Deep eutectic solvent-assisted synthesis of Cu-doped ZnO for robust photocatalytic degradation of rhodamine B toward sustainable wastewater purification

Deep eutectic solvents (DESs) have gained increasing attention as alternatives reaction media to conventional solvents due to their simple preparation, tunable physicochemical properties, low vapor pressure, low environmental impact, and excellent compatibility with metal and semiconductor synthesis. Their ability to enhance reaction efficiency and selectivity makes them particularly attractive for photocatalytic water treatment technologies. In this study, pristine ZnO and Cu-doped ZnO photocatalysts were synthesised via co-precipitation method using an environmentally benign deep eutectic solvent, employing zinc nitrate and copper nitrate precursors, followed by annealing at 400 °C. Structural and morphological analyses confirmed the formation of the hexagonal wurtzite structure of ZnO in both undoped and Cu-doped samples. The materials exhibited a predominantly spherical morphology, and the most active photocatalyst showed a surface area of 30m 2 g⁻ 1 , indicating enhanced surface availability for photocatalytic reactions. Photocatalytic performance was evaluated using rhodamine B as a model dye pollutant in an aqueous medium. Among the samples, 5% Cu-doped ZnO exhibited superior activity, achieving 97% degradation of a 10-ppm dye solution within the investigated time of 180 min using 0.020 g of catalyst. Kinetic analysis demonstrated pseudo-first-order behaviour, and the photocatalyst maintained good stability over three successive degradation cycles. Liquid chromatography–mass spectrometry (LC–MS) analysis enabled the identification of intermediate products and the proposal of a plausible degradation pathway, while scavenger studies indicated that photogenerated holes and superoxide radicals dominate the degradation process. This study demonstrates the potential of DES-assisted synthesis as a sustainable strategy for photocatalytic wastewater purification.