ∼360 K and ∼450 K depends on the crystal reduction state. For high-reduced TiO2(1 1 0) crystals characterized by large terraces most O2 molecules desorb at ∼386 K, whereas O2 desorption is peaking at ∼410 K for low- and medium-reduced crystals. Furthermore, the O2-TPD experiments revealed a highly non-linear behavior of the O2 desorption peak integrals as function of the crystal reduction state. The presented results point to an ionosorption model where the adsorbates withdraw the excess charge (Ti3+) from the near-surface region at temperatures < ∼360 K and where Ti interstitials react with oxygen species on the surface at temperatures ≥ ∼360 K.