Morphological evaluation of green synthesized, modified TiO2 and ZnO nanomaterials and their photocatalytic behavior in water treatment

The emergence of nanotechnology has opened new avenues for various nano-structured materials, especially in wastewater treatment applications. However, the chemical reagents and processes for developing nanomaterials often lead to secondary pollution. Thus, environmentally friendly fabrication methods, particularly for TiO 2 and ZnO photocatalysts, have been of interest. These methods include preparation of the photocatalysts with biomolecules derived from various biological sources resulting in lower toxicity, higher stability and enhanced physicochemical, biological and optical features. Thus, this review focuses on advances in green synthetic approaches utilizing various biomolecule-containing sources such as plants, fungi, algae and bacteria to develop TiO 2 and ZnO semiconductor photocatalysts. Furthermore, it gives insight into the photocatalytic applications of TiO 2 and ZnO towards the removal of textile and pharmaceutical-based organic pollutants in simulated and real water samples. This is followed by the evaluation of the advantages and limitations of green synthesized TiO 2 and ZnO materials over conventional approaches through synthesis mechanisms, light absorption properties, removal efficiencies and degradation pathways. Finally, the review outlines future prospects, challenges, and potential directions of these advanced green methods and materials. Illustration of the mechanism of photocatalysis. Reproduced with permission. [37] 2015, Universiti Kebangsaan Malaysia. • Green synthesis of photocatalysts: The review highlights environmentally friendly fabrication methods for TiO 2 and ZnO photocatalysts using biomolecules from plants, fungi, algae, and bacteria • Enhanced material properties: Green-synthesized photocatalysts exhibit improved toxicity profiles, stability and enhanced physicochemical and optical properties • Pollutant removal applications: The photocatalysts are evaluated for their effectiveness in degrading textile and pharmaceutical organic pollutants in both simulated and real wastewater samples • Comparison and outlook: The review compares green and conventional synthesis methods, discussing advantages, limitations and future challenges in terms of synthesis mechanisms, light absorption, removal efficiency and degradation pathways