Nitrogen-induced reconstruction and faceting of Re($11\bar 21$112¯1)

Abstract

The surface morphology of Re(11̄21), tailored on the nanometer scale by kinetic control of nitrogen, has been investigated using low energy electron diffraction, scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory (DFT) in combination with the ab initio atomistic thermodynamics approach. Experiments show that when exposing to NH3 (>0.5 L) at 300 K followed by annealing in ultra-high vacuum at 700 K or 900 K, the initially planar Re(11̄21) surface becomes (2 × 1) reconstructed or partially faceted, respectively. Upon annealing in 100 L NH3 at 900 K, Re(11̄21) becomes fully faceted and covered by N. The fully faceted surface consists of two-sided ridges formed by (13̄42) and (31̄42) facets. The (2 × 1) reconstruction may serve as a precursor state for faceting of Re(11̄21). The DFT calculations provide an atomistic understanding of facet formation in terms of binding sites and energies of N on Re surfaces of the substrate and facets as well as the corresponding surface phase diagram. The N-covered faceted Re(11̄21) surfaces are promising nanoscale model catalysts and nanotemplates. Our findings should be of importance for the design and development of Re-based heterogeneous catalysts operating under nitrogen-rich conditions.