Intrinsic defects on a TiO2(110)(1×1) surface and their reaction with oxygen: a scanning tunneling microscopy study

Abstract

We report a scanning tunneling microscopy (STM) study of the rutile TiO2(110) surface. The surface was prepared by sputtering and annealing in an ultrahigh vacuum (UHV). After annealing to 1100K in UHV, a (1×1) surface with a terrace width of ∼100Å is obtained. The terraces are separated by monoatomic step edges running predominantly parallel to 〈001〉 and 〈11̄1〉 type directions. Approximately half of the 〈001〉-type steps have a kinked appearance that is attributed to a (4×1)-reconstructed step edge. Atomic models for autocompensated step edges are presented. Oxygen vacancies (point defects) in the bridging oxygen rows are created by the high-temperature anneal in UHV. In STM images, these oxygen vacancies appear as bright features centered on dark rows. Their density is 7±3% per surface unit cell and is reduced upon exposure to molecular oxygen at room temperature. Dark features on bright rows are also seen; these are not affected by molecular oxygen and are tentatively assigned to subsurface defects. Hydroxyl groups from spurious water in the oxygen gas stream are observed to adsorb dissociatively at step edges and on the in-plane Ti rows on the terraces. The appearance of the surface oxygen vacancies depends on the state of the STM tip; asymmetric tips skew the appearance of the point defects and may even render images where they are invisible. Tip changes occur frequently, especially when the surface has been exposed to oxygen, and may lead to images that are hard to interpret. The “normal” tip state where the vacancies appear as bright spots connecting bright rows can be regained reproducibly by scanning with a high (up to +10V) tip voltage; the tip is then possibly covered with substrate material. The oxygen vacancies show strong interactions with the STM tip, i.e. tip-induced oxidation and mobility. These interactions depend strongly on the state of the tip, and are enhanced by the presence of oxygen in the ambient. A model for the tip-induced oxidation is presented where oxygen atoms hop between the tip and sample to explain these effects.