Synthesis and characterization of Co_xFe_1−xFe₂O₄ nanoparticles by anionic, cationic, and non-ionic surfactant templates via co-precipitation

Abstract

The cobalt ferrite nanoparticles (Co_xFe_1−xFe₂O₄) were synthesized by the surfactant templated co-precipitation method using various surfactants namely sodium dodecyl sulfate (SDS), hexadecyltrimethylammonium bromide (CTAB), and Tween20. Under the substitution, the Co_xFe_1−xFe₂O₄ particles were synthesized at various Co²⁺ and Fe²⁺ mole ratios (x = 1, 0.6, 0.2, and 0) with the SDS. The cobalt ferrite nanoparticles were characterized for their morphology, structure, magnetic, and electrical properties. All Co_xFe_1−xFe₂O₄ nanoparticles showed the nanoparticle sizes varying from 16 to 43 nm. In the synthesis of CoFe₂O₄, the SDS template provided the smallest particle size, whereas the saturated magnetization (M_s) of CoFe₂O₄ was reduced by using CTAB, SDS, and Tween20. For the Co_xFe_1−xFe₂O₄ as synthesized by the SDS template at 1.2 CMC, the M_s increased with increasing Fe²⁺ mole ratio. The highest M_s of 100.4 emu/g was obtained from the Fe₃O₄ using the SDS template. The Fe₃O₄ nanoparticle is potential to be used in various actuator and biomedical devices.

Introduction

Magnetic nanoparticles have been widely investigated for many applications such as magnetic fluid¹, catalysis², magnetic resonance imaging (MRI)³, proton exchange membrane⁴, actuator⁵, hyperthermia⁶, and drug delivery⁷. Substitution of various divalent cations (M²⁺) namely Co²⁺, Mn²⁺, Zn²⁺, Mg²⁺, and Ni²⁺ into ferrite nanoparticles can significantly alter their magnetic properties⁸. Among the ferrites magnetic nanoparticles with the spinel structures, CoFe₂O₄ provides the notable properties namely: chemical stability, high coercivity (H_c), and high Curie temperature⁹. Moreover, CoFe₂O₄ possesses a good anisotropic property as the Co²⁺ substitution provides a higher degree of anisotropy relative to Fe²⁺ and Fe³⁺¹⁰. However, the bulk saturated magnetization (M_s) of CoFe₂O₄ (80 emu/g) obtained so far is still lower than Fe₃O₄ (presently at ~ 89 emu/g)¹¹.

The shape, size, and properties of magnetic particles are generally dictated by the synthesis method^12–14. There are various methods to synthesize magnetic nanoparticles such as hydrothermal¹⁵, sol-gel¹⁶, micro-emulsion¹⁷, thermal decomposition¹⁸, and co-precipitation¹⁹. Among these techniques, the co-precipitation is a simple method as it is inexpensive, with a short reaction time and a lower reaction temperature. The important factors namely the reaction temperature, stirring speed, and pH of the reactant are essential in controlling the particle shape and size as related to the particle nucleation and growth rates. Ideally, the nucleation rate should be higher than the growth rate to obtain smaller particles.

Alternatively, the particle shape and size can be manipulated by using surface-active agents, namely surfactants, because of their electrostatic repulsion and steric hindrance properties. In particular, the surfactant could reduce the agglomeration of the magnetic nanoparticles from the magnetic interaction and with high surface reactivity. Vadivel et al. used sodium dodecyl sulfate (SDS) as the surfactant for the co-precipitation synthesis of CoFe₂O₄ under various SDS concentrations. SDS improved the particles size distribution and magnetic property of CoFe₂O₄²⁰.

Nanomagnetic particles (NMPs) have been utilized in various applications, in particular actuators^21–25 and biomedical devices^26–30. The important and required features of NMP for these applications are the high magnetization, superparamagnetic behavior, and non-toxicity towards human.

In this work, the effect of surfactant types, namely sodium dodecyl sulfate (SDS), hexadecyltrimethylammonium bromide (CTAB), and Tween20 as anionic, cationic, and non-ionic surfactants, were investigated on the synthesis of Co_xFe_1−xFe₂O₄ with 0 ≤ x ≤ 1 and on the resultant magnetic properties. It will be shown that SDS was the most suitable surfactant for the synthesis of CoFe₂O₄ with the nanoparticle size of 16 ± 3 nm, whereas the highest magnetization as obtained from the Fe₃O₄ by the SDS template was as high as 100.41 emu/g with the superparamagnetic behavior. The synthesized Fe₃O₄ particle possesses magnetic properties which are potential to be used in various actuator and biomedical devices.

Methods

Materials

Iron (III) chloride (99% purity, Sigma Aldrish), cobalt (II) chloride (AR grade, Merck), and iron (II) sulfate heptahydrate (99% purity, Univar) were used as the precursors. Sodium dodecyl sulfate, SDS, (98.5% purity, Sigma Aldrich), hexadecyltrimethylammonium bromide, CTAB, (96% purity, Sigma Aldrich), and Tween20 (AR grade, Sigma Aldrich) were the surfactants used. Sodium hydroxide, NaOH (AR grade, Univar) was utilized as a precipitating agent.

Synthesis of CoFe₂O₄ magnetic nanoparticles by surfactant assisted co-precipitation under various surfactant types

Metal precursors including iron (III) chloride (Fe³⁺), and cobalt (II) chloride (Co²⁺) with the Fe³⁺: Co²⁺ molar ratio of 0.10: 0.05 (0.81 g: 0.33 g) were put in 25 ml deionized water. The metal ion solution was separately mixed with 25 ml of various surfactant solutions namely: SDS (8.2 mM³¹, 0.12 g), CTAB (0.92 mM³², 0.003 g), and Tween 20 (0.06 mM³³, 0.02 g) at their critical micelle concentrations (CMC): To obtain the CMC data, the surfactant solution in water was tested at 25 °C^31–33. Each surfactant was dissolved in the deionized water and was stirred for 30 min to form micelles before adding the metal ions at room temperature. The mixture solution was continuously stirred at room temperature for 30 min. After that, 3 M NaOH solution (15 ml) was added dropwise and then continuously stirred for 4 h at 80 °C. The obtained dark precipitate was washed with water and ethanol to eliminate the remaining surfactant, and then dried at 80 °C for 24 h. The synthesized CoFe₂O₄ by SDS, CTAB, and Tween20 as the surfactants and no surfactant are coded as CoFe₂O₄_SDS_1CMC, CoFe₂O₄_CTAB_1CMC, CoFe₂O₄_Tween20_1CMC, and CoFe₂O₄_Bare, respectively.

Synthesis of Co_xFe_1−xFe₂O₄ magnetic nanoparticles by surfactant assisted co-precipitation under various molar ratio of Co²⁺ and Fe²⁺

CoFe₂O₄, Co_0.6Fe_0.4Fe₂O₄, Co_0.2Fe_0.8Fe₂O₄, and Fe₃O₄ were synthesized with the metal precursors including iron (III) chloride (Fe³⁺), cobalt (II) chloride (Co²⁺), and iron (II) sulfate (Fe²⁺) at the Fe³⁺: Co²⁺: Fe²⁺ molar ratios of 0.10: 0.05: 0.00 (0.81 g: 0.33 g: –), 0.10: 0.03: 0.02 (0.81 g, 0.26 g, 0.14 g), 0.10: 0.01: 0.04 (0.811 g: 0.07 g: 0.56 g), and 0.10: 0.00: 0.05 (0.811 g: –: 0.70 g), where they were dissolved in 25 ml deionized water. The SDS (10 mM, 0. 14 g) was dissolved in 25 ml deionized water for 30 min and then each metal precursor solution was put in the SDS solution and stirred at room temperature for 30 min to obtain a homogeneous solution. After that, 3 M NaOH solution (15 ml) was added and then continuously stirred for 4 h at 80 °C. The obtained dark precipitate was washed with water and ethanol to eliminate the remaining surfactant and then dried at 80 °C for 24 h. The synthesized CoFe₂O₄, Co_0.6Fe_0.4Fe₂O₄, Co_0.2Fe_0.8Fe₂O₄, and Fe₃O₄ are coded as CoFe₂O₄_SDS_1.2CMC, Co_0.6Fe_0.4Fe₂O₄_SDS_1.2CMC, Co_0.2Fe_0.8Fe₂O₄_SDS_1.2CMC, and Fe₃O₄_SDS_1.2CMC, respectively.

Cobalt ferrite nanoparticles characterization

A wide angle X-ray diffractometer, XRD, (Rigaku, SmartLab) was utilized to investigate the crystalline structures of the magnetic nanoparticles. The CuK-alpha radiation source was employed at 40 kV/30 mA using the K-beta filter to eliminate interference peaks. The diffractometer was fitted with the Bragg–Brentano geometry, the graphite monochromator and the diffracted beam, and operated at a scan rate of 2°/min and a scan step of 0.02°. Each sample was dried and grinded to obtain a fine powder. The sample was put into a mold and then compressed by a hydraulic machine.

A Fourier transform infrared spectrometer, FT-IR, (Nicolet, iS5) was employed to measure spectra of the magnetic nanoparticles using potassium bromide (KBr) as the background material. To prepare a sample, a small amount of sample powder was mixed and grinded with KBr. The mixture powder was put into a mold and then compressed by a hydraulic pressure machine for 15 s. The spectra were measured in the wavenumber range of 650 cm⁻¹ to 4000 cm⁻¹.

A scanning electron microscope, SEM, (Hitachi, S-4800) was used to study the morphological structure and to measure the magnetic nanoparticle sizes. Each sample was coated with a thin layer of platinum. The images were obtained at the acceleration voltage of 5 kV and at the magnifications of 100,000 and 150,000.

An electron dispersive spectrometer, EDS, (FE-SEM Hitachi, S-4800) was used to determine the atomic percentages of the cobalt ferrite nanoparticles. Each sample was coated with a thin layer of platinum.

An X-ray photoelectron spectroscope, XPS, (Kratos, Axis Ultra DLD) was employed to determine the atomic percentages of Co_xFe_1−xFe₂O₄ using the monochromatized Al K. Each sample was distributed on a carbon tape on the sample holder, and a copper grid was used as the reference for the elemental analysis.

A vibrating sample magnetometer, VSM, (LakeShore, Series 7400 model 7404) was employed to measure the saturated magnetization (M_s), and coercivity (H_c) of the cobalt ferrite nanoparticles. The measurements were taken under a magnetic field strength of 10,000 Gauss at room temperature, with 80 points/loop and with a scan speed of 10 s/point.

Results and discussion

Cobalt ferrite synthesis and characterization

The synthesis scheme is shown in Fig. 1. After the complete micelle formation at equal or above the critical micelle concentration (CMC), the metal ions (Fe³⁺, Fe²⁺, and Co²⁺) were added into the surfactant solution. The metal ions were stabilized with the spherical micelles of surfactant by the interaction between the polar groups of the surfactants and the metal cation precursors^34,35. The synthesis reaction was carried out by adding NaOH (at the pH of 13) for 4 h under the nitrogen atmosphere to prevent the oxidation of ferrous ions (Fe²⁺) to ferric ions (Fe³⁺) by the oxygen atmosphere. In the case of SDS as an anionic surfactant, it could stabilize the metal cations by the micelle formation via the interaction between the polar group of SO₄^–2 and the metal cations³⁵. After the adding NaOH to precipitate the ferrite particle, the OH^– from NaOH interacted with the metal cations to form the hydroxide precipitant and the SDS interacted with the hydroxide precipitant on the surface. The co-precipitation reaction is shown in Eq. (1)³⁶.

$$\begin{array}{c}{\text{2Fe}}^{3+}+{\text{6OH}}^{-}\to \text{2Fe}{\left(\text{OH}\right)}_3\\ {\text{xCo}}^{2+}+{\text{2OH}}^{-}\to \text{xCo}{\left(\text{OH}\right)}_2\\ {\text{(1-x)Fe}}^{2+}+{\text{2OH}}^{-}\to \left(1-\text{x}\right)\text{Fe}{\left(\text{OH}\right)}_2\\ \text{xCo}{\left(\text{OH}\right)}_2+\left(1-\text{x}\right)\text{Fe}{\left(\text{OH}\right)}_2+\text{2Fe}{\left(\text{OH}\right)}_3\to {\text{Co}}_{\text{x}}{\text{Fe}}_{1-\text{x}}{\text{Fe}}_2{\text{O}}_4+{\text{4H}}_2\text{O}\\ \end{array}$$ 1

Figure 1.

Surfactant assisted co-precipitation for synthesis of Co_xFe_1−xFe₂O₄.

The crystalline structure of cobalt ferrite nanoparticles was characterized by the x-ray diffraction technique. Normally, magnetite nanoparticles are of a cubic spinel structure (AB₂X) which composes of a divalent cation (A), a trivalent cation (B), and a divalent anion (X). The cations A and B occupy the octahedral or tetrahedral site of the spinel structure. Nevertheless, the ferrite nanoparticles can also form a reverse spinel structure, where the tetrahedral site is occupied by a trivalent cation and the octahedral site is occupied by a divalent cation and the remaining trivalent cation³⁷. The XRD patterns of the CoFe₂O₄ as synthesized by SDS, CTAB, Tween20 and without surfactant are shown in Fig. 2a. The patterns of CoFe₂O₄ synthesized by all surfactants show the major characteristic peaks at (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) which reflect a cubic spinel structure³⁸. Table 1 lists the calculated average crystallite sizes. The average crystallite size was calculated by using the (3 1 1) peak and Eq. (2):

$$\text{t}=\frac{\text{k}\mathrm{\lambda }}{\mathrm{\beta }cos\theta }$$ 2

where k is the dimensionless shape factor (k = 0.9), λ is the X-ray wavelength (CuKa = 1.5405 Å), β is the full width at the half maximum of diffraction peak (3 1 1), and θ is the angle of diffraction (2θ/2). The lattice constant (a) was calculated by using the (3 1 1) peak and Eq. (3):

$$\text{a}=\text{d}\sqrt{{\text{h}}^2+{\text{k}}^2+{\text{l}}^2}$$ 3

where d is the interplanar spacing, and (h l k) are the Miller indices. The volume unit cell was calculated by Eq. (4):

$${\text{V}}_{\text{cell}}={\text{a}}^3$$ 4

Figure 2.

XRD patterns of CoFe₂O₄ and Co_xFe_1−xFe₂O₄: (a) under various surfactant types; (b) under various Fe²⁺ contents.

Table 1.

Co: Fe mole ratios, average crystallite sizes (t₃₁₁), lattice constants (a), unit volume cells (V_cell), hopping lengths for tetrahedral site (L_A), hopping lengths for octahedral site (L_B), particle sizes, and the Co: Fe atomic ratios from EDS and XPS of cobalt ferrite nanoparticles.

Sample	Co: Fe mole ratio	XRD					SEM	EDS	XPS
		Average crystallite size (t311) (nm)	Lattice constant (a) (Å)	Volume (Vcell)	LA (nm)	LB (nm)	Particle size (nm)	Co: Fe atomic ratio	Co: Fe atomic ratio
CoFe2O4_Bare	–	10.9	8.32	596.22	3.60	2.94	42 ± 8	–	–
CoFe2O4_SDS_1CMC	–	15.9	8.34	579.05	3.61	2.95	16 ± 3	–	–
CoFe2O4_CTAB_1CMC	–	12.5	8.34	579.68	3.61	2.95	20 ± 3	–	–
CoFe2O4_Tween20_1CMC	–	9.21	8.32	597.60	3.60	2.94	21 ± 3	–	–
CoFe2O4_SDS_1.2CMC	1:2	16.8	8.35	581.96	3.62	2.95	22 ± 3	1:1.8	1:1.9
Co0.6Fe0.4Fe2O4_SDS_1.2CMC	1:4	18.7	8.41	596.18	3.64	2.98	24 ± 3	1:3.5	1:3.9
Co0.2Fe0.8Fe2O4_SDS_1.2CMC	1:14	11.7	8.36	585.51	3.62	2.96	32 ± 4	1:12	1:14
Fe3O4_SDS_1.2CMC	0:1	9.81	8.35	583.11	3.62	2.95	43 ± 8	0:1	0:1

The hopping lengths for the tetrahedral site (L_A) and octahedral site (L_B) were calculated by Eqs. (5–6)³⁹:

$${\text{L}}_{\text{A}}\text{= a}\frac{\sqrt{3}}{4}$$ 5

and

$${\text{L}}_{\text{B}}\text{= a}\frac{\sqrt{2}}{4}$$ 6

Table 1 also lists the calculated average crystallite sizes (t₃₁₁), lattice constants (a), volumes (V_cell), and hopping lengths (L_A and L_B) of the cobalt ferrite nanoparticles synthesized. From the calculated crystallite sizes in Table 1, the CoFe₂O₄ synthesized using SDS as the surfactant possesses the largest crystallite size relative to other surfactant types which suggests that SDS improves the crystallinity of the CoFe₂O₄ as the negative charge of the SDS micelles stabilizes the cation and confine the space for crystallization⁴⁰. However, the CoFe₂O₄ as synthesized by Tween20 and without surfactant show lower crystalline sizes than the CoFe₂O₄ with SDS or CTAB. This is because Tween20 (a non-ionic surfactant) and no surfactant could not stabilize the magnetic nanoparticles during the synthesis reaction resulting in a random crystallization.

The XRD patterns of Co_xFe_1−xFe₂O₄ are shown in Fig. 2b. From Table 1, the crystalline size of Co_xFe_1−xFe₂O₄ increases from 16.8 nm to 18.7 nm with x varying from 1.0 to 0.6, and then decreases to 9.81 nm at x equal to 0. This result suggests that the crystalline size decreases with increasing Fe²⁺ content or decreasing x from 0.6 to 0.0 due to the smaller grain size and the nanoparticle crystallinity⁴¹.

The FT-IR spectra of the synthesized cobalt ferrite magnetic nanoparticles under various surfactants and Co_xFe_1−xFe₂O₄ are shown in Fig. 3 and Fig. 4, respectively. All spectra show the identical peaks at around 1600 cm⁻¹ and 3400 cm⁻¹, corresponding the hydroxyl groups on the surface of the cobalt ferrite magnetic nanoparticles from the humidity⁴². In addition, there is no surfactant peak present which confirms the elimination of surfactants after washing out with water and ethanol. The SDS surfactant peaks should appear at 1113 cm⁻¹, corresponding to the S–O stretching vibration; 1460 cm⁻¹, corresponding to the C–O stretching; and 2923 and 2865 cm⁻¹, corresponding to the C–H stretching vibration³⁵.

Figure 3.

FT-IR spectra of the CoFe₂O₄ under various surfactant types: (a) CoFe₂O₄_SDS_1CMC; (b) CoFe₂O₄_CTAB_1CMC; (c) CoFe₂O₄_Tween20_1CMC; and (d) CoFe₂O₄_Bare.

Figure 4.

FT-IR spectra of the Co_xFe_1−xFe₂O₄: (a) CoFe₂O₄_SDS_1.2CMC; (b) Co_0.2Fe_0.8Fe₂O₄_SDS_1.2CMC; and (c) Fe₃O₄_SDS_1.2CMC.

The EDS technique was used to measure the atomic percentages corresponding to the Co: Fe mole ratio of the Co_xFe_1−xFe₂O₄ magnetic nanoparticles as shown in Table 1. The result shows that the EDS experimental Co: Fe mole ratios of Co_xFe_1−xFe₂O₄ are 1: 1.8, 1: 3.5, 1: 12 and 0: 1 for the CoFe₂O₄, Co_0.6Fe_0.4Fe₂O₄, and Co_0.2Fe_0.8Fe₂O₄, respectively. The calculated synthesis values of Co: Fe mole ratios are 1: 2, 1: 4, 1: 14, and 0: 1 respectively; thus, the EDS experimental values are quite close to the theoretical values.

The XPS technique was also used to confirm the Co: Fe mole ratio and the XPS spectra are shown in Fig. 5. The visible peaks can be observed at 778.3 eV, 706.7 eV, and 529.2 eV corresponding to the Co 2p, Fe 2p and O 1s respectively. The corresponding Co: Fe mole ratios of Co_xFe_1−xFe₂O₄ are 1: 1.9, 1: 3.9, 1: 14, and 0: 1, respectively. These mole ratio values from the EDS and XPS techniques are quite close thus confirming that the synthesized Co_xFe_1−xFe₂O₄ mole ratios match their theoretical stoichiometric values.

Figure 5.

XPS spectra of the Co_xFe_1−xFe₂O₄: (a) CoFe₂O₄_SDS_1.2CMC; (b) Co_0.6Fe_0.4Fe₂O₄_SDS_1.2CMC; (c) Co_0.2Fe_0.8Fe₂O₄_SDS_1.2CMC; and (d) Fe₃O₄_SDS_1.2CMC.

Morphology of the cobalt ferrite nanoparticles was investigated by the scanning electron microscope. In the case of CoFe₂O₄ under various surfactant types, the nearly spherical shapes of CoFe₂O₄ were obtained from all surfactants as shown in Fig. 6. The particle sizes of CoFe₂O₄ synthesized without surfactant, and with SDS, CTAB, and Tween20 are 42 nm, 16 nm, 20 nm, and 21 nm and, respectively. It appears that the particle size of cobalt ferrite nanoparticles as synthesized by the co-precipitation method was reduced by employing a surfactant because of the steric hindrance effect from the surfactant contributing to a slower nucleation and growth rate. Interestingly, SDS as an anionic surfactant provides the smaller particle size of 16 nm along with a narrow size distribution as the anion from SDS could stabilize the metal cations and the cobalt ferrite nanoparticles. For cases of CTAB and Tween20, the particle sizes are 20 nm and 21 nm, respectively, thus their sizes are comparable. However, the CoFe₂O₄ particle as synthesized by CTAB (cationic surfactant) tended to agglomerate and formed a larger flake, as shown in Fig. 6b. Figure 7 shows the nearly spherical shapes of CoFe₂O₄, Co_0.6Fe_0.4Fe₂O₄, Co_0.2Fe_0.8Fe₂O₄, and Fe₃O₄ with SDS at the surfactant concentration of 1.2 times the critical micelle concentration. The particle sizes are 22 nm, 24 nm, 32 nm, and 43 nm, respectively. For the different particle sizes of the Co_xFe_1−xFe₂O₄ ferrite particles, the particle sizes increased with increasing the Fe²⁺ substitution, indicating that the addition of Fe²⁺ effectively increases the crystal growth rate of Co_xFe_1−xFe₂O₄ with a larger particle size⁴³. The smaller particles can be obtained when the nucleation rate is higher than the growth rate⁴⁴.

Figure 6.

SEM images of CoFe₂O₄ under various surfactant types: (a) CoFe₂O₄_ SDS_1CMC; (b) CoFe₂O₄_CTAB_1CMC; (c) CoFe₂O₄_Tween20_1CMC; and (d) CoFe₂O₄_Bare.

Figure 7.

SEM images of Co_xFe_1−xFe₂O₄: (a) CoFe₂O₄_SDS_1.2CMC; (b) Co_0.6Fe_0.4Fe₂O₄_SDS_1.2CMC; (c) Co_0.2Fe_0.8Fe₂O₄_SDS_1.2CMC; and (d) Fe₃O₄_SDS_1.2CMC.

Magnetic property of cobalt ferrite nanoparticles

The magnetic properties of cobalt ferrite nanoparticles were measured by the VSM at room temperature (300 K). The saturated magnetization (M_s), coercivity (H_c) and magnetic remanence (M_r) values were obtained from the hysteresis curves in Fig. 8a,b, and are tabulated in Table 2. The hysteresis curves show the large loops of cobalt ferrite nanoparticles with the presence of cobalt atoms namely: CoFe₂O₄, Co_0.6Fe_0.4Fe₂O₄, and Co_0.2Fe_0.8Fe₂O₄ with the high H_c and M_r values; thus, the synthesized cobalt ferrite nanoparticles are hard or ferromagnetic materials⁴⁵. On the other hand, the Fe₃O₄ hysteresis curve shows the superparamagnetic behavior where the H_c and M_r values were close to zero⁴⁶.

Figure 8.

Hysteresis loops of CoFe₂O₄ and Co_xFe_1−xFe₂O₄: (a) under various surfactant types; (b) under various Fe²⁺ contents.

Table 2.

Magnetic and electrical properties of Co_xFe_1−xFe₂O₄ nanoparticles.

Sample	VSM			Electrical conductivity (S/cm)	Ref
	Ms (emu/g)	Hc (Oe)	Mr (emu/g)
CoFe2O4_Bare	13.30	786.66	4.31	1.11 × 10–2 ± 9.16 × 10–4	This work
CoFe2O4_SDS_1CMC	28.06	448.58	8.18	1.41 × 10–2 ± 1.48 × 10–3
CoFe2O4_CTAB_1CMC	31.26	232.52	7.48	1.33 × 10–2 ± 1.41 × 10–3
CoFe2O4_Tween20_1CMC	15.15	53.52	1.01	1.13 × 10–2 ± 7.07 × 10–4
CoFe2O4_SDS_1.2CMC	46.19	263.02	11.83	2.06 × 10–2 ± 9.44 × 10–5
Co0.6Fe0.4Fe2O4_SDS_1.2CMC	74.19	877.76	24.78	3.94 × 10–2 ± 3.03 × 10–3
Co0.2Fe0.8Fe2O4_SDS_1.2CMC	80.62	190.76	13.56	5.33 × 10–2 ± 8.64 × 10–4
Fe3O4_SDS_1.2CMC	100.41	43.03	4.37	1.18 × 10–1 ± 1.82 × 10–2
CoFe2O4	74.08	527.97	23.81	–	54
CoFe2O4	58.40	286.00	12.45	–	55
CoFe2O4	34.70	233.00	47.20	–	56
Fe3O4	63.36	–	–	–	57
Fe3O4	61.92	–	–	–	58
Fe3O4	78.00	–	–	–	59
Fe3O4	87.00	31.00	4.60	9.68 × 10–3	43

Figure 8a shows the hysteresis curves of CoFe₂O₄ as synthesized by various surfactant types. The M_s values are 13.30 emu/g, 28.06 emu/g, 31.25 emu/g, and 15.15 emu/g, for the CoFe₂O₄ synthesized by using no surfactant, SDS, CTAB and Tween20 with the particle sizes of 42 nm, 16 nm, 20 nm, and 21 nm, respectively. For the CoFe₂O₄ as synthesized by SDS and CTAB, it appears that the M_s value depends on the particle size, it increases slightly with increasing particle size; a smaller particle has a weaker coordination of surface atoms resulting in a disorder in the surface spins⁴⁷. However, the CoFe₂O₄ as synthesized by Tween20 and no surfactant show the lower M_s values due to the lower crystallinity⁴⁸, which can be observed from the (311) plane of the XRD patterns in Fig. 2a. The XRD patterns of CoFe₂O₄ as synthesized by Tween20 and no surfactant show the weak and broad peaks due to the lower crystallinity relative to the XRD patterns of CoFe₂O₄ as synthesized by SDS and CTAB as shown in Fig. 2a.

In the case of Co_xFe_1−xFe₂O₄ as shown in Fig. 8b, the M_s values are 46.19 emu/g, 74.19 emu/g, 80.62 emu/g, and 100.41 emu/g for the CoFe₂O₄, Co_0.6Fe_0.4Fe₂O₄, and Co_0.2Fe_0.8Fe₂O₄, and Fe₃O₄, respectively. On comparing with the previous M_s values of the bulk CoFe₂O₄ (80 emu/g)⁴⁹ and Fe₃O₄ (90 emu/g)⁴⁷, the present M_s value of Co_xFe_1−xFe₂O₄ increases with increasing Fe²⁺ substitution due to fact that Fe²⁺ provides more unpaired electrons in the 3d orbital leading to the higher number of magnetic moments in the metal ion of the magnetic nanoparticles^50,51. On comparing the Fe²⁺ and Co²⁺ 3d orbitals, Fe²⁺ has a higher number of unpaired electrons in the 3d orbital resulting in a higher magnetic moment and Bohr magneton which can be approximately by Eq. (7)⁴⁵.

$${\mathrm{\mu }}_{\text{s}}=\text{g}\sqrt{\text{S}\left(\text{S}+1\right)}$$ 7

where μ_s is the magnetic moment (Bohr magneton), g is the gyromagnetic ratio or the ratio of the magnetic moment to the angular momentum. For a free electron, g = 2, and S is the sum of the spin quantum numbers where each electron contributes ± 1/2. The S values of Co²⁺ and Fe²⁺ are 3/2 and 4/2, respectively. Thus, the calculated magnetic moments of Co²⁺ and Fe²⁺ are 3.87 magnetons and 4.90 magnetons, respectively. Other previous works also showed the increase of M_s values under the substitution of increasing Fe²⁺ in the Co_xFe_1−xFe₂O₄^11,41.

The H_c values of the cobalt ferrite nanoparticles are 263.02 Oe, 877.76 Oe, 190.76 Oe, and 43.03 Oe for CoFe₂O₄, Co_0.6Fe_0.4Fe₂O₄, and Co_0.2Fe_0.8Fe₂O₄, and Fe₃O₄, respectively. Comparing with previous work as shown in Table 2, the H_c values of the synthesized CoFe₂O₄ and Fe₃O₄ are comparable to the previous work. It can be noted that the H_c value increases with decreasing x values from 1 to 0.4, along with the increase of the Fe²⁺ mole ratio. Below x value of 0.4, the H_c value decreases to the lowest value for Fe₃O₄ (x = 0). The result is consistent with the previous work as the highest H_c value was found in the case of Co_0.5Fe_0.5Fe₂O₄ (x = 0.5)^11,52.

Lastly, it may be noted that the M_s values of Fe₃O₄ from previous works^43,57–59 as tabulated in Table 2 were 63.36, 61.92, and 78.00 emu/g, respectively. The presently obtained M_s value of Fe₃O₄_SDS_1.2CMC is 100.41 emu/g which is relatively higher.

Electrical conductivity of cobalt ferrite nanoparticles

Electrical conductivity of cobalt ferrite nanoparticles was investigated by using a two-point probe meter. The electrical conductivity values of cobalt ferrite nanoparticles are shown in Table 2. For the CoFe₂O₄ under various surfactant types, the electrical conductivity values are 1.11 × 10^–2 S/cm, 1.41 × 10^–2 S/cm, 1.33 × 10^–2 S/cm, and 1.13 × 10^–2 S/cm for the CoFe₂O₄ synthesized by using no surfactant, SDS, CTAB and Tween20, respectively. From the electrical conductivity results, CoFe₂O₄ can be categorized as a semiconducting material⁵³. Under various Fe²⁺ and Co²⁺ substitution, the electrical conductivities are 2.06 × 10^–2 S/cm, 3.94 × 10^–2 S/cm, 5.33 × 10^–2, S/cm and 1.18 × 10^–1 S/cm for the CoFe₂O₄, Co_0.6Fe_0.4Fe₂O₄, Co_0.2Fe_0.8Fe₂O₄, and Fe₃O₄, respectively. Thus, the electrical conductivity increases with increasing Fe²⁺ mole ratio as shown in Table 2. The electrical conductivity of Fe₃O₄ can be attributed to the electron hopping between Fe³⁺ and Fe²⁺ in the octahedral site of the inverse spinel structure. With the substitution of Fe²⁺ by Co²⁺, the electrical conductivity decreases due to the loss of closed-neighbor pairs (Fe²⁺ and Fe³⁺).

Conclusions

The cobalt ferrite nanoparticles were successfully synthesized by the simple surfactant templated co-precipitation method. The cobalt ferrite nanoparticles show the cubic spinel structure with the nano-sizes varying between 16 and 43 nm with the nearly spherical shapes. The most suitable surfactant for the synthesis of CoFe₂O₄ was SDS with the smallest particle size of 16 ± 3 nm. The experimental stoichiometry of cobalt ferrite nanoparticles as obtained by EDS and XPS agreed with the theoretical stoichiometry. The magnetization of cobalt ferrite nanoparticles depended on the size of the nanoparticles and the Fe²⁺ and Co²⁺ ratio. The currently highest magnetization value, M_s, was obtained from the synthesized Fe₃O₄ using the SDS template at 100.41 emu/g. The synthesized Fe₃O₄ nanoparticle with high M_s is potential to be utilized in various actuator devices and biomedical applications.

Author contributions

K.S.: Investigation, Formal analysis, Writing - original draft preparation. N.P. and K.R.: Writing - review and editing. A.S.: Writing - review and editing, Supervision. All authors confirm that manuscript “Synthesis and characterization of Co_xFe_1-xFe₂O₄ nanoparticles by anionic, cationic, and non-ionic surfactant templates via co-precipitation” represents original research work. The manuscript or its content article has not been published or considered for publication elsewhere. That all authors have checked the manuscript and have agreed to the submission.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.