Surface nitrogen induced evolution of the electronic structure of gadolinium

Lanthanide nitride materials have recently been shown to act as catalysts for mild condition ammonia synthesis through reaction mechanisms differing from existing industrial catalysts. However, an in-depth study of the electronic structure of GdN, the prototypical lanthanide nitride, has not yet been completed. We report on an extensive in situ x-ray photoelectron spectroscopy and near-edge x-ray absorption study of the surface of epitaxial gadolinium (Gd) pre and post exposure to molecular nitrogen (${\mathrm{N}}_{2}$), compared with a near-stoichiometric gadolinium nitride (GdN) thin film. In this study, the nitrogen $1s$ core level is used as a proxy to probe the facile breaking of molecular nitrogen (${\mathrm{N}}_{2}$) by the pure gadolinium (Gd) surface at ambient temperature. We also investigate the evolution of Gd $3d, 4d$, and $4p$ core levels with nitrogen exposure compared with Gd ${3}^{+}$ atomic multiplet calculations. We find that the exchange splitting of the Gd $4s$ and $5s$ levels suggests that $4f$ electrons are not significantly involved in bonding with nitrogen, which is well aligned with ongoing computational assumptions in the literature. This confirms that Gd-N bonding is predominately ionic, with the valence and conduction bands being of N $2p$ and $\mathrm{Ln} 5d$ and $6s$ character, respectively. The valence band suggests that the formation of GdN by exposure of a Gd surface to ${\mathrm{N}}_{2}$ does not result in complete nitridation, at least for the dose used and depth studied. Indeed, the surface of epitaxial Gd, post exposure to ${\mathrm{N}}_{2}$, suggests the coexistence of both pure Gd and GdN, as confirmed by density of states calculations.