Experimental and simulated STM images of stoichiometric and partially reduced RuO2(1 1 0) surfaces including adsorbates

We present experimental and DFT-simulated STM images of ultrathin RuO2(1 1 0) films on Ru(0 0 0 1), including adsorbates such as oxygen and CO. We are able to identify the under-coordinated O atoms on the RuO2(1 1 0) surface with STM, i.e. the bridging O atoms and the on-top O atoms. The partial reduction of the RuO2(1 1 0) surface by CO exposure at room temperature leads to a surface where part of the bridging O atoms have been removed and some of the vacancies are occupied by bridging CO. When dosing 10 L of CO at room temperature, the RuO2(1 1 0) surface becomes fully mildly reduced in that all bridging O atoms are replaced by bridging CO molecules. Annealing the surface to 600 K produces holes on the terraces of such a mildly reduced RuO2(1 1 0) surface. These pits are not generated by the recombination of lattice O with CO, but rather these pits are assigned to a complex temperature-induced rearrangement of surface atoms in the topmost RuO2 double layer of RuO2(1 1 0). With this process the bridging O atoms are again populated and surplus Ru atoms agglomerate in small islands at the rims of the holes.