Heterostructured photocatalysts in the removal of methylene blue dye: Mechanisms, recent advances, and design principles

Abstract

Summary Methylene blue (MB) is a persistent dye pollutant widely used to evaluate photocatalytic wastewater treatment because of its environmental persistence and well-defined degradation behavior. This review examines the design principles, charge-transfer mechanisms, and performance of more than 50 binary and ternary heterostructured photocatalysts, including Type II, Z-scheme, S-scheme, p-n, and Schottky architectures based on TiO2, ZnO, bismuth-based semiconductors, g-C3N4, and MXene-containing composites. Comparative analysis shows that S-scheme heterojunctions best preserve the redox potentials required for reactive oxygen species generation, while pore architecture strongly influences adsorption and degradation kinetics. The review also demonstrates that decolorization at 664 nm frequently overestimates treatment efficiency because organic intermediates can remain after color removal, thereby highlighting the importance of total organic carbon measurements. These findings provide practical guidance for photocatalyst design, mechanistic verification, and standardized performance assessment to support the development of more effective solar-driven wastewater treatment technologies.