Most metallic surfaces reconstruct upon dissociative O2 adsorption. Such rearrangements of the surface, bearing an enormous importance in determining the catalytic activity in oxidative environments, consist most often in the formation of added rows of oxygen and metal atoms. The source of substrate atoms needed for the reconstruction has been heavily debated. The currently accepted mechanism is that they are released from preexisting step edges and diffuse eventually to the terraces, where they react with oxygen atoms generated by dissociation of oxygen molecules impinging from the gas phase. Evidence for this mechanism comes from the retraction of step edges of the substrate terraces upon oxidation, observed under controlled ultra high vacuum conditions, e.g., for Cu. At odds with this explanation, scanning tunneling microscopy (STM) investigations show also added rows located in the middle of terraces, far away from atomic steps. Oxidation induces, moreover, surface roughening with the formation of pits, as reported for both Agand Cu. For the latter case, an excavation of the substrate to produce the adatoms needed for the reconstruction was suggested to dominate when the supply from steps is inhibited, albeit without unraveling the mechanism. We show  by low temperature Scanning Tunneling Microscopy that Ag atom excavation occurs indeed on Ag(110) already at a sample temperature T < 200 K, leading to single vacancy formation or to more complex structures depending on atomic oxygen coverage. Density functional theory (DFT) based calculations allow us to describe the detailed mechanisms involved in the extraction process. The substrate roughening generates undercoordinated atoms and opens pathways to the Ag subsurface layer. The mechanism is expected to be very general depending on the delicate interplay of energy and entropy, so that it may be active also for other materials at different (higher) temperatures.
 Jagriti Pal,Takat B. Rawal,Marco Smerieri,Sampyo Hong,Matti Alatalo,Letizia Savio,Luca Vattuone,Talat S. Rahman,and Mario Rocca, Phys. Rev. Lett. XX, XX (2017)