Magnetically separable Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites: stable and efficient sunlight-driven photocatalyst for environmental remediation

Abstract

Nanomaterials have recently gained significant interest as they are believed to offer an outstanding prospect for use in environmental remediation. Among many possible candidates, due to their useful properties including magnetic nature, wide surface area, and high absorptivity, ferrite materials hold tremendous appeal, allowing them to be used for multifaceted applications. In the present study, using a sol–gel auto combustion process, a magnetically separable Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0, 0.25, 0.50, 0.75, 1.0) ferrite with superior photocatalytic activity for dye degradation was manufactured. Rietveld refinement and FTIR studies confirm that a single-phase cubic spinel system was built for all samples with crystallite sizes of 34–57 nm. VSM has determined the magnetic properties of the samples at room temperature. With the introduction of Mg²⁺ and Co²⁺ in the Zn ferrites, a transformation from the soft superparamagnetic activity to the hard ferromagnetic character was reported. Considering the band structure in the visible region, the photocatalytic activities of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites for the degradation of the MB dye under natural sunlight were investigated. Zn_0.25Co_0.375Mg_0.375Fe₂O₄ showed an efficiency of degradation of 99.23% for MB dye with a quick 40 min irradiation period with high reusability of up to four cycles.

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1. Introduction

Spinel ferrites have been extensively studied for several decades because of their versatile and promising magnetic, optical, electrical, and chemical properties, which often vary from their bulk counterpart.^1,2 Some important applications (including magnetic sensors, catalysis, MRI imaging, supercapacitors, and antibacterial agents) attract spinal ferrites.^3–7 Recently, research studies have more focused on the synthesis of spinel ferrites with a general formula of AFe₂O₄, where A stands for metals, such as Co, Mg, Zn, Ni, Mn. Several forms of magnetic ferrites, such as manganese ferrite (MnFe₂O₄), cobalt ferrite (CoFe₂O₄), zinc ferrite (ZnFe₂O₄), and copper ferrite (CuFe₂O₄), have been reported to exhibit excellent catalytic and photocatalytic operation. Various chemical methods have been thoroughly investigated for the synthesis of well-dispersed spinel ferries, such as solvothermal, chemical co-precipitation, microwave, and sol–gel auto combustion.^8–11 Due to its simplicity, lower time and energy, and environmental safety, the sol–gel auto combustion method has attracted considerable attention among the various methods.

In the past decade, environmental pollution has increased the extreme overall anxiety. For instance, the release of industrial wastewater containing synthetic pigments, phenolic derivatives, and organic colouring contributes to significant environmental hazards. These contaminants are non-degradable and thus cause the aquatic life to have a serious problem. Nanomaterials play a very vital role in the degradation of effluents released from dyes and textile industries.^12,13 As photocatalysts that act as sensitizers for light-induced redox processes, various semiconducting materials (such as TiO₂, ZnO, and Sn₃O₄) have customarily been used.^14–16 Recently, spinel ferrites are important photocatalysts used for environmental remediation.^17,18 Its stability, abundant availability, non-toxicity, and simple synthesis make it an economically and environmentally interesting choice for the removal of aqueous organic contaminants in AOPs.^19–21 The spinel ferrites exhibit a bandgap in the visible region. In view of the bandgap in the visible region, several researchers have synthesized various ferrites, and researched their morphological and optical properties. For example, Cao et al. studied the photocatalytic degradation of the MB dye using the CES-derived zinc ferrite,²² and Yang et al. demonstrated the fabrication of Cr and Mn-doped zinc ferrite nanospheres using industrial waste. The synthesized ferrites were tested for the degradation of Congo red dye.²³ Mohammad et al. reported the hydrothermal route for the synthesis of cobalt ferrites, revealing an enhanced activity for the degradation of Reactive red 4 dye.²⁴ Borhan et al. designed the ZnFe_2−xAl_xO₄ ferrites using the sol–gel auto combustion method for the photodegradation of Orange I dye.²⁵ Mahmoodi et al. reported the degradation of Reactive red 120 and Reactive red 198 dye using magnetically separable zinc ferrites.²⁶ Satya et al. studied the removal of cationic triarylmethane dyes in contaminated water using spinel cobalt ferrites.²⁷ Ikram et al. have reported on CoFe₂O₄ and MnFe₂O₄ nanocrystals by microwave-assisted and co-precipitation methods, respectively, and the catalyst showed excellent results in the photodegradation of the MB dye.²⁸ These studies highlighted that while some great work has been done on photocatalysis, the catalytic performance remains unsatisfactory, and further work is still needed to explore new approaches to improve it. In this context, the present work is the first report on the synthesis of Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites and its photocatalytic activity under natural sunlight.

Herein, we synthesized Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites using a cost-effective and simple sol–gel auto combustion method. The photocatalytic activity of the as-synthesized ferrites under solar light irradiation was tested for degradation of the methylene blue dye. The magnetic, physicochemical, optical, and morphological properties of the as-synthesized Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites were elucidated by characterization using various analytical techniques. A plausible photocatalytic mechanism for the degradation of the MB dye in the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites was also suggested.

2. Experimental details

2.1 Synthesis

Zn(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O, Mg(NO₃)₂·6H₂O, and Fe(NO₃)₃·9H₂O were procured from Merck (India). Cobalt and magnesium-doped Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0, 0.25, 0.50, 0.75, 1.0) ferrites were synthesized using the sol–gel auto combustion method. During synthesis, the aqueous solution was prepared by taking all of the nitrates and glycine in a 1 : 3 molar ratio in water. The as-formed homogeneous mixture was stirred well, and the pH of the reaction mixture was adjusted to 7–8 with liq. NH₃ to form “sol”. To obtain a viscous gel, the viscous fluid was condensed or hardened to form the ‘gel’, and the sol was placed on the hot plate at 90 °C. On the hot plate, this homogeneous gel was ignited. The glycine was then completely combusted, yielding Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites. The as-synthesized samples of ferrites were ground well, and then annealed in the furnace at 800 °C for five hours. The obtained samples were labeled as ZF1 (X = 0.0), ZCMF1 (X = 0.25), ZCMF2 (X = 0.50), ZCMF3 (X = 0.75) and as CMF (X = 1.0), respectively, and were characterized using various techniques.

2.2 Characterization

This XRD pattern of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites was recorded on Philips (Xpert) X-ray diffractometer using Cu K_α radiation having wavelength 1.540 Å at room temperature. The morphology investigation of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites can be done using JEOL JSM-7600F, FEG-SEM instrumentation. The optical properties were recorded by UV-visible spectrophotometer (Shimadzu make model 1800). The analysis of the magnetic properties was carried out by Quantum Design USA to make the SQUID system (Model MPMS XL). FTIR spectra of all samples were recorded on a 3000 Hyperion Microscope with vertex 80 FTIR using KBr pellets in the range of 400 to 4000 cm⁻¹.

2.3 Photocatalytic activity

The as-synthesized Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites were tested using methylene blue (MB) dye as a model contaminant for its photodegradation behavior. For the photoreactor, the Pyrex beaker used was 250 mL. To ensure healthy adsorption, 100 mL, 10 ppm aqueous MB dye solution containing 0.2 g of the catalyst was stirred in the dark before sunlight irradiation. They were exposed to sunlight after 1 hour of suspension. Reaction samples were collected at regular intervals, and a UV-visible spectrophotometer was used to check the residual MB dye content.

3. Results & discussion

The crystalline structure of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites was investigated by the Rietveld refinement XRD pattern. Fig. 1 shows the XRD and Rietveld refinement patterns of cobalt and magnesium-doped Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0 to 1.0) ferrites. Form the figure, all diffraction peaks in the XRD spectra were indexed to the (111), (220), (311), (222), (400), (422), (511), and (440) planes of the cubic spinel phase. The measured diffraction peaks of ZF, ZCMF1, ZCMF2, ZCMF3, and CMF ferrites are in accordance with JCPDS card no. 22-1012.²⁹ The as-synthesized ferrites are polycrystalline in a single phase without any other impurity phases. It can also be found that the substitution of Zn by the substance of Co–Mg does not alter the structure of the cubic spinel zinc ferrite.

Fig. 1 (a) XRD and (b) Rietveld refinement spectra of Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0 to 1.0) ferrites.

The efficiency of refinement is defined by the consideration of the goodness-of-fit (χ²). The findings obtained in the Rietveld refinement on the X-ray diffraction of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites systems are listed in Table 1. From Table 1, the Rietveld refinement of the goodness-of-fit (χ²) ranged from 1.05 to 1.82, and the low value of the fitting factor is attributed as an excellent result value. The profile fitting is better with lower χ² values; hence, the procedure adopted for the profile fitting is by minimizing the χ² function. Fig. 2 and Table 1 demonstrate the theoretical and observed lattice constant of Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0 to 1.0) ferrites, and the values are very much in agreement. Fig. 2 shows that the lattice parameter decreases from (x = 0.0 to 1.0), and that the decrease in the lattice parameters is in accordance with Vegards law.³⁰ Furthermore, the close observations from the figure indicate that the a_th value is greater than the a_obs value. This is because the ideal spinel structure was considered a rigid sphere for measuring the a_th value with cations and anions. The crystallite size of the as-synthesized ferrites was calculated using the Debye–Scherrer equation, and the most intense 311 peak was chosen for measuring the crystallite size:³⁰

d = 0.9λ/β cos θ

where d is the size of the crystallite (nm), θ is the peak angle, λ is the wavelength of the X-ray beam, and β is the full width at half maximum (FWHM) of prominent intense peak measured in radians. The size of the crystallite decreases (x = 0.0 to 1.0), as shown in Table 1, from 57.25 nm to 34.19 nm. The decrease in the crystallite size is due to the substitution of a large ionic radius of Zn²⁺ ions with a smaller ionic radius of Co²⁺ and Mg²⁺.³¹ Sharma et al. reported similar variation behavior in the size of the crystallite, with very limited ion doping.³² From the lattice parameter, the X-ray density of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites was measured. With the increasing concentration of Co²⁺ and Mg²⁺ (x = 0.0 to 1.0), there is a steady decrease in the X-ray density. The decrease in the crystallinity with a decrease in the lattice parameter may be the key cause for decreasing the X-ray density.³³

Table 1 Goodness of fit (χ²), lattice parameter (observed (a_obs) and theoretical (a_th)), radii of the tetrahedral (r_A) and octahedral (r_B) sites, oxygen position parameter (u), crystallite size (d), and X-ray density

Comp.	χ^2	aobs (Å)	ath (Å)	rA (Å)	rB (Å)	u (Å)	d (nm)	dx (g cm^−3)
ZF	1.82	8.4539	8.463	0.74	0.67	0.3906	57.25	5.2739
ZCMF1	1.4	8.4412	8.448	0.739	0.6702	0.3908	52.43	5.1736
ZCMF2	1.26	8.4280	8.429	0.737	0.6704	0.391	48.57	5.0878
ZCMF3	1.05	8.4062	8.407	0.736	0.6705	0.3912	42.13	4.9872
CMF	1.47	8.3856	8.385	0.735	0.6707	0.3915	34.19	4.8974

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Fig. 2 Theoretical and observed lattice parameters of Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0 to 1.0) ferrites.

The Bertaut method was used in spinel ferrite to assess the site occupancy.³⁴ This is calculated from the study of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0 to 1.0) X-ray diffraction pattern. The exact information on the cation distribution was obtained by comparing the intensity ratio (experimental and calculated) for the reflections, whose intensities: (i) vary with cation distribution in opposite ways, (ii) are almost independent of the oxygen position parameter, and (iii) are not significantly different. The distribution of cations at the tetrahedral site and octahedral site is shown in Table 1. The trivalent Fe³⁺ ions are distributed both at the tetrahedral and octahedral sites due to the simultaneous doping of cobalt and magnesium ions. Cobalt ions push Fe³⁺ ions to the tetrahedral sites, and this results in the distribution of Fe³⁺ ions over the tetrahedral A and octahedral B sites. However, it does show maximum occupancy at the B site. This is due to the greater ionic radius of the Zn²⁺ ion (0.82 Å). It appears to live on the tetrahedral site, and displaces the smaller Fe³⁺ (0.67 Å) ions and Co²⁺ (0.74 Å) ions at the expense of the Mg²⁺ (0.66 Å) ions from the tetrahedral sites to the octahedral sites. The data given in Table 2 shows that the octahedral sites are primarily occupied by Fe³⁺ ions. This is simply due to the increased energy from the octahedral sites. The radius of the tetrahedral A site (r_A) and the radius of the octahedral B site (r_B) were calculated, and are depicted in Table 1 using a modified relation discussed elsewhere.³⁵ With the increased concentration of cobalt and magnesium, and the decreased concentration of zinc, the r_A and r_B values increased. Similar results have been reported in the cobalt substituted in Ni–Zn ferrites.³⁶ The oxygen positional parameter (u) values are tabulated in Table 1. The value of the oxygen positional parameter is nearly equal to 0.375 Å in the spinel structure. The as-synthesized samples show a small increment in the oxygen positional parameter (u) value in comparison to the ideal value. These may be attributed to different causes, experimental mistakes or failures in the calculation. For most samples, u > 0.375 is obtained from the expansion of the tetrahedral interstitials due to the slight displacement of anions. The spatial coordinates of oxygen (u) in this series are always greater than 0.375, which can be due to the displacement of anions from the ideal positions.³⁵ The disturbance in the lattice is confirmed from the data of the lattice constants and oxygen positional parameters (u).

Table 2 Cation distribution of Zn_1−xCo_0.5xMg_0.5xFe₂O₄

Comp.	Chemical formulae	Occupancy of cations
		A site	B site
ZF	ZnFe2O4	Zn1	Fe2
ZCMF1	Zn0.75Co0.125Mg0.125Fe2O4	Zn0.75Mg0.0125Co0.1125Fe0.225	Mg0.1125Co0.0125Fe1.775
ZCMF2	Zn0.50Co0.25Mg0.25Fe2O4	Zn0.50Mg0.12Co0.015Fe0.365	Mg0.13Co0.235Fe1.635
ZCMF3	Zn0.25Co0.375Mg0.375Fe2O4	Zn0.25Mg0.2Co0.02Fe0.53	Mg0.175Co0.355Fe1.47
CMF	Co0.50Mg0.50Fe2O4	Mg0.3Co0.025Fe0.675	Mg0.2Co0.350Fe1.325

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Infrared spectroscopy is an effective spectroscopic technique for obtaining information about the position of the metal ions in the crystal lattice through the presence of various vibrational modes in the crystal lattice.³⁷ Fig. 3a shows the FTIR spectra of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites. Two characteristic bands are present; the lower frequency band at 424 cm⁻¹ corresponds to the intrinsic vibrations of the octahedral sites, whereas the higher frequency band at 545 cm⁻¹ corresponds to the intrinsic vibrations of the tetrahedral sites. From Fig. 3b, it can be observed that with an increase in the content of Co²⁺ and Mg²⁺ (ZF to CMF), the low-frequency band (M_tet–O) slightly shifted to the lower frequency side, whereas the higher frequency band (M_oct–O) slightly shifted to the higher frequency side. These shifts are observed due to the changes in mass at sites A and B, such as replacing the tetrahedral Zn²⁺ ion with the lighter Co²⁺ and Mg²⁺ ions, at the same time as both ions transfer a few Fe³⁺ ions from the octahedral site to the tetrahedral site. The shift in the peaks of the A sites is greater than B sites, which indicates that the Co²⁺ and Mg²⁺ ions preferably go to the B sites, and some of the Fe³⁺ ions transfer to the A sites, resulting in substantial increases in the stretching tetrahedral vibrations. The same results are also confirmed from the cation distribution analysis.³⁸

Fig. 3 FTIR absorption spectra of Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0 to 1.0) ferrites (a) in the range of 400–4000 cm⁻¹ and (b) in the range of 400 to 700 cm⁻¹.

Morphological analysis of the as-synthesized Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites was achieved using field emission scanning electron microscopy (FESEM). Fig. 4 displays the FESEM images of the as-synthesized cobalt and magnesium-doped Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites, respectively. From Fig. 4a, it can be seen that the high magnification image at 200 nm depicts the agglomerated morphology made up of secondary irregular shaped spherical nanoparticles. The size of these nanoparticles is found in the range of 40–70 nm. With cobalt and magnesium-doping in Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (ZCMF1, ZCMF2, ZCMF3, CMF), the greater agglomeration of the spherically formed nanoparticles was observed with a subsequent reduction in the crystalline size. The agglomeration for the higher concentrations of cobalt and magnesium is due to the ferromagnetic Co²⁺ substitution and diamagnetic Zn²⁺ replacement. More crystallinity was observed for the undoped ZF1 sample. The agglomeration can be attributed to the magnetic nature of the samples, and this result was also confirmed by VSM analysis. Energy-dispersive X-ray analysis of the as-synthesized Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrite was performed to investigate the elemental composition. Fig. 4b depicts the peaks of the Zn, Fe, O elements for undoped ZnFe₂O₄ (ZF1). Fig. 4d, f and h showed the peaks of the Zn, Co, Mg, Fe, O elements for ZCMF1, ZCMF2, and ZCMF3 ferrites, respectively. The sample CMF shows the peaks of Co, Mg, Fe, O elements. The stoichiometric ratios used for the synthesis of nanoferrites closely matches with the corresponding EDS results. No impurity peaks observed in the as-synthesized nanoferrites indicates the purity of the synthesized nanoferrites.

Fig. 4 FESEM and EDS images of the as-synthesized (a and b) ZF, (c and d) ZCMF1, (e and f) ZCMF2, (g and h) ZCMF3, and (i and j) CMF, respectively.

Fig. 5 shows the magnetic hysteresis loops that were recorded at room temperature for the as-synthesized Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites. The magnetic parameters obtained from the hysteresis loops, such as the remanent magnetization (M_r), saturation magnetization (M_s), coercivity (H_c), squareness ratio (M_r/M_s), and magnetic moment (n_B), are summarized in Table 3. When the Co²⁺ and Mg²⁺ dopants are introduced into a soft paramagnetic ZnFe₂O₄, they are transformed into hard ferrimagnetic materials. The M_s value of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrite nanoparticles depends on the distribution of the magnetic Co²⁺ (d⁷) and Fe³⁺ (d⁵) ions, as well as both Mg²⁺ (d⁰) and Zn²⁺ (d¹⁰) ions, which are non-magnetic in nature between the A sites (tetrahedral) and B sites (octahedral). According to the previous literature, ZnFe₂O₄ displays a regular spinel structure, where the Zn²⁺ ions occupy the tetrahedral (A) sites, and the octahedral (B) sites are occupied by Fe³⁺ ions. When the Zn²⁺ ions are replaced by Co²⁺ or Mg²⁺ ions, they always prefer the octahedral positions. This leads to a reverse spinal structure, and move the Fe³⁺ ions from the octahedral B sites to tetrahedral A sites.³⁹ This will contribute to Fe³⁺ ion redistribution on the tetrahedral and octahedral sites. In the present work, it was found that the M_s value (Fig. 6a) of the nanoparticles slowly increased with the increase of the content of Co²⁺ and Mg²⁺ until x = 0.75, which is observed due to the magnetic Co²⁺ ions occupying the octahedral B sites preferentially, and then unexpectedly decreased if x was greater than 0.75, i.e., CMF as indicated in Table 3. The decrease in the magnetization was observed as the concentration of the Mg²⁺ ions increased, and the non-magnetic Mg²⁺ tends to occupy the octahedral B sites that move the magnetic Fe³⁺ ions to the tetrahedral A site. The remnant magnetization (Fig. 6b) and coercivity (Fig. 6c) are gradually increased with the introduction of Co²⁺ and Mg²⁺ dopants in ZnFe₂O₄, which suggests that the soft magnetic materials are transformed into hard magnetic materials. Furthermore, Table 3 demonstrates the superparamagnetic nature of samples ZF to ZCMF2 (x = 0.0 to x = 0.50), and the composition x = 0.75 and 1.0 (ZCMF3 and CMF) have M_r/M_s ratios greater than 0.10. Hence, these samples exhibit ferromagnetism.⁴⁰

Fig. 5 Magnetic hysteresis loops of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites.

Table 3 Saturation magnetization (M_s), remnant magnetization (M_r), coercivity (H_c), squareness ratio (M_r/M_s), and magnetic moment (n_B) of the synthesized Zn_1−xCo_0.5xMg_0.5xFe₂O₄ nanoferrites at room temperature

Comp.	Ms emu g^−1	Mr emu g^−1	Mr/Ms	Hc Oe	nB μB
ZF	6.16	0.0019	0.00031	8.32	0.26
ZCMF1	22.59	0.0097	0.00043	12.05	0.94
ZCMF2	46.27	4.1493	0.08968	26.29	1.88
ZCMF3	76.13	21.8424	0.28691	92.34	3.02
CMF	63.09	28.1948	0.4469	216.47	2.43

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Fig. 6 VSM results of Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0 to 1.0) ferrites: (a) saturation magnetization (M_s), (b) remanent magnetization (M_r), and (c) coercivity (H_c).

The UV-visible spectra of the as-synthesized Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites are shown in Fig. 7. The Tauc plot (inset: Fig. 7) of (αhν)² against the photon energy (hν) was plotted to clarify the bandgap. The spectra show the broad absorption in the UV-visible region for the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites, and commonly observed cubic spinel phase. The approximate ZF1, ZCMF1, ZCMF2, ZCMF3, and CMF band gaps were found to be 2.43, 2.31, 2.18, 2.01, and 2.07 eV, respectively. In addition, as the cobalt and magnesium content increased (ZCMF1 to ZCMF3), the ferrite band gap continued to decrease, supporting the substitution of the Co²⁺ and Mg²⁺ ions for Zn²⁺. Nevertheless, a further rise in the band gap was observed at the CMF ferrite. This may be due to the complete replacement of Zn²⁺ by Co²⁺ and Mg²⁺. All of the samples of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites could absorb a significant amount of sunlight due to an electronic transition from Fe 3d–O 2p.⁴¹

Fig. 7 UV-visible absorption spectra of Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.0 to 1.0) ferrites (inset: (αhν)² vs. photon energy (hν) spectra for the calculation of the optical band gap by extrapolating on the hν axis at α = 0).

The photocatalytic efficiency of the Zn_1−xCo_0.5xMg_0.5xFe₂O₄ (x = 0.00, 0.25, 0.50, 0.75, 1.0) ferrites was studied for the degradation of methylene blue dye under natural sunlight. Fig. 8a shows the degradation of the MB dye with respect to time over the Co and Mg doped Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites. Furthermore, a controlled experiment without any catalyst was conducted for comparison. The MB dye was scarcely degraded in the absence of a catalyst, but MB dye degradation was observed in the presence of a photocatalyst. We note that the degradation rate of ZF is 93% within 80 min. By comparison, when the ZF ferrites are changed with the doping of cobalt and magnesium, the degradation levels of MB over the doped Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites also surprisingly increase. The ZCMF3 sample shows the greatest amount of degradation under sunlight irradiation. The degradation rate of MB reaches up to a maximal 75% within 20 min, and completely disappeared after irradiation for 40 min. Moreover, the degradation rate was substantially higher than previous records.^42–44 However, upon further increasing the content of Co and Mg, the degradation rate starts to reduce, which could be caused by the cation distribution, as well as increase in the bandgap of CMF. The kinetic equation of the first-order reaction is used to match the photocatalytic degradation reaction of the MB dye, in order to allow a direct quantitative comparison. The ZF, ZCMF1, ZCMF2, ZCMF3 and CMF reaction rate constants were 0.032, 0.039, 0.054, 0.108 and 0.072 min⁻¹, respectively. The ferrite rate constant of ZCMF3 is greater than that of other ferrites. Thus, the ZCMF3 ferrite has shown strong photodegradation speed with excellent linear correlation (Table 4), indicating pseudo-first-order kinetics. The improved photodegradation activity of the as-synthesized ZCMF3 ferrites may be attributed to proper band structure, morphology, and crystallinity.

Fig. 8 (a) A plot of the change in absorbance vs. irradiation time in the presence of the different Zn_1−xCo_0.5xMg_0.5xFe₂O₄ photocatalysts. (b) Effect of catalyst loading on the Zn_0.25Co_0.375Mg_0.375Fe₂O₄ photocatalyst. (c) Recyclability of the Zn_0.25Co_0.375Mg_0.375Fe₂O₄ photocatalyst. (d) XRD spectra of Zn_0.25Co_0.375Mg_0.375Fe₂O₄ after photocatalytic treatment.

Table 4 Summary of the pseudo-first-order kinetics for the photocatalytic MB dye degradation using different Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites

Catalyst	k (min^−1)	R^2
ZF	0.032	0.94
ZCMF1	0.039	0.94
ZCMF2	0.054	0.95
ZCMF3	0.108	0.99
CMF	0.072	0.98

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The strong spectral response of the Zn_0.25Co_0.375Mg_0.375Fe₂O₄ photocatalyst was investigated for the degradation of MB dye. Fig. 8b displays the effect of catalyst loading on the MB dye degradation. The Zn_0.25Co_0.375Mg_0.375Fe₂O₄ percentage of catalyst loading by weight ranged from 0.1 g to 0.25 g. When the catalyst load is 0.15 g, the degradation of the MB dye reaches maximum (completely disappear within 45 min). Nevertheless, when increasing the content of Zn_0.25Co_0.375Mg_0.375Fe₂O₄, the rate of degradation starts to decrease, which may be attributable to the shielding effect that weakens the light harvesting ability of the photocatalyst.⁴⁵ In addition, given the stability and recyclability of the photocatalysts, we test the stability and reproducibility of Zn_0.25Co_0.375Mg_0.375Fe₂O₄ by the cyclic degradation of the MB dye under sunlight irradiation. Fig. 8c indicates that the degradation rates of the MB dye have evidently not decreased as a result of a four-fold consecutive photocatalytic reaction, indicating that Zn_0.25Co_0.375Mg_0.375Fe₂O₄ is superior in stability and reusability. In addition, as seen in Fig. 8d, the Zn_0.25Co_0.375Mg_0.375Fe₂O₄ XRD was performed before and after the photocatalytic degradation reaction of the MB dye. It is worth noting that after four consecutive cycles, the Zn_0.25Co_0.375Mg_0.375Fe₂O₄ XRD pattern has almost no difference, which further shows that it has outstanding reliability and reusability.

Based on the above results (Fig. 9a), we proposed the photocatalytic mechanism using Zn_0.25Co_0.375Mg_0.375Fe₂O₄ under sunlight irradiation. Initially, the sunlight falls on the Zn_0.25Co_0.375Mg_0.375Fe₂O₄ photocatalyst surface, activating the ferrite, and pairs of e⁻/h⁺ are formed. The electrons in the Zn_0.25Co_0.375Mg_0.375Fe₂O₄ valence band become excited as the holes are left in the valence band. The holes are used for the oxidation process in the valence band, while the electrons are used for the reduction process in the conduction band. The reactive species (OH˙, O₂⁻˙) that are produced decay the methylene blue dye in CO₂ and H₂O.⁴⁶ In this degradation, the hydroxyl ion plays a significant role in the photodegradation of the methylene blue dye. The presence and absence of the hydroxyl radical scavenger, such as isopropyl alcohol, was monitored to investigate the development of a photodegradation reaction. The rate constant decreased significantly with the inclusion of 0.1 mL of IPA, as shown in Fig. 9b. The photodegradation of the MB dye in the presence of Zn_0.25Co_0.375Mg_0.375Fe₂O₄ and sunlight is responsible for promoting the formation.

Fig. 9 (a) Possible mechanism for the photocatalytic MB dye degradation under sunlight irradiation using Zn_0.25Co_0.375Mg_0.375Fe₂O₄. (b) Effect of the IPA scavenger on the photodegradation efficiency of methylene blue using Zn_0.25Co_0.375Mg_0.375Fe₂O₄.

The bandgap of the ferrites Zn_1−xCo_0.5xMg_0.5xFe₂O₄ lies in the visible light field (2.01–2.43 eV), which is known from optical measurements. As described in the optical properties section, the narrowing of the bandgap of ZnCMF3 was responsible for the greater activity. This makes the band structure of ZnCMF3 better for solar light optical absorption, as well as charge carrier separation. As a result, ZnCMF3 exhibits stronger MB dye degradation over other ferrites. In addition, the VSM study of the ZnCMF3 ferrites reveals that the increased cobalt ions in the octahedral sites are responsible for the high photocatalytic activity. More precisely, the MB degradation of ZnCMF3 is much higher than in previous studies (Table 5).^22,42,43,47

Table 5 Comparison of previous research reports of methylene blue dye degradation using ferrites

Sr. no.	Photocatalysts material used	Dye soln. tested	Optimized catalyst & quantity	Light source used	Remark	References
1	MnFe2O4/rGO	10 ppm MB dye	0.03 g	40 W UV lamp	97% degradation within 60 min	Mandal et al.;^42
2	CoFe2O4	10 ppm MB dye	0.01 g	Visible light	80% degradation in 140 min	Kalam et al.;^43
3	ZnO/ZnFe2O4	10 ppm MB dye	0.08 g	300 W Xe arc lamp (UV & visible light)	98% degradation in 240 min	Yan Xu et al.;^47
4	ZnFe2O4/Fe2O3	10 ppm MB dye	0.5 g	75 W Hg lamp as UV source	88.27% degradation in 120 min	Cao et al.;^22
5	Zn1−xCo0.5xMg0.5xFe2O4	10 ppm MB dye	0.2 g	Sunlight	99% degradation 40 min	Current work

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4. Conclusion

The present study reports the synthesis of Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites through the sol–gel auto combustion method. The Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites demonstrate high photocatalytic activity under sunlight irradiation for the degradation of the MB dye. Different characterization techniques characterized the prepared Zn_1−xCo_0.5xMg_0.5xFe₂O₄ ferrites, which confirmed the formation of the single-phase cubic spinel structure with space group Fd3m. When compared to other ferrites, the Zn_0.25Co_0.375Mg_0.375Fe₂O₄ ferrite displays excellent degradation capability for the MB dye. The Zn_0.25Co_0.375Mg_0.375Fe₂O₄ ferrite could be regenerated up to four times with the photocatalytic output negligibly degrading. The present low-cost magnetic recyclable Zn_0.25Co_0.375Mg_0.375Fe₂O₄ ferrite can be used to mitigate the multi-industry environmental problems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

K. S. J. and K. D. P. are very thankful to TIFR for use of the instrumentation facilities, Mr Nilesh Kulkarni for the XRD facility, Mrs Bhagyashri Chalke for SEM-EDAX, Mr Ganesh Jangam for the SQUID data and SAIF IIT BOMBAY for FTIR. P. S. M. is grateful to Principal Dr B. B. Sharma for the provision of all infrastructure facilities and assistance during this work.

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