Structure and thermal stability of fully oxidized TiO2/Pt(111) polymorphs

We present a thorough investigation of TiO2 films of different thicknesses grown on Pt(111) surface by scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). When the thickness is below 10 monolayers (ML) the film, growing on top of a wetting monolayer, has the structure of a TiO2(B) polymorph, related to the rect′-TiO2 reported in the ultrathin regime. The film is characterized by islands with a high defect density and with grain boundaries where the lattice is shifted between two adjacent islands. Thermal treatment in oxidizing conditions reduces the number of surface defects without modifying the average island size. Above 10 ML, a different rutile-like oxide structure appears, labelled as quasi-(1 × 2) according to the LEED pattern. STM shows a quite rough and granular morphology, which is largely improved by thermal treatment, resulting in ordered islands covering more than 75% of the film. STM reveals, on the ordered islands, the incommensurate (1 × 2) surface unit cell, as well as a new (7 × 1) periodicity that does not find a counterpart in the LEED pattern. Beyond 20 ML coverage a higher grade of surface roughness and defectivity is observed, which can be improved only after very long thermal treatment. This gives rise to ordered patches covering an area of about 45%, and with the same surface structure observed at lower coverage. The (7 × 1) periodicity is not compatible considering previous models of bulk oxide surfaces, suggesting that a stress-mediated mechanism could play a role, during the growth of each island, in defining the observed periodical corrugation.