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One-step hydrothermal microwave-assisted synthesis of LaFeO3 nanoparticles
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
One-step hydrothermal microwave-assisted synthesis of LaFeO3 nanoparticles Egor M. Kostyukhina,b, Alexander L. Kustova,b,c, Leonid M. Kustova,b,c,∗ a
N.D. Zelinsky Institute of Organic Chemistry, 47 Leninsky prosp, Moscow, 119991, Russia National University of Science and Technology MISiS, 4 Leninsky prosp, Moscow, 119991, Russia c Chemistry Department, Moscow State University, 1 Leninskie Gory, 3, Moscow, 119992, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskites Microwave-assisted synthesis Lanthanum orthoferrite
Lanthanum orthoferrite (LaFeO3) powders were synthesized via a highly efficient one-step hydrothermal microwave-assisted synthesis at relatively low temperatures of 240 °C and pressure of 60 bar. The use of microwave irradiation for heating during the synthesis intensifies the LaFeO3 crystallization process leading to reduced synthesis duration at least 16 times as compared with conventional heating (3 versus 48 h).
1. Introduction
2. Experimental methods
Due to good thermal stability, redox behavior, oxygen mobility, electronic and ionic conductivity, perovskite-type bimetallic systems (particularly LaFeO3) are successfully used as catalysts in oxidation processes, electrodes, sensors, etc. [1–3]. As a rule, their preparation process is relatively time-consuming and complex, and often involves several synthesis steps. Clear examples illustrating these disadvantages are long-term (up to 14 days) hydrothermal synthesis [4–6] and hightemperature (400–1000 °C) decomposition processes [7–9]. Therefore, there is a demand to use high-efficient preparation methods to overcome these limitations. Microwave-assisted synthesis became a powerful approach in materials science and was applied in obtaining different types of materials, including MOFs, metals, oxides and complex oxide systems [10–12]. Iron-containing perovskite-type oxides of various compositions were synthesized via hydrothermal microwave-assisted synthesis at relatively short times and low temperatures [13,14]. Recently, LaFeO3 was obtained in one-step synthesis at low temperatures (200–240 °C) by adding urea into a reaction mixture [15,16]. However, the synthesis time is still long (about 48 h). This disadvantage can be solved by synthesis at microwave-heating conditions. Moreover, to date, one-step hydrothermal microwave-assisted LaFeO3 synthesis has not been performed. Thus, this work is dedicated to one-step hydrothermal microwaveassisted synthesis of lanthanum orthoferrite. It will be shown that the use of microwave irradiation during the hydrothermal synthesis leads to significant crystallization rate acceleration of the LaFeO3 phase.
2.1. Synthesis of LaFeO3 samples
∗
All samples were prepared using a Multiwave Pro (Anton Paar GmbH, Austria) microwave reaction system equipped with a Rotor 8 N for 8 pressure TFM vessels in accordance to the following procedure: 20 mL of 0.4 M water solutions of La(NO3)3 ⋅ 6H2O (Acros Organics, 99%+) and Fe(NO3)3 ⋅ 9H2O (Acros Organics, 99%+) were mixed together, then 22 g KOH (Acros Organics, 85%) were dissolved gradually in the above-noted salts solution while stirring. When the alkaline solution was cooled to room temperature, 5 g of CO(NH2)2 (Fischer Scientific, 99%+) were dissolved in this solution. The obtained colloid solution was transferred in pressure vessels to perform hydrothermal microwave-assisted synthesis. All hydrothermal reactions were carried out at the power-controlled regime at the power of 900 W, the same temperature (240 °C) and autothermic pressure (gradually increasing to 60 bar during the reaction) but for a different synthesis time: 1, 3, 9 h for LFO-1h, LFO-3h, LFO-9h samples, respectively. In 20 min, the desired temperature was reached, the microwave power was reduced and maintained at about 500 W for the whole synthesis time. After the hydrothermal reactions, the samples were washed with distilled water several times and additionally a part of LFO-3h sample was washed with diluted nitric acid (labeled as LFO-3h-a). The obtained powders were dried for 12 h at 100 °C.
Corresponding author. N.D. Zelinsky Institute of Organic Chemistry, 47 Leninsky prosp, Moscow, 119991, Russia. E-mail address: [email protected] (L.M. Kustov).
https://doi.org/10.1016/j.ceramint.2019.04.155 Received 24 January 2019; Received in revised form 14 April 2019; Accepted 17 April 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Egor M. Kostyukhin, Alexander L. Kustov and Leonid M. Kustov, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.04.155
Ceramics International xxx (xxxx) xxx–xxx
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Fig. 1. XRD patterns of the obtained samples (a) LFO-1h, (b) LFO-3h, (c) LFO-9h, (d) LFO-3h-a.
not disappear with time. Probably, this observation can be explained by the dissolution of La2O3 from the perovskite phase in water condensing during the cooling process when the hydrothermal reaction stops by the following reaction:
2.2. Characterization X-Ray diffraction (XRD) patterns were recorded at room temperature over the scanning range (2ϴ) of 25.0–60.0° with a step of 0.020° and scan speed of 2° min−1 using a DRON-2 powder diffractometer equipped with a Cu anode (Kα irradiation, λ = 1.540562 Å). Morphological analysis was performed using a scanning electron microscope LEO EVO 50 XVP equipped with an energy-dispersive detector INCA – energy 450 at a residual pressure of 10−6 Torr. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were obtained by using a LEO 912 AB Omega device with the accelerating voltage of 120 kV. Thermal analysis was performed by the TG-DTA method [17] using a Derivatograph-C instrument (MOM Company) in air from 20 to 800 °C at a heating rate 10 °C min−1.
La2O3 + 3H2O → 2La(OH)3 which derives a Fe2O3 phase as the result of LaFeO3 leaching. In order to purify the powder from the residuals, we have performed an additional washing step with diluted HNO3 of the LFO-3h sample, obtained within the 3-h synthesis (Fig. 1d). On the pattern there are no additional peaks, which confirms its high purity. Based on literature, this reaction does not proceed at the temperatures below 280 °C without using of urea [4], however, it's utilization allows one to obtain the samples at 200 °C but with low yield [16]. Probably, the reaction takes place due to a presence of CO2, obtained as result of urea decomposing which can be confirmed by earlier work [6] where sodium carbonate was used. Nevertheless, to date the formation mechanism of LaFeO3 in hydrothermal conditions with using of urea is still not studied. Depending on the XRD results, one can conclude that a formation of perovskite phase proceeds through metastable compounds possessing a low stability which can be obtained at high temperatures only. Fig. 2 shows SEM microphotographs of the LFO-3h sample obtained without acid treatment (Fig. 2a) and the LFO-3h-a sample additionally washed by diluted nitric acid (Fig. 2b) with corresponding energy-dispersive spectroscopy (EDS) spectra of certain areas. It is well seen that both samples mostly contain only cube-like LaFeO3 crystals with a size of 5–15 μm, which confirms the orthorhombic crystal system formation.
3. Results and discussion Fig. 1 shows the XRD patterns of LaFeO3 powders obtained at different synthesis times. One can see the low yield of orthorhombic LaFeO3 (crystallographic card PDF#37–1493) during a 1-h hydrothermal microwave-assisted synthesis (Fig. 1a). Additionally, in this sample there is a significant amount of initial La(OH)3 (PDF#83–2034) and Fe2O3 (PDF#33–0664) obtained at the base dissolution stage in the salts mixture. The powders obtained within 3 and 9 h (Fig. 1b and c) show similar patterns with a good purity. However, at closer examination, it is getting clear that these samples contain traces of initial above-mentioned compounds. Also, it is evident that their amounts do 2
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Fig. 2. SEM microphotographs of (a) LFO-3h and (b) LFO-3h-a samples and EDS spectra at certain areas.
It is necessary to note, a size of the prepared particles earlier via conventional hydrothermal synthesis is about 20 μm [15]. In our opinion, the reduced particle size is due to the reduced duration of microwaveassisted synthesis. The efficiency of acid treatment for the La(OH)3 removal process is confirmed by a dramatically reduced amount of cylindrical crystals, which probably correspond to La(OH)3, in the LFO-3h-a sample (Fig. 2b). This fact can be confirmed by EDS analysis performed for two points (for both samples): (1) on the cube-like crystal surface, which has a La/Fe ratio of 1.03 (LFO-3h sample) and 1.02 (LFO-3h-a sample); (2) on the cylinder-like crystal surface with the La/Fe ratio of 8.74 (LFO-3h sample) and 7.17 (LFO-3h-a sample). The atomic ratios for both samples at the points (1) obviously correspond to the LaFeO3 compound, while the increased La/Fe ratio at the points (2) corresponds to La (OH)3; the presence of Fe atoms at the points (2) is explained by the shielding of the LaFeO3 surface on which the crystals of La(OH)3 are located. Comparing the results of the samples obtained at different times (microphotographs are not presented), one can conclude that the crystals aggregation takes place during the time. The LFO-1h sample is a single particle powder, while LFO-9h sample mostly consists of aggregates of these particles. To understand the crystal nature of LaFeO3 particles, we obtained TEM micrographs with selected area electron diffraction (SAED) patterns (Fig. 3) for the LFO-3h-a sample. Due to the thickness of LaFeO3 particles, electron beam is not able to transmit through the whole volume of a particle, so it is problematic to obtain a clear information on the crystallite size, however, the SAED pattern of a crystal edge provides insight into this issue: the directional alignment of the reflexes confirms the single-crystal nature of LaFeO3 cube-like particles presented on SEM images (Fig. 2). In order to evaluate the amount of La(OH)3 in the final powder of LFO-3h-a sample, TG-DTA study was performed (Fig. 4). The sample
Fig. 3. TEM image (SAED pattern inserted) of the LFO-3h-a sample.
has an extremally low total weight loss of 0.7%: up to 200 °C adsorbed water evaporates from the perovskite surface following by the second loss attributed to La(OH)3 into LaOOH conversion, which is accompanied by a subtle exo-effect. From the temperature of about 430 °C, smooth LaOOH decomposition into an oxide phase (La2O3) takes place [18]. Although the sample was washed by acid, unfortunately, a small amount of lanthanum hydroxide remains in the sample, which is confirmed by SEM and DLS studies. Anyway, the TG-DTA study 3
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Fig. 4. TG-DTA curves for the LFO-3h-a sample.
financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (grant no. К2-2019-005).
demonstrates that the sample has a high thermal stability and high purity. The presented results confirm the microwave irradiation efficiency in the hydrothermal preparation of LaFeO3. Comparing to the previous reports on the one-step hydrothermal synthesis of lanthanum orthoferrite with the similar methodology [6,15,16], pure LaFeO3 powder with reduced particle size (5–15 μm) has been obtained with a high yield at least 16 times faster (3 h versus 48 h). Acceleration of the synthesis process under microwave conditions is probably observed due to several reasons. First of all, rapid and homogeneous heating of the reaction mixture takes place primarily because of dipolar polarization of a polar solvent – water in our case. However, as it is well known [10], the contribution of dipolar polarization decreases with a temperature increase. At the same time, formed perovskite particles start to interact with the microwave field, and heat is produced also due to surface polarization of LaFeO3 particles (the Maxwell-Wagner effect). This mechanism perhaps prevails over the dipolar polarization and causes the local overheating on their surface which, as a result, accelerates the crystallization process and the particle growth. This microwave-assisted hydrothermal approach for the synthesis of LaFeO3 shows potential and is worthy for further investigations and optimizations.
References [1] X. Huang, G. Zhao, G. Wang, J.T.S. Irvine, Synthesis and applications of nanoporous perovskite metal oxides, Chem. Sci. 9 (2018) 3623–3637, https://doi.org/10.1039/ C7SC03920D. [2] N.A. Davshan, A.L. Kustov, O.P. Tkachenko, L.M. Kustov, C.H. Kim, Oxidation of carbon monoxide over MLaO x perovskites supported on mesoporous zirconia, ChemCatChem 6 (2014) 1990–1997, https://doi.org/10.1002/cctc.201400013. [3] A.L. Kustov, O.P. Tkachenko, L.M. Kustov, B.V. Romanovsky, Lanthanum cobaltite perovskite supported onto mesoporous zirconium dioxide: nature of active sites of VOC oxidation, Environ. Int. 37 (2011) 1053–1056, https://doi.org/10.1016/j. envint.2011.05.002. [4] J.A. Gómez-Cuaspud, E. Vera-López, J.B. Carda-Castelló, E. Barrachina-Albert, Onestep hydrothermal synthesis of LaFeO3 perovskite for methane steam reforming, React. Kinet. Mech. Catal. 120 (2017) 167–179, https://doi.org/10.1007/s11144016-1092-8. [5] K.O. Ogunniran, G. Murugadoss, R. Thangamuthu, P. Periasamy, All inorganic based Nd0.9Mn0.1FeO3 perovskite for Li-ion battery application: synthesis, structural and morphological investigation, J. Alloy. Comp. 766 (2018) 1014–1023, https://doi.org/10.1016/j.jallcom.2018.06.340. [6] W. Zheng, R. Liu, D. Peng, G. Meng, Hydrothermal synthesis of LaFeO3 under carbonate-containing medium, Mater. Lett. 43 (2000) 19–22, https://doi.org/10. 1016/S0167-577X(99)00223-2. [7] Z. Li, W. Zhang, C. Yuan, Y. Su, Controlled synthesis of perovskite lanthanum ferrite nanotubes with excellent electrochemical properties, RSC Adv. 7 (2017) 12931–12937, https://doi.org/10.1039/C6RA27423D. [8] S.L.C. Pinho, J.S. Amaral, A. Wattiaux, M. Duttine, M.-H. Delville, C.F.G.C. Geraldes, Synthesis and characterization of rare-earth orthoferrite LnFeO 3 nanoparticles for bioimaging, Eur. J. Inorg. Chem. 2018 (2018) 3570–3578, https://doi.org/10.1002/ejic.201800468. [9] Z. Wang, R. Gao, X. Deng, G. Chen, W. Cai, C. Fu, Dielectric and ferroelectric properties of LaFeO3 particles derived from metal organic frameworks precursor, Ceram. Int. 45 (2019) 1825–1830, https://doi.org/10.1016/j.ceramint.2018.10. 071. [10] M. Baghbanzadeh, L. Carbone, P.D. Cozzoli, C.O. Kappe, Microwave-assisted synthesis of colloidal inorganic nanocrystals, Angew. Chem. Int. Ed. 50 (2011) 11312–11359, https://doi.org/10.1002/anie.201101274. [11] E.M. Kostyukhin, L.M. Kustov, Microwave-assisted synthesis of magnetite nanoparticles possessing superior magnetic properties, Mendeleev Commun. 28 (2018) 559–561, https://doi.org/10.1016/j.mencom.2018.09.038. [12] E.M. Kostyukhin, Synthesis of magnetite nanoparticles upon microwave and convection heating, Russ. J. Phys. Chem. A. 92 (2018) 2399–2402, https://doi.org/10. 1134/S0036024418120233. [13] C. Ponzoni, M. Cannio, D.N. Boccaccini, C.R.H. Bahl, K. Agersted, C. Leonelli, Ultrafast microwave hydrothermal synthesis and characterization of
4. Conclusion In this work, we have synthesized lanthanum orthoferrite (LaFeO3) in one step via low-temperature microwave-assisted hydrothermal synthesis without an annealing step. It has been shown that the use of microwave irradiation as a heating source is highly efficient and leads to increasing the crystallization rate of nanoparticles. In comparison with a conventionally heated hydrothermal synthesis of LaFeO3, the total synthesis time is dramatically reduced to 3 h. Also it has been shown that the final LaFeO3 sample contains a small amount of La(OH)3 which can be successfully removed by diluted nitric acid. Acknowledgement This work was supported by the Russian Foundation for Basic Research, project no. 18-29-24182 in the part related to the synthesis of the materials. The characterization of materials was carried out with a 4
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LaCrO3, and La0.75Sr0.25MnO3, CrystEngComm 16 (2014) 2874, https://doi.org/ 10.1039/c3ce42554a. [17] O.A. Kirichenko, G.I. Kapustin, O.P. Tkachenko, V.D. Nissenbaum, I.V. Mishin, N.A. Davshan, E.A. Redina, L.M. Kustov, Synthesis and characterization of novel Au/θ-Al 2-x Fe x O 3 nanomaterials with high thermal stability in catalytic oxidation of carbon monoxide, Mater. Res. Bull. 80 (2016) 139–149, https://doi.org/10. 1016/j.materresbull.2016.03.042. [18] M. Ozawa, R. Onoe, H. Kato, Formation and decomposition of some rare earth (RE=La, Ce, Pr) hydroxides and oxides by homogeneous precipitation, J. Alloy. Comp. 408–412 (2006) 556–559, https://doi.org/10.1016/j.jallcom.2004.12.073.
Bi1−xLaxFeO3 micronized particles, Mater. Chem. Phys. 162 (2015) 69–75, https://doi.org/10.1016/j.matchemphys.2015.05.002. [14] X. Sun, Z. Liu, H. Yu, Z. Zheng, D. Zeng, Facile synthesis of BiFeO 3 nanoparticles by modified microwave-assisted hydrothermal method as visible light driven photocatalysts, Mater. Lett. 219 (2018) 225–228, https://doi.org/10.1016/j.matlet.2018. 02.052. [15] L. Yuan, K. Huang, S. Wang, C. Hou, X. Wu, B. Zou, S. Feng, Crystal shape tailoring in perovskite structure rare-earth ferrites REFeO 3 (RE = La, Pr, Sm, Dy, Er, and Y) and shape-dependent magnetic properties of YFeO 3, Cryst. Growth Des. 16 (2016) 6522–6530, https://doi.org/10.1021/acs.cgd.6b01219. [16] C. Hou, W. Feng, L. Yuan, K. Huang, S. Feng, Crystal facet control of LaFeO3,
5
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
One-step hydrothermal microwave-assisted synthesis of LaFeO3 nanoparticles Egor M. Kostyukhina,b, Alexander L. Kustova,b,c, Leonid M. Kustova,b,c,∗ a
N.D. Zelinsky Institute of Organic Chemistry, 47 Leninsky prosp, Moscow, 119991, Russia National University of Science and Technology MISiS, 4 Leninsky prosp, Moscow, 119991, Russia c Chemistry Department, Moscow State University, 1 Leninskie Gory, 3, Moscow, 119992, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskites Microwave-assisted synthesis Lanthanum orthoferrite
Lanthanum orthoferrite (LaFeO3) powders were synthesized via a highly efficient one-step hydrothermal microwave-assisted synthesis at relatively low temperatures of 240 °C and pressure of 60 bar. The use of microwave irradiation for heating during the synthesis intensifies the LaFeO3 crystallization process leading to reduced synthesis duration at least 16 times as compared with conventional heating (3 versus 48 h).
1. Introduction
2. Experimental methods
Due to good thermal stability, redox behavior, oxygen mobility, electronic and ionic conductivity, perovskite-type bimetallic systems (particularly LaFeO3) are successfully used as catalysts in oxidation processes, electrodes, sensors, etc. [1–3]. As a rule, their preparation process is relatively time-consuming and complex, and often involves several synthesis steps. Clear examples illustrating these disadvantages are long-term (up to 14 days) hydrothermal synthesis [4–6] and hightemperature (400–1000 °C) decomposition processes [7–9]. Therefore, there is a demand to use high-efficient preparation methods to overcome these limitations. Microwave-assisted synthesis became a powerful approach in materials science and was applied in obtaining different types of materials, including MOFs, metals, oxides and complex oxide systems [10–12]. Iron-containing perovskite-type oxides of various compositions were synthesized via hydrothermal microwave-assisted synthesis at relatively short times and low temperatures [13,14]. Recently, LaFeO3 was obtained in one-step synthesis at low temperatures (200–240 °C) by adding urea into a reaction mixture [15,16]. However, the synthesis time is still long (about 48 h). This disadvantage can be solved by synthesis at microwave-heating conditions. Moreover, to date, one-step hydrothermal microwave-assisted LaFeO3 synthesis has not been performed. Thus, this work is dedicated to one-step hydrothermal microwaveassisted synthesis of lanthanum orthoferrite. It will be shown that the use of microwave irradiation during the hydrothermal synthesis leads to significant crystallization rate acceleration of the LaFeO3 phase.
2.1. Synthesis of LaFeO3 samples
∗
All samples were prepared using a Multiwave Pro (Anton Paar GmbH, Austria) microwave reaction system equipped with a Rotor 8 N for 8 pressure TFM vessels in accordance to the following procedure: 20 mL of 0.4 M water solutions of La(NO3)3 ⋅ 6H2O (Acros Organics, 99%+) and Fe(NO3)3 ⋅ 9H2O (Acros Organics, 99%+) were mixed together, then 22 g KOH (Acros Organics, 85%) were dissolved gradually in the above-noted salts solution while stirring. When the alkaline solution was cooled to room temperature, 5 g of CO(NH2)2 (Fischer Scientific, 99%+) were dissolved in this solution. The obtained colloid solution was transferred in pressure vessels to perform hydrothermal microwave-assisted synthesis. All hydrothermal reactions were carried out at the power-controlled regime at the power of 900 W, the same temperature (240 °C) and autothermic pressure (gradually increasing to 60 bar during the reaction) but for a different synthesis time: 1, 3, 9 h for LFO-1h, LFO-3h, LFO-9h samples, respectively. In 20 min, the desired temperature was reached, the microwave power was reduced and maintained at about 500 W for the whole synthesis time. After the hydrothermal reactions, the samples were washed with distilled water several times and additionally a part of LFO-3h sample was washed with diluted nitric acid (labeled as LFO-3h-a). The obtained powders were dried for 12 h at 100 °C.
Corresponding author. N.D. Zelinsky Institute of Organic Chemistry, 47 Leninsky prosp, Moscow, 119991, Russia. E-mail address: [email protected] (L.M. Kustov).
https://doi.org/10.1016/j.ceramint.2019.04.155 Received 24 January 2019; Received in revised form 14 April 2019; Accepted 17 April 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Egor M. Kostyukhin, Alexander L. Kustov and Leonid M. Kustov, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.04.155
Ceramics International xxx (xxxx) xxx–xxx
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Fig. 1. XRD patterns of the obtained samples (a) LFO-1h, (b) LFO-3h, (c) LFO-9h, (d) LFO-3h-a.
not disappear with time. Probably, this observation can be explained by the dissolution of La2O3 from the perovskite phase in water condensing during the cooling process when the hydrothermal reaction stops by the following reaction:
2.2. Characterization X-Ray diffraction (XRD) patterns were recorded at room temperature over the scanning range (2ϴ) of 25.0–60.0° with a step of 0.020° and scan speed of 2° min−1 using a DRON-2 powder diffractometer equipped with a Cu anode (Kα irradiation, λ = 1.540562 Å). Morphological analysis was performed using a scanning electron microscope LEO EVO 50 XVP equipped with an energy-dispersive detector INCA – energy 450 at a residual pressure of 10−6 Torr. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were obtained by using a LEO 912 AB Omega device with the accelerating voltage of 120 kV. Thermal analysis was performed by the TG-DTA method [17] using a Derivatograph-C instrument (MOM Company) in air from 20 to 800 °C at a heating rate 10 °C min−1.
La2O3 + 3H2O → 2La(OH)3 which derives a Fe2O3 phase as the result of LaFeO3 leaching. In order to purify the powder from the residuals, we have performed an additional washing step with diluted HNO3 of the LFO-3h sample, obtained within the 3-h synthesis (Fig. 1d). On the pattern there are no additional peaks, which confirms its high purity. Based on literature, this reaction does not proceed at the temperatures below 280 °C without using of urea [4], however, it's utilization allows one to obtain the samples at 200 °C but with low yield [16]. Probably, the reaction takes place due to a presence of CO2, obtained as result of urea decomposing which can be confirmed by earlier work [6] where sodium carbonate was used. Nevertheless, to date the formation mechanism of LaFeO3 in hydrothermal conditions with using of urea is still not studied. Depending on the XRD results, one can conclude that a formation of perovskite phase proceeds through metastable compounds possessing a low stability which can be obtained at high temperatures only. Fig. 2 shows SEM microphotographs of the LFO-3h sample obtained without acid treatment (Fig. 2a) and the LFO-3h-a sample additionally washed by diluted nitric acid (Fig. 2b) with corresponding energy-dispersive spectroscopy (EDS) spectra of certain areas. It is well seen that both samples mostly contain only cube-like LaFeO3 crystals with a size of 5–15 μm, which confirms the orthorhombic crystal system formation.
3. Results and discussion Fig. 1 shows the XRD patterns of LaFeO3 powders obtained at different synthesis times. One can see the low yield of orthorhombic LaFeO3 (crystallographic card PDF#37–1493) during a 1-h hydrothermal microwave-assisted synthesis (Fig. 1a). Additionally, in this sample there is a significant amount of initial La(OH)3 (PDF#83–2034) and Fe2O3 (PDF#33–0664) obtained at the base dissolution stage in the salts mixture. The powders obtained within 3 and 9 h (Fig. 1b and c) show similar patterns with a good purity. However, at closer examination, it is getting clear that these samples contain traces of initial above-mentioned compounds. Also, it is evident that their amounts do 2
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Fig. 2. SEM microphotographs of (a) LFO-3h and (b) LFO-3h-a samples and EDS spectra at certain areas.
It is necessary to note, a size of the prepared particles earlier via conventional hydrothermal synthesis is about 20 μm [15]. In our opinion, the reduced particle size is due to the reduced duration of microwaveassisted synthesis. The efficiency of acid treatment for the La(OH)3 removal process is confirmed by a dramatically reduced amount of cylindrical crystals, which probably correspond to La(OH)3, in the LFO-3h-a sample (Fig. 2b). This fact can be confirmed by EDS analysis performed for two points (for both samples): (1) on the cube-like crystal surface, which has a La/Fe ratio of 1.03 (LFO-3h sample) and 1.02 (LFO-3h-a sample); (2) on the cylinder-like crystal surface with the La/Fe ratio of 8.74 (LFO-3h sample) and 7.17 (LFO-3h-a sample). The atomic ratios for both samples at the points (1) obviously correspond to the LaFeO3 compound, while the increased La/Fe ratio at the points (2) corresponds to La (OH)3; the presence of Fe atoms at the points (2) is explained by the shielding of the LaFeO3 surface on which the crystals of La(OH)3 are located. Comparing the results of the samples obtained at different times (microphotographs are not presented), one can conclude that the crystals aggregation takes place during the time. The LFO-1h sample is a single particle powder, while LFO-9h sample mostly consists of aggregates of these particles. To understand the crystal nature of LaFeO3 particles, we obtained TEM micrographs with selected area electron diffraction (SAED) patterns (Fig. 3) for the LFO-3h-a sample. Due to the thickness of LaFeO3 particles, electron beam is not able to transmit through the whole volume of a particle, so it is problematic to obtain a clear information on the crystallite size, however, the SAED pattern of a crystal edge provides insight into this issue: the directional alignment of the reflexes confirms the single-crystal nature of LaFeO3 cube-like particles presented on SEM images (Fig. 2). In order to evaluate the amount of La(OH)3 in the final powder of LFO-3h-a sample, TG-DTA study was performed (Fig. 4). The sample
Fig. 3. TEM image (SAED pattern inserted) of the LFO-3h-a sample.
has an extremally low total weight loss of 0.7%: up to 200 °C adsorbed water evaporates from the perovskite surface following by the second loss attributed to La(OH)3 into LaOOH conversion, which is accompanied by a subtle exo-effect. From the temperature of about 430 °C, smooth LaOOH decomposition into an oxide phase (La2O3) takes place [18]. Although the sample was washed by acid, unfortunately, a small amount of lanthanum hydroxide remains in the sample, which is confirmed by SEM and DLS studies. Anyway, the TG-DTA study 3
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Fig. 4. TG-DTA curves for the LFO-3h-a sample.
financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (grant no. К2-2019-005).
demonstrates that the sample has a high thermal stability and high purity. The presented results confirm the microwave irradiation efficiency in the hydrothermal preparation of LaFeO3. Comparing to the previous reports on the one-step hydrothermal synthesis of lanthanum orthoferrite with the similar methodology [6,15,16], pure LaFeO3 powder with reduced particle size (5–15 μm) has been obtained with a high yield at least 16 times faster (3 h versus 48 h). Acceleration of the synthesis process under microwave conditions is probably observed due to several reasons. First of all, rapid and homogeneous heating of the reaction mixture takes place primarily because of dipolar polarization of a polar solvent – water in our case. However, as it is well known [10], the contribution of dipolar polarization decreases with a temperature increase. At the same time, formed perovskite particles start to interact with the microwave field, and heat is produced also due to surface polarization of LaFeO3 particles (the Maxwell-Wagner effect). This mechanism perhaps prevails over the dipolar polarization and causes the local overheating on their surface which, as a result, accelerates the crystallization process and the particle growth. This microwave-assisted hydrothermal approach for the synthesis of LaFeO3 shows potential and is worthy for further investigations and optimizations.
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4. Conclusion In this work, we have synthesized lanthanum orthoferrite (LaFeO3) in one step via low-temperature microwave-assisted hydrothermal synthesis without an annealing step. It has been shown that the use of microwave irradiation as a heating source is highly efficient and leads to increasing the crystallization rate of nanoparticles. In comparison with a conventionally heated hydrothermal synthesis of LaFeO3, the total synthesis time is dramatically reduced to 3 h. Also it has been shown that the final LaFeO3 sample contains a small amount of La(OH)3 which can be successfully removed by diluted nitric acid. Acknowledgement This work was supported by the Russian Foundation for Basic Research, project no. 18-29-24182 in the part related to the synthesis of the materials. The characterization of materials was carried out with a 4
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