Structure of Reduced Ultrathin TiO_x Polar Films on Pt(111)

Abstract

The structure of well-ordered reduced TiOx nanolayers (NLs) on Pt(111), prepared by reactive evaporation of Ti in an oxygen background and postannealed in different conditions, is discussed on the basis of experimental and theoretical data. All of the observed NLs are formed by a wetting 2D Ti−O bilayer where the Ti atoms are at the interface and the O atoms form the topmost layer. Different structures and stoichiometries depend on the actual Ti coverage and on the conditions of the postannealing. They represent the final products of a self-assembling process where the Ti atoms tend to organize in pseudoepitaxial regions, whereas the O atoms in the topmost layer (more abundant due to stoichiometry constraints) solve the crowding problem by creating pseudoepitaxial regions that exhibit dislocation lines and alternate with regions where Ti vacancies (defects or holes) occur. Given the inherent similarity among such reduced phases, as confirmed by an analysis of their electronic structure, it turns out that subtle effects are at the origin of the large variety of possible structures and that kinetic effects during their synthesis can play a role in driving toward a specific phase

Cation site environment in ultrathin TiOx films grown on Pt(111) probed by X-ray absorption spectroscopy at the Ti 2p edge

“…The BE parameter allows a straightforward classification of the TiO x structures into reduced and oxidised, respectively. Density functional (DF) calculations, that provided an accurate estimate of the TiO x stoichiometry, confirm the XPS results [13][14][15][16]. Recently we also investigated the structure of the occupied electronic levels of the TiO x ultrathin films by means of valence band (VB) photoemission spectroscopy (VB-PES) [11].…”

“…The notation used to label the phases is determined by to the shape of the surface unit cell or to their appearance in STM. This notation is consistent with the one used in previous experimental and theoretical studies [10][11][12][13][14][15][16]20]. The Ti coverage is gauged from Ti 2p XPS intensity (peak area) normalised to the average XPS intensity of the wagon-wheel w structure, which serves as a reference (see Fig.…”

“…[13][14][15][16]). Structural characterisation is available for all the phases reported except k 0 [10, [13][14][15][16], for which XPS data is the only tool for chemical characterisation yet available. The notation used to label the phases is determined by to the shape of the surface unit cell or to their appearance in STM.…”

Stability of TiO2 Polymorphs: Exploring the Extreme Frontier of the Nanoscale

“…[9,10] On the other hand, when a higher oxygen background is used, fully stoichiometric NSs of different thickness can be prepared, where oxygen atoms are located at the interface with the Pt substrate. [11][12][13] In particular, we have already shown that a lepidocrocitelike [14] fully stoichiometric NS (named rect-TiO 2 according to the previously reported nomenclature) can be formed under strongly oxidative conditions (P O2 = 1 10 À4 Pa) on both Pt (111) and (1 2)-Pt(110) surfaces: [11][12][13] On the former it has been found that the NS is characterized by a rectangular unit cell and that the film is incommensurate with respect to the substrate.…”

Atomistic and Electronic Structure Methods for Nanostructured Oxide Interfaces

“…These have been extensively studied by the authors and reviewed previously [11,12,15,55,134], so that we do not repeat their discussion here and we refer the reader to these previous reviews.…”