*TITLE: Exploring Calcium Oxide (CaO) Nanostructures: Eco-Friendly Synthesis, Structural Insights, and Emerging Functional Applications* *Author:* Hazrat Bilal *Affiliation:* Department of Chemistry, Abdul Wali Khan University Mardan, Pakistan *Email

This study presents a comprehensive overview of eco-friendly synthesis routes for Calcium Oxide (CaO) nanostructures and their emerging functional applications. CaO nanoparticles were discussed with focus on green synthesis using plant extracts, sol-gel, co-precipitation, and thermal decomposition methods. Structural properties were analyzed by XRD, SEM/TEM, and FTIR techniques. The paper highlights the correlation between synthesis conditions, particle size (20-50 nm), morphology, and surface basicity. Furthermore, potential applications of CaO nanostructures are critically reviewed, including CO2 capture (CaO + CO2 -> CaCO3), biodiesel production as heterogeneous solid base catalyst, wastewater treatment for heavy metal and dye removal, antimicrobial activity against E. coli and S. aureus, and enhancement of cement and construction materials. Author: Hazrat Bilal, Department of Chemistry, Abdul Wali Khan University Mardan, Pakistan. Email: [email protected] Keywords: Calcium Oxide, CaO Nanoparticles, Green Synthesis, CO2 Capture, Biodiesel