Design and Application of a Capsule-like Zn–Cu-Based Catalyst for the Reduction of 4-Nitrophenol and Degradation of Methylene Blue Dye: A Green and Multifunctional Approach

Herein, the catalytic reduction of 4-nitrophenol (4-NP) and the photocatalytic degradation of methylene blue (MB) dye using an easily synthesizable capsule-like A-ZnO-CuO (AZC) nanocomposite as an S-scheme heterojunction photocatalyst are reported. The properties of A-ZnO (AZ) and AZC nanocomposite samples were adjusted by the addition of the leaf extract of the <i>Saraca asoca</i> plant as a stabilizing and reducing agent. The nanocomposite was synthesized using an eco-friendly approach, ensuring sustainability and minimizing environmental impact. The formation of AZ and AZC nanocomposite samples was confirmed by different spectroscopic and microscopic techniques (UV-vis diffuse reflectance spectroscopy (DRS), field-emission scanning electron microscopy associated with energy-dispersive X-ray, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, and X-ray diffraction). By plotting the DRS absorbance data in Tauc's equation, the bandgap energy of all of the as-prepared samples was investigated. Among all of the as-prepared samples, the capsule-like AZC3 nanocomposite material was found to show excellent photocatalytic activity in breaking down the MB dye up to 96.2% using a modest catalyst loading and 20 mg/L dye concentration that follows pseudo-first-order kinetics. Further, this novel nanocomposite is proven to be an effective catalyst in the reduction of 4-NP to 4-aminophenol using NaBH₄ as a reducing agent. The integration of CuO with ZnO facilitates enhanced electron transfer, thereby accelerating both the photocatalytic degradation of the MB dye and the catalytic reduction of 4-NP. Moreover, the catalyst exhibits exceptional stability over multiple cycles, demonstrating its potential for large-scale wastewater treatment applications. Additionally, the dual functionality of the catalyst in addressing multiple organic pollutants further enhances its feasibility for practical environmental remediation.