An all-solid-state Z-scheme mechanism in the enhanced photocatalytic performance of NiO–Au–ZnO triple-layer thin films

Although thin-film photocatalysts offer practical advantages such as high recoverability and minimal secondary pollution, their performance is often hampered by their low surface area compared to their nanoparticle-based counterparts. This study demonstrates the fabrication of a triple-layer NiO-Au-ZnO heterostructure with significantly enhanced photocatalytic activity. The heterostructure, composed of solution-processed NiO and ZnO layers with a controlled sputtered Au interlayer, exhibits a well-defined layered architecture comprising NiO nanoparticles, a dense Au film, and ZnO nanorods. The continuity of this Au interlayer, achieved through optimized sputtering, was identified as the critical factor governing performance. The optimized NiO-Au-ZnO film demonstrates superior photocatalytic activity, achieving 98% degradation of methylene blue after 180 minutes alongside excellent reusability over multiple cycles. Comprehensive characterization using UV-vis, PL, XPS, and scavenger tests indicates that the enhancement is driven by an all-solid-state Z-scheme mechanism. Facilitated by the continuous Au interlayer, this mechanism promotes efficient charge carrier separation while preserving the strong redox potentials of the individual semiconductors. Notably, a discontinuous Au interlayer led to an unexpected decrease in photocatalytic performance, an effect that was attributed to a proposed antagonistic interaction between coexisting Z-scheme and type II p-n heterojunction pathways. This work underscores the critical importance of interface engineering and precise nanostructural control in thin-film systems for the rational design of advanced photocatalytic materials.