Hybrid synthesis strategies of NiO nanostructures for photocatalytic dye removal from contaminated water sources: A critical review

The continued release of synthetic dyes by the textile and allied industries is of great ecological and public-health relevance, because, in most cases, the traditional infrastructures of waste water treatment do not curb the complex chromophoric molecules. NiO NPs have also been found to be one of the most effective remediation agents due to their large surface area, chemical stability, and strong photocatalytic capabilities. This is a critical review of the current advances in chemical, green, and hybrid synthesis protocols, which expound how particle size, morphology, and surface-functionalisation dictate the efficacy of adsorption and the photocatalytic degradation pathways. The special focus is given to doped NiO systems and NiO based composites, which are characterized by an increased charge separation, modulation of the band-gap, and the production of reactive oxygen species. Comparative studies have shown how optimised NiO nanostructures can have removal efficiencies of more than 90 - 99% of primary classes of dyes with visible or solar light. The review also outlines the current issues such as nanoparticle aggregation, low recoverability, and possible ecotoxicological effects, and suggests future research directions that could be useful to allow scalable, cost-effective, and sustainable application of NiO-based materials in industrial wastewater purification. Comprehensively, this synthesis provides a comprehensive mechanism structure that guides rational development of next generation NiO nanomaterials to be effectively used in the removal of dyes.