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23Condensed Matter and Interphases. 2021;23(2): 196-203
ISSN 1606-867Х (Print) ISSN 2687-0711 (Online)
Condensed Matter and Interphases
Kondensirovannye Sredy i Mezhfaznye Granitsy https://journals.vsu.ru/kcmf/
Original articles
Research article
https://doi.org/10.17308/kcmf.2021.23/3429
Sol-gel synthesis, crystal structure and magnetic properties of nanocrystalline praseodymium orthoferrite
Bui Xuan Vuong1, Nguyen Anh Tien2^
1Faculty of Natural Sciences, Sai Gon University, Ho Chi Minh City 700000, Vietnam
2Faculty of Chemistry, Ho Chi Minh City University of Education, Ho Chi Minh City 700000, Vietnam
Abstract
In this work, nano-sized crystalline praseodymium orthoferrite was successfully synthesized via sol-gel method using water - methanol co-solvent. Single-phase PrFeO3 nanoparticles were formed after annealing the precursors at 650, 750, 850, and 950 °C during 60 min. The crystal size, lattice volume and coercivity (Hc) of nanocrystalline PrFeO3 increase with the annealing temperature. The obtained praseodymium orthoferrite exhibited paramagnetic properties with Hc = 28 - 34 Oe.
Keywords: Sol-gel synthesis, Methanol, Praseodymium orthoferrite, Magnetic property
For citation: Bui X. V., Nguyen A. T. Sol-gel synthesis, crystal structure and magnetic properties of nanocrystalline praseodymium orthoferrite. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2021;23(2): 196-203. https://doi.org/10.17308/kcmf.2021.23/3429
Для цитирования: Буи Х. В., Нгуен А. Т. Золь-гель синтез, кристаллическая структура и магнитные свойства нанокристаллического ортоферрита празеодима. Конденсированные среды и межфазные границы. 2021;23(2): 196-203. https://doi.org/10.17308/kcmf.2021.23/3429
И Nguyen Anh Tien, e-mail: tienna@hcmue.edu.vn © Bui Xuan Vuong, Nguyen Anh Tien, 2021
The content is available under Creative Commons Attribution 4.0 License.
Bui Xuan Vuong, Nguyen Anh Tien
Original article
1. Introduction
Amongst nano-sized metal oxide semiconductors, rare earth orthoferrites AFeO3 (A = La, Y, Pr, Sm, Ho) have been studied for application in many fields such as inorganic dyes, optical catalysts, gas sensors, magnetic materials or electrodes for Li-ion battery [1-8]. The properties of this type of materials depend on not only the particle size and morphology, but also the dopant concentration and preparation methods [5-9].
Recently, the sol-gel method has been used for preparation of AFeO3 orthoferrite nanomaterials owing to many advantage of this method: low annealing temperature, narrow particle size distribution, high purity, facile synthesize highly doped AFeO3 materials, [1-3, 10-12]. However, the challenge of this method lies in the selection of appropriate organic polymer for gel formation and the experimental time is usually prolonged. In previous works [13-14], orthoferrite AFeO3 (A=Nd and Ho) nanomaterials of particle size < 100 nm were synthesized by co-precipitation method using hot ethanol via the hydrolysis of A(III) and Fe(III) cations in hot water (T > 95 °C) and NH3 5 % solution as the precipitant. Methanol and ethanol have similar dipole moments ((|(C2H5OH) = 1.66 D, |m(CH3OH) = 1.69 D) [15], which are lower than that of water (|(H2O) = 1.85 D) [16]. Meanwhile, the viscosity of CH3OH (5.9-10-4 Pa- s) is lower than that of C2H5OH (1.2- 10-3 Pa-s), and is also very low compared to organic polymers [15]. As a result, the interaction between A(III) and Fe(III) cations with CH3OH is smaller than with C2H5OH, which leads to the decrease of the size of orthoferrite AFeO3 particles synthesized by sol-gel method using methanol.
In this work, the formation, as well as structural and magnetic properties of nano-sized orthoferrite praseodymium (o-PrFeO3) prepared by sol-gel method using methanol have been studied and characterized.
2. Experimental and methods
All solvents and chemicals for the synthesis of used nanocrystalline praseodymium orthoferrite were purchased and used as-received: Pr(NO3)3- 6H2O (99.8 % purity, Merck), Fe(NO3)3 9H2O (99.6 % purity, Sigma-
Aldrich), methanol absolute (99.7 % purity, d = 0.792 g/mL), ammonia solution (Xilong purity, 85 %, d = 0.901 g/mL).
A mixture of Pr(NO3)3- 6H2O and Fe(NO3)3- 9H2O (1:1 mol to mol ratio) was dissolved in 50 mL solvent of H2O - CH3OH (1:1, V/V). The mixture solution was then added dropwise to a round-bottom flask containing 150 mL boiling H2O -CH3OH co-solvent (T ~ 85 °C). The slow addition of Pr (III) and Fe (III) mixture to the co-solvent at 85 °C would increase the hydrolysis process, thus hinder and control the particle size of orthoferrite PrFeO3. Details of optimized conditions can be found in previous reports on the synthesis of LnFeO3 (Ln = Y, La, Nd) orthoferrites [1719]. The system continued to be refluxed for an additional 30 minutes before cooling down to ~ 30 °C, resulted in a brownish-yellow mixture. By refluxing, the solvent volume was maintained and the diffusion of toxic CH3OH vapor to the environment could be minimized. Next, NH3 5 % solution was added dropwise to the system until pH ~ 9^10 (tested by pH paper). The system was stirred for 30 minutes, then vacuum filtered. After removing all the filtrate, the residue was dry at 50 °C during 3 hours and grounded to obtain brownish-yellow powder (precursor for the synthesis of o-PrFeO3).
Thermogravimetry and differential scanning calorimetry (TG-DSC) curves were recorded under dried air at the heating rate of 10 K-min-1, maximum temperature 950 °C, platinum crucibles, using Labsys Evo - TG-DSC 1600 °C (France).
X-ray diffraction (XRD) patterns of PrFeO3 samples were recorded using X-ray powder diffractometer (XRD, D8-ADVANCE, Germany) with CuKa radiation (l = 1.5406 A), range 20 = 1075°, step size 0.019 °/s. Crystal size (Dxrd, nm) of PrFeO3 samples was determined according to Debye-Scherrer equation, lattice parameters (a, b, c, V) were calculate according to previous works [12, 19-20]. Phase composition was determined by Rietveld refinement, Fullprof 2009.
The content and surface distribution of the elements (Pr, Fe, O) were studied by energy-dispersive X-ray spectroscopy (EDX and EDX-mapping), FE-SEM S-4800 (Japan). The quantitative elemental composition were taken as the average of 5 different positions of each sample.
Bui Xuan Vuong, Nguyen Anh Tien
Original article
Crystal size and morphology of PrFeO3 samples were characterized by transmission electron spectroscopy (TEM), Joel JEM-1400 (Japan). The crystal size distribution of were determined by IMAGE J.
Hysteresis loop and magnetic properties including coercive force (Hc, Oe), remanent magnetization (M., emu/g) and saturation magnetization (Ms, emu/g) were recorded on vibrating sample magnetometer (VSM, MICROSENE EV11) under the magnetic field in the range of -21 000 Oe to +21 000 Oe.
3. Results and discussion
Fig. 1 shows the TG-DSC curves of precursors for the synthesis of o-PrFeO3 nanomaterial. The total mass loss from room temperature to 950 °C was 23.67 %. This result proves the formation of bonds between Pr(III) and Fe(III) cation with CH3- group in the precipitate [21]. Indeed, if this precipitate had only included Pr(OH)3j and Fe(OH)3j, mass loss deduced from equation (1) would have been 18.07 %.
Fe(OH)3 + Pr(OH)
3 ^ PrFeO3 + 3H2O
(1)
The mass loss by decomposition of M3+-CH3 (M = Pr, Fe) bonds corresponds to the exothermic peak at 270.56 °C on the DSC curve (Fig. 1). The endothermic peaks at 113.37 and 358.52 °C are the
dehydrate and decomposition of praseodymium (III) and iron (III) hydroxides. Similar results were also observed in previous works [13, 19] for HoFeO3 and NdFeO3 orthoferrite. The exothermic peak at 617.31 °C correspond to the phase formation of PrFeO3 orthoferrite from Pr2O3 and Fe2O3 according to equation (2). This inference is in good agreement with the mass change on the TG curve (there were no observable changes in the sample's mass from ~650 °C). From the TG-DSC results, the sample was annealed at 650, 750, 850, and 950 °C for 60 min to characterize the structural properties of PrFeO3 crystals by XRD.
Fe2O3 + Pr2O3 ^ 2PrFeO3 (2)
XRD patterns of praseodymium orthoferrite precursor after annealing at different temperatures for 60 min were shown in Fig. 2. The results give single phase orthorhombic PrFeO3. All obtained peaks match well with the standard peaks of PrFeO3 (JCPDS: 74-1472), without any observable oxide peaks such as Pr2O3, Pr6O11 or Fe2O3. The degree of crystallinity and crystal phase content of PrFeO3 samples increased with the annealing temperature, however, this increment was not linear (Table 1). The crystallinity of the sample annealed at 750 °C (592.04 cts) and that annealed at 950 °C (614.66 cts) were approximate, but the PrFeO3 crystal phase content of the sample
Fig. 1. TG-DSC curves of the dried gel powders
Bui Xuan Vuong, Nguyen Anh Tien
Original article
Fig. 2. PXRD patterns of PrFeO, nanopowders annealed at 650, 750, 850, and 950 °C for 60 min
Table 1. Characteristics of PrFeO, samples annealed at different temperatures for 60 min
T 650 °C 750 °C 850 °C 950 °C
20, ° 32.6092 32.5659 32.6185 32.5584
Height, cts 288.26 592.04 490.86 614.66
Crystal phase, % 68.8 93.4 80.5 89.7
Amorphous phase, % 31.2 6.6 19.5 10.3
FWHM, ° 0.1309 0.1683 0.1122 0.1122
d-spacing, Â 2.74604 2.74960 2.74529 2.75022
D, nm 62.5 48.6 73.0 73.0
a, Â 5.4556 5.4521 5.4509 5.4501
b, Â 5.5753 5.5840 5.6206 5.6218
с, Â 7.8113 7.7245 7.8169 7.8145
V, Â3 237.59 235.17 239.49 239.43
H, Oe c7 28.0 30.8 33.7 -
Mr, emu/g 0.22 0.13 0.76 -
Ms, emu/g 0.24 1.10 0.73 -
annealed at 750 °C was much higher than the others. The full-width at half maximum (FWHM, °) of the sample annealed at 750 °C was the widest, leading to the smallest Debye-Scherrer crystal size (Dxrd = 48.6 nm) and lattice volume (V = 235.17 A3) (Table 1). Thus, it can be
assumed that 750 °C for 60 min is the appropriate conditions for the formation of single phase praseodymium orthoferrite (o-PrFeO3) by sol-gel method using water-methanol co-solvent.
From the EDX and EDX-mapping analysis of PrFeO, sample annealed at 750 °C, only
Bui Xuan Vuong, Nguyen Anh Tien Original article
praseodymium, iron and oxygen peaks were observable without any other signals of impurity elements (Fig. 3). The averages of weight percentage and atomic percentage of the elements Pr, Fe, O from five different positions are shown in Table 2. The obtained results are consistent with expected chemical composition (Table 2).
Particle size and morphology of PrFeO3 powder annealed at 750 °C are shown in the TEM image (Fig. 4a). The obtained particles have slightly angular spherical shape with the size mostly in the range of 20-60 nm (Fig. 4b). Average
size calculated by IMAGE J was 46.28 nm. This result is rather close to the crystal size by Debye-Scherrer equation (Dxrd = 48.6 nm) (Table 1).
Field dependence of the magnetization of PrFeO3 nanomaterials at 300K are shown in Fig. 5. The coercive force (Hc = 20.8 v 30.7 Oe) and saturation magnetization (M. = 0.13 v 0.76 emu/g) (Table 1) of all three PrFeO3 samples in this work are much lower than those of PrFeO3 prepared by co-precipitation method reported by Sudandararaj T. S. A. et. al. [22] (Hc = 505.45 Oe, M. = 27.63 emu/g). The low value of Hc and
Table 2. EDX analysis of PrFeO, nano-sized powders annealed at 750 °C
Pr Fe O
Wt% At% Wt% At% Wt% At%
56.39 21.02 22.35 18.28 21.36 60.70
Fig 3. EDX and EDX-mapping images of PrFeO, sample annealed at 750 °C
Fig. 4. (a) TEM image of PrFeO3 sample annealed at 750 °C and (b) Particle size distribution
Bui Xuan Vuong, Nguyen Anh Tien
Original article
Mr could be originated from the homogeneity in shape and size of the PrFeO3 nanoparticles with clear particle boundaries (see Fig. 4) while in the TEM image of the corresponding PrFeO3 in [22], the particle boundaries are not observable with severe aggregation that joined the entire area of the material despite the particle size of 36.0 nm (by Image J).
Most interestingly, the magnetic parameters of PrFeO3 nanomaterials changed irregularly with the annealing temperature (Table 1). The PrFeO3 sample annealed at 750 °C has the lowest Mr (0.13 emu/g) while its Ms (1.10 emu/g) has the highest value. This can be ascribed by the highest crystallinity and crystal phase content of the PrFeO3 sample annealed at 750 °C (see Table 1) which decreased the magnetocrystalline anisotropy of the material, leading to the rise in Ms and the decreased in Mv [23-24].
Thus, with low Hc, Mr and high Ms, obtained PrFeO3 nanomaterials are soft magnetics that can be applied as material working under the external field as transformer cores, electromagnet cores, and conductive cores [24].
4. Conclusions
In this study, nanocrystalline praseodymium orthoferrite (o-PrFeO3) was successfully synthesized by sol-gel method using watermethanol co-solvent. The PrFeO3 nanocrystal
formed after annealing the precursor at different temperatures (650, 750, 850, and 950 °C) for 1 hour. The crystal size of PrFeO3 samples are in the range of 45^70 nm (XRD, TEM). The PrFeO3 sample annealed at 750 °C had the highest crystallinity (592.04 cts) and crystal phase content (93.4 %) (XRD) with smallest particle size (46.28 nm, TEM). The obtained PrFeO3 nanomaterials are soft magnetic materials with low coercive force and remanent magnetization, high saturation magnetization.
Contribution of the authors
The authors contributed equally to this article.
Conflict of interests
The authors maintain that they have no conflict of interest to be described in this communication.
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Information about the authors
Xuan Vuong Bui, PhD in Chemistry, Lecturer of Faculty of Natural Sciences, Sai Gon University, Ho Chi Minh City, Vietnam; e-mail: bxvuong@sgu.edu.vn. ORCID iD: https://orcid.org/0000-0002-3757-1099.
Anh Tien Nguyen, PhD in Chemistry, Chief of General and Inorganic Chemistry Department, Ho Chi Minh City University of Education, Vietnam; E-mail: tienna@hcmue.edu.vn. ORCID iD: http://orcid. org/0000-0002-4396-0349.
Received 13 April 2021; Approved after reviewing 30 April 2021; Accepted 15 May 2021; Published online 25 June 2021.
