Fundamental studies of titanium oxide–Pt(1 0 0) interfaces: I. Stable high temperature structures formed by annealing TiOx films on Pt(1 0 0)

Surface structures formed by titanium oxide thin films at a Pt(1 0 0) surface have been studied by STM (scanning tunneling microscopy), LEED (low energy electron diffraction), AES (Auger electron spectroscopy), XPS (X-ray photoelectron spectroscopy), XPD (X-ray photoelectron diffraction) and He+-ISS (He+ ion scattering spectrometry) in order to explore and elucidate stable high-temperature structures that may be formed at titania–Pt interfaces and provide a basis for characterizing the chemistry of titania thin films on Pt(1 0 0). Titanium oxide films were produced by two different methods. First, titanium oxide films on Pt(1 0 0) were produced by oxidation using ozone (O3) of a Pt3Ti surface alloy at 300 K and annealing at 1000 K. Smooth thin films with a (3 × 5) structure were observed at 1 ML (monolayer) using this procedure, and we propose that the (3 × 5) structure is due to one layer of a Ti2O3 film that is similar to the (1 × 2) strands formed on reduced TiO2(1 1 0) surfaces. Structures prepared by this method showed particularly “flat” terraces without islands. Second, TiOx films were formed by Ti evaporation and deposition on Pt(1 0 0) in 6.7 × 10−5 Pa O2 and annealing the substrate above 750 K in vacuum. A (35) structure was produced also for these films below ∼1 ML. A (4 × 3√5R60°) structure, referred to as the “24-structure”, was observed after deposition of ∼2 ML and after annealing at 850–1000 K. We propose a model for this structure composed of TiO2 tetragonal nets with some O atoms in the second layer. The (4 × 3√5R60°) film changed to one with a (3 × 5) structure after annealing above 960 K in vacuum. TiO2 clusters were observed in our investigations, but mainly after Ti deposition sufficient to form several monolayers and after annealing at 1000 K. Under those conditions, a small number of (3 × 5) domains were observed also, but it was surprising that clean (1 × 1)- and (5 × 20)-Pt(1 0 0) domains still existed over ∼2% of the surface. TiO2 clusters decomposed to form a (2√2 × 2√2)R45° structure, which we propose to be Ti5O8, and (3 × 5) domains after annealing at 1300 K. Within our models, the composition of all titanium oxide ultrathin films on a Pt(1 0 0) surface are TiO2−1.5 after annealing at 850–1300 K. Chemical changes occurring during annealing of titanium oxide films include Ti dissolution (alloying) into the bulk of the Pt(1 0 0) crystal. We suggest that these results will have important consequences for the understanding and modeling of catalysis of related systems.