Green Synthesis of Magnetic Fe–Co Bimetallic Nanoparticles and Their Photocatalytic Activity

Abstract

The leaves of the Murraya koenigii aromatic plant contain various specific phytochemicals, including lutein, β-carotene, vitamin C, nicotinic acids, and other polyphenols, which act as reducing agents to produce metallic nanoparticles from their respective precursors. In this study, we report the green synthesis of iron–cobalt bimetallic nanoparticles (Fe–Co BMNPs) using natural resources of reducing and capping agents from aqueous extract of Murraya koenigii leaves. The synthesized Fe–Co BMNPs were characterized using SEM, EDS, BET surface area, TGA, XRD, TEM, and VSM techniques, revealing their crystalline structure with a surface area of 83.22 m² g⁻¹ and particle sizes <50 nm. Furthermore, the photocatalytic ability of the synthesized Fe–Co BMNPs was examined concerning methylene blue dye (MBD) aqueous solution. The synthesized Fe–Co BMNPs exhibited promising potential for dye removal from aqueous solution in acidic and basic medium (>97% of 10 mg L⁻¹).

1. Introduction

Dyes are widely used as raw materials in various applications including paper and textile industries. For instance, methylene blue dye (MBD) bearing an aromatic ring is a cationic thiazine dye used to color silk, wool, cotton, etc. [1]. Moreover, MBD is water-soluble, carcinogenic, non-biodegradable, and resistant to light and temperature in water [1]. Its inhalation causes respiratory problems [2]. Thus, MBD-contaminated water is a severe threat to the aquatic ecosystem and human beings’ health [1]. Among various reported techniques for treating dye-contaminated water, photocatalysis based on metal oxide nanoparticles has attracted significant attention because of the optoelectronic properties, chemical stability, and ability to generate e⁻/h⁺ pairs based on species very reactive to degrade dyes [3,4]. Metal oxides such as cobalt oxide nanoparticles are useful in diverse applications, including photocatalysis, sensing, medical, and electrical devices [4,5,6]. The specific properties such as high surface area, favorable band gap, multivalent oxidation states, and catalytic ability make them attractive transition metal nanoparticles [5]. Several methods, such as sol–gel, sonochemical, thermal decomposition, laser ablation, hydrothermal, co-precipitation, and green synthesis, have been reported in the literature to synthesize cobalt nanoparticles [5,7,8,9]. Among these, the plant-extract-mediated green approach provides several advantages for iron and cobalt nanoparticle synthesis compared to other methods, such as being simple, inexpensive, eco-friendly, non-toxic, versatile, and cost-effective [9,10,11]. The plant-extract-mediated green synthesis approach eliminated the requirement of reducing and stabilizing chemical agents during the synthesis of nanoparticles from their respective metal ion precursors [12,13]. The specific biomolecules in plant extracts, such as polyphenols, flavonoids, and alkaloids, act as good reducing and stabilizing agents for synthesizing stable metallic nanoparticles [14]. For instance, Vinyagam et al. [5] reported cobalt oxide nanoparticle synthesis using Muntingia calabura leaf extract and the synthesized nanoparticles’ photocatalytic ability was examined for dye-contaminated water under natural sunlight irradiation. The prepared nanoparticles showed significant photocatalytic activity with a specific surface area of 10.31 m² g⁻¹ and particle size of 27.59 nm. Shanmuganathan et al. [15] reported cobalt oxide nanoparticle synthesis via a green approach using Curcuma longa root extract. SEM, XRD, and DLS analysis confirmed the formation of spherical, crystalline cobalt oxide nanoparticles with an average size distribution of 97.5 ± 35.1 nm. In addition, synthesized nanoparticles exhibited excellent photocatalytic degradation of methyl orange, methyl red, and methyl blue dye in aqueous solution along with antifungal, antibacterial, and antioxidant activity. Nevertheless, metallic nanoparticles alone as a catalyst or an adsorbent also have limitations. For instance, metallic nanoparticles are very small in size (<100 nm). Thus, during their application in dye degradation from contaminated water, there is greater chance of secondary contamination by nanoparticles because of the insufficient recovery of metallic nanoparticles from treated water [16]. Therefore, effective and easily separable metallic nanoparticles are highly essential for the remediation of contaminated water without the creation of secondary pollution. Considering the advantages of the plant-mediated green synthesis approach and need for an easily separable photocatalyst, iron–cobalt bimetallic nanoparticles (Fe–Co BMNPs) were prepared using the extract of Murraya koenigii leaf. Bimetallic nanoparticles are attracting more attention for catalytic reactions [17,18]. The combination of iron along with the cobalt nanoparticles can provide better separation ability from treated water along with catalytic/adsorption ability [19,20]. Murraya koenigii (curry leaves) is a commonly available medicinal plant with various bioactive compounds such as flavonoids, essential oils, alkaloids, carotene, nicotinic acids, and a high content of oxalic acids [21,22]. In recent years, green synthesis of various metallic nanoparticles (Fe, Cu, Zn, Ag, etc.) has been reported using extract of curry leaves [21,23,24]. However, magnetically recoverable green synthesis of Fe–Co BMNPs (surface area of 83.22 m² g⁻¹ and saturation magnetization value of 28.38 emu g⁻¹) has not been attempted yet using Murraya koenigii leaf extract. The photocatalytic dye degradation ability of the synthesized Fe–Co BMNPs was examined for removal of MBD from aqueous solution.

2. Materials and Methods

2.1. Materials

Cobalt nitrate (>96%) was purchased from Molychem. Ferrous chloride (>99%) was purchased from Molychem. Sodium hydroxide (pellets, ≥75%) and hydrochloric acid (HCl, ≥35%) were purchased from Loba Chemie. All the chemicals were used as received without further purification. Deionized (DI) water (18 MΩ) prepared all aqueous solutions. Murraya koenigii plant’s leaves were collected at the university’s campus (MM(DU), Mullana, Ambala) and washed 3–5 times with DI water.

2.2. Procedures for Preparation of Fe–Co BMNPs

2.2.1. Preparation of Murraya koenigii Plant Leaves Aqueous Extract

A total of 25 g of Murraya koenigii plant leaves was converted into small pieces by grinding, added to a beaker containing 100 mL DI water, and heated to boil at 80 °C for 1–2 h. The resultant extract was cooled, filtered via Whatman filter paper and stored in a clean glass vial for Fe–Co BMNPs synthesis [23].

2.2.2. Synthesis of Fe–Co BMNPs Using Murraya koenigii Plant Leaf Extract

Initially, 50 mL of aqueous leaf extract was heated at 80 °C with stirring on a magnetic stirrer [22,23]. Cobalt nitrate and ferrous chloride (0.5 M of each) were added to the heated aqueous extract, and the pH of the resulting solution was adjusted to approximately 11 by the appropriate addition of 1 M aqueous solution of NaOH. The heating process of this solution was continued along with stirring to form a dense product. The obtained viscous product was subjected to calcination at 400 °C for 2 h in a furnace (muffle). The resultant product of calcination process was washed several times using ethanol and water and then dried (Figure 1).

2.3. Characterization of Synthesized Fe–Co BMNPs

Fe–Co BMNPs were characterized using a Scanning Electron Microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively, for the analysis of morphological structure and elemental composition. The XRD (X-ray diffractometer) instrument was used for sample pattern recording from 10° to 80° at a scanning speed of 2°, 2θ per min. TGA (SII 6300 EXSTAR) analysis was performed (under N₂ atmosphere) to assess the thermal stability of the Fe–Co BMNPs at a heating rate of 10 °C min⁻¹. The particle size ranges of the synthesized Fe–Co BMNPs were determined through transmission electron microscopy (TEM; JEOL, JEM–1400).

The Quantachrome (BET method, Quantachrome, NOVA, 2200e, Boynton Beach, FL, USA) and VSM instruments (Make: Lakeshore, Model: 7410 series VSM system, Westerville, OH, USA) with magnetic fields were employed to analyze the surface area and magnetic properties of the synthesized Fe–Co based material, respectively [25,26].

2.4. Photocatalytic Ability of Fe–Co BMNPs

The dye degradation activity (photocatalytically) of the synthesized Fe–Co BMNPs was evaluated with the use of MBD in aqueous solution and natural sunlight exposure. In this experiment, 10 mL of 10 mg L⁻¹ MBD aqueous solution was taken in separate glass vials, and 25 mg of Fe–Co BMNPs were added in each vial and placed in dark for adsorption–desorption equilibrium (30 min). Later, the resultant suspension was subjected to sunlight exposure (in an open atmosphere). At definite intervals (0, 30, 60, 90, 120, and 135 min, etc.), equilibrated samples were collected and Fe–Co BMNPs were separated using a simple magnet, because of their magnetic properties. The final MBD concentration in each separated samples was determined using an ultraviolet–visible spectrophotometer (Spectramax ID3, Molecular Devices, LLC., San Jose, CA, USA) at room temperature in the scanning range of 350 nm to 700 nm. The pH of the aqueous MBD solution was maintained in acidic (4.0) and basic (9.0) medium in order to assess the effect of pH during the photocatalytic dye degradation study.

3. Results and Discussion

The surface morphology, elemental composition, and crystallinity of the synthesized Fe–Co BMNPs were examined using FESEM, EDS, and X-ray diffractometer analysis (XRD patterns). The Fe–Co BMNPs SEM image (Figure 2a) demonstrates the irregular and rough morphology appearance. The extent of aggregation may be due to the interaction with the phytochemicals of plant extract (hydrogen bonding), resulting in magnetostatic interaction (high surface energy of the nanoparticles) [5,27]. Figure 2b shows the EDS spectra of the synthesized Fe–Co BMNPs acquired from different portions of the synthesized nanoparticles. The obtained spectrum displays the peak area of Fe, Co, O, and C in the analyzed samples. The high signal and strong peak of 38.74% for Fe, 21.23% for Co, and 29.35% for O indicate the presence of Co and Fe in the form of oxide as main components of the synthesized materials. The obtained X-ray diffraction pattern from the X-ray diffractometer is expressed in Figure 2c. The crystalline phase of iron and cobalt oxides was identified. The appearance of strong peaks (2θ = 28.54, 33.06, 47.48, 56.32, 58.98,69.28, and 76.60) in the XRD pattern confirmed the crystallinity and high purity of the iron and cobalt oxide in the synthesized Fe–Co BMNPs [5,28,29]. The observed XRD pattern is similar to those obtained for cobalt iron oxide nanoparticles already reported in the literature [30,31]. It can be seen that >94% weight remains constant even after exposure to temperatures up to 800 °C. The Fe–Co BMNPs’ TGA curve is shown in Figure 2d. The observed small reduction in their mass may be due to moisture (water molecules) evaporation into a gaseous state as well as the combustion of organic molecules [32,33]. This observation inferred the high thermal stability of Fe–Co BMNPs. TEM image analysis shows the average particle size of the synthesized Fe–Co BMNPs was below 50 nm (Figure 3a,b). The plot of magnetization (M) and magnetic field (Figure 3c) shows a 28.38 emu g⁻¹ value of saturation magnetization (Ms). The Ms value is a key factor used to represent the magnetic strength of the prepared particles from the liquid phase (during separation of treated water) via simple external magnet [34]. This observed high value of Ms indicates that the prepared Fe–Co BMNPs possess magnetism and could be easily separated from the liquid phase with the help of a simple conventional magnet [26]. In addition, BET surface area was also measured via a N₂ adsorption–desorption isotherm (Figure 3d) [35]. The specific surface area was found to be 83.22 m² g⁻¹. The large surface area of the metallic nanoparticles is favorable to facilitate the higher active sites for catalytic reaction or sorption of water contaminants [13,30,36,37].

The photocatalytic dye removal activity of the synthesized Fe–Co BMNPs was evaluated under natural sunlight exposure by observing UV–Vis absorption spectra. The blue dye color disappeared within a certain period when 25 mg and 50 mg Fe–Co BMNPs were added separately in the 10 mg L⁻¹ MBD aqueous solution with a pH value of 4.0. Figure 4a,b depicts the absorbance spectra of changes in absorbance value (in the range of wavelength from 400 to 800 nm) for residual MBD concentration for different intervals of time (0, 30, 60, 90, 120, and 135 min, etc.) of sunlight exposure. The wavelength of 664 nm was observed as the lambda max of MBD aqueous solution [38,39]. Interestingly, it was also observed that the degradation efficacy of Fe–Co BMNPs was further increased when the pH of the MBD (concentration = 10 mg L⁻¹) aqueous solution was kept higher at approximately 9.0 (as depicted in Figure 4c,d).

At the pH of 9.0 of aqueous solution, the blue color of MBD was found to approximately disappear with an efficiency of 97% over 135 min and confirmed the effective catalytic activity for degradation of dye molecules. With the possibility of the higher number of adsorptive/reactive sites on the surface Fe–Co BMNPs with large surface area (83.22 m² g⁻¹), a moderate band gap and the formation of photo-generated electron–hole pairs at the hetero-junction surface may be the main reasons for MBD degradation in the presence of sunlight with high efficiency [36,40,41,42]. MBD is a cationic dye. The higher pH of the solution favors the electrostatic attraction between cationic MBD and the photocatalyst surface and enhances the generation of reactive oxygen species by photon-induced electron-hole pairs [9,40].

4. Conclusions

This study reported the Murraya koenigii leaf-extract-mediated green synthesis of magnetically separable Fe–Co BMNPs. The results of characterization analysis using XRD pattern, VSM, and SEM-EDS revealed the formation of the crystalline structure of Fe–Co BMNPs with the major components of Fe (38.74%), Co (21.23%), oxygen (29.35%), and a saturation magnetization value of 28.38 emu g⁻¹. Furthermore, results of photocatalytic analysis inferred that the Fe–Co BMNPs degraded >97% of 10 mg L⁻¹ MBD within 135 min of exposure to sunlight. Thus, the study’s results confirmed the efficient photocatalytic activity of the Fe–Co BMNPs.

5. Patents

The “preparation methodology of magnetic Fe–Co bimetallic photocatalyst” described in this study is subject of the Indian patent application number: 202411028940.