Soft Chemical Conversion of Layered Double Hydroxides to Superparamagnetic Spinel Platelets

We report the synthesis of a wide range of single-crystal spinel platelets with exposed (111) faces, lateral dimensions in the micrometer range, and thicknesses of 20–50 nm, prepared by soft chemical dehydration of well crystallized layered precursors. This method enables the synthesis of the metastable composition NiCoAlO4, which cannot be prepared by conventional solid state synthesis. Because of their nanoscale thickness, mosaic structure, shape anisotropy, and surface porosity, NiCoAlO4 and NiCo2O4 platelets exhibit room-temperature superparamagnetism (T B = 40 and 250 K, respectively) despite the fact that they have micrometer-size lateral dimensions. The accessibility of a wide range of superparamagnetic spinel platelets (as opposed to cubes or spheres) should be useful for studies in ferrofluids and related composite magnetic materials.