Microwave-assisted green synthesis of ZnO nanoparticles utilizing Litchi chinensis leaf waste for the 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 method for the utilization of litchi leaf waste through a green route to synthesize 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 is 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 is examined. The synthesized ZnO nanoparticles were characterized through XRD, UV–vis spectroscopy, SEM, EDX spectroscopy, and FTIR spectroscopy 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 the Scherrer equation. The UV–vis spectrum showed a prominent absorption wavelength at 374 nm, corresponding to a band gap energy of 3.33 eV. SEM micrographs showed uniformly distributed spherical nanoparticles, while EDX analysis confirmed that the material was of the highest purity, detecting only Zn and O. FTIR spectra confirmed the Zn–O stretching because of the presence of a characteristic peak at 664 cm⁻¹. The synthesized nanoparticles demonstrated promising photocatalytic activity under natural sunlight, achieving a 98% removal efficiency along with an adsorption capacity of 49.4 mg g⁻¹, 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.