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.

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 x Fe 1−x Fe 2 O 4 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 CoFe 2 O 4 with the nanoparticle size of 16 ± 3 nm, whereas the highest magnetization as obtained from the Fe 3 O 4 by the SDS template was as high as 100.41 emu/g with the superparamagnetic behavior. The synthesized Fe 3 O 4 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. To obtain the CMC data, the surfactant solution in water was tested at 25 °C [31][32][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 synthesized with the metal precursors including iron (III) chloride (Fe 3+ ), cobalt (II) chloride (Co 2+ ), and iron (II) sulfate (Fe 2+ ) at the Fe 3+ : Co 2+ : Fe 2+ 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 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.

Synthesis of CoFe
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 x Fe 1−x Fe 2 O 4 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. www.nature.com/scientificreports/

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 (Fe 3+ , Fe 2+ , and Co 2+ ) 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 (Fe 2+ ) to ferric ions (Fe 3+ ) 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 SO 4 -2 and the metal cations 35 . After the adding NaOH to precipitate the ferrite particle, the OHfrom 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 .
The crystalline structure of cobalt ferrite nanoparticles was characterized by the x-ray diffraction technique. Normally, magnetite nanoparticles are of a cubic spinel structure (AB 2 X) 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 CoFe 2 O 4 as synthesized by SDS, CTAB, Tween20 and without surfactant are shown in Fig. 2a. The patterns of CoFe 2 O 4 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):  www.nature.com/scientificreports/ 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)  and Table 1 also lists the calculated average crystallite sizes (t 311 ), 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 CoFe 2 O 4 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 CoFe 2 O 4 as the negative charge of the SDS micelles stabilizes the cation and confine the space for crystallization 40 . However, the CoFe 2 O 4 as synthesized by Tween20 and without surfactant show lower crystalline sizes than the CoFe 2 O 4 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 x Fe 1−x Fe 2 O 4 are shown in Fig. 2b. From Table 1, the crystalline size of Co x Fe 1−x Fe 2 O 4 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 Fe 2+ 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 Co x Fe 1−x Fe 2 O 4 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 Fig. 6. The particle sizes of CoFe 2 O 4 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 CoFe 2 O 4 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 CoFe 2 O 4 , Co 0.6 Fe 0.4 Fe 2 O 4 , Co 0.2 Fe 0.8 Fe 2 O 4 , and Fe 3 O 4 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 x Fe 1−x Fe 2 O 4 ferrite particles, the particle sizes increased with increasing the Fe 2+ substitution, indicating that  www.nature.com/scientificreports/ the addition of Fe 2+ effectively increases the crystal growth rate of Co x Fe 1−x Fe 2 O 4 with a larger particle size 43 . The smaller particles can be obtained when the nucleation rate is higher than the growth rate 44 .

Lattice constant (a) (Å) Volume (V cell ) L A (nm) L B (nm) Particle size (nm)
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  46 . Figure 8a shows the hysteresis curves of CoFe 2 O 4 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 CoFe 2 O 4 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 CoFe 2 O 4 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 spins 47 . However, the CoFe 2 O 4 as synthesized by Tween20 and no surfactant show the lower M s 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 CoFe 2 O 4 as synthesized by Tween20 and no surfactant show the weak and broad peaks due to the lower crystallinity relative to the XRD patterns of CoFe 2 O 4 as synthesized by SDS and CTAB as shown in Fig. 2a.
In the case of Co x Fe 1−x Fe 2 O 4 as shown in Fig. 8b 49 and Fe 3 O 4 (90 emu/g) 47 , the present M s value of Co x Fe 1−x Fe 2 O 4 increases with increasing Fe 2+ substitution due to fact that Fe 2+ 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 Fe 2+ and Co 2+ 3d orbitals, Fe 2+ 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 .
(7) µ s = g S(S + 1) www.nature.com/scientificreports/ 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.

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 CoFe 2 O 4 was SDS with the smallest particle size of 16 ± 3 nm. The experimental stoichiometry of cobalt ferrite nanoparticles    www.nature.com/scientificreports/

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.  www.nature.com/scientificreports/