TiO₂–ZnO semiconductor oxide coatings obtained via plasma electrolytic oxidation for solar-driven photocatalytic degradation: A comparison with conventional nanopowders

The increasing variety of persistent organic pollutants in water and the limited effectiveness of traditional wastewater treatment technologies require the development of sustainable photocatalyst synthesis and usage strategies. Directive 2024/3019 required advanced micropollutant removal methods in European wastewater treatment systems, thereby increasing interest in photocatalytic technologies. Semiconductor photocatalysts, such as TiO₂ and ZnO, exhibit high degradation efficiency. However, their practical implementation is limited by difficulties in recovering nanoparticles, aggregation, and potential ecotoxicological risks. This study presents a novel approach to synthesizing immobilized TiO₂-ZnO photocatalysts using plasma electrolytic oxidation (PEO), emphasizing this method's simplicity and environmental friendliness. Unlike conventional multistep immobilization strategies, PEO enables the direct formation of microporous oxide coatings in aqueous electrolytes without complex processing or hazardous reagents. This provides a green platform for developing photocatalytic materials with potential safety advantages. The resulting TiO₂-ZnO coatings effectively immobilize ZnO nanoparticles and exhibit a synergistic interaction between the two semiconductors, as evidenced by a band gap shift toward the visible range (3.08 eV). Under simulated solar irradiation, the coatings achieved nearly 80% dye degradation efficiency, approaching the performance of commercial TiO₂-P25 and outperforming ZnO powder. These findings highlight PEO as a promising method with potential for designing efficient and safe photocatalysts for environmental purposes.