Formation of Ni_xFe_3-xO₄ on Fe₃O₄(001)

Magnetite (Fe3O4) doped with earth-abundant metals has become a promising catalyst material. In particular, Ni-doped magnetite (Ni/Fe3O4) has been demonstrated as a cheap, robust, and catalytically active material in photocatalytic and electrochemical water oxidation. Recently, the incorporation of Ni atoms in Fe3O4 single crystalline surfaces was studied extensively with scanning tunneling microscopy and density-functional theory calculations. However, because of its importance for catalytic activity, this incorporation process and the determination of the lattice sites occupied by the Ni atoms require further experimental study. In this work, we investigated the surface structure of as-grown and annealed Ni/Fe3O4(001) as a function of Ni coverage under ultra-high vacuum conditions with temperature-dependent x-ray and ultraviolet photoelectron spectroscopy, x-ray photoelectron diffraction, and low-energy electron diffraction. We observe that octahedrally coordinated subsurface cation vacancy sites are already occupied upon Ni deposition at room temperature and that Ni atoms start to diffuse further into the octahedral subsurface sites with increasing temperature.