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Advances in Materials Science and Engineering
VolumeΒ 2012Β (2012), Article IDΒ 380306, 6 pages
http://dx.doi.org/10.1155/2012/380306
Research Article

Size Effect on the Structural and Magnetic Properties of Nanosized Perovskite LaFeO3 Prepared by Different Methods

1Department of Physics, College of Education, Hue University, 34 Le Loi, Hue City, Vietnam
2Faculty of Physics, Hanoi University of Sciences, VNU, 334 Nguyen Trai, Thanh Xuan, Hanoi City, Vietnam

Received 24 April 2012; Accepted 19 June 2012

Academic Editor: DavidΒ Cann

Copyright Β© 2012 Nguyen Thi Thuy and Dang Le Minh. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Nanosized LaFeO3 material was prepared by 3 methods: high energy milling, citrate gel, and coprecipitation. The X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) show that the orthorhombic LaFeO3 phase was well formed at a low sintering temperature of 500Β°C in the citrate-gel and co-precipitation methods. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations indicate that the particle size of the LaFeO3 powder varies from 10 nm to 50 nm depending on the preparation method. The magnetic properties through magnetization versus temperature 𝑀(𝑇) and magnetization verses magnetic field 𝑀(𝐻) characteristics show that the nano-LaFeO3 exhibits a weak ferromagnetic behavior in the room temperature, and the 𝑀(𝐻) curves are well fitted by Langevin functions.

1. Introduction

The perovskite-type oxides (general, formula ABO3, A, and B are the metallic ions) have been attracting much attention for more than two decades due to their potential commercial applications as catalysts for various reactions. Moreover, the modified perovskite compounds such as La1βˆ’xSrxMnO3, La1βˆ’xPrxMnO3, La0.7Sr0.3Mn1βˆ’xNixO3, Ca1βˆ’xNdxMnO3, CaMn1βˆ’xFexO3, and so forth [1–7] have received much attention because of their interesting physical effects: colossal magnetoresistance (CMR), giant magnetocaloric effect (GMCE), and high thermoelectric performance (TEP) at high temperature. In recent years, many laboratories in the world have studied LaFeO3 as a thermoelectric material with high Seebeck coefficient and high power factor and it can be used as catalyst for methane combustion, the thin film gas sensors, and so forth. The LaFeO3 thin film can be used as sensitive O2 gas sensors [8] and nano-sized LaFeO3 powder can be used as catalyst for the autoreforming of sulfur-containing fuels or for partial oxidation of methane (POM) to (H2/CO) [9–12]. For preparation of those nano-materials, various technological methods are used such as co-precipitation, sol-gel, hydrothermal reactions, mechanical alloying, pulsed wire discharge, shock wave, spray drying, and so forth.

In the present study, the nano-sized LaFeO3 has been prepared by 3 methods: high energy milling, citrate gel, and co-precipitation. Beside determination of the particle size, crystalline, and microstructures, the magnetic properties were also investigated. The particle size of the samples prepared by different methods influenced strongly on the structural and magnetic properties of the material.

2. Experimental Procedure

The nano-LaFeO3 was prepared using sol-gel, co-precipitation, and high-energy milling methods. These methods were performed as the following.

In the sol-gel method, the analytical grade La(NO)3Β·6H2O, Fe(NO3)3Β·9H2O, and citric acid (CA) C6H8O7Β·H2O were used as starting materials. The same mole equivalent amounts of metal nitrates were weighed according to the nominal composition LaFeO3 and then dissolved in distilled water. The citric acid with the ratios (CA)/Ξ£(Metal ions) = (1.2–1.5) was then proportionally added to the metal nitrates solution. In the above ratio, (CA) and Ξ£(Metal ions) are concentration of (CA) and sum of concentration of metallic ions, respectively. The solution was concentrated by evaporation at 60–70Β°C with continuous stirring and pH controlled by NH3 solution. The nanocrystals of perovskite LaFeO3 were obtained by decomposition of the dried gel complex at selected temperatures: 300, 500, and 700Β°C in air.

In the co-precipitation method, La(NO3)Β·6H2O, Fe(NO3)3Β·4H2O were raw materials. NH3 solution was added to the metal nitrates solution. The La(OH)3 and Fe(OH)3 were co-precipitated as hydroxide gel [13] at 80Β°C under continuous stirring and pHβ‰ˆ10 to ensure the completely precipitation. Then, the hydroxide gel was filtered and dried. The dried powders were calcined at different temperatures ranging from 100 to 700Β°C for 3 h in air.

In the high-energy milling method, firstly, the bulk sample was prepared by ceramic method and then it was milled into the nanopowder using the high-energy milling equipment SPEX 8000D for 5 h.

Various techniques such as thermal analysis (DSC and TGA with SDT-2960-TA Instrument-USA.), XRD (Diffractometer D5005-Bruker), SEM (S-4800-Hitachi-Japan), and TEM (JEM1011-Jeol-Japan) were employed to characterize the nano-sized LaFeO3 powder. The magnetic properties of the samples were examined by a vibrating sample magnetometer (VSM) DDS-880 (USA).

3. Results and Discussion

Figure 1 shows the DSC and TGA curves for the sample prepared by sol-gel method. It can be seen from Figure 1 that TGA curve exhibits a weight loss of about 65% corresponding to an exothermic peak in DSC curve at 240.55°C, those are the removal of the water from crystallization and decomposition process of the organic substances. Heating at higher temperature led to a small weight loss (~8.3%) at 250°C and finishing at 500°C associated with a peak at 457.01°C in the DSC curve. The weight loss (~65%) is due to the chemical changes as shown in the following equation [14]: La(NO3)3⋅6H2O+FeNO3⋅9H2O+C6H8O7⋅H2O→LaFeO3+6CO2+2N2+2NO2+20H2O(1) During the evaporation of the solvent, a reddish-brown gas corresponding to NO2 comes out of the solution. The above chemical formula only shows the result of chemical reaction but the nature of the sol-gel method is not pointed out. In the used sol-gel method, before creating the solid solution of LaFeO3, the La and Fe ions have been presented in a gel complex. The Fourier transform infrared (FTIR) spectra of the citric acid, gel, and LaFeO3 have been measured for demonstration of the process mentioned above [15].

380306.fig.001
Figure 1: The DSC-TGA curves of the gel complex.

The FTIR spectra of the citric acid, gel complex, and LaFeO3 nanoparticles are shown in Figure 2. In Figure 2(b) (black line), two vibrational bands can be observed at 1572 cmβˆ’1 and 1385 cmβˆ’1 that are assigned to the stretching of C–O bonds. The bands occurred at 551 cmβˆ’1 and 646 cmβˆ’1 are corresponding to Fe–O and La–O bonds, respectively, and the wide band around 3137 cmβˆ’1 in Figure 2(b) (black-line) and 3364 cmβˆ’1  in Figure 2(a) correspond to the hydroxyl group. From the above spectroscopic observations it was suggested that the as-prepared gel consists of an intermediate/complex of citric acid, water, and metal ions. On the basis of the above FTIR results, the expected molecular structure of the complex of metal ions and citric acid is shown in Figure 3.

fig2
Figure 2: (a) FTIR spectra of citric acid; (b) FTIR spectra of gel complex (black line) and LaFeO3 (red line).
fig3
Figure 3: Molecular structure for the citric acid (a) and for a possible complex of metal ions and citric acid (b) in gel precursor of LaFeO3 nanoparticles.

Figure 4 shows the XRD patterns of the nano-sized LaFeO3 powders obtained after heating at different temperatures of 300Β°C (line 1), 500Β°C (line 2), and 700Β°C (line 3) for 3 hours. At 700Β°C the XRD pattern shows that the major phase is LaFeO3 with orthorhombic crystalline structure. The lattice parameters are π‘Ž=5.546 Å; 𝑏=5.5497 Å; 𝑐=7.8573 Å. The gel complex which was heated at 500Β°C for 3 hours has not yet changed to the LaFeO3 phase, as shown in Figure 4 (line 2) and Figure 5 (red line). It seems to be amorphous, but with further heating at 500Β°C for 7 hours, the LaFeO3 phase was completely formed (Figure 5β€”black line). Figure 6 shows the XRD pattern of LaFeO3 prepared by the co-precipitation method. The complex precipitate was heated at different temperatures for 3 hours. The phase states are similar to the case of the sol-gel method (Figure 5). The XRD patterns of hydroxide gel show that the LaFeO3 phase does not appear at 300Β°C or 500Β°C; however, at 700Β°C a major phase as LaFeO3 is formed (Figure 6).

380306.fig.004
Figure 4: The powder X-ray diffraction patterns of gel complex heated at 300Β°C (line 1); 500Β°C (line 2); 700Β°C (line 3) for 3 hours.
380306.fig.005
Figure 5: The powder X-ray diffraction patterns of gel complex heated at 500Β°C for 3 hours (red line) and for 10 hours (black line).
380306.fig.006
Figure 6: The powder X-ray diffraction patterns of hydroxide gel heated at 300Β°C (line 1); 500Β°C (line 2); 700Β°C (line 3) for 3 hours.

The average crystalline particle size calculated from Scherrer’s formula 𝐷=π‘˜πœ†/𝐡cosπœƒ is about 30 nm, where 𝐷 is the average size of crystalline particle, assuming that particles are spherical, π‘˜=0.9 [14], πœ† is the wavelength of X-ray radiation, 𝐡 is full width at half maximum of the diffracted peak, and πœƒ is angle of diffraction.

The particle size and morphology of the calcined powders examined by TEM and SEM are shown in Figures 7(a), 7(b), and 8, respectively. It can be estimated from these figures that the particle size is varying from about 10 to 30 nm.

fig7
Figure 7: TEM (a) and SEM (b) micrographs of LaFeO3 prepared by sol-gel method, followed by calcining process at 700Β°C.
380306.fig.008
Figure 8: SEM micrograph of nano-LaFeO3 prepared by high-energy milling method.

The magnetic properties of the samples were examined by Vibrating Sample Magnetometer (VSM) in the field of 13.5 kOe from room temperature to 800 K. The Curie temperature determined by the 𝑀(𝑇) curve (Figure 9) is around 730 K, which is corresponding to the peak in the DSC curve at about 457Β°C (Figure 1). The 𝑀(𝐻) curve of nano-LaFeO3 prepared by sol-gel method is shown in Figure 10.

380306.fig.009
Figure 9: The 𝑀(𝑇) curve of nano-LaFeO3 prepared by sol-gel method.
380306.fig.0010
Figure 10: The 𝑀(𝐻) curve at room temperature of nano-LaFeO3 prepared by sol-gel method.

As for the sample prepared by high-energy milling the powders after milling were heated at 500Β°C in 3 hours to eliminate inner stress in the samples. Figure 8 shows the SEM image for the LaFeO3 powder after milling and heat treatment. The average size of particle is about 50 nm. The 𝑀(𝐻) curve of nano-sized LaFeO3 prepared by milling method is shown in Figure 11.

380306.fig.0011
Figure 11: The 𝑀(𝐻) curve at room temperature of nano-LaFeO3 prepared by high-energy milling method.

It is well known that the perovskite LaFeO3 displays antiferromagnetic and insulator behavior in room temperature [16]. However, the 𝑀(𝑇) and 𝑀(𝐻) curves of the prepared LaFeO3 show that LaFeO3 exhibits weak ferromagnetism. It may be caused by the antiferromagnetic order with canted spins [17]. In addition, during heating at high temperature some couples of Fe3+-Fe2+ may be appeared in LaFeO3 due to the losing of oxygen. The difference between magnetic moment of Fe3+ ions (5 μB) and Fe2+ (4 μB) has contributed to magnetic behaviors of the samples and they became an electrical conducting materials as semiconductor.

The parameters of hysteresis loop of the samples prepared by sol-gel and milling methods are listed in Table 1.

tab1
Table 1: The parameters of hysteresis loop of the samples prepared by sol-gel and milling methods.

The results listed in the above table show that the preparation method and particle size influence on the magnetic properties. Although after milling the samples have been annealed, it seems that the inner press could not be eliminated completely; thus the magnetization π‘€π‘š of the sample prepared by milling method is less than that of the samples prepared by sol-gel method. The particle size of the powders prepared by the milling method is larger than the one obtained by the sol-gel method. The bigger particles give a higher coercivity 𝐻𝑐. This is in good agreement with the law (π»π‘βˆΌπ·6) of the nanomagnetic particles [18, 19]. It is noted that the nanosized, and single-domain ferromagnetic powder could be superparamagnetic with 𝐻𝑐=0 and π‘€π‘Ÿ=0; 𝑆=(π‘€π‘Ÿ/𝑀𝑠)=0 [20]. If the prepared nano-sized powder has some of particles with multiple domain sizes, 𝐻𝑐, π‘€π‘Ÿ, and 𝑆 will differ from zero. The larger particle size gives higher 𝑆 and the ferromagnetic behavior is more clear. That is why we suggested that the ratio 𝑆=π‘€π‘Ÿ/𝑀𝑠 could be used as a functionally parameter for evaluating the homogeneity on dimension of nanoparticles and the limit of single domain size of the magnetic nano-sized powder materials.

As mentioned above, the prepared nano-sized LaFeO3 powder is weakly ferromagnetic (π‘€π‘Ÿβ‰ 0). It is a multi-disperse system consisting of the single-domain and multiple-domain particles. The magnetization of the sample is considered as the sum of two terms: 𝑀(𝐻)=𝑀sp(𝐻)+𝑀𝑓(𝐻),(2) where 𝑀sp(𝐻) is the contribution from the superparamagnetic (sp) nanoparticles (single domain), 𝑀𝑓(𝐻) is the contribution of ferromagnetic (𝑓) nanoparticles (multiple domains): 𝑀𝑓(𝐻)=2π‘€π‘“π‘ πœ‹tanβˆ’1𝐻±𝐻𝑐𝐻𝑐tanπœ‹π‘†2,(3)𝑀𝑓𝑠: saturation magnetization of ferromagnetic phase (𝑀𝑓𝑠=π‘€π‘Ÿ/0.866). 𝑆: rectangular coefficence of ferromagetic hysteresis loop.

The noninteraction magnetization process of the superparamagnetic monodisperse nanoparticles can be shown by the expression: 𝑀(𝐻)=𝑀(∞)πΏπ‘šπ»π‘˜π΅π‘‡ξ‚Ή,(4) where π‘š is magnetic moment and 𝐿(π‘₯)=coth(π‘₯)βˆ’1/π‘₯ is the Langevin function, π‘₯=π‘šπ»/π‘˜π΅π‘‡, [21]. To take into account the effects of size dispersion that are always presented in any real system, the magnetization of superparamagnetic particles, in this case, it is better to use the expression: 𝑀sp(𝐻)=𝑀sp(ξ“βˆž)π‘—π‘“ξ€·π‘šπ‘—ξ€ΈπΏξ‚Έπ‘šπ‘—π»π‘˜π΅π‘‡ξ‚Ή.(5)π‘šπ‘— is magnetic moment of the particle, 𝑓(π‘šπ‘—) is weighted terms in Langevin functions [22].

It is suggested that the particles are spherical shape, the distribution of particle size 𝑓(𝐷) is shown by the expression [23]: 1𝑓(𝐷)=βˆšξƒ©βˆ’2πœ‹πœŽπ·expln(𝐷/𝐷)22𝜎2ξƒͺ,(6) where 𝜎 is standard deviation and 𝐷 is the average particle size. 𝑓(π‘šπ‘—) can be calculated from 𝐷. Figure 12 shows the Langevin function fitting result for the magnetization curve of the nano-sized LaFeO3.

380306.fig.0012
Figure 12: The result of the fitting of the 𝑀(𝐻) curve of the nano-LaFeO3 prepared by sol-gel method based on the Langevin function.

4. Conclusion

The nano-sized LaFeO3 has been successfully prepared by different methods. The particle size of nano-LaFeO3 is varying from about 10 to 50 nm depending on the preparation method. The prepared nano-LaFeO3 exhibited a ferromagnetic behavior and the particle size influences the magnetic properties of nano-LaFeO3. The 𝑀(𝐻) curve was well fitted by Langevin function. We have proposed that by using parameter 𝑆=π‘€π‘Ÿ/𝑀𝑠 one could evaluate the homogeneity of the dimensions of nanoparticles and the critical size of single domain of the nano-magnetic materials.

Acknowledgment

This work was supported by Vietnam’s National Foundation For Science and Technology Development (NAFOSTED), with the project code β€œ103.03.69.09”.

References

  1. V. Caignaert, A. Maignan, and B. Raveau, β€œUp to 50 000 per cent resistance variation in magnetoresistive polycrystalline perovskites Ln2/3Sr1/3MnO3 (Ln=Nd; Sm),” Solid State Communications, vol. 95, no. 6, pp. 357–359, 1995. View at Google Scholar Β· View at Scopus
  2. N. Gayathri, A. K. Raychaudhuri, and S. K. Tiwary, β€œElectrical transport, magnetism, and magnetoresistance in ferromagnetic oxides with mixed exchange interactions: a study of the La0.7Ca0.3Mn1−xCoxO3 system,” Physical Review B, vol. 56, pp. 1345–1353, 1997. View at Google Scholar
  3. H. Taguchi, M. Nagao, and M. Shimada, β€œMechanism of metal-insulator transition in the systems (Ln1−xCax)MnO3−δ (Ln: La, Nd, and Gd) and (Nd0.1Ca0.9−ySry)MnO2.97,” Journal of Solid State Chemistry, vol. 97, no. 2, pp. 476–480, 1992. View at Google Scholar Β· View at Scopus
  4. Md. A. Choudhury, S. Akhter, D. L. Minh, N. D. Tho, and N. Chau, β€œLarge magnetic-entropy change above room temperature in the colossal magnetoresistance La0.7Sr0.3Mn1−xNixO3 materials,” Journal of Magnetism and Magnetic Materials, vol. 272, pp. 1295–1297, 2004. View at Publisher Β· View at Google Scholar
  5. K. Iwasaki, T. Ito, M. Yoshino, T. Matsui, T. Nagasaki, and Y. Arita, β€œPower factor of La1−xSrxFeO3 and LaFe1−yNiyO3,” Journal of Alloys and Compounds, vol. 430, no. 1-2, pp. 297–301, 2007. View at Publisher Β· View at Google Scholar Β· View at Scopus
  6. M.-H. Hung, M. V. M. Rao, and D.-S. Tsai, β€œMicrostructures and electrical properties of calcium substituted LaFeO3 as SOFC cathode,” Materials Chemistry and Physics, vol. 101, pp. 297–302, 2007. View at Publisher Β· View at Google Scholar
  7. D. Bayraktar, F. Clemens, S. Diethelm, T. Graule, J. Van herle, and P. Holtappels, β€œProduction and properties of substituted LaFeO3-perovskite tubular membranes for partial oxidation of methane to syngas,” Journal of the European Ceramic Society, vol. 27, no. 6, pp. 2455–2461, 2007. View at Publisher Β· View at Google Scholar Β· View at Scopus
  8. K. Iwasaki, T. Ito, M. Yoshino et al., β€œPower factor of La1−xSrxFeO3 and LaFe1−yNiyO3,” Journal of Alloys and Compounds, vol. 430, pp. 297–301, 2007. View at Publisher Β· View at Google Scholar
  9. P. Dinka and A. S. Mukasyan, β€œPerovskite catalysts for the auto-reforming of sulfur containing fuels,” Journal of Power Sources, vol. 167, no. 2, pp. 472–481, 2007. View at Publisher Β· View at Google Scholar Β· View at Scopus
  10. M. Yang, A. Xu, and H. Du, β€œRemoval of salicylic acid on perovskite-type oxide LaFeO3 catalyst in catalytic wet air oxidation process,” Journal of Hazardous Materials B, vol. 139, pp. 86–92, 2007. View at Publisher Β· View at Google Scholar
  11. X. P. Dai, R. J. Li, C. C. Yu, and Z. P. Hao, β€œUnsteady-state direct partial oxidation of methane to synthesis gas in a fixed-bed reactor using AFeO3 (A=La, Nd, Eu) perovskite-type oxides as oxygen storage,” Journal of Physical Chemistry B, vol. 110, no. 45, pp. 22525–22531, 2006. View at Publisher Β· View at Google Scholar Β· View at Scopus
  12. M. Søgaard, P. V. Hendriksen, and M. Mogensen, β€œOxygen nonstoichiometry and transport properties of strontium substituted lanthanum ferrite,” Journal of Solid State Chemistry, vol. 180, no. 4, pp. 1489–1503, 2007. View at Publisher Β· View at Google Scholar Β· View at Scopus
  13. A. D. Jadhav, A. B. Gaikwad, V. Samuel, and V. Ravi, β€œA low temperature route to prepare LaFeO3 and LaCoO3,” Materials Letters, vol. 61, no. 10, pp. 2030–2032, 2007. View at Publisher Β· View at Google Scholar Β· View at Scopus
  14. G. Shabbir, A. H. Qureshi, and K. Saeed, β€œNano-crystalline LaFeO3 powders synthesized by the citrate-gel method,” Materials Letters, vol. 60, pp. 3706–3709, 2006. View at Publisher Β· View at Google Scholar
  15. M. Srivastava, S. Chaubey, and A. K. Ojha, β€œInvestigation on size dependent structural and magnetic behavior of nickel ferrite nanoparticles prepared by sol-gel and hydrothermal methods,” Materials Chemistry and Physics, vol. 118, no. 1, pp. 174–180, 2009. View at Publisher Β· View at Google Scholar Β· View at Scopus
  16. S. Komine and E. Iguchi, β€œDielectric properties in LaFe0.5 Ga0.5 O3,” Journal of Physics and Chemistry of Solids, vol. 68, no. 8, pp. 1504–1507, 2007. View at Publisher Β· View at Google Scholar Β· View at Scopus
  17. A. V. Galubkov, E. V. Goncharova, V. P. Zhuze, and I. G. Manilove, β€œTransport mechanism in samarium sulfide,” Soviet Physics Solid State, vol. 7, no. 8, pp. 1963–1967, 1966. View at Google Scholar
  18. G. Herzer, β€œGrain size dependence of coercivity and permeability in nanocrystalline ferromagnets,” IEEE Transactions on Magnetics, vol. 26, no. 5, pp. 1397–1402, 1990. View at Publisher Β· View at Google Scholar
  19. D. Xue, G. Chai, X. Li, and X. Fan, β€œEffects of grain size distribution on coercivity and permeability of ferromagnets,” Journal of Magnetism and Magnetic Materials, vol. 320, no. 8, pp. 1541–1543, 2008. View at Publisher Β· View at Google Scholar Β· View at Scopus
  20. J. P. Vejpravova, D. Niznnasky, J. Plocek, A. Hutlova, and J.-L. Rehspringer, β€œSuperparamagnetism of co-ferrite nanoparticles,” in Proceeding of Contributed Paper, Part III (WDS '05), pp. 518–523, 2005.
  21. G. F. Goya, T. S. Berquo, and F. C. Fonseca, β€œStatic and dynamic magnetic properties of spherical magnetite nanoparticles,” Journal of Applied Physics, vol. 94, Article ID 3520, 9 pages, 2003. View at Publisher Β· View at Google Scholar
  22. F. C. Fonseca, A. S. Ferlauto, F. Alvarez, G. F. Goya, and R. F. Jardim, β€œMorphological and magnetic properties of carbon-nickel nanocomposite thin films,” Journal of Applied Physics, vol. 97, Article ID 044313, 7 pages, 2005. View at Publisher Β· View at Google Scholar
  23. S.-J. Lee, J.-R. Jeong, S.-C. Shin, J.-C. Kim, and J.-D. Kim, β€œSynthesis and characterization of superparamagnetic maghemite nanoparticles prepared by coprecipitation technique,” Journal of Magnetism and Magnetic Materials, vol. 282, no. 1–3, pp. 147–150, 2004. View at Publisher Β· View at Google Scholar Β· View at Scopus