Microwave-assisted starch stabilization & Chitosan green synthesis of zinc oxide nanoparticles for the photocatalytic applications

Abstract Zinc oxide (ZnO) nanoparticles were successfully synthesized via a microwave-assisted green route employing starch as a stabilizing matrix and chitosan as a biopolymeric capping and surface-functionalizing agent. Glucose served as a mild reducing agent, while microwave irradiation provided rapid and energy-efficient heating. The synthesis strategy aligns with green chemistry principles by minimizing hazardous reagents, reducing reaction time, and enabling controlled nanoparticle growth. UV–Vis spectroscopy revealed a characteristic excitonic absorption peak at 373 nm, confirming nanoscale ZnO formation. The optical band gap estimated from the Tauc plot was approximately 3.26 eV, indicating slight quantum confinement effects. FT-IR analysis showed a distinct Zn–O stretching vibration at 476.52 cm⁻¹ along with residual –OH and –NH₂ surface groups derived from starch and chitosan. XRD patterns confirmed the formation of phase-pure hexagonal wurtzite ZnO with an average crystallite size of 18–25 nm. SEM and TEM analyses revealed near-spherical nanoparticles with moderate aggregation, while EDX confirmed stoichiometric Zn and O composition with minor carbon and nitrogen contributions from surface biopolymers. Photocatalytic performance was evaluated using methylene blue (MB) under natural sunlight irradiation. The ZnO nanoparticles achieved over 90% degradation within 90 minutes and followed pseudo-first-order kinetics with an apparent rate constant of 0.024 min⁻¹. The enhanced photocatalytic activity is attributed to efficient charge carrier generation, suppressed electron–hole recombination mediated by surface functional groups, and increased reactive oxygen species (ROS) formation. The results demonstrate that microwave-assisted starch–chitosan stabilization offers a sustainable and scalable strategy for producing high-performance ZnO photocatalysts suitable for environmental remediation applications.