Tuning Magnetic and Catalytic Properties in a Family of Compositionally Complex ACo₂O₄ Spinel Nanostructures

Nanostructured materials offer promising physicochemical properties across diverse applications in the fields of energy storage, sensors, catalysis, biomedicine, and more. Compositionally complex oxides (CCO) may offer increased structure-property tunability due to their unique structure diversity, tunable compositions, and possible property enhancements/modifications through increased entropic disorder. Here, an eco-friendly low temperature soft-templating reaction route is applied to form a large family of spinel nanostructures with formula <i>A</i>Co₂O₄, where <i>A</i> includes <i>n</i> different equiatomic combinations of one to seven transition metal cations. The phase selectivity and temperature stability windows for the series of nano-compositions are found to be dominated by the inclusion (or exclusion) of specific cations, with the temperature required for formation influenced by increasing <i>n</i> (increasing configurational entropy). We apply a wide array of characterization techniques, including high resolution transmission electron microscopy, neutron diffraction and pair distribution function analysis, and Special Quasi-Random Structures Density Functional Theory approaches, to probe the influence of cation selection and processing parameters on structural, catalytic, and magnetic properties. Specifically, we demonstrate water splitting catalyst activity can be tuned through <i>A</i> cation selection and post heat treatment. Certain metastable <i>A</i>Co₂O₄compositions are found to be novel precursor phases for synthesis of bulk hard-soft exchange magnets: we demonstrate that tunable magnetic performance can be achieved through control of the nanoscale heterogeneity of annealed products. Links between specific lattice defects, chemical short-range order, nanoscale heterogeneity and magnetic/catalytic performance in this ACo₂O₄ spinel family demonstrates a wider approach to promoting design principles and strategies for nanostructured CCOs and related materials.