Green Synthesis of Zinc Oxide Nanoparticles Using Plant and Agricultural Waste Resources - A Review

Abstract

Zinc oxide nanoparticles (ZnO NPs) have attracted considerable attention owing to their unique optical, catalytic, antimicrobial, and semiconductor properties. Conventional synthesis methods often involve hazardous chemicals, high energy consumption, and environmentally undesirable reaction conditions. Consequently, green synthesis approaches have emerged as sustainable alternatives for ZnO nanoparticle production. Plant extracts and agricultural wastes contain diverse phytochemicals such as polyphenols, flavonoids, tannins, proteins, and carbohydrates that can function as reducing, stabilizing, and capping agents during nanoparticle synthesis. Recent studies have demonstrated the successful utilization of coconut wastes, banana peels, citrus peels, rice husk, tea waste, coffee waste, and medicinal plant extracts for the green synthesis of ZnO nanoparticles. These biologically synthesized nanoparticles exhibit promising applications in photocatalytic degradation of pollutants, antimicrobial activity, wastewater treatment, agriculture, and energy related technologies. This mini review summarizes recent developments in plant and agricultural waste mediated synthesis of ZnO nanoparticles, discusses the underlying formation mechanisms, highlights major characterization techniques, and evaluates key environmental and technological applications. The review also identifies current challenges related to reproducibility, standardization, and large scale production. Green synthesis offers a sustainable pathway for the development of environmentally friendly ZnO nanomaterials and supports the principles of circular bioeconomy and green chemistry.

Introduction

The text discusses green synthesis of zinc oxide nanoparticles (ZnO NPs), focusing on their properties, preparation methods, characterization techniques, and applications. ZnO nanoparticles are widely studied due to their excellent optical, catalytic, antimicrobial, semiconducting, and photocatalytic properties. Their wide band gap, chemical stability, and environmental compatibility make them useful in fields such as environmental remediation, agriculture, healthcare, sensors, and energy technologies.

Traditional ZnO nanoparticle production methods, including sol-gel, hydrothermal, chemical precipitation, and vapor deposition, provide good control over nanoparticle properties but often require toxic chemicals, high energy input, and environmentally harmful conditions. To overcome these limitations, green synthesis methods have been developed using biological resources such as plant extracts, agricultural wastes, fungi, algae, and microorganisms. Plant-based synthesis is especially attractive because plants contain natural phytochemicals like flavonoids, phenolics, tannins, proteins, and carbohydrates that act as reducing, stabilizing, and capping agents.

The green synthesis mechanism generally involves:

1. Complexation of zinc ions with plant compounds.
2. Formation of zinc hydroxide intermediates through hydrolysis.
3. Conversion into ZnO nanoparticles through heating or calcination.

Factors such as pH, temperature, precursor concentration, extract amount, and calcination temperature influence particle size, morphology, crystallinity, and performance.

Various sustainable resources are used for ZnO synthesis, including:

• Fruit and vegetable wastes: banana peel, citrus peel, pomegranate peel, and other plant residues.
• Medicinal plants: such as Cassia fistula, Aloe vera, Melia azedarach, and others, which provide bioactive compounds for nanoparticle formation.
• Agricultural wastes: rice husk, sugarcane bagasse, tea waste, coffee waste, and coconut residues, supporting waste recycling and circular economy approaches.

Characterization of green-synthesized ZnO nanoparticles is carried out using multiple techniques:

• XRD: Confirms crystal structure and estimates crystallite size.
• FTIR: Identifies functional groups involved in synthesis and stabilization.
• UV–Visible spectroscopy: Determines optical properties and band gap.
• SEM and TEM: Analyze particle size and morphology.
• EDX: Confirms elemental composition and purity.

Green ZnO nanoparticles have several important applications:

• Photocatalytic degradation: They degrade organic pollutants such as industrial dyes through reactive oxygen species generated under UV light.
• Antimicrobial activity: They inhibit bacteria, fungi, and some viruses through oxidative stress, membrane disruption, and zinc ion release.
• Wastewater treatment: Their adsorption and photocatalytic properties help remove contaminants from water.
• Agriculture: They are explored as nano-fertilizers, plant growth promoters, and antimicrobial agents.
• Energy applications: Their semiconducting nature makes them useful in batteries, supercapacitors, solar cells, and hydrogen production.

Conclusion

Green synthesis has emerged as a sustainable and environmentally friendly approach for the production of zinc oxide nanoparticles. Unlike conventional chemical and physical methods, plant mediated synthesis utilizes naturally occurring phytochemicals as reducing, stabilizing, and capping agents, thereby minimizing the use of hazardous chemicals and reducing environmental impact. Plant extracts and agricultural wastes such as banana peels, citrus peels, rice husk, sugarcane bagasse, tea waste, coffee waste, coconut residues, and medicinal plants have been successfully employed for ZnO nanoparticle synthesis. These biological resources are abundant, renewable, inexpensive, and rich in phytochemicals that facilitate nanoparticle formation and stabilization. The synthesized ZnO nanoparticles have been characterized using various analytical techniques including X ray diffraction, Fourier transform infrared spectroscopy, UV Visible spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy dispersive X ray spectroscopy. These studies have confirmed the successful production of crystalline ZnO nanoparticles with diverse morphologies and functional properties. Green synthesized ZnO nanoparticles have demonstrated considerable potential in photocatalytic degradation of organic pollutants, antimicrobial activity, wastewater treatment, agriculture, and emerging energy applications. Their high surface area, photocatalytic efficiency, and multifunctional characteristics make them promising materials for sustainable environmental and technological solutions. Despite significant progress, challenges related to reproducibility, standardization of synthesis protocols, mechanistic understanding, scalability, and long term environmental safety remain important areas for future investigation. Addressing these challenges will facilitate the translation of laboratory scale research into practical applications. The green synthesis of ZnO nanoparticles using plant and agricultural waste resources represents a promising strategy that combines nanotechnology, waste valorization, and sustainable development. Continued research in this field is expected to contribute significantly to environmental remediation, resource utilization, and advanced functional materials.

References

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Copyright

Copyright © 2026 K. Kamalaveni, A. Ilamparithi . This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.