Paper SS1-TuM10
Morphology and Electronic Structure of Ultrathin Oxide Films on Pt₃Ti(111)

Tuesday, November 10, 2009, 11:00 am, Room M

TiO₂ supported metal catalysts are known to have interesting catalytic properties, e.g. for oxygen reduction of water in fuel cells and for CO hydrogenation. The formation of titanium oxides on Pt-Ti alloy surfaces is one promising approach to develop new TiO₂ based model catalysts. In this work the crystallographic and electronic structure as well as the surface composition of ultrathin titanium oxide films grown on a Pt₃Ti(111) single crystal surface have been investigated as a function of oxidation temperature (700 K to 1000 K) and oxygen exposure (up to 9000 L) using Low Energy Electron Diffraction (LEED), Ultraviolet Photoelectron Spectroscopy (UPS) and Auger Electron Spectroscopy (AES). The composition of the titanium oxide films formed on this alloy surface strongly depends on the preparation conditions. From AES and LEED measurements two different stable oxide phases have been identified at high temperatures (above 900 K), depending on the oxygen exposure. At lower oxygen exposures (below 135 L) the most stable structure is a quasi-hexagonal one (a = 3.35 Å, b = 3.25 Å, α = 124°) rotated by 4° with respect to one of the substrate symmetry axes. The p(2×2) pattern, which is attributed to the Pt₃Ti(111) substrate, is still visible after this mild oxidation; suggesting that some areas of the substrate remain unoxidized. At higher oxygen exposures (between 220 L and 9000 L) the predominant structure is a commensurate hexagonal structure (a = 3.20 Å) rotated by 4° with respect to one of the substrate symmetry axes. In this case the p(2×2) substrate structure is no longer visible. Apart from these two stable structures, several additional LEED patterns have been observed, which are related to metastable oxide structures. However, this great diversity of LEED patterns is not reflected in the UPS measurements. Comparing the clean Pt₃Ti(111) surface with Pt(111) shows a significant broadening of the Pt related intensity between 1 and 4 eV as well as the appearance of a strong Ti related signal around 6 eV. For the clean Pt₃Ti(111) surface a work function of 5.02 eV has been determined. Oxidizing the alloy leads to additional peaks at a binding energy of 5.5 and 7.5 eV. The work function of the oxidized surface is increased by 0.3 eV related to clean Pt₃Ti(111). Remarkably, neither the spectral shape nor the work function significantly depends on the temperature and oxygen pressure used during oxidation.