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 Fe x O to oxidize, the single-crystal nanoparticles are nonetheless kinetically persistent even at elevated temperatures (200 °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.