A comprehensive review of solution-synthesized ZnO-based ternary nanocomposites for photocatalytic degradation: Challenges, limitations, and solution approaches

Photocatalytic degradation stands out as an effective and sustainable wastewater treatment method for the removal of organic pollutants. This approach offers an environmentally friendly alternative by using free, renewable solar energy and relatively low-cost photocatalysts. Metal oxides are widely preferred in photocatalytic systems because of their facile synthesis and structural stability. However, limited light absorption, rapid electron–hole recombination, and a spectral-activity range in single-component and binary oxides highlight the need for more advanced materials design. ZnO-based ternary heterostructures developed in this direction offer high photocatalytic degradation efficiencies owing to bandgap engineering, increased visible-light absorption, and stepwise charge transport provided by multiple heterojunctions. Published studies show that degradation efficiencies of 98–100% can be achieved with appropriate synthesis methods and selection of oxide combinations, and reaction times can be significantly shortened. These developments have been made possible by optimizing parameters that directly affect photocatalytic activity, such as morphological control, interface quality, defect density, and band alignment. This review comprehensively evaluates, over the past five years, the performance of ZnO-based ternary metal-oxide composites synthesized using solution-based methods for the removal of organic and inorganic pollutants. The effects of co-precipitation, hydrothermal, solvothermal, and microwave-assisted synthesis approaches on photocatalytic activity have been compared, and the findings provide a comprehensive reference framework for designing future high-performance photocatalysts.