Efficient photo-Fenton degradation of an organic dye by reusable magnetic (Al_0.6Mn_0.6Fe_0.6Co_0.6Ni_0.6)O₄ high entropy oxides

Abstract

High entropy oxides (HEOs) have recently emerged as potential candidates for the catalytic degradation of organic dyes due to their interesting functional properties, such as high structural integrity and diversified elemental compositions. Herein, we report the development of an HEO nanostructure comprising Al, Mn, Fe, Co, and Ni with a chemical composition of (Al_0.6Mn_0.6Fe_0.6Co_0.6Ni_0.6)O₄ using the sol–gel method, followed by calcination at ∼700 °C in air. The formation of single-phased HEO nanostructures was evident from XRD and TEM analyses, whereas the presence of Al, Mn, Fe, Co, and Ni cations was confirmed by STEM-EDS, flame atomic absorption spectroscopy and XPS studies. This HEO nanostructure (average size of ∼25 nm) exhibited ferrimagnetic ordering behavior at room temperature (∼20 emu g⁻¹) with a specific surface area of 22.9 m² g⁻¹. The UV-visible DRS, Mott–Schottky and impedance results demonstrated the ability of the HEO nanostructure to degrade organic dyes through Fenton-type catalytic reactions, photon absorption, or photo-Fenton-type reactions. It is observed that the organic dye, Methylene Blue (MB), was degraded up to 94% within 90 min under the photo-Fenton catalytic process, while only 60% and 85% degradation of MB was observed under photocatalytic and Fenton-type reactions, respectively. The successful mineralization of MB into smaller molecules is evident from mass spectroscopic analysis. Specifically, the magnetic HEO with multivalent cations (Fe, Mn, Ni and Co) could make it a reusable catalyst for the photo-Fenton degradation of organic dyes.

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1. Introduction

Dyes are vital substances used in many industries, such as textiles, printing, cosmetics, and food, to give different colours to substrates. They are chromophoric compounds that can interact physically or chemically with substrates to absorb certain light wavelengths and display specific colours selectively. These compounds usually have conjugated systems of π-electrons, which help them to absorb light and display specific colors. When a dye is exposed to energy, usually heat or light, the transitions of electrons within the conjugated system cause the dye to absorb light.¹ An essential aspect of human civilization has long been the use of dyes. The study of dyes covers a wide variety of topics, from the early usage of natural sources to contemporary synthetic complex chemical compounds. The evolution of dyes highlights human ingenuity in utilizing natural resources. Early civilizations used plants, minerals, and animal-derived materials to colour fabrics, serving practical purposes while reflecting cultural significance.² Numerous industries, including food, cosmetics, paper printing, and textiles, utilize synthetic colours extensively. The textile sector uses them the most.³ Nevertheless, over the past ten years, environmental concerns related to the manufacture and use of dyestuffs have increased dramatically, and they are unquestionably one of the main concerns influencing human health today.^3,4

Wastewater from textile dyeing is a significant part of industrial pollutants. An estimated 40% of the seventy thousand tons of dye employed annually in the textile sector are predicted to become contaminants and pose a threat to environmental health eventually.⁵ The textile dye business contributes 17%–20% of all industrial dye sectors.⁶ These synthetic dyes are organic substances classified as direct, reactive, acidic, and basic but are highly soluble in water. Due to their high-water solubility, dyes are more challenging to remove from water using standard methods.⁷

Textile dyes are categorized based on their industrial usage or chemical structure (such as triphenyl methyl, nitrated, phthalein, anthraquinone, nitro, azo and indigo dyes).^8,9 Most of these dyes have carcinogenic, poisonous, and mutagenic properties. As a result, they negatively impact ecology.^10–12 These technologies are beneficial; however, they are unable to break down organic contaminants in industrial effluent fully.^13,14 However, the removal of dyes from water bodies requires the application of effective wastewater treatment methods, such as adsorption, filtration, and advanced oxidation processes.^15,16

Advanced oxidation processes (AOPs) frequently produce radicals, including ˙OH and ˙SO₄⁻, although inactivated oxidants exhibit less oxidation activity. Three main reaction types, addition, hydrogen abstraction, and electron transfer, have been identified to be involved in the breakdown of organic molecules by free radicals. The addition and hydrogen abstraction reactions are more likely to occur in ˙OH, while the electron transfer reaction is more likely to occur in ˙SO₄⁻.¹⁷ AOPs are more effective at treating water than traditional treatment technologies because they have a greater oxidation capacity, a faster rate of reaction, fewer secondary pollutants, softer reaction conditions, and a wider range of applications.¹⁸

Amongst the AOPs, the Fenton and photo-Fenton reactions are exciting because they produce hydroxyl radicals (˙OH) when the Fe²⁺ ion combines with hydrogen peroxide (H₂O₂). The organic molecules are rapidly broken down into smaller components, including CO₂ and H₂O molecules, by the highly reactive ˙OH radical.¹⁹ However, properly restoring the catalyst following each cycle is a significant challenge in the Fenton and photo-Fenton processes. However, magnetic catalysts are easily isolated from treated water solutions using an external permanent magnet. Thus, they may be recycled due to their inherent magnetic qualities, which also greatly lowers the chance of secondary contamination.²⁰ When compared to alternative techniques, transition metal activation is simple to scale up and offers the benefits of high activity and low energy usage. Cobalt, manganese, copper, iron, and specific compounds containing one or more of these transition metals are often used as catalysts.^21,22

In this context, high entropy oxides (HEOs) offer a promising alternative to current catalysts. These materials, composed of five or more cations in equiatomic or non-equiatomic ratios, share similarities with high entropy alloys (HEAs). The key criteria for HEAs are as follows: (i) at least five elements with 5–35 at % composition, (ii) mixing entropy exceeding 1.5R, (iii) mixing enthalpy ranging from −16 to 5 kJ mol⁻¹, (iv) atomic size mismatch below 8.5%, and (v) valence electron concentration under 6.87 or above 8.²³ With five or more metallic elements in a single oxide structure, HEOs have a range of improved features like mechanical strength, electrical and magnetic capabilities, and thermal stability. Rost et al. found the first high entropy oxide (HEO), (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)O, in 2015. The conventional solid-state method created this single-phase rock salt-structured HEO.²⁴ Recent studies have reported HEOs with perovskite, fluorite, and spinel structures, demonstrating their potential for applications such as energy storage, hydrogen production, supercapacitors, water splitting, microwave absorption, CO₂ conversion, and electrocatalysis. Due to their nanostructured form and unique physical and chemical properties, these materials stand out. Among them, spinel-structured HEOs exhibit comparatively superior performance in Li-ion batteries, catalysis, electrolysis, microwave absorption, electrocatalysis, supercapacitors, and water splitting.^25–28

In this study, we report the development of (Al_0.6Mn_0.6Fe_0.6Co_0.6Ni_0.6)O₄ HEOs using the sol–gel method, followed by calcination at 700 °C in open air, and investigate their effectiveness for treating organic dye, methylene blue (MB), using photocatalytic, Fenton, and photo-Fenton catalytic processes. Additionally, the molecular mechanisms of MB degradation and its precise structural, magnetic, optical and electrical properties were investigated. It is noteworthy that up to 95% of the MB was degraded within 90 min under the photo-Fenton catalytic process. Specifically, multivalent elements such as Fe, Mn, Ni and Co facilitate redox reactions and generate electron (e⁻)–hole (h⁺) under UV light to disintegrate the MB dye. Moreover, magnetic HEO with multivalent cations can be reused as an efficient catalyst for the photo-Fenton degradation of organic dyes.

2. Experimental details

2.1 Materials and method

The sol–gel method was utilized to produce a material characterized by the chemical formula (Al_0.6Mn_0.6Fe_0.6Co_0.6Ni_0.6)O₄. This synthesis involved the use of precise quantities of iron chloride (FeCl₃, 99%), nickel chloride hexahydrate (NiCl₂·6H₂O, 98%), manganese chloride tetrahydrate (MnCl₂·4H₂O, 99%), cobalt chloride hexahydrate (CoCl₂·6H₂O, 98%), and aluminium chloride (AlCl₃, 98%) as precursor materials. These chemicals were procured from the Central Drug House (CDH India). The synthesis commenced with the dissolution of the metal components in an equiatomic ratio in deionized (DI) water at ambient temperature, accompanied by continuous magnetic stirring. Following the complete dissolution of the metal chlorides, citric acid was introduced into the solution as a coordinating agent, maintaining a molar ratio of metal ions to citric acid at 1 : 3. The mixture was stirred for 2 h at room temperature to achieve homogeneity. The solution was then heated on a hot plate to around 80 °C, resulting in a transparent sol, which transitioned into a dark, viscous gel. This gel was dried in an oven at 200 °C for 24 h, yielding a foam-like material. The dried foam was then pulverized using a pestle and mortar to produce a fine powder, which was ultimately calcined in air at a temperature of 700 °C for 1.5 h.

2.2 Characterization

The X-ray diffraction (XRD) patterns were obtained using a BT-Rigaku Miniflex X-ray diffractometer with a wavelength of 1.54056 Å. Data acquisition was performed over a 10–90° range utilizing a step size of 0.02° and a scan rate of 5° min⁻¹. The crystalline phases in the samples were identified through an analysis of the powder diffraction patterns, and the Scherrer equation was applied to estimate the crystallite size. For transmission electron microscopy (TEM) characterization, CM 200 (PHILIPS) and TALOS F200X (FEI) microscopes were used, and these microscopes were operated at an acceleration voltage of 200 kV. Selected area electron diffraction (SAED) patterns were also recorded to examine the structural properties. The TEM sample preparation involved dispersing a small amount of the powder in ethanol using an ultrasonicator and then depositing a drop of the dispersion onto a carbon-coated copper grid for imaging. Additionally, elemental mapping was performed in high-angular annular dark field (HAADF) mode. X-ray photoelectron spectroscopy (XPS) was conducted using a PHI5000 Versa Probe II spectrometer (ULVAC-PHI) to analyze the surface composition and oxidation states. The sample for XPS was prepared by compressing the powder into a pellet using a hydraulic press. A dual-anode Al Kα X-ray source (1486.61 eV) was utilized with a filament current of 6.5 mA, and measurements were carried out under a vacuum pressure below 10⁻⁷ Pa. As an internal reference for the absolute binding energy, the Ag 3d_5/2 peak (368.21 eV) was used. The deconvolution of the peaks was performed using the CasaXPS software after performing the Shirley background subtraction. To determine the exact elemental composition of the HEO, we performed flame atomic absorption spectroscopy using a Contra AA-300 Flame Atomic Absorption Spectrometer, Analytik Jena, Germany. Furthermore, the magnetic properties of the samples were assessed at room temperature using a SQUID magnetometer (Quantum Design, MPMS5) under an applied magnetic field of ±50 kOe. The Brunauer–Emmett–Teller (BET) technique (BELLSORP MAX II and BELCAT-II, Microtrac Bel Corporation) was employed to measure the specific surface area of the sample. This analysis used nitrogen adsorption–desorption isotherms at 77 K within a volumetric system in a nitrogen environment. Prior to the measurements, the samples were subjected to drying at 150 °C for 8 h to remove any residual moisture. The volume of nitrogen gas adsorbed onto the sample surface was determined at its boiling point (−196 °C), which correlates directly with the total surface area, including surface pores. A UV-visible diffuse reflectance (DRS) study was performed using a JASCO V-550 double beam spectrophotometer. Mott–Schottky and electrochemical impedance spectroscopy (EIS) analyses were performed using a PARSTAT 2273 advanced electrochemical system.

2.3 Fenton and photo-Fenton degradation of MB

Methylene blue (MB), a blue cationic thiazine dye, was used as a model dye in the catalytic experiments. First, 20 mg of HEO catalyst and 1 mL of H₂O₂ were dispersed in 50 mL of aqueous solution containing 10 ppm of MB. The reaction mixture was thoroughly agitated in an ultrasonic water bath for 20 min. The photo-Fenton catalytic studies were conducted at room temperature using a homemade photo irradiator with three 6 W UV tube lights (Philips TUV 6W/G6T5), which were placed horizontally over the solution surface at a wavelength of 254 nm. When conducting catalytic studies, the effect of visible light was eliminated by covering the inner surfaces of the homemade photo irradiator with black-coloured thick papers. To create a homogeneous colloidal solution, it was magnetically agitated in the dark for 90 min throughout the photo-Fenton catalytic experiment (MB + HEO + H₂O₂ + UV). Following the experiment, a 1 mL aliquot was removed every 15 minutes. Utilizing Milli Q water as a reference, the absorbance of the supernatant at 665 nm was measured using a JASCO V-650 UV-vis spectrophotometer to monitor the MB degradation process.

Similar procedures were also used for the photocatalytic (MB + HEO + UV) and Fenton catalytic (MB + HEO + H₂O₂ under dark mode conditions) degradation of MB. To ensure comprehensive control tests, MB degradation experiments were conducted under various conditions: in the absence of HEO, H₂O₂, and UV light (i.e., MB alone), with HEO only (MB + HEO), and with UV light alone (MB + UV). Recycling tests were performed to evaluate the reusability of the HEO catalyst for the photo-Fenton degradation of MB in the presence of H₂O₂ and UV light over four cycles. At the end of each cycle, the dispersed HEO catalyst was separated from the MB solution using a tabletop magnet, washed four times with Milli-Q water, and air-dried at room temperature. This catalyst was then reused in subsequent cycles following the same procedure. A mass spectrometry (MS) analysis of the decolorized dye product obtained after a 90-minute photo-Fenton catalytic reaction was performed using a Waters™ Xevo TQD LC–MS/MS system equipped with an electrospray ionization (ESI) source.

3. Results and discussion

3.1 XRD

Fig. 1 shows the Rietveld refined XRD pattern of the HEO sample using Fullprof Suite Toolbar software. We utilized the Thompson-Cox-Hastings pseudo-Voigt function to model the XRD peak profile. The obtained parameters after Rietveld refinement, along with factors relating to the reliability of fit, are illustrated in Table 1. The analysis of the HEO indicates the formation of a single-phase cubic spinel structure belonging to the space group Fd3̄m (227) following calcination at 700 °C in an air atmosphere (Fig. 1). The preparation involved equiatomic proportions of all elements, resulting in a final composition of (Al_0.6Mn_0.6Fe_0.6Co_0.6Ni_0.6)O₄. The XRD results confirm the absence of any additional phases.

Fig. 1 Rietveld refined XRD pattern of Al-HEO after calcining at 700 °C in an open atmosphere.

Table 1 Summary of Rietveld refined structural data of the Al-HEO sample

Parameters	Values
Space group	Fd3̄m (No.: 227)
a (Å)	8.34
V (Å^3)	581.259
Atomic position of Mn1/Ni1/Fe1/Al1/Cu1 (x = y = z)	0.125
Atomic position of Mn2/Ni2/Fe2/Al2/Cu2 (x = y = z)	0.5
Atomic position of O (x = y = z)	0.25873
R wp	15.4
R e	10.4
χ ^2	1.92

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The diffraction peaks were indexed using JCPDS (#01-074-0748), which corresponds to the face-centered cubic spinel structure. The observed peaks at angles 18.67°, 30.9°, 36.7°, 38°, 44.1°, 54.8°, 58.4°, 64.2°, 71.45°, 77.1°, and 81.2° are associated with the crystallographic planes of (111), (220), (311), (222), (400), (422), (511), (440), (620), (622), and (444), respectively. The crystallite size of the HEO sample is measured to be ∼9.5 nm using Scherrer's formula.

3.2 Magnetic properties

The hysteresis loops (M vs. H) were recorded across a field range of ±50 kOe, as shown in Fig. 2. These loops were measured at two different temperatures: 300 and 5 K. The purpose of these measurements was to assess the magnetic response of the sample. The key magnetic properties, including coercive field (H_C), remanent magnetization (M_r), and saturation magnetization (M_S), are summarized in Table 2. At 5 K, the sample exhibits a significant coercivity of 1176 Oe, highlighting its hard magnetic nature. Notably, Co₃O₄ and NiO have Néel temperatures (T_N) of 40 and 523 K, respectively, rendering them antiferromagnetic at 5 K. Conversely, at this temperature, Mn₃O₄ and Fe₃O₄ have T_c of 43 and 858 K, respectively, resulting in ferrimagnetic.

Fig. 2 Hysteresis loop (M vs. H plot) of HEO at 5 and 300 K. The expanded curve near the origin is shown as an inset.

Table 2 Magnetic parameters of the HEO measured at 5 and 300 K

Temperature (K)	M S (emu g^−1)	M r (emu g^−1)	H C (Oe)
300	19.94	2.14	47
5	42.75	25.80	1176

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However, at room temperature, Mn₃O₄ is paramagnetic and Co₃O₄ is weak ferromagnetic, but nickel oxide is antiferromagnetic and Fe₃O₄ is ferrimagnetic.^29–32 Overall, this HEO shows ferrimagnetic behavior with an M_S value of ∼20 emu g⁻¹ at ambient temperature, which could be connected to the magnetic cation distribution at the voids in the spinel structure. At 5 K, the sample's M_S value increased by 22.81 emu g⁻¹, and the M_r value increased by 23.66 emu g⁻¹, similar to some of the earlier HEO magnetic materials reported by our group. However, at lower temperatures, the H_C value sharply increased from 47 Oe to 1176 Oe (Table 2), indicating that the material's magnetic nature changed from soft to hard.

3.3 TEM

As depicted in Fig. 3(a), the SAED pattern of HEO displayed a series of discrete rings, indexed to the (222), (400), (422), (333), (531) and (622) planes, which align with the Fd3̄m cubic spinel structure, further corroborating the XRD findings. The bright-field TEM image (Fig. 3(b)) of the sample calcined at 700 °C revealed aggregated particles with irregular shapes.

Fig. 3 TEM analysis for the synthesized HEO: (a) SAED pattern, (b) TEM image, (c) HR-TEM image, (d) particle size distribution, (e) STEM image, and EDS (Kα) elemental mapping for (f) overlay of Al, Fe, Co, Ni, Mn and O, (g) Al, (h) Fe, (i) Co, (j) Ni, (k) Mn and (l) O.

The particles exhibited a size distribution that is predominantly smaller than 100 nm. Additionally, the inset in Fig. 3(c) presents a high-resolution (HR-TEM) image showing distinct lattice fringes, and the observed lattice fringe widths were measured to be 0.24 and 0.29 nm, corresponding to the (220) and (311) planes of the spinel M₃O₄ structure, respectively. Fig. 3(d) shows the results of an analysis of the nanoparticle size distribution using Image-J software, which showed an average size of roughly 25 nm and a range of 10–70 nm (a typical TEM micrograph is given as S1). STEM-EDS elemental mapping was carried out on HEO utilizing the Kα-emission energies of Mn, Cr, Co, Al, and Fe. The analysis confirmed that these elements were evenly distributed at the nano-meter scale throughout the sample, demonstrating excellent homogeneity (Fig. 3(e)–(l)).

3.4 XPS

XPS analysis was performed on HEO to validate the oxidation states of the components. XPS survey scan shows the presence of Al, Mn, Fe, Co, Ni and O elements in HEO. Fig. 3 shows high resolution XPS spectra of (a) Mn 2p, (b) Ni 2p, (c) Fe 2p, (d) Al 2p, (e) Co 2p and (f) O 1s. The Mn 2p, Ni 2p, Fe 2p and Co 2p peaks of HEO can be resolved into two sets of doublets. The peaks observed at 641.88 (Mn 2p_3/2) and 653.74 eV (Mn 2p_1/2) are consistent with the expected binding energies of the Mn³⁺ oxidation state, while the peaks observed at 644.18 (Mn 2p_3/2) and 655.02 eV (Mn 2p_1/2) match well with the binding energies of the Mn⁴⁺ oxidation state. Further, a shake-up satellite peak attributed to the ligand-to-metal 3d charge transfer was observed at 646.2 eV in the XPS spectrum of Mn 2p.³³ The peaks corresponding to Ni²⁺ oxidation state were observed at 854.7 eV (Ni 2p_3/2) and 872.5 eV (Ni 2p_1/2), while those of Ni³⁺ were found at 856.4 eV and 874.2 eV, respectively, in the Ni 2p spectrum.^34,35 The peaks observed at 861.23 and 879.48 eV in the Ni 2p XPS spectrum can be assigned to shake-up peaks. The Fe 2p spectrum showed peaks at 711.08 and 724.48 eV, corresponding to Fe 2p_3/2 and Fe 2p_1/2, respectively, which can be further deconvoluted into peaks associated with Fe²⁺ (710.68 and 723.78 eV) and Fe³⁺ (712.78 and 726.18 eV). Moreover, the shake-up satellite peaks of Fe 2p are visible at 720.08 and 732.5 eV.^36,37 The Al 2p fitted spectrum showed that there is an oxide peak at the binding energy of 74.9 eV.³⁸

The peaks observed at (782.1 and 796.89) eV and (780.03 and 795.43) eV were attributed to the Co²⁺ and Co³⁺ ion states, respectively, for the Co 2p peak envelope. The shake-ups related to Co 2p_3/2 and Co 2p_1/2 were represented by small peaks at 785.67 eV and 803.15 eV, respectively.³² The peaks found at 529.8 eV, 531.21 eV, and 532.5 eV in the XPS spectrum of O 1s can be correlated to metal–oxygen (MO) bonds, surface-adsorbed hydroxides/oxy-hydroxides (MO–OH/OH⁻), and physically absorbed water (H₂O), respectively.³⁹ Therefore, it is possible that multivalent cations of Fe, Mn, Ni and Co could supply a constant flow of electrons and holes (h⁺) for the mineralization of organic dye, multi-metal synergistic action, and subsequently generate a significant amount of [˙OH] radicals, which could have accelerated the degradation reaction and served as the fundamental cause of dye decolorization (Fig. 4).

Fig. 4 High resolution XPS spectra of (a) Mn 2p, (b) Ni 2p, (c) Fe 2p, (d) Al 2p, (e) Co 2p and (f) O 1s of HEO.

The compositional analysis was performed with the present HEO using flame atomic absorption spectroscopy, which suggests the atomic concentrations of Al, Mn, Fe, Co and Ni to be 0.5, 0.65, 0.61, 0.57 and 0.67, respectively. The instrumental relative standard deviation (RSD) was around 5%. The difference in the theoretical and experimental concentrations could be due to differences in the activities of the individual cations.

3.5 Brunauer–Emmett–Teller (BET) analysis

The specific surface area of the HEO was determined using the BET technique. The total surface area, including the surface pores, was correlated with the volume of N₂ adsorbed onto the sample surface. Based on the adsorption isotherm presented in Fig. 5, the specific surface area of the HEO material, as calculated using the BET method, was found to be 22.9 m² g⁻¹. The average pore diameter was found to be ∼49 nm, showing the mesoporous nature of the particles. The isotherm of the sample indicates that the adsorption process is fully reversible, as evidenced by its typical shape and the absence of a hysteresis loop.²⁰

Fig. 5 (a) Nitrogen adsorption–desorption isotherm curves and (b) pore size distribution profiles for HEO.

3.6 Optical and electrical properties

Fig. 6 shows (a) UV-Vis DRS (inset shows the [F(R)·hν]²vs. hν plot), (b) Mott–Schottky and (c) EIS (fitted data are shown by line, while symbols represent the experimental data, top and bottom insets show the equivalent circuit used for fitting of Nyquist plots and semicircle in the high-frequency region, respectively) plots of HEO. The band gap energy (E_g) of HEO was found to be 2.15 eV from the transformed Kubelka–Munk function [F(R)·hν]²vs. hν, where R represents the proportion of light reflected, h is Planck's constant, and ν is the photon frequency.⁴⁰

Fig. 6 (a) UV-Vis DRS (inset shows the [F(R)·hν]²vs. hν plot), (b) Mott–Schottky, and (c) EIS (top and bottom insets show the equivalent circuit used for fitting Nyquist plots and semicircle in the high-frequency region, respectively) plots of HEO. A line represents the fitted data, while symbols represent the experimental data.

Mott–Schottky measurement was performed at a frequency of 1 kHz in the potential range of −1 to +1 V (vs. Pt electrode in 0.1 M KCl). Depending on the applied potential, the HEO shows a duplex character with both n-type and p-type semiconducting. Below −0.35 V, the positive slope of the Mott–Schottky plot indicates that the HEO shows an n-type behavior. Above this potential, the semiconducting character changes into a p-type, as denoted by the negative slope and implicated by the 1/C²vs. V variation. There are various reports on the duplex semiconducting characteristics of high-entropy alloys.^41,42 Based on the point defect model, it is demonstrated that defects, such as oxygen vacancies and cation interstitials, impart n-type behavior, while cation vacancies generate a p-type character.⁴³ Therefore, the observed p–n heterojunction can be attributed to the accumulation of oxygen vacancies and cation interstitials below −0.35 V, whereas cation vacancies are the main dopants beyond this value.

In order to understand the electrochemical behaviour of HEO, an EIS study was carried out using a small signal AC voltage of 10 mV. The EIS plot consists of two semicircles representing R₁ parallel Q₁ in series with R₂ parallel Q₂. In this plot, the presence of a semicircle in the high-frequency region (R₁ = 0.2 kΩ, Q₁ = 1.54 nF) could be assigned to the interface between the electrode and electrolyte double layer effects, while a semicircle in the low-frequency region (R₂ = 13.9 kΩ, Q₂ = 3.88 µFsⁿ⁻¹, where n = 0.68) could be associated with the formation of a solid electrolyte phase in HEO. The observed relatively lower values of R₁ and R₂ indicate better charge transfer ability. The UV-vis DRS, Mott–Schottky and EIS results demonstrated that the developed HEO can serve as a catalyst for the degradation of organic dyes through Fenton-type reaction or absorbing photons or involving both photo-Fenton type reactions.

3.7 Degradation of MB by photocatalysis, Fenton, and photo-Fenton

The MB degradation over HEO catalyst (S) during Fenton (with H₂O₂ only), photo (with UV light irradiation), and photo-Fenton (with H₂O₂ and UV light irradiation) conditions was performed along with a number of control catalytic tests (Fig. 7). The starting concentrations in Fig. 7(a) were C₀ and C, respectively, following the catalyst's equilibrium adsorption and the MB reaction concentration at time t. An insignificant degradation of MB alone was observed under dark conditions without the HEO catalyst and H₂O₂ [MB(D)]. The degradation of MB was minimal in both the presence of the HEO catalyst (S) alone under dark conditions [MB + S (D)] and the presence of H₂O₂ and UV light but without HEO [MB + H₂O₂ (UV)]. Both UV radiation alone [MB (UV)] and H₂O₂ alone under dark [MB + H₂O₂ (D)] and UV light [MB + H₂O₂ (UV)] conditions caused slight MB deterioration.

Fig. 7 (a) Degradation of MB over HEO by the Fenton, photo, and photo-Fenton catalytic reaction conditions (for simplicity, the HEO catalyst is denoted by the letter “S”); (b) time-dependent absorption spectra of photo-Fenton catalytic degradation of MB over HEO and (c) its ln (C₀/C) vs. time curve; and (d) HEO recyclability data for photo-Fenton MB deterioration after four cycles.

MB was shown to degrade by about 60% when photocatalytic conditions were met, i.e., in the presence of both HEO catalyst and UV radiation [MB + S (UV)]. However, the degradation of MB occurred rapidly as a function of time in the presence of the HEO catalyst and H₂O₂ under dark [S + MB + H₂O₂ (D)] and UV light [S + MB + H₂O₂ (UV)]. The degradation efficiency was found to be around 85% and 94% in Fenton [S + MB + H₂O₂ (D)] and photo-Fenton [S + MB + H₂O₂ (UV)] conditions, respectively. Fig. 7(b) displays the time-dependent absorption spectra of MB under its photo-Fenton degradation. Furthermore, MB degradation kinetics of HEO obey a pseudo-first-order model, as evidenced by the linear ln (C₀/C) – time plot (Fig. 7(c)), with a rate constant of 0.0306 ± 0.0023 min⁻¹. It is noted that when H₂O₂ and UV light are present, the degradation of MB over HEO occurs incrementally over time. The photocatalytic process begins when incident photons with energy matching or exceeding the material's bandgap promote electron transitions from the valence band to the conduction band, creating separated electron–hole pairs (e⁻–h⁺).⁴⁴ These photoinduced charge carriers then diffuse toward the HEO surface,⁴⁵ where valence band holes (h⁺) oxidize adsorbed molecules through direct electron transfer and conduction band electrons (e⁻) reduce surface species by accepting electrons.⁴⁶ This dual redox capability facilitates the breakdown of various organic pollutants and water molecules at the catalyst interface.⁴⁷ Multiple oxidation states of Mn, Ni, Fe and Co are identified in HEO based on the XPS study. By encouraging the photo-Fenton type reaction, the multioxidation states of these metals may help to explain the high catalytic activity of this HEO. Reactive oxygen species (˙OH) produced from H₂O₂ by a photo-Fenton-type reaction are the primary means by which the HEO breaks down the MB molecule.^48,49 A few likely reaction mechanisms that use HEO catalysts to degrade MB are displayed in Fig. 8.

Fig. 8 Reaction mechanism involved in the photo-Fenton degradation of MB by HEO.

Our investigation into the reusability of the HEO was prompted by its exceptional photo-Fenton catalytic breakdown and its facile magnetic separation (Fig. 7(d)). The photo-Fenton degradation efficiency of this HEO showed a negligible decrease (from 93.43% in the first cycle to 89.93% in the fourth cycle), confirming its high chemical stability with respect to MB degradation. Furthermore, the XRD measurement of the magnetically separated catalyst after dye degradation verified its structural stability, meaning that the used catalyst's crystal phase matched that of the unused one (Fig. S2). Even the XPS studies for the obtained HEO after 4^th cycle suggest that the oxidation states of the cations remain unchanged (Fig. S3). According to these findings, the developed HEO catalyst is unquestionably appropriate for recycling.

To investigate the catalytic degradation of MB by HEO nanoparticles through a photo-Fenton-like reaction, we analysed sample aliquots of the dye solution obtained after 90 min of treatment using an LC–MS/MS system. The MS data were used to identify intermediate products and assess the decomposition pathway. By analysing the key intermediates, fragment patterns, and existing literature, we identified three plausible degradation pathways for MB, as illustrated in Fig. 9(b). The MS spectrum of pristine MB generally showed a characteristic peak at m/z 284.⁵⁰ Notably, this signature peak was absent in the spectra of intermediate products generated during HEO nanoparticle-catalysed MB degradation in the presence of H₂O₂. These intermediate findings demonstrate that MB degradation proceeds through multiple simultaneous processes, including hydroxylation, dihydroxylation, demethylation, dehydroxylation, sulfonation, deamination, and ultimately aromatic ring cleavage.^50,51 The intermediates are produced by photo-Fenton-like reactions at m/z = 243.75, 215, 255, 169.47, 115.56, 203.05, 107.59, 79.94, 135.37, 114.78, 91.02, 60.05, 52.95, 148.99, and 130.05. As reported by Zaied et al.,⁵² the mass peaks at m/z = 270, 256, 241, and 227 correspond to N-demethylated MB derivatives, including azure B, azure A, azure C, and thionin, respectively. Further, thionin (m/z = 229) undergoes oxidative deamination and subsequent ring modification, leading to the formation of phenothiazine (m/z = 149) through the loss of amino substituents and partial cleavage of the aromatic amine structure. After that, the final cleavage yields two separate fragments (m/z = 129 and m/z = 115), corresponding to sulfur- and nitrogen-containing aromatic rings (path C), respectively. In path A, the intermediate at m/z = 245 is formed through ˙OH radical attack at the sulfur (S) heteroatom, oxidizing the C–S⁺=O moiety to a sulfoxide (C–S(=O)–C) structure with m/z = 301. Now, m/z = 245 undergoes oxidative ring cleavage to form m/z = 215, which further degrades via desulfonation and deamination into smaller aromatic fragments at m/z = 169 and m/z = 115. The m/z 169 intermediate undergoes hydroxylation and deamination to form m/z = 203, 93 (aniline), and 94 (phenol). Further oxidation of m/z = 203 yields catechol (m/z = 109), which undergoes ring cleavage and decarboxylation, forming smaller hydrocarbons at m/z = 78 (benzene), 98, and 84.^53,54 The aromatic fragment at m/z = 78 (benzene) undergoes ring-opening and oxidation, forming oxalic acid (m/z = 90) and malonic acid (m/z = 116). Further oxidative degradation of these intermediates leads to the formation of acetic acid (m/z = 60), formic acid (m/z = 46), and smaller ring fragments (m/z = 52), indicating advanced mineralization stages in the photo-Fenton process and eventually mineralized into inorganic end-products. In path B, hydroxyl radicals (˙OH) attack the heterocyclic S–N bridge, leading to the opening of the aromatic system and the formation of a partially degraded intermediate at m/z = 215. Further, the intermediate at m/z = 215 undergoes oxidative desulfonation and deamination and leads to the formation of smaller aromatic derivatives, such as m/z = 136, 114, and 108. Later hydroxylation and fragmentation of these compounds produce phenol (m/z = 94) and aniline (m/z = 93).

Fig. 9 (a) MS spectra of the photo-Fenton degradation of MB by HEO and (b) proposed mechanism of photo-Fenton degradation by HEO.

These in turn undergo dihydroxylation to form catechol or hydroquinone (m/z = 109), followed by oxidative ring opening and decarboxylation, resulting in low-molecular-weight compounds, such as benzene (m/z = 78), and fragments at m/z = 84 and 98. Ultimately, all intermediate compounds underwent full mineralization, converting to simple inorganic species, such as CO₂, H₂O, SO₄²⁻, and NO³⁻. Specifically, the present study demonstrated the development of a reusable magnetic HEO catalyst and investigated its potential application in the degradation of organic dyes involving Fenton-type, photocatalytic and photo-Fenton-type reactions. No change in the structural, morphological and oxidation states of the HEO sample was observed after the 4th cycle of recycling. This was confirmed from the XRD pattern and XPS data illustrated in Fig. S2 and S3, respectively. Additionally, for comparison purposes, we provided the list of various catalysts used to degrade the dyes, such as methylene blue, rhodamine B and Tetracycline, under various conditions in Table 3.

Table 3 Comparison of photocatalytic degradation of methylene blue dye, rhodamine B and tetracycline dyes by different catalysts

Dye	Photocatalyst	Light source	Catalyst (mg L^−1)	Pollutant (mg L^−1)	Degradation time (min)
Methylene blue	AgBr/ZnO	UV	1000	10	240
Methylene blue	ZnO	UV	250	20	180
Methylene blue	(α-Mn3O4/MnO)@rGO	UV	20	20	45
Methylene blue	(CeGdHfPrZr)O2	UV	500	20	240
Methylene blue	TiO2	UV	500	10	300
Methylene blue	Ce0.2Gd0.2Hf0.2La0.2Zr0.2O2	Sunlight	500	10	180
Tetracycline (TC)	SnO2/g-C3N4 (3 wt% SnO2)	Visible	50	30	120
Methylene blue	(Co, Ni, Cu, Zn, Mg)0.9Ca0.10	Visible	60	10	80
Rhodamine B	TiO2	UV	20	10	120
Rhodamine B	Ba0.4Zn0.6O2	UV	50	25	—
Rhodamine B	(FeNiCuZnCo)O/ZnO	Mercury lamp	30	30	60
Rhodamine B	BiFeO3	UV	50	10	60–120
Tetracycline	(La0.2Ce0.2Gd0.2Zr0.2Fex)O2	Xenon lamp	30	20	180
Tetracycline	TiO2/(FeCoGaCrAl)2O3	UV	0.4	20	—
Tetracycline	(Mn0·2Fe0·2Co0·2Ni0·2Zr0.2)3O4	UV	2	4	130
Methylene blue (Present work)	(Al0.6Mn0.6Fe0.6Co0.6Ni0.6)O4	UV	20	20	90

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4. Conclusions

Room temperature ferrimagnetic HEO comprising multivalent metal ions, such as Al, Mn, Fe, Co, and Ni, with a chemical composition of (Al_0.6Mn_0.6Fe_0.6Co_0.6Ni_0.6)O₄ was prepared using the sol–gel method, followed by calcination at ∼700 °C. The formation of single-phase cubic spinel-structured HEO with an average particle size of around 25 nm was verified by XRD and TEM investigations. The presence of Al, Mn, Fe, Co, and Ni cations in HEO was confirmed by STEM-EDS and XPS analyses, while the specific surface area was found to be about 22.9 m² g⁻¹ from the BET measurement. The UV-visible DRS, Mott–Schottky and impedance results demonstrated the ability of HEO nanostructures for the catalytic degradation of organic dyes, such as MB, through Fenton-type catalytic reactions, photon absorption, or photo-Fenton-type reactions. It is observed that the developed HEO decomposed the MB dye into smaller and non-hazardous molecules within 90 min under photo-Fenton conditions. Specifically, magnetic HEO with multivalent cations and duplex semiconducting characteristics could serve as an efficient recyclable catalyst for photo-Fenton degradation of organic dyes.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this work.

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information includs figures for the HEO sample obtained after four cycle of photo-Fenton treatment. Fig. S1: TEM image of HEO sample. Fig. S2: XRD pattern of HEO. Fig. S3: Post-experimental XPS of HEO. See DOI: https://doi.org/10.1039/d5nj03007b.

Acknowledgements

Sanjula Pradhan is grateful to the Bhabha Atomic Research Centre in Mumbai and the Indian Institute of Technology (Banaras Hindu University), Varanasi, for allowing her to conduct a part of her research in the Chemistry Division of the Bhabha Atomic Research Centre, Mumbai. N. K. Prasad would like to thank the Anusandhan National Research Foundation (ANRF), India, for providing financial support (CRG/2023/000644). Vasundhara Mutta would like to thank the Department of K&IM, Indian Institute of Chemical Technology, for their assistance (IICT/Pubs./2025/419).

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