Thermal stability and the role of oxygen vacancy defects in strong metal support interaction - Pt on Nb-doped TiO2(100)

We have investigated the relative thermal stability of MBE-grown TiO2(100) and Nb-doped TiO2(100) surfaces grown on fully stoichiometric TiO2(100) substrates, along with the comparative interaction of Pt with these surfaces. Vacuum-annealed TiO2(100) films exhibit a well-ordered (1 x 3) surface reconstruction at temperatures above 500 degrees C, which is associated with partial reduction of the surface and the formation of [110]-oriented microfacets. However, the (1 x 3) reconstruction does not occur on the Nb0.1Ti0.9O2(100) surface after comparable vacuum annealing. Thermal stabilization of the TiO2(100) surface by Nb doping is due to strong and stable bonding between substitutional Nb and O in the mixed-metal rutile lattice. The interface between vacuum-deposited Pt and essentially defect-free TiO2(100) is much more stable than that formed between Pt and surfaces of vacuum-reduced TiO2(110) bulk specimens after comparable anneals in UHV. Moreover, interface chemistry with Pt is further suppressed when the TiO2 film is doped with Nb to form Nb0.3Ti0.7O2(100). These results suggest that diffusion of oxygen vacancies from the bulk to the surface plays a critical and, thus far, unappreciated role in promoting reduction to TiOx (x less than or similar to 2) at the Pt/TiO2 interface. Significantly, these results suggest that considerable suppression of the strong metal-support interaction for Pt on TiO2(100) can be achieved by reducing oxygen vacancy defect densities in the bulk and doping the near-surface region with Nb.