The behavior of Cr–Ni nanofilms on NiO(100) upon annealing and substrate-controlled oxidation studied by quantitative X-ray Photoelectron Spectroscopy

Abstract

The interplay between metallic and oxidized phases of Cr, Ni nanodeposits has been investigated via a model system. The evolution upon annealing in ultra-high vacuum of metallic Cr ultra-thin or nanoparticulate films of less than 0.7 nm equivalent thickness on a NiO(100) single crystal substrate has been studied by means of X-ray Photoelectron Spectroscopy (XPS), angle resolved XPS and Low Energy Ion scattering. The NiO substrate provided oxygen in a highly controllable fashion, at first to the metallic Cr film, causing its fast oxidation at 550 K with parallel stoichiometric surface reduction of NiO to metallic Ni. The latter was subsequently subjected to slow re-oxidation by oxygen from the NiO bulk upon annealing up to 940 K, thus leading to complex interfaces that may include simultaneously metallic Cr, Ni and their respective oxides, all found together within a few atomic layers. The step-by-step monitoring of the evolution of the interfaces by XPS at various stages of annealing and the simulation of quantitative XPS data for alternative spatial distribution models of the co-existing phases yield structural information about the stacked layers produced by oxidation of Cr to Cr2O3 and metallic Ni, the re-oxidation of Ni0 via oxygen supplied by the NiO substrate and finally a mixing of the two oxides at 940 K with probable formation of a NiCr2O4 spinel.