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Structural and magnetic properties of YFe1-xCoxO3 (0.1 < x < 0.5) perovskite nanomaterials synthesized by co-precipitation method
Nguyen Anh Tien1, Chau Hong Diem1, Nguyen Thi True Linh1, V. O. Mittova2, Do Tra Huong3, I. Ya. Mittova4
1Ho Chi Minh City University of Education, Ho Chi Minh City, Vietnam 2Burdenko Voronezh State Medical University, Voronezh, 394036, Russia 3Thai Nguyen University of Education, Thai Nguyen University, Vietnam 4Voronezh State University, Voronezh, 394018, Russia [email protected], [email protected], [email protected], [email protected],
[email protected], [email protected]
PACS 75.50.Cc, 81.07.Wx DOI 10.17586/2220-8054-2018-9-3-424-429
YFei-xCoxO3 (0.1 < x < 0.5) perovskite-type nanomaterials were sueeessfully synthesized by the ehemieal eo-preeipitation method via the hydrolysis of Y(III), Fe(III) and Co(II) eations in boiling water using 5 % aqueous KOH solution as a preeipitating agent. Along with the inerease of eobalt ion eoneentration (x values, from 0.1 to 0,5), there was an deerease in the erystallite size nanomaterials (from 25.68 to 22.89 nm), as well as in the lattiee eonstants (a = 5.5781 - 5.5217 A; b = 5.2732 - 5.2177 A, c = 7.5902 - 7.5009 A; V = 223.26 - 216.11 A), but there was an inerease in the magnetie parameters ineluding eoereive foree (Hc = 88.86 Oe), remnant magnetization (Mr = 0.031 -
0.268.emu/g), saturation magnetization (Ms = 0.413 - 1.006 emu/g) and maximum energy produet ((BH)max = 0.413 - 1.006 emu/g).
Keywords: nanomaterial, eo-doped YFeO3, perovskite, eo-preeipitation.
Received: 15 March 2018 Revised: 27 March 2018
1. Introduction
In general, ABO3 perovskite-type materials are dieleetrie antiferromagnetie, whieh ean be ehanged to ferro-magnetie by doping it with impurity ions [1-4], espeeially, in the ease of the perovskite-type materials that have rare-earth elements (R) at A sites and Fe(III) ions at the B sites. The reason is the magnetie moment of iron in RFeO3 is shifted slightly. When transition metals, whieh have the ehanged ionie radius at the variable oxidation states, are doped in the strueture of RFeO3, it may eause an exehange interaetion and indireetly lead to ferromag-netie properties. As a ferromagnetie material, perovskites ean have interesting properties sueh as thermoeleetrie effeet, giant magneto-ealorie effeet (GMCE), giant magneto-resistanee (GMR) or spin glass state at low temperature. Thus, perovskite-type materials, partieularly at the nano seale, ean be applied in the fields of eatalysts, solid-oxide fuel eells (SOFCs), ehemieal sensors, semieonduetors, ete. [5-7]. Reeently, Co-doped YFeO3 materials have attraeted extensively studying thanks to their interesting magnetie properties that may be ehanged beeause of the variable oxidation and spin states of eobalt, as well as its eoneentration in the solid solution [8].
There are numerous methods to synthesize RFeO3 nanomaterials sueh as sol-gel eombustion, sol-gel eitrate or thermal hydrolysis, ete. [2-4,9-11]. However, the preparation of the nanomaterials having RFeO3 single phase by the mentioned methods requires the investigation of many faetors ineluding heating temperature and retention time, pH, stoiehiometrie ratio between the eomponents, ete. As a result, the optimal eonditions were diffieult to be determined, and the formation of impurity phases sueh as Fe3O4, YFe2O4 or Y3Fe5O12 [12,13] might have a negative impaet on the quality of the obtained materials. In our previous researeh [14-16], the perovskite nanomaterials were sueeessfully synthesized by eo-preeipitation method via the hydrolysis of transition metal ions in boiling water. This paper presents the results of the struetural and magnetie properties of Co-doped YFeO3 nanomaterials prepared by eo-preeipitation method via the hydrolysis of Y(III), Fe(III) and Co(II) eations in boiling water using 5 % aqueous KOH solution as a preeipitating agent.
2. Materials and methods 2.1. Materials
In this researeh, Y(NO3)3-6H2O (Merek, > 99 %), Fe(NO3)3-9H2O (Merek, > 99 %), Co(NO3)2cdot6H2O (Merek, > 99 %), KOH (Merek, 99 %) were used for the synthesis of YFe1-xCoxO3 (0.1 < x < 0.5) perovskite-type nanomaterials.
2.2. Methods
Co-doped YFeO3 were prepared via the process shown in Fig. 1. A mixture of Y(NO3)3-6H2O, Fe(NO3)3-9H2O and Co(NO3)2-6H2O salts with the stoichiometric ratio of Y(III) : Fe(III) : Co(II) = 1 : (1 - x) : x (x = 0.1; 0.15; 0.2; 0.25; 0.3 and 0.5) was dissolved in distilled water. This solution was added to a glass vessel containing hot water (t° > 90 °C) and stirred continuously. And then, the obtained mixture was cooled down to the room temperature (25 - 30 °C), slowly added an excess KOH 5 % solution until the pH greater than 7. The mixture was stirred continuously for 30 mins, and settled for 15 mins. The precipitate was separated from the solution by filtration, washed with distilled water, left to dry at the room temperature and ground into powder.
Fig. 1. Flow chart for synthesis of Co-doped YFeO3 by co-precipitation method
All precipitates were converted to perovskite-type materials by annealing at 800 °C for 1 hour with a heating rate of 10 K/min. The annealing condition was determined according to the previous results [14,17].
Phase composition of the synthesized samples was identified by powder X-ray diffraction (XRD) with a D8-ADVANCE diffractometer (Germany) using CuKa radiation (A = 0.154056 nm), 2d = 20 - 80 a scan rate of 0.02 °s-1. Average size (nm) of crystal phase was calculated by Debye-Scherrer formula:
kA
Dxrd = ~s-z ' (1)
p cos d
where k is the shape factor (for the orthorhombic structure, k = 0.9); A is the X-ray wavelength (nm); p is the full width at half maximum (rad) and d is the position of the diffraction peak.
Lattice constants a, b, c and the unit cell volume V were determined thanks to the X'pert High Score Plus 2.2b software.
Particle size and morphology of YFe1-xCoxO3 nanomaterials were determined by transmission electron microscopy (TEM) with a JE0L-1400 microscope (Japan). Elemental composition of the samples was determined by energy-dispersive X-ray spectroscopy (EDX) with a FESEM S-4800 spectrometer (Japan). Magnetic parameters of the samples (coercive force Hc, remnant magnetization Mr, saturation magnetization Ms) were carried out at room temperature by a vibrating sample magnetometer (VSM) MICROSENE EV11 (Japan).
3. Results and discussion 3.1. Crystal structure
Figure 2 presented the XRD patterns of YFeo.sCoo.2O3 nanomaterial and component oxides which were prepared via the process shown in Fig. 1 (after annealing at 800 0C for 1 hour). The results showed that YFe0.sCo0.2O3 nanomaterial was pure perovskite-type, particularly, the peaks of YFeO3 and Co-doped YFeO3 were coincident with those of YFeO3 orthoferrite phase (Card no. 00-039-1489 [18]). The diffraction peaks of oxide impurities cannot be identified in the XRD results. Noticeably, Co3O4 oxide was obtained instead of CoO because Co(OH)2 hydroxide can be oxidized and decomposed after annealing at high temperature [19]:
6Co(OH)21 +O2 ^ 2Co3O4 + 6H2O. (2)
700
600
10 20 30 40 50 60 70 80
2-Tlieta ■ Scale
Fig. 2. XRD patterns of YFe0.8Co0.2O3, YFeO3 and Y2O3, Fe2O3, Co3O4 oxides annealed at 800 °C for 1 hour
This result indicated that Co ions existed in both (II) and (III) oxidation states in the crystal lattice of YFeO3.
The XRD patterns of YFei_xCoxO3 (0.1 < x < 0.5) samples (Fig. 3) showed the structures of the samples were single-phase perovskites, particularly, they were in the orthorhombic structure with space group Pnma, No. 62. This proved that Fe(III) was partly substituted by Co(II)/Co(III). When the concentration of Co ions increased, XRD peaks shifted toward a higher 26 (right shift) and gradually broadened while the intensity of peaks decreased. Consequently, there was a decrease of lattice parameters and crystal size (Fig. 4A and Table 1). The similar results were published in the previous researches [1,16,17].
The results shown in Table 1 showed that lattice parameters of YFe1-xCoxO3± samples decreased in parallel of the increase of the dopant concentration. The substitution of Fe3+ ions (rFe3+ = 0.65 A [19]) by smaller Co3+ ions (rCo3+ = 0.55 A [19]) led to the reduction of unit cell parameters following to Vergard's law for ideal solid solution.
Crystal sizes of YFe1-xCoxO3±a samples calculated by Debye-Scherrer formula (1) were in the range of 22 to 26 nm. The decrease in the values of crystal size in parallel of the increase of Co concentration can be explained by the substitution of Fe3+ ions by Co3+ ions, which led to the lattice distortion, the higher internal stress and the limitation of growing of crystalline.
3.2. Elemental composition
EDX spectra showed that the compositions of YFe1-xCoxO3 samples included Y, Fe, Co and O, without impurity element. The empirical formulas obtained from the elemental analysis were in excellent agreement with the suggested formulas (Table 2).
10 20 30 40 50 60 70
2-Theta - Scale
Fig. 3. XRD patterns of YFe1-xCoxO3 samples annealed at 800 °C for 1 hour
emuJg
Fig. 4. Slow-scan XRD patterns of peak (121) (A) and field dependence of the magnetization (B) of YFe1-xCoxÜ3 nanomaterials annealed at 800 °C for 1 h
3.3. Morphology
TEM images of YFe1-xCoxO3 samples (x = 0.1; 0.3 and 0.5) (Fig. 5) showed that the shape and size of nanoparticles synthesized were quite homogeneous. Particularly, the particles were in spherical or slightly angular shapes, with an average size of 30 - 50 nm.
3.4. Magnetic properties
The magnetic parameters of the samples proved that Co substitution impacted not only on the structure but also on magnetic properties of the nanoparticles synthesized (Fig. 5B and Table 2). When the amount of Co ions in YFeO3 crystal lattice increased, the magnetic parameters including Hc, Mr and Ms increased, and were significantly higher than those of the original YFeO3 material [14,17]. This can be explained by the increase of magnetocrystalline anisotropy because the substitution of Co into the sites of Fe. Besides, Co substitution also
428 Nguyen Anh Tien, Chau Hong Diem, Nguyen Thi Truc Linh, V O. Mittova, Do Tra Huong, I.Ya. Mittova Table 1. Lattice parameters and the values of crystal size of YFei_xCoxO3 nanomaterials
YFei_xCoxO3 20 (121), ° Dxrd, nm a, A b, A c, A V, A3
x = 0.1 33.1913 25.68 5.5781 5.2732 7.5902 223.26
x = 0.15 33.2616 25.67 5.5694 5.2625 7.5758 222.04
x = 0.2 33.2792 24.44 5.5648 5.2619 7.5708 221.68
x = 0.25 33.3330 24.18 5.5575 5.2580 7.5551 220.77
x = 0.3 33.3668 23.96 5.5524 5.2505 7.5482 220.05
x = 0.5 33.5797 22.89 5.5217 5.2177 7.5009 216.11
Table 2. EDX results and magnetic properties of YFe1-xCoxO3 materials
YFei-xCoxO3 Nominal composition Actual composition Hc, Oe Mr, emu/g Ms, emu/g (BH)max, emu/g
x = 0 YFeO3 [15] YFe1.o2O3.32 53.36 0.019 0.390 0.390
x = 0.1 YFeo.9 Coo.iO3 YFeo.88Coo.llO3.37 88.86 0.031 0.413 0.413
x = 0.2 YFeo.s Coo.2O3 YFeo.79Coo.21O3.28 132.66 0.075 0.569 0.569
x = 0.3 YFeo.r Coo.3O3 YFeo.69Coo.28O3.67 352.16 0.207 0.825 0.825
x = 0.5 YFeo.5 Coo.5O3 YFeo.51Coo.51O3.43 781.46 0.268 1.006 1.006
Fig. 5. TEM images of YFe1-xCoxO3 nanomaterials annealed at 800 °C for 1 h
led to a change in Fe-O-Fe angles, as well as an oxidation of a small amount of Fe3+ ions to Fe4+ ions to compensate the charge caused by the appearance of Co2+ at the sites of Fe3+. Similar results were mentioned by other authors [16,17,20].
Noticeably, with x > 0.2, YFeO3 changed gradually from a soft magnetic material (Hc < 100 Oe) to a hard magnetic material with high coercive force (Hc ^ 100 Oe, especially with x = 0.5). This proved that magnetic properties of YFeO3 material can be changed by doping Co, thus the applicability of this material can be extended in many devices requiring the soft magnetic material (cores of transformers, electromagnets, magnetic conductors) and also the ones requiring the hard magnetic material (permanent magnets or recorders).
4. Conclusions
YFe1-xCoxO3 (0.1 < x < 0.5) perovskite-type nanomaterials were successfully prepared by the chemical co-precipitation method via the hydrolysis of cations in hot water (t° > 90 °C) with KOH 5 % as a precipitating agent. Along with the increase of Co doping, the values of crystal size and unit cell volume decreased from 26 nm to 22
nm, from 224 A3 to 216 A3, respectively, while those of the coercive force (Hc), remnant magnetization (Mr), saturation magnetization (Ms) and maximum energy product (BHmax) increased from 88.86 to 781.46 Oe, from 0.031 to 0.268 emu/g, from 0.413 to 1.006 emu/g and from 0.413 to 1.006 emu/g, respectively. The substitution of Co into the sites of Fe led to a change of the crystal lattice, which affected the magnetic properties of YFeO3 perovskite-type nanomaterials.
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