INTERFACE-REACTIONS OF PT/TIO2 - COMPARATIVE ELECTRICAL, XPS-DEPTH, AND AES-DEPTH PROFILE INVESTIGATIONS

Pt/TiO2-interfaces show Schottky-barrier or ohmic behavior depending on the diffusion of Pt into TiO2 subsurface layers. Diffusion effects are negligible for TiO2 rutile surface structures in the absence of oxygen at T less-than-or-equal-to 1070 K. This leads to Schottky-barrier electrical behavior. If bulk diffusion of Pt is pronounced we observe ohmic behavior. This is the case for all TiO2 samples with non-stoichiometric Magneli-subsurface phases and for TiO2 rutile surface phases at T greater-than-or-equal-to 1070 K in the presence of oxygen. The latter is explained by reduced activation barriers of surface platinum during the first step of in-diffusion if platinum is present as an ion (Pt4+ or Pt2+) at the surface. Ions are formed as intermediates during the formation and decomposition of platinum oxide at the three-phase-boundary O2/Pt/TiO2.