Substituted Co–Cu–Zn nanoferrites: synthesis, fundamental and redox catalytic properties for the degradation of methyl orange

Abstract

Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Zn²⁺, Co²⁺, Ni²⁺ and Mn³⁺. x = 0.2, 0.4, 0.6 and 0.8) magnetically recyclable catalysts have been synthesized via a sol–gel auto combustion method. The structural and magnetic properties of the prepared samples were investigated using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and vibrating sample magnetometry (VSM). The XRD analysis of the synthesized samples confirmed the formation of a single-phase cubic spinel structure and the average crystallite sizes of the nanoparticles estimated using the Debye–Scherrer's equation were found to be 40–60 nm after annealing at 1000 °C (within an error of ±2 nm). The lattice constant increases with an increase in all metal ion substitution. The hysteresis curves of the samples exhibited reduction of the saturation magnetization and coercivity with substitution of all the metal ions in Co–Cu–Zn nano ferrites. The DC electrical resistivity decreased with an increase in temperature, indicating the semiconducting nature of the ferrite samples. Manganese substituted Co–Cu–Zn nanoferrites showed the best catalytic activity among all the magnetic nanoferrites. All the magnetic nanoferrites can be easily recovered by using a magnet and no decrease in the efficiency was observed after several consecutive rounds of reaction.

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1. Introduction

Over the past few decades, magnetic nanoferrites have been well cultivated due to their colossal properties which are novel in comparison with the corresponding bulk part^1,2 such as superparamagnetism, high surface area, large surface-to-volume ratio and easy separation under external magnetic fields and strong adsorption ability. They have budding applications in electrical components, memory devices, magnetostriction, microwave devices and high-density magnetic recording.^3,4 Amid the nanoferrites cobalt ferrites have drawn wide spread attention because of their high, large magnetocrystalline anisotropy, moderate saturation magnetization, large magnetostrictive coefficient at room temperature, chemical stability, mechanical hardness, flexible magnetic properties.^5–7 The properties of the magnetic nanoferrites can be customized by the composition of the ferrites. This is achieved through the substitution of different metal ions into the ferrite lattice.

Various researchers have reported the substitution of metal ions into the cobalt ferrites.^8–20 Sundararajan et al.⁸ synthesized the zinc doped cobalt ferrite Co_1−xZn_xFe₂O₄ (0 ≤ x ≤ 0.5) by microwave combustion method by employing urea as a fuel. The authors reported the crystalline size ∼3.07–11.30 nm and the saturation magnetization was higher for Co_0.7Zn_0.3Fe₂O₄ composition. Raut et al.⁹ prepared Co_1−xZn_xFe₂O₄ (0.0 ≤ x ≤ 1.0) ferrite by sol–gel auto-combustion technique and obtained nano-particles had grain size in range of 52–62 nm. The authors also stated that the saturation magnetization, coercivity, magneton number increased after gamma irradiation.

Co–Zn ferrite with formula Co_0.5Zn_0.5Fe₂O₄ was synthesized by Mozaffari et al.¹¹ via co-precipitation method. During the synthesis of ferrite, solution temperature varied from room temperature to 363 K. It was concluded that as the solution temperature was increased, the crystallite size also increased. Patil et al.¹² synthesized ferrites having formula Co_1−xZn_xFe₂O₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) and studied the effect of Zn substitution on the elastic properties. The elastic properties were found to decrease with increase in zinc composition. However, the Poisson's ratios almost remained constant with increasing zinc content. From the above discussion it is clear that the substitution of Zn ion in the cobalt ferrite lattice is promising tool to change the structural, electrical and magnetic properties. Therefore, there is a need to thoroughly study the transition metal substitution in Co–Zn ferrite with new composition possessing desired properties for various applications.

Substitution of non magnetic zinc into the cobalt ferrite lattice is expected to produce distorted spinel structures. The introduction of zinc in the cobalt ferrite lattice leads to an interesting cation distribution over tetrahedral A-sites and octahedral B-sites. This is because Zn²⁺ ions seek to the tetrahedral sites while Co²⁺ ions preferably occupy the octahedral sites. Co–Zn is a hard magnetic material having high Curie temperature and good chemical stability.¹² From applications point of view, cobalt–zinc ferrites are used in transformer cores, electric motors and generators.

Very few researchers have carried out their research on the substituted Co–Zn nanoferrites. Khan et al.¹³ synthesized Co_0.50−xMn_xZn_0.5Fe₂O₄ (0 < x < 0.4) ferrites and concluded that the average grain size increased from 0.84195 to 0.84429 nm with increase in Mn concentration. The authors also conclude that the saturation magnetization increased up to x = 0.1 and then decreased with increase in Mn content. Sharma et al.¹⁴ prepared the Cr_xCo_0.5−xZn_0.5Fe₂O₄ (x = 0.1 to 0.5) nano particles using co-precipitation method and showed that the particle size as well as the superparamagnetic blocking temperature decreased with increasing Cr concentration. Bhargava et al.¹⁵ investigated the large magnetic moment in nano-sized Cu_0.25Co_0.25Zn_0.5Fe₂O₄ and reported that the sample have moment 81 A m² kg⁻¹ which was comparable to largest reported values in spinel ferrites as that found in bulk CoFe₂O₄. Sharifi and Shokrollahi¹⁶ evaluated the structural, magnetic and Mossbauer studies of Mn substituted Co–Zn ferrite nanoparticles synthesized by co-precipitation and mentioned that the lattice parameter increased with increase in the Mn. The authors also reported that the Curie point, and saturation magnetization decreased with increase in Mn content up to 0.4.

The magnetic nano-ferrite materials exhibit catalytic activity owing to the presence of transition metal ions which are stable in more than one oxidation states. Due to this reason, the constituent metal ion/ions undergo a cyclic electron transfer process, enabling ferrites to exhibit catalytic activity. Ferrites are used in various reactions which are addressing environmental concerns^17–20 and those involving degradation of organic pollutants and harmful dyes.^21–24 Limited literature is however available on the dye degradation using substituted ferrites. The removal of RhB by Co_xFe_3−xO₄ magnetic nanoparticles activated oxone was performed by Su et al.²⁵ The authors reported that removal performance increased with increase in the cobalt content in the catalyst. The authors also mentioned that 80% of the dye was degraded within 60 min even after the Co_xFe_3−xO₄ was used for the fourth time. Fan et al.²⁶ studied the photo-catalytic degradation of MB under visible light irritation with cobalt doped zinc ferrite (Zn_1−xCo_xFe₂O₄). The authors concluded that the photo catalytic activity for the degradation of MB dye was increased with increase in cobalt concentration which was mainly attributed to decrease in band gap with increase in cobalt ion concentration. Borhan et al.²⁷ examined the effect of Al³⁺ substituted zinc ferrite on photo catalytic degradation of Orange I azo dye. The authors observed that Orange I azo dye removal efficiency was maximum when the Al³⁺ and Fe³⁺ were present in equimolar amounts. Various transition metal cations like Pb²⁺, Co²⁺, Cu²⁺, Mn²⁺, Bi³⁺, Cd²⁺, Mg²⁺, Zn²⁺ and Ni²⁺ enhance the exchange capacity and rate of the reaction towards the degradation of dye.^28,29

From the above literature it is clear that the introduction of zinc metallic ions into CoFe₂O₄ lattice alters the structural, electrical and magnetic properties. The magnetic properties such as; saturation magnetization (M_s), remanent magnetization (M_r), (H_c) and curie temperatures of these materials can be tuned by substituting Zn²⁺ for Fe³⁺ on the tetrahedral A or the octahedral, B sites. The properties can be further improved by substituting various transitions metals ions in to the Co–Zn nano ferrites. However very few reports are available on the metals substituted cobalt zinc ferrite and their use for the degradation of dye have been studied to a very limited extent. Keeping in mind the immense importance of Co–Zn nano ferrites, the present work explore the effect of various transition metals ions in to the cobalt zinc nanoferrites and their photo catalytic activity for the degradation of methyl orange dye. The transition metal ions chosen for this purpose are Zn²⁺, Co²⁺, Ni²⁺ and Mn³⁺.

2. Experimental

2.1. Fabrication of the nanoferrites

For the preparation of Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Zn²⁺, Co²⁺, Ni²⁺ and Mn³⁺. x = 0.2, 0.4, 0.6 and 0.8) stoichiometric amounts of Co(NO₃)₂·6H₂O (S.D Fine-Chem Limited), Fe(NO₃)₃·9H₂O (Thermo Fischer-Scientific India Pvt. Ltd.), copper nitrate Cu(NO₃)₂·3H₂O (S.D. Fine-Chem Limited), nickel nitrate Ni(NO₃)₂·6H₂O (CDH chemicals), zinc nitrate Zn(NO₃)₂·6H₂O (Qualigens Fine Chemicals) and manganous chloride MnCl₂·4H₂O (S.D. fine-chem limited), citric acid were of A. R. Grade were utilized. All chemicals are used without any further purification. For the synthesized of ferrites, the same experimental procedure was followed as reported in our previous work.³⁰

2.2. Photo-catalytic activity evaluation

Photo-catalytic activity of all the magnetic nano-particles was evaluated by measuring the degradation of methyl orange (MO) in the aqueous solution under visible light irradiation. A 400 W visible lamp was employed as light source. For each experiment, 0.05 g of photo-catalyst was dispersed in 100 ml of MO aqueous solution. The pH of the dye solution was adjusted by adding sulphuric acid solution. Prior to irradiation, the suspension was magnetically stirred in the dark for 30 min to ensure the adsorption–desorption equilibrium between MO aqueous solution and the photo-catalyst. The solution was exposed to visible light under stirring after addition of 0.1 ml of 30% H₂O₂. At given time intervals, 3 ml of aliquots were withdrawn and centrifuged to remove ferrite particles. The concentration of MO in aqueous solution was determined with the help of UV-vis spectrophotometer. The solution of methyl orange shows bands at 463 nm. The absorption peak at 463 nm decreased considerably throughout the irradiation time which indicated the degradation of MO. Percentage of the dye degraded was calculated by using equation:³¹where C₀ is initial MO concentration and C_t is the concentration of MO at time t.

2.2.1. Determination of OH˙ radical. To determine the formation of hydroxyl radicals (OH˙) in the photo catalytic reaction solution, terephthalic acid photoluminescence (TAPL) probing technique has been used. This is a very sensitive and reliable technique for the detection of hydroxyl radicals. In this experiment, the alkaline solution of terephthalic acid (TA) (1 × 10⁻⁴ M) in 0.1 M in aqueous NaOH, having ferrite catalyst was irradiated with UV-visible light. Sample was withdrawn from the reaction mixture after 10 min of irradiation and centrifuged to separate catalyst particles. TA reacts with OH˙ to produce highly fluorescent 2-hydroxyterephthalic acid. As shown in Fig. 1 the product of TA hydroxylation gave a peak at 425 nm with excitation at a wavelength of 315 nm monitored using Fluorescence Spectrometer. The intensity of the PL signal at 425 nm of 2-hydroxyterephthalic acid was in proportion to the amount of hydroxyl radicals produced in water.

Fig. 1 Fluorescence spectra of Co_0.6Zn_0.4Cu_0.2Mn_0.8Fe_1.0O₄ annealed at 1000 °C.

3. Physical techniques

The powder X-ray diffraction (XRD) was performed using Powder X-ray Diffractometer (PANanalytical) with Cu-Kα radiation of wavelength 1.5404 Å, in steps of 0.02°. The observed patterns were cross-matched with those in the JCPDS database. The IR spectra were recorded by transmission through KBr pellets containing about 1% of the compounds by use of a Perkin Elmer RX-1 FT-IR spectrophotometer. The electrical properties were studied using two probe method. The magnetism of the samples was measured in the solid state at room temperature using a vibrating sample magnetometer (VSM) (155, PAR). The hysteresis loops of each sample were measured in a magnetic field in the range of ±15 kOe. The concentration of MO was analyzed using a UV-vis spectrophotometer (JASCO, V-530). Determination of hydroxyl radicals was done using Perkin Elmer LS 55 Fluorescence Spectrometer.

4. Results and discussion

4.1. Fourier transform infrared (FT-IR) characterization

The FT-IR bands are assigned to the vibration of metal ions in the crystal lattice. It is well known that the cubic spinel have two absorption bands in the range of 400–600 cm⁻¹ corresponding to tetrahedral and octahedral clusters. The higher frequency band (ν₁) attributes to the intrinsic stretching vibration of the tetrahedral site, where as the lower frequency band (ν₂) corresponds to intrinsic stretching vibration of octahedral site.^32,33 FT-IR spectra were recorded for Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Zn²⁺, Co²⁺, Ni²⁺ and Mn³⁺. x = 0.2, 0.4, 0.6 and 0.8) powders samples annealed at 1000 °C. The FT-IR spectra of the entire composition are shown in Fig. 2. All the ferrites samples showed bands at 450–550 cm⁻¹ and 550–600 cm⁻¹ corresponding to octahedral and tetrahedral cluster respectively. But in the case of zinc and nickel substituted Co–Cu–Zn ferrite, only one band in the range of 500–600 cm⁻¹ corresponding to the tetrahedral stretching vibration was observed. This may be due to the reason that sometimes the band due to octahedral M–O stretching is observed slightly below 400 cm⁻¹. Since the given FT-IR spectra ranged between 400 and 800 cm⁻¹, any peaks below 400 cm could not be observed. A sharp peak at 720 cm⁻¹ corresponded to Nujol.

Fig. 2 Typical FT-IR spectra of for Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Mn³⁺, Co²⁺, Ni²⁺ and Zn²⁺) annealed at 1000 °C for 2 h.

4.2. Powder X-ray diffraction (XRD) studies

The identification of the crystal structure and detection of phase and purity of the entire substituted Co–Cu–Zn ferrite nanoferrites was performed using X-ray diffraction (XRD) studies. XRD patterns of the entire ferrite compositions annealed at various temperatures were recorded and typical XRD pattern of Co_0.6Zn_0.4Cu_0.2Ni_0.6Fe_1.2O₄ is shown in Fig. 3. The positions and relative intensities of all detectable peaks matched well with those from the standard CoFe₂O₄ (JCPDS card no. 00-001-1121). The peaks were found to grow sharper with increased annealing temperature because of an increase in crystallite size. The reflections from the planes (220), (311), (222), (400), (422), (511) and (440) observed for the samples confirmed the formation of a cubic spinel structure (Fd3̄m) of the nanoferrites samples. The crystallite size of all the nano ferrite samples was calculated by using Scherrer equation,³⁴ D_hkl = 0.9λ/B cos θ, where D_hkl is the crystallite size, B is the half maximum line width, λ is the wavelength of the radiation used, θ is the angle of diffraction. The average crystallite size for the entire ferrite compositions was found to be ∼20–30 nm and 40–60 nm after annealed at 400 and 1000 °C respectively (within an error of ±2 nm). Powder X-ray diffraction for all the Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO_4x (M = Zn²⁺, Co²⁺, Ni²⁺ and Mn³⁺. x = 0.2, 0.4, 0.6 and 0.8) nano ferrite samples annealed at 1000 °C are presented in Fig. 4. Fig. 4 showed well developed diffraction lines, which could be assigned to the cubic spinel structure. The peaks could be indexed to the (220), (311), (222), (400), (422), (511) and (440) planes corresponding to the standard XRD pattern of CoFe₂O₄ (JCPDS card no. 00-001-1121). In addition, a secondary phase corresponding to metal oxides, Co₃O₄ (JCPDS card no. 00-043-1003) for Co_0.6Zn_0.4Cu_0.2Co_xFe_1.8−xO₄ (x ≥ 0.6), NiO (JCPDS card no. 00-036-1451) for Co_0.6Zn_0.4Cu_0.2Ni_xFe_1.8−xO₄ (x = 0.8) and ZnO (JCPDS card no. 00-036-1451) for Co_0.6Zn_0.4Cu_0.2Zn_xFe_1.8−xO₄ (x = 0.6) was observed. So, it can be said that while going to the higher concentration of dopant metal ions, there is a probability of single phase composition to get altered to biphasic.^35–37 The lattice parameters of all the samples were calculated using Le bail refinement method (built in TOPAS V2.1 of BRUKER AXS) and its values is given in Table 1. The lattice parameter was found to increase with increasing Mn³⁺ (8.36–8.38 Å), Ni²⁺ (8.37–8.40 Å), Co²⁺ (8.379–8.390 Å) and Zn²⁺ (8.379–8.42 Å) substitution^34,38 which was attributed to the larger ionic radii of Mn³⁺ (0.645 Å), Ni²⁺ (0.78 Å), Co²⁺ (0.82 Å) and Zn²⁺ (0.82 Å) substituted ions as compared that of iron Fe³⁺ (0.64 Å). The increase in lattice constant obeys the Vegard's law.

Fig. 3 XRD patterns of Co_0.6Zn_0.4Cu_0.2Ni_0.6Fe_1.2O₄ samples annealed at various temperature.

Fig. 4 XRD patterns of Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Mn³⁺, Co²⁺, Ni²⁺ and Zn²⁺) samples annealed at 1000 °C.

Table 1 Lattice parameter, a (Å); crystallite size, D_hkl (nm); X-ray density, ρ_x (g cm⁻³); and porosity (%) of Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ annealed at 1000 °C for 2 h

Ferrite composition	X	a (Å)	Dhkl ± 2 (nm)	ρx (g cm^−3)	Porosity (%)
Co0.6Zn0.4Cu0.2MnxFe1.8−xO4	0.2	8.360	20	5.42	59.4
	0.4	8.367	15	5.41	59.3
	0.6	8.377	11	5.38	59.1
	0.8	8.380	8	5.37	59.0
Co0.6Zn0.4Cu0.2CoxFe1.8−xO4	0.2	8.379	57	5.40	59.2
	0.4	8.382	37	5.41	59.3
	0.6	8.389	31	5.41	59.3
	0.8	8.390	30	5.42	58.1
Co0.6Zn0.4Cu0.2NixFe1.8−xO4	0.2	8.370	47	5.42	59.4
	0.4	8.387	44	5.40	59.2
	0.6	8.389	42	5.41	59.3
	0.8	8.400	42	5.42	59.4
Co0.6Zn0.4Cu0.2ZnxFe1.8−xO4	0.2	8.379	54	5.43	59.4
	0.4	8.380	38	5.47	59.7
	0.6	8.399	40	5.48	59.8
	0.8	8.420	57	5.49	59.9

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The value of X-ray density (ρ_x) was determined using the following relation: ρ_x = ZM/Na³, where Z represents the number of molecules in a unit cell of spinel lattice and in this case Z is 8, M represents the molecular weight of the sample, N is Avogadro number and ‘a’ is the lattice parameter. It was found that the X-ray density, ρ_x decreased with increasing Mn³⁺ concentration, whereas it increased with increasing Zn²⁺ and Co³⁺ concentration. The decrease in the X-ray density values on increasing Mn³⁺ concentration may be attributed to the lower atomic mass as compared to that of iron; and their larger ionic radii, which leads to expansion of the lattice, thereby increasing the volume. However, the increase in density with increasing Co³⁺ and Zn²⁺ substitution may be due to larger mass of ions.

The percent porosity of the prepared ferrite samples was calculated by using the following formula:

% porosity = (1 − bulk density/X-ray density) × 100 (2)

The % porosity decreased with increasing in magnesium concentration. However, it increased with increase in nickel and zinc ion concentration.³⁹ All the calculated parameters are given in Table 1.

4.3. Magnetic studies

In order to study the magnetic properties of the entire ferrite compositions, their room temperature hysteresis loops were recorded using Vibrating Sample Magnetometer. From the hysteresis loops, the values of saturation magnetization (M_s), (H_c), retentivity (M_r), squareness (S_q) and anisotropy constant (K) were obtained. Typical hysteresis loops of the Co_0.6Zn_0.4Cu_0.2Mn_0.4Fe_1.6O₄, Co_0.6Zn_0.4Cu_0.2Co_0.2Fe_1.6O₄, Co_0.6Zn_0.4Cu_0.2Ni_0.8Fe_1.6O₄ and Co_0.6Zn_0.4Cu_0.2Zn_0.2Fe_1.6O₄ ferrite compositions annealed at 400 °C and 1000 °C are shown in Fig. 5.

Fig. 5 Hysteresis loops of Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Mn³⁺, Co²⁺, Ni²⁺ and Zn²⁺) nanoferrites annealed at 400 °C and 1000 °C.

As expected, the hysteresis loops of the ferrite compositions annealed at different temperatures, exhibited increase in saturation magnetization with increase in annealing temperature.⁴⁰ The increase in saturation magnetization with increase in annealing temperature was attributed to the increase in particle size, which led to the decrease in super-paramagnetic relaxations.⁴¹ However, the was found to be decrease with increase in annealing temperature for all the ferrite compositions.

The hysteresis loops recorded for the different metal ion substituted ferrite composition Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Zn²⁺, Co²⁺, Ni²⁺ and Mn³⁺. x = 0.2, 0.4, 0.6 and 0.8) are shown in Fig. 6 which confirmed ferromagnetic character for all the synthesized samples after annealed at 1000 °C.

Fig. 6 Hysteresis loops of Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Mn³⁺, Co²⁺, Ni²⁺ and Zn²⁺) nanoferrites annealed at 1000 °C.

The saturation magnetization of the soft nanoferrites mainly depends upon the net magnetization of the spinel lattice. Magnetic properties of the Mⁿ⁺ ions substituted cobalt zinc ferrites depend inherently on the cation distribution in the lattice of the nanoferrites. The magnetic properties such as, saturation magnetization (M_s), coercivity, (H_c) and remanent magnetization (M_r) were affected differently by Mⁿ⁺ ion concentration. The values of saturation magnetization, coercivity, remanence, anisotropy constant and squareness ratio are given in Table 2. The saturation magnetization was found to decrease with increase in substitution of all metal ions in Co–Cu–Zn nano ferrites. This could be due to the smaller magnetic moment of Zn²⁺ ion (0 μ_B), Mn³⁺ ion (4 μ_B),⁴² Ni²⁺ ion (2 μ_B) (ref. 43) and Co³⁺ (4 μ_B) as compared to that of Fe³⁺ ion (5 μ_B).⁴⁴ The coercivity (H_c) and remanence (M_r) values exhibited similar decreasing trend with increasing Mⁿ⁺ concentration. Chae et al.⁴⁵ also reported the same trend with increase in Mⁿ⁺ ion concentration. The decrease in saturation magnetization with increase in Mn³⁺ ions substitution have also been reported by Hankare et al.³² in manganese substituted lithium ferrites (Li_0.5Fe_2.5−xMn_xO₄).

Table 2 Saturation magnetization (M_s), (H_c), remanence (M_r), anisotropy constant (K) and squareness (S_q) of substituted cobalt–zinc ferrites annealed at 1000 °C for 2 h

Ferrite composition	x	Mr [emu g^−1]	Ms [emu g^−1]	Hc [Oe]	[K] (erg Oe^−1)	Sq
Co0.6Zn0.4Cu0.2MnxFe1.8−xO4	0.2	7.0	58.15	95	5754.42	0.120
	0.4	4.5	50.04	60	3133.75	0.089
	0.6	5.0	44.01	50	2292.18	0.113
	0.8	5.5	34.74	40	1447.50	0.158
Co0.6Zn0.4Cu0.2CoxFe1.8−xO4	0.2	9.0	69.90	105	7645.31	0.128
	0.4	10.5	51.97	140	7578.45	0.202
	0.6	4.5	28.09	120	3511.25	0.160
	0.8	3.5	22.32	115	2673.75	0.156
Co0.6Zn0.4Cu0.2NixFe1.8−xO4	0.2	13	52.35	210	11 451.56	0.247
	0.4	9.5	46.96	150	7337.50	0.202
	0.6	9.0	40.14	210	8780.62	0.224
	0.8	8.0	33.14	185	6386.35	0.241
Co0.6Zn0.4Cu0.2ZnxFe1.8−xO4	0.2	4.0	62.42	70	4551.45	0.064
	0.4	18	58.59	25	811.71	0.307
	0.6	1.5	31.17	50	708.07	0.048
	0.8	2.1	27.19	25	464.32	0.077

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The behavior of saturation magnetization can be explained by using cation distribution of all the compositions. The proposed cation distribution is given in Table 3 by correlating the magnetic moment calculated from the Neel's two sub-lattice model,⁴⁶ n^N_B (calculated magnetic moment) and the one observed using the values of saturation magnetization,⁴⁷ n_B (observed magnetic moment).

Table 3 Cation distribution, observed magnetic moment (n_B) and calculated magnetic moment (n^N_B) of Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ annealed at 1000 °C

Ferrite composition	x	Cation distribution	nB	n^NB
Co0.6Zn0.4Cu0.2MnxFe1.8−xO4	0.2	(Fe0.893Zn0.101)^A[Co0.6Fe0.707Mn0.2Zn0.293Cu0.2]^BO4	2.48	2.47
	0.4	(Fe0.91Zn0.09)^A[Co0.6Fe0.49Mn0.4Zn0.31Cu0.2]^BO4	2.13	2.10
	0.6	(Fe0.915Zn0.085)^A[Co0.6Fe0.285Mn0.6Zn0.315Cu0.2]^BO4	1.87	1.85
	0.8	(Fe0.932Zn0.068)^A[Co0.6Fe0.068Mn0.8Zn0.322Cu0.2]^BO4	1.48	1.48
Co0.6Zn0.4Cu0.2NixFe1.8−xO4	0.2	(Fe0.875Zn0.125)^A[Co0.6Fe0.725Ni0.2Zn0.275Cu0.2]^BO4	2.24	2.25
	0.4	(Fe0.839Zn0.161)^A[Co0.6Fe0.561Ni0.4Zn0.239Cu0.2]^BO4	2.01	2.01
	0.6	(Fe0.81Zn0.19)^A[Co0.6Fe0.39Ni0.6Zn0.21Cu0.2]^BO4	1.72	1.70
	0.8	(Fe0.81Zn0.19)^A[Co0.6Fe0.19Ni0.8Zn0.21Cu0.2]^BO4	1.14	1.10
Co0.6Zn0.4Cu0.2ZnxFe1.8−xO4	0.2	(Fe0.732Zn.268)^A[Co0.6Fe0.868Zn0.332Cu0.2]^BO4	2.68	2.68
	0.4	(Fe0.768Zn0.232)^A[Co0.6Fe0.632Zn0.568Cu0.2]^BO4	1.35	1.32
	0.6	(Fe0.682Zn0.318)^A[Co0.6Fe0.518Zn0.682Cu0.2]^BO4	1.19	1.18
	0.8	(Fe0.622Zn0.378)^A[Co0.6Fe0.378Zn0.822Cu0.2]^BO4	0.78	0.78
Co0.6Zn0.4Cu0.2CoxFe1.8−xO4	0.2	(Fe0.65Co0.2Zn.15)^A[Co0.6Fe0.95Zn0.25Cu0.2]^BO4	2.99	2.90
	0.4	(Fe0.55Co0.4Zn0.05)^A[Co0.6Fe0.85Zn0.35Cu0.2]^BO4	2.32	2.30
	0.6	(Fe0.752Co0.18Zn.068)^A[Co1.02Fe0.448Zn0.332Cu0.2]^BO4	1.20	1.20
	0.8	(Fe0.565Co0.35Zn.085)^A[Co1.05Fe0.435Zn0.315Cu0.2]^BO4	0.96	0.95

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In the present study, the Mn³⁺, Co³⁺ and Ni²⁺ ions predominantly occupied the octahedral sites, which was consistent with their preference for large octahedral site. This contributed to decrease in the net magnetic moment. The substitution of Fe³⁺ ions by Mⁿ⁺ ions is shown in Scheme 1 and it can be seen that all the Mn³⁺, Co³⁺ and Ni²⁺ ions predominantly occupy the octahedral sites of the ferrite sub lattice where as only Zn²⁺ ion have strong preference for tetrahedral site.

Scheme 1 Sketch map of Fe³⁺ ions substituted by M⁺ ions.

The values of anisotropy constant, K were calculated using the values of, H_c and the saturation magnetization (M_s), using the relation,⁴⁶ H_c = 0.96K/M_s; and the squareness ratio was calculated using the formula,⁴⁶ S_q = M_r/M_s. The anisotropy constant was observed to decrease with increasing Mⁿ⁺ content. The squareness ratio was found to vary from 0.048 to 0.247 for all the ferrite compositions.

The proposed cation distribution was correlated with the theoretical lattice constant. The theoretical lattice constant ‘a_th’ was calculated using the following relation.⁴⁸

a_th = (8/3√3)[(r_A + r_o) + √3(r_B + r_o)] (3)

where r_o is the radius of oxygen ion (1.32 Å), r_A and r_B are the ionic radii of tetrahedral (A) and octahedral (B) site respectively. The values of r_A and r_B depend critically on the cation distribution of the given system. The ionic radii for each site were calculated using following relation:⁴⁸

r_A = [C_AFer(Fe²⁺) + C_AZnr(Zn²⁺)] (4)

r_B = [C_BCor(Co²⁺) + C_BFer(Fe²⁺) + C_BCrr(Mn⁺) + C_AZnr(Zn²⁺) + C_BCur(Cu²⁺)]/2 (5)

where r(Fe²⁺), r(Zn²⁺), r(Co²⁺), r(Mn⁺) and r(Cu²⁺) are the cationic radii of Fe, Zn, Co, Mn and Cu respectively; C_AFe and C_AZn are the concentrations of metallic ions in A-site, and C_BCo, C_BMn, C_BCu and C_BFe are the concentrations on metal ion in B-sites. The values of r_A, r_B and a_th are given in Table 4. The observed decrease or increase in the r_A and r_B values might be due to the substitution process. There is a good agreement between the values of experimental and theoretical lattice constants, indicating that the proposed cation distribution was correct.

Table 4 Ionic radius (r_A and r_B) and theoretical lattice constant ‘a_th’ (Å) for Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ annealed at 1000 °C

Ferrites composition	x	Ionic radius [Å]		ath [Å]
		rA	rB
Co0.6Zn0.4Cu0.2MnxFe1.8−xO4	0.2	0.659	0.696	8.422
	0.4	0.656	0.698	8.423
	0.6	0.655	0.699	8.424
	0.8	0.652	0.701	8.425
Co0.6Zn0.4Cu0.2CoxFe1.8−xO4	0.2	0.683	0.692	8.450
	0.4	0.681	0.701	8.471
	0.6	0.666	0.716	8.489
	0.8	0.676	0.716	8.503
Co0.6Zn0.4Cu0.2NixFe1.8−xO4	0.2	0.662	0.708	8.459
	0.4	0.668	0.719	8.497
	0.6	0.674	0.721	8.512
	0.8	0.674	0.744	8.573
Co0.6Zn0.4Cu0.2ZnxFe1.8−xO4	0.2	0.688	0.699	8.478
	0.4	0.681	0.721	8.524
	0.6	0.697	0.731	8.575
	0.8	0.708	0.743	8.626

---

The oxygen parameter (u) was calculated using following equation:⁴⁹

u = [(r_A + r_o)1/√3a + 1/4] (6)

The oxygen parameter (u) varies from 0.3868 Å to 0.3890 Å in the entire compositions.

The inter ionic distances and the radii of the tetrahedral and octahedral bonds are calculated using experimental value of lattice and oxygen position parameter.⁵⁰

The calculated values of entire composition are presented in Table 5.

Table 5 Cation–anion distance in Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ ferrite samples annealed at annealed at 1000 °C

Ferrite composition	x	Tet. bond dAX	Oct. bond dBX	Octa. edge dXX		Tet. edge	Oxygen (u) [Å]
				Shared	Unshared
Co0.6Zn0.4Cu0.2MnxFe1.8−xO4	0.2	1.9807	1.9946	2.6724	2.2775	3.2346	0.3868
	0.4	1.9781	1.9988	2.6860	2.2782	3.2303	0.3865
	0.6	1.9761	2.0035	2.6963	2.2797	3.2270	0.3862
	0.8	1.9739	2.2390	2.7020	2.2801	3.2234	0.3860
Co0.6Zn0.4Cu0.2CoxFe1.8−xO4	0.2	2.0029	2.0934	2.6538	2.9704	3.2709	0.3880
	0.4	2.0009	2.0955	2.6592	2.9712	3.2676	0.3878
	0.6	1.9865	2.0023	2.6877	2.9724	3.2441	0.3867
	0.8	1.9962	1.9977	2.6725	2.9735	3.2599	0.3873
Co0.6Zn0.4Cu0.2NixFe1.8−xO4	0.2	1.9831	2.2378	2.6798	2.2786	3.2385	0.3868
	0.4	1.9901	2.2404	2.6805	2.2812	3.2498	0.3870
	0.6	1.9949	2.2406	2.6740	2.2815	3.2577	0.3873
	0.8	1.9960	2.001	2.6799	2.2976	3.2596	0.3872
Co0.6Zn0.4Cu0.2ZnxFe1.8−xO4	0.2	2.0070	1.9882	2.6453	2.9708	3.2775	0.3883
	0.4	2.0015	1.9913	2.6567	2.9980	3.2687	0.3879
	0.6	2.0162	1.9903	2.6449	2.9783	3.2939	0.3886
	0.8	2.0270	1.9922	2.6419	2.9863	3.3115	0.3890

---

4.4. Electrical studies

Nano-ferrites have more than 50% of iron content in it and is one of the most important material for application in telecommunication and high frequency devices.⁵¹ Ferrites have high electrical resistivity, low dielectric losses, high permeability, thermal stability and are semiconductor in nature.⁵² The electrical resistivity gives useful information about conduction mechanism. The conduction in the nanoferrites sample could be due to three reasons: impurities (extrinsic type), polaron hopping (intrinsic type), and magnetic ordering. According to Chaudhari et al.⁵³ the conduction mechanism involves the electrons exchange between the elements having more than one valence state in the equivalent sites in the crystalline lattice. Knowing the current flowing through the circuit and voltage across the sample, resistivity of the sample was calculated by using the following equation:⁵⁴where R is the resistance of the sample, A = πr², surface area, r is radius of the pellet, l is the thickness of the pellet and ρ is the resistivity of the sample. The variation of DC resistivity with concentration for all the compositions is expounded in Fig. 7 and the values are given in Table 6.

Fig. 7 Variation of DC resistivity with temperature of Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Mn³⁺, Co²⁺, Ni²⁺ and Zn²⁺) annealed at 1000 °C.

Table 6 D.C. electrical resistivity (ρ), drift mobility (μ_d) and energy of activation (E_a), (ρ) at 323 K for all the ferrite compositions annealed at 1000 °C

Ferrite composition	x	Ea (eV)	ρ × 10^9 (ohm cm)	μd × 10^−13 (cm^2 V^−1 s^−1)
Co0.6Zn0.4Cu0.2MnxFe1.8−xO4	0.2	3.56	0.62	11.32
	0.4	3.27	0.89	8.96
	0.6	3.17	1.19	7.86
	0.8	2.45	1.68	6.66
Co0.6Zn0.4Cu0.2CoxFe1.8−xO4	0.2	3.06	0.24	47.40
	0.4	3.02	0.40	23.23
	0.6	2.58	0.62	1303
	0.8	2.65	0.81	8.69
Co0.6Zn0.4Cu0.2NixFe1.8−xO4	0.2	4.82	17.30	0.40
	0.4	3.39	26.00	0.31
	0.6	3.17	39.40	0.20
	0.8	2.45	55.20	0.20
Co0.6Zn0.4Cu0.2ZnxFe1.8−xO4	0.2	2.48	43.51	0.16
	0.4	2.39	24.86	0.32
	0.6	2.62	18.25	0.52
	0.8	2.93	12.30	0.75

---

Fig. 7 clarify that the DC resistivity increased with increase in manganese, nickel and cobalt metal ion concentration from x = 0.2 to x = 0.8 except zinc, which was due to strong preference of zinc for tetrahedral site. The DC electrical resistivity decreased with increase in temperature, indicating semiconducting nature of the ferrite samples.

Drift mobility (μ_d) of the prepared samples was calculated using relation:

μ_d = 1/ηeρ,

where e is the charge on the electron, ρ is the DC electrical resistivity and η is the concentration of charge carriers that can be calculated using relation:

η = N_Aρ_SP_Fe/M,

where M is the molecular weight of ferrite, N_A is the Avagadro's number, P_Fe is number atoms in the formula and ρ_S is bulk density. The drift mobility was inversely proportional to the DC resistivity. Thus the drift mobility decrease with increase in manganese, nickel and cobalt metal ion concentration from x = 0.2 to x = 0.8 except zinc. The relation between DC resistivity and temperature can be expressed as:⁵⁵

ρ = ρ₀e^ΔE/kT,

where ρ₀ is the resistivity extrapolated to infinite temperature, T is the absolute temperature, k is Boltzmann constant, ΔE is activation energy.

The activation energy of all the samples was calculated from the slope of (log ρ) versus (1000/T) (Fig. 8). Thus, energy of activation was obtained from log ρ vs. 1000/T plots and was calculated to be ∼4.0 eV. The values of different parameter are tabulated in Table 6.

Fig. 8 Variation of the logarithm of resistivity (log ρ) with the reciprocal of temperature (1000/T) Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Mn³⁺, Co²⁺, Ni²⁺ and Zn²⁺) annealed at 1000 °C.

4.5. Catalytic performance of substituted cobalt zinc nanoferrites

The magnetic nano-ferrite materials exhibit catalytic activity owing to the presence of transition metal ions which are stable in more than one oxidation states. Due to this reason, the constituent metal ion/ions undergo a cyclic electron transfer process, enabling ferrites to exhibit catalytic activity. In addition, the catalytic activity of spinel ferrites is influenced by various other factors like particle size, redox properties of metal ions and their distribution among the tetrahedral (A) and octahedral (B) co-ordination sites. In general, the transition metal ions occupying A-sites are catalytically inactive; consequently, the catalytic activity is crucially related to the metal cations on B-sites. This is due to the fact that the metal ions present at B-sites are placed at sufficiently large distances from each other, so that the reactant molecule can interact with B-cations.⁵⁶

4.5.1. Control experiments. The control experiment was done in the presence of all the nanoferrites. Typical control experiment in the presence of Co_0.6Zn_0.4Cu_0.2M_0.8Fe_1.0−xO₄ is shown in Fig. 9. Fig. 9 illustrates that negligible dye was degraded both in a direct photolytic process under mercury lamp as well as in the absence of ferrite (MO + H₂O₂ only). The dye degradation efficiency was very less even in the absence of H₂O₂ (MO + ferrite only) both in dark and under light. The removal efficiency was enhanced when Fenton's reagent was generated. However the rate of reaction was significantly enhanced to 99% when reaction was done in the presence of light. Similar trend was found for all the other substituted nano ferrites.

Fig. 9 Typical control experiments for the degradation of MO in the presence of Co_0.6Zn_0.4Cu_0.2Mn_0.8Fe_1.0O₄ ferrites. The reaction conditions are (a) MO + light (b) MO + H₂O₂ + dark (c) MO + H₂O₂ + light (d) MO + ferrite + dark (e) MO + ferrite + light (f) MO + ferrite + H₂O₂ + dark (g) MO + ferrite + H₂O₂ + light.

4.5.2. Catalysis by (Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Zn²⁺, Co²⁺, Ni²⁺ and Mn³⁺. x = 0.2, 0.4, 0.6 and 0.8)) annealed at 1000 °C. The catalytic oxidation of MO was studied by using substituted Co–Cu–Zn nano-catalysts to investigate their performance as catalysts. The dye degradation activity of MO dye with Mn³⁺ substitution Co–Cu–Zn ferrite (Co_0.6Zn_0.4Cu_0.2Mn_xFe_1.8−xO₄ = 0.2, 0.4, 0.6 and 0.8) is shown in Fig. 10. The catalytic activity increased with increase in manganese substitution. The increase might be attributed to the ability of manganese to itself participate in the cyclic electron transfer process. In the case of Mn³⁺ ion substituted compounds, the dye was completely degraded in 40 min. The Mn³⁺ ions also have octahedral site preference. Costa et al.⁵⁷ reported that species at the octahedral site in the magnetite strongly affects the reactivity towards H₂O₂. Other reason of dye degradation is that Mn³⁺ ion (1.51 V) has higher reduction potential as compared to that of Fe³⁺ ions (0.77 V) which endorse the reduction of Mn³⁺ ions and produce Mn²⁺ ions which again continue the catalytic cycle of H₂O₂ decomposition. Similar results were reported by other authors also.^58,59

Fig. 10 Change in absorption with time of methyl orange solution in the presence of Co_0.6Zn_0.4Cu_0.2Mn_xFe_1.8−xO₄ annealed at 1000 °C.

Photo catalytic activity of Co and Zn substituted Co–Cu–Zn compositions annealed at 1000 °C were also evaluated and change in absorption spectra with time after one hour is exemplified in Fig. S1 and S2† respectively. Cobalt preference for the octahedral B site was also reported earlier.⁶⁰ Zinc ion has both tetrahedral and octahedral site preference.^61,62 The increase in the catalytic activity with increasing Co and Zn substituent ions concentration may be attributed to the substitution of trivalent iron ions with divalent zinc ions, which might have led to the development of oxygen vacancies on the surface of catalyst, thereby, increasing their degradation capacity.⁶³

The effect of nickel metal ion substituted Co–Cu–Zn nano ferrites on the dye degradation was shown in Fig. S3.† It is clear from figure that the photo-catalytic activity was best for the Co_0.6Zn_0.4Cu_0.2Ni_0.2Fe_1.6O₄ composition where Fe : Ni ratio is maximum and thereafter it decreases with further decreased in this ratio. Similar results were reported by Pawar et al.⁶⁴ in the Sr_xCa_1−xFe₂O₄ (0.0 ≤ x ≤ 1.0) composition.

4.6. Mechanism of degradation of dye using ferrites

On the basis of the experimental results and literature⁶⁵ the photo-catalytic degradation mechanism of dyes by the Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ under visible light irradiation was proposed and given in Scheme 2 and eqn (13)–(17).

Scheme 2 Schematic representation of mechanism of nano ferrite as photo catalyst terephthalic acid 2-hydroxyterephthalic acid.

Step-1: the reaction of H₂O₂ with Fe(III) and Fe(II) generated a Fenton reagent which produced a hydroxyl radical.

Fe³⁺ + H₂O₂ → Fe²⁺ + HOO˙ + H⁺ (13)

Fe²⁺ + H₂O₂ → Fe³⁺ + OH˙ + OH⁻ (14)

Step-2: photo-excitation of semiconductor valence band electron creating hole (h⁺) and free electron (e⁻).

Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ + hν → Co_0.6Zn_0.4Cu_0.2M_xFe_2−xO₄(e_cb⁻ + h_vb⁺) (15)

Step-3: formation of HO˙ radicals by the reaction of valence band hole (h_vb⁺) with adsorbed H₂O or HO⁻ ions.

Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄(h_vb⁺) + H₂O → H⁺ + OH˙ (16)

Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄(h_vb⁺) + OH⁻ → OH˙ (17)

Step-4: degradation of substrate (MO dye) of valence band hole (h⁺) or by HO˙ radical h⁺.

OH˙, HO^˙₂, O₂˙⁻, h_vb⁺ + MO → degradation product

In order to validate the proposed mechanism terephthalic acid photoluminescence (TAPL) probing technique has been used. Terephthalic acid (TA) reacts with OH˙ to produce highly fluorescent 2-hydroxyterephthalic acid which was monitored using Fluorescence Spectrometer, confirming the proposed mechanism.

4.7. Chemical kinetics

The kinetics of dye degradation can be expressed according to Langmuir–Hinshelwood model.⁶⁶

As the concentration of dye MO in our reactions was relatively low, the rate equation can be re-written as

where k (min⁻¹) is rate constant for pseudo first order reaction and by integrating in limit of C = C₀ at t = 0, above equation can be written as

Thus, the reaction of MO degradation followed the pseudo first order reaction as supported by literature.^67,68 Fig. 11 shows that the linear relationship between ln(C₀/C_t) and irradiation time, suggesting a pseudo first order reaction for typical Co_0.6Zn_0.4Cu_0.2Mn_0.8Fe_1.0O₄ composition.

Fig. 11 Typical plot of ln C₀/C_t vs. irradiation time for photo catalytic degradation of MO for Co_0.6Zn_0.4Cu_0.2M_0.8Fe_1.0O₄ (M = Mn, Co, Zn, Ni).

Similar behavior was observed for all other compositions of ferrites. The calculated rate constant values of all the compositions of ferrites are given in Table 7. The collective effect of all the metal ions substitutes was shown in Fig. 12. It is clear from Table 7 and Fig. 12 that the rate constant values enhanced with increase in all metal ion concentration except nickel. Among all the metals the best catalytic results were obtained in case of Mn. However in the case of nickel substituted Co–Cu–Zn nano ferrite the catalytic activity was maximum at x = 0.2 concentration. Thereafter a decrease in rate constant was observed as discussed earlier also.

Table 7 First order rate constant, k (min⁻¹) for the degradation of MO dye by substituted cobalt zinc ferrites, Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄

Ferrite composition	x	k (min^−1)
Co0.6Zn0.4Cu0.2MnxFe1.8−xO4	0.2	0.037
	0.4	0.102
	0.6	0.123
	0.8	0.216
Co0.6Zn0.4Cu0.2CoxFe1.8−xO4	0.2	0.009
	0.4	0.012
	0.6	0.020
	0.8	0.024
Co0.6Zn0.4Cu0.2ZnxFe1.8−xO4	0.2	0.027
	0.4	0.037
	0.6	0.056
	0.8	0.061
Co0.6Zn0.4Cu0.2NixFe1.8−xO4	0.2	0.107
	0.4	0.036
	0.6	0.008
	0.8	0.004

---

Fig. 12 Plots of rate constant with metal ion concentration in cobalt zinc ferrites for photo catalytic degradation of MO.

4.8. Recyclability and magnetically separability

One of the venerable advantages of using ferrites as heterogeneous catalysts is the easy magnetic separation, which establishes the stability of the catalyst over various catalytic runs. For executing the recyclability experiment the catalyst was separated using a magnet after the completion of the photo catalytic reaction. In order to test the stability and long-term use of ferrites as heterogeneous photo-Fenton catalysts, the same sample was reused four times after separation as shown in Fig. 13. Slight decrease in the activity of samples was observed due to the loss occurred during recovery process.⁶⁹ The XRD of the recovered ferrite samples (Fig. 14(b)) did not show any additional peaks and thus the possibility of decrease in activity due to metal leaching was ruled out.

Fig. 13 Recyclability of Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Mn³⁺, Co²⁺, Ni²⁺ and Zn²⁺) photo catalyst for the degradation of MO for 4 cycles.

Fig. 14 (a) Separation of the nanoferrites by applied magnetic field, (b) XRD pattern of the recovered Co_0.6Zn_0.4Cu_0.2Mn_0.8Fe_1.0O₄.

All the ferrites presented good catalytic behavior without any significant loss in degradation efficiency even after four cycles. Thus the results illustrated ferrites to be stable magnetically separable heterogeneous photo-Fenton catalysts with potential application in the long-term process of waste water treatment. The magnetic separation of all the catalyst was after the reaction is shown in Fig. 14.

5. Conclusions

Different metal substituted Co_0.6Zn_0.4Cu_0.2M_xFe_1.8−xO₄ (M = Zn²⁺, Co²⁺, Ni²⁺ and Mn³⁺. x = 0.2, 0.4, 0.6 and 0.8) have been prepared successfully using sol–gel auto combustion method. X-ray diffraction patterns confirmed the formation of single cubic spinel phase with Fd3̄m space group for all the ferrite samples. Magnetic measurements show that the decrease in both the magnetization and coercivity with increase in substituted all metal ions in Co–Cu–Zn nano ferrites. The drift mobility decrease with increase in manganese, nickel and cobalt metal ion concentration from x = 0.2 to x = 0.8 except zinc. The catalytic activity of the ferrites was found to be dependent upon the redox properties of metal ions and their distribution of cations among the tetrahedral and octahedral sites of the ferrite sub-lattice. Manganese substituted nanoferrites shows best catalytic activity among the prepared substituted nanoferrites due to their higher redox potential and octahedral site preference.

Acknowledgements

The authors express their deep gratitude to DST (SERB) for providing grant under major research project and University Grants Commission (UGC) for providing fellowship to Santosh Bhukal.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22561b

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