A nickel(ii) complex of a naphthaldehyde-derived bis-imine ligand for sunlight-driven dye remediation: mechanistic, intermediate identification, and recyclability studies

Abstract

Dyes have been proven to act as persistent organic pollutants in both freshwater bodies and the marine environment. Thus, suitable dye remediation measures have become obligatory to save aquatic biosystems and maintain human health. This work presents a study on the photocatalytic remediation of cationic dyes using a nickel(II) complex (NiL₁Et). The NiL₁Et complex was synthesized using the ONS donor ligand (L₁) and confirmed using spectroscopic techniques and elemental analysis. The Ni(II) complex NiL₁Et adopts a square planar geometry, which was envisioned through a computational study. The experimentally estimated direct band gap of 2.19 eV is in agreement with the HOMO–LUMO energy obtained in the computational study. This establishes the semiconductor-like property of the NiL₁Et complex. Moreover, the slower electron–hole recombination rate of the excited complex, as inferred from the emission intensities, further demonstrates the potential of NiL₁Et as a photocatalyst. Methylene blue (MB) was used as the model dye, and a maximum degradation efficiency of 80.15% was achieved using NiL₁Et within 60 min under natural sunlight and without any artificial light source. The effects of various parameters, including the catalyst amount, dye concentration, reaction time, pH, and H₂O₂ dose, were analyzed. It was observed that the complex could be efficiently used as a photocatalyst for 5 catalytic cycles. Furthermore, insights into the mechanistic pathway of MB degradation were obtained. The path of MB degradation was predicted by determining the intermediate species generated during the degradation process using LC-MS.