Reusable ZnO/g-C3N4 thin film photocatalysts for efficient dye removal from wastewater

The increasing contamination of water resources by persistent organic pollutants poses a serious environmental challenge, necessitating the development of efficient and practically deployable photocatalytic systems. Although semiconductor-based photocatalysts have shown promise for pollutant degradation, most reported systems are in powder form, which suffer from poor recyclability, difficult recovery, and limited scalability, restricting their real-world application. In this work, ZnO/g-C 3 N 4 composite thin films with varying g-C 3 N 4 loadings (5–20 wt%) were successfully fabricated via a hydrothermal-assisted method followed by spin-coating deposition. Structural and morphological analyses confirmed the formation of a well-defined heterojunction with enhanced surface area and defect density. Optical studies revealed improved light absorption and suppressed charge carrier recombination in the composite films. Among the prepared samples, the ZNC20 thin film exhibited superior photocatalytic activity, achieving 56.3% degradation of methylene blue under neutral conditions and a maximum of 97.6% under alkaline conditions (pH 11) under UV irradiation. The kinetic studies followed pseudo-first-order behaviour, while scavenger experiments identified ●O 2 - and ●OH as the dominant reactive species, supporting a direct Z-scheme charge transfer mechanism. Importantly, the immobilised thin film configuration eliminates the need for catalyst recovery, prevents material loss, and enables facile reuse, retaining 85.3% efficiency after five cycles. Compared to previously reported systems, this work demonstrates competitive performance with lower g-C 3 N 4 loading and a simplified architecture.