Chemisorption on Single-Crystal Planes: Nitrogen on Tungsten

Abstract

The effects of surface structure on the adsorption of nitrogen have been examined by contact-potential measurements on macroscopic planes cut from single-crystal tungsten. On the (100), room-temperature chemisorption lowers the work function by 0.4 eV, confirming previous field-emission results. Flash desorption reveals only one state, β, in which nitrogen is bound as adatoms. On the considerably less reactive (111), the work function φ rises by 0.15 eV. Two states contribute: α, which is probably molecular, and β; both raise φ. At higher concentrations, a second β state is isolated by flash desorption. Nitrogen does not chemisorb at all on the (110) at 300°K and p∼10−7 mm. Adsorption of γ nitrogen can, however, be observed by lowering the temperature to T∼130°K or raising the pressure above 10−3 mm. On the (110), this state is atomic and lowers φ. On other planes, γ nitrogen has different properties and forms because of the heterogeneity induced in the surface layer by the presence of adatoms. Macroscopic surfaces can therefore be prepared showing extreme structural specificity. Previous correlations between the work-function changes and electronegativities are examined and are found insufficient. The effects of structure on the surface dipole are, however, correlated by the empirical rule that nitrogen atoms on flat planes lower the work function, whereas on rough surfaces the work function is increased.