Nanoscopic nickel aluminate spinel (NiAl2O4) formation during NiAl(1 1 1) oxidation

Oxidation of a NiAl(1 1 1) single crystal surface was investigated using high resolution soft X-ray photoelectron spectroscopy (HRSXPS), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectroscopy (EDS), X-ray mapping, and atomic force microscopy (AFM). After repeated oxygen exposure, annealing, and cleaning cycles under ultrahigh vacuum conditions, a new oxide phase in the form of tiny 3-dimensional surface structures was detected. These features are several micrometers long and ∼300 nm high and oriented along low index 〈1¯01〉 directions in the plane of the substrate; they have nickel aluminate spinel (NiAl2O4) stoichiometry. We propose that repeated cycles of oxygen dosing and annealing of the NiAl(1 1 1) surface leads to oxygen diffusion into the bulk and nucleation of spinel below the surface.