A general hierarchical flower-shaped cobalt oxide spinel template: facile method, morphology control, and enhanced saturation magnetization

Abstract

Nanostructures of materials with the spinel structure and A_xB_3−xO₄ stoichiometry have been intensively studied due to their many useful magnetic, electronic, and catalytic properties. However, few studies have systematically considered nanostructures of a complete spinel series due to lack of stable and general synthesis strategies. This limits the development of rational structure–property-application pathways. Here, we provide a facile and general method to synthesize hierarchical flower-shaped spinel cobalt oxide (SCO) based materials by a one-step solution-phase thermal decomposition. This SCO material serves as an autonomous template to form bimetallic oxide spinels Fe_xCo_3−xO₄ (Fe-SCO), Mn_xCo_3−xO₄ (Mn-SCO), Ni_xCo_3−xO₄ (Ni-SCO), and even the ternary Fe–Ni–Co–O (FNC) spinel in a single step. Morphology control was explored by tuning multiple input parameters such as precursors, ligands, additives, and solvents. The ternary FNC spinel achieved the highest saturation magnetization (98.6 emu g⁻¹ at 10 kOe) reported for any cobalt spinel. This complete understanding of the SCO template with well-controlled shape and magnetization provides a general pathway to flower-like nanostructures of Co-dominant spinels. The SCO templated structures considered in detail here are only the tip of the iceberg. We have also produced Cu-, Ag-, Zn-, Pd-, Mo-, Gd-, Pt-, Al-, K-, Li-, and Cr- containing SCO, all showing flower shapes, and combining magnetic properties of Co spinel with properties of the doping elements. We believe this template and approach will be of broad utility for incorporating multiple elements in spinel nanostructures.