Adsorbate-induced nanoscale faceting of rhenium surfaces.

摘要

In this dissertation, we report the first systematic study of adsorbate-induced faceting of hexagonal close-packed (hcp) metal surfaces. Focusing on two atomically rough rhenium surfaces: Re(1231) and Re(11 21), we reveal the dependence of their surface morphology on adsorbate coverage and species by means of low energy electron diffraction (LEED), scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), temperature programmed desorption (TPD) and high resolution soft X-ray photoemission spectroscopy (HRSXPS) based on synchrotron radiation.;Re(1231) becomes completely faceted when oxygen coverage is greater than 0.7 monolayer (ML) and the surface is annealed at T > 700K. As oxygen coverage further increases, the surface morphology evolves from long ridges formed by (01 1 0) and (11 21) facets, to truncated ridges due to sequential emergence of (10 1 0) and (01 1 1), and eventually to complex structures formed by (01 1 0) (0 1 0) (01 1 1) and (10 1 1) facets. All facets disappear when the surface is annealed at T > 1300K due to oxygen desorption and the surface reverts to planar.;Drastic differences have also been found between oxygen and nitrogen-induced faceting of Re(11 21). For O/Re(11 21), the morphology evolves as a function of oxygen coverage from a partially faceted surface with zigzag chains formed by (01 1 0) and (10 1 0) to a completely faceted surface with four-sided pyramids formed by (01 1 0) (10 1 0) (01 1 1) and (10 1 1). Two metastable facets, (3364) and (2x1) reconstructed (1122) are also observed in the evolution process. In contrast, for N/Re(11 21), a fully faceted surface shows ridges formed by (13 42) and (3142) facets upon exposure to ammonia at 800-900K; ammonia dissociates on Re and only nitrogen remains on the surface at T > 600K. A (2x1) reconstructed N/Re(11 21) surface is also observed in LEED when the surface is annealed at 600-700K. Temperature-pressure phase diagrams from first principles calculations are consistent with the experimental results.;Our work has implications for Re-based catalysts that operate under oxygen or nitrogen-rich conditions because the structure of the catalysts often affects their performance. The results show great promise of tailoring the surface morphology at the nanometer scale by choosing appropriate adsorbate-substrate combinations, adsorbate coverages and annealing conditions.