Unveiling a reline deep eutectic solvent (DES) for controlled synthesis of phase-pure CuFeS2 towards dielectric and electrocatalytic applications

Abstract

CuFeS₂, a heavy-metal free transition metal chalcogenide (TMC), has remained unexplored despite having dielectric and electrocatalytic properties, primarily due to difficulty in controlling its phase and composition. Herein, we have developed a deep eutectic solvent (DES)-based solvothermal method for synthesizing stable and phase-pure chalcopyrite CuFeS₂ with flower-like morphology consisting of triangular nanoplates. Different sulphur precursors such as thiourea, thioacetamide, sodium sulphide, and thio-reline DES were investigated, and among them, only thiourea resulted in the formation of phase-pure CuFeS₂. The solvent “reline DES” serves as a reducer, solvent, and stabilizer for CuFeS₂ nanocrystals and remains thermally stable up to 200 °C under solvothermal conditions, as verified by visual and spectroscopic analyses. The synthesized CuFeS₂ exhibits a frequency and temperature-dependent dielectric constant and dielectric loss behavior. Furthermore, alternating current (AC) conductivity measurements indicated semiconductor-like behavior as conductivity increased with temperature, consistent with the reduction in real and imaginary parts of impedance. Also, CuFeS₂ achieved an optimal oxygen evolution reaction (OER) performance with an overpotential of 360 mV at 10 mA cm⁻² and a Tafel slope of 148.7 mV dec⁻¹. This work presents a simple, eco-friendly route to synthesize phase-pure CuFeS₂ with a high dielectric constant, low loss, excellent thermal stability, and good OER activity.