Structural stability and local electronic properties of some EC synthesized magnetite nanopowders

Structural and electronic properties, oxidation and aging effect of electrochemically (EC) synthesized magnetite nanopowders (NPs) are studied by means of X–ray diffraction (XRD), X–ray absorption fine structure (XAFS) and X–ray magnetic circular dichroism (XMCD). The obtained results enabled to get a direct insight into the structure and electronic properties of Fe immediate surrounding and to elucidate the influence of preparation conditions on stoichiometry of NPs and their stability in ambient conditions. All investigated NPs are produced as non–stoichiometric Fe3−δO4 oxide phases, with the lattice constant and the Fe2+/Fe3+ ratio both in–between the values for bulk maghemite and magnetite. NPs synthesized under smaller current density (J = 200 mA/dm2) are more magnetite–alike, whereas larger current density (J = 1000 mA/dm2) has led to formation of NPs closer to maghemite. Oxidation of magnetite–like NPs is slower, although in the course of time particles agglomerate and oxide penetrates into the core. Maghemite–like NPs oxidize much faster and the oxide layer which is confined close to the particles' surface protects the core from further oxidation. In all NPs the fist coordination around Fe is pretty stable against both temperature and oxidation process. The temperature change from 293 K to 20 K considerably affects the second coordination around Fe, which is most likely a consequence of the Verwey transition present in all investigated samples. [Display omitted] •EC synthesized magnetite nanopowders are composed of non–stoichiometric Fe3−δO4.•NPs synthesized under smaller current density are closer to bulk magnetite.•Larger current density has led to formation of NPs closer to bulk maghemite.•Oxidation of magnetite–like NPs is slow but in time oxide penetrates into the core.•Maghemite–like NPs oxidize faster and the oxide layer is confined to the surface.