Adsorption of sulfur onTiC(001): Photoemission and first-principles studies

Abstract

High-resolution photoemission, thermal desorption mass spectroscopy, and first-principles density-functional slab calculations were used to study the interaction of sulfur with a TiC(001) surface. At small coverages (0.3 ML), atomic sulfur is present on the TiC(001) substrate. A saturated overlayer at 300 K contains S and molecular ${\mathrm{S}}_{2}.$ For 100-K deposition, a multilayer of sulfur can be grown and it contains ${\mathrm{S}}_{8},$ ${\mathrm{S}}_{4},$ and ${\mathrm{S}}_{2}$ species. These species desorb at temperatures between 250 and 500 K, leaving only atomic sulfur on the carbide surface. The adsorption of sulfur (0.3--0.8 ML) induced small positive shifts (0.1--0.25 eV) in the Ti $2p$ core levels, while a big positive shift (1.0--1.3 eV) was detected for the C $1s$ core level. These photoemission studies indicate the existence of strong $\mathrm{S}\ensuremath{\leftrightarrow}\mathrm{C}$ interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CTiTi hollow as the most stable site for the adsorption of sulfur. The $\mathrm{S}\ensuremath{\leftrightarrow}\mathrm{C}$ interactions determine the type of sulfur species present on the surface. In general, the bond between S and the TiC(001) surface is best described as covalent, with a small degree of ionic character. The deposition of sulfur induces a significant reduction in the Ti density of states near the Fermi level.