Cobalt Iron Oxides Prepared by Acidic Redox-Assisted Precipitation: Characterization, Applications, and New Opportunities.

The microscopic homogeneity of mixed metals in a single-phase oxide is a critical issue in improving material performance. Aqueous alkaline precipitation is the most common approach but it has the limits of microscopic inhomogeneity because of intrinsically different precipitation rates between metal cations. Herein, we demonstrate a new preparation of uniformly structural substituted cobalt iron oxides via acidic redox-assisted precipitation (ARP) upon the interaction of CoII and K2FeO4. This low-pH synthesis features the redox process between Co and Fe, presumably through the formation of inner-sphere complexes such as [(H2O)5CoII-O-FeVIO3]. With the nucleation starting from such complexes, one obtains a product with predominantly mixed-metal Co-O-Fe moieties, which improves the electrical conductivity of the product. This work further analyzes how the properties of the product species evolve during the hydrothermal synthesis step in the ARP process. We see that the Co/Fe ratio slowly increases from about 1:1 to a final value of 2:1, but does not reach the expected redox stoichiometry of 3:1. At the same time, the magnetization also increases, reaching a value of 16.9 emu g-1 for the final superparamagnetic product, which is three times higher than the value of monometallic Co3O4 and Fe2O3. The cobalt iron oxide samples obtained from ARP also possess superior oxygen evolution activity (307 mV overpotential at 10 mA cm-2 μg-1) compared to a mixture of Co3O4 and Fe2O3 (422 mV) or pure cobalt oxide (350 mV), highlighting the structure-induced enhancement of the catalytic activity. The difficult synthesis of evenly blended trinary/quaternary metals in a single-oxide phase may become possible in the future via ARP.