Microwave-assisted Green Synthesis of ZnO Nanoparticles Utilizing Litchi Chinensis Leaf Waste for Efficient Photocatalytic degradation of Methylene Blue

Abstract

The accumulation of fruit residues contributes significantly to environmental pollution, prompting extensive research aimed at managing this waste while generating economic value. In this context, the present study introduces a novel utilization of litchi waste leaves through a green route to synthesis ZnO nanoparticles. This work focuses on a simple, eco-friendly, and energy-efficient approach in which litchi leaf extract functions simultaneously as the reducing, capping, and stabilizing agent during nanoparticle formation. The objective was to develop a non-toxic, facile and sustainable approach for fabricating photo-catalytically active ZnO nanoparticles and evaluating their efficiency in degrading methylene blue dye. Additionally, the effect of varying microwave power levels on the characteristics of the nanoparticles was examined. The synthesized ZnO nanoparticles were characterized through XRD, UV–Vis, SEM, EDX, and FTIR to analyze their crystallite size, optical property, morphology, elemental composition, and functional groups respectively. XRD analysis verified the formation of hexagonal wurtzite ZnO with an average crystallite size of 17.76 nm according to Scherrer equation. UV–Vis spectroscopy spectrum showed a prominent absorption wavelength at 374 nm, corresponding to a band energy gap of 3.33 eV. SEM micrographs showed uniformly distributed spherical nanoparticles, while EDX analysis confirmed highest purity, detecting only Zn and O. FTIR spectra confirmed the Zn–O stretching by exhibited a characteristic peak at 664 cm⁻1. The synthesized nanoparticles demonstrated promising photocatalytic activity under natural sunlight, achieving a 98% removal efficiency along an adsorption capacity of 49.4 mg/g, following first-order kinetics. However, increasing the microwave power resulted in reduced photocatalytic efficiency due to the promoted particle growth and enhanced agglomeration. Overall, the findings demonstrate that this green and sustainable synthesis approach provides efficient ZnO photo-catalysts suitable for wastewater treatment applications.