Composition and Oxidation State of Cobalt- and Nickel-Iron Oxide Colloidal Nanoparticles in Liquid Phase

摘要

Magnetic nanoparticles of cobalt- and nickel-iron oxide have been extensive interest due to their superparamagnetic properties and their potential applications in many fields. Iron, cobalt and nickel can stay in many oxidation states and are easily oxidized especially in ambient air therefore the composition and oxidation states of these oxides can be unintentionally modified. Usually, the composition and oxidation states in these magnetic nanoparticles are determined by various experimental techniques required a sample in solid phase. This may lead the nanoparticles to directly contact with air and change the state. In this study, the magnetic nanoparticles in colloidal phase with concentration of 24mg/ml, derived from co-precipitation process, were directly injected to a liquid cell for X-ray absorption near-edge structure (XANES) measurement. The iron-, cobalt- or nickel-iron oxide nanoparticles were prepared by dissolving CoCl_2/FeCl_3 or NiCl_2/FeCl_3, respectively, in deionized water with various atomic ratios. The average iron oxide nanoparticle size is about 4.2 nm with polydispersity of 0.987 which was obtained by dynamic light scattering. Spherical shape with some stabilizer layer was observed by transmission electron microscope. The iron content in various composition nanoparticles was estimated in liquid phase by the ratio between the Fe edge peak and Co or Ni edge peak. The oxidation states of metal ions were also derived from the linear fitting of standard compounds at particular oxidation states. The shifts of peak positions were examined to indicate the variation of oxidation state as well.