Electronic Structure of Nanostructured Nickel Oxide Using Ni 2p XPS Analysis

We examine the electronic structure of nanostructured nickel oxide with an average particle size of 4-5 nm using Ni 2p X-ray photoelectron spectrum. The most striking features of the spectrum are the Ni 2p main line broadening and an increase in the relative intensity of the ∼ 1.5 eV satellite. We explain the observations as due to an enhancement in the non-local screening process on the basis of Ni7O36 cluster model by taking into account the large surface area to volume ratio and high Ni2+ vacancy concentration in nanostructured nickel oxide. The important contribution of an enhanced Ni 3d-O 2p hybridization and the origin of shake-up peaks above the ∼ 7 eV satellite is also discussed. The study underlines the importance of factors such as the actual local environment of the core hole site, defect density and distribution, possible structural transitions, etc. in determining the Ni 2p core level X-ray photoelectron spectrum of nanostructured nickel oxide and shows that the actual synthesis routes and the thermal history greatly influence the electronic structure of nanostructured nickel oxide