Role of oxygen defects on the magnetic properties of ultra-small Sn FeO nanoparticles

Although the role of oxygen defects in the magnetism of metal oxide semiconductors has been widely discussed, it is been difficult to directly measure the oxygen defect concentration of samples to verify this. This work demonstrates a direct correlation between the photocatalytic activity of Sn FeO nanoparticles and their magnetic properties. For this, a series of ∼2.6 nm sized, well characterized, single-phase SnFeO crystallites with x 0-0.20 were synthesized using tin acetate, urea, and appropriate amounts of iron acetate. X-ray photoelectron spectroscopy confirmed the concentration and 3 oxidation state of the doped Fe ions. The maximum magnetic moment/Fe ion, μ, of 1.6 × 10 μ observed for the 0.1 Fe doped sample is smaller than the expected spin-only contribution from either high or low spin Fe ions, and μ decreases with increasing Fe concentration. This behavior cannot be explained by the existing models of magnetic exchange. Photocatalytic studies of pure and Fe-doped SnO were used to understand the roles of doped Fe ions and of the oxygen vacancies and defects. The photocatalytic rate constant k also showed an increase when SnO nanoparticles were doped with low concentrations of Fe, reaching a maximum at 0.1 Fe, followed by a rapid decrease of k for further increase in Fe. Fe doping presumably increases the concentration of oxygen vacancies, and both Fe ions and oxygen vacancies act as electron acceptors to reduce e-h recombination and promote transfer of electrons (and/or holes) to the nanoparticle surface, where they participate in redox reactions. This electron transfer from the Fe ions to local defect density of states at the nanoparticle surface could develop a magnetic moment at the surface states and leads to spontaneous ferromagnetic ordering of the surface shell under favorable conditions. However, at higher doping levels, the same Fe ions might act as recombination centers causing a decrease of both k and magnetic moment μ. © 2013 American Institute of Physics.