Magnetic, morphological, and photocatalytic studies of Cu2+ doped Mg-Zn ferrite nanoparticles

Due to their distinctive characteristics, including their optical, catalytic, electrical, and magnetic properties, spinel ferrite nanoparticles attract more interest. Also, the substitution of transition metals like copper in ferrites has the potential to control their physical characteristics and could improve their catalytic and magnetic capabilities. Cu2+ doped Mg-Zn ferrite samples show a change in behaviour from superparamagnetic to soft ferrimagnetic. The photocatalytic studies for the CuxMg0.5Zn0.5-xFe2O4 (x= 0.1 to 0.5, and Δx= 0.1) nano-ferrites are conducted in visible light to investigate the methylene blue photodecomposition capability. The Cu-Mg-Zn nano-ferrites displayed unique behaviour in terms of Magnetic, and photocatalytic activity. These outcomes show that the Cu-Mg-Zn ferrite samples are apply to water remediation.


Introduction
For around 55 years, researchers have studied and used ferrites, a large class of oxides with notable magnetic properties.The majority of ferrites have aided in the development of new applications in nanotechnology.Due to its notable modification in the physical and chemical properties compared to that of the bulk ferrites [1], nano-ferrites are used in photocatalysis [2], gas sensors [3], magnetic resonance imaging [4], photochemical hydrogen generation from water [5], and drug delivery [6].Nano-ferrites can be produced using a variety of methods to yield their three distinct crystal systems, and the potential to produce nearly infinite solid solutions of the nano-ferrites opens up the possibility of modifying and adapting their properties for a variety of uses.These ferrite nanoparticles have evolved into a new and fascinating era in the research field of dye degradation.
Several synthetic dyes are currently employed in various sectors, including rubber, leather, pharmaceuticals, plastic, etc., but the textile business uses them the most frequently.Dyes are categorized as cationic, anionic, and non-ionic (disperse dyes).The most common synthetic dye used in the majority of the coloring industries is a cationic dye like methylene blue (MB).It is employed primarily due to its distinctive characteristics of high thermal and light-withstanding behavior [7].It is exceedingly difficult to eliminate MB from water when it is drained out with wastewater and it has negative impacts.To remove the MB, a variety of traditional methods have been developed, such as coagulation, nanofiltration, adsorption, etc. which are slower and time-consuming, and did not show good output efficiency [7].These days, nanoparticles are utilized to degrade dyes because they have a high surface area, good adsorption, and a quick rate of equilibrium [8].1291 (2023) 012007 IOP Publishing doi:10.1088/1757-899X/1291/1/012007 2 Spinel ferrites have the potential to be used in visible-light-driven photocatalytic applications, according to certain recent publications [9][10][11].To enhance the characteristics of ferrites, several substitutions have been incorporated.The size, magnetic stability, and purity of these nanoparticles further determine their applicability [12].The substitution of magnetic ions like Cu 2+ for Zn 2+ has a substantial impact on the characteristics of Mg-Zn ferrites [13].A soft magnetic ion, Cu 2+ has a magnetic moment of 1 μB.According to M. Suleman et al. [14], adding cobalt ions to Mg-Zn ferrites increased their photocatalytic activity against benzimidazole and methylene blue.In the study by S. B. Somvanshi et al. [15], a sufficient amount of Gd 3+ considerably increases the number of hydroxyl radicals that the ferrite produces, which in turn increases the photocatalytic activity of the material against Rhodamine B dye. M. A. Abdo et al. [16] synthesized Co0.5Cu0.25Zn0.25YxFe2-xO4;(0≤x≤0.1;step 0.02) (CCZY) spinel ferrites by citrate technique and demonstrated their photocatalytic activity against MB.The nano-ferrite CCZY (0.1), which has a degradation efficiency of 95% within only 60 minutes, exhibits the best performance for the elimination of MB dye.Zn0.7Mg0.24Cu0.06Fe2O4(x = 0.06) nanophotocatalyst demonstrated 90% photocatalytic degradation efficiency in the photocatalytic degradation of methylene blue with a maximum rate constant of 0.0252 min -1 [17].The literature [18,19] supports the use of Cu as a dopant, showing that the photocatalytic performance of the catalysts was enhanced by doping Cu into ferrites.
In this present work, magnetic and morphological characteristics of Cu 2+ doped Mg-Zn ferrite nanoparticles were analyzed using Vibrating Sample Magnetometer (VSM) and Field Emission Scanning Electron Microscopy (FE-SEM).The photocatalytic activity on MB dye degradation was further explored with visible light irradiation.

Experimental procedure
The preparation of CuxMg0.5Zn0.5-xFe2O4(with x ranging from 0.1 to 0.5 at an increase of 0.1) ferrites nano-particles [Cu-Mg-Zn NPs] by chemical co-precipitation technique (potassium oxalate precipitant) has been already published elsewhere [20].The batch mode photocatalytic experiments were conducted in visible light (160 W lamp with 535 lux) for 180 minutes.Using 25 mL of a 10-ppm pollutant aqueous solution (MB) and 25 mg of catalyst, adsorption-desorption equilibrium was established in the dark.
The solution was then subjected to visible light, and 2 mL aliquots were taken at specific intervals with the solid catalyst particles, which were subsequently removed with an external magnet.As seen in Figure 1, a magnet can be used to quickly separate the catalyst from the reaction mixture.Using a UV-Vis spectrophotometer (Make: LabIndia UV 3000+), the photocatalytic degradation is determined by the subsequent equation: Where, Co and Ct stand for initial and after absorbance time t, respectively, and % D for percent degradation.

Results and discussion
The detailed structural, vibrational, and optical absorption properties of Cu-Mg-Zn NPs which were investigated by X-ray diffraction, FT-IR, and UV-DRS have been also published elsewhere [20].

3.1
Magnetization studies by VSM analysis Figure 2 displays the M-H curves for samples of CuxMg0.5Zn0.5-xFe2O4(with x ranging from 0.0 to 0.5 at an increase of 0.1) at room temperature.Synthesized ferrites with Cu 2+ doping exhibit a shift from superparamagnetic to soft ferrimagnetic behavior.The residence of diamagnetic Mg 2+ and Zn 2+ ions in the x= 0 sample, which have no electron spin and do not contribute to magneto-crystalline anisotropy, results in super-paramagnetic behavior [21].The ferrimagnetic property is induced by replacing diamagnetic Zn 2+ with magnetic Cu 2+ ions possessing a magnetic moment of 1 μB.Samples can display soft ferrimagnetic behavior and maintain magnetic order by boosting magneto-crystalline anisotropy with the help of magnetic Cu 2+ ions [22].
It is also evident from the figure that all of the curves furnished narrow loops accompanied by a behavior typical of soft magnetic ferrites (in the current samples, easy magnetization and demagnetization accompanied with low coercivity meant fewer hysteresis losses, which is a crucial condition for a successful electromagnet).These hysteresis curves were employed to evaluate magnetic properties such as coercivity (Hc), remanence magnetization (Mr), and saturation magnetization (Ms) (Table 1).As seen in Table 1, the Ms values rise linearly with rising copper content up to x=0.3, at which point it starts to noticeably decline.Based on the exchange interactions and the cation distribution at the A and B sites, respectively, it is possible to explain the observed fluctuation in saturation magnetization.Although A-B super-exchange interactions are more significant than those of A-A and B-B, all super-exchange interactions, including A-B, A-A, and B-B, affect ferrite magnetization.The magnetization of spinel ferrites is calculated by assuming the magnetizations at the B-and A-sites are MB and MA in Neel's two sub-lattice model [22]: The first rise in the saturation magnetization value is ascribed to the preference of Cu 2+ ions with magnetic moments of 1 μB to migrate to the B-sites over non-magnetic Zn 2+ ions with magnetic moments of 0 μB, which exhibit high priority for the tetrahedral (A) site.As a result, Zn 2+ moves Fe 3+ from site A to B site, and the addition of Cu 2+ ions to site B raises the magnetization of these sublattice sites.Because of the super exchange interaction in the ferrite lattice, which causes the magnetic spin of nearby A and B sites to be anti-ferromagnetically connected, the net effect is the rise in the magnetic moment on the B-site, and the net magnetic moment of the Cu-Mg-Zn NPs rises with Cu 2+ ion content up to x = 0.3 as shown in Figure 2. The presence of some Fe 2+ ions in addition to their trivalent state at B-sites will cause the magnetization of ferrites with compositions x = 0.4 and x = 0.5 to decrease.For samples with x=0.4 and x=0.5, the steady decrease in magnetization would be due to an increase in the occupation of B sites by Fe 2+ ions.Additionally, the movement of Fe 3+ ions to the B sub-lattice in conjunction with the movement of Mg 2+ ions to A-sites will subsequently increase the magnetization of B-sites up to x = 0.3, increasing the Ms values of the Cu-Mg-Zn NPs.If the Cu 2+ ion can push some Fe 3+ ions to move from B-site to A-site during cation redistribution, the drop in the saturation magnetization for x ≥ 0.4 may also be described.In addition, the production of some Fe 2+ with a lower magnetic moment (4 μB) will diminish the magnetization of the B sites, which will lower the overall net magnetization of synthesized samples.A similar explanation for the decrease in Ms of coppersubstituted magnesium zinc ferrite has been provided by M. H. Zaki et al. [23].One can conclude that this technique's claimed saturation magnetization of the tested ferrites is superior to that of previous ceramic techniques [24] and auto-combustion routes [25].This indicates that the employed coprecipitation technique has the potential to enhance the ferrite with higher saturation magnetization, making this ferrite significant throughout a broad range of frequencies.
Coercivity and remanence magnetization values are shown to increase with rising Cu concentrations.The significant magneto crystalline anisotropy of the Cu 2+ ion, carried by one unpaired electron, contributes to the rise in coercivity with Cu substitution.The coercivity and remanence magnetization values are in line with what Sharma et al. [22] and Zaki et al. [23] have previously published.As Cu 2+ concentration increases, the Ms values increase in direct proportion to the expansion of ηB [26].It is shown that K and Hc behave in the same way.
Table 1 displays the results of the calculations made for the other parameters, including squareness ratio (R), magnetic moment (ηB), and anisotropy constant (K), which were calculated using the following equations [26,27].According to Table 1, the remanence ratio rises from 0.1380 for x = 0.0 to 0.3442 for x = 0.5, with the low value indicating the isotropic nature of samples [28].Multi-magnetic domains are present when R for prepared samples is measured and is smaller than 0.5 [29].

𝑅 = 𝑀 𝑟 𝑀 𝑠
(3) The Yafet-Kittel three sub-lattice model can also be used to explain the fluctuation in magnetic moment.The B sub-lattice can be split into two sub-lattices, B1 and B2, each of which has a triangle spin configuration (canting angle, αY-K), according to the Yafet-Kittel model.Understanding the triangular or non-collinear arrangement of spins is provided by Yafet-Kittel's model.The following relation was employed to calculate the Yafet-Kittel (Y-K) angles [30]: The αY-K angles become smaller as the substitution of Cu 2+ increases (Table 1).The rise in superexchange interaction (A-B) and fall in the non-collinear arrangement of spins are both confirmed by a decrease in αY-K angles with Cu 2+ doping [29].The increase in wave function overlaps adjacent by surrounding magnetic ions is indicated by a decrease in αY-K angles, which increases A-B superexchange interactions and decreases B-B interactions [31,32].

FE-SEM, EDS, and elemental mapping
A representative Cu0.2Mg0.5Zn0.3Fe2O4ferrite sample was analyzed using an FE-SEM microscope together with EDS and elemental mapping, and the results are depicted in Figure 3.
Figure 3 (A) of the FE-SEM micrograph shows the non-uniform grain growth of a representative ferrite sample.FE-SEM micrograph shows aggregated nanoparticles with almost spherical shapes and a range of sizes.Theoretically, the development of large and irregularly distributed agglomerated nanocrystals is influenced by annealing, the synthesis method, and flaws in Cu-doped Mg-Zn ferrites [33].An energy-dispersive X-ray (EDS) spectrometer is employed to assess the purity and chemical composition of the representative sample.The EDS spectra of Cu0.2Mg0.5Zn0.3Fe2O4nanoparticles are shown in Figure 3 (B).Cu, Mg, Fe, O, and Zn are present in the sample, proving that these elements are present.The sample contains no additional peaks.It demonstrates the sample's purity.The inset table shows the calculated atomic percentages of Cu, Mg, Fe, O, and Zn, which are pretty close to the theoretical value.The uniform distribution of Cu, Mg, Fe, O, and Zn was confirmed with elemental mapping from a portion of the sample that was randomly chosen.

3.3
Photocatalytic activity of CuxMg0.5Zn0.5-xFe2O4Cu-Mg-Zn NPs are suitable for photocatalytic activity because of their low band gap (~ 1.6 eV) [20].The efficiency of photo-catalytic degradation was determined using equation (1).To track the effects of CuxMg0.5Zn0.5-xFe2O4(x=0.1 to 0.5, and Δx=0.1) on the photocatalytic degradation of methylene blue (MB) dye, absorption spectra were used.As a result of the proportionality between concentration (C) and absorbance (A), the Co/Ct can be replaced by Ao/At [34].When Cu-Mg-Zn ferrites are exposed to light [240 V (160 W) 535 lux], electron-hole pairs are furnished.The production of more electron-hole pairs as a result of lengthening the exposure duration for each sample causes the degradation to occur more quickly.Due to stable electronic configuration (d 10 ) and low electron availability of Zn 2+ ions, Cu 2+ ions should be swapped out for them to increase electron availability.Because there are more available electrons at x=0.5, the most electron-hole pairs are produced.The basic variables that affect photodegradation are the quantity of the photocatalyst, the temperature of the reaction medium, the pH of the solution, the duration and intensity of the light source, the nature of the substrate as well as the photocatalyst, the surface area of the photocatalyst, the doping of non-metals or metals, and the structural characteristics of the photocatalyst, such as band gap, particle size, surface defects, etc. [35].Ferrite becomes a more promising material for photodegradation applications as its band gap shrinks.The photocatalytic degradation of MB by Cu-Mg-Zn NPs under visible light irradiation was proposed and outlined below based on the literature [33].
Step-I: Cu-Mg-Zn NP valence band electron is excited by light, generating a free electron (e -) and a hole (h + ).
ℎ + +  2  →  .+  + (8) In addition, by interacting with the adsorbed molecule O2, the electron from the conduction band can produce an anionic super-oxide radical: These super-oxide radicals react with H + to produce HO2 ., which subsequently reacts with electrons to produce OH .(Free radicals). .,  2 .−,  2 .+  →   (13) Afore-discussed reaction mechanism for MB photodegradation can also be shown as a schematic given below (Figure 4): Figure 5 shows the highest absorbance of MB measured utilizing a UV-vis spectrophotometer with a wavelength range of 200-800 nm [36].The operational factors affecting the photocatalytic degradation of MB are as follows:

Effect of contact time
In the presence of CuxMg0.5Zn0.5-xFe2O4photocatalysts (x = 0.1 to 0.5, and Δx = 0.1), the effect of contact time (i.e., 30-180 min) on the photocatalytic degradation of MB dye was assessed.A blank experiment without any photocatalysts was used as the control in the experiment.We also conducted a photocatalytic test in the dark, which subjects MB solution with the catalyst to degradation under similar conditions but without the presence of visible light.The findings show that as the irradiation time interval is increased, the degradation rate of MB increases rapidly up to 90 min.The initial concentration of the dye molecules was higher for degradation, but it gradually decreased with time.This may be because the dye molecule is initially attacked by a large number of anionic radicals in the solution, but as their quantity slowly decreases over time after 90 min, the breakdown rate slows.The results of the degradation of MB dye by varying the contact duration were evaluated using a catalyst dosage of 25 mg in 25 mL (10 ppm) of dye solution.The degrading effectiveness of CuxMg0.5Zn0.5-xFe2O4(x=0.1 to 0.5, and Δx=0.1) was plotted against the contact time as shown in Figure 6.It demonstrates that the MB degradation rises considerably up to 120 minutes and gets saturated after the ideal period.N. Ali et al. observed similar findings for contact time variation [37].

Effect of Cu-Mg-Zn NPs dosing
The amount of catalyst used in the photocatalysis method is a critical factor from an economic standpoint [38].Figure 7 displays the percentage of MB dye degradation (10 ppm) at different CuxMg0.5Zn0.5-xFe2O4(x = 0.1 to 0.5, and Δx = 0.1) photocatalyst dosages (10-35 mg).When the catalyst dosage is raised from 10 to 25 mg, the percentage of MB dye degradation rises linearly to 88.74 percent.The photocatalytic degradation declines after this photocatalyst dosage.The active sites on the catalyst surface grew as their quantity rose, which in turn improved superoxide and hydroxyl radical generation and accelerated dye degradation.Furthermore, over-catalyzing can lead to catalyst particle aggregation and turbidity, which reduce photon absorption and decrease the effectiveness of photocatalytic degradation [39,40].Therefore, a modified dosage may be utilized to more efficiently to degrade the MB dye.Under visible light, the control experiment (blank) showed the least amount of MB selfdegradation.The effectiveness of the catalyst dose change was evaluated over a contact time of 120 minutes with 25 mL of dye solution (10 ppm).The production of hydroxyl radicals on catalyst surfaces and their interactions with dye species play a major role in controlling the photodegradation of MB [41].The quantity of dye molecules that are first adsorbed on the catalyst surface increases as the initial dye concentration rises.As a result, the majority of the active sites are covered by more dye molecules, which reduces the generation of hydroxyl radicals.The photodegradation efficiency consequently fell.H. S. B. Naik [42] and G. Nagaraju et al. [43] claim that there is still another scenario that could be the source of the issue.The catalyst, according to the claimed explanation, was able to absorb fewer photons by providing a shielding effect for light that entered the solution at high dye concentrations.This, in turn, decreased 1291 (2023) 012007 IOP Publishing doi:10.1088/1757-899X/1291/1/01200710 the quantity of hydroxyl radicals that were produced on the catalyst's surface.The consequences of the initial dye concentration variation were investigated using a catalyst dosage of 25 mg in 25 mL of dye solution and a contact time of 120 min.

Effect of pH
The adsorption and degradation of MB dye are influenced by the surface charge of the catalyst and pH, two crucial parameters in dye degradation [44].Figure 9    Since pH 10 had the highest percentage of degradation (53.07 to 92.25%) and the shortest time is taken to degrade, it was determined in this study to be the optimal pH.Cu0.2Mg0.3Zn0.5Fe2O4nanoparticles have a point of zero charge (pzc) of 7.4, according to Dhiman et al. [45].Electrostatic repulsions arise because the catalyst surface and the positively charged cationic dye molecules are at an acidic pH, which minimizes degradation.The pzc of Cu-Mg-Zn ferrite relates the pH of the solution to the photodegradation of aqueous MB dye solutions, which shows increased activity at pH 10. Above the pzc value, the surface of the Cu-Mg-Zn NPs material turns negative.Due to the cationic dye MB being drawn to the oppositely charged Cu-Mg-Zn ferrite species as result, the pollutant can be quickly destroyed.

Effect of different acids
The photocatalytic breakdown of the dye may be impacted by the acidic environment.Various acids were tested to achieve this. Figure 11 illustrates the addition of various acids to the reaction solution in varying volumes (2-8 mL) to quantify photocatalytic degradation.Because the MB solution contains a lot of hydrogen ions (H + ), HNO3 (0.01 mol/L) causes the most deterioration in terms of percent.The hydrogen ions H + interact with the electrons in the aqueous medium to form hydrogen radicals H • , which then interact with the MB dye to demolish it.
Because H2SO4 (0.01 mol/L) produces sulfate SO4 2-ions, which act as radical scavengers as shown below, MB dye degrades relatively more slowly than HNO3.
There is less photocatalytic degradation of the MB solution as a consequence of the HCl (0.01 mol/L) producing Cl -ions in the MB solution, which more effectively scavenges OH .radicles.
Additionally, we compared our results with those that have already been reported for the removal of MB employing ferrite-based photocatalysts (Table 2).
IOP Publishing doi:10.1088/1757-899X/1291/1/01200713 4. Conclusions In CuxMg0.5Zn0.5-xFe2O4(x= 0.1 to 0.5, and Δx= 0.1) nano-ferrites, as the concentration of Cu doping increases, the transition from superparamagnetic to soft ferrimagnetic behavior has been observed.FE-SEM reveals the formation of spherical, non-homogeneous, agglomerated nanoparticles for Cu0.2Mg0.5Zn0.3Fe2O4sample.An amount of 1 g/L photocatalyst was found to provide the optimum dosage for 10 ppm MB solution, resulting in maximum degradation (92.25 %) under visible light irradiation in 120 minutes of contact time.The Cu0.5Mg0.5Fe2O4photocatalyst showed MB dye degradation efficiency.The photocatalysts showed remarkable performance in alkaline medium (pH 10), it might be due to the influence of surface charge properties.

5.
Declaration of competing interest: The researchers state that they have no known competing financial interests or personal relationships that could have appeared to affect the research reported in this paper.

Figure 1 .
Figure 1.Images for MB solution (a) without photocatalyst (b) with photocatalyst, and (c) after degradation showing magnetic separation.

7
: Degradation of MB by active species:

Figure 8
displays the degradation efficiency (%) as a function of the initial MB dye concentration.When the starting MB dye concentration is increased from 5 to 10 ppm, the degradation efficiency (%) increases to 87.58 %.The effectiveness of the degradation later decreases as the original concentration of MB dye rises.

Figure 8 .
Figure 8. Degradation efficiency (%) against methylene blue as a function of initial dye concentration.
displays the degradation efficiency (%) as a function of pH after tests were conducted at three distinct pH levels (4, 7, and 10).Contact duration of 120 minutes, 25 mL of 10 ppm MB dye solution, and 25 mg of a photocatalyst were used in the experiment.Three different pH values were used to measure the breakdown of the MB (4, 7, and 10).While Figure10depicts the representative absorbance spectra for MB degradation at optimum pH 10.As shown in Figure9, the rate of MB breakdown under visible light dramatically increased from pH 4 to pH 10.

Figure 9 .
Figure 9.Effect of pH on photodegradation of methylene blue.

Fig. 10 .
Fig. 10.Absorbance spectra for effect of pH on the methylene blue dye degradation.

Figure 11 .
Figure 11.Influence of different acids on the methylene blue dye.