Solution Synthesis of Thiospinel CuCo₂S₄ Nanoparticles

CuCo₂S₄ is an important mixed-metal spinel-type sulfide that is typically synthesized using high-temperature solid-state reactions, which produce agglomerated particles with low surface areas that are not optimal for applications such as heterogeneous catalysis. Here, we show that highly crystalline and nonagglomerated colloidal CuCo₂S₄ nanoparticles can be synthesized in solution at 200°C, which is significantly lower than previously reported methods. The CuCo₂S₄ nanoparticles were found to be highly active electrocatalysts for the oxygen evolution reaction (OER) under strongly alkaline conditions (1.0 M KOH, pH 14), requiring an OER overpotential of 395 mV to produce a current density of 10 mA cm^-2. X-ray photoelectron spectroscopy (XPS) studies showed evidence of oxide formation, suggesting, in conjunction with the observed electrocatalytic properties, that the mixed-metal sulfides may serve as precursors to oxides and/or hydroxides, which are likely to be the catalytically active species.