Photocatalytic activity of Ni (II) Complexes Derived from p-substituted Isonitrosoacetophenone-4 Phenylthiosemicarbazone as Catalyst for Degradation of Azo Dye

Abstract

A series of p-substituted isonitrosoacetophenone-4-phenylthiosemicarbazone Ni(II) complexes were successfully synthesized and investigated for their photocatalytic potential in the degradation of the azo dye methyl orange, a common organic pollutant found in industrial wastewater. The synthesized complexes were thoroughly characterized using various physicochemical and spectroscopic techniques, including elemental analysis, molar conductance measurements, magnetic susceptibility studies, electronic absorption spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, and thermogravimetric–differential thermal analysis (TG-DTA). The photocatalytic activity of the Ni(II) complexes was evaluated under heterogeneous catalytic conditions by monitoring the degradation of methyl orange under light irradiation. The influence of important operational parameters such as irradiation time, catalyst dosage, and catalyst reusability was systematically examined to optimize the degradation process. The results revealed that the synthesized Ni(II) complexes exhibited significant photocatalytic efficiency toward methyl orange degradation, leading to substantial dye removal within the studied time period. Furthermore, the catalysts retained considerable activity after repeated catalytic cycles, demonstrating good stability and reusability. Based on the experimental findings, a possible photocatalytic degradation mechanism involving the generation of reactive oxygen species has been proposed. These results suggest that the synthesized Ni(II) complexes can serve as effective and environmentally friendly photocatalysts for the treatment of dye-contaminated wastewater.

Introduction

The study investigates the photocatalytic degradation of methyl orange (MO), a persistent azo dye commonly found in industrial wastewater, using synthesized nickel(II) thiosemicarbazone complexes under visible-light irradiation. Industrial dye pollution poses significant environmental and health risks, and conventional treatment methods often suffer from drawbacks such as incomplete degradation, high cost, and secondary pollution. Photocatalysis offers a sustainable alternative by generating reactive oxygen species that mineralize organic pollutants.

Nickel(II) complexes were synthesized from p-substituted isonitrosoacetophenone phenyl thiosemicarbazone ligands (NL1–NL6) using a 2:1 ligand-to-metal ratio. The complexes were characterized by elemental analysis, molar conductance, magnetic susceptibility, UV–Visible spectroscopy, FT-IR spectroscopy, and TG-DTA analysis.

Photocatalytic activity was evaluated using a 10 ppm methyl orange solution irradiated with a tungsten filament lamp. The maximum absorption wavelength (λmax) of MO was determined to be 480 nm, and degradation efficiency was calculated from changes in absorbance.

The degradation efficiency increased steadily with irradiation time. After 120 min, all complexes exhibited high photocatalytic activity, with degradation efficiencies ranging from 85.71% to 90.47%, while the NL6 (p-NO? substituted) complex showed the highest performance. Increasing the catalyst dosage from 25 mg to 100 mg also enhanced dye degradation, with efficiencies increasing from 18.09–23.80% to 77.14–81.90%.

The representative NL1 complex was evaluated for reusability over five catalytic cycles. Although a gradual decline in degradation efficiency was observed due to catalyst loss and surface fouling, the catalyst retained appreciable activity, demonstrating good stability and recyclability.

The proposed photocatalytic mechanism involves visible-light excitation of the nickel complex, generating electron–hole pairs that produce hydroxyl (•OH) and superoxide (O?•?) radicals. These reactive oxygen species attack the azo bond and aromatic rings of methyl orange, leading to progressive degradation and eventual mineralization into environmentally benign products such as CO?, H?O, and inorganic ions.

Conclusion

The photocatalytic degradation of Methyl Orange was investigated using a series of nickel(II) complexes containing p-substituted isonitrosoacetophenones phenyl thiosemicarbazone ligands. All the synthesized complexes exhibited significant photocatalytic activity, achieving more than 85% degradation of Methyl Orange within 120 min of irradiation. Among the investigated complexes, NL6, containing the p-nitro substituted ligand, displayed the highest photocatalytic activity. The enhanced performance of NL6 can be attributed to the strong electron-withdrawing nature of the nitro group, which facilitates efficient charge separation and suppresses electron–hole recombination. This promotes the generation of reactive oxygen species such as hydroxyl and superoxide radicals, leading to improved degradation of Methyl Orange. Similarly, the p-chloro substituted complex NL5 exhibited superior activity compared to the unsubstituted and electron-donating substituted analogues, further highlighting the beneficial effect of electron-withdrawing substituents on photocatalytic performance. In contrast, complexes containing electron-donating substituents such as methyl, methoxy, and hydroxyl groups showed comparatively lower degradation efficiencies. Although these substituents may enhance electron density within the ligand framework, they are less effective in facilitating charge separation during the photocatalytic process. The unsubstituted complex NL1 displayed the lowest activity among the series. The results demonstrate that the electronic properties of ligand substituents play a crucial role in determining the photocatalytic efficiency of nickel(II) complexes. The present study suggests that incorporation of electron-withdrawing groups into the ligand framework is an effective strategy for enhancing photocatalytic degradation of organic dyes. These nickel(II) complexes therefore

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Copyright

Copyright © 2026 Shivani B. Malap, Sayali V. Kadam, Raju M. Patil. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.