Synthesis and Characterization of Fe_0.8Mn_0.2Fe₂O₄ Ferrite Nanoparticle with High Saturation Magnetization via the Surfactant Assisted Co-Precipitation

Abstract

Manganese ferrite nanoparticles (MnFe₂O₄) were synthesized via surfactant-assisted co-precipitation, where sodium dodecyl sulfate (SDS) was used as the template to control particle size at various SDS concentrations. The substitutions of iron (II) (Fe²⁺) into the MnFe₂O₄ ferrite nanoparticles were carried out to obtain Fe₍₁–_x)Mn_xFe₂O₄, with various Mn²⁺: Fe²⁺ molar ratios. The synthesized ferrite nanoparticles were characterized by the Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analyzer (TGA), X-ray diffractometer (XRD), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), two-point probe, and vibrating sample magnetometer (VSM) techniques. The experimental Mn:Fe mole ratios of the Fe_(1−x)Mn_xFe₂O₄ ferrite nanoparticles were verified to be in agreement with the theoretical values. The synthesized MnFe₂O₄ and Fe_(1−x)Mn_xFe₂O₄ ferrite nanoparticles were of mixed spinel structures, with average spherical particle sizes between 17–22 nm, whereas the magnetite ferrite nanoparticles (Fe₃O₄) were of the inverse spinel structure. They showed soft ferromagnetic behavior. The synthesized Fe_0.8Mn_0.2Fe₂O₄ ferrite nanoparticle possessed the highest saturation magnetization of 88 emu/g relative to previously reported work to date.

1. Introduction

Magnetic transition oxides or spinel ferrite nanoparticles are among the magnetic nanoparticles widely used in industrial and biomedical applications such as drug delivery [1], drug release [2], removal of heavy metal from water [3], catalyst [4], permanent magnet [5], tissue engineering [6], hyperthermia [7,8,9,10], magnetic resonance imaging (MRI) [11], gas sensor [12], and actuator [13], due to their thermal, optical, electrical, and magnetic properties [14,15]. The general spinel ferrite formula is AB₂O₄, where A and B are metallic cations residing at two crystallographic sites: the tetrahedral (A site is a divalent ion such as Mn²⁺, Ni²⁺, Co²⁺, Fe²⁺, Cu²⁺, and Zn²⁺) and the octahedral (B site is a trivalent ion, usually is Fe³⁺) [16]. Among the spinel ferrite nanoparticles, manganese ferrite (MnFe₂O₄) is a mixed (partially inverse) spinel structure with good properties such as high anisotropy, high Curie temperature, size-dependent saturation magnetization, low coercivity, and superparamagnetic behavior at room temperature [17,18]. Nevertheless, the properties of the manganese ferrite nanoparticles depend on their particle sizes and shapes, which are controlled by various synthesis conditions: the type of salt used, the reaction temperature, the pH value, the stirring rate, the ionic strength of the media, the M (divalent ion) to Fe³⁺ ratio, and the synthesis method [19,20,21].

Many methods have been used to synthesize manganese ferrite nanoparticles, such as sol-gel [22], thermal decomposition [23], solvothermal [24], micro-emulsion [25], and co-precipitation [18]. Among the synthesis routes, co-precipitation is one preferred method due to its simplicity, low cost, high yield, ease to control particle size, processibility at lower temperature, and environmentally friendly synthesis route [18,26,27]. However, the magnetic nanoparticles as obtained from this method normally aggregate, thus altering magnetic properties. Therefore, the functionalization and modification, such as using metals, polymer coating, and surfactants absorption on the surface, have been found to reduce the aggregation [21,28,29]. Recently, a cobalt ferrite nanoparticle (CoFe₂O₄) was synthesized by the co-precipitation method at various cetyltrimethylammonium bromide (CTAB) concentrations. The aggregation decreased with increasing CTAB surfactant [30]. Furthermore, the substitution at the tetrahedral with different divalent cations led to the changes in the magnetic properties. Modaresi et al. synthesized Mn_xFe_3−xO₄ ferrite nanoparticles by the co-precipitation method, with x varying from 0 to 1. The saturation magnetization increased with increasing Mn²⁺ ion content, as the Mn²⁺ ion magnetic moment is higher than the Fe²⁺ ion [31]. Other works on the manganese ferrite synthesis were reported with the saturation magnetizations and particle sizes to be between 15–69 emu/g and 5–46 nm, respectively [9,17,18,23,24,32,33,34,35].

In this work, the synthesis of Fe_(1−x)Mn_xFe₂O₄ ferrite nanoparticles was carried out via the surfactant-assisted co-precipitation method. The physical, thermal, electrical, magnetic, and dispersion properties of the synthesized Fe_(1−x)Mn_xFe₂O₄ ferrite nanoparticles were investigated under the effects of the surfactant type, the SDS surfactant concentration, and the Mn²⁺: Fe²⁺ molar ratio substitution, where x’s value was varied from 1 to 0 (x = 1, 0.6, 0.2, and 0). It will be shown that SDS was successfully used as the template to control the particle size and hence the saturation magnetization; the highest saturation magnetization value to date of 88 emu/g was obtained with x equal to 0.2 and with the nanoparticle size of 18 ± 3 nm exhibiting soft ferromagnetic behavior.

2. Materials and Methods

2.1. Materials

Iron (III) chloride (FeCl₃∙2H₂O, 99%, Sigma-Aldrich, Munich, Germany), manganese (II) chloride (MnCl₂, Synthesis grade, Merck, Darmstadt, Germany), iron (II) sulfate (FeSO₄∙7H₂O, 99%, Univar, Ajax Finechem Pty Ltd, Australia), hexadecyltrimethylammonium bromide (CTAB, ≥96%, Fluka, Fluka Chemiclas Limited, Dorset Gillingham, UK), sodium dodecyl sulfate (SDS, Omnipur, Merck, Darmstadt, Germany), polysorbate20 (Tween20, Sigma-Aldrich, Lyon, France), 25 wt% Ammonium hydroxide solution (NH₄OH, AR grade, Merck, Darmstadt, Germany), deionized water, and ethanol (AR grade, RCI Labscan, Bangkok, Thailand) were obtained and used without further purification.

2.2. Synthesis of Ferrite Nanoparticles

The synthesis of ferrite nanoparticles via the surfactant-assisted co-precipitation method was carried out in two parts.

2.2.1. Effect of Surfactant Types and Concentrations

The synthesis of MnFe₂O₄ ferrite nanoparticles was carried out at various SDS concentrations. First, 0.81 g of FeCl₃∙2H₂O was mixed with 0.32 g of MnCl₂ at the Fe³⁺:Mn²⁺ molar ratio of 0.10:0.05 and then dissolved in 25 mL deionized water. The SDS solutions, at the 0.5, 1.0, and 1.2 CMC concentrations (CMC is the Critical Micelle Concentrations of surfactant), were separately prepared by dissolving 0.058 g, 0.12 g, and 0.15 g in 25 mL deionized water at room temperature. Then, the prepared metal precursor solution was mixed with SDS solution. The mixture solution was vigorously stirred at 150 rpm and room temperature of 27 °C for 1 h to obtain a homogeneous solution and then heated up to 80 °C under a nitrogen gas flow. Then, 25 wt% NH₄OH solution (19 mL) was added into the mixture solution dropwise, in which an orange solution changed into a black solution. The mixture solution was vigorously stirred at 80 °C for 4 h for the precipitation to proceed. Finally, the black precipitate was obtained through cooling down to room temperature, centrifugation, washing several times with deionized water and ethanol to remove the residue surfactant, and drying in an oven at 80 °C for 24 h to obtain the MnFe₂O₄ ferrite nanoparticles. The MnFe₂O₄ ferrite nanoparticles with CTAB and Tween20 were prepared by the same procedure at the surfactant concentration of 1.0 CMC.

2.2.2. Effect of Mn²⁺:Fe²⁺ Molar Ratios

The Fe_(1−x)Mn_xFe₂O₄ ferrite nanoparticles were synthesized at various Mn²⁺: Fe²⁺ molar ratios, where x’s value was varied from 1 to 0 (x = 1, 0.6, 0.2, and 0). The Fe³⁺: Mn²⁺: Fe²⁺ molar ratios of the metal precursor solutions were 0.10:0.05:0.00 (0.81 g:0.32 g:0.0 g), 0.10:0.03:0.02 (0.81 g:0.19 g:0.28 g), 0.10:0.01:0.04 (0.81 g:0.063 g:0.56 g), and 0.10:0.00:0.05 (0.81 g:0.0 g:0.70 g); they were prepared under the same procedure as above but at the 1.2 SDS CMC, in which FeSO₄∙7H₂O was mixed with FeCl₃∙2H₂O and MnCl₂ and then dissolved in 25 mL deionized water at room temperature. The bare MnFe₂O₄ ferrite nanoparticle was also prepared at the same condition without the surfactant to compare with the MnFe₂O₄ ferrite nanoparticles synthesized using SDS. The synthesized Fe_(1-x)Mn_xFe₂O₄ ferrite nanoparticles were coded as Fe_(1−x)Mn_xFe₂O₄_yCMC_SDS, where x is the fractional molar ratio of Mn²⁺: Fe²⁺ and y is the fraction of the CMC of SDS. The synthesis schematic of the Fe_(1−x)Mn_xFe₂O₄ ferrite nanoparticles at various SDS concentrations is shown in Scheme 1.

2.3. Characterization of Ferrite Nanoparticles

The structure of the MnFe₂O₄ ferrite nanoparticles was characterized for the functional groups by a Fourier transformed infraredspectrometer, FT-IR (Thermo Scientific, Nicolet iS5, Therno Fisher Scientific, Waltham, MA, USA) in the wavenumber range of 650 cm⁻¹ to 4000 cm⁻¹. Each sample was mixed with the ground KBr and then compressed.

A thermogravimetric analyzer, TGA (Perkin Elmer, TGA7, PerkinElmer, Waltham, MA, USA) was used to determine the thermal behavior and to confirm the structure of the MnFe₂O₄ ferrite nanoparticles. Each sample powder of 5–10 mg was loaded into a platinum pan as temperature was varied from 37 °C to 700 °C with the heating rate of 10 °C/min under a nitrogen flow.

A wide-angle X-ray diffractometer, XRD (Rigaku, SmartLab, Rigaku Corporation, Tokyo, Japan) was used to study the crystal structure of the ferrite nanoparticles. The CuK-alpha radiation source was operated at 40 kV/30 m and a K-beta filter was used to eliminate the interference peak. Each sample was dried, ground, compressed by a hydraulic machine to obtain a disc shape, and then placed on the sample holder. The 2θ range was from 10 deg. to 80 deg. with the scan speed of 2°/min and the scan step of 0.02°.

A high-resolution field-emission scanning electron microscope with energy-dispersive X-ray, FE-SEM-EDX (Hitachi, S-4800 FE-SEM-EDX, Hitachi High-Tech Science Corporation, Tokyo, Japan) was used to determine the chemical composition ratios of the ferrite nanoparticles. Each sample was distributed on the sample holder and then coated with a thin platinum layer by ion-sputtering. The copper grid was used as the reference.

An X-ray photoelectron spectroscopy, XPS (Kratos, Axis Ultra DLD, Manchester, UK) was used to examine the atomic percentages of Fe, Mn, and O atoms of the ferrite nanoparticles. The experiment was operated by a monochromatized A1 K source in the wide scan mode.

A high-resolution field-emission scanning electron microscope, FE-SEM (Hitachi, S-4800 FE-SEM, Hitachi High-Tech Science Corporation, Tokyo, Japan) was used to investigate the morphology and to measure the particle sizes of the ferrite nanoparticles. Each sample was distributed on a sample holder and then coated with a thin platinum layer by ion-sputtering. The SEM images were obtained by using an acceleration voltage of 2 kV, a current of 10 mA, and a magnification of 70 kx. The average particle sizes were determined from the SEM images from at least 100 random nanoparticles.

A transmission electron microscope, TEM (Hitachi, H-7650, Hitachi High-Technologies Corporation, Tokyo, Japan) was also used to determine the particle shapes and sizes of the ferrite nanoparticles. Each sample was dispersed in ethanol, and the solution was coated onto a copper grid. The instrument was operated at 100 kV.

The electrical conductivity of the ferrite nanoparticles was measured by an electrometer (Keithley, 6517A, Tektronic, Portland, OR, USA) connected to a custom-built two-point probe. Each sample was ground and compressed by a hydraulic machine to obtain a disc shape. The applied voltage and the resultant current were plotted to obtain the slope, which was used to calculate the electrical conductivity of the ferrite nanoparticles by using Equation (1) [13]:

σ = 1/ρ = 1/(R_s × t) = I/(K × V × t) = slope/(K × t),

(1)

where σ is the specific conductivity (S/cm), ρ is the specific resistivity (Ω cm), R_s is the sheet resistivity (Ω), I is the measured current (A), K is the geometric correction factor (1.00 × 10⁻⁴), V is the applied voltage, and t is the disc thickness (cm).

A vibrating sample magnetometer, VSM (LakeShore, Series 7400 model 7404, Lake Shore Cryotronics, Inc., Westerville, OH, USA) with a 4 in electromagnet was used to measure the magnetic properties of the ferrite nanoparticles, namely the saturation magnetization (M_s), coercivity (H_c), and remanent magnetization (M_r). The hysteresis loops were measured under the magnetic field strength of 10,000 Gauss at room temperature. The data were taken at 80 points/loop with a scan speed of 10 s/point.

3. Results

3.1. Characterization of Ferrite Nanoparticles

In the surfactant assisted co-precipitation method of the ferrite nanoparticles, SDS was used as the surfactant template. The mechanism of co-precipitation reaction is represented by the following Equation (2) [36]:

2Fe³⁺ + 6OH⁻ 2Fe(OH)₃

xMn²⁺ + 2OH⁻ xMn(OH)₂

(1 − x)Fe²⁺ + 2OH⁻ (1 − x)Fe(OH)₂

2Fe(OH)₃ + xMn(OH)₂ + (1 − x)Fe(OH)₂ Fe_(1−x)Mn_xFe₂O₄ + 4H₂O

(2)

In the synthesis mechanism, the base solution (NH₄OH) was added into the mixed metal ion precursor solution (Fe³⁺, Mn²⁺, and Fe²⁺) to precipitate the ferrite nanoparticles; the OH^- from NH₄OH interacted with the metal ion precursors to form the hydroxide precipitant and by-product water. The presence of SDS in the synthesis provided the micelles which stabilized the metal ion precursors by the interaction between the SO₄²⁻ polar group of SDS and the metal cation precursors [37]. SDS interacted with the precipitant surface by the bridges of hydroxide precipitant; thus, SDS prevented the aggregation of ferrite particles at a nano-scale [38]. Finally, SDS was removed by washing with water and ethanol. The synthesis reaction under N₂ atmosphere prevented the oxidation of Fe²⁺ ions to Fe³⁺ ions by the O₂ atmosphere [21]. The substitution of MnFe₂O₄ ferrite nanoparticles with Fe²⁺ ions by varying the Mn²⁺: Fe²⁺ molar ratio was known to affect the magnetic properties [39].

3.1.1. FT-IR Analysis

The chemical structure of the MnFe₂O₄ was investigated by FT-IR spectroscopy as shown in Figure 1. The FT-IR spectrum of SDS showed peaks at 1081 cm⁻¹, corresponding to the S-O stretching vibration; 1468 cm⁻¹, corresponding to the C-O stretching vibration; and 2955 and 2850 cm⁻¹, corresponding to the C-H stretching vibration [37]. The FT-IR spectra of the bare MnFe₂O₄ and MnFe₂O₄_1.2CMC_SDS show the peaks at 3344–3387 cm⁻¹ and 1518–1630 cm⁻¹, corresponding to the bending and stretching vibrations of the O-H groups, respectively. These peaks occurred from the humidity [31]. The FT-IR spectrum of MnFe₂O₄_1.2CMC_SDS indicates that SDS was not present. Thus, it can be concluded that the SDS was completely eliminated by washing with water and ethanol.

3.1.2. TGA Analysis

The thermograms (TGA) and derivative thermograms (DTG) of the MnFe₂O₄ are shown in Figure 2a,b. The SDS thermogram shows a significant weight loss between 180 and 350 °C of about 69.18%, due to the degradation of SDS [40]. The degradation temperature of the SDS surfactant is identified at 212.34 °C from the DTG curve in Figure 2b. The bare MnFe₂O₄ has a weight loss of 3.66% between 100 and 350 °C from the removal of adsorbed water, whereas the MnFe₂O₄_1.2CMC_SDS has the weight loss of 4.79% between 60 and 160 °C due to the water evaporation [41]. The thermograms suggest that the ferrite nanoparticles consist of over 95% inorganic phase of the MnFe₂O₄, and the synthesized MnFe₂O₄ is of good thermal stability. Furthermore, the MnFe₂O₄_1.2CMC_SDS has no weight loss related to the SDS surfactant. This confirms that the SDS surfactant was totally removed from the MnFe₂O₄_1.2CMC_SDS.

3.1.3. X-Ray Diffraction (XRD) Analysis

Generally, the spinel ferrite has a structural formula of AB₂O₄ (where A and B are the divalent and trivalent metallic cations in the tetrahedral and octahedral sites, respectively) with the spinel ferrite crystal structure. For the MnFe₂O₄ ferrite, it is a face-centered cubic mixed spinel structure (Fd3m) with a structural formula of (Mn²⁺_1−cFe³⁺_c)_A[Mn²⁺_cFe³⁺_2−c]_BO₄. The substitution of divalent iron ions (Fe²⁺) changes the occupancy of the mixed (partially inverse) spinel in the MnFe₂O₄ crystal to the inverse spinel crystal structure of Fe₃O₄ crystal with the structural formula of (Fe³⁺)_A[Fe²⁺Fe³⁺]_BO₄ [27,42,43].

The XRD patterns of the bare MnFe₂O₄ and Fe_(1-x)Mn_xFe₂O₄_1.2CMC_SDS, with various Mn²⁺: Fe²⁺ molar ratios, where x’s value varies from 1 to 0 (x = 1, 0.6, 0.2, and 0), are shown in Figure 3a. The XRD patterns of the Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS at various Mn²⁺:Fe²⁺ molar ratios show the same XRD patterns as the bare MnFe₂O₄ with the diffraction peaks at the (111), (220), (311), (400), (422), (511), (440), and (533) crystal planes indicating the cubic spinel structure with the Fd3m space group [27,31,39,44]. Furthermore, each pattern does not show the diffraction peaks of other components, which may occur from the oxidation reaction. This confirms that the metal ion precursors (Fe³⁺, Mn²⁺, and Fe²⁺) were completely bounded to the ferrite nanoparticles [27].

All of the magnetic ferrite XRD patterns show the main diffraction peak at around 2θ ≈ 35.1°–35.9° corresponding to the (311) crystal plane, and the averaged crystalline size (D) was calculated from this peak by using the Debye-Scherrer’s Equation (3) [45]:

D = Kλ/(βcosθ)

(3)

where D is the average crystallite size (nm), K is the shape factor that depends on the structure of the crystallite (in this case, the crystallite shape is spherical; K = 0.9), λ is the X-ray wavelength (Cu Kα-radiation; 0.154056 nm), β is the full width at half-maximum (FWHM) of the main diffraction peak (331) (radian), and θ is the Bragg’s angle (radian). The averaged crystallite sizes are tabulated in

Table 1. Average crystallite size (D), lattice constant (a), unit volume cell (V_cell), hopping length for tetrahedral site (L_A), hopping length for octahedral site (L_B), X-ray density (${\mathsf{\rho }}_{\mathrm{x}-\mathrm{ray}}$ ), specific surface area (S), particle size (d), aggregated particle size (d’) of the synthesized bare MnFe₂O₄, MnFe₂O₄ at various SDS surfactant concentrations, and Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS ferrite nanoparticles.

Sample	D (nm)	a (Å)	Vcell (Å3)	LA (Å)	LB (Å)	${\mathsf{\rho }}_{\mathbf{X-ray}}$ (×103 Kg/m3)	S (m2/g)	d (nm)	d’ (nm)
Bare MnFe2O4	10.6	8.39	591	3.66	2.99	5.19	111	36 ± 6	-
MnFe2O4_0.5CMC_SDS	10.5	8.39	591	3.66	2.99	5.19	113	22 ± 5	-
MnFe2O4_1.0CMC_SDS	8.47	8.39	591	3.63	2.97	5.19	137	17 ± 3	49 ± 8
MnFe2O4_1.2CMC_SDS	7.80	8.46	604	3.66	2.99	5.07	152	17 ± 3	40 ± 8
Fe0.4Mn0.6Fe2O4_1.2CMC_SDS	7.82	8.39	590	3.63	2.97	5.20	148	18 ± 2	-
Fe0.8Mn0.2Fe2O4_1.2CMC_SDS	10.8	8.35	582	3.62	2.95	5.28	106	18 ± 3	-
Fe3O4_1.2CMC_SDS	13.3	8.30	572	3.60	2.94	5.38	83.9	22 ± 4	-

. From Table 1, the averaged crystallite size of the MnFe₂O₄ ferrite nanoparticles at various SDS concentrations decreases with increasing SDS concentration from 10.6 nm to 7.80 nm. This is because the surfactant influences the particle–particle interaction at the nano-scale; the higher surfactant concentration corresponds to the weaker particle–particle interaction, and the crystallite size is reduced [30]. The averaged crystallite size of Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS increases with increasing Fe²⁺ cation substitution (with x = 1 to 0) from 7.80 nm to 13.3 nm, as the increase in Fe²⁺ ions substitution increases the crystal growth rate, leading to the crystallite size enlargement [46]. Moreover, the (311) crystal plane peak is shifted toward the higher angle with increasing Fe²⁺ ions, and the broadened peaks imply smaller crystallite sizes, as shown in Figure 3b [47]. The lattice constant (a) was calculated from the (311) crystal plane by using Equation (4) [14];

$$\mathrm{a}=\mathrm{d}\sqrt{h^2+k^2+l^2}$$

(4)

where d is the interplane distance, and (hkl) are the Miller indices of the crystal planes. The volume of unit cell (V_cell) was calculated by using Equation (5) [45];

V_cell = a³

(5)

The lattice constants and the volumes of the unit cell are listed in Table 1; these two parameters decrease with increasing SDS surfactant concentration, and the Fe²⁺ ions substitution (x). The decrease of the lattice constant with increasing Fe²⁺ ions substitution (x) is due to the ionic radius effect. The larger ionic radius of Mn²⁺ (0.83 Å) in the ferrite nanoparticles structure is replaced by the smaller ionic radius of Fe²⁺ (0.78 Å), resulting in a decrease in the lattice constant and consequently the volume of unit cell [48]. The hopping length for tetrahedral site (L_A) and octahedral site (L_B) were calculated by Equations (6) and (7), respectively [49]:

$${\mathrm{L}}_{\mathrm{A}}=\mathrm{a}\sqrt{3}/4$$

(6)

$$\mathrm{L}{\mathrm{B}}_{}=\mathrm{a}\sqrt{2}/4$$

(7)

The hopping length L_A and L_B values are tabulated in Table 1. The hopping lengths L_A and L_B under the effect of SDS concentrations change insignificantly, while the hopping lengths L_A and L_B under the effect of increasing Fe²⁺ ions substitution (x) decrease slightly. The L_A and L_B values are related to the lattice constant values as shown in Table 1. The X-ray density (${\mathsf{\rho }}_{\mathrm{x-ray}}$) and the specific surface area of nanoparticles are calculated by Equations (8) and (9), respectively [50]:

$${\mathsf{\rho }}_{\mathrm{x-ray}}=8\mathrm{M}/{\mathrm{N}}_{\mathrm{A}}{\mathrm{a}}^3$$

(8)

S = 6/Dρ_x-ray

(9)

where M is the molecular weight of the sample (g/mol), N_A is Avogadro’s number (6.02 × 10²³ mol⁻¹), and D is the crystallite size (nm). The X-ray density values and specific surface areas are tabulated in Table 1. The specific surface area of Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS decreases with increasing Fe²⁺ ions content, as the surface area is inversely related to the particle size that increases. A larger particle size corresponds to a smaller specific surface area and vice versa [31]. From the XRD results, the differences in crystallite size, lattice constant, the volume of unit cell, hopping length of L_A, hopping length of L_B, and specific surface area confirm that the Fe²⁺ ions were successfully substituted into the MnFe₂O₄, consistent with previous works [31,39,41,50].

3.1.4. EDX and XPS Measurements

The chemical compositions of Mn:Fe in each Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS were verified by the atomic percentages of Fe, Mn, and O atoms from the EDX and XPS measurements. From the EDX results, the Mn:Fe molar ratios of MnFe₂O₄, Fe_0.4Mn_0.6Fe₂O₄, and Fe_0.8Mn_0.2Fe₂O₄ were 1:2.02, 1:3.82, and 1:13.64, respectively. These results indicate that the Fe element increases with an increasing amount of Fe²⁺ ions substitution in the MnFe₂O₄, and the experimental Mn:Fe ratios were quite close to the theoretical molar ratios, namely 1:2, 1:4, and 1:14, respectively. The survey scan XPS spectra of the ferrite nanoparticles are displayed in Figure 4. Each XPS spectrum illustrates the binding energy peaks of 706.1 eV, 650.1 eV, and 529.2 eV, corresponding to the Fe 2p, Mn 2p, and O 1s, respectively [51]. The Mn:Fe molar ratios for the MnFe₂O₄, Fe_0.4Mn_0.6Fe₂O₄, and Fe_0.8Mn_0.2Fe₂O₄ from the XPS results are 1:2.02, 1:4.09, and 1:14.03, respectively. These values are close to the EDX results and the theoretical molar ratios. Hence, the EDX and XPS results confirm that the Fe²⁺ ions were successfully substituted into the MnFe₂O₄.

3.1.5. SEM and TEM Analysis

The morphological characteristics namely the shape, size, and degree of aggregation of the MnFe₂O₄ and Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS of various synthesized conditions were examined by the SEM and TEM techniques, as shown in Figure 5a–d″ or Figure 6a–c.

From the SEM images, all ferrite nanoparticles are of spherical shapes with the apparent individual particle sizes of 36 ± 6 nm (bare MnFe₂O₄), 22 ± 5 nm (MnFe₂O₄_0.5CMC_SDS), 17 ± 3 nm (MnFe₂O₄_1.0CMC_SDS), and 17 ± 3 nm (MnFe₂O₄_1.2CMC_SDS), as tabulated in Table 1. The individual particle size decreases with increasing SDS surfactant concentration, consistent with the decreasing crystallite size in Table 1. Adding the SDS surfactant reduces the crystallite and particle sizes as the SDS micelles disrupt the particle–particle interaction and the crystal growth rate [21,30,44,49]. For the MnFe₂O₄_1.0CMC_SDS and Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS, the aggregated particle sizes (d’) are 49 ± 8 nm and 40 ± 8 nm, respectively. The particle aggregation possibly occurred from the removal of SDS after the synthesis to form the larger particle sizes.

For the effect of increasing Fe²⁺ ions substitution using SDS at 1.2 CMC, the apparent individual particle sizes of the MnFe₂O₄_1.2CMC_SDS, Fe_0.4Mn_0.6Fe₂O₄_1.2CMC_SDS, Fe_0.8Mn_0.2Fe₂O₄_1.2CMC_SDS, and Fe₃O₄_1.2CMC_SDS are 17 ± 3, 18 ± 2, 18 ± 3, and 22 ± 4 nm, respectively, as tabulated in Table 1. The individual particle size increases slightly with increasing the Fe²⁺ ions substitution, possibly due to the relatively higher crystal growth rates, consistent with the increase in crystallite size as shown in Table 1 [46,52].

The TEM images of the MnFe₂O₄_1.2CMC_SDS and Fe_0.8Mn_0.2Fe₂O₄_1.2CMC_SDS are shown in Figure 5d″ or Figure 6b’, respectively. The individual spherical particle sizes of MnFe₂O₄_1.2CMC_SDS and Fe_0.8Mn_0.2Fe₂O₄_1.2CMC_SDS are 8 ± 2 and 11 ± 2 nm, respectively. Thus, the TEM images indicate that the particle size also increases with increasing Fe²⁺ ions substituted consistent with the SEM and XRD results.

3.2. Electrical Conductivity Analysis

Generally, the spinel ferrites are classified as semiconductor materials in which the electrical conduction takes place by the electron exchange between the same ion elements. For the MnFe₂O₄ ferrite nanoparticles, it has two electrical conduction hopping processes: (i) the hole hopping between Mn²⁺ and Mn³⁺ ions; and (ii) the electron hopping between Fe²⁺ and Fe³⁺ ions [14,53,54,55,56]. The electrical conductivity of MnFe₂O₄_1.2CMC_SDS, Fe_0.4Mn_0.6Fe₂O₄_1.2CMC_SDS, Fe_0.8Mn_0.2Fe₂O₄_1.2CMC_SDS, and Fe₃O₄_1.2CMC_SDS are 2.04 × 10⁻³, 3.26 × 10⁻³, 4.08 × 10⁻³, and 9.68 × 10⁻³ S/cm, respectively, as shown in Figure S2. The MnFe₂O₄_1.2CMC_SDS has a lower electrical conductivity than others, simply because it has no Mn³⁺ and Fe²⁺ ions in the ferrite structure; it has only the hopping process between Mn²⁺ and Fe³⁺ ions. The electrical conductivity increases with increasing the Fe²⁺ substitution where the Fe₃O₄ attains the highest electrical conductivity. The increase in the number of Fe²⁺ substituted allows the Fe³⁺ ions to remain at the octahedral site (B site). Therefore, the electron hopping between Fe²⁺ and Fe³⁺ ions is promoted, resulting in the higher electrical conductivity; a high electrical conductivity provides a potential for uses in actuator, bioengineering, and biomedical applications, particularly the actuations under an applied electric field, as shown in previous publications. Petcharoen et al. studied the actuation under applied electric and electromagnetic field of magnetite nanoparticles (Fe₃O₄)/polyurethane (PU) composites; the storage modulus (G’) and the storage modulus sensitivity (ΔG’/G’₀) of the composites increased with increasing Fe₃O₄ contents under applied electric field, and the deflection responses also increased with increasing Fe₃O₄ contents under applied electric and electromagnetic fields [13]. Mohanty et al. developed the polyvinyl alcohol (PVA)/magnetite (Fe₃O₄) nanocomposite membrane; the increase of Fe₃O₄ contents increased the electrical conductivity, resulting in the increase in charge transport of the nanocomposite membrane under applied electric field [57].

3.3. Vibrating Sample Magnetometer (VSM) Analysis

The magnetic properties of magnetic materials are known to depend on the structure, size, shape, and composition [58]. The magnetization versus magnetic field plots (M-H hysteresis loop) provide the magnetism behavior as well as the values of saturation magnetization (M_s), coercivity (H_c), and remanent magnetization (M_r). The magnetic properties of the bare MnFe₂O₄ and Fe_(1-x)Mn_xFe₂O₄_1.2CMC_SDS are shown in Figure 7 and tabulated in

Table 2. The saturation magnetization (M_s), coercivity (H_c), remanent magnetization (M_r), and magnetic moment in the units of Bohr magneton of the synthesized bare MnFe₂O₄ and MnFe₂O₄ at various SDS surfactant concentrations, and Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS ferrite nanoparticles.

Sample	Ms (emu/g)	Hc (Oe)	Mr (emu/g)	Magnetic Moment in the Units of Bohr Magneton
Bare MnFe2O4	27	27	1.6	1.1
MnFe2O4_0.5CMC_SDS	55-6	19	2.4	2.3
MnFe2O4_1.0CMC_SDS	73	20	2.8	3.1
MnFe2O4_1.2CMC_SDS	57	17	1.5	2.4
Fe0.4Mn0.6Fe2O4_1.2CMC_SDS	85	16	2.4	3.6
Fe0.8Mn0.2Fe2O4_1.2CMC_SDS	88	23	2.8	3.7
Fe3O4_1.2CMC_SDS	87	31	4.6	3.7

. All hysteresis loops of the ferrite nanoparticles are quite narrow with small amounts of energy dissipated, thus exhibiting the soft ferromagnetic behavior at room temperature, as also suggested by the inset plots of Figure 7, Figures S3 and S4 [21,58,59]. Moreover, the small H_c could imply that the energy barriers are larger than the thermal, energy and hence the magnetic moments will be blocked in the parallel or anti-parallel directions, resulting in the small hysteresis loop (not superparamagnetic). When the energy barriers are smaller than the thermal energy, the energy barriers are overcome and the rotation of the magnetic moments becomes spontaneous under applied the magnetic field, resulting in the non-hysteresis loop (superparamagnetic).

3.3.1. Effect of Surfactant Types

The effect of surfactant types, namely CTAB, SDS, and Tween20, on the magnetic properties are shown in Figure S3. For the MnFe₂O₄ ferrite nanoparticle with SDS, the M_s value is 73 emu/g, relatively higher than CTAB (67 emu/g) and Tween20 (57 emu/g). This is because only the SDS molecules can provide the steric repulsive force to balance the magnetic and van der Waals attractive forces between the nanoparticles [44,60]. All MnFe₂O₄ with surfactants possesses higher M_s values than the bare MnFe₂O₄ (27 emu/g), as the surfactants can influence the crystallite and particle sizes, the nanoparticle aggregation, and consequently the surface spin disorder [44,49,60]. The SDS surfactant appears to be the most effective surfactant to provide the high M_s value.

3.3.2. Effect of SDS Surfactant Concentrations

The M_s values of the bare MnFe₂O₄ and the MnFe₂O₄ with various SDS concentrations are tabulated in Table 2 and Figure S4. The M_s value increases from 27 emu/g to 73 emu/g, with increasing SDS concentration from 0.0 to 1.0 CMC. At 1.2 CMC, the M_s value decreases to 57 emu/g. The corresponding individual particle sizes (d) are 36 nm, 22 nm, and the aggregated particle sizes (d’) are 49 nm, and 40 nm, with increasing SDS concentration, as tabulated in Table 1. The SDS presence affects the M_s value as the SDS molecules can influence the particle–particle interaction and consequently the surface spin disorder [30,49]. The individual particle size (d) here refers to the visibly apparent particle sizes in Figure 5a,b, and the aggregated particle sizes (d’) can be seen in Figure 5c’,d’; the latter particles consist of many smaller and aggregated particles. The M_s values in Table 2 can be correlated with the particle size (d) of bare MnFe₂O₄ and MnFe₂O₄_0.5CMC_SDS and the aggregated particle sizes (d’) of MnFe₂O₄_1.0CMC_SDS and MnFe₂O₄_1.2CMC_SDS, as shown in Table 1. Here, M_s increases with particle size due to the reduction in spin disorders from the particle aggregation, after the removal of SDS [49].

The optimum surfactant concentrations to obtain the highest M_s values have been reported. Zhang et al. reported the M_s value of MnFe₂O₄ nanoparticles increased with increasing CTAB surfactant concentration as synthesized by a co-precipitation method; the highest M_s value was 54.5 emu/g at 0.5 M of CTAB surfactant; it became lower at higher concentrations [44]. Vadivel et al. reported that the M_s value of CoFe₂O₄ nanoparticles increased with increasing SDS surfactant concentration; the highest M_s value was 138.75 emu/g at 0.08 M of SDS surfactant [49]. The M_s values of the synthesized MnFe₂O₄ in this work are in the range of 15–69 emu/g, consistent with other previous reports [11,17,18,23,24,32,33,34,35,39,41,61]. When comparing the M_s values in the bulk or film forms, the M_s value of the presently synthesized MnFe₂O₄ nanoparticle is lower than that of the bulk form at 82 emu/g [61], but higher than the film form at 38 emu/g [61].

3.3.3. Effect of Mn²⁺:Fe²⁺ Molar Ratio Substitutions

Under the effect of Fe²⁺ ion substitutions, the M_s value increases with increasing Fe²⁺ substituted into the MnFe₂O₄; the Fe_0.8Mn_0.2Fe₂O₄ and Fe₃O₄ attain the highest M_s values of 88 and 87 emu/g, respectively, as tabulated in Table 2. The results can be explained by three factors. (i) The first factor is the cation distributions of Fe³⁺, Mn²⁺, and Fe²⁺ substituted into the A and B sites. As the Fe²⁺ ions are substituted for the Mn²⁺ ions, the Fe²⁺ and Mn²⁺ ions preferably stay at the A site, while the Fe³⁺ ions prefer to stay at the B site [31,41,62,63]. (ii) The second factor is the crystallite and particle sizes: the substitution of Fe²⁺ ions into the MnFe₂O₄ slightly increases the crystallite and particle sizes (Table 1) along with the lower surface-to-volume ratio and the lower surface spin disorder [31,44,47]. Hence, the M_s value increases with increasing crystallite and particle sizes, as observed from the XRD and SEM results. (iii) The third factor is the spin–spin interaction; as more Fe²⁺ ions are added into the MnFe₂O₄, the M_s value also increases because of the stronger spin–spin interaction as related to the decrease in lattice constant as shown in Table 1. The spin–spin interactions determine the Curie temperature (T_c) and hence the temperature variation of the magnetization, resulting in the different M_s values at room temperature. In short, the M_s value increases with increasing spin–spin interaction [63,64]. Moreover, the H_c values in Table 2 are close to zero, which suggests that the energy barriers are larger than the thermal energy, and the magnetic moments will be blocked in the parallel or anti-parallel directions, resulting in the small hysteresis loop and indicating the soft ferromagnetic behavior in all samples.

It may be noted that the M_s value of the synthesized Fe_0.8Mn_0.2Fe₂O₄_1.2CMC_SDS (88 emu/g) is higher than those of other works. Modaresi et al. and Doaga et al. synthesized the Fe_0.8Mn_0.2Fe₂O₄ ferrite nanoparticles by using the co-precipitation method; the M_s values were 18.2 and 56.3 emu/g, respectively [31,39]. Guner et al. synthesized the Fe_0.8Mn_0.2Fe₂O₄ ferrite nanoparticles by using the polyol method; the M_s value was 48.32 emu/g [41].

The magnetic moments in the units of Bohr magneton of the synthesized ferrites were calculated by Equation (10) [41]:

Magnetic moments in the units of Bohr magneton = (M × σ_s)/(N_A × µ_B)

(10)

where M is the molecular weight of each sample (g/mol), σ_s is the specific magnetization (emu/g) as evaluated from the specific magnetization as a function of 1/H² plot, N_A is the Avogadro’s number (6.02 × 10²³ mol⁻¹), and µ_B is the Bohr magneton (9.27 × 10⁻²¹ Erg/Oe). All magnetic moments of the MnFe₂O₄ ferrite nanoparticles under various SDS surfactant concentrations and the Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS ferrite nanoparticles under various Fe²⁺ ions substituted are tabulated in Table 2. The magnetic moment increases with increasing SDS concentration and Fe²⁺ substituted into the MnFe₂O₄ ferrite nanoparticles. A low magnetic moment occurs with a low M_s value resulting from a specifically preferred cation distribution of the metal ions in the A and B sites, and a surface spin disorder [31,39,41]. The theoretical magnetic moments of Mn²⁺, Fe³⁺, and Fe²⁺ ions are 5 µ_B, 5 µ_B, and 4 µ_B, respectively, as derived from the numbers of unpaired electrons in the 3D orbitals [31].

3.4. Dispersion Property Fe_(1−x)Mn_xFe₂O₄ Ferrite Nanoparticles

The dispersion property in water of the synthesized bare MnFe₂O₄ and Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS ferrite nanoparticles with various Fe²⁺ ions substituted was investigated next, as this property is an important feature in various applications such as magnetic fluid, catalysis, bio-sensing, magnetic resonance imaging (MRI), and hyperthermia treatment [65,66]. In a recent work, Mei et al. used sodium oleate (NaOL) to modify the surface of magnetite nanoparticles to improve the selective and functional water dispersion to enhance the efficiency of plasmonic bio-sensing [67]. In this work, all ferrite nanoparticles were dispersed in water by using the sonication bath for 15 min, and then the sedimentation of precipitate was observed during a period of 7 days. The results of the sedimentation experiment are shown in Figure 8. The Fe_0.8Mn_0.2Fe₂O₄_1.2CMC_SDS ferrite nanoparticles (C) provided a longer-lasting dispersion in water than the bare MnFe₂O₄ (A), MnFe₂O₄_1.2CMC_SDS (B), and Fe₃O₄_1.2CMC_SDS ferrite nanoparticles (D). Generally, the dispersion is promoted by the Brownian motion of the nanoparticles [68]. On the other hand, the rapidly precipitated magnetic nanoparticles are due to the strong magnetic dipole–dipole interaction between nanoparticles and the gravity force [29,65].

All ferrite nanoparticles can be separated from the water by using an external magnetic field. The sedimentation ratios of the bare MnFe₂O₄ and Fe_(1−x)Mn_xFe₂O₄_1.2CMC_SDS ferrite nanoparticles were calculated from the heights of the precipitates divided by the initial heights as a function of time, as shown in Figure 9. The results support the dispersion stability results of magnetic nanoparticles in Figure 8. The dispersions of the bare MnFe₂O₄, MnFe₂O₄_1.2CMC_SDS, and Fe₃O₄_1.2CMC_SDS ferrite nanoparticles are relatively poor as the sedimentation ratios drop close to zero within a short time, whereas the Fe_0.8Mn_0.2Fe₂O₄_1.2CMC_SDS ferrite nanoparticle can be dispersed in water for a period of 360 min. Surfactants, polymeric compounds, organic materials, or inorganic materials have been commonly used to improve the dispersion stability of magnetic materials [67,68]. Petcharoen et al. used two fatty acids (oleic acid (OA) and hexanoic acid (HA)) as coating agents for the synthesized magnetite nanoparticles; they illustrated that the oleic acid-coated magnetite nanoparticle was more stable for a longer period of time in the water than the hexanoic acid and bare samples [43].

4. Conclusions

The MnFe₂O₄ ferrite nanoparticles were successfully synthesized by the surfactant-assisted co-precipitation under the effects of surfactant types, surfactant concentrations, and Mn²⁺:Fe²⁺ molar ratio substitutions. The presence of SDS generally reduced the crystallite and individual particle sizes whereas the increase of Fe²⁺ ions substituted increased the crystallite and particle sizes due to the increased crystal growth rate. Adding Fe²⁺ ions changed the crystal structure from the mixed spinel (MnFe₂O₄) to the inverse spinel (Fe₃O₄). All of the ferrite nanoparticles showed a spherical nanoparticle shape, with individual sizes between 17–22 nm depending on the synthesis conditions. The electrical conductivity increased by increasing the number of Fe²⁺ ions substituted. The highest electrical conductivity of 9.68 × 10⁻³ S/cm was obtained from the Fe₃O₄ ferrite nanoparticle. For the magnetic properties, all ferrite nanoparticles demonstrated the soft ferromagnetic behavior. The highest M_s value of 88 emu/g was obtained from the Fe_0.8Mn_0.2Fe₂O₄_1.2CMC_SDS, with the nanoparticle size of 18 ± 3 nm. The Fe_0.8Mn_0.2Fe₂O₄_1.2CMC_SDS ferrite nanoparticle took the longest time to sediment in water. The present results suggest that the SDS surfactant was influential as the template to control the crystallite and particle sizes to improve the magnetic properties, and the Fe²⁺ ions substitution was shown to be a significant route to improve the electrical and magnetic properties. The synthesized nano-magnetic particles have the potential to be used [7,13,69,70,71,72,73,74,75,76,77,78,79]: in soft ferrogels and ferro-elastomers for the magnetic-deformation or electro-magnetic-deformation in actuator applications; in the drug-loaded magnetic nanocomposites for controlling the amount of drug release in drug release systems; in magnetic nanocarriers or magnetic nanocomposites for killing cancer cells by thermotherapy via ultrasounds, microwaves, or radiofrequency radiation in hyperthermia applications, based on natural and synthetic polymers.