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Phase composition and magnetic properties of Ni1-xCoxFe2O4 nanocrystals with spinel
structure, synthesized by Co-precipiation
Anh Tien Nguyen1, Tien Dat Nguyen1, V. O. Mittova2, M. V. Berezhnaya3, I. Ya. Mittova3 1Ho Chi Minh City Pedagogical University, Ho Chi Minh City, Vietnam 2Burdenko Voronezh State Medical University, Voronezh 394036, Russia 3Voronezh State University, Voronezh 394018, Russia [email protected], [email protected], [email protected], [email protected], [email protected]
PACS 75.50.Cc, 81.07.Wx DOI 10.17586/2220-8054-2017-8-3-371-377
Nanopowders of nickel ferrite doped with cobalt were synthesized by co-precipitation using a 3 % KOH solution as a precipitant. The effects of different annealing regimes on the composition and particle size of Nii-xCoxFe2O4 were studied. It was established that with annealing at t = 1000 °C for 2 h single-phase products were formed with a crystallite size of 30-50 nm. The saturation magnetization and the coercive force increased as the content of the dopant increased from 50.3 emu/g and 51.94 Oe for NiFe2O4 to 80.45 emu/g and 848.32 Oe for CoFe2O4. Keywords: ferrites, spinel, nanocrystals, co-precipitation, magnetic properties. Received: 7 April 2017 Revised: 12 May 2017
1. Introduction
Among magnetic materials, nanocrystalline ferrites with a spinel structure of type MFe2O4 (M = Co, Ni, Zn, Mn) are distinguished by high permeability, saturation magnetization and are used to produce new multifunctional materials, such as high-frequency devices, due to the decreased Foucault currents and increased duration of their functioning [1-5]. In addition, ferrites with a spinel structure are cheaper and more stable (in terms of time and temperature) compared to metals and alloys.
The magnetic and electrical characteristics of these ferrites depend on their chemical composition, cation location, particle size, dopant content and the synthesis method [1-9]. CoFe2O4 is hard magnetic ferrite with a high coercive force Hc > 1500 Oe, with an average saturation magnetization of Ms ~ 40 emu/g [10]. NiFe2O4 is a soft magnetic material characterized by small values of coercive force, saturation magnetization and excessive magnetization (Hc = 46.46 Oe, Ms = 8.8 emu/g, Mr = 0.2 emu/g) [1-3]. Different magnetic properties of ferrites are required for different applications and can be achieved in several ways: either by controlling the particle size, or by changing the concentrations of hard and soft magnetic phases in the material by introduction of another element (dopant) or by the formation of coatings on SiO2 [11-14].
Sol-gel processes, including the formation of complexes with the addition of surfactants are promising for the synthesis of ferromagnetic oxide nanomaterials MFe2O4 (M = Co, Ni). An important advantage of such complexes is the lower annealing temperature in comparison to solid-phase synthesis, leading to decreased size of the obtained particles [2-5,10,11]. However, the formation of a single-phase product synthesized by the sol-gel method is affected by many factors, such as the annealing temperature and time, the gel formation temperature, the stoichiometry of the surfactant/metal ions, pH value, and others.
In previous studies [1,12], MFe2O4 nanocrystals (M = Co, Ni) were obtained by co-precipitation of M2+ and Fe3+ cations at room temperature with the addition of an aqueous ammonia at pH = 11. However, at pH = 11, Co(OH)2 and Ni(OH)2 are dissolved with the formation of complexes with ammonia in accordance with the following equation:
M(OH)2 I +6NH3 ^ [M(NH3)6](OH)2, therefore, it is impossible to completely precipitate cobalt and nickel hydroxides with an aqueous solution of ammonia, which makes the achievement of the molar ratio M2+ : Fe3+ = 1 : 2 in the composition of precipitation complicated.
In this study, we investigated conditions for the formation of nanocrystals with the spinel structure Ni1-xCoxFe2O4 (x = 0; 0.2; 0.4; 0.6; 0.8; 1) by co-precipitation during the hydrolysis of M(II) and Fe(III) cations in boiling water, followed by cooling and addition of 3% aqueous KOH as a precipitant, as well as the phase composition and magnetic properties of the formed compounds. Hydrolysis of M(II) and Fe(III) cations in
boiling water leads to the formation of a stable precipitate and decreased particle size compared to co-precipitation of the hydroxide ions at room temperature. Some nanosystems of LnFeO3 (Ln = La, Y) type were obtained by this co-precipitation method [15,16].
2. Materials and methods
2.1. Initial substances
The precursors for these syntheses were aqueous solutions of nickel (II), cobalt (II) and iron (III) nitrates (all -chemically pure) with a molar ratio of Ni2+ : Co2+ : Fe3+ = (1 - x) : x : 2. The precipitant was an aqueous solution of potassium hydroxide (3 %).
2.2. The synthesis of Ni1_xCoxFe2O4 nanoparticles
Ni1-xCoxFe2O4 nanoparticles were synthesized by adding 50 mL of a mixture containing Ni(NO3)2 0.1 M, Co(NO3)2 0.1 M and Fe(NO3)3 0.2 M into 500 mL of boiling water with magnetic stirring. The obtained colloidal system was cooled to room temperature. The system acquired a reddish-brown color which it retained upon cooling. Then, 3 % aqueous solution of potassium hydroxide in the amount required for complete precipitation of the cations (until the disappearance of the phenolphthalein color) was added with magnetic stirring.
The precipitated hydroxides were stirred for 30-40 min with magnetic stirring. The obtained precipitate was separated using a vacuum filter, washed several times with distilled water and dried at room temperature to a constant weight.
2.3. Methods of investigation
The obtained samples were finely ground into a powder and subjected to complex thermal analysis using thermal analyzer Labsys Evo (TG-DSC 1600 °C, France), including thermogravimetric analysis (TGA) and differential-scanning calorimetry (DSC). The thermal analysis was carried out in the dry air. The heating rate was 10 °/min; the maximum heating temperature was 1100 °C.
The phase compositions of the samples were determined by X-ray phase analysis (XPA, diffractometer SIEMEN D - 5000 Brucker, Germany), CuKa-radiation, A = 0.15406 nm, 26 = 10-80 °, measurement interval 0.02 °/sec. The obtained diffractograms were analyzed using the JCPDS database.
The average crystal size according to XPA data and the constant parameter of the spinel cubic lattice Ni1-xCoxFe2O4 were calculated by following equations:
d 0.89 • A (1)
d = JCOSe, (1)
A Vh2 + k2 + l2, (2)
2 sin 6
where d - average crystal size, A; A - X-ray tube wavelength (for the copper tube used in this recording, A = 0.15406 nm); 26 - is the position of the maximum of the peak, deg; 3 - is the true physical broadening of the diffraction maximum, rad.; h, k, l - Miller indices, correspond to the peaks with the highest intensity.
The sizes and morphologies of the nanoparticles were determined by transmission electron microscopy (TEM-JEOL 1400, Japan).
Magnetic characteristics of Ni1-xCoxFe2O4 nanopowders were studied using a Microsene EV11 (Japan) vibration magnetometer at room temperature.
2.4. Results and discussion
The results of complex thermal analysis of the obtained precipitate for Nio.6Coo.4Fe2O4 nanopowder before annealing, shown in Fig. 1, demonstrated that mass loss of the sample during all heat treatment processes from room temperature to 1100 °C was 16.61 % (TGA curve). The mass loss of the sample heated from 65 °C to 320 °C occurs rapidly, as indicated by the slope of TGA curve. Two peaks of endothelial effects at 200 °C and 27 ° C, characteristic for desorption, evaporation and decomposition of nickel (II), cobalt (II) and iron (III) hydroxides were detected on DSC curve.
After that, the mass loss of the sample is slower, terminating at ^ 650 °C due to the continuation of decomposition of hydroxides with formation of NiO, CoO and Fe2O3. At this point, a large peak corresponding to heat release at 500-650 ° C appeared.
Subsequently, in accordance with the thermal analysis data of Ni0.6Co0.4Fe2O4, thermal treatment was carried out at 700, 800 and 1000° C for 2 h in order to establish the conditions for the formation of single-phase Ni1-xCoxFe2O4 products.
Fig. 1. DSC and TGA curves of precipitate sample for production of Ni0.6Co0.4Fe2O4 before annealing
The diffractograms of undoped NiFe2O4 (Fig. 2) after annealing at 700, 800 and 1000 °C showed that the background of lines of diffractograms are stable, the intensities of the peaks increase and their widths decrease at higher temperatures. Consequently, when the annealing temperature increased from 700 °C to 1000 °C, the degree of crystallization of the samples increased with decreasing average crystal size. The presence of only one phase of NiFe2O4 spinel with a cubic structure belonging to Fd3m spatial group (JCPDS 00-010-0325) was detected in the diffractograms of all samples.
Fig. 2. Diffractograms of NiFe2O4 samples, obtained by the co-precipitation method after annealing at 700, 800 and 1000 °C for 2 h
In diffractograms of CoFe2O4 samples after annealing at 700, 800 and 1000 °C (Fig. 3), peaks corresponding to only one phase - CoFe2O4 with a cubic structure belonging to the spatial Fd3m group (JCPDS 00-022-1086) were also present.
Later, in accordance with the XPA data of MFe2O4 samples (M = Ni, Co), the following annealing modes were selected for the formation of Ni1-xCoxFe2O4 (x = 0.2, 0.4, 0.6 and 0.8): temperature 800 and 1000 °C, time - 2 h.
X-ray phase analysis (Figs. 4, 5) of the synthesized nanopowders demonstrated the formation of solid solutions between NiFe2O4 and CoFe2O4 [12,13], peaks with different x values coincided with each other and with standard
Fig. 3. Diffractograms of CoFe2O4 samples, obtained by the co-precipitation method after annealing at 700, 800 and 1000 °C for 2 h
diffractograms of JCPDS base. However, the slow scanning of the peak angles with the highest intensity (26 was from 34.5 to 35.5 deg., Miller's indices (311)) (Fig. 4) showed the shift of X-ray diffraction lines to the left (decrease of 26) as x values increased (Fig. 5). This fact proves the substitution of nickel by cobalt in Ni1-xCoxFe2O4 spinel [14]. The average size of crystals of Ni1-xCoxFe2O4 spinel after annealing at 800 and 1000 °C for 2 h, calculated by formula (1), was from 19 to 50 nm and it gradually increased with an increase in the degree of doping from x = 0 to x = 0.6, and then decreased. The parameter of the crystalline cubic lattice (a) of Ni1-xCoxFe2O4 nanocrystals, calculated from formula (2), increased with increased annealing temperatures and cobalt content in the samples (Tables 1 and 2). The last trend can be explained by the fact that the ionic radius of Ni2+ (0.72 A) is lower than that of Co2+ (0.74 A) [11]. Similar results were obtained in studies [11,12,14].
Fig. 4. Diffractograms of Ni1-xCoxFe2O4 samples obtained by the co-precipitation method after annealing at 1000 °C for 2 h
Fig. 5. Displacement of X-ray diffraction lines of Ni1-xCoxFe2O4 samples after annealing at 1000 °C for 2 h
Table 1. Parameters of crystalline structure of Ni1-xCoxFe2O4 samples, synthesized by the co-precipitation method after annealing at 800 °C for 2 h
x 0 0.2 0.4 0.6 0.8 1
20maX, deg 35.6714 35.6293 35.5707 35.5636 35.5244 35.4663
FWHM, rad. 0.311 0.276 0.223 0.192 0.378 0.432
D, nm 26.540 33.202 38.195 42.969 22.187 19.092
a, A 8.311 8.351 8.364 8.366 8.375 8.387
Table 2. Parameters of crystalline structure of Ni1-xCoxFe2O4 samples synthesized by the co-precipitation method after annealing at 1000 °C for 2 h
x 0 0.2 0.4 0.6 0.8 1
26>max, deg 35.6756 35.6058 35.5881 35.5292 35.4835 35.4499
FWHM, rad. 0.251 0.221 0.195 0.168 0.314 0.382
D, nm 32.880 39.139 42.311 49.113 26.204 21.590
a, A 8.340 8.356 8.366 8.373 8.384 8.392
TEM images of NiFe2O4, Ni0.6Co0.4Fe2O4 and CoFe2O4 samples after annealing at 800 °C for 2 h (Fig. 6) showed that the size of the synthesized particles was 30-50 nm.
The magnetization curves of Ni1-xCoxFe2O4 samples, measured at room temperature are shown in Fig. 7, while their magnetic characteristics are shown in Table 3. Analysis of the obtained results shows that saturation magnetization (Ms), excess magnetization (Mr), and coercive force (Hc) of Ni1-xCoxFe2O4 samples increase with increased cobalt content in NiFe2O4 lattice. The increase in Mr and Hc values was due to an increase in the size of Ni1-xCoxFe2O4 crystals as the cobalt content increased (see Table 2). The increase in saturation magnetization (Ms) is explained by the fact that the magnetic moment of Co2+ cation (u = 3^b) is higher than the magnetic moment of Ni2+ion (u = 2^b) [17].
Excess magnetization values of the samples (Mr = 7.58-38.45 emu/g) obtained in this study were not different from data published earlier [10-12], but coercive force value (Hc = 51.94-838.42 Oe) decreased 2-3-fold depending on the value of x. The saturation magnetization (Ms = 50.3-80.45 emu/g) was not only higher than the saturation magnetization of CoFe2O4 spinel, but also much higher than the saturation magnetization of NiFe2O4 spinel [3, 4,10,17]. In addition, the saturation magnetization value of CoFe2O4 nanocrystals synthesized in this study, after annealing at 1000 °C for 2 h (Ms = 80.45 emu/g) was higher than the standard saturation magnetization value of CoFe2O4 multidimensional materials (Ms = 80 emu/g) [18].
Thus, Ni1-xCoxFe2O4 nanocrystals, synthesized by simple co-precipitation method, take precedence over CoFe2O4 nanocrystals by Hc value and over NiFe2O4 - by the value of Ms.
Fig. 6. TEM-images of Ni1-xCoxFe2O4 powders after annealing at 800 °C during 2 h
Applied Fieid, Oe Applied Field, Oe Applied Field, Oe
Fig. 7. Field dependence of magnetization of Ni1-xCoxFe2O4, nanocrystals synthesized by the co-precipitation method after annealing at 1000 °C for 2 h
Table 3. Magnetic characteristics of Ni1-xCoxFe2O4 nanocrystals synthesized by the co-precipitation method after annealing at 1000 °C for 2 h
Nii-xCoxFe2O4 Ms, emu/g Mr, emu/g Hc, Oe
x = 0 50.3 7.58 51.94
x = 0.2 57.57 9.93 70.49
x = 0.4 66.07 10.26 73.41
x = 0.6 71.44 26.41 547.89
x = 0.8 78.16 30.96 706.26
x =1 80.45 38.45 848.32
3. Conclusions
Ni1-xCoxFe2O4 nanopowders (x = 0; 0.2; 0.4; 0.6; 0.8; 1) with a spinel structure were synthesized by a simple co-precipitation method using a 3 % aqueous solution of KOH as a precipitant. Single-phase samples were formed by annealing the precipitates at 700, 800 and 1000 °C for 2 h. The obtained Ni1-xCoxFe2O4 nanocrystals had a particle size of about 30-50 nm. The size of their crystals, the cubic lattice, and the magnetic characteristics (Hc, Mr and Ms) increased with the increase of cobalt content in NiFe2O4 spinel.
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