Millet-Extract-Mediated Green Synthesis of Fe₂O₃ Nanoparticles for Visible-Light-Driven Photocatalytic Degradation of Rhodamine B

Millet-Extract-Mediated Green Synthesis of FeO Nanoparticles for Visible-Light-Driven Photocatalytic Degradation of Rhodamine B

Bandi Ashok (1) , Bandari Raju (1), (2) M. Satyanarayana, (1)A. Rajasri

(1) Department of Physics, SRR Government Arts & Science College, Karimnagar-505 001

(2) Department of Physics, Government Degree College, Rajendranagar, Rangareddy, Hydereabad-505 218

Abstract – Iron oxide nanoparticles (FeO NPs) were successfully synthesized through a sustainable green synthesis route employing aqueous extracts of finger millet, pearl millet, and foxtail millet. The naturally occurring phytochemicals in the millet extracts acted as reducing, capping, and stabilizing agents, facilitating the formation and stabilization of the nanoparticles. The synthesized FeO NPs were systematically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier-transform infrared spectroscopy (FTIR). Their photocatalytic activity was investigated by monitoring the degradation of Rhodamine B (RhB) dye under visible-light irradiation. Among the synthesized materials, FeO NPs prepared using finger millet extract exhibited the highest photocatalytic efficiency, achieving

stability, and remarkable magnetic properties. These characteristics enable their application in several fields, including biomedicine, heterogeneous catalysis, environmental remediation, chemical sensing, cosmetics, and wastewater treatment. Several conventional techniques have been developed for the synthesis of iron-based nanomaterials, including chemical reduction, thermal decomposition, pyrolysis, photochemical and electrochemical methods, ultraviolet irradiation, laser ablation, and lithographic approaches [4,5].

Although these methods can produce nanoparticles with controlled properties, they commonly involve toxic or hazardous chemicals, including hydrazine, sodium borohydride, sodium citrate, and hydroxylamine, as reducing or stabilizing agents. The extensive use of such chemical reagents can result in undesirable environmental impacts and

approximately

%92degradation of RhB within 120min.

may pose potential risks to biological systems. Consequently,

The enhanced photocatalytic performance demonstrates the potential of the synthesized FeO NPs as effective photocatalysts for the treatment of dye-contaminated wastewater. Furthermore, the utilization of readily available millet extracts provides a simple, economical, environmentally benign, and sustainable alternative to conventional chemical synthesis methods

Key Words: Green synthesis, Nanoparticles, Millet extract, Photocatalysis, RhB degradation

1. INTRODUCTION

   Nanomaterials have emerged as promising materials of scientific and technological importance owing to their distinctive physicochemical characteristics and diverse reaction pathways, offering considerable potential for addressing contemporary environmental and biomedical challenges [13]. Among the various classes of nanomaterials, iron oxide-based nanoparticles have received significant research interest because of their natural abundance, high surface-area-to-volume ratio, excellent chemical and thermal

   green synthesis strategies have gained considerable attention as safer and more sustainable alternatives for producing iron- based nanoparticles. These approaches generally offer advantages such as reduced environmental impact, lower energy consumption, simplicity, and cost-effectiveness. In particular, plant-mediated nanoparticle synthesis has become increasingly attractive because plants are readily available, renewable, and rich in naturally occurring phytochemicals. Plant extracts contain a diverse range of biologically active constituents, including polyphenols, flavonoids, proteins, terpenoids, tannins, alkaloids, and other antioxidant compounds. These phytochemicals can participate in nanoparticle formation by facilitating the reduction of metal ions while also contributing to particle stabilization and surface functionalization. Thus, plant-based synthesis provides an environmentally compatible route for the preparation of iron oxide nanoparticles with potential applications in environmental remediation and other advanced technologies.

2. MATERIALS AND METHODS

   1. Materials

      Millet grains (finger millet, pearl millet, foxtail millet) were procured from a local market. Ferric chloride (FeCl) and ferrous sulfate (FeSO) of analytical grade were used without further purification. Distilled water was used throughout the experiment

   2. Preparation of Millet Extract

      The millet grains were initially washed several times with distilled water to eliminate adhering impurities and subsequently dried at room temperature. The dried grains were then pulverized into a fine powder using a mechanical grinder. Approximately 10 g of the resulting millet powder was dispersed in 100 mL of distilled water and maintained at

      70 °C for 20 min under heating. After cooling to room temperature, the suspension was filtered through Whatman No. 1 filter paper to remove insoluble residues and obtain a clear aqueous millet extract for further synthesis.

   3. Synthesis of Iron Nanoparticles

   For the green synthesis of FeO nanoparticles, 50 mL of a 0.05 M aqueous FeCl solution was prepared as the iron precursor. Subsequently, 50 mL of the freshly prepared aqueous millet extract was added slowly, dropwise, to the FeCl solution under continuous magnetic stirring at 60 °C. The reaction mixture was maintained under constant stirring for 45 min to facilitate the interaction of iron ions with the phytochemical constituents present in the millet extract. During the reaction, the color of the mixture gradually changed from pale brown to dark brown, indicating the formation of iron-based nanoparticles. The resulting suspension was then allowed to age at room temperature to promote further nucleation and stabilization of the nanoparticles. The product was subsequently separated, washed thoroughly with distilled water, dried, and calcined to obtain FeO nanoparticles for further characterization and photocatalytic studies.

3. RESULT AND DESCUSSION

   1. XRD Analysis

      The X-ray diffraction (XRD) patterns of the FeO nanoparticles synthesized using finger millet, pearl millet, and foxtail millet extracts are presented in Fig. 1. All three samples display well-defined diffraction reflections at approximately 24°, 33°, 35°, 49°, and 54° in the 2 range.

      Fig-1. XRD of Iron oxide nanoparticles exctrcts of finger, pearl and foxtail millets

      These reflections can be indexed to the (012), (104), (110), (024), and (116) crystallographic planes, respectively, and are characteristic of the rhombohedral hematite (-FeO) phase. The observed peak positions are consistent with the standard JCPDS reference pattern No. 33-0664, confirming the formation of crystalline -FeO nanoparticles through the millet-mediated green synthesis route [4,6].

   2. Scanning Electron Microscope (SEM)

      The scanning electron microscopy (SEM) micrographs provide information on the surface morphology of the FeO nanoparticles synthesized using three different millet extracts, namely finger millet (Ragi), pearl millet (Bajra), and foxtail millet. As shown in Fig. 2(a), the FeO nanoparticles synthesized using finger millet extract exhibit a predominantly sherical morphology with relatively uniform particle distribution and smooth surfaces. The particles are well dispersed, with sizes mainly within the nanometer range of approximately 1030 nm. Only a limited degree of particle agglomeration is observed, suggesting that the phytochemical constituents present in the finger millet extract may have contributed effectively to the capping and stabilization of the nanoparticles during synthesis [710].

      Fig-2. SEM of Iron oxide nanoparticles of (a) finger(b) pearl and(c) foxtail millets exctracts

      The SEM image in Fig.2(b) reveals that the FeO nanoparticles synthesized using pearl millet (Bajra) extract possess predominantly spherical to semi- spherical morphology, with particle sizes ranging from approximately 20 50nm. Compared with the nanoparticles obtained from finger millet extract, these particles are relatively larger and show a moderate degree of agglomeration, which may be attributed to the comparatively lower stabilization of the nanoparticles by phytochemical constituents present in the extract. In contrast, the FeO nanoparticles synthesized using foxtail millet extract exhibit irregular and semi-spherical morphologies, along with partially hollow structures and pronounced aggregation. The particles display a broader size distribution and comparatively lower uniformity, suggesting that the phytochemicals in the foxtail millet extract provide less effective capping and stabilization during nanoparticle formation [11,.]12

   3. Energy Dispersive X-ray (EDX) spectra

      The Energy-Dispersive X-ray (EDX) spectra shown in Fig. 3 provide evidence of the elemental composition of the iron oxide nanoparticles synthesized using the extracts of three different millets, namely finger millet (Ragi), pearl millet (Bajra), and foxtail millet

      Fig-3. EDX of Iron oxide nanoparticles of (a) finger(b) pearl and(c) foxtail millets exctracts

   4. Fourier transform infrared Spectra (FTIR)

   The FTIR spectra of the iron oxide (FeO) nanoparticles synthesized using finger millet (Ragi), pearl millet (Bajra), and foxtail millet extracts provide valuable information regarding the functional groups involved in nanoparticle synthesis and stabilization. As illustrated in Fig. 4, all three samples display a broad absorption band near 3400 cm¹, which can be attributed to OH stretching vibrations associated with phenolic and alcoholic groups. The absorption bands observed

   around 2920 cm¹ are assigned to CH stretching vibrations, while the band near 1650 cm¹ can be related to C=O stretching vibrations of protein-associated groups. The band appearing at approximately 1380 cm¹ corresponds to CN stretching, whereas the absorption around 1150 cm¹ is associated with CO stretching vibrations of polysaccharide-related compounds. These characteristic bands indicate the participation of various phytochemicals and biomolecules present in the millet extracts during nanoparticle formation. In addition, the absorption band observed in the 550600 cm¹ region is attributed to FeO vibrational modes, providing evidence for the formation of iron oxide nanoparticles [12].

   Among the three samples, the nanoparticles synthesized with finger millet extract exhibit relatively intense and well-defined FTIR bands, suggesting a greater contribution of phytochemical constituents to the reduction and stabilization processes. The pearl millet- derived nanoparticles display intermediate peak intensities, whereas the foxtail millet sample shows comparatively broader and less intense absorption bands, which may indicate differences in phytochemical composition and capping ability. Overall, the FTIR findings demonstrate that biomolecules present in millet extracts contribute significantly to the green synthesis of FeO nanoparticles by acting as both reducing and stabilizing/capping agents [1315].

   Fig-4. FTIR spectra of Iron oxide nanoparticles of all millets exctracts

4. PHOTOCATALYSIS

   The photocatalytic performance of the FeO nanoparticles synthesized using finger millet (Ragi), pearl millet (Bajra), and foxtail millet extracts was investigated by monitoring the degradation of Rhodamine B (RhB), a widely studied organic dye pollutant, under visible-light irradiation using a 300 W lamp. As shown in Fig. 5(a), the RhB solution without a photocatalyst exhibited only about 10 % degradation, indicating that direct photolysis under the employed irradiation conditions was negligible. In contrast, the FeO nanoparticles prepared with the different millet extracts demonstrated considerably enhanced photocatalytic activity, although their efficiencies varied depending on the extract employed during synthesis. The FeO nanoparticles synthesized using finger millet extract exhibited the highest degradation efficiency, reaching approximately 92% after 240 min of visible- light irradiation, as presented in Fig. 5(b). This was followed by the nanoparticles synthesized using pearl millet and foxtail millet extracts, which achieved degradation efficiencies of about 85% and 78%, respectively.

   Fig-5. Concentration changes of Rh B over Iron oxidenanoparticles of millets exctracts along with the blank experiment. (b).

   Degradation % of Rh B in the presence of Iron oxidenanoparticles of millets exctracts along with pure Rh B.

   Figure 6(ab) presents the time-dependent UV-Vis absorption profiles used to monitor the photocatalytic performance of Iron oxidenanoparticles of finger millets exctracts which are show highest PCA against Rh B under visible-light illumination. The steady, pronounced attenuation of the characteristic Rh B.absorption peak with increasing exposure time directly demonstrates the progressive breakdown of the dye’s chromophoric structure. This continuous decline in absorbance intensity confirms the active visible-light- driven degradation of Rh B.while the comparative

   curves in Figure 6(a) and 6(b) clearly illustrate the relative kinetic efficiencies and superior catalytic performance Iron oxidenanoparticlesof finger millets exctracts

   The enhanced photocatalytic performance of the finger millet-mediated FeO nanoparticles may be associated with their relatively smaller particle size, greater accessible surface area, and increased availability of surface-active sites. These characteristics can promote more effective interaction between the photocatalyst and RhB molecules and facilitate the generation of reactive oxygen species (ROS), particularly hydroxyl radicals (OH) and superoxide radicals (O), under visible-light irradiation. The generated reactive species contribute to the oxidative decomposition of RhB into smaller and potentially less harmful intermediates.

   Fig-6. UV-VIS absorption spectra of )a (Pure RhB )b (RhB with Iron oxide nanoparticlesof finger millets exctracts

   The comparatively lower photocatalytic efficiencies of the pearl millet- and foxtail millet-derived nanoparticles may be related to greater particle aggregation and reduced accessibility of active surface sites. Overall, the results indicate that the type of millet extract used during green synthesis has a significant influence on the physicochemical characteristics and photocatalytic behavior of FeO nanoparticles. Among the investigated extracts, finger millet exhibited the most favorable performance as a biogenic reducing and stabilizing medium for producing FeO nanoparticles with promising potential for photocatalytic environmental remediation [1417].

5. SCAVENGER TEST

   The scavenger experiments conducted with the millet-mediated FeO nanoparticles provide valuable insight into the reactive species involved in the

   photocatalytic degradation of Rhodamine B. As shown in Fig. 7, the addition of benzoquinone (BQ) resulted in the most pronounced decrease in degradation efficiency for the finger millet (Ragi)-mediated FeO nanoparticles, which exhibited the highest photocatlytic performance.

   This significant inhibition indicates that superoxide radicals (O) are the predominant reactive species contributing to RhB degradation. The introduction of isopropanol (IPA) also caused a substantial reduction in photocatalytic efficiency, demonstrating the important contribution of hydroxyl radicals (OH) to the degradation process.

   Fig-7. Scavenger test of RhB with Iron oxide nanoparticlesof finger millets exctracts

   In comparison, the presence of EDTA produced a relatively smaller decline in degradation efficiency, suggesting that photogenerated holes (h) have a comparatively secondary role in the photocatalytic reaction. Among the synthesized samples, the Ragi- mediated FeO nanoparticles demonstrated the highest photocatalytic activity, followed by the Bajra- and foxtail millet-derived nanoparticles, consistent with the degradation trends observed in the photocatalytic experiments [1819].

6. CONCLUSION

The present investigation establishes an effective green synthesis route for iron oxide nanoparticles using extracts of finger millet (Ragi), pearl millet (Bajra), and foxtail millet. This biological approach offers a simple, economical, and environmentally benign alternative to conventional

nanoparticle synthesis, with naturally occurring phytochemicals serving as reducing and capping agents. The resulting nanoparticles exhibited notable photocatalytic efficiency toward Rhodamine B degradation under visible-light irradiation, achieving removal efficiencies of 92%, 85%, and 78% for the Ragi-, Bajra-, and foxtail millet-derived nanoparticles, respectively. Scavenger experiments indicated that benzoquinone produced the greatest suppression of photocatalytic activity, suggesting that superoxide radicals (O) are the major reactive species involved in RhB degradation, while hydroxyl radicals (OH) and photogenerated holes (h) also contribute to the reaction. Among the investigated samples, the Ragi-mediated FeO nanoparticles exhibited the highest photocatalytic performance, followed by the Bajra- and foxtail millet- derived materials. These findings demonstrate that the choice of millet extract plays an important role in determining the properties and photocatalytic performance of the synthesized nanoparticles, highlighting their potential application in sustainable wastewater treatment and environmental remediation.

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