Green Synthesis of ZnO Nanoparticles Using Terminalia catappa Leaf Extract: Effect of Precursor Ratio on Cluster Size and Optical Band Gap Tuning

Abstract

Zinc oxide (ZnO) nanoparticles have attracted considerable attention due to their unique optical, electronic, and photocatalytic properties; however, conventional synthesis methods often involve hazardous chemicals and generate environmentally harmful waste. Therefore, this study aimed to synthesize ZnO nanoparticles through a green synthesis approach using aqueous ketapang leaf extract (Terminalia catappa) as a natural reducing and stabilizing agent and to evaluate the effects of precursor-to-extract ratio on the optical properties of the synthesized nanoparticles. Qualitative phytochemical screening confirmed the presence of alkaloids, flavonoids, tannins, saponins, and triterpenoids, indicating the extract's potential to facilitate nanoparticle formation. ZnO nanoparticles were synthesized using 0.1 M zinc acetate dihydrate with precursor-to-extract volume ratios of 1:1, 1:2, and 1:3. UV–Vis spectroscopic analysis showed that the optimum synthesis condition was achieved at a precursor-to-extract ratio of 1:2, yielding the highest absorbance value of 0.90 at a maximum absorption wavelength of 372 nm. The optical band gap energy determined from Tauc plot analysis was 3.31 eV under optimum conditions.  The structural features and functional groups capping the nanoparticles were verified using Fourier Transform Infrared Spectroscopy (FTIR). Furthermore, the effective optical cluster dimensions were estimated using the quantum confinement model via the Brus equation, indicating calculated sizes within the range of 10.20–14.42 nm. These findings demonstrate that ketapang leaf extract can serve as an effective bioreducing and stabilizing agent for the environmentally friendly synthesis of ZnO nanoparticles with tunable optical properties.