CTAB Influence on the Hydrolytic Stripping of Nickel Ferrite

The role of the CTAB concentration (0.0 M, NS; 5x10^-4 M, ½CMC; 2x10^-3 M, 2CMC) and the residence time on the composition, morphology and magnetic properties for the nickel nanoferrite formation by hydrolytic stripping (33% v/v naphthenic/kerosene) at 200°C were studied. X-ray, SEM/EDS, FTIR, VSM, and ICP/OES were used to characterise solids and solutions. Regardless of the experimental conditions, all samples present nickel spinel ferrite as the only phase, with similar Fe/Ni molar ratio. However, those precipitates in the presence of CTAB show more homogeneous morphology than in its absence. CTAB at high concentration (2CMC) increases the discharge rate, reaching 100% at 40 min; 14.79 nm average size nanoparticles were obtained, with better magnetic properties (Ms=53.02 emu/g and Hc=35.59 Oe) than those achieved at low concentration (½CMC) and in the absence of CTAB.

Keywords:
Nickel nanoferrites; CTAB; Hydrolytic stripping; Surfactant; Magnetization

Visual abstract

1. Introduction

In recent years, the preparation and characterization of nanoscale magnetic materials is combined with the study of fundamental aspects of the magnetic arrangement phenomenon in small dimensions and with the development of new applications, such as high-density information storage systems, electronic devices, gas sensors, catalysts, microwave devices¹,² ferrofluids³,⁴, magneto-optical films⁵, drug carriers⁶ and cancer treatment materials⁷. Among the magnetic materials, nickel ferrites combine a wide range of magnetic properties with relatively low electrical conductivity. As an inverse spinel ferrite, half of the ferric ions fill the tetrahedral sites (A) in the NiFe₂O₄ unit cell and the rest, along with the Ni²⁺ ions, occupy the octahedral sites (B). Thus, the compound can be represented by the formula [Fe(III)]_A[Ni Fe(III)]_BO₄, where A and B represent the tetrahedral and octahedral sites, respectively⁸.

Several procedures have been used to synthesize ferrites, such as coprecipitation⁹, vapor phase deposition¹⁰, hydrothermal¹¹, mechanical milling¹², polymer precursor¹³, microwave-solvothermal¹⁴, sol-gel¹⁵ and hydrolytic stripping¹⁶,¹⁷, among others. The magnetic properties of spinel ferrites strongly depend on the microstructure characteristics and the composition variability, which are affected by the preparation method¹⁸. The hydrolytic stripping route is a process that combines stripping and hydrolysis steps in one operation; certain metal ions can be precipitated directly as oxides, including hematite, zirconium oxide and the nickel and cobalt ferrites, from loaded organic solvents by reaction with water at temperatures in the range 130 - 200 °C¹⁷,¹⁹. The hydrolysis reaction of the metal carboxylates is carried out in the organic phase and it is non-reversible¹⁶. Cobalt ferrite nanoparticles have been obtained by this method in a stainless-steel autoclave at 190°C and 1.3 MPa; the particles size (Feret´s diameter) was reduced from 110 to 13 nm by increasing stirring speed from 200 to 800 rpm²⁰.

The addition of surfactants in the synthesis of magnetic nanoparticles helps to control the particle growth and agglomeration, due to steric hindrance and stabilization properties²¹,²². In particular, Cetyl trimethylammonium bromide (CTAB), a cationic surfactant with CMC = 0.88 - 1.02 mM at 25°C²³, plays a key role in controlling the nucleation and growth of polar compounds. In the presence of CTAB, interfacial and surface tensions reduce, lowering the energy needed for the formation of a new phase. Additionally, the steric stabilization given by the surfactant adsorption has shown that it may serve as both a growth controller and an agglomeration inhibitor, by forming a covering film on the newly formed crystal²⁴,²⁵.

The CTAB surfactant concentration role on the structural, morphological and magnetic properties of spinel ferrites synthesized by different methodologies has been reported. Baykal et al.²⁶ synthesized nickel ferrite nanoparticles through a CTAB assisted hydrothermal and annealing route. They claim that CTAB helps to stabilize the material in both aqueous and organic phases, acting as a material transfer agent between such phases. Alamolhoda et al.²⁷ report the CTAB concentration effect on both purity and particle size of nano-sized nickel ferrites synthesized by sol-gel auto-combustion. At increasing CTAB concentration, a crystallite size decreasing trend was observed; however, an increase in FeNi₃ formation was also observed, attributed to the surfactant hydrocarbon chain acting as fuel, therefore increasing the fuel/oxygen ratio. Vadivel et al.²⁸ investigated the CTAB concentration effect on the properties of CoFe₂O₄ obtained by co-precipitation. They found that the particle-particle interaction is unaffected at concentrations below 0.16 M CTAB; at higher surfactant concentration, increment in both crystallite size (from 15.57 nm to 18.39 nm) and the saturation magnetization (61.45 to 79.79) were achieved, along to crystallinity improvement of precipitates. Zhao and Nan²⁹ studied the CTAB impact on Lanthanum (La³⁺) dispersion in ZnLa_0.02Fe_1.98O₄ crystals synthesized by hydrothermal method. The particle size increases with the surfactant concentration up to 0.5 mM, decreasing with further CTAB concentration increments. La-doped Zn ferrite powders were obtained at the highest CTAB concentration tested (1.2 mM). However, the ferrite structure was affected by the addition of the surfactant: a second phase (LaFeO₃) was formed due to reactivity of both cations on the grain boundaries; additionally, magnetic properties decreased at increasing concentration, due to the CTAB effect on La³⁺ dispersion in the crystals and the presence of the second phase.

This paper reports and discusses the CTAB concentration impact on the growth and properties of nano-sized, single-phase crystalline nickel ferrite, NiFe₂O₄, obtained by hydrolytic stripping at 200°C using naphthenic acid as reaction medium. X-ray powder diffraction (XRD), inductively coupled plasma–optical emission spectrometry (ICP-OES), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) and vibrating sample magnetometry (VSM) were used for the characterization of the precipitates.

2. Methodology

Hexahydrated ferric chloride (FeCl₃·6H₂O, Fermont), hexahydrated nickel sulfate (NiSO₄·6H₂O, J.T. Baker) and cetyltrimethylammonium bromide (CTAB, C₁₉H₄₂BrN, Sigma-Aldrich) were used as source materials to prepare NiFe₂O₄ nanoparticles by CTAB assisted hydrolytic stripping. 0.15 M iron and nickel carboxylate solutions were prepared by solvent extraction, using naphthenic acid (C₁₀H₁₈O₂, Fluka Chemika) in kerosene (33%, v/v) solutions (O/A = 1/1); 4M sodium hydroxide (NaOH, Fermont) was added drop to drop to the two-phase mixture under magnetic stirring, until reaching a stable pH of 6.8 and 2.2 for the Ni and Fe solvent extraction, respectively. The iron and nickel loaded organic solutions were mixed (Fe/Ni molar ratio = 2) and brought into contact with a CTAB aqueous solution (0.0, 5x10^-4 and 2x10^-3 M), O/A = 10/1, into rimless medium walled culture tubes, sealed with an oxygen-butane flame. The tubes were placed in a preheated oven (180°C) and brought to 200°C in less than 5 minutes, kept at 200°C for a predetermined time (5 to 60 minutes), opened and centrifuged (Thermo electron 1L GP). The precipitates were filtered (Whatman # 542), washed with acetone to remove entraining organic and dried at 80°C for 60 minutes.

Elemental chemical analysis of solids and solutions (aqueous and organic) were carried out by inductively coupled plasma–optical emission spectrometry (ICP-OES, Perkin Elmer Optima 8000). An aliquot of each powder was dissolved in HCl and diluted for the analysis as required. The crystalline structure and phase purity of the prepared nanoferrites was confirmed by X-ray powder diffraction (XRD, Bruker D8 Focus) using Cu K_α radiation (λ = 1.54056 Å), 35 kV and 25 mA and scanning at 4°min^-1 from 5° to 80° 2θ. The morphology and particle size of the solids were determined by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS, JEOL-JSM 6300). Samples were subjected to Au-Pd coating to make them conducting, as required for the FESEM images. Speciation analysis of the solids was performed by attenuated total reflectance Fourier transform infrared spectroscopy (FTIR-ATR) in the middle region (4000-400 cm^-1), 1 cm^-1 resolution, using a Perkin Elmer Frontier spectrometer equipped with a PIKE germanium ATR. Magnetic characterization was carried out at 300 K with a maximum applied field of 70 kOe using a Quantum Design magnetic property measurement system (MPMS 3).

3. Results

Nickel nanoferrites were synthesized at 200°C by hydrolysis of metal carboxylates in the presence (½CMC, 5x10^-4 M; 2CMC, 2x10^-3 M) and absence of CTAB (NS). The decreasing nickel and iron concentration trend in the organic solutions were very similar in all three cases, indicating that the Fe/Ni molar ratio was kept constant throughout the hydrolysis reaction. The reaction rate is not affected by the presence of CTAB at low concentration (½CMC), as can be appreciated in Figure 1, where the strong effect of the surfactant at high concentration (2CMC) is also evident. In both NS and ½CMC systems, a 25 min induction period is observed, after which the hydrolysis reaction rate accelerates (progressive conversion period), reaching 90% yield at 45 and 40 min respectively. Regarding the 2CMC system trend, the hydrolysis reaction proceeds at a constant, faster rate, achieving 90% yield at 30 min; in this case, total hydrolysis of the metal carboxylates is reached at 40 min, whilst 55 min is necessary to obtain complete conversion in the NS and ½CMC systems.

Figure 1
Iron carboxylate discharge during hydrolytic stripping.

The decrease in induction period before the hydrolytic stripping reaction onset with the addition of 2CMC CTAB could be attributed to the surfactant-organic complex interaction during the process. The decrease in surface tension as a result of the CTAB adsorption on the aqueous/organic interface, coupled with the presence of micelles as a natural effect having exceeded the critical micelle concentration of CTAB³⁰, has caused a better water transport process towards the carboxylate species in the organic medium and probably increased the nucleation rate. A similar effect was observed by Baykal et al.²⁶ in the synthesis of nickel nanoferrite by hydrothermal route, using CTAB as surfactant and NH₃ and NaOH as hydrolyzing agents.

Nickel nanoferrite precipitation started at 25, 35 and 10 minutes for the NS, ½CMC and 2CMC systems, respectively; the diffraction patterns obtained are shown in Figure 2. All the peaks match well with the single-phase cubic spinel NiFe₂O₄ crystal structure belonging to Fd3m space group (JCPDS file 10-325), indicating that CTAB did not promote the formation of another phase. The broadened peaks, associated with nanosized particles, became sharper, narrower, and more intense with time, indicating the high crystallinity and fine grain size achieved in all cases³¹.

Figure 2
Evolution of XRD patterns of NiFe₂O₄ obtained in the presence and absence of CTAB; (a) NS, (b) ½CMC, (c) 2CMC.

The composition of the precipitated solids was estimated using the following molar ratio relationship, based on the general stoichiometry of the spinel ferrites:

$O_T=Ni+\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.Fe$ (1)

where O_T, corresponding to the oxygen present in the solid samples, is set to 4. In general, the calculated Fe/Ni molar ratio prevailing in the systems with no added surfactant and ½CMC was 2.2 / 0.7. In the third scheme analyzed, 2CMC, Fe/Ni = 2.3/0.6. Despite the obvious effect on the speed of the reaction, the change shown in the stoichiometry of the products is minimal.

In general, the hydrolytic stripping process can be described by the following stoichiometric reactions:

a) Formation of a Ni-Fe carboxylate complex in the mixed organic solution

(RCOO)₂Ni + 2(RCOO)₃Fe + → (RCOO)₈Fe₂Ni

b) Hydrolysis reaction in the organic/aqueous interfase

(RCOO)₈Fe₂Ni + 4H₂O → NiFe₂O₄ + 8RCOOH

FTIR studies have evidenced the formation of Fe/Ni/Zn mixed carboxylates as precursors for (Ni, Zn)Fe₂O₄ ferrites precipitated by hydrolytic stripping³². Likewise, it has been claimed that magnetical coupling between the metals in a hydroxo-centred trinuclear complex¹⁶, Fe₂Ni(OH)₄(RCOO)₄·2RCOOH formed during hydrolysis, results in the precipitation of the spinel phase, instead of the single species Fe₂O₃ and Ni(OH)₂. The fact that neither hematite nor nickel hydroxide precipitated although the Fe/Ni ratio was not 2, is the result that the mixed iron-nickel carboxylate complexes are stable, and the metal centers in the organic complex are magnetically coupled¹⁶.

According to the XRD results, neither hematite nor nickel hydroxide are present in the precipitates, indicating that a mixed iron-nickel carboxylate complex is formed in the organic phase³²; the iron excess in the solids does not imply therefore that the phase reported in X-ray diffraction is not present, since there may be a redistribution of cations in the spinel cell as a result of the size or vacancies present in the structure³³-³⁵.

The average crystallite size, lattice constant, X-ray density, lattice strain, specific surface area and the dislocation density, calculated using the (311) peak reflection according to Equations (2), (3), (4), (5), (6) and (7), respectively³⁶,³⁷, are presented in Table 1.

Table 1
Crystallite size, lattice constant, X-ray density, lattice strain, dislocation density, specific surface area, bond length, tetrahedral edge, octahedral shared and unshared edge and ionic radii of NiFe₂O₄ as a function of time and CTAB concentration.

SAMPLE	Time (min)	Crystallite size (nm)	Lattice constant (Å)	X-ray density (g/cm3)	Lattice strain (x10-3)	Surface area (m2/g)	Dislocation density (x10-3 lines/cm2)	A-O Bond length (Å)	B-O Bond length(Å)	A-O Edge (Å)	B-O Shared edge (Å)	B-O Unshared edge (Å)	Ionic radii rA (Å)	Ionic radii rB (Å)
NS	25	11.1199	8.41	5.2294	25.0920	103.1812	8.0872	1.9089	2.0527	3.1171	2.8316	2.9761	0.5589	0.7027
	30	11.4835	8.45	5.1664	24.3959	101.1318	7.5832	1.9166	2.0610	3.1297	2.8431	2.9881	0.5666	0.7110
	35	13.9596	8.42	5.2158	20.0051	82.4065	5.1316	1.9105	2.0545	3.1199	2.8341	2.9787	0.5605	0.7045
	40	14.7823	8.40	5.2577	18.8414	77.1997	4.5763	1.9054	2.0490	3.1115	2.8265	2.9707	0.5554	0.6990
	45	14.8797	8.38	5.2830	18.6881	76.3269	4.5166	1.9024	2.0458	3.1066	2.8220	2.9660	0.5524	0.6958
	50	14.8688	8.39	5.2797	18.7057	76.4302	4.5233	1.9028	2.0462	3.1072	2.8226	2.9666	0.5528	0.6962
	55	15.7559	8.38	5.2936	17.6371	71.9381	4.0282	1.9011	2.0444	3.1045	2.8201	2.9640	0.5511	0.6944
	60	15.4651	8.39	5.2632	18.0032	73.7136	4.1811	1.9048	2.0483	3.1104	2.8255	2.9697	0.5548	0.6983
½CMC	35	13.3805	8.39	5.2628	20.8086	85.2048	5.5854	1.9048	2.0484	3.1105	2.8256	2.9698	0.5548	0.6984
	40	13.3329	8.40	5.2564	20.8913	85.6130	5.6254	1.9056	2.0492	3.1118	2.8267	2.9710	0.5556	0.6992
	45	13.7923	8.40	5.2542	20.1983	82.7960	5.2569	1.9058	2.0495	3.1122	2.8271	2.9714	0.5558	0.6995
	50	13.8644	8.40	5.2535	20.0941	82.3766	5.2023	1.9059	2.0496	3.1124	2.8273	2.9715	0.5559	0.6996
	55	14.1178	8.39	5.2659	19.7180	80.7067	5.0173	1.9044	2.0480	3.1099	2.8250	2.9692	0.5544	0.6980
	60	14.1418	8.40	5.2541	19.6992	80.7503	5.0002	1.9058	2.0495	3.1122	2.8271	2.9714	0.5558	0.6995
2CMC	10	9.9449	8.42	5.2084	28.0335	115.8360	10.1111	1.9114	2.0555	3.1213	2.8354	2.9801	0.5614	0.7055
	15	11.4772	8.42	5.2180	24.3435	100.1878	7.5916	1.9102	2.0542	3.1194	2.8337	2.9782	0.5602	0.7042
	20	12.5277	8.42	5.2171	22.2884	91.8017	6.3717	1.9103	2.0543	3.1196	2.8338	2.9784	0.5603	0.7043
	25	12.0490	8.40	5.2448	23.1753	94.9445	6.8881	1.9070	2.0507	3.1141	2.8288	2.9732	0.5570	0.7007
	30	13.2979	8.39	5.2703	20.9617	85.6113	5.6550	1.9039	2.0474	3.1090	2.8242	2.9684	0.5539	0.6974
	35	13.0832	8.39	5.2756	21.2712	86.9282	5.8421	1.9033	2.0467	3.1080	2.8233	2.9674	0.5533	0.6967
	40	14.7854	8.39	5.2756	18.8161	76.9207	4.5744	1.9033	2.0467	3.1080	2.8233	2.9674	0.5533	0.6967
	45	14.9206	8.40	5.2602	18.6638	76.4470	4.4919	1.9051	2.0487	3.1110	2.8261	2.9703	0.5551	0.6987
	50	15.1697	8.39	5.2753	18.3397	74.9761	4.3456	1.9033	2.0467	3.1081	2.8234	2.9674	0.5533	0.6967
	55	15.1458	8.39	5.2704	18.3744	75.1647	4.3593	1.9039	2.0474	3.1090	2.8242	2.9683	0.5539	0.6974
	60	15.1570	8.38	5.2862	18.3425	74.8845	4.3528	1.9020	2.0453	3.1059	2.8214	2.9654	0.5520	0.6953

Scherrer´s formula:

$D_{\left(311\right)}=\frac{0.9\lambda }{Bcos{\theta }_B}$ (2)

Where D₍₃₁₁₎ is the crystallite size, λ is the Cu K_α (0.154 nm) X-ray wavelength, B is the full width at half maximum (FWHM) measured in radians, and θ is the Bragg angle.

$a=d_{311}\sqrt{h^2+l^2+k^2}$ (3)

Where a is the lattice constant, (hkl) are the Miller indices and d₃₁₁ the spacing values, determined for the recorded diffraction peaks using Bragg´s law.

$d_X=\frac{8M}{N_A{a}^3}$ (4)

Where d_X is the X-ray density, “a³” is the spinel ferrite unit cell volume, 8 is the number of formula units in the unit cell and M and N_A are the molecular weight and Avogadro´s number, respectively.

The lattice strain (η) was measured using and altering Williamson and Hall equation,

$\eta =\frac{d_{311}\left(K-1\right)}{D_{\left(311\right)}{d}_X}$ (5)

where, d₃₁₁ is lattice spacing for (311) planes and K is the shape factor, equal to 0.89.

The specific surface area (S) was calculated using the following equation

$\text{S}=\frac{6}{D_{\left(311\right)}}$ (6)

Finally, the equation used for calculating the dislocation density is the following:

$\sigma =\frac{1}{D^2}$ (7)

Figures 3, 4 and 5 present the profile of all the calculated parameters.

Figure 3
Effect of CTAB on the crystallite size developing profile of nickel ferrite synthesized at 200°C.

Figure 4
Effect of CTAB concentration and reaction time on NiFe₂O₄ obtained at 200°C. (a) lattice constant, (b) X-ray density.

Figure 5
Effect of CTAB concentration role and reaction time on the precipitates. (a) lattice strain, (b) dislocation density, (c) specific surface area.

As it can be seen, the crystallite size increase with time, from 11 to 15 nm, 13 to 14 and 9 to 15 nm for the NS, ½CMC and 2CMC precipitates, respectively. The lattice constant decreased as the time increased until 40, 35 and 25 minutes for the NS, ½CMC and 2CMC samples respectively, and remains constant at 8.40 thereafter. These values are in good agreement with those reported for nickel nanoferrites obtained by sol-gel ³⁸ and the citrate precursor method³⁹, and larger than the reported value of 8.3390 Å for bulk particles³⁹, indicating the lattice expansion as the size of the spinel ferrite particles is reduced. The X-ray density of the samples increases with the increasing time, caused by the decrease in the lattice constant, inversely proportional to a³.

The shrinking crystallite size observed in the NS and 2CMC in the samples precipitated before 30 minutes, led to an increase in the lattice strain and dislocation density, which could affect the growth and crystalline nature of the particles³⁷. The specific surface area for the ½CMC sample was the highest at 100% iron discharge. Even more, the specific surface area for all ½CMC precipitates, independently of the time, showed similar values. These values could indicate the interaction of CTAB, at low concentrations, to promote constant particle growth.

Additionally, the Equations (8), (9), (10), (11)²⁹, (12), (13) and (14)⁴⁰ were used to estimate ionic radii (r_A, r_B), bond lengths (A-O, B-O) in the tetrahedral and octahedral sites, tetrahedral edge [(A-O)_edge], shared and unshared octahedral edge [(B-O)_sedge, (B-O)_uedge] of the cubic spinel structure using the X-ray diffraction data; these results are also shown in Table 1.

$r_A=\left(u-\frac{1}{4}\right)a\sqrt{3}-R_{O^{2-}}$ (8)

$r_B=\left(\frac{5}{8}-u\right)a-R_{O^{2-}}$ (9)

$A-O=\left(u-\frac{1}{4}\right)a\sqrt{3}$ (10)

$B-O=\left(\frac{5}{8}-u\right)a$ (11)

${\left(A-O\right)}_{edge}=\left(2u-\frac{1}{2}\right)a\sqrt{2}$ (12)

${\left(B-O\right)}_{sedge}=\left(1-2u\right)a\sqrt{2}$ (13)

${\left(B-O\right)}_{uedge}={\left[4u^2-3u+\left(\frac{11}{16}\right)\right]}^{\frac{1}{2}}a$ (14)

Where $R_{O^{2-}}$ is the oxygen ionic radius (1.35 Å) and “u” is the oxygen ionic parameter (0.381 Å). The structural parameters such as ionic radii (r_A, r_B), bond lengths in the tetrahedral and octahedral sites, tetrahedral edge and shared and unshared octahedral edge, showed a slight decrease with increasing time for all the NS and 2CMC precipitates, whilst the values for the four parameters, tetrahedral and octahedral bond length and ionic radii, remain constant for all the 2CMC powders. This effect on the 2CMC samples could be related to the cation redistribution as the grain size increased⁴⁰.

The formation of the spinel structure and its interaction with the surfactant was confirmed by the results from FTIR spectroscopy. The infrared spectra of complete discharge samples are shown as an example (Figure 6). The bands appearing at 570, 575 and 577 cm^-1correspond to v₁, the stretching vibrations of the oxygen-metal ion bonds in the tetrahedral sites of the spinel structure⁴¹,⁴²; the vibration corresponding to the octahedrally coordinated metal ions, around 400 cm^-1, is not evident. The variations observed in the tetrahedral vibration band for the ½CMC and 2CMC samples, are associated with the particle-CTAB interaction and the differences in particle size in general²⁸.

Figure 6
FTIR spectra of nickel ferrite for samples precipitated at 55 minutes for NS and ½CMC, and 40 minutes for 2CMC. (a) 4000-400 cm^-1, (b) the 1000-400 cm^-1.

For CTAB interacting with the spinel ferrite nanoparticles, the weak intensity bands at 958 and 909 cm^-1 are attributed to the symmetric and asymmetric C-N⁺ stretching vibration⁴³-⁴⁵. The bands near 1736, 1290 and 1223 cm^-1 are assigned to the vibration of the carbonyl group⁴⁶, indicating the presence of naphthenic acid in the sample. The bands appearing in the region 2849-2927 cm^-1, correspond to the symmetric and asymmetric stretching vibrations of alkyl radicals, while the bands at 1418 cm^-1 are assigned to the flexural stretching vibration of that same group⁴⁷. The band around 1505 cm-1 could also be assigned to CH₃⁺ stretching⁴⁸. The presence of hydrocarbon chains is due to residual naphthenic and the surfactant. Finally, the bands in the region around 3378 cm^-1 are associated with the stretching vibration of the OH- group⁴⁹ and the N-H stretching vibration of pure CTAB molecules⁴⁴. The adsorption band near 2348 cm^-1 are due to the atmospheric CO₂ in the environment⁵⁰.

The structural morphology of the NiFe₂O₄ nanoparticles was examined through FESEM analysis. Figures 7, 8, 9 present the SEM images of the particles obtained, corresponding to the onset time of precipitation (Figure 7, 8, 9-a) and the time for complete conversion (Figure 7, 8, 9-b); the corresponding EDX spectra are also shown in Figure 7, 8, 9 - c. In general, the particles consist of nano aggregates (20-30 nm) of nearly spherical shape, as a result of their magnetic interaction and the thermodynamic requirement for decreasing the nanocomposites high surface/volume ratio generated⁵¹,⁵². CTAB, at both concentrations, helps to preserve the homogeneity during the interaction for longer times; the ½CMC samples (Figure 8-a, b) show higher morphology homogeneity than those corresponding to the higher concentration (Figure 9-a, b); however, both present fewer agglomerates than in the powders obtained without surfactant (Figure 7-a, b). Even more, if all 6 microphotographs are compared, it is clear that, regardless of CTAB concentration, the tensoactive present at the organic/water interface promotes a stable growth. Comparing the particle size obtained in SEM with the crystallite size reported by DRX, it can be observed that the aggregates size is similar regardless of the system and time analyzed. This is due to the magnetic interactions of the particles and the coalescence period; although in DRX a change in the crystallite size is observed as the reaction progresses, it ends up been stabilized and so ½CMC samples present a smaller size compared to the others; this effect can be seen in the size dispersion diagram presented in Figures 7, 8, 9-c.

Figure 7
FESEM micrographs for the NS system precipitates. (a) Onset reaction, (b) 100% discharge, (c) particle size distribution and (d) EDX spectra for 100% discharge.

Figure 8
FESEM micrographs for the ½CMC system precipitates. (a) Onset reaction, (b) 100% discharge, (c) particle size distribution and (d) EDX spectra for 100% discharge.

Figure 9
FESEM micrographs for the 2CMC system precipitates. (a) Onset reaction, (b) 100% discharge, (c) particle size distribution and (d) EDX spectra for 100% discharge.

Comparing the tendency of discharge (Figure 1) with the microphotographs, for the ½CMC and NS samples, the reaction rate is slow and constant, but the onset of precipitation is occurring 10 minutes before for the latter. These results could indicate that, even though the hydrolysis has taken place, nanoparticles remain in the organic fluid because they have not reached the suitable size to settle. Although the yields are similar, the NS powders have growth patterns by agglomeration and coalescence between particles⁵³,⁵⁴, while the surfactant could help to avoid the interaction between nanoparticles in both ½CMC and 2CMC systems.

Figure 10-a show the magnetization profile at 300K as a function of applied magnetic field for the NS, ½CMC and 2CMC at 55, 55 and 40 minutes (Figure 10-a) and 60 minutes (Figure 10-b), respectively. As can be observed, the highest M_s value correspond to the ½CMC sample at 100% discharge time. In all cases, the nickel nanoferrites synthetized exhibit superparamagnetic behaviour, as indicated by their low coercivity and retentivity⁵⁵. NS precipitates reach an equilibrium point around M_s = 45 emu/g, whilst the magnetization shows a slight stagnation (50 to 51 emu / g) for the ½CMC sample and increases slowly from 43 to 53 emu/g for the 2CMC precipitates, achieving the highest M_s value of the whole systems.

Figure 10
VSM curves for spinel ferrites. (a) 100% discharge time (b) 60 minutes.

The experimental magnetic moment per formula unit, ${\eta }_B$, expressed in Bohr Magneton, was calculated for the 100% discharge samples (55 minutes for NS and ½CMC; 40 minutes for 2CMC) by the following formula⁵⁶:

${\eta }_B=\frac{M_W{M}_S}{5585}$ (15)

Where $M_W$ is the molecular weight of the sample and $M_S$ is the saturation magnetization in emu/g. The experimental magnetic moment values vary from 1.7 to 2.1 ${\mu }_B$. Changes in the magnetic properties of nanoferrites are due to the interaction of the spins with the exchange of nearby atoms⁵⁷, and depend largely on the method of preparation, microstructure and particle size⁵⁸, these changes can be accentuated by doping the nickel ferrite with Zn(II)⁵⁹,⁶⁰ and Co(II)⁶¹, among other ions⁶²,⁶³, due to their strong tendency for occupying tetrahedral sites.

The changes in M_s values can be attributed to the crystallite size⁶⁴, since Ms decreased with decreasing crystallite size for mono-domain particles, due to the surface spin canting and thermal fluctuation⁶⁵. Furthermore, remanent magnetization is similar in all samples, being about 2 emu/g, whereby the M_r/M_s ratio is less than 0.05, indicating the formation of a single magnetic domain⁶⁶.

It is known that the energy of a magnetic particle in an external field is proportional to its particle size via the number of magnetic molecules in a single magnetic domain⁶⁶. At the nanoscale, many phenomena such as spin canting, pinning of surface spins and the presence of a spin glass-like surface layer around the magnetic core of the nanoparticle, tend to decrease the number of aligned magnetic moments in the particles as a whole, and therefore the magnetisation in NS nanocrystals decreases⁶⁷.

Although all precipitates present spinel ferrite single phase, the CTAB concentration plays a vital role in the crystal´s growth, promoting a redistribution of Fe³⁺ cations on the particles surface, so affecting the magnetic properties, as suggested and observed in previous studies²⁷,³⁰.

4. Conclusions

According to the results obtained, CTAB favors the physical and magnetic properties of Ni ferrites obtained by hydrolytic stripping. Both the ½CMC and NS samples present similar behavior in the naphthenoates discharge rate, but CTAB might promote the formation of nanoparticles suspended in the organic medium, as evidenced by the appearance of precipitates 10 minutes before in the latter set. The mass transport improvement shown by the 2CMC samples, with precipitation starting 15 min earlier than NS samples, indicates a strong carboxylate-surfactant interaction. Nickel nanoferrite precipitates as the only crystalline phase in all cases, with a constant Fe/Ni molar ratio. In the presence of surfactant, the precipitates show better homogeneity than those without it, indicating the adsorption of CTAB on the particle surface during the growing stage, leading to an increase in the saturation magnetization caused by the cations rearrangement in the crystalline structure.

5. References

• ¹ Kodama RH. Magnetic nanoparticles. J Magn Magn Mater. 1999;200(1):359-72.
• ² Gomes JA, Azevedo GM, Depeyrot J, Mestnik-Filho J, Silva GJ, Tourinho FA, et al. ZnFe₂O₄ nanoparticles for ferrofluids: a combined XANES and XRD study. J Magn Magn Mater. 2011;323(10):1203-6.
• ³ Kim EH, Lee HS, Kwak BK, Kim BK. Synthesis of ferrofluid with magnetic nanoparticles by sonochemical method for MRI contrast agent. J Magn Magn Mater. 2005;289:328-30.
• ⁴ Arulmurugan R, Vaidyanathan G, Sendhilnathan S, Jeyadevan B. Co-Zn ferrite nanoparticles for ferrofluid preparation: study on magnetic properties. Physica B. 2005;363(1-4):225-31.
• ⁵ Holinsworth BS, Mazumdar D, Sims H, Sun Q, Yurtisigi MK, Sarker SK, et al. Chemical tuning of the optical band gap in spinel ferrites: CoFe₂O₄ vs NiFe₂O₄ Appl Phys Lett. 2014;103:082406.
• ⁶ Rana S, Gallo A, Srivastava RS, Misra RDK. On the suitability of nanocrystalline ferrites as a magnetic carrier for drug delivery: Functionalization, conjugation and drug release kinetics. Acta Biomater. 2007;3(2):233-42.
• ⁷ Brusentsov NA, Nikitin LV, Brusentsova TN, Kuznetsov AA, Bayburtskiy FS, Shumakov LI, et al. Magnetic fluid hyperthermia of the mouse experimental tumor. J Magn Magn Mater. 2002;252:378-80.
• ⁸ Pubby K, Meena SS, Yusuf SM, Bindra Narang S. Cobalt substituted nickel ferrites via Pechini’s sol–gel citrate route: x-band electromagnetic characterization. J Magn Magn Mater. 2018;466:430-45.
• ⁹ Shaikh PA, Kambale RC, Rao AV, Kolekar YD. Structural, magnetic and electrical properties of Co-Ni-Mn ferrites synthesized by co-precipitation method. J Alloys Compd. 2010;492(1-2):590-6.
• ¹⁰ Shen L, Althammer M, Pachauri N, Loukya B, Datta R, Iliev M, et al. Epitaxial growth of spinel cobalt ferrite films on MgAl₂O₄ substrates by direct liquid injection chemical vapor deposition growth. J Cryst Growth. 2014;390:61-6.
• ¹¹ Zahraei M, Monshi A, Morales MDP, Shahbazi-Gahrouei D, Amirnasr M, Behdadfar B. Hydrothermal synthesis of fine stabilized superparamagnetic nanoparticles of Zn²⁺ substituted manganese ferrite. J Magn Magn Mater. 2015;393:429-36.
• ¹² Raju P, Murthy SR. Preparation and characterization of Ni–Zn ferrite + polymer nanocomposites using mechanical milling method. Appl Nanosci. 2013;3:469-75.
• ¹³ Gharagozlou M. Synthesis, characterization and influence of calcination temperature on magnetic properties of nanocrystalline spinel Co-ferrite prepared by polymeric precursor method. J Alloys Compd. 2009;486(1-2):660-5.
• ¹⁴ Anchieta CG, Severo EC, Rigo C, Mazutti MA, Kuhn RC, Muller EI, et al. Rapid and facile preparation of zinc ferrite (ZnFe₂O₄) oxide by microwave-solvothermal technique and its catalytic activity in heterogeneous photo-Fenton reaction. Mater Chem Phys. 2015;160:141-7.
• ¹⁵ Raut AV, Barkule RS, Shengule DR, Jadhav KM. Synthesis, structural investigation and magnetic properties of Zn²⁺ substituted cobalt ferrite nanoparticles prepared by the sol–gel auto-combustion technique. J Magn Magn Mater. 2014;358-359:87-92.
• ¹⁶ Doyle FM, Monhemius AJ. Kinetics and mechanisms of precipitation of nickel ferrite by hydrolytic stripping of iron (III)-nickel carboxylate solutions. Hydrometallurgy. 1994;35(2):251-65.
• ¹⁷ Doyle-Garner FM, Monhemius AJ. Hydrolytic stripping of single and mixed metal-versatic solutions. Metall Mater Trans B: Process Metall Mater Proc Sci. 1985;16:671-7.
• ¹⁸ Hajalilou A, Mazlan SA. A review on preparation techniques for synthesis of nanocrystalline soft magnetic ferrites and investigation on the effects of microstructure features on magnetic properties. Appl Phys A: Mater Sci Process. 2016;122:680.
• ¹⁹ Doyle FM, Ye W. ZrO₂ Powders from Zirconium (IV) Carboxylates. J Miner Met Mater Soc. 1987;39:34-7.
• ²⁰ Konishi Y, Nomura T, Mizoe K, Nakata K. Preparation of cobalt ferrite nanoparticles by hydrolysis of cobalt-iron (III) Carboxylate dissolved in organic solvent. Mater Trans. 2004;45(1):81-5.
• ²¹ Bayoumy WAA. Synthesis and characterization of nano-crystalline Zn-substituted Mg-Ni-Fe-Cr ferrites via surfactant-assisted route. J Mol Struct. 2014;1056-1057:285-91.
• ²² Lu HF, Hong RY, Li HZ. Influence of surfactants on co-precipitation synthesis of strontium ferrite. J Alloys Compd. 2011;509(41):10127-31.
• ²³ Goronja JM, Janošević Ležaić AM, Dimitrijević BM, Malenović AM, Stanisavljev DR, Pejić ND. Determination of critical micelle concentration of cetyltrimethylammonium bromide: different procedures for analysis of experimental data. Hem Ind. 2016;70(4):485-92.
• ²⁴ Ji GB, Tang SL, Ren SK, Zhang FM, Gu BX, Du YW. Simplified synthesis of single-crystalline magnetic CoFe₂O₄ nanorods by a surfactant-assisted hydrothermal process. J Cryst Growth. 2004;270(1-2):156-61.
• ²⁵ Chandekar KV, Kant KM. Estimation of the spin-spin relaxation time of surfactant coated CoFe₂O₄ nanoparticles by electron paramagnetic resonance spectroscopy. Physica E. 2018;104:192-205.
• ²⁶ Baykal A, Kasapoǧlu N, Köseoǧlu Y, Toprak MS, Bayrakdar H. CTAB-assisted hydrothermal synthesis of NiFe₂O₄ and its magnetic characterization. J Alloys Compd. 2008;464(1-2):514-8.
• ²⁷ Alamolhoda S, Mirkazemi SM, Shahjooyi T, Benvidi N. Effect of Cetyl trimethylammonium bromide (CTAB) amount on phase constituents and magnetic properties of nano-sized NiFe₂O₄ powders synthesized by sol-gel auto-combustion method. J Alloys Compd. 2015;638:121-6.
• ²⁸ Vadivel M, Babu RR, Ramamurthi K, Arivanandhan M. CTAB cationic surfactant assisted synthesis of CoFe₂O₄ magnetic nanoparticles. Ceram Int. 2016;42(16):19320-8.
• ²⁹ Zhao B, Nan Z. Effects of CTAB on magnetic properties of ZnLa_0.02Fe_1.98O₄ crystals. J Alloys Compd. 2013;580:321-6.
• ³⁰ Amirthavalli C, Thomas JM, Nagaraj K, Prince AAM. Facile room temperature CTAB-assisted synthesis of mesoporous nano-cobalt ferrites for enhanced magnetic behaviour. Mater Res Bull. 2018;100:289-94.
• ³¹ Prabhakaran T, Hemalatha J. Combustion synthesis and characterization of highly crystalline single phase nickel ferrite nanoparticles. J Alloys Compd. 2011;509(25):7071-7.
• ³² Zepeda-Mendoza YA. Precipitación de materiales magnéticamente suaves mediante despojado hidrolítico [dissertation]. Mexico City: National Polytechnic Institute; 2005.
• ³³ Rocha AKS, Magnago LB, Pegoretti VCB, Freitas MBJG, Lelis MFF, Fabris JD, et al. Copper local structure in spinel ferrites determined by X-ray absorption and Mössbauer spectroscopy and their catalytic performance. Mater Res Bull. 2019;109:117-23.
• ³⁴ Gao JM, Cheng F. Study on the preparation of spinel ferrites with enhanced magnetic properties using limonite laterite ore as raw materials. J Magn Magn Mater. 2018;460:213-22.
• ³⁵ Raju K, Venkataiah G, Yoon DH. Effect of Zn substitution on the structural and magnetic properties of Ni–Co ferrites. Ceram Int. 2014;40(7):9337-44.
• ³⁶ Cullity BD, Stock SR. Elements of X-ray diffraction. 3rd ed. Upper Saddle River: Prentice Hall; 2001.
• ³⁷ Kakde AS, Belekar RM, Wakde GC, Borikar MA, Rewatkar KG, Shingade BA. Evidence of magnetic dilution due to unusual occupancy of zinc on B-site in NiFe₂O₄ spinel nano-ferrite. J Solid State Chem. 2021;300:122279.
• ³⁸ Srivastava M, Chaubey S, Ojha AK. Investigation on size dependent structural and magnetic behavior of nickel ferrite nanoparticles prepared by sol-gel and hydrothermal methods. Mater Chem Phys. 2009;118(1):174-80.
• ³⁹ Prasad S, Gajbhiye NS. Magnetic studies of nanosized nickel ferrite particles synthesized by the citrate precursor technique. J Alloys Compd. 1998;265(1-2):87-92.
• ⁴⁰ Yadav RS, Kuřitka I, Vilcakova J, Havlica J, Masilko J, Kalina L, et al. Structural, magnetic, dielectric, and electrical properties of NiFe₂O₄ spinel ferrite nanoparticles prepared by honey-mediated sol-gel combustion. J Phys Chem Solids. 2017;107:150-61.
• ⁴¹ Waldron RD. Infrared spectra of ferrites. Phys Rev. 1955;99:1727-35.
• ⁴² Baykal A, Kasapo N, Glu, Durmu Z, Kavas H, Toprak MS, et al. CTAB-assisted hydrothermal synthesis and magnetic characterization of Ni_xCo_1−xFe₂O₄ Nanoparticles (x = 0.0, 0.6, 1.0). Turk J Chem. 2009;33:33-45.
• ⁴³ Chandekar KV, Kant KM. Relaxation phenomenon and relaxivity of cetrimonium bromide (CTAB) coated CoFe₂O₄ nanoplatelets. Physica B. 2018;545:536-48.
• ⁴⁴ Nithiyanantham U, Ozaydin MF, Tazebay S, Kundu S. Low temperature formation of rectangular PbTe nanocrystals and their thermoelectric properties. New J Chem. 2016;40:265-77.
• ⁴⁵ Li K, Zeng Z, Xiong J, Yan L, Guo H, Liu S, et al. Fabrication of mesoporous Fe₃O₄@SiO₂@CTAB-SiO₂ magnetic microspheres with a core/shell structure and their efficient adsorption performance for the removal of trace PFOS from water. Colloids Surf A Physicochem Eng Asp. 2015;465:113-23.
• ⁴⁶ Cyboran-Mikołajczyk S, Żyłka R, Jurkiewicz P, Pruchnik H, Oszmiański J, Hof M, et al. Interaction of procyanidin B₃ with membrane lipids – Fluorescence, DSC and FTIR studies. Biochimica et Biophysica Acta (BBA) -. Biomembranes. 2017;1859(8):1362-71.
• ⁴⁷ Tan DT, Shao X, Pang SF, Zhang YH. The effect of CTAB on Na₂SO₄ nucleation in mixed Na₂SO₄/CTAB aerosols by FTIR-ATR technology. Chin Chem Lett. 2016;27(7):1073-6.
• ⁴⁸ Ramos Guivar JA, Sanches EA, Magon CJ, Ramos Fernandes EG. Preparation and characterization of cetyltrimethylammonium bromide (CTAB)-stabilized Fe₃O₄ nanoparticles for electrochemistry detection of citric acid. J Electroanal Chem. 2015;755:158-66.
• ⁴⁹ Frost RL. An infrared and Raman spectroscopic study of the uranyl micas. Spectrochim Acta A Mol Biomol Spectrosc. 2004;60(7):1469-80.
• ⁵⁰ Ilhan S, Izotova SG, Komlev AA. Synthesis and characterization of MgFe₂O₄ nanoparticles prepared by hydrothermal decomposition of co-precipitated magnesium and iron hydroxides. Ceram Int. 2015;41(1):577-85.
• ⁵¹ Humbe AV, Nawle AC, Shinde AB, Jadhav KM. Impact of Jahn Teller ion on magnetic and semiconducting behaviour of Ni-Zn spinel ferrite synthesized by nitrate-citrate route. J Alloys Compd. 2017;691:343-54.
• ⁵² Aslibeiki B, Kameli P, Manouchehri I, Salamati H. Strongly interacting superspins in Fe₃O₄ nanoparticles. Curr Appl Phys. 2012;12(3):812-6.
• ⁵³ Konishi Y, Doyle FM. Kinetics of hydrolysis and precipitation in aqueous-organic systems. I. Analysis of heterogeneous growth. Hydrometallurgy. 1994;35(2):203-22.
• ⁵⁴ Konishi Y, Doyle FM. Kinetics of hydrolysis and precipitation in aqueous-organic systems. II. Analysis of heterogeneous growth. Hydrometallurgy. 1994;35(2):223-49.
• ⁵⁵ Arévalo-Cid P, Isasi J, Alcolea Palafox M, Martín-Hernández F. Comparative structural, morphological and magnetic study of MFe₂O₄ nanopowders prepared by different synthesis routes. Mater Res Bull. 2020;123:110726.
• ⁵⁶ Yehia M, Labib S, Ismail SM. Structural and magnetic properties of nano-NiFe₂O₄ prepared using green nanotechnology. Physica B. 2014;446:49-54.
• ⁵⁷ Salih SJ, Mahmood WM. Review on magnetic spinel ferrite (MFe₂O₄) nanoparticles: from synthesis to application. Heliyon. 2023;9:e16601.
• ⁵⁸ Amirabadizadeh A, Salighe Z, Sarhaddi R, Lotfollahi Z. Synthesis of ferrofluids based on cobalt ferrite nanoparticles: influence of reaction time on structural, morphological and magnetic properties. J Magn Magn Mater. 2017;434:78-85.
• ⁵⁹ Sarita, Anchal, Priya, Beniwal RK, Rulaniya MS, Saini PM, et al. Development and characterization of superparamagnetic Zn-Doped Nickel ferrite nanoparticles. J Magn Magn Mater. 2014;610:172547.
• ⁶⁰ Velmurugan K, Venkatachalapathy VSK, Sendhilnathan S. Thermogravimetric and magneticproperties of Ni_1-XZn_xFe₂O₄ nanoparticles synthesized by coprecipitation. Mater Res. 2009;12(4):529-34.
• ⁶¹ Shelar MB, Patange SM. Studies on structural and magnetic properties of cobalt substituted nickel nano ferrites prepared by microwave assisted self-propagating high temperature route. Mater Today Proc. 2022;59:1196-201.
• ⁶² Gaba S, Rana PS, Kumar A, Pant RP. Structural and paramagnetic resonance properties correlation in lanthanum ion doped nickel ferrite nanoparticles. J Magn Magn Mater. 2020;508:166866.
• ⁶³ Sinha A, Dutta A. Structural, optical, and electrical transport properties of some rare-earth-doped nickel ferrites: A study on effect of ionic radii of dopants. J Phys Chem Solids. 2020;145:109534.
• ⁶⁴ Baabe D, Mienert D, Schultze D, Krumeich F, Sepel V, Litterst FJ, et al. Evolution of structure and magnetic properties with annealing temperature in nanoscale high-energy-milled nickel ferrite. J Magn Magn Mater. 2003;257(2-3):377-86.
• ⁶⁵ Zhang D, Tong Z, Xu G, Li S, Ma J. Templated fabrication of NiFe₂O₄ nanorods: Characterization, magnetic and electrochemical properties. Solid State Sci. 2009;11:113-7.
• ⁶⁶ Zhao L, Yang H, Zhao X, Yu L, Cui Y, Feng S. Magnetic properties of CoFe₂O₄ ferrite doped with rare earth ion. Mater Lett. 2006;60(1):1-6.
• ⁶⁷ Debnath S, Das A, Das R. Effect of cobalt doping on structural parameters, cation distribution and magnetic properties of nickel ferrite nanocrystals. Ceram Int. 2021;47:16467-82.

Publication Dates

• Publication in this collection
  19 May 2025
• Date of issue
  2025

History

• Received
  22 Jan 2025
• Reviewed
  26 Mar 2025
• Accepted
  25 Apr 2025