Synthesis and characterization of CoxFe1-xFe2O4 nanoparticles by anionic, cationic, and non-ionic surfactant templates via co-precipitation.

The cobalt ferrite nanoparticles (CoxFe1-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 CoxFe1-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 CoxFe1-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 (Ms) of CoFe2O4 was reduced by using CTAB, SDS, and Tween20. For the CoxFe1-xFe2O4 as synthesized by the SDS template at 1.2 CMC, the Ms increased with increasing Fe2+ mole ratio. The highest Ms 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 fluid[1], catalysis[2], magnetic resonance imaging (MRI)[3], proton exchange membrane[4], actuator[5], hyperthermia[6], and drug delivery[7]. Substitution of various divalent cations (M2+) namely Co2+, Mn2+, Zn2+, Mg2+, and Ni2+ into ferrite nanoparticles can significantly alter their magnetic properties[8]. Among the ferrites magnetic nanoparticles with the spinel structures, CoFe2O4 provides the notable properties namely: chemical stability, high coercivity (Hc), and high Curie temperature[9]. 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 (Ms) 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 hydrothermal[15], sol-gel[16], micro-emulsion[17], thermal decomposition[18], and co-precipitation[19]. 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 CoFe2O4[20].

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 CoxFe1−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 mM[31], 0.12 g), CTAB (0.92 mM[32], 0.003 g), and Tween 20 (0.06 mM[33], 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 CoxFe1−xFe2O4 magnetic nanoparticles by surfactant assisted co-precipitation under various molar ratio of Co2+ and Fe2+

CoFe2O4, Co0.6Fe0.4Fe2O4, Co0.2Fe0.8Fe2O4, 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, Co0.6Fe0.4Fe2O4, Co0.2Fe0.8Fe2O4, and Fe3O4 are coded as CoFe2O4_SDS_1.2CMC, Co0.6Fe0.4Fe2O4_SDS_1.2CMC, Co0.2Fe0.8Fe2O4_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 CoxFe1−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 (Ms), and coercivity (Hc) 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 cations[35]. 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].

Figure 1

Surfactant assisted co-precipitation for synthesis of CoxFe1−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 cation[37]. 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 structure[38]. Table 1 lists the calculated average crystallite sizes. The average crystallite size was calculated by using the (3 1 1) peak and Eq. (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): where d is the interplanar spacing, and (h l k) are the Miller indices. The volume unit cell was calculated by Eq. (4):

Figure 2

XRD patterns of CoFe2O4 and CoxFe1−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 (Vcell), hopping lengths for tetrahedral site (LA), hopping lengths for octahedral site (LB), 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 (LA) and octahedral site (LB) were calculated by Eqs. (5–6)[39]:

and

Table 1 also lists the calculated average crystallite sizes (t311), lattice constants (a), volumes (Vcell), and hopping lengths (LA and LB) 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 crystallization[40]. 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 CoxFe1−xFe2O4 are shown in Fig. 2b. From Table 1, the crystalline size of CoxFe1−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 crystallinity[41].

The FT-IR spectra of the synthesized cobalt ferrite magnetic nanoparticles under various surfactants and CoxFe1−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 humidity[42]. 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 vibration[35].

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 CoxFe1−xFe2O4: (a) CoFe2O4_SDS_1.2CMC; (b) Co0.2Fe0.8Fe2O4_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 CoxFe1−xFe2O4 magnetic nanoparticles as shown in Table 1. The result shows that the EDS experimental Co: Fe mole ratios of CoxFe1−xFe2O4 are 1: 1.8, 1: 3.5, 1: 12 and 0: 1 for the CoFe2O4, Co0.6Fe0.4Fe2O4, and Co0.2Fe0.8Fe2O4, 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 CoxFe1−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 CoxFe1−xFe2O4 mole ratios match their theoretical stoichiometric values.

Figure 5

XPS spectra of the CoxFe1−xFe2O4: (a) CoFe2O4_SDS_1.2CMC; (b) Co0.6Fe0.4Fe2O4_SDS_1.2CMC; (c) Co0.2Fe0.8Fe2O4_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, Co0.6Fe0.4Fe2O4, Co0.2Fe0.8Fe2O4, 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 CoxFe1−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 CoxFe1−xFe2O4 with a larger particle size[43]. The smaller particles can be obtained when the nucleation rate is higher than the growth rate[44].

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 CoxFe1−xFe2O4: (a) CoFe2O4_SDS_1.2CMC; (b) Co0.6Fe0.4Fe2O4_SDS_1.2CMC; (c) Co0.2Fe0.8Fe2O4_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 (Ms), coercivity (Hc) and magnetic remanence (Mr) 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, Co0.6Fe0.4Fe2O4, and Co0.2Fe0.8Fe2O4 with the high Hc and Mr values; thus, the synthesized cobalt ferrite nanoparticles are hard or ferromagnetic materials[45]. On the other hand, the Fe3O4 hysteresis curve shows the superparamagnetic behavior where the Hc and Mr values were close to zero[46].

Figure 8

Hysteresis loops of CoFe2O4 and CoxFe1−xFe2O4: (a) under various surfactant types; (b) under various Fe2+ contents.

Table 2

Magnetic and electrical properties of CoxFe1−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 Ms 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 Ms 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 spins[47]. However, the CoFe2O4 as synthesized by Tween20 and no surfactant show the lower Ms values due to the lower crystallinity[48], 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 CoxFe1−xFe2O4 as shown in Fig. 8b, the Ms values are 46.19 emu/g, 74.19 emu/g, 80.62 emu/g, and 100.41 emu/g for the CoFe2O4, Co0.6Fe0.4Fe2O4, and Co0.2Fe0.8Fe2O4, and Fe3O4, respectively. On comparing with the previous Ms values of the bulk CoFe2O4 (80 emu/g)[49] and Fe3O4 (90 emu/g)[47], the present Ms value of CoxFe1−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]. where μs 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 Ms values under the substitution of increasing Fe2+ in the CoxFe1−xFe2O4[11,41].

The Hc values of the cobalt ferrite nanoparticles are 263.02 Oe, 877.76 Oe, 190.76 Oe, and 43.03 Oe for CoFe2O4, Co0.6Fe0.4Fe2O4, and Co0.2Fe0.8Fe2O4, and Fe3O4, respectively. Comparing with previous work as shown in Table 2, the Hc values of the synthesized CoFe2O4 and Fe3O4 are comparable to the previous work. It can be noted that the Hc 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 Hc value decreases to the lowest value for Fe3O4 (x = 0). The result is consistent with the previous work as the highest Hc value was found in the case of Co0.5Fe0.5Fe2O4 (x = 0.5)[11,52].

Lastly, it may be noted that the Ms 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 Ms 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 material[53]. 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, Co0.6Fe0.4Fe2O4, Co0.2Fe0.8Fe2O4, 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, Ms, was obtained from the synthesized Fe3O4 using the SDS template at 100.41 emu/g. The synthesized Fe3O4 nanoparticle with high Ms is potential to be utilized in various actuator devices and biomedical applications.