Synthesis of FeCoCrO4 Nanoparticles Using ginger and Malva sylvestris Extracts as Green Capping Agents: Investigation Their Magnetic Properties

INTRODUCTION
In recent years, metal oxide nanoparticles have received considerable attention due to their unique physical and chemical properties, especially in terms of magnetic, electrical, and catalytic behavior [1-3]. Among these, mixed metal oxides stand out owing to synergistic effects arising from the combination of multiple metal components, which often result in enhanced thermal stability, improved structural integrity, and superior functional performance compared to single-metal oxides [4,5].
Magnetic properties of nanoparticles are of particular interest, especially as particle size is reduced to the nanoscale where the high surface-to-volume ratio and altered electronic band structures emerge. These changes have enabled the application of magnetic nanoparticles in a variety of fields including targeted drug delivery, magnetic resonance imaging (MRI), magnetic hyperthermia, and magnetic separation [6-8].
However, these properties are highly sensitive to factors such as chemical composition, particle size, synthesis method, and surface modifications [9,10]. Various synthetic routes have been employed to the preparation of mixed metal oxides, including sol-gel, auto-combustion sol-gel, co-precipitation, hydrothermal and thermal decomposition [11,12]. Of these, co-precipitation method has received tremendous attraction due to cost-effective, simple, scalability, and versatility. Recently, numerous works have been devoted to the co-precipitation of nanoparticles. For example, preparation of CdO/CuO/ZnO mixed metal oxide nanocomposite via co-precipitation method has been reported for anticancer activity application [13]. Kandasamy et al. used co-precipitation method for synthesis of ZnO/NiO/Co3O4 nanocomposite for solar cell application [14].
Despite its significant advantages, controlling size and stabilization of nanoparticles during synthesis by co-precipitation method remains challenging [15]. To address this, it is imperative to use of chemicals such as surfactants, polymers, and other capping agents. Due to environmental and toxicological concerns, green alternatives with lower ecological impacts are attracting considerable interest. The surface modification of nanoparticles using green capping agents has recently gained momentum due to its environmentally friendly and ability to enhance nanoparticle dispersion, reduce agglomeration, and tailor surface-related properties [16,17].
In the present work, two natural plant extracts, ginger and Malva sylvestris, were employed as capping and stabilizing agents during the synthesis process. The plant extracts contain wide variety of organic compounds, including flavonoids, terpenoids, alkaloids, and polyphenols, which have potential capability serving as steric stabilizer [18,19]. This study presents the co-precipitation synthesis of the ternary metal oxide nanoparticles, FeCoCrO4 (FCCO), in the presence of different green extracts. For comparison, the FCCO was synthesized in pure aqueous medium to assess the effectiveness of the used plant extracts in stabilizing and controlling the particle size. The vibrating sample magnetometer (VSM) analysis was employed to investigate the effect of particle size on the magnetic behavior of the synthesized nanoparticles. 

MATERIALS AND METHODS
Preparation of green extract
5.0 g of the ginger powder and Malva sylvestris were separately weighted and dispersed into 100 mL of deionized water and then heated at 80 °C for 1 h. After that, the mixture was isolated from the solids using paper filter. The filtrates were kept in refrigerator for following uses.

Synthesis of FeCoCrO4 nanoparticles (FCCO)
The co-precipitation method was used to synthesize the FCCO nanoparticles. Different green capping agents were used to control the size of nanoparticles. The metal precursors were dissolved into 50 mL of the as-prepared green extract, including Cr2(SO4)3.xH2O (1.0 mmol), Fe(NO3).9H2O (1.0 mmol) and Co(NO3).6H2O (1.0 mmol). The solution was stirred for 1 h at 50 °C. Then, pH of the solution was adjusted to 9 using addition of NaOH (0.1 M). The solution was heated at 80 °C under constant stirring to remove water by evaporation, leading to formation of the dark brown sticky gel. The gel was dried at 120 °C and then calcined at 600 °C for 6 h. The similar route was followed in pure water medium to synthesize of FCCO without addition of capping agent. Fig. 1 shows the schematic illustration of the steric stabilization of the synthesized nanoparticles using green capping agent.    

Characterization
Composition and crystalline phase of the prepared nanoparticles were studied using X-ray diffraction (XRD) patterns by X’pert Pro MPD Philipls (Cu Kα, λ = 1.54 Å). Functional groups of the nanoparticles were detected using Fourier transform infrared (FTIR) spectroscopy by Bruker Tensor 27 FTIR spectrometer. Morphology of the prepared nanoparticles was studied using field emission scanning microscope (FESEM) (TESCAN BRNO-Mira3 LMU). Magnetic measurements were studied using vibrating sample magnetometer (VSM) by VSM MDKB.  

RESULTS AND DISCUSSION
The XRD patterns of the synthesized nanoparticles are presented in Fig. 2, revealing the successful synthesis of cubic phase of FeCoCrO4 nanoparticles. All the diffraction planes corresponded to the Bragg positions in JCPDS file no. 01-076-2496 without observing secondary phase. For a cubic structure lattice constants (a = b = c; α = β = γ = 90°) can be determined as follows:

, where d is the lattice distance, a is the lattice constant, h, k, and l are indexes of diffraction planes. By considering the reflection of (311) at 2ϴ = 35.6°, the lattice constants for the samples synthesizing in the different conditions were obtained, which are tabulated in Table1. Also, the average crystallite size for the different nanoparticles was calculated using Scherrer equation [20], reported in Table 1.
The results are in agreement with expectations, the lattice constants and crystallite sizes decreased by using of the green extract as the capping agent. The used green extracts have the significant potential serving as the efficient steric agents to limit the particle growth by hampering their accumulations.
The composition of the synthesized nanoparticles was further studied using the FTIR spectroscopy. Fig. 3a-c show the FTIR spectra of the nanoparticles before calcination at 600 °C. The existence of the functional groups corresponded to the green extracts clearly confirmed the surface modification of the nanoparticles using the capping agents. The O-H bending vibration, C-O stretching vibration, and C-H bonds are occurred at around 1600, 1400, and 2900 cm-1, respectively, which are attributed to the organic compounds within the green extracts [21,22]. The sample synthesized without addition of capping agents is obviously lack of these characteristic vibration bands. After calcination of the nanoparticles, the FTIR spectra (Fig. 3d-f) exhibit the pure mixed metal oxide composition, revealing removal of the surface adsorbed organic compounds as the result of the heat treatment. The broad peaks centered 3400 cm-1 are related to the O-H vibrational modes due to the adsorbed water molecules [22]. The strong bands in the range 600-400 cm-1 are assigned to the metallic bonds, confirming the formation of the mixed metal oxides. According to the literature [23], the FCCO nanoparticles possess the spinel structure; therefore, the observed bands at around 600 and 400 cm-1 are reasonably attributed to the metal-oxygen vibrations at the tetrahedral (A) and octahedral (B) sites of the spinel lattice, respectively [24,25].
The FESEM images in Fig. 4 show the morphology and surface properties of the synthesized nanoparticles using the different green capping agents. In accordance with XRD results, the observed decrement in the particle sizes highlighted the capability of the used capping agents to limit crystallite aggregation by controlling the nucleation ant growth processes. As can be seen, the sample synthesized in pure water medium (Fig. 4a) exhibits the particle size in the hundreds of nanometers along with strong agglomeration, whereas the appreciable size  reduction  is observed for the nanoparticles synthesized by the ginger (Fig. 4b) and Malva sylvestris (Fig. 4c) extracts. The FESEM images for the samples synthesized using the green capping agent exhibit very fine nanoparticles with the sizes below 50 nm. 
Due to the high surface energy of the fine nanoparticles, they tend to agglomerate, leading to the formation of secondary fragments at the micrometer scale. These fragments consist of the spherical nanoparticles, which are more clearly observed at the higher magnification (Fig. 4d and 4e). Additionally, Fig. 4f-g present the particle size distribution for the FCCO nanoparticles synthesized in different conditions. As seen, the sample prepared using the ginger extract has the narrower size distribution. Compared to the Malva sylvestris extract, the organic compounds within the ginger extract seem to provide the greater steric hindrance, resulting in the smaller FCCO nanoparticles. Moreover, the compositional analysis for the synthesized FCCO nanoparticles was performed using the EDX spectroscopy (Fig. 4h-j), confirming the existence of the constituents of the synthesized nanoparticles. 
Given that the FCCO nanoparticles are composed of the Cr3+ (d3), Fe3+ (d5), and Co2+ (d7), the magnetic behavior is primarily attributed to the magnetic moments originating from unpaired d electrons of these ions, which contribute to the net magnetization of the system [26,27]. The magnetic behavior of the synthesized CCFZO nanoparticles was studied using VSM analysis. Fig. 5 shows the room temperature M-H curves for the different FCCO nanoparticles, confirming the ferromagnetic properties for the nanoparticles. 
According to the literature [28,29], the saturation magnetization (Ms), coercivity (Hc), and remnant magnetization (Mr) are affected by the particle size. As seen from Fig. 5, the Ms decreases with reduction in particle size, so that FCCO nanoparticles synthesized by ginger extract has the lowest value of Ms (29.5 emu/g). The Ms values for the FCCO nanoparticles synthesized in pure water and Malva sylvestris extract are 88.9 and 43.4 emu/g, respectively. Due to the enhanced surface spin disorder, decrement in the particle size lead to decrease in the saturation magnetization [30].  Moreover, the decrease in the particle size leads to the change in the term of multi-domain particle into single-domain particle, reducing the coercivity (Hc) [31]. The Hc values are 215.45, 191.51, and 211.37 Oe for the different FCCO nanoparticles, respectively synthesized in pure water, ginger, and Malva sylvestris extract. Additionally, the remnant magnetization exhibits the size-dependent behavior, where the Mr decreases with the reduction in particle size, attributed to the utilization of the green extracts. In this regard, using ginger extract as capping agent led to the decrease in the Mr from 7.9 to 1.8 emu/g. Moreover, the magnetic moment (nB) was affected by the size of the FCCO nanoparticle, as calculated by the following equation (Eq. 2) [32]: 

where M is the molecular weight of the nanoparticles, Ms is the saturation magnetization, and the constant 5585 is used to convert the magnetization and molecular weight into the magnetic moment in Bohr magnetons per metal ion.  
In addition, the anisotropy constant K was determined for the different synthesized FCCO nanoparticles. To end this, the following equation (Eq. 3) was employed [33]:

  
As expected, the variations in both the magnetic moment and anisotropy constant were consistent with the changes in the FCCO particle size. Table 2 summarizes the magnetic measurements for the synthesized FCCO nanoparticles.

CONCLUSION
In this work, the FeCoCrO4 (FCCO) nanoparticles were synthesized using the simple co-precipitation method. Different green capping agents, ginger and Malva sylvestris extracts, were used to stabilize the nanoparticles and control their sizes. The crystal structure investigations were performed for the synthesized nanoparticles, which confirmed the cubic phase for the FCCO nanoparticles. It was found that crystallite size and lattice constants changed through application of green capping agents. The FESEM images showed that the FCCO particle size decreased when ginger and Malva sylvestris extracts were used as the reaction medium for synthesizing the nanoparticles. The magnetic behavior of the FCCO nanoparticles was studied using VSM analysis, showing the size dependence of magnetic properties. The Ms for the FCCO synthesized in aqueous medium was 88.9 emu/g, whereas this value decreased to 29.5 emu/g by using of ginger extract. Similar trends were observed for the Hc and Mr, where FCCO synthesized in pure water and ginger extract media exhibited the coercivities of 215. 45 and 191.51 Oe and remnant magnetizations of 7.9 and 1.8 emu/g, respectively.

CONFLICT OF INTEREST 
The authors declare that there is no conflict of interest regarding the publication of this manuscript.