Photocatalytic Degradation of Methylene Blue and Rhodamine B by Mononuclear Copper (II) Complex with Phenanthroline-Based Ligand

Abstract

Photocatalytic degradation under ultraviolet irradiation provides an efficient strategy for removing toxic organic dyes such as Methylene Blue (MB) and Rhodamine B (RhB), which pose serious risks to aquatic ecosystems and human health. Copper-based photocatalysts are attractive alternatives to noble metals due to copper’s natural abundance, low cost, and accessible redox states. In this study, a mononuclear copper(II) complex, Cu(II)(2,9-dmp)Cl₂, was investigated as a homogeneous photocatalyst for MB and RhB degradation under UVC irradiation. The complex was characterised via CHN elemental analysis, FTIR, UV-Vis spectroscopy, and single-crystal X-ray diffraction (SCXRD), which confirmed a distorted square pyramidal geometry (τ = 0.26) within a monoclinic crystal system (space group P2₁). Photocatalytic performance was evaluated in a custom-built reactor (22 W UVC), with degradation kinetics assessed across varying dye concentrations, catalyst dosages, and H₂O₂ levels. Under optimal conditions, the catalyst achieved degradation efficiencies of 95% for MB and 90% for RhB within 100 min, following second-order kinetics. The higher rate constant for MB (0.242 M⁻¹min⁻¹) compared to RhB is tentatively attributed to the greater structural susceptibility of the thiazine framework relative to RhB’s rigid xanthene core. The complex is proposed to operate via a dual-pathway mechanism involving ligand-to-metal charge transfer (LMCT) and Cu(II)/Cu(I) redox cycling to generate hydroxyl radicals, although direct radical-identification experiments remain to be performed. These findings establish phenanthroline-based copper complexes as cost-effective, environmentally compatible photocatalysts, providing a foundation for future structural optimisation and heterogeneous system development for industrial wastewater treatment.