Synthesis of Complex Oxide Powders Using Deep Eutectic Solvents as Reaction Media

Abstract

This study demonstrates the synthesis of zinc-transition metal composite oxides ZnM₂O₄ (M = Co, Mn, Fe) using deep eutectic solvents (DES) as environmentally friendly reaction media. A coprecipitation reaction was conducted in a complete DES environment combining choline chloride-urea DES and choline chloride-oxalic acid DES to obtain metal oxalate precursors, which were subsequently converted to the target spinel oxides. XRD analysis and SEM observations revealed that the precursors exhibited distinctive needle-like or columnar morphologies depending on the transition metal species. Upon heat treatment at 400°C, these precursors transformed into corn-like porous secondary structures composed of aggregated nanoparticles with sizes of 10–40 nm. The obtained oxide powders demonstrated temperature-dependent responses to various gases including ethanol and acetone, with sensor behaviors reflecting different semiconductor characteristics (p-type/n-type) depending on the material composition. This approach enables particle morphology control that is difficult to achieve with conventional aqueous solution methods, providing a novel synthetic guideline for functional oxide materials with potential applications in gas sensing and catalysis.

Graphical Abstract

TEM images of spinel oxides.