Surface Structures in the SMSI State; Pd on (1 × 2) Reconstructed TiO2(110)

Abstract

We have used scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) to investigate the thermal stability of Pd(111) islands and thin films on TiO2(110)-(1 x 2). Two new nano-structures were observed to form on the surface of the Pd by annealing to 973 K. Atomic resolution STM images show a super-structure. One domain has a ((72)(57)) unit cell with respect to the Pd(111) whereas the other domain has a ((72)(57)) unit cell. Coexisting with this phase is a structure consisting of zigzag rows that run along the close-packed directions of the Pd(111) islands. This has a rectangular unit cell incommensurate with both the substrate TiO2(110) and the Pd(111) islands. STM shows these two structures merge with no noticeable domain barriers or steps, suggesting a close relationship between the two. LEED shows several distinct, overlapping patterns that can be identified with the surface structures observed, TiO2( 110)-( 1 x 2), Pd(111)-(1 x 1), the hexagonal pinwheel structure, and the rectangular zigzag unit cell. XPS at normal and grazing emission show the encapsulating layer to be composed of TiO2 with Ti predominantly in similar to2(+) or similar to3(+) oxidation states. The proposed models are structurally consistent with the LEED and STM data and have stoichiometries of TiO and TiO1.4, chemically consistent with the XPS spectra. The STM images of the zigzags bear a strong similarity to structures seen for annealed Pt islands on TiO2(110)-(1 x 1) and TiOx supported on Pt(111), while the pinwheel structure is similar to annealed Cr on Pt(111). We discuss the similarities of our structures to those seen before for these related systems.