*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:* chemistrystudent549@gmail.com *ABSTRACT* Calcium oxide (CaO) nanostructures have gained significant attention as a low-cost, non-toxic, and highly reactive alkaline earth metal oxide. This paper provides a comprehensive overview of recent advances in the eco-friendly fabrication of CaO nanostructures. We discuss various green synthesis routes, including plant-extract mediated, sol-gel, co-precipitation, and thermal decomposition methods that reduce the use of hazardous chemicals. The structural properties of CaO with different morphologies such as nanoparticles, nanorods, and nanoflakes are examined through characterization techniques including X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Fourier Transform Infrared Spectroscopy (FTIR). Furthermore, the correlation between synthesis parameters, crystallite size, and surface basicity is critically analyzed. Finally, the emerging applications of CaO nanostructures in CO2 capture, biodiesel production as a solid base catalyst, antimicrobial activity, cement and construction, and environmental remediation for wastewater treatment are highlighted. *Keywords:* Calcium Oxide, CaO Nanoparticles, Green Synthesis, CO2 Capture, Biodiesel Catalyst *1. INTRODUCTION* Calcium oxide, commonly known as quicklime, is one of the most widely available alkaline earth metal oxides. When the particle size is reduced to the nanoscale (<100 nm), its surface area, basicity, and chemical reactivity increase dramatically compared to bulk CaO. Due to its low cost and environmental compatibility, nano-CaO is considered a promising material for sustainable technologies.[1] *2. ECO-FRIENDLY FABRICATION ROUTES* a) *Green Synthesis:* Use of plant leaf extracts as capping agents. Calcium nitrate is mixed with extract and then calcined at 500-700°C. b) *Sol-Gel Method:* A simple, low-temperature method providing high purity and controlled particle size. c) *Co-precipitation Method:* Calcium chloride and sodium hydroxide are reacted to form Ca(OH)2, which on calcination gives CaO nanoparticles. d) *Thermal Decomposition:* Decomposition of calcium carbonate at high temperature.[2][3][4] *3. STRUCTURAL INSIGHTS* *3.1 XRD Analysis:* Cubic structure of CaO is confirmed with peaks at 2-theta = 32.2°, 37.3°, 53.8°. Crystallite size is 20-50 nm. *3.2 SEM/TEM Analysis:* Green synthesized particles are mostly spherical and agglomerated due to high surface energy. *3.3 FTIR Spectroscopy:* Ca-O stretching band appears around 400-500 cm-1. OH band around 3640 cm-1 shows moisture absorption.[3][4][5] *4. EMERGING FUNCTIONAL APPLICATIONS* *1. CO2 Capture:* CaO + CO2 -> CaCO3. It can capture up to 0.78 g CO2 / g CaO theoretically. *2. Biodiesel Production:* Acts as a heterogeneous solid base catalyst for transesterification. *3. Environmental Remediation:* Effective for removal of heavy metals and dyes. *4. Antimicrobial Activity:* Nano-CaO shows activity against E. coli and S. aureus. *5. Construction Industry:* Improves mechanical strength of cement composites.[2][6][4][5][1] *5. CONCLUSION* Eco-friendly synthesis of CaO nanostructures offers a sustainable pathway for producing a highly functional material. Future research should focus on improving stability against moisture and reusability. *REFERENCES* Habte, L., et al. (2019). Sustainability, 11(12), 3332. Liu, W., et al. (2023). Journal of CO2 Utilization, 68, 102355. Reddy, K. S., et al. (2021). Materials Today: Proceedings, 42, 1202-1208. Samanta, A., et al. (2020). Journal of Environmental Chemical Engineering, 8(5), 104162. Roy, A., & Bhattacharya, (2010). International Journal of Nanoscience, 9(3), 123-131. Kouzu, M., & Hidaka, J. S. (2012). Fuel, 93, 1-12. Zhang, Y., et al. (2022). Bioresource Technology Reports, 18, 101042.