Ferromagnetic nature and solar-induced fast catalytic properties for green environment: New Fe/Cu/V-Mn₃O₄ p-type compounds

2.1. Synthesis procedure

To synthesise pure Fe-Cu, and Fe-V substituted Mn₃O₄ nanoparticles (Hausmannite), highly analytical grade chemical reagents were utilized without further purification. MnCl₂·4H₂O, Cu(NO₃)₂·3H₂O, Fe(NO₃)₂·9H₂O, and NH₄VO₃ were used as sources for Mn, Fe, Cu, and V, with NaOH as the precipitating agent. Milli-Q Water was used as a solvent during the chemical reaction to synthesise pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ compositions [43,44]. For the synthesis process, required amounts of chemical materials were dissolved into 200 mL of Milli-Q Water, as illustrated in Table 1. Then, the beakers were stirred at 1000 rpm for 30 mins on a magnetic hotplate at room temperature to make homogenous solutions. Later, the NaOH solution was added drop wise under constant stirring (1000 rpm) until the mixture attained pH-10. Finally, the resultant precipitates were intensively washed 7 times using Milli-Q Water to remove the dissolved contaminants like sodium Na or Cl ions. The washed precipitates were dried at 80°C for 6 hrs and then calcined at 500°C for 3 hrs under atmospheric air with increasing rate of 5 C min⁻¹. The obtained powders of the pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ nanoparticles were analyzed using different techniques.

2.2. Physical characterization and applied measurements

Purity, crystalline nature, and phases of the synthesized pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ nanoparticles were examined through the Bruker D8 Advance XRD apparatus using cooper K-alpha radiation (λ = 1.5406 Å). The particle shape of pure, Fe-Cu, and Fe-V substituted Mn₃O₄ nanoparticles was studied by an HR-TEM (JEOL, JEM-100). The valence states of Mn, O, Fe, Cu, and V constituents were investigated by XPS (Al K-alpha radiation, Fisher Scientific, USA). The room temperature magnetic performance of pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ samples was detected using a LakeShore 7410 vibrating sample magnetometer. The optical characteristics of pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ samples were recorded by a DR tool (Shimadzu UV-2050).

The light-activated removal properties of pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ photocatalysts for 25 mg/L Congo red (CR) dye were studied using the natural sunlight as a radiation source. The role of photolysis process (self-degradation) and the adsorption on the surface of the particles were measured and discussed. The main measurement of the photocatalytic performance was done by adding 0.06 g of pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ nanoparticles to 100 mL solution containing dye concentration of 25 mg/L. The solutions of dye containing catalyst particles were irradiated for definite times under continuous stirring. The absorbance of the irradiated solutions was measured at fixed intervals to estimate the degradation value based on the relation (Eq. 1) [45]:

In relation (1), A₀ and A_t codes specify the initial concentration of CR dye solution (25 mg/L) and the concentration of irradiated solution for the CR contaminant.

3.1. XRD analysis

The crystallinity phase of pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ nanoparticles synthesized via the coprecipitation route was well-identified using the XRD technique, as represented in Figure 1. As clarified in the XRD pattern of pure Mn₃O₄ powder, the characteristic peaks of (101), (112), (200), (103), (211), (004), (220), (105), (312), (303), (321), (224), (400), (305), (413), (422), and (404) planes were evidently matched to tetragonal structure of manganese (II,III) oxide, Mn₃O₄, (hausmannite, I41/amd, JCPDS file No. 80-0382) with calculated lattice constant “a” = “b” = 5.7734 Å and “c” = 9.4257 Å. The appearance of the same characteristics, crystallographic lines in the XRD patterns of Fe-Cu and Fe-V co-doped Mn₃O₄ samples confirmed the production of hausmannite structure with tetragonal phase. The high XRD intensity of the Fe-Cu co-doped Mn₃O₄ sample reflects the enhanced crystallinity, while the broadening of peaks of both co-doped samples can be related to the crystallite size reduction. No additional XRD peaks were observed in XRD patterns of pure and Fe-Cu and Fe-V co-doped samples, demonstrating the absence of impurities or any other phases of manganese oxides. Moreover, the Debye–Scherrer method, as given in Eq. (2), was applied to measure the average crystallite size (L) of pure, Fe-Cu, and Fe-V-modified Mn₃O₄ nanoparticles [46].

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Figure 1.

XRD patterns of pure, Cu-Fe, and V-Fe co-doped Mn₃O₄ nanoparticles.

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The θ value in the Debye–Scherrer equation indicates the angle of the XRD peaks, β reflects the full width at half maximum and λ signifies the wavelength of the copper alpha radiation (1.5406 Å). For pure, Fe-Cu, and Fe-V-modified Mn₃O₄ nanoparticles, the average crystallite sizes were computed based on three main XRD peaks for each sample to be 50, 22, and 31 nm, respectively.

3.2. XPS

Figure 2 depicts the XPS survey spectra of Fe-Cu and Fe-V co-doped Mn₃O₄ samples, illuminating the occurrence of carbon (C), manganese (Mn), oxygen (O), iron (Fe), copper (Cu) and vanadium (V) elements. The nonappearance of any other elements point to the purity of the Fe-Cu and Fe-V co-doped Mn₃O₄ compositions. Figure 3 represents the high-resolution deconvoluted XPS spectra of the Mn2p state, where binary twofold binding energy peaks were identified as Mn 2p_1/2 and Mn 2p_3/2. Based on the XPS spectra of Mn 2p for Fe-Cu-modified Mn₃O₄ composition, the two valence states are distinguished as Mn²⁺ (641.15 and 652.09 eV) and Mn³⁺ state (644.45 and 652.94 eV). For the Fe-V co-doped Mn₃O₄ sample, the deconvoluted core level spectrum of Mn 2p gives binding energy peaks at 641.35 and 652.11 eV related to the Mn²⁺ state, while the binding energies at 643.74 and 653.24 were linked to the Mn³⁺ state [47-49]. As given in Figure 4, the XPS of O 1s for Fe-Cu co-doped Mn₃O₄ composition reveals three different deconvolution signals at binding energies equal to 528.97, 530.05, and 531.42 eV, which are indexed to lattice oxygen, oxygen vacancy, and chemisorbed oxygen, respectively [50]. The deconvoluted core level splitting pattern of O 1s for Fe-V co-doped Mn₃O₄ powder displays binding energy peaks at 530.04, 531.54 and 532.69 eV, which also attributed to lattice oxygen, oxygen vacancy, and chemisorbed oxygen species, respectively [50]. The XPS signals of the O 1s state verified the presence of oxygen vacancy defects in both co-doped Mn₃O₄ compositions.

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Figure 2.

XPS survey spectra of Fe-V and Fe-Cu co-doped Mn₃O₄ compositions.

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Figure 3.

XPS of Mn 2p for (a) Fe-Cu co-doped Mn₃O₄ and (b) Fe-V co-doped Mn₃O₄ compositions.

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Figure 4.

XPS of O 1s for Fe-V and Fe-Cu co-doped Mn₃O₄ compositions.

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Figure 5 gives the high-resolution deconvoluted core-level splitting patterns of Fe, Cu, and V dopants used to modify the Mn₃O₄ structure. The pattern of the XPS spectrum of the magnetic Fe ions of Fe-Cu co-doped Mn₃O₄ structure displays that the divalent Fe²⁺ and the trivalent Fe³⁺ ions are present in this composition [51,52]. The binding energy peaks located at 709.16, 713.65 (satellite), and 722.99 eV are in agreement with the XPS analysis of Fe²⁺-2p, while the binding energy peaks situated at 711.29, 718.22 (satellite), and 725.37 eV were related to Fe³⁺-2p XPS data. As noticed in Figure 5(a), the nonappearance of a peak around 706-707 eV rules out the occurrence of metallic iron (Fe) element in Fe-Cu co-doped Mn₃O₄ composition. In Figure 5(b), the high-resolution deconvoluted core level splitting of Cu²⁺ XPS approves the doping of Cu²⁺ ions by revealing two binding energy peaks at 933.79 eV and a satellite peak at 944.43 eV of Cu 2p state [48]. As shown in Figure 5(c), the high-resolution deconvoluted spectrum of V dopant gives a peak located at 516.71 eV, which indicates the presence of V⁴⁺ oxidation state inside the Fe-V co-doped Mn₃O₄ sample [53]. These results verify that the dopants of Fe, Cu, and V ions hold the +2/+3, +2 and +4 oxidation states without the presence of any metallic forms in these constituents, respectively. The absence of any metallic form of Fe dopant ruled out its effect on the magnetic properties of Mn₃O₄ sample. Besides, the electronic configurations of Fe²⁺, Fe³⁺, Cu²⁺, and V⁴⁺ ions are [Ar] 3d⁶, [Ar] 3d⁵, [Ar] 3d⁹, and [Ar] 3d¹, respectively. The presence of unpaired electrons in the 3d-orbitals of these dopants, besides the existence of oxygen vacancies, can greatly improve the magnetic properties of the Mn₃O₄ sample.

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Figure 5.

Deconvoluted core-level splitting patterns of (a) Fe, (b) Cu and (c) V dopants used to modify Mn₃O₄ nanoparticles.

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3.3. TEM

The detailed morphological particles size and shape of pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ powders were analyzed by a TEM device, as presented Figure 6. The details from the TEM image of the pure Mn₃O₄ sample illustrate the formation of agglomerated particles with an irregular shape and an average size of 46 nm. As noticed in the TEM image of Fe-Cu co-doped Mn₃O₄ powder, quasi-spherical homogenous particles formed with an average size of 19 nm, Figure 6(b). The morphological TEM image of Fe-V incorporated into Mn₃O₄ powder displayed the manufacture of uniform particles with an average size of 35 nm and a high agglomeration ratio.

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Figure 6.

Images of TEM analysis of (a) pure, (b) Fe-Cu, and (c) Fe-V incorporated Mn₃O₄ structure.

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3.4. DR study

Figure 7 displays the pattern of DR analysis of pure, Cu-Fe, and V-Fe modified Mn₃O₄ nanoparticles in the spectral area from 200 to 2400 nm, which is parallel to UV, visible light, and IR regions. The curves of the samples give obvious absorption band edges at different wavelengths, indicating the dissimilarity in the band gap energies. As seen in Figure 7, the absorption band edge of pure Mn₃O₄ compound was moved to a lower energy region due to the introducing of Cu-Fe and V-Fe ions into the host lattices, besides the observed decreases in the reflectance intensity. The assessment of the band gap energy values of pure, Cu-Fe, and V-Fe modified Mn₃O₄ nanoparticles can be calculated accurately by using the Kubelka-Munk (Eq. 3) [54]:

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Figure 7.

DR spectra of pure, Cu-Fe, and V-Fe co-doped Mn₃O₄ nanoparticles.

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In Eq. (3), hv is the energy of the incident photon, F(R) is the Kubelka-Munk function, E_g stands for the band gap energy, and A is a proportional constant. In this formula, “n” was considered as 2 for a direct transition and 1/2 for an indirect transition. As well, the Kubelka–Munk function can be expressed by formula (Eq. 4) [54]:

Where S, K, and R indicate the scattering coefficient, absorption coefficient, and reflection coefficient, respectively. As illustrated in Figure 8, ([F(R)hv]² vs. hv), from the linear extrapolation of [F(R)hv]² to zero point, the band gap energy values of pure, Cu-Fe, and V-Fe modified Mn₃O₄ nanoparticles were estimated to be 2.1, 1.78, and 1.65 eV, respectively. The band gap of the prepared Mn₃O₄ nanoparticles is in agreement with published values [55-57]. The introducing of impurity levels by Cu-Fe and V-Fe ions reduces the band gap energy of pure Mn₃O₄ nanoparticles and gives obvious signs to the active substitution process.

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Figure 8.

Band gap energy of (a) pure, (b) Fe-Cu, and (c) Fe-V co-doped Mn₃O₄ nanoparticles.

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3.5. Room temperature magnetic analysis

To study the influence of Cu-Fe and V-Fe ions on the magnetic order of Mn₃O₄ nanoparticles, the vibrating sample magnetometer analysis was carried out at room temperature. Figure 9 displays the applied magnetic field dependent magnetization (M–H curves) of pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ nanoparticles at room temperature (300 K). As shown in Figure 9(a), the M–H plot of pure Mn₃O₄ nanoparticles shows semi-linear performance with a narrow and weak hysteresis loop within ±1000 Oe, indicating the presence of general paramagnetic performance with very weak ferromagnetic order at low magnetic field. The measured coercivity and remanence (retentivity) values from the hysteresis part of the curve of pure Mn₃O₄ nanoparticles were 138 Oe and 0.063 emu/g. However, the M–H plots of Fe-Cu and Fe-V co-doped Mn₃O₄ nanoparticles exhibit markedly different magnetization loops compared to pure Mn₃O₄ nanoparticles. Clearly, Fe-Cu and Fe-V modified Mn₃O₄ nanoparticles revealed room temperature ferromagnetism with characterized hysteresis shape. As demonstrated in Figure 9(b), the magnetic curve of Fe-Cu co-doped Mn₃O₄ powder gives high coercivity and remanence (retentivity) values of 1668 Oe and 0.42 emu/g with measured saturation magnetization equal to 1.49 emu/g, respectively. In the same direction, the hysteresis curve of Fe-V co-doped Mn₃O₄ semiconductor exhibits coercivity, remanence, and saturation magnetization values of 378 Oe, 0.23 emu/g, and 1.67 emu/g, respectively. At room temperature, the pure Mn₃O₄ powder displayed a paramagnetic, with weak hysteresis at low field, which is in agreement with the previous published reports [12,54]. The detected high saturation magnetization and coercivity values of Fe-Cu and Fe-V modified Mn₃O₄ nanoparticles are an important issue, which highlight the potential appropriateness of co-doped Mn₃O₄ nanoparticles for use in magnetic data storage, MRI, biomedical imaging and drug delivery [12,58].

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Figure 9.

Room temperature magnetic properties of (a) pure, (b) Fe-Cu, and (c) Fe-V co-doped Mn₃O₄ nanoparticles.

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Nevertheless, the origin of room temperature ferromagnetism can include a very complex process. At present, there is no agreement on the origin of room temperature ferromagnetism in doped oxide materials, although many theoretical concepts have been suggested to explain this ferromagnetic ordering [12,58]. The model of the free-carrier-mediated ferromagnetism was suggested to clarify the magnetic characteristics of Mn-doped III-V semiconductors. However, room temperature ferromagnetism based on the model of the free-carrier predicted for Mn-doped III-V semiconductors may not be appropriate to Mn₃O₄, as it is highly resistive. Also, the presence of small Fe, Cu, and V clusters or any second phases was ruled out since the characterization analysis of XRD, DR, and XPS did not give any sign for these constituents. The XPS results confirmed that the Fe, Cu, and V ions present as +3/+2, +2, and +4 oxidation states without any metallic form, respectively. The XPS analysis recommends that the Fe^2+/3+, Cu²⁺, and V⁴⁺ ions replace the Mn sites in the Mn₃O₄ lattice. The presence of oxygen vacancies inside Fe-Cu and Fe-V co-doped Mn₃O₄ nanoparticles was verified based on XPS patterns of the O 1s orbital. As a result, according to our organized characterization analyses, the oxygen vacancies mediated bound magnetic polarons (BMPs) exchange interaction can be recommended for the origin of room temperature ferromagnetism in these compositions [58]. The oxygen vacancies inside Fe-Cu and Fe-V modified Mn₃O₄ nanoparticles can actively align with the Fe^2+/3+, Cu²⁺, and V⁴⁺ cations, leading to operative hybridization for dopants and defects, which can induce room temperature ferromagnetic order in Fe-Cu and Fe-V co-doped Mn₃O₄ nanoparticles. The exchange interaction between the Fe^2+/3+, Cu²⁺, and V⁴⁺ ions and the contamination band made by these defects (oxygen vacancies) proposes the construction of BMPs, which regulate the ferromagnetic ordering, leading to this improved room temperature ferromagnetic performance.

3.6. Photocatalytic study

The photocatalytic investigation of the obtained compositions was estimated by the photocatalytic elimination of CR (25 mg/L) in an aqueous solution under ordinary solar irradiation. The 25 mg/L CR dye solutions (100 mL) were readied from the stock solution of CR pollutant, and 0.6 g of pure, Fe-Cu, and Fe-V modified Mn₃O₄ catalysts were suspended. The obtained solutions were stirred for 5 mins in a dark environment before being exposed to solar radiation. At ordered time intervals (5 mins), ∼ 3–4 mL of dark and irradiated solution were taken out using a 10 mL syringe, and the particles of the pure, Fe/Cu, and Fe/V modified Mn₃O₄ catalysts were separated through a centrifuge device. The absorbance opposed to wavelength (λ, nm) of the photodegraded CR samples was measured from 250 to 650 nm via using UV-visible spectrophotometer, as revealed in Figure 10. The absorbance peak at 497 nm was utilized to calculate the concentration value of organic CR pollutant for each interval time (5 mins). Primarily, pure, Fe-Cu, and Fe-V modified Mn₃O₄ semiconductor catalysts had adsorption values of CR dye of 16, 27, and 11%, respectively. Also, the photolysis process of CR dye in the absence of the catalyst ruled out the self-degradation under sunlight irradiation. After 25 mins of solar irradiation, the photocatalytic process was completed for the Fe-Cu modified Mn₃O₄ catalyst. Exactly, the absorbance vs. wavelength plot of the CR solution using pure Mn₃O₄ semiconductor catalyst reflects a degradation efficiency of 59% after 25 mins. For the Fe-Cu modified Mn₃O₄ catalyst, the changes in the absorbance peak at 497 nm confirmed a photo-removal activity of 98% during 25 mins of solar light irradiation. On contrast, the variations of absorbance peak at 497 nm of CR solution using Fe-V modified Mn₃O₄ semiconductor catalyst provides a photodegradation activity of 56% in the same period. The sequence of degradation efficiency was Fe-Cu modified Mn₃O₄ > Mn₃O₄, > Fe-V modified Mn₃O₄ catalysts. The results clarify that the addition of Fe-Cu ions remarkably trigger the photo-removal efficiency of Mn₃O₄ semiconductor towards CR dye, while the Fe-V blend slightly weak the activity.

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Figure 10.

The absorbance spectra depicting degradation of CR dye under solar light at room temperature for (a) pure, (b) Fe-Cu, and (c) Fe-V co-doped Mn₃O₄ nanoparticles.

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Two of the main factors that influence the degradation activity of the photocatalytic process are the broad absorption of light energy (photons) and the effective separation of charge carriers. Both co-doped samples have broad absorption properties, but it seems that the addition of Fe-Cu supports the charge carriers’ separation, which limits their recombination. The presence of V⁴⁺ ions seems to increase the recombination rate of the charge carriers. Furthermore, the TEM images confirmed that Fe-Cu modified Mn₃O₄ catalysts have the smallest particle sizes, which can increase the reaction surface.

As displayed in Figure 11(a), the plots of ln(C₀/C_t) vs. time of irradiation represent the kinetic performance for the elimination of CR dye by the prepared pure, Fe-Cu, and Fe-V modified Mn₃O₄ catalysts. The kinetics of photodegradation reaction of pure, Fe-Cu and Fe-V modified Mn₃O₄ catalysts has been analyzed by using the well-known pseudo-first-order kinetic formula using Langmuir-Hinshelwood (LH) model, as shown in (Eq. 5) [59]:

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Figure 11.

Shows (a) Kinetic plots of photodegradation of CR dye under solar light for pure, Fe-Cu, and Fe-V co-doped Mn₃O₄ nanoparticles and (b) impact of dose of Fe-Cu co-doped Mn₃O₄ catalyst on removal efficiency for CR dye.

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In this formula, “k_app” stands for the apparent rate constant of Langmuir-Hinshelwood first-order model, “t” represents the time of each interval, C₀ is the main concentration, and C_t is the concentration of degraded solutions at different interval times. The values of apparent rate constant (k_app) were calculated by linear fit of points of ln(C₀/C_t) vs. time (min) plots. The achieved values of apparent rate constant (k_app) for pure, Fe-Cu, and Fe-V modified Mn₃O₄ catalysts are ∼ 0.044, 0.12 and 0.03 min⁻¹, respectively. The photodegradation first-order apparent rate constant (k_app) of Fe-Cu modified Mn₃O₄ catalyst is higher than that of Mn₃O₄ and Fe-V modified Mn₃O₄ catalysts. Moreover, the best catalyst dose of Fe-Cu modified Mn₃O₄ sample for photo-removal of CR dye has also been identified by varying the photocatalyst quantity from 0.02 to 0.1 g under solar irradiation with a fixed CR concentration of 25 mg/L, and the achieved photodegradation efficiencies were graphically plotted in Figure 11(b). As noticed in Figure 11(b), the photo-removal of CR dye was intensely affected by the dose of Fe-Cu modified Mn₃O₄ catalyst. 47% photodegradation efficiency of CR dye was detected when 0.02 g of Fe-Cu modified Mn₃O₄ catalyst was used. When the amount of Fe-Cu modified Mn₃O₄ catalyst was increased to 0.04, 0.06, 0.08, and 0.1 g, 74%, 98%, 81%, and 67% degradation efficiencies of CR dye were recorded, respectively. It is noticeable that by increasing the Fe-Cu modified Mn₃O₄ catalyst from 0.02 g to 0.06 g, the photo-removal efficiency of CR dye was improved from 47% to 98% under the same conditions. These improvements can be seemingly attributed to the availability of more energetic sites that can yield extra reactive radicals by increasing the Fe-Cu modified Mn₃O₄ photocatalyst. But the further growth of Fe-Cu modified Mn₃O₄ catalyst may increase the opaqueness of the solution, leading to the hindering of solar light penetration, which reduces the activity of the removal of CR dye. As a result, the suitable amount of Fe-Cu modified Mn₃O₄ photocatalyst was estimated to be 0.06 g.

The superoxide (O₂^•–) and hydroxyl ions (·OH) radicals are highly energetic and non-stable reactive species that commonly possess a remarkable influence on the photo-removal of organic pollutants such as dyes. To know the roles of the superoxide (O₂^•–), hydroxyl (·OH) ions, holes or negative electrons species on the activity of Fe-Cu modified Mn₃O₄ photocatalyst for decomposition of CR dye, different chemical trapping agents were added. Herein, the K₂Cr₂O₇ was used to capture electrons, isopropyl alcohol to attack ·OH, EDTA to detect the positive holes and benzoquinone to identify the superoxide (O₂^•–). As illustrated in Figure 12(a), the activity of the degradation of Fe-Cu co-doped Mn₃O₄ photocatalyst for CR dye was noticeably dropped after using the isopropyl alcohol and benzoquinone scavengers which indicate that the hydroxyl (·OH) and superoxide (O₂^•–) radicals are the power species to remove this dye using Fe-Cu co-doped Mn₃O₄ photocatalyst.

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Figure 12.

Illustrates (a) effect of chemical scavengers on degradation efficiency of Fe-Cu co-doped Mn₃O₄ catalyst and (b) TOC removal (%) test of Fe-Cu co-doped Mn₃O₄ catalyst for analysis of CR mineralization. EDTA: Ethylenediamine tetraacetic acid.

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To examine the real applicability of Fe-Cu co-doped Mn₃O₄ photocatalyst, the total organic carbon (TOC) was calculated based on the relation (Eq. 6) [60]:

In relation (6), TOC₀ and TOC_t represent the primary TOC and their values at different solar irradiation, respectively. As illustrated in Figure 12(b), the % TOC elimination of Fe-Cu cdoped Mn₃O₄ photocatalyst is 94.5% for 25 mg/L CR dye, demonstrating that the major hazardous and non-hazardous by-products were converted (mineralized) into H₂O and CO₂ after 40 min, which confirms the perfect mineralization for CR dye. The mineralization process takes more time compared to the decolorization of CR dye. This difference can be related to the production of intermediate products during the mineralization process, and finally, these compounds are converted to H₂O and CO₂ molecules.

The photodegradation reusability of the Fe-Cu modified Mn₃O₄ photocatalyst was examined under similar treatment conditions. The recycled photodegradation processes were symbolized as 1R, 2R, 3R, and 4R, indicating the first, second, third, and fourth cycles, respectively. The reusability test is used to explore the stability of the Fe-Cu modified Mn₃O₄ photocatalyst for the green environmental remediation process. As shown in Figure 13, the Fe-Cu modified Mn₃O₄ photocatalyst revealed good reusability and stability properties until the fourth cycle. The measured photodegradation efficiencies of Fe-Cu modified Mn₃O₄ photocatalyst for 1R, 2R, 3R, and 4R cycles were 98%, 93%, 86%, and 79%, respectively. The decreases in the photocatalytic efficiency of Fe-Cu co-doped Mn₃O₄ photocatalyst may be ascribed to the amount of catalyst lost during the washing and drying processes for each cycle. These findings recommend that Fe-Cu co-doped Mn₃O₄ photocatalyst has the potential to be utilized for a long time in the degradation reaction of CR dye.

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Figure 13.

Stability and reusability test of Fe-Cu co-doped Mn₃O₄ catalyst towards CR pollutant.

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Commonly, the photodegradation of organic dyes, pharmaceutical compounds, or other contaminants by oxides semiconductor photocatalysts includes three stage of reactions; (I): producing of negative electron (e-) and positive holes (h⁺) at the conduction band (CB) and valence band (VB) of Fe-Cu modified Mn₃O₄ photocatalyst under solar irradiation, respectively; (II): the produced e- and h⁺ transfer to the surface of Fe-Cu co-doped Mn₃O₄ particles and react with oxygen (O₂) and water (H₂O) to produce other superoxide (O₂^•–), and hydroxyl (·OH) reactive species; and finally (III): the superoxide (O₂^•–), and hydroxyl (·OH) reactive species attack and degrade CR molecules via robust oxidation and reduction reactions, as illustrated in Figure 14 and clarified in the following steps (Eq. 7)[61-63]:

${\text{e}}^{-}+{\text{O}}_2\to {\text{O}}^{-.}$

${\text{h}}^{+}+{\text{H}}_2\text{O }\to {\text{OH}}^{.}+{\text{ H}}^{+}$

${\text{h}}^{+}+{\text{OH}}^{-}\to {\text{OH}}^{.}$

${\text{Cu}}^{2+}+{\text{e}}^{-}\to {\text{Cu}}^{+}$

${\text{Fe}}^{3+}+{\text{e}}^{-}\to {\text{Fe}}^{2+}$

${\text{Cu}}^{+}{\text{or Fe}}^{2+}+{\text{O}}_2\to {\text{Cu}}^{2+}+{\text{ Fe}}^{3+}+{\text{ O}}^{-.}$

${\text{O}}^{-.}/{\text{ OH}}^{.}+\text{ CR moleclues}\to {\text{H}}_2\text{O }+{\text{ CO}}_2$

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Figure 14.

Diagram depicting a proposed mechanism of CR degradation using Fe-Cu co-doped Mn₃O₄ catalyst.

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