Spectroscopic and kinetic analysis of textile dyes and antibiotic photodegradation via hydrothermally fabricated CuO/ZnO p/n heterojunction system

In the present study, a CuO/ZnO p/n heterojunction has been rationally fabricated using the hydrothermal-ultrasonication route, thereby developing a heterojunction-based photocatalyst for the efficient degradation of the Congo red (CR) dye and the Tetracycline (TC) drug. The synthesized pure CuO and ZnO and their CuO/ZnO heterojunction were characterized to investigate their structural, interfacial, morphological, chemical purity, optical, and charge-transport properties. The X-ray diffraction (XRD) confirmed the development of monoclinic CuO (9.75 nm) and ZnO (18.63 nm) with phase purity, and CuO/ZnO (16.01 nm) heterojunction fabrication with strong interfacial contact as verified by X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM), energy-dispersive X-ray (EDS) spectroscopy, and elemental mapping indicate irregularly sized nanoparticles in CuO/ZnO, verifying uniform mixing and interfacial contact between CuO and ZnO. Ultraviolet-visible (UV-vis) spectroscopy revealed substantial visible-light harvesting by the heterojunction compared to its pure counterparts, with optical bandgaps of 2.18 eV (CuO), 2.83 eV (ZnO) and 2.44 eV (CuO/ZnO). The photocatalytic degradation test revealed 88.1% Congo Red (CR) and 91.7% Tetracycline hydrochloride (TC) degradation over CuO/ZnO in 60 min of visible-light irradiation, outperforming CuO (41.86% TC and 44.53% CR) and ZnO (50.68% TC and 53.09% CR) with substantial mineralization (TOC analysis) of 85.2% TC and 82.7% CR (CuO/ZnO), compared to 37.3% TC and 40.6% CR (CuO), and 46.2% TC and 48.9% CR (ZnO). The kinetics study presents the pseudo-first-order kinetics with rate constants of 0.0406 min−1 (TC) and 0.0314 min−1 (CR) for CuO/ZnO, 0.0089 min−1 (TC) and 0.0094 min−1 (CR) for CuO, and 0.0125 min−1 (TC) and 0.0129 min−1 (CR) for ZnO.