CC BY
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FORMATION MECHANISM OF GdFeO3 NANOPARTICLES UNDER THE HYDROTHERMAL CONDITIONS
E. A. Tugova1'2, I.A. Zvereva3
1Ioffe Physical Technical Institute of RAS, Saint Petersburg, Russia 2Saint Petersburg Electrotechnical University "LETI", Saint Petersburg, Russia 3Saint Petersburg State University, Saint Petersburg, Russia [email protected]; [email protected]
PACS 61.46.+w
The formation mechanism of GdFeÜ3 nanoparticles by varying of the hydrothermal conditions has been investigated. The mean size of coherent scattering regions of GdFeO3 was determined to be equal to 53, 68 and 73 nm. The observed regularities allowed us to assume the oriented attachment of nanocrystals.
Keywords: nanoparticles, hydrothermal synthesis, nucleation, phase formation.
1. Introduction
Perovskite-type compounds possess unique electrical, magnetic, thermal properties [1-6]. The potential exists for materials production based on the application of perovskite-like oxide nanoparticles. However, in the literature there is little data concerning investigations into the chemical pre-history and synthetic technique's influence on size, morphology and properties of obtained LnFeO3 (Ln = rare earth element) [7, 8]. The sonochemical method is suggested [9] for the synthesis of nanoparticles of the rare earth series of orthoferrites, using iron pentacarbonyl and rare earth carbonates as precursors. In this manner, GdFeO3, TbFeO3 nanoparticles of 60 nm and EuFeO3, ErFeO3 of 40 nm were obtained. According to the presented data [10] LaFeO3 was synthesized by three different preparation methods, i.e., by the calcination of both mixtures of La2O3 and Fe2O3 (I), co-precipitated La(OH)3 and Fe(OH) 3 hydroxides (II) and La[Fe(CN)a]-5H2O heteronuclear complex (III). The formation of LaFeO3 is recognized for I, II and III cases at calcining temperatures above 1000, 800 and 600°C, respectively. The mean particle diameter of LaFeO3 after heat treatment of La[Fe(CN)6]-5H2O at 600°C for 2 hours was 30 nm [10].
It was also shown [11, 12] that the mean size of coherent scattering regions, morphology and magnetic characteristics of YFeO3 target product were strongly dependent upon the synthetic techniques. Besides, it is well known [13, 14], that hydrothermal synthesis allows the production of highly crystallized and well dispersed powders at relatively low temperatures. There is little literature concerning the hydrothermal synthesis of LnFeO3 (Ln = rare earth element), particularly, GdFeO3.
These reasons demonstrate the importance of systematic investigations of the peculiarities of nanocrystalline GdFeO3 formation under hydrothermal conditions.
2. Experimental
2.1. Synthesis procedure
The initial mixture, corresponding to the stoichiometry of GdFeO3 was prepared by precipitation method from aqueous solutions of stoichiometric amounts of 1M Gd(NO3)3-5H2O and Fe(NO3) 3-9H2O by a previously published procedure [15]. The obtained powders were then transferred to autoclaves and heated at 300-480°C for 1-3 h under 60-90 MPa pressure in distilled aqueous media. The required pressure was determined by temperature and water filling content and produced on Kennedy table data [16]. After cooling, the product was unloaded and then dried at the ambient temperature.
2.2. Characterization of prepared nanocrystals
Purity and crystallization of GdFeO3 samples were characterized by powder X-ray diffraction (XRD) using a Shimadzu XRD-7000 with monochromatic CuKa radiation (A= 154.178 pm). Crystallite sizes of the obtained powders were calculated by the X-ray line broadening technique based on Scherer's formula.
The microstructure of the specimen, elemental composition and the composition of separate phases were analyzed by means of scanning electron microscopy (SEM) using Quanta 200, coupled with ED AX microprobe analyzer. The error in determining the elements content by this method varies with the atomic number and equals to ±0.3 mass% on average.
3. Results and discussion
The performed X-Ray and SEM/EDAX analysis of co-precipitated initial mixture corresponding to the stoichiometry of GdFeO3 shows the amorphous state and heterogeneity of the produced powders. But, it should be noted, that X-Ray diffraction pattern related to the initial mixture demonstrates the weak affect which can be attributed to hexagonal modification of Gd(OH)3 (Fig. 1(1)).
Table 1. Electron probe microanalysis data for the regions indicated in Fig. 1(b - d)
Sample Sintering temperature°C Examined region Components content, mol% Phases
GdOi.5 FeOi.5
b 300 SQ1 36.58 63.42 Gd3Fe5Oi2
P1 38.67 61.33 Gd3Fe5Oi2
P2 17.92 82.08 Fe2O3
P3 37.44 62.56 Gd3Fe5Oi2
c 400 SQ2 43.97 56.03 GdFeO3 + Gd3Fe5Oi2
P4 50.30 49.70 GdFeO3
P5 41.02 58.98 GdFeO3 + Gd3Fe5Oi2
P6 46.71 53.29 GdFeO3
d 480 SQ3 48.85 51.15 GdFeO3
Based on X-ray and SEM/EDX data (Fig. 1(2,3), Table 1), samples treated at 300°C and 400°C under 70 MPa pressure for 1 hour contain Gd(OH)3, FeOOH and small amounts ofGd3 Fe5O 12 and GdFeO3. At the same time, the presence of Gd2O3 and Fe2O3 are observed
26
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Am, c-GdjOj oFejOj • GdFeO] ■ Gd(OH)i °FeO(OH) 0Gd,FeAj
Fig. 1. a) X-Ray diffraction patterns and b-d) SEM photographs of: 1) initial mixture, 2-4), b-d) initial mixtures after hydrothermal treatment at 300, 400, 480°C under 70 MPa for 1 h
Table 2. Electron probe microanalysis data for the regions indicated in Fig. 2(b,c)
Sample Hydrothermal Examined Components content, mol% Phases
conditions region GdOi.5 FeOi.5
SQ 46.71 53.29
b 600 SQ1 46.79 53.21
1 47.64 52.36
SQ 48.18 51.82
1 41.60 58.40 GdFeOs
900 2 41.08 58.92
c 3 48.58 51.42
4 45.76 54.24
5 47.63 52.37
Fig. 2. a) X-Ray diffraction patterns and b,c) SEM photographs of initial mixtures after hydrothermal treatment at 480°C under 60 MPa (1,b) and 90 MPa (2,c) for 1 h
Fig. 3. Phase formation scheme, describing processes which are taken place under initial mixture hydrothermal treatment to yield GdFeO3
(Fig. 1(2,3); Table 1). Raising the temperature rising to 480°C leads to homogeneous GdFeO3 formation (Fig. 1(4), Table 1).
The formation of GdFeO3 nanoparticles was investigated by varying pressure from 60 to 90 MPa and was carried out at the same temperature, 480°C. Fig. 2 and Table 2 present results for X-ray and SEM/EDX data of the initial mixture samples treated under 60 and 90 MPa at 480°C for 1 hour. According to the presented data (Fig. 2,a and Table 2), all characteristic reflects corresponded to the target product. The mean size of coherent scattering regions (D111) was determined from X-ray data for peak with (111) index for samples of GdFeO3 produced after initial mixture treatment under 60, 70, 90 MPa at 480°C. The D 111 values were equal to 53, 68 and 73 nm, respectively. Figure 2(b,c) shows that the product was entirely composed of crystals with a relatively uniform, rod-like morphology.
Thus, according to presented and literature data [17, 18], the formation mechanism for GdFeO3 nanoparticles under the hydrothermal conditions can be illustrated as the shown scheme (Fig. 3).
The large values of mean size of coherent scattering regions of GdFeO3 nanoparti-cles can be explained by oriented attachment of nanocrystals proceeding via the described mechanism [19, 20].
4. Conclusion
These results showed that the mechanism by which GdFeO3 nanoparticles were formed proceeded through the dehydration stages of Gd(OH)3 and FeOOH. The target product was entirely composed of crystals with a relatively uniform, rod-like morphology. The large values of mean size of coherent scattering regions of GdFeO3 nanoparticles ranging from 53-73 nm size were obviously attributed to oriented attachment of nanocrystals.
Acknowledgments
The authors would like to thank Prof. V. V. Gusarov for useful discussions. This work was financially supported by the Russian Foundation for Basic Research, project N 13-03-12470 ofi_m2.
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