Synthesis of zinc oxide nanoparticles using sol-gel method and their characterization

Introduction

Nanotechnology has emerged as an innovative approach to improve the bio-efficacy of a large group of materials including zinc. Zinc oxide nanoparticles (ZnO NPs) have received considerable attention because of their small particle size, large specific surface area, enhanced reactivity and improved nutrient delivery efficiency compared with conventional zinc fertilizers. These unique properties facilitate better absorption, translocation and utilization of zinc by plants, thereby enhancing nutrient use efficiency and crop productivity (Prasad et al., 2012; Raliya et al., 2018) [7, 8] . A wide range of synthesis methods has been developed for the preparation of metal and metalloid nanoparticles, including chemical precipitation, sol-gel, hydrothermal, sonochemical, microemulsion and green synthesis techniques (Namakka et al., 2023) [6] . Among these approaches, chemical synthesis methods are widely preferred because they are simple, economical, reproducible and suitable for large-scale production of nanoparticles with controlled particle size and morphology. However, the physicochemical properties of nanoparticles are greatly influenced by synthesis parameters such as precursor concentration, pH, reaction temperature, reaction time, drying and calcination conditions. These parameters determine the particle size, crystallinity, and stability and surface characteristics of the synthesized nanoparticles. Therefore, optimization of the synthesis process followed by detailed characterization is essential before their use in agricultural applications. Comprehensive characterization of nanoparticles is necessary to confirm their successful synthesis and to understand their reactivity and applicability (Namakka et al., 2023) [6] . In the present study, ultraviolet-visible (UV-Vis) spectroscopy was employed as a preliminary technique to confirm the formation of ZnO nanoparticles based on their characteristic absorption behavior. Scanning electron microscopy (SEM) was used to examine the surface morphology, particle shape and aggregation behavior, whereas dynamic light scattering (DLS) was used to determine the hydrodynamic particle size distribution and dispersion characteristics of the synthesized nanoparticles. The combined application of these analytical techniques provides reliable information on the quality, stability and suitability of ZnO nanoparticles for agricultural use (Raliya et al., 2018) [8] . Although the synthesis and physicochemical characterization of ZnO nanoparticles have been reported by several researchers (Jain et al., 2013; Sun et al., 2020; Lian et al., 2024) [2,10,4] , relatively few studies have integrated controlled nanoparticle synthesis and comprehensive physicochemical characterization with their subsequent evaluation as foliar nano-fertilizers under field conditions (Zhang et [11,10,4] . Moreover, variations in synthesis protocols, particle size, morphology, surface charge, dissolution and aggregation behaviour, as well as the characterization techniques employed, often hinder direct comparison of the properties and agronomic performance of ZnO nanoparticles across studies. Therefore, systematic synthesis followed by detailed physicochemical characterization is essential for developing stable, efficient and agronomically suitable nano-zinc formulations. The present investigation was undertaken to synthesize zinc oxide nanoparticles (ZnO NPs) using the oxalate decomposition (solgel) method, a chemical synthesis technique, and to characterize the synthesized nanoparticles using UV-Visible spectroscopy, scanning electron microscopy (SEM) and dynamic light scattering (DLS).

Materials and Methods

Synthesis of nanoscale ZnO

Preparation of nanoscale ZnO: Nanoscale ZnO was prepared by using the oxalate decomposition technique as described by Prasad et al., 2012 [7] .

Characterization of nanoscale ZnO UV -Vis (UV-Visible spectra) analysis:

The UV-visible spectra of this solution were recorded in Spectrophotometer (UV-1800, SHIMADZU), from 200 to 1100 nm. This analysis was useful to identify the presence of nutrients and its phase. Sample was prepared by suspending 1mg of the nanoparticles in 5 mL of water and then estimated using UV-Vis spectrophotometer.

DLS (Dynamic Light Scattering) analysis:

The aqueous suspension of the synthesized nanoparticles was filtered through a 0.22 μm syringe driven filter unit and the size and zeta potential of the distributed nanoparticles were measured by using the principle of Dynamic Light Scattering (DLS) technique made in a Nanopartica (HORIBA, SZ-100) compact scattering spectrometer. This information is useful to know the dispersion capacity of nanoparticles and stability of nanoparticles by negative zeta potential values.

Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) analysis: The surface morphology of the synthesized zinc oxide (ZnO) nanoparticles was examined using a ZEISS Scanning Electron Microscope (SEM) at the Department of Nanotechnology, University of Agricultural Sciences, Raichur, Karnataka, India. A small quantity of the dried ZnO nanoparticle powder was mounted on an aluminium stub using conductive carbon tape and sputter-coated with a thin layer of gold prior to imaging. The micrographs were recorded using a secondary electron (SE1) detector at an accelerating voltage of 10.0 kV, a working distance of 13.5 mm, and a magnification of 20, 000×. The SEM analysis was used to determine the particle morphology, surface characteristics, degree of agglomeration and approximate particle size of the synthesized ZnO nanoparticles.

Results

Synthesis of nanoscale ZnO

The zinc oxide (ZnO) nanoparticles were synthesized using the oxalate decomposition method as described in the Materials and Methods section, and the synthesis procedure is presented in Fig. 1. The synthesized nanoparticles were characterized using UV-Visible spectroscopy, Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS) to assess their optical behaviour, morphological characteristics and hydrodynamic particle size, respectively. The combined characterization confirmed the successful formation of ZnO nanoparticles with nanoscale dimensions, desirable surface morphology and stable dispersion characteristics, suggesting their suitability for foliar application and for further investigations in agriculture.

Characterization of nanoscale ZnO UV -Vis (UV-Visible spectra) analysis for nanoscale ZnO

The optical absorption characteristics of the synthesized zinc oxide nanoparticles were investigated via UV-Visible spectroscopy within a scanning range of 200 nm to 1100 nm. The recorded absorption spectrum exhibits two primary distinct features:

• Characteristic Excitonic Absorption Peak (373.00 nm): A well-defined, prominent absorption peak was recorded at 373.00 nm with a corresponding absorbance value of 0.67 (Fig. 2.). This sharp band edge absorption stems directly from the basic intrinsic surface plasmon resonance transition of electrons moving from the valence band to the conduction band. The occurrence of this sharp absorption maximum below the typical 380 nm threshold value observed in bulk zinc oxide validates a significant blue-shift, which firmly confirms the successful synthesis of particles well within the nanoscale boundaries experiencing clear quantum confinement attributes. • High-Energy Secondary Absorption Peak (267.00 nm): A second distinctive absorption peak is recorded at 267.00 nm with a higher relative absorbance value of 0.95. In nanoparticle synthesis setups, auxiliary deep-UV absorption peaks in the high-energy 230 to 270 nm range typically indicate light interaction with localized electronic surface defect states, or the electronic excitation of organic functional capping agents present during synthesis that act as necessary stabilizing elements (Loganathan et al., 2021) [3] .

Scanning electron microscopy (SEM) analysis for nanoscale ZnO

SEM was performed to investigate the surface morphology and particle size of the synthesized zinc oxide (ZnO) nanoparticles. The representative SEM micrograph (Fig. 3.) revealed the formation of densely packed nanoparticles exhibiting predominantly irregular spherical to polyhedral morphology with moderate agglomeration. The observed agglomeration is a characteristic feature of ZnO nanoparticles and can be attributed to their high surface energy and strong van der Waals attractive forces during the drying process (Sangeetha et al., 2011) [9] . Despite the formation of aggregates, the individual nanoparticles remained distinguishable, indicating successful synthesis of nanosized ZnO particles. Direct particle size measurements obtained from the SEM image showed particle diameters of 32.55, 23.02 and 45.01 nm, corresponding to an average particle size of 33.53 nm. The particle size distribution (approximately 23-45 nm) confirmed the nanoscale dimensions of the synthesized ZnO. Nanoparticles within this size range possess a high surface area-to-volume ratio, thereby enhancing surface reactivity, foliar adhesion and nutrient availability following application to crop foliage (Dimkpa et al., 2018) [1] . The relatively small particle size is expected to improve zinc uptake efficiency by facilitating better penetration through the leaf cuticle and increasing interaction with plant tissues compared with conventional zinc fertilizers. Furthermore, the SEM micrograph exhibited a rough and porous surface texture, indicating extensive nucleation and crystal growth during synthesis. Such porous morphology is advantageous for nano-fertilizer applications because it increases the contact surface between nanoparticles and leaf tissues, thereby promoting gradual dissolution and sustained release of Zn²⁺ ions after foliar application (Kah et al., 2018) [3] . Similar morphological characteristics of irregular spherical ZnO nanoparticles with moderate agglomeration have been reported by Sangeetha et al. (2011) [9] who observed particle sizes ranging from 30-50 nm for chemically synthesized ZnO nanoparticles.

The morphology observed in the present investigation agrees well with previous reports on ZnO nanoparticles synthesized through chemical precipitation and sol-gel methods, confirming the successful formation of nanocrystalline ZnO. Collectively, the SEM observations, together with UV-Visible spectroscopy analyses, confirmed the synthesis of high-purity ZnO nanoparticles with an average particle size of 33.53 nm, making them suitable for foliar application to improve micronutrient use efficiency and crop performance.

Dynamic Light Scattering (DLS) Analysis of ZnO Nanoparticles

The particle size distribution of the synthesized zinc oxide (ZnO) nanoparticles determined through Dynamic Light Scattering (DLS) is shown in Fig. 4. The DLS profile exhibited a single intense peak, indicating the formation of a relatively homogeneous nanoparticle suspension. The synthesized ZnO nanoparticles recorded an average hydrodynamic particle size of 35.5 nmconfirming that the particles were within the nanoscale range. The narrow particle size distribution reflected the uniformity of the synthesized nanoparticles and suggested that the adopted synthesis method produced particles with consistent dimensions. The absence of secondary peaks in the distribution curve indicated that particle agglomeration was negligible and the nanoparticles remained well dispersed in the suspension. [7,9] .

Conclusion

Zinc oxide nanoparticles were successfully synthesized using the oxalate decomposition method and characterized by UV-Visible spectroscopy, SEM and DLS analyses. The UV-Visible spectrum confirmed nanoparticle formation with a characteristic absorption peak at 373 nm and an optical band gap of 3.32 eV. SEM analysis revealed irregular spherical to polyhedral nanoparticles with an average particle size of 37.09 nm, while DLS analysis showed a uniform particle size distribution with an average hydrodynamic diameter of 35.5 nm. The combined characterization confirmed the successful synthesis of stable ZnO nanoparticles with desirable optical, morphological and dispersion properties. These results indicate that the synthesized ZnO nanoparticles are suitable for foliar application as a nano-zinc fertilizer in sustainable crop production.

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