Theoretical investigations of oxygen vacancy effects in nickel-doped zirconia from ab initio XANES spectroscopy at the oxygen K-edge

Abstract

In this study, we present theoretical X-ray absorption near-edge structure (XANES) spectra at the K-edge of oxygen in zirconia containing Ni dopant atoms and O vacancies at varying concentrations. Specifically, our model system consist of a supercell composed of a zirconia (ZrO 2 ) matrix containing two nickel dopants (2Ni), which substitute two Zr atoms at a finite separation. We found the 2Ni atoms to be most stable in a ferromagnetic configuration in the absence of oxygen vacancies. In this system, each Ni atom is surrounded by two shells of O with tetrahedral geometry, in a similar way as in bulk cubic zirconia. The oxygen K-edge XANES spectrum of this configuration shows a pre-edge peak, which is attributable to dipole transitions from O 1s to O 2p states that are hybridized with unoccupied Ni 3d states. The intensity of this pre-edge peak, however, reduces upon the introduction of a single vacancy in the 2Ni-doped zirconia matrix. The corresponding ground state remains ferromagnetic, while one of the nickel

Probing the Local Environment of Active Sites during 2-Propanol Oxidation to Acetone on the Co3O4 (001) Surface: Insights from First-Principles O K-Edge XANES Spectroscopy

“…The parameters usually invoked to explain the increase of pre-edge peak intensity of the ligand K-edge spectrum are the increase of metal oxidation state as well as the hybridization of ligand p and metal 3 d empty states. − Indeed, the stronger is the pre-edge, the emptier are the 3 d states, which may suggest a higher metal oxidation state. In order to verify this presumption, we examined more closely the electronic states of oxygen and cobalt atoms.…”

“…Another parameter influencing the intensity of the pre-edge peak is the extent of the overlapping with the 3 d orbitals. Indeed, the increase in the covalency of metal–oxygen bond implies a better overlap between the O 2 p orbitals and the Co 3 d orbitals, and hence a higher intensity of the 3 d contribution in the O K-edge spectrum. ,, It worth noticing that from the 2-propanol to 2-propoxide, there is the formation of a metal-alkoxide bond with a stronger covalent character than in the parent alcohol . Such a change could explain the increase of the intensity of the pre-edge peak while the formal oxidation state does not change.…”

“…Theoretically, as also successfully done on other relevant oxides, , we gained insight by tuning the incident photon energy to the K-edge energy of the O atom in the material and used the ligand K-edge to scan unoccupied 3 d Co states, instead of the L 23 -edge of Co atoms themselves as done in experiment. This stems from shortcomings of DFT-based single-electron methods that prohibit correct reproduction of experimental L 23 -edge spectra.…”

“…We recently used theoretical XANES spectroscopy to provide insight into the abovementioned information for bulk Co 3 O 4 combining experiment and theory. 10 Theoretically, as also successfully done on other relevant oxides, 11,12 we gained insight by tuning the incident photon energy to the K-edge energy of the O atom in the material and used the ligand K-edge to scan unoccupied 3d Co states, 13 instead of the L 23 single-electron methods that prohibit correct reproduction of experimental L 23 -edge spectra. Some of the limitations: the considerable overlap of the 2p-hole and 3d radial wave functions, the dependency of the spectral shape and the incident energy on the covalency of the metal−ligand bonds, and the change in the metal spin and oxidation state.…”