Plasma treatment of ZnO tetrapod–BiOBr heterojunction supported on PET waste for photocatalytic degradation of oil in water

In this study, a ZnO tetrapod–BiOBr heterojunction was ultrasonically immobilized onto floating recycled polyethylene terephthalate (PET) at different weight ratios (5, 10, and 15%) and evaluated for the photocatalytic degradation of hexane as a refractory oily pollutant under simulated solar light. The ZnOT(5)-B/P composite exhibited outstanding performance, achieving 96.6% degradation within 40 min due to its high surface area, strong ZnO–BiOBr interfacial contact, and efficient visible-light utilization. Comprehensive characterization (XRD, UV–Vis DRS, FESEM, BET/BJH, PL, AFM, CV, FTIR, Mott Schottky, ICP, and WCA) confirmed the formation of highly crystalline ZnO tetrapods and BiOBr microspheres with robust adhesion to the PET substrate and favorable bandgap reduction (3.1 → 1.8 eV), promoting enhanced charge separation. In the subsequent phase, the ZnOT(15)-B/P sample, which initially exhibited the lowest activity, was subjected to air-plasma surface engineering to address its weaker interfacial and textural features. Post-treatment structural and surface analyses (FESEM, FTIR, CV, and WCA) revealed that plasma exposure generated nanoscale surface etching, increased roughness and interparticle connectivity, and improved hydrophobicity, thereby facilitating stronger hexane adsorption and accelerated interfacial charge transfer. Furthermore, CV and XPS results indicated enhanced redox kinetics and increased density of surface oxygen vacancies, while AFM showed a ≈ 2.3-fold increase in surface roughness. As a result, the photocatalytic performance of ZnOT(15)-B/P improved substantially, rising from 67.5% to 84.8%, demonstrating the effectiveness of plasma post-activation in restoring and boosting the activity of an initially underperforming photocatalyst.