Recent Advances In Metal Co-Doped TiO₂ Photocatalysts For Enhanced Dye Degradation: A Systematic Review

The persistent release of synthetic dyes from industrial effluents poses severe environmental and health threats due to their toxicity, stability, and resistance to conventional treatments. Titanium dioxide (TiO₂) has emerged as a promising photocatalyst; however, its wide band gap and rapid electron–hole recombination limit its performance under visible light. Recent advances highlight metal co-doping as an effective strategy to overcome these drawbacks through synergistic band gap narrowing, improved charge carrier separation, and enhanced structural stability. This systematic review consolidates experimental findings from 2015–2025 on metal co-doped TiO₂ photocatalysts for dye degradation, encompassing 30 peer-reviewed studies selected via PRISMA guidelines. The analysis reveals dominant trends in Fe–Cu, Ce–Nd, and Co–Mn co-doped systems, which consistently achieve >90% degradation efficiencies under visible light due to optimized redox interactions and stabilized anatase phases. Sol–gel and hydrothermal routes remain the most widely adopted synthesis methods, while innovative approaches such as flash sintering and green sol–gel demonstrate potential for sustainable scale-up. Band gap engineering within the optimal range of 2.3–2.9 eV and multipathway radical generation are identified as critical factors driving superior photocatalytic activity. Despite notable progress, challenges remain in achieving long-term stability, scalable production, and standardized performance metrics. This review provides a comprehensive framework for correlating co-dopant selection, synthesis strategy, and photocatalytic performance, offering insights into the rational design of next-generation TiO₂-based photocatalysts for sustainable wastewater treatment. Keywords: TiO₂ photocatalyst, metal co-doping, dye degradation, visible light, band gap engineering, systematic review