Critical perspective review on green synthesis of zinc oxide nanoparticles: Explored wurtzite phase, characterization and real world applications

The current article critically reviews the green synthesis of Zinc oxide nanoparticles (ZnO NPs) by focusing on the formation, stability and importance of the hexagonal wurtzite (zincite) phase, the most thermodynamically stable polymorph of ZnO NPs. Green synthesis methods employing plant extracts, microorganisms, fungi and algae have come out as eco-friendly and non-toxic substitutes to traditional chemical processes. The work illustrates the importance of precursors, pH, temperature, capping agents and annealing process for controlling particle size, shape and phase purity. Among all biological systems, Allium sativum (garlic skin), Aspalathus linearis (rooibos) and Rosmarinus officinalis (rosemary) resulted in well-crystalline hexagonal wurtzite ZnO NPs of 7.0 to 15.0 nm with excellent reproducibility and phase homogenization. The XRD, TEM, SEM, FTIR, UV–Vis, XPS and DLS analyses confirm the wurtzite structure. The observed lattice parameters are close to the typical bulk values (a ≈ b ≈ 3.25 Å, c ≈ 5.21 Å), with expected nanoscale deviations arising from lattice relaxation, surface strain and defect density, with optical band gaps between 2.65 and 3.2 eV, thus indicating a strong quantum confinement effect. The dominance of the (0 0 2) and (1 0 1) planes was related to the improved photocatalytic, antibacterial, and optoelectronic properties. Green synthesized ZnO NPs have excellent potential applications in environmental remediation, sensors, photocatalysis, drug delivery and cosmetics industries, fulfilling cleaner/safer nanotechnology. Despite the significant advances, there are challenges of scale-up, reproducibility and toxicity evaluation. This review concludes that advancing green synthesis of wurtzite ZnO NPs requires improved mechanistic insight, standardized protocols and rigorous ecotoxicological evaluation. • Wurtzite hexagonal structure oriented (1 0 1) plane. • Lattice parameters a=b= 3.253 Å, c= 5.212 Å, α= β= 90°,γ= 120 °. • ZnO NPs vary in size and shape for surface-based biological capping. • TEM explored for Wurtzite hexagonal shape and orientation. • XPS explore binding energy and state of the Wurtzite structure.