Design and Application of Hetero-Multicomponent Metal Oxide Photocatalysts for Wastewater Treatment: Ti–Cu–Zn Catalysts and Future Research Directions

Hetero-multicomponent metal oxide catalysts are attracting increasing attention for wastewater remediation due to their tunable band structures, synergistic redox activity, and enhanced stability. This review thoroughly evaluates recent progress in the synthesis and application of such catalysts, highlighting Ti-Cu-Zn nanostructures as a representative case study. We examine synthesis approaches-including hydrothermal, biosynthesis, precipitation, and spray-based methods, with additional insight into sol-gel and other less commonly applied techniques-with emphasis on their suitability for constructing layered and multicomponent heterostructures. Mechanistic aspects of photocatalysis, Fenton and Fenton-like processes, adsorption, and electrochemical routes are discussed, with particular focus on charge separation, reactive oxygen species (ROS) generation, and pollutant-specific degradation pathways. Comparative performance metrics against antibiotics, pesticides, dyes, and fertilizers are analyzed, alongside considerations of leaching, reusability, and scale-up potential. Importantly, while significant progress has been made for organic micropollutants, applications in heavy metal remediation remain scarce, highlighting an urgent research gap. By situating Ti-Cu-Zn systems within the broader class of multicomponent catalysts, this review not only synthesizes current advances but also identifies opportunities to expand their role in sustainable wastewater management, including field deployment, regulatory compliance, and integration into decentralized treatment systems.