Nickel/Iron Oxide Nanocrystals with a Nonequilibrium Phase: Controlling Size, Shape, and Composition

Mixed nickel iron oxide nanomaterials have great potential for use in fields as wide-ranging as optics, water oxidation catalysis, and biomedical applications; large-scale deployment for any of these applications is possible due to the abundance of iron and nickel in the earth's crust. Here, we describe single-crystal mixed nickel iron oxide nanoparticles with a rocksalt crystal structure and an iron content greater than >10 at%. The nanoparticles were synthesized via thermal decomposition of nickel and iron oleates, yielding monodisperse nanoparticles <12 nm in size. A range of compositions were accessible, ranging from iron-poor to iron-rich. Despite the predicted poor solubility of iron in the NiO lattice, as based upon the equilibrium phase diagram, and the general tendency of pure FexO to oxidize, the single-crystal nanoparticles are nonetheless kinetically persistent even at elevated temperatures (200 degrees C). Furthermore, by controlling the drying conditions and decomposition time of the oleate precursors as well as the initial ratio of nickel to iron in the precursor oleate mixture a variety of nanoparticle shapes namely stars, cubes, and spheres were formed. This control of shape and composition affords tunability of physical properties, exemplified by magnetic properties herein.