Yttrium Orthoferrite YFeO3 Nanopowders Formation ... - Springer Link

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2014, Vol. 87, No. 2, pp. 167−171. © Pleiades Publishing, Ltd., 2014. Original Russian Text © V.I. Popkov, O.V. Almjasheva, 2014, published in Zhurnal Prikladnoi Khimii, 2014, Vol. 87, No. 2, pp. 185−189.

INORGANIC SYNTHESIS AND INDUSTRIAL INORGANIC CHEMISTRY

Yttrium Orthoferrite YFeO3 Nanopowders Formation under Glycine-Nitrate Combustion Conditions V. I. Popkova and O. V. Almjashevab St. Petersburg State Technological Institute (Technical University), Moskovskii pr. 26, St. Petersburg, 198013 Russia b St. Petersburg Electrotechnical University LETU, ul. Professora Popova 5, St. Petersburg, 197376 Russia e-mail: [email protected], [email protected] a

Received March 3, 2014

Abstract—Glycine-nitrate combustion method was used to obtain nanopowders of orthorhombic and hexagonal yttrium orthoferrite with particles sizes in the ranges from 30 to 53 nm and from 6 to 14 nm, respectively. It is shown that the structure and size of the particles are determined by the process temperature, which depends on the relative amounts of glycine and nitrates in the initial mixture. It is shown that yttrium orthoferrite nanopowders with a certain crystal structure and particle size can be obtained by varying the glycine/nitrate ratio. The nanopowders and materials on their basis can be used as photocatalysts. DOI: 10.1134/S1070427214020074

a high homogeneity of the products due to the mixing of the starting components on the molecular level, enables control over the stoichiometric composition of samples [25], and can yield nanosize substances at comparatively low temperatures. However, despite the large number of studies concerned with processes occurring in the course of a glycine-nitrate synthesis (GNS) [9, 25, 26], data on the formation and phase transformations in nanoparticles of complex oxides, including yttrium orthoferrite, are only fragmentary. The process of glycine-nitrate combustion may occur both via an exclusively nitrate oxidation of glycine, and via mixed oxidation involving atmospheric oxygen [26, 28]:

Yttrium orthoferrite has the perovskite structure, is a ferromagnetic with high coercive force and substantial mobility of domain walls, exhibits a high catalytic activity, and possesses a number of other practically important properties [1–10]. It has been shown that the properties of yttrium orthoferrite largely depend on the synthesis method and particle size [11–16]. In this context, it is of interest to study how YFeO3 nanopowders are formed. A rather large number of methods are known for obtaining nanosize orthoferrites of rare-earth metals and, in particular, yttrium orthoferrite [17]. Of these, the following are most frequently used: solid-phase synthesis [18–20], sol-gel process [21, 22], hydrothermal synthesis [11], coprecipitation method [8, 12, 15, 16], and thermaldecomposition process [23]. With these methods employed, it is possible to obtain under certain conditions nanopowders with particles sizes in the range from several nanometers to tens of nanometers. Recently, the method of combustion of a glycine-nitrate precursor has gained a wide recognition. In this technique, nanopowders of oxide substances are produced in a combustion wave [9, 21, 24–28]. This method provides

3Y(NO3)3 + 3Fe(NO3)3 + 10C2H5NO2 = 3YFeO3 + 14N2 + 20CO2 + 25H2O (G/N = 1.7),

(1)

Y(NO3)3 + Fe(NO3)3 + 6C2H5NO2 + 6O2 = YFeO3 + 6N2 + 12CO2 + 15H2O (G/N = 3.0),

(2)

where G/N is the molar ratio between glycine and sum of nitrate ions in yttrium and iron nitrates. 167

168

POPKOV, ALMJASHEVA

In [26, 28], the process in which YFeO3 nanocrystals are formed was studied in comparative detail only in the glycine deficiency region, i.e., the G/N ratio was 0.85–1.7, whereas data for the excess of the reducing agent hardly reported [26]. Therefore, it was of considerable interest to study process in which YFeO3 nanoparticles are formed under the glycine-nitrate combustion conditions and analyze the effect of synthesis conditions on the phase composition, morphology, and properties of the resulting yttrium orthoferrite in a wide range of glycine/nitrate ratios. In our study, we consider the formation of YFeO3 in an excess of the reducing agent (With G/N varied from 2.4 to 3.6). EXPERIMENTAL Yttrium orthoferrite nanoparticles were produced by combustion of a glycine-nitrate precursor by the procedure described in [26, 27], with the G/N ratio varied from 0.85 to 3.6. To obtain the precursor, glycine H2NCH2COOH (analytically pure) was added in an amount providing the prescribed G/N ratio to a solution of iron(III) nitrate Fe(NO3)3 (pure grade) and yttrium nitrate Y(NO3)3 (chemically pure grade), taken in equimolar amounts. The resulting solution was thermostated at 70°C for 15 min until the solution color changed due to the formation of glycine complexes with Fe3+ and Y3+ cations [29]. Then, the solution was heated until water was nearly completely evaporated and then self-ignition occurred. The relative change in the temperature in the combustion zone of the glycine-nitrate mixture was evaluated with a chromelalumel thermocouple. The elemental composition of the samples was determined by X-ray fluorescence microanalysis (FEI Quanta 200 scanning electron microscope with EDAX energydispersive analyzer). The phase composition of the samples was monitored by X-ray diffraction analysis with a Shimadzu XRD-7000 diffractometer (CuKα radiation, λ = 0.154051 nm). X-ray diffraction patterns were measured at Bragg angles 2θ = 25–55°, with scanning step of 0.02° and exposure of 1 s at a point. The qualitative X-ray phase analysis was made with a PDF2–2012 powder diffraction database. The size of crystallites (coherent-scattering regions) was calculated from the broadening of the diffraction peaks by the Scherrer formula [30].

RESULTS AND DISCUSSION According to the X-ray fluorescence microanalysis data, the Fe : Y ratio in all the samples corresponds to the stoichiometric composition YFeO3 with an error of up to 5%. According to the results of an X-ray phase analysis (Fig. 1), the glycine-nitrate synthesis yielded at a G/N ratio of 2.4 a orthorhombic modification of yttrium orthoferrite. Raising the G/N ratio to 2.7 results in that reflections corresponding to hexagonal yttrium orthoferrite appear in X-ray diffraction patterns in addition to the reflections of orthorhombic YFeO3. With the G/N ratio raised to 3.3, peaks corresponding to crystallization of exclusively the metastable hexagonal modification of yttrium orthoferrite are observed. At the maximum G/N ratio of 3.6, the sample is fully X-ray-amorphous. The results of calculation of the average size of the YFeO3 crystallites being synthesized (Fig. 2) indicate that, for both the hexagonal and orthorhombic yttrium orthoferrite phases, a decrease in the G/N ratio leads to a monotonic increase in the average size of YFeO3 crystallites. The average crystallite size varies from 30 ± 3 to 53 ± 3 nm for the orthorhombic YFeO3 phase with G/N in the range 2.4–3.0, and from 6 ± 3 to 14 ± 3 nm for the hexagonal YFeO3 phase at G/N = 2.7–3.3. Analysis of how the phase composition (Fig. 1) and average crystallite size (Fig. 2) depends on the glycinenitrate ratio suggests that lowering the G/N ratio is accompanied by a rise in temperature in the reaction zone. In the process, the mass transfer is activated, which leads to a noticeable increase in the particle size and intensifies phase transformations in the order amorphous substance– metastable hexagonal modification of YFeO3–stable orthorhombic modification of YFeO3. To reveal how the temperature of the glycine-nitrate combustion depends on the G/N ratio in the precursor, we measured the relative temperature in the combustion front at various ratios between glycine and the nitrates (Fig. 3). The values of temperature, recorded in this case, are presumably understated as compared with the real process temperature because of the thermocouple inertia. However, relative values of temperature measured in this way as a function of the G/N ratio make it possible to assess the tendencies of variation of the real temperature in the reaction zone. Analysis of the data in Fig. 3 indicates that the temperature in the reaction zone depends on the glycine-nitrate

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 2 2014

YTTRIUM ORTHOFERRITE YFeO3 NANOPOWDERS

169

(ort)

G/N = 2.4 G/N = 2.7 G/N = 3.0 G/N = 3.3 G/N = 3.6

G/N = 2.4 2θ, deg Fig. 1. X-ray diffraction patterns of YFeO3 samples produced by the glycine-nitrate synthesis at various G/N ratios. (2θ) Bragg angle. YFeO3 modification: (1) orthorhombic and (2) hexagonal.

ratio in an extremal manner. As the G/N ratio is raised within the range 0.85–2.4, the temperature increases and then, passing through a maximum at G/N = 2.4, decreases at G/N within the range 2.4–3.6, as recorded with the thermocouple. A possible explanation of this run of the dependence is that the rise in the relative temperature in the nitrate-combustion region [reaction (1)] is predominantly due to the increase in the amount of the organic fuel (glycine) in the precursor at an excess of the oxidizing agent. As, however, the G/N ratio is shifted into the

mixed combustion region [reaction (2)], this tendency gives way to that exactly opposite, which may be due to the insufficient amount of nitrates in the mixture for full oxidation of glycine at these G/N ratios. This makes necessary a partial oxidation of glycine by atmospheric oxygen whose access to the active-combustion zone is hindered. It should be noted that, according to published data [26, 31], the maximum combustion temperature for oxide T, °C Nitrate combustion

d, nm

YFeO3(ort)

YFeO3(ort)

YFeO3(ort)

Mixed combustion YFeO3(hex) amor

ort

amor

hex+ amor+ ort amor hex

amor

G/N

G/N Fig. 2. Average size d of yttrium orthoferrite crystallites vs. the G/N ratio.

ort + hex hex + amor

YFeO3(hex)

YFeO3(hex)

Fig. 3. Temperature T recorded by the thermocouple in the reaction zone vs. the G/N ratio.


170

POPKOV, ALMJASHEVA

systems of this kind corresponds to the stoichiometric ratio G/N = 1.7. However, the maximum in the relative temperature is observed in this case at a G/N ratio different from the stoichiometric value (G/N = 2.4), which may indicate that processes of nitrate and mixed combustion are conjugated in the system under study, which leads to the above-mentioned shift of the temperature extremum. Comparing the X-ray phase analysis data with the results of calculations of the average size of yttrium orthoferrite crystallites, we can distinguish several ranges of G/N ratios with different phase compositions (Fig. 3). At G/N ≤ 0.85 G/N ≥ 3.6, the samples are X-ray-amorphous substances. At 0.85 ≤ G/N ≤ 1.3 and 3.3 ≤ G/N ≤ 3.6, the X-ray-amorphous phase and the hexagonal modification of YFeO3 coexist. The orthorhombic and hexagonal modifications of yttrium orthoferrite coexist in ratio ranges 1.3 ≤ G/N ≤ 1.7 and 2.4 ≤ G/N ≤ 3.3. In the range 1.7 ≤ G/N ≤ 2.4, the samples contain exclusively the orthorhombic modification of YFeO3. The observed distribution of phases over the ratios in the range under study is in good agreement with the assumption that the value of G/N determines the temperature of the glycine-nitrate combustion, and the temperature, in turn, governs the phase state of a sample. It should be noted, however, that the relative temperature at which each of the recorded phases is formed is substantially higher in the mixed-combustion region than that for the nitrate combustion region. This phenomenon is accounted for by the presence in the mixed-combustion region of a substantial excess of glycine in the mixture being burned, which leads to two effects. On the one hand, the excess of glycine complicates mass-transfer processes in the glycine-nitrate combustion, which leads to a rise in the temperature necessary for the crystalline phase to be formed, compared with the nitrate combustion in which no effect of this kind is observed. On the other hand, filling regions between phase-forming components, the excess glycine makes larger the distance between the iron- and yttrium-containing phases. This, in turn, restricts the growth of the newly formed yttrium orthoferrite crystallites and results in the formation of nanopowders with substantially smaller average particle sizes. It should be noted that the yttrium orthoferrite nanopowders synthesized by the method under consideration can be used as high-efficiency photocatalysts [8, 9, 32], and its ferromagnetic properties enable magnetic regeneration of YFeO3-based catalysts [32]. The whole set of these properties give reason to believe that yttrium

orthoferrite can be actively used in practice in “green” technologies. CONCLUSIONS (1) The method of self-propagating glycine-nitrate combustion was used to obtain yttrium orthoferrite nanopowders of two modifications, orthorhombic and metastable hexagonal, with average crystallite sizes of 30 ± 3 to 53 ± 3 nm and 6 ± 3 to 14 ± 3 nm, respectively, depending on the G/N ratio. (2) It was found that the principal factor affecting the formation of yttrium orthoferrite of particular modification is the G/N ratio governing the combustion temperature of the glycine-nitrate mixture and thereby affecting the phase composition and the average crystallite size of the yttrium orthoferrite modifications obtained. (3) It was shown that, in the mixed-combustion region of the glycine-nitrate mixture, the formation of the YFeO3 crystalline phase is complicated by a substantial excess of glycine in the mixture, which hinders effective mass transfer and spatially separates the phase-forming components in the course of the self-propagating combustion. ACKNOWLEDGMENTS The authors are grateful to V.V. Gusarov for his attention to the study, active discussions, and assistance in interpretation of the results. The study was financially supported by the Russian Foundation for Basic Research (grant no. 13-0312470). REFERENCES 1. Maiti, R., Basu, S., and Chakravorty, D., J. Magnetism Magnetic Materials, 2009, vol. 321, no. 19, pp. 3274– 3277. 2. Wu, A., Shen, H., Xu, J., et al., Bull. Materials Sci., 2012, vol. 35, no. 2, pp. 259–263. 3. Didosyan, Y.S., Hauser, H., Reider, G.A., et al., J. Appl. Phys., 2003, vol. 93, no. 10, pp. 8755–8757. 4. Didosyan, Y.S., Hauser, H., Nicolics, J., et al., J. Appl. Phys., 2000, vol. 87, no. 9, pp. 7079–7081. 5. Didosyan, Y.S., J. Appl. Phys., 1993, vol. 7, no. 10, pp. 6828. 6. Didosyan, Y.S., Hauser, H., Papanicolaou, N., et al., J. Appl. Phys., 2002, vol. 91, no. 10, pp. 7302–7304.


YTTRIUM ORTHOFERRITE YFeO3 NANOPOWDERS 7. Markova-Velichkova, M., Lazarova, T., Tumbalev, V., et al., Chem. Eng. J., 2013, vol. 231, pp. 236–244. 8. Wang, W., Li, S., Wen, Y., et al., Acta Physico-Chimica Sinica, 2008, vol. 24, no. 10, pp. 1761–1766.

171

2013, vol. 83, no. 12, pp. 1999–2001. 21. Zhang, W., Fang, C., Yin, W., et al., Mater. Chem. Phys., 2013, vol. 137, no. 3, pp. 877–883.

9. Zhang, Y., Yang, J., Xu, J., et al., Materials Lett., 2012, vol. 81, pp. 1–4.

22. Tac, D.V., Mittova, V.O., Fedchuk, I.V., and Mittova, I.Ya., Condensed Matter Interphases, 2011, vol. 3, no. 3, pp. 42–48

10. Liu, J., Zhang, H., Xie, J., et al., Materials Sci. Forum, 2007, vols. 561–565, pp. 1085–1088.

23. Berbenni, V., Milanese, C., Bruni, G., et al., Thermochim. Acta, 2011, vol. 521, nos. 1–2, pp. 218–223.

11. Tien, N.A., Almjasheva, O.V., Mittova, I.Y., et al., Inorganic Materials, 2009, vol. 45, no. 11, pp. 1304–1308.

24. Zhuravlev, V., Vasilyev, V., Vladimirova, E., et al., Glass Physics Chemistry, 2010, vol. 36, no. 4, pp. 506–512

12. Tac, D.V., Mittova, V.O., Almjasheva, O.V., et al., Inorganic Materials, 2011, vol. 47, no. 10, pp. 1141–1146. 13. Tac, D.V., Mittova, V.O., and Mittova, I.Y., Inorganic Materials, 2011, vol. 47, no. 5, pp. 590–595. 14. Tien, N.A., Mittova, I.Ya., and Almjasheva, O.V., Russ. J. Appl. Chem., 2009, vol. 82, no. 11, pp. 1915–1918. 15. Tien, N.A., Almjasheva, O.V., Mittova, I.Y., et al., Glass Physics Chemistry, 2008, vol. 34, no. 6, pp. 756–761. 16. Tac, D.V., Mittova, V.O., Almjasheva, O.V., et al., Inorganic Materials, 2011, vol. 48, no. 1, pp. 74–78. 17. Shen, H., Xu, J., and Wu, A., J. Rare Earths, 2010, vol. 28, no. 3, pp. 416–419. 18. Jacob, K.T. and Rajitha, G., Solid State Ionics, 2012, vol. 224, pp. 32–40. 19. Lomanova, N.A. and Gusarov, V.V., Nanosystems: Physics, Chemistry, Mathematics, 2013, vol. 4, no. 5, pp. 696–705. 20. Lomanova, N.A. and Gusarov, V.V., Russ. J. Gen. Chem.,

25. Komlev, A.A. and Vilezhaninov, E.F., Russ. J. Appl. Chem., 2013, vol. 86, no. 9, pp. 1344–1350. 26. Wu, L., Yu, J.C., Zhang, L., et al., J. Solid State Chem., 2004, vol. 177, no. 10, pp. 3666–3674. 27. Sychev, A.E. and Merzhanov, A.G., Russ. Chem. Rev., 2004, vol. 73, no. 2, pp. 147–159. 28. Munir, Z. and Holt, J., Combustion and Plasma Synthesis of High Temperature Materials, New York: VCH Publishers, 1989, p. 501. 29. Mandado, M. and Cordeiro, M., J. Comput. Chem., 2010, vol. 31, no. 15, pp. 2735–2745. 30. Patterson, A., Phys. Rev., 1939, vol. 56, pp. 978–982. 31. Ye, T., Guiwen, Z., Weiping, Z., et al., Materials Research Bull., 1997, vol. 32, no. 5, pp. 501–506. 32. Tang, P., Chen, H., Cao, F., et al., Catalysis Sci. & Technol., 2011, vol. 1, no. 7, pp. 1145–1148.
