Subsurface oxygen at transition metal surfaces: Its direct atom-resolved imaging and role in metal oxidation

Clarifying how oxygen is incorporated into the subsurface region of metal surfaces is a crucial step towards a robust understanding of the initial oxidation of metals. Due to lack of direct atomic-resolution imaging of subsurface oxygen, the atomistic mechanisms of oxygen-metal interactions and the role of subsurface oxygen remain elusive. Here, by state-of-the-art transmission electron microscopy, we demonstrate a direct atom-resolved imaging of subsurface oxygen that is dissolved in the subsurface region of the Fe (001) and Ti (0001) surfaces. It is found that subsurface oxygen occupies the octahedral sites of the Fe and Ti lattices, rather than the tetrahedral sites. First-principles calculations reveal that the octahedral-site occupancy of subsurface oxygen is energetically more favorable compared to the tetrahedral-site occupancy and will facilitate the process of metal oxidation by reducing the formation energy of metal oxides. Our findings clarify the atomistic mechanism of how oxygen attacks metal surfaces for the initiation of oxidation that occurs everywhere in nature.