Structural, optical, and electronic structure properties of Cu substituted NiO solid solutions

A comprehensive investigation of the electronic structure, both occupied and unoccupied states, of phase-pure Ni1−xCuxO (x = 0, 0.06, 0.10, 0.15, 0.22, 0.25) solid solutions is presented through the analysis of x-ray absorption near edge structure (XANES) spectra along with valence band (VB) structure using off-resonant and resonant photoelectron spectroscopy (PES). Structural analysis using x-ray diffraction and extended x-ray absorption fine structure (EXAFS) confirm a linear variation in the lattice parameter and bond lengths with increasing Cu concentration. A minor change (∼0.2 eV) in the optical bandgap, even at 25% of Cu substitution, is attributed primarily to the localized Ni 3d states in the conduction band. Resonant PES identifies distinct Ni and Cu derived states in the VB, confirming the charge transfer insulating nature of the solid solution. XANES study confirms the presence of Ni and Cu ions in the 2+ oxidation state.