Thermal stability and the role of oxygen vacancy defects in strong metal support interaction — Pt on Nb-doped TiO 2(100)

Abstract

We have investigated the relative thermal stability of MBE-grown TiO 2(100) and Nb-doped TiO 2(100) surfaces grown on fully stoichiometric TiO 2(100) substrates, along with the comparative interaction of Pt with these surfaces. Vacuum-annealed TiO 2(100) films exhibit a well-ordered (1 × 3) surface reconstruction at temperatures above 500°C, which is associated with partial reduction of the surface and the formation of [110]-oriented microfacets. However, the (1 × 3) reconstruction does not occur on the Nb 0.1Ti 0.9O 2(100) surface after comparable vacuum annealing. Thermal stabilization of the TiO 2(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 TiO 2(100) is much more stable than that formed between Pt and surfaces of vacuum-reduced TiO 2(110) bulk specimens after comparable anneals in UHV. Moreover, interface chemistry with Pt is further suppressed when the TiO 2 film is doped with Nb to form Nb 0.3Ti 0.7O 2(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 TiO x ( x ≲ 2) at the Pt TiO 2 interface. Significantly, these results suggest that considerable suppression of the strong metal-support interaction for Pt on TiO 2(100) can be achieved by reducing oxygen vacancy defect densities in the bulk and doping the near-surface region with Nb.