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

## TL;DR

A ZnO tetrapod–BiOBr photocatalyst on recycled PET was developed and improved via plasma treatment to efficiently degrade oil pollutants in water.

## Contribution

A novel ZnO tetrapod–BiOBr heterojunction on PET waste was developed, with plasma treatment enhancing its photocatalytic performance for oil degradation.

## Key findings

- The ZnOT(5)-B/P composite achieved 96.6% oil degradation within 40 minutes due to strong interfacial contact and visible-light utilization.
- Plasma treatment improved the ZnOT(15)-B/P sample's performance from 67.5% to 84.8% by enhancing surface roughness and charge transfer.
- Plasma exposure increased surface oxygen vacancies and hydrophobicity, facilitating stronger oil adsorption and photocatalytic activity.

## Abstract

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.

The online version contains supplementary material available at 10.1038/s41598-025-32882-2.