Facile synthesis of NiMn₂O₄/ZnMn₂O₄ heterostructure nanocomposite for visible-light-driven degradation of methylene blue dye

Abstract

Nowadays, waste discharge and contaminants in drinking water have emerged as a significant global problem. Therefore, it is necessary to remove these pollutants from the water and photocatalysis is the best technique for this purpose. The co-precipitation technique was employed to produce NiMn₂O₄ and ZnMn₂O₄ and a photocatalyst containing NiMn₂O₄/ZnMn₂O₄ heterostructure nanocomposite. Various analytical techniques have been ascribed to examine the physical, morphological and optical features of the fabricated catalysts. XRD diffraction peak patterns were utilized to confirm the presence of cubic NiMn₂O₄, tetragonal ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ phases in a nanocomposite. In photodegradation tests, the nanocomposite catalyst outperformed the individual catalysts. After 60 minutes of visible light exposure, this nanocomposite catalyst eliminated the methylene blue (MB) dye, attaining substantially higher degradation rates than pure NiMn₂O₄ (66%) and ZnMn₂O₄ (77%). The nanocomposite catalyst was highly effective against methylene blue (MB) dye with a degradation efficiency of 92%.

KEYWORDS:

• Methylene blue
• heterostructure
• spinel compounds
• photocatalysis
• dye degradation

1. Introduction

In addition to dyes and pharmaceutical chemicals, industrial pollutants contribute significantly to the pollution problem because their hazardous components are complex and challenging to degrade [1–5]. Developing cutting-edge wastewater treatment technology is essential to safeguarding the environment, public health, and sustainability [2, 6–8]. Photocatalysis modifies the redox potentials of light-generated charge pairs and is a standard technique for degrading substances [9, 10]. Another benefit of the approach is its negligible environmental influence [11–15]. Developing and manufacturing photocatalysts capable of treating dye and pharmaceutical effluents at high efficiency is essential for ecological remediation and public health [16, 17]. Recently, spinel has gained popularity for its use in H₂ evolution from splitting of water, supercapacitors, carbon dioxide (CO₂) reduction, and organic pollutant degradation [18–20].

Modifying the structure and composition of spinel semiconductor materials can influence their physical and chemical properties, such as economical, higher chemical stability, different chemical states, durability, and they can easily attribute to dyes surfaces [21–24]. NiMn₂O₄ [25], CoMn₂O₄ [26], CuAl₂O₄ [27], NiAl₂O₄ [28], and ZnMn₂O₄ [29] are examples of spinel-type metal manganese oxides that have recently attained remarkable attention due to their potential uses [17, 23, 30–32]. Due to their adaptable structure, morphology, and chemistry, metal manganese oxides have been widely utilized in various actions [33–36]. However, nickel magnetite (NiMnO₄) spinel oxide has only been examined and reported on by a few scientists. NiMn₂O₄ (NMO) rapidly recombines the photogenerated electron–hole pair (e^-/h⁺), producing modest photocatalytic performance in visible light. Several ways have been discovered to intensify the photocatalytic performance of NMO, such as noble metal deposition, bandgap modification, and morphological alteration [19, 37–39]. Recently, Luo et al. created and designed a LaFeO₃/SnS₂ nanomaterial utilizing a basic hydrothermal process using the direct Z-scheme. Due to presence of Z-scheme heterojunction among LaFeO₃ & SnS₂, this catalyst exhibits exceptional photocatalytic properties [40]. As a result, the material was able to absorb more visible light, meanwhile, recombination occurring among electrons as well as holes was reduced [41–43]. Building heterostructures using the Z-scheme is an excellent method for modifying charge separation [44]. A Z-scheme heterojunction consists of dual semiconductors with appropriately positioned energy bands to maximize the uncoupled efficacy of photo-induced carriers while maintaining their robust redox capabilities. It is crucial to locate appropriate semiconductors for establishing a Z-scheme heterojunction [17, 45]. In addition to keeping the correct charge redox potential, this would allow for strong redox behaviour at the interface of light-generated isolated carriers [46, 47].

Heterojunction coupling has increased photocatalytic efficiency because interfacial charge transfer in NiMn₂O₄ facilitates the separation of photogenerated charge carriers [48]. Two-component heterojunctions exhibited more photocatalytic activity than their single-component counterparts. Consequently, the photocatalytic effectiveness of Z-scheme composites containing nanoparticles was dramatically enhanced. This technology enhanced the carrier separation efficiency of Z-scheme heterojunctions [49]. The two-component oxide dramatically increased visible light absorption, which significantly enhanced the system's photocatalytic activity [50]. Consequently, when exposed to visible light, the binary nanocomposite exhibited exceptional MB degrading properties [51].

The NiMn₂O₄/ZnMn₂O₄ nanocomposite offers a novel and exceptionally successful advancement in field of photocatalytic degradation of dye. The synergistic combination of two unique spinel oxide materials, NiMn₂O₄ and ZnMn₂O₄, results in improved photocatalytic activity due to their complimentary optical and electrical characteristics. The heterostructured nanocomposite not only broadens the absorption spectrum, allowing for the utilization of a wider variety of light wavelengths, but it also allows for separation of charge and its transfer, lowering recombination rates and increasing the formation of reactive species. This complex interaction at the nanoscale between NiMn₂O₄ and ZnMn₂O₄ not only improves the degradation efficiency of various organic dyes under visible light irradiation, but also shows the potential for designing advanced multifunctional nanomaterials with exceptional photocatalytic performance for environmental remediation applications.

A two-step approach was employed to study the photocatalytic activity of ZnMn₂O₄/NiMn₂O₄ under photodegradation of (MB) organic dye. According to our knowledge, it has not been reported as a composite for photocatalytic degradation of organic dye. Furthermore, being a superior visible-light photocatalyst, NiMn₂O₄ (NMO) is a p-type semiconductor having a small bandgap. Despite its limited specific surface area, this catalyst has a high electron–hole recombination efficiency. The present material shows the linear value of slope “k”, denotes the kinetic rate constant for the deprivation of organic pollutants, as well as the regression coefficient (R²) values for all the nano-catalysts NiMn₂O₄/ZnMn₂O₄ (0.993) nanocomposite, because the values are so near to one, the absence of organic dye is considered a first-order response. This resultant nanocomposite has high degradation efficiency of 92% under visible light compared to the others.

2. Experimental procedure

2.1. Chemicals and reagents

All the chemicals including nickel nitrate hexahydrate (Ni(NO₃)₂. 6H₂O, Merck, 98.5%, CAS# 13478-00-7), manganese nitrate hexahydrate (Mn(NO₃)₂. 6H₂O, Sigma Aldrich, 98%, CAS# 15710-66-4), zinc chloride (ZnCl₂, Sigma Aldrich, 98%, CAS#7646-85-7), potassium hydroxide (KOH, Duksan, 85%, CAS#1310-58-3), ethanol (C₂H₅OH, Merck, 99%, CAS#64-17-5) were utilized as they received without any prior purification.

2.2. Synthesis of NiMn₂O₄ and ZnMn₂O₄ nanoparticles

For the fabrication of NiMn₂O₄, nickel nitrate (0.1 M) and manganese nitrate (0.1 M) were mixed in 100 ml of ultrapure deionized water and 20 ml of 4 M KOH solution was also poured into it under constant stirring to achieve a pH of 11 to obtained required precipitates. These precipitates were cleaned with deionized water as well as with ethanol through filtration. These obtained precipitates were allowed to  dehydrate in an electric oven at 80°C for 12 h and then sintered at 700°C in a muffle electric furnace for three hours. The annealed product was kept to conduct characterization. The same procedure has been adopted for synthesizing the ZnMn₂O₄ using zinc chloride and manganese nitrate hexahydrate as a precursor.

2.3. Synthesis of NiMn₂O₄/ZnMn₂O₄ nanocomposite

As a part of this experiment, 100 ml of ethanol was poured into a beaker along with 1 mg of NiMn₂O₄ and ZnMn₂O₄ nanoparticles prepared in advance. Following the ultrasonic agitation for 45 min with heat treatment of 60°C, the nanoparticles were evenly distributed throughout the beaker, and for 7 h, the reaction mixture was stirred ferociously. After washing, the finally obtained product was dehydrated in the oven at 80°C for 12 h. The nanocomposite synthesis has been revealed in Scheme 1.

Scheme 1. Illustration for the synthesis of NiMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ nanocomposite

2.4. Physical characterization

Powder X-ray diffraction (XRD) was used to validate the crystalline and structural characteristics of the synthesized NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ nanocomposite. An Advanced Bruker D-8 X-ray system was used to record the transition peak pattern using a Cu-Kα radiations diffractometer. Fourier transform infrared spectroscopy (FTIR) performed on (Nicolet 170SXFTIR) provides the functional group information. To evaluate the nanoparticle size and dispersion, topographical images of the nanocomposite material were obtained using scanning electron microscopy (SEM) joined with energy dispersive spectroscopy (EDS). A diffuse reflectance spectrum (DRS) was performed by Shimadzu UV-2550 to investigate the photocatalytic dye degradation identified by UV-visible spectrophotometry. For the Electrochemical impedance spectroscopy (EIS), performance was analysed on the Autolab PGSTAT-204 workstation.

2.5. Photocatalytic setup

All photocatalytic properties were performed in a commercial photo-reactor system having a 200 W tungsten lamp. Furthermore, the photocatalytic performance of the created catalysts (NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄) was investigated at room temperature. For this challenge, (MB) was nominated as the representative organic dye. To accelerate the degradation process, 100 ml of 20 mg/L of solution of dye was used with 10 mg of each nanocatalyst. Firstly, this solution and catalyst were mixed for 15 min to maintain the equilibrium process. After that, to track the degradation process, the changes in dye concentration were checked using a UV-visible spectrophotometer after every 15 min for two hours under visible light. The percentage degradation of resultant solution was estimated via Equation (1). (1) $\mathrm{Degradation}(\mathrm{\%})=C_0-\mathrm{Ct}/C_0$(1) In this C₀ specifies the initial concentration, C_t shows its concentration at a given time of the results of polluted water.

3. Results and discussion

3.1. Structural analysis

The PXRD peak patterns of NiMn₂O₄, ZnMn₂O₄, and NiMn₂O₄/ZnMn₂O₄ nanocomposite were exhibited in Figure 1(a). There are 2θ values of 18.01°, 30.15°, 35.08°, 42.94°, 47.04°, 52.96°, 65.01°, 70.17°, and 74.11°, which are related to the lattice planes (111), (220), (311), (400), (331), (422), (531), (620), and (622), respectively and matched with the JCPDS No. 01-071-0852, the diffraction peaks related with the phase of cubic NiMn₂O₄ with the Fd-3 m space group [52]. The ZnMn₂O₄ diffraction peaks of 2θ at 18.06°, 29.16°, 32.94°, 36.05°, 44.09°, 50.17°, 56.07°, 63.95°, 68.03°, and 76.07°, may also be associated to the lattice planes of (101), (112), (103), (211), (220), (204), (303), (116), (411), and (413), given a tetragonal structure with a I41/ space group (JCPDS: 01-077-0470) [53]. There were no additional crystalline phases in the nanomaterials produced confirming the single phase of each material. The presence of peaks of these materials in the XRD pattern of nanocomposite confirm its successful formation. The mean crystallite size of NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ was estimated using Scherer's Equation (2) [54–56]. (2) $\mathrm{Dc}=\frac{\mathrm{K\lambda }}{\mathrm{\beta cos\theta }}$(2) In this, D, λ, β, θ, and k stand for size of the crystal, X-ray beam wavelength, the width of the diffraction line at its maximum half-intensity, Bragg angle along with geometric form factor, usually 0.90. The diameters of the crystallites of NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ were 62, 48, and 39 nm, accordingly.

Figure 1. (a) XRD peak pattern, (b) FTIR pattern of the prepared NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄.

Figure 1(b) depicts the FT-IR spectra of NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ between 4000 and 500 cm⁻¹. The absorption of surface-bound hydroxyl ions by water molecules may result in the two peaks at 3420 cm⁻¹ & 1827 cm⁻¹, which are thought to be ?>produced by the O-H stretching mode and H-O-H bending mode [57], correspondingly. There are some additional vibration peaks ranging 1400-1500 cm⁻¹ indicating C = O due to the carbon dioxide available in the atmosphere. The metal–oxide (M-O) vibration of the Nickel-Oxide & Manganese-Oxide ions at tetrahedral sites is responsible for the 586 and 975 cm⁻¹ bands in NiMn₂O₄ [58]. All metal oxide (M-O) stretching vibration of the Zinc-Oxide and Manganese-Oxide are attributed at 655 cm⁻¹ & 1133 cm⁻¹ [59]. The composite (NiMn₂O₄/ZnMn₂O₄) include all the peaks present in the pure NiMn₂O₄ and ZnMn₂O₄.

3.2. Morphological analysis

The shape and size of generated NiMn₂O₄, ZnMn₂O₄, NiMn₂O₄/ZnMn₂O₄ nanocrystals were assessed via FESEM analysis. The NiMn₂O₄ has the nanocrystal like morphology (Figure 2a) while ZnMn₂O₄ has the nanoflake like structure (Figure 2b) and then the NiMn₂O₄/ZnMn₂O₄ nanocomposite has the nanoflake embedded nanocrystal like structure (Figure 2c). This type of the morphology was also confirmed via TEM analysis as mentioned in Figure 2(d), indicating the internal sight view of the composite formation due to the presence of both phases. Which is helpful for degradation of MB dye because of presence of porous sites at the catalyst surface. The purity of this nanocomposite was assessed via energy-dispersive X-rays spectroscopy. The peaks of Mn, Zn, Ni, Mn, and O components of the synthesized nano-catalysts present in the EDX spectrum of NiMn₂O₄/ZnMn₂O₄ nanocomposite (Figure 2e) confirms the successful synthesis as also confirmed via XRD analysis. The ratio of the individual element has been confirmed by using following relation: $\begin{array}{rl}& \mathrm{Composition}\\ & =\frac{\begin{array}{c}\mathrm{Molecular}\mathrm{mass}\mathrm{of}\mathrm{the}\mathrm{material}\\ \times \mathrm{Atomic}(\mathrm{\%})\mathrm{from}EDX\end{array}}{100\times \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{individual}\mathrm{element}}\end{array}$The ratio of elements calculated from the above relation is given in Table 1 and their experimental ratios are in better agreement with that of the theoretical one.

Figure 2. SEM images of (a) NiMn₂O₄, (b) ZnMn₂O₄, (c) NiMn₂O₄/ZnMn₂O₄, (d) TEM image of the composite material and (e) EDX of the synthesized composite.

Table 1. EDX elemental compositional analysis for all the fabricated materials.

Element	NiMn2O4	ZnMn2O4	NiMn2O4/ZnMn2O4
Ni	0.96	—	1.01
Mn	1.93	1.97	2.06
Zn	—	1.02	0.95

The specific surface area of NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ composites was determined using BET study. The nitrogen sorption data collected from the samples showed type IV isotherms, as shown in Figure 3(a). NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ specific BET surface areas were determined around 33.6, 25 m²/g &15 m²/g, accordingly. The specific surface area of the NiMn₂O₄/ZnMn₂O₄ nanocomposite reduced following decorating with ZnMn₂O₄ nanoparticles due to the limited exposure of the NiMn₂O₄ surface. The calculated pore volume of the synthesized samples, Such as NiMn₂O₄, ZnMn₂O₄, and NiMn₂O₄/ZnMn₂O₄ were 0.005, 0.013, 0.038 cm³/g, respectively, as mentioned in Figure 3(b). The results indicate higher pore volume of the nanocomposite causes many voids on its surface, increasing the amount of active sites, which are helpful for the photochemical reaction.

Figure 3. BET and BJH pore volume of the NiMn₂O₄/ZnMn₂O₄.

3.3. Optical study

The optical band gap (Eg) of all designed materials was examined using UV-visible spectra, as shown in Figure 4. The resultant spectra indicated an absorbance peak at 412, 425, and 441 nm for NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄, respectively, confirming that the peaks were present in the visible region, and nanocomposite shows the higher capability for absorption. In general, the formula of Tauc is given in Equation (3) to calculate the band gap value [60]. (3) $(\mathrm{\alpha hv}{)}^2=A(\mathrm{hv}-E_g)$(3) In this A is an optical transition-dependent constant, hv denotes the input photon energy, and α is absorption coefficient. According to Figure 4, Tauc's plot indicates the deviation between the (αhv)² and photon energy (hv) plots govern the Eg values for generated material. The fabricated materials like NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ were found to have Eg values of 2.6, 2.5 eV, &2.01 eV, accordingly. NiMn₂O₄/ZnMn₂O₄ displayed a smaller energy bandgap than NiMn₂O₄ and ZnMn₂O₄, according to the findings. This indicates that the smaller band gap will be responsible for the maximum visible light absorption in NiMn₂O₄/ZnMn₂O₄, which confirms the catalyst applicability for photocatalytic activity under sunlight, as 45% part of our solar spectrum is visible light.

Figure 4. (a) Absorption spectra, (b-d) Tauc plots, of all prepared materials.

The EIS spectra of NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ are shown in Figure 5. The charge’s speeds were determined using the size of the arcs in the EIS spectra. It is widely assumed that the smaller the charge transfer resistance, the smaller the arc radius. Because of its smaller arc radius, nanocomposite has less resistance to charge migration than pure NiMn₂O₄ and ZnMn₂O₄. The resultant charge transfer resistance of all the synthesized products like NiMn₂O₄, ZnMn₂O₄ and NiMn₂O₄/ZnMn₂O₄ nanocomposite were 9, 5.6, and 0.5 Ω, accordingly. The lower the Rct (charge transfer resistance) greater will be the photocatalytic activity of this fabricated materials. The electron lifetime before recombination was calculated using the following Equation [61]: (4) $t_e=\frac{1}{2\pi f_{\max }}$(4) In this t_e is the average time perior to the recombination of e^-/h⁺ and f_max denotes maximum frequency at the mid-frequency range's peak. The fmax and te values for the synthesized nanofibers are displayed in Table 2. The NiMn₂O₄/ZnMn₂O₄ has the longest lifetime of the group, mostly due to lowered rate of e^-/h⁺  recombination. ZnMn₂O₄ may serve as traps, speeding up the charge transition process.

Figure 5. EIS spectra of all the synthesized products.

Table 2. Charge transfer resistance and lifetime of electron calculated from EIS of all the synthesized materials.

Sr. No.	Sample	Rct Ω	Te ms
1	NiMn2O4	9	14.75
2	ZnMn2O4	5.6	21.9
3	NiMn2O4/ZnMn2O4	0.5	50.0

3.4. Evaluation of photocatalytic activity of all the synthesized products

The photocatalytic mechanism comprisis the three phases occurring on the surfaces of semiconductor materials during photocatalytic processes: (1) effective charge carrier separation, (2) photocatalytic reaction, and (3) significant light absorption [62–64]. The photocatalytic experiments on NiMn₂O₄, ZnMn₂O₄, and NiMn₂O₄/ZnMn₂O₄ nanocomposite were performed. The degradation of organic methylene blue dye in occurrence of visible light was studied. The addition of NiMn₂O₄, ZnMn₂O₄, and NiMn₂O₄/ZnMn₂O₄ gradually decreases the UV-Vis maximum absorption of MB (670 nm) dye over the duration of 80 min under the visible light illumination as depicted in Figure 6(a–c). The absorbance of MB declines as exposure time increases, and no new absorbance bands resulted in the UV-Vis absorption. This demonstrates that when exposed to visible light, MB organic dye practically entirely degrade, implying that the NiMn₂O₄/ZnMn₂O₄ heterostructures formed are effective in photodegrading organic dye as mentioned in Figure 6(d). The C/C_o of the MB dye was plotted versus time, and the photocatalyst samples spent absorbing light as shown in Figure 6(e). The percentage degradation efficiency of (MB) dye has calculated using this formula R = [(C^o - C_t) / C^o] × 100%, where C^o and C_t indicate the starting concentrations of MB and the dye concentration at time (t), accordingly.

Figure 6. Absorption spectra variation of MB dye in existence of (a) NiMn₂O₄/ZnMn₂O₄, (b) ZnMn₂O₄, (c), NiMn₂O₄, (d) degradation efficiency (%), (e) plot of C_o/C_t vs. irradiation time, (f) rate constant plots.

Table 1 shows the degradation efficacy of NiMn₂O₄, ZnMn₂O₄, and NiMn₂O₄/ZnMn₂O₄ nano catalysts for MB dye. The NiMn₂O₄/ZnMn₂O₄ heterostructure nanocomposite displays remarkable photodegradation efficiency as compared to already reported materials as shown in Table 4. The Langmuir Hinshelwood first-order kinetic model was seen when MB was photo-catalytically degraded in the presence of NiMn₂O₄/ZnMn₂O₄ nano-catalysts, and ln (C_t/C^o) = kt gives the reaction rate. Figures 6(f) depict the evolution of ln (C_t/C^o). Table 3 shows the regression coefficient (R²) values of the NiMn₂O₄ (0.989), ZnMn₂O₄ (0.992), and NiMn₂O₄/ZnMn₂O₄ (0.984) nano-catalysts, as well as linear slope value “k”, denotes the kinetic rate constant for deprivation of organic contaminants. These values suggest the deprivation of organic dye is the first order reaction as values are nearly equal to unity. Based on the experiment data and explanation presented, Figure 7 depicts a proposed photocatalytic reaction pathway that may assist to explain why the current NiMn₂O₄/ZnMn₂O₄ have superior photocatalytic activity. NiMn₂O₄/ZnMn₂O₄ had higher photocatalytic activity than pure NiMn₂O₄ and ZnMn₂O₄. This might occur if h⁺ from ZnMn₂O₄ valence band (VB) goes to NiMn₂O₄ and e^- from NiMn₂O₄ conduction band (CB) moves to ZnMn₂O₄ surface. Each of these phenomena is due to exposure of visible light. As a result, the production of NiMn₂O₄/ZnMn₂O₄ heterostructure substantially reduces the electron/hole recombination while increasing their photocatalytic activity. Furthermore, holes inside VB of NiMn₂O₄ generate active species that aids in oxidation of surface-attached water molecules, and as a result •OH are created. The photo-excited electrons of the ZnMn₂O₄ in the CB will combine with oxygen to form superoxide anions, that are required for dye reduction. UV experiment demonstrate that NiMn₂O₄/ZnMn₂O₄ nano-catalysts have remarkable photocatalytic properties, and this is because of the band gap of NiMn₂O₄/ZnMn₂O₄ as they delay the electron recombination, and the concentration of NiMn₂O₄/ZnMn₂O₄ nano-catalyst grows, the reconnection rate of e^-/h⁺ pairs decrease.

Figure 7. Photocatalytic mechanism of the MB dye degradation using NiMn₂O₄/ ZnMn₂O₄

Table 3. Comparison of the percent degradation and kinetic study.

Sr. No.	Materials	% Degradation	K min^−1	Person’s r	R^2
1	NiMn2O4	66	0.0058	0.997	0.995
2	ZnMn2O4	77	0.0079	0.992	0.984
3	NiMn2O4/ZnMn2O4	92	0.0127	0.994	0.981

Table 4. Percentage degradation comparison of prepared nanocomposite with already reported data.

Sr. No.	Material	Dye	% Degraded	Time (min)	Light Source	Reference
1	MgFe2O4	MB	80	120	Sun-light	[65]
2	ZnFe2O4/Cellulose	MB	60	180	Sun-light	[66]
3	ZnFe2-0.1Y0.1O4	MB	94.03	180	Tungsten lamp	[67]
4	Ag3PO4/MnFe2O4	MB	98	82	Sun-light	[68]
5	Ni0.6Zn0.4Fe2O4	MB	94	275	Mercury vapour Lamp	[69]
6	Ni0.5Cu0.5Fe2O4	MB	91.2	40	Xenon arc lamp	[70]
7	ZnFe2O4/ZnO	MB	100	180	UV lamp	[71]
8	ZnFe2O4	MB	99.22	110	Hg lamp	[72]
9	NiFe2O4/CdO	MB	72	80	Sun-light	[73]
10	NiMn2O4/ZnMn2O4	MB	92	80	Tungsten lamp	This work

A trapping test was conducted to better comprehend the operation of the nanocomposite which decreases the degradation efficiency from 92% to 38.0%, 72%, and 30.4%, engaging the p-benzoquinone as an •O^-₂ scavenger, EDTA-2Na acting as a h⁺ scavenger, and tertiary-butanol as scavenger of •OH [74] as depicted in Figure 8(a). In this case, the •OH is the principal reactant species in the decomposition of MB, while •O₂ and h⁺ are subsidiary reactant species. This was supported by prior investigations that shown the use of a •OH scavenger decreased photocatalysis efficiency by 30.4% as displayed in Figure 7(a).

Figure 8. (a) Scavenger test, (b) cyclic stability, (c) MB and TOC removal at optimized process and (d) post stability test XRD

Its stability and reusability of a photocatalytic catalyst are crucial. As a result, the composite material underwent five consecutive runs. Figure 8(b) demonstrates that the photocatalytic performance of composite materials does not vary significantly over time. Total organic carbon (TOC) analysis has performed to evaluate mineralization efficiency. Figure 8(c) shows the methylene blue dye and TOC removal under optimized circumstances. It was observed that dye degraded under these particular circumstances, with a degradation efficiency of 92% and a TOC removal of 78%. The results indicate that MB could Figure 8(d) illustrates the XRD pattern of the nanocomposite material after five consecutive runs. This was done to determine the structural stability of nanocomposite material. According to the XRD pattern, all peaks associated with composite materials were still visible, but their intensities were significantly less. This demonstrates the stability and reproducibility of the photocatalyst.

4. Conclusion

Herein, by employing the efficient, low-cost coprecipitation approach, novel NiMn₂O₄/ZnMn₂O₄ heterojunction nano-catalyst was created. All the fabricated materials were analysed via various analytical tools to confirms the structure, purity, morphology, and optical properties. Using visible light, fabricated nano-catalysts were subsequently employed to destroy MB dye molecules as an organic pollutant. Among all the NiMn₂O₄/ZnMn₂O₄ exhibit a higher photocatalytic degradation efficiency of 92% than pure ZnMn₂O₄ (77%) and NiMn₂O₄ (66%) due to lower band gap of nanocomposite (2.01 eV). Also, this may be due to their ability to separate charge carriers, diminish high light absorption, and allow photo-excited e^-/h⁺ couples to migrate away from the constrictive heterojunction interface. As a result, the NiMn₂O₄/ZnMn₂O₄ act as visible-light-sensitive photocatalysts and might be utilized to clean the environment by photo-catalytically decomposing organic contaminants.

Acknowledgements

K.F.F express appreciation to the Deanship of Scientific Research at King Khalid University Saudi Arabia for funding through research groups program under grant number R.G.P. 2/376/44. Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R55), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

K.F.F express appreciation to the Deanship of Scientific Research at King Khalid University Saudi Arabia for funding through research groups programme under [grant number R.G.P. 2/376/44]. Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R55), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia