Eucalyptus Leaf Extract-Mediated Green Synthesis of Zinc Oxide Nanoparticles: Phytochemical-Driven Band Gap Engineering and Structural Optimization

Abstract

This study reports a single-step, eco-friendly precipitation route for synthesizing zinc oxide (ZnO)
nanoparticles using aqueous Eucalyptus leaf extract as a simultaneous bio-reducing, capping, and
stabilizing agent, in place of hazardous chemical reductants and synthetic surfactants. Phytochemical
screening confirmed a phenolic- and flavonoid-rich extract matrix, with a total phenolic content of 174.52 ±
4.85 mg gallic acid equivalent (GAE)/g and a total flavonoid content of 58.31 ± 2.14 mg quercetin
equivalent (QE)/g, while GC-MS profiling identified 1,8-cineole (46.85%), α-pinene (14.22%), α-terpineol
(9.18%), globulol (6.41%), and a gallic acid derivative (5.74%) as the principal constituents. Synthesis
parameters were systematically optimized, with an extract-to-precursor ratio of 1:2 (v/v), 0.10 M zinc
nitrate, pH 10, and calcination at 400 °C identified as optimal. X-ray diffraction confirmed a phase-pure
hexagonal wurtzite structure (JCPDS 36-1451) with an average crystallite size of 18.35 nm, corroborated
by FE-SEM/HR-TEM particle sizing (20.42 ± 2.58 nm) and a near-stoichiometric Zn:O ratio of 1:1.01
(EDX). Comparative FTIR analysis confirmed retention of a deprotonated polyphenolic capping layer,
conferring a zeta potential of −32.40 mV and a narrow hydrodynamic size distribution (PDI = 0.214). BET
analysis revealed a mesoporous structure with a specific surface area of 34.82 m²/g. Critically, UV-Vis
diffuse reflectance spectroscopy revealed a narrowed optical band gap of 3.25 eV relative to bulk ZnO
(3.37 eV), accompanied by intense defect-related photoluminescence at 535 nm, indicating a high density
of singly ionized oxygen vacancies. Tasleem (2017) These findings demonstrate that Eucalyptus-derived
phytochemicals function not merely as passive capping agents but as an active, non-chemical route to band
gap engineering, establishing a mechanistically substantiated foundation for the visible-light photocatalytic
application of this material.