Insights into the Selective Sulfurization of NiMn₂O₄ Ternary Metal Oxide and Enhanced OER Activity

The chemical transformation of ternary metal oxide nanocrystals offers a versatile strategy for tailoring their physiochemical properties. We investigated the controlled chemical transformation of ternary metal oxide NiMn 2 O 4 to metal sulfides by employing different sulfur precursors to alter the composition and morphology of the resulting products. The distinct pH values of the precursor solutions, neutral for thiourea and alkaline for sodium sulfide, influence the selective precipitation of metal sulfides due to their pH-dependent solubility. Specifically, thiourea promotes the formation of intergrown NiS 2 cubes via the dissolution of NiMn 2 O 4, followed by nucleation and growth, as evidenced by time-dependent studies. On the other hand, the use of sodium sulfide under alkaline conditions results in the formation of MnS that inherits the morphology from the parent ternary metal oxide. Employing an equimolar mixture of sulfurizing agents facilitates the synthesis of a NiS 2 /MnS heterostructure. This transformation to sulfide enhanced the oxygen evolution reaction (OER) activity as compared with the activity using the ternary metal oxide. The overpotential decreased from 391 mV of NiMn 2 O 4 to 352 and 340 mV for NiS 2 and MnS, respectively, at a current density of 10 mA cm –2 in 1 M KOH solution. Notably, during a chronoamperometry test, the NiS 2 /MnS heterostructure showed significant stability at 1.57 mV potential to achieve a current density of 10 mA cm –2 for 25 h. This work establishes a synthetic route for the selective sulfurization of ternary metal oxide NiMn 2 O 4 and the NiS 2 /MnS heterostructure, which can be utilized for other materials.