Intercalation of Hexacyanoferrate(III) Ions in Layered Double Hydroxides: A Novel Precursor To Form Ferri-/Antiferromagnetic Exchange Coupled Oxides and Monodisperse Nanogram Spinel Ferrites

摘要

We report the formation and characterization of [Fe(CN)_6]~(3-) intercalated Ni~(II)/Fe~(III) layered double hydroxides (LDHs) to form exchange coupled oxides and single crystalline monodisperse spinel ferrite in the absence of organic surfactant. The presence of excess divalent Ni ions can act as an inorganic surfactant by forming core-shell type structure during thermal decomposition of layered double hydroxide. Monodisperse spinel oxides having an average size of 76 nm were successfully separated by leaching excess nickel oxide. Transmission electron microscopy and magnetization measurements indicate the dispersion of spinel oxide having ionic interaction with NiO surrounding the spinel phase, which is further supported through electron spin resonance spectroscopy. The average crystallite/particle size estimated from X-ray line broadening and microscopic techniques along with the effective g value (2.29) obtained from electron spin resonance (ESR) spectroscopy strongly suggested the formation of single grain NiFe2O4. The advantage of this method is that the excess divalent metal ions in LDHs can be used as high-temperature inorganic surfactant to restrict the growth and agglomeration of the magnetic nanoparticles by forming a divalent oxide protective layer on the surface during heat treatment. These surface modified exchange coupled magnetic oxides with antiferromagnetic NiO phase exhibit some interesting structural and magnetic properties and have some useful application.