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Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid
Advanced Powder Technology xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid Shin Mimuro a,b,⇑, Yuki Makinose a, Nobuhiro Matsushita a a b
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan EV System Laboratory, Nissan Research Center, Nissan Motor Co., Ltd., 1, Natsushima-cho, Yokosuka 237-8523, Japan
a r t i c l e
i n f o
Article history: Received 15 September 2014 Received in revised form 8 June 2015 Accepted 19 June 2015 Available online xxxx Keywords: Lanthanum strontium cobalt ferrite Low-temperature synthesis Citrate complex method Oleic acid Surfactant
a b s t r a c t Crystalline lanthanum strontium cobalt ferrite (LSCF) was obtained by calcination at a low temperature of 500 °C from precursor nanoparticles. The precursor nanoparticles were synthesized by the citrate complex method adding oleic acid as a surfactant, and dynamic laser scattering measurements suggested that they exhibited high dispersibility in the solution with an average diameter less than around 20 nm. The crystallite size decreased and the total specific area increased as the calcination temperature was decreased from 800 to 500 °C. It was found that the oleic acid functioned as a surfactant to form homogeneous fine precursor, and then assisted in the crystallization of LSCF even at a low calcination temperature of 500 °C. Such a low temperature synthesis suppressed the agglomeration and growth of LSCF particles so that fine powders with particle size less than 20 nm were obtained. Ó 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Lanthanum strontium cobalt ferrite (LSCF, La1xSrxCo1yFeyO3, 0 < x < 1, 0 < y < 1) is one of the main cathode materials for solid oxide fuel cells because of its relatively high electronic and oxygen ion mixed conductivities [1,2] and high catalytic activity for the oxygen reduction reaction [3]. For decades, the solid-state reaction method has been used to synthesize single-phase materials because of its simplicity. However, it requires a high process temperature greater than 1000 °C to diffuse and to react the raw materials and to form the single-phase crystal [4]. Because such high temperature calcination increases production costs, the development of a low-temperature process that enables the crystallization of LSCF has long been anticipated [4]. In recent years, several wet chemical syntheses have been investigated to solve this issue [5]. Fan and Liu [6] and Ghouse et al. [7] prepared single-phase LSCF at 700 °C by citric acid combustion and polymerized complex methods, respectively. Baqué et al. [8] reported that single-phase LSCF was obtained by calcination at 750 °C using citric acid and hexamethylenetetramine as combustion fuels. To further decrease the calcination temperature, we propose a new type of calcination process using oleic acid in addition to citric ⇑ Corresponding author at: EV System Laboratory, Nissan Research Center, Nissan Motor Co., Ltd., 1, Natsushima-cho, Yokosuka 226-8523, Japan. Tel.: +81 50 3751 6810. E-mail address: [email protected] (S. Mimuro).
acid. Taniguchi et al. reported that uniform-sized nano CeO2 was successfully synthesized by the hydrothermal method using oleic acid as a surfactant [9]. From the result, we selected the oleic acid to synthesize nano-sized LSCF in this study. In our new process, oleic acid functions as a surfactant to improve the dispersibility of the precursor complex in the solution and to promote the formation of a fine and highly homogeneous precipitate. Such a fine precipitate improves the surface energy and reactivity so that the crystallization at a low temperature can be expected. The objective of this study is to prepare single-phase LSCF at a low calcination temperature and to investigate its formation mechanism.
2. Experimental 2.1. Sample preparation We selected the composition of La:Sr:Co:Fe = 0.6:0.4:0.2:0.8 because the composition is widely selected in terms of the consistency of the thermal expansion coefficient with the electrolyte(YSZ) and higher electron and oxide ion mixed conductivities. The precursor powders with composition La0.6Sr0.4Co0.2Fe0.8O3 were prepared by a citrate complex method using La(NO3)36H2O (99.9%), Sr(NO3)2 (98%), Fe(NO3)39H2O (99.9%), Co(NO3)36H2O (99.5%) and citric acid (C6H8O7) purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan).
http://dx.doi.org/10.1016/j.apt.2015.06.006 0921-8831/Ó 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006
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S. Mimuro et al. / Advanced Powder Technology xxx (2015) xxx–xxx
Stoichiometric amounts of the nitrates were dissolved in diluted water at 60 °C on the hotplate and mixed at 200 rpm using a magnetic stirrer, and then citric acid in equal molar amount to total metal ions was added to the solution. The sample prepared with the above process is called LS-cit, and the sample with an equal molar amount of oleic acid (C18H34O2, Wako Pure Chemical Industries, Ltd.) also added to LS-cit is called LS-ol. The pH of the solution was adjusted between 7 and 9 using dilute nitric acid and aqueous ammonia. The solution was gradually dried to form the precursor gel. The precursor gel was heated at 130 °C for 1 h and then at 400 °C for 1 h, and finally it was calcined in the range from 400 to 900 °C for 2 h in air. The composition ratio of some of the resulting powders was checked by inductively coupled plasma (ICP) spectroscopy and it was almost consistent with that of the fed raw materials. 2.2. Characterization The samples were analyzed by X-ray diffraction (XRD) for phase identification with Cu Ka radiation of 40 kV/40 mA at room temperature. The lattice parameters were refined by the Rietveld method [10]. The compositions of the samples were confirmed by ICP at room temperature. Raman spectroscopy analysis was performed at room temperature. The particle size distribution and zeta potential of the precursor solution were examined using a dynamic laser scattering (DLS) system. Thermogravimetry–differ ential thermal analysis (TG/DTA) measurements of the precursor powder were performed with temperature in the range from room temperature to 900 °C in air. The crystallite sizes of the LSCF sample powders were observed by transmission electron microscopy (TEM). 3. Results and discussion 3.1. Phase purity Figs. 1 and 2 show the XRD profiles of the LS-ol and LS-cit samples calcined at various temperatures. The simulated profile of single-phase LSCF with rhombohedral symmetry of the space group R3c was calculated using the reported data [11] and is shown at the bottom in Figs. 1 and 2. No secondary phases were observed for the LS-ol sample calcined in the range 500–800 °C, and the XRD pattern is in good agreement with the simulated profile, as shown in Fig. 1. While the sample calcined at 400 °C was not a single phase, the impurity phase was not assigned. The desired composition of
Fig. 2. XRD profiles of LS-cit samples calcined at various temperatures. The impurity phase peaks are indicated by arrows.
La0.6Sr0.4Co0.2Fe0.8O3 was confirmed for the samples calcined at 500–800 °C by ICP. As one of representatives, the results of LS-ol calcined at 800 °C are shown in Table 1. Conversely, a single phase was not obtained for the LS-cit samples even at a high calcination temperature of 900 °C, as shown in Fig. 2. The calculated lattice parameters of the LS-ol samples by the Rietveld method are shown in Fig. 3. They agreed well with the reported values [12,13,18]. In the case of LS-ol calcined at 700 °C, for example, the reliable factors Rwp, Rp and S were 17.765, 13.187 and 1.4169, respectively. Although those values were relatively high to investigate precise crystal parameters, it was enough low for our main objective to judge whether LSCF was successfully synthesized or not by comparing the obtained lattice parameters to the reported ones. From these results, the calcination of precursors prepared by the citrate complex method adding oleic acid enabled the formation of LSCF powders in a single phase even at a low temperature of 500 °C. However, a single phase was not obtained for LS-cit precursors even after calcination at a high temperature of 900 °C, as shown in Fig. 2. We examined the particle size distribution of LS-ol and LS-cit precursors in the solution to explain these results (Fig. 4). The profile of the LS-ol precursor in Fig. 4 shows a single sharp peak, suggesting a small particle size of several tens of nanometers and a narrow size distribution. Conversely, the profile of the LS-cit sample was split into two peaks, suggesting a much larger particle size than LS-ol. These results indicate that oleic acid on the particle surface functioned as a surfactant in the case of LS-ol to promote the dispersibility of the precursor in the solution and helped to attain high homogeneity. We believe that the high dispersibility and homogeneity enabled the formation of single-phase LSCF even at a low calcination temperature of 500 °C. Another possibility is that LSCF crystallization was also assisted by combustion heat from oleic acid during the calcination, as shown in Fig. 5. Table 1 ICP results of LS-ol calcined at 800 °C. A11 data were averaged values for 10 times measurements. Sample weight 10.34 mg.
Concentration (mg l1) R S Cv Molar ratio
Fig. 1. XRD profiles of LS-ol samples calcined at various temperatures.
Fe
Co
Sr
La
40.26 0.65 0.20 0.49 4.38
9.70 0.17 0.06 0.57 1.00
31.36 0.53 0.17 0.54 2.18
72.65 1.13 0.46 0.63 3.18
R – resolution of the each element. S – standard delation of measurements. Cv – coefficient of the variation.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006
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Fig. 5. TG/DTA profile of sodium oleate.
Fig. 3. Lattice parameters of LS-ol samples plotted as a function of calcination temperature.
Fig. 6. Zeta potential of LS-ol and LS-cit precursor samples measured at 25 °C.
Fig. 4. Particle size distribution of LS-ol and LS-cit precursor samples measured at pH 10 and 25 °C.
Fig. 7 shows the dependence of calculated crystallite size on the calcination temperature. The crystallite size almost linearly decreased as the calcination temperature decreased to 500 °C. The crystallite sizes of around 15–20 nm for the samples calcined below 800 °C were much smaller than those of 20–40 nm for samples prepared by combustion syntheses using glycine
We examined the zeta potential of the precursor solutions to investigate the high dispersibility of LS-ol. The LS-ol precursor exhibited a more negative zeta potential than LS-cit at pH > 7, as shown in Fig. 6. This result can be explained by considering the ionization of the side-chain carboxyl group of oleic acid. Because the carboxyl group is deprotonated in alkaline conditions, the precursor is negatively charged at high pH. Ignoring steric repulsion, electrostatic repulsion increased as the negative charge on the nanoparticle surface increased, resulting in the LS-ol precursor being well-dispersed in the solution (pH 10), as shown in Fig. 4. 3.2. Size and morphology The crystallite sizes of the calcined samples were calculated using the XRD results in Fig. 1 and following equation:
D ¼ Kk=B cos h; where D is the volume mean diameter, K is a constant (=1.33), B is the integral width, k is the wavelength (=1.54055 Å), and h is the diffraction angle [19].
Fig. 7. Crystallite size of LS-ol samples plotted as a function of calcination temperature.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006
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(calcined at 850 °C) [14] and EDTA–citrate (calcined at 900 °C) [15], and the polymerization complex process using citric acid and ethylene glycol (calcined at 900 °C) [16]. This is because crystallite growth is suppressed at lower temperatures. Furthermore, oleic acid surrounding LSCF nanoparticles and/or its derived substance around the precursor might interfere with the direct contact of the particles and suppress particle growth during calcination. Fig. 8 shows TEM images of the LS-ol sample calcined at various temperatures. The crystallite sizes were slightly smaller for lower calcination temperatures, and they were much smaller than those of LS-cit (>100 nm, Fig. 9). The results can be explained as follows. In the case of LS-cit, without inhibitor between the LSCF precursor
Fig. 9. TEM image of LS-cit sample calcined at 900 °C.
particles, volume diffusion and particle growth were promoted because of the larger contact area between the particles compared to the LS-ol. The approximate crystallite sizes of LS-ol observed by TEM were around 20 nm, and are consistent with the calculated sizes (Fig. 7). The specific surface areas of LS-ol calcined at 500 and 800 °C were 10.73 and 7.95 m2/g, respectively, which are much larger than the reported values of 3.4–5.4 m2/g [14,16,17]. This result can be attributed to the smaller crystallite sizes, as shown in Figs. 4, 7 and 8. 3.3. TG/DTA Fig. 10 shows the TG/DTA spectra of the LS-ol precursor measured up to 900 °C. The weight loss up to 150 °C was assigned to the dehydration of the absorbed water on the precursors. The broad bands in the range 150–500 °C were assumed to be the decompositions of citric acid [18] and oleic acid, as shown in Fig. 5. The two sharp peaks observed between 500 and 600 °C in Fig. 5 are ascribed to oleic acid decomposition. da Conceição et al. reported that weight loss at around 800 °C can be attributed to LSCF phase formation [18]. However, we confirmed that the LSCF phase was obtained even at a calcination temperature of 500 °C, and suggest that the weight loss at around 800 °C might be because of the burning of residual organic molecules surrounding the nanoparticle surfaces, because a similar weight loss was observed for sodium oleate. Raman spectra of the LS-ol samples calcined at 600–800 °C are shown in Fig. 11. Although the graphite has high
Fig. 8. TEM images of LS-ol samples calcined at (a) 500, (b) 800, and (c) 900 °C.
Fig. 10. TG/DTA profile of LS-ol precursor sample.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006
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References
Fig.11. Raman spectra of LS-ol samples calcined at 600, 700 and 800 °C.
Raman activity, no D band structure around 1350 cm1 reflecting the defects and distortion was observed in each spectrum. From these results, we thought that the amount of residual carbon was surprisingly low in LS-ol samples calcined at 600–800 °C. 4. Conclusions LSCF precursor was synthesized by the citrate complex method adding oleic acid as a surfactant. From the XRD results, we succeeded in obtaining crystalline LSCF powders by calcining the precursor at a temperature as low as 500 °C in air. This temperature is much lower than that required for the crystallization of LSCF powders synthesized by the conventional citrate complex method. DLS results showed that the oleic acids around the precursor nanoparticles improve their dispersibility, enabling the formation of a homogeneous fine precipitate that promotes the crystallization of LSCF at low temperature. The crystallite sizes of the calcined samples were around 20 nm by TEM, which is consistent with the sizes calculated by Scherer’s equation. The specific surface area increased from 7.9 to 10.95 m2/g as the calcination temperature was decreased from 800 to 500 °C. These results indicate that a low calcination temperature suppresses the growth and agglomeration of LSCF particles and enabled the formation of nanosized particles.
[1] H.J. Hwang, J.W. Moon, S. Lee, E.A. Lee, Electrochemical performance of LSCFbased composite cathodes for intermediate temperature SOFCs, J. Power Sources 145 (2005) 243–248. [2] C.C. Chen, M.M. Nasrallah, H.U. Anderson, M.A. Alim, Immittance response of La0.6Sr0.4Co0.2Fe0.8O3 based electrochemical cells, J. Electrochem. Soc. 142 (1995) 491–496. [3] G. DiGiuseppe, L. Sun, Electrochemical performance of a solid oxide fuel cell with an LSCF cathode under different oxygen concentrations, Int. J. Hydrogen Energy 36 (2011) 5076–5087. [4] M. Kumar, S. Srikantha, B. Ravikumar, T.C. Alex, S.K. Das, Synthesis of pure and Sr-doped LaGaO3, LaFeO3 and LaCoO3 and Sr, Mg-doped LaGaO3 for ITSOFC application using different wet chemical routes, Mater. Chem. Phys. 113 (2009) 803–815. [5] Z. Shao, W. Zhou, Z. Zhu, Advanced synthesis of materials for intermediatetemperature solid oxide fuel cells, Prog. Mater Sci. 57 (2012) 804–874. [6] B. Fan, X. Liu, A-deficit LSCF for intermediate temperature solid oxide fuel cells, Solid State Ionics 180 (2009) 973–977. [7] M. Ghouse, Y. Al-Yousef, A. Al-Musa, M.F. Al-Otaibi, Preparation of La0.6Sr0.4Co0.2Fe0.8O3 nanoceramic cathode powders for solid oxide fuel cell (SOFC) application, Int. J. Hydrogen Energy 35 (2010) 9411–9419. [8] L. Baqué, A. Caneiro, M.S. Moreno, Adriana Serquis, High performance nanostructured IT-SOFC cathodes prepared by novel chemical method, Electrochem. Commun. 10 (2008) 1905–1908. [9] T. Taniguchi, K. Katsumata, S. Omata, K. Okada, N. Matsushita, Tuning growth modes of ceria-based nanocubes by a hydrothermal method, Cryst. Growth Des. 11 (9) (2011) 3754–3760. [10] F. Izum, K. Momma, Three-dimensional visualization in powder diffraction, Solid State Phenom. 130 (2007) 15–20. [11] T. Itoh, M. Nakayama, Using in situ X-ray absorption spectroscopy to study the local structure and oxygen ion conduction mechanism in (La0.6Sr0.4)(Co0.2Fe0.8)O3d, J. Solid State Chem. 192 (2012) 38–46. [12] A. Dutta, J. Mukhopadhyay, R.N. Basu, Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells, J. Eur. Ceram. Soc. 29 (2009) 2003–2011. [13] S. Hashimoto, Y. Fukuda, M. Kuhn, K. Sato, K. Yashiro, J. Mizusaki, Oxygen nonstoichiometry and thermo-chemical stability of La0.6Sr0.4Co1yFeyO3d (y = 0.2, 0.4, 0.6, 0.8), Solid State Ionics 181 (2010) 1713–1719. [14] M. Zawadzki, H. Grabowska, J. Trawczyn´ski, Effect of synthesis method of LSCF perovskite on its catalytic properties for phenol methylation, Solid State Ionics 181 (2010) 1131–1139. [15] C. Nityanand, W.B. Nalin, B.S. Rajkumar, C.M. Chandr, Synthesis and physicochemical characterization of nanocrystalline cobalt doped lanthanum strontium ferrite, Solid State Sci. 13 (2011) 1022–1030. [16] L. Ge, Z. Zhu, Z. Shao, S. Wang, S. Liu, Effects of preparation methods on the oxygen nonstoichiometry, B-site cation valences and catalytic efficiency of perovskite La0.6Sr0.4Co0.2Fe0.8O3d, Ceram. Int. 35 (2009) 3201–3206. [17] M. Zawadzki, J. Trawczyn´ski, Synthesis, characterization and catalytic performance of LSCF perovskite for VOC combustion, Catal. Today 176 (2011) 449–452. [18] L. Conceição, A.M. Silva, N.F.P. Ribeiro, M.M.V.M. Souza, Combustion synthesis of La0.7Sr0.3Co0.5Fe0.5O3 (LSCF) porous materials for application as cathode in IT-SOFC, Mater. Res. Bull. 46 (2011) 308–314. [19] T. Ida, S. Shimazaki, H. Hibino, H. Toraya, Diffraction peak profiles from spherical crystallites with lognormal size distribution, J. Appl. Crystallogr. 36 (2003) 1107–1115.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid Shin Mimuro a,b,⇑, Yuki Makinose a, Nobuhiro Matsushita a a b
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan EV System Laboratory, Nissan Research Center, Nissan Motor Co., Ltd., 1, Natsushima-cho, Yokosuka 237-8523, Japan
a r t i c l e
i n f o
Article history: Received 15 September 2014 Received in revised form 8 June 2015 Accepted 19 June 2015 Available online xxxx Keywords: Lanthanum strontium cobalt ferrite Low-temperature synthesis Citrate complex method Oleic acid Surfactant
a b s t r a c t Crystalline lanthanum strontium cobalt ferrite (LSCF) was obtained by calcination at a low temperature of 500 °C from precursor nanoparticles. The precursor nanoparticles were synthesized by the citrate complex method adding oleic acid as a surfactant, and dynamic laser scattering measurements suggested that they exhibited high dispersibility in the solution with an average diameter less than around 20 nm. The crystallite size decreased and the total specific area increased as the calcination temperature was decreased from 800 to 500 °C. It was found that the oleic acid functioned as a surfactant to form homogeneous fine precursor, and then assisted in the crystallization of LSCF even at a low calcination temperature of 500 °C. Such a low temperature synthesis suppressed the agglomeration and growth of LSCF particles so that fine powders with particle size less than 20 nm were obtained. Ó 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Lanthanum strontium cobalt ferrite (LSCF, La1xSrxCo1yFeyO3, 0 < x < 1, 0 < y < 1) is one of the main cathode materials for solid oxide fuel cells because of its relatively high electronic and oxygen ion mixed conductivities [1,2] and high catalytic activity for the oxygen reduction reaction [3]. For decades, the solid-state reaction method has been used to synthesize single-phase materials because of its simplicity. However, it requires a high process temperature greater than 1000 °C to diffuse and to react the raw materials and to form the single-phase crystal [4]. Because such high temperature calcination increases production costs, the development of a low-temperature process that enables the crystallization of LSCF has long been anticipated [4]. In recent years, several wet chemical syntheses have been investigated to solve this issue [5]. Fan and Liu [6] and Ghouse et al. [7] prepared single-phase LSCF at 700 °C by citric acid combustion and polymerized complex methods, respectively. Baqué et al. [8] reported that single-phase LSCF was obtained by calcination at 750 °C using citric acid and hexamethylenetetramine as combustion fuels. To further decrease the calcination temperature, we propose a new type of calcination process using oleic acid in addition to citric ⇑ Corresponding author at: EV System Laboratory, Nissan Research Center, Nissan Motor Co., Ltd., 1, Natsushima-cho, Yokosuka 226-8523, Japan. Tel.: +81 50 3751 6810. E-mail address: [email protected] (S. Mimuro).
acid. Taniguchi et al. reported that uniform-sized nano CeO2 was successfully synthesized by the hydrothermal method using oleic acid as a surfactant [9]. From the result, we selected the oleic acid to synthesize nano-sized LSCF in this study. In our new process, oleic acid functions as a surfactant to improve the dispersibility of the precursor complex in the solution and to promote the formation of a fine and highly homogeneous precipitate. Such a fine precipitate improves the surface energy and reactivity so that the crystallization at a low temperature can be expected. The objective of this study is to prepare single-phase LSCF at a low calcination temperature and to investigate its formation mechanism.
2. Experimental 2.1. Sample preparation We selected the composition of La:Sr:Co:Fe = 0.6:0.4:0.2:0.8 because the composition is widely selected in terms of the consistency of the thermal expansion coefficient with the electrolyte(YSZ) and higher electron and oxide ion mixed conductivities. The precursor powders with composition La0.6Sr0.4Co0.2Fe0.8O3 were prepared by a citrate complex method using La(NO3)36H2O (99.9%), Sr(NO3)2 (98%), Fe(NO3)39H2O (99.9%), Co(NO3)36H2O (99.5%) and citric acid (C6H8O7) purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan).
http://dx.doi.org/10.1016/j.apt.2015.06.006 0921-8831/Ó 2015 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006
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S. Mimuro et al. / Advanced Powder Technology xxx (2015) xxx–xxx
Stoichiometric amounts of the nitrates were dissolved in diluted water at 60 °C on the hotplate and mixed at 200 rpm using a magnetic stirrer, and then citric acid in equal molar amount to total metal ions was added to the solution. The sample prepared with the above process is called LS-cit, and the sample with an equal molar amount of oleic acid (C18H34O2, Wako Pure Chemical Industries, Ltd.) also added to LS-cit is called LS-ol. The pH of the solution was adjusted between 7 and 9 using dilute nitric acid and aqueous ammonia. The solution was gradually dried to form the precursor gel. The precursor gel was heated at 130 °C for 1 h and then at 400 °C for 1 h, and finally it was calcined in the range from 400 to 900 °C for 2 h in air. The composition ratio of some of the resulting powders was checked by inductively coupled plasma (ICP) spectroscopy and it was almost consistent with that of the fed raw materials. 2.2. Characterization The samples were analyzed by X-ray diffraction (XRD) for phase identification with Cu Ka radiation of 40 kV/40 mA at room temperature. The lattice parameters were refined by the Rietveld method [10]. The compositions of the samples were confirmed by ICP at room temperature. Raman spectroscopy analysis was performed at room temperature. The particle size distribution and zeta potential of the precursor solution were examined using a dynamic laser scattering (DLS) system. Thermogravimetry–differ ential thermal analysis (TG/DTA) measurements of the precursor powder were performed with temperature in the range from room temperature to 900 °C in air. The crystallite sizes of the LSCF sample powders were observed by transmission electron microscopy (TEM). 3. Results and discussion 3.1. Phase purity Figs. 1 and 2 show the XRD profiles of the LS-ol and LS-cit samples calcined at various temperatures. The simulated profile of single-phase LSCF with rhombohedral symmetry of the space group R3c was calculated using the reported data [11] and is shown at the bottom in Figs. 1 and 2. No secondary phases were observed for the LS-ol sample calcined in the range 500–800 °C, and the XRD pattern is in good agreement with the simulated profile, as shown in Fig. 1. While the sample calcined at 400 °C was not a single phase, the impurity phase was not assigned. The desired composition of
Fig. 2. XRD profiles of LS-cit samples calcined at various temperatures. The impurity phase peaks are indicated by arrows.
La0.6Sr0.4Co0.2Fe0.8O3 was confirmed for the samples calcined at 500–800 °C by ICP. As one of representatives, the results of LS-ol calcined at 800 °C are shown in Table 1. Conversely, a single phase was not obtained for the LS-cit samples even at a high calcination temperature of 900 °C, as shown in Fig. 2. The calculated lattice parameters of the LS-ol samples by the Rietveld method are shown in Fig. 3. They agreed well with the reported values [12,13,18]. In the case of LS-ol calcined at 700 °C, for example, the reliable factors Rwp, Rp and S were 17.765, 13.187 and 1.4169, respectively. Although those values were relatively high to investigate precise crystal parameters, it was enough low for our main objective to judge whether LSCF was successfully synthesized or not by comparing the obtained lattice parameters to the reported ones. From these results, the calcination of precursors prepared by the citrate complex method adding oleic acid enabled the formation of LSCF powders in a single phase even at a low temperature of 500 °C. However, a single phase was not obtained for LS-cit precursors even after calcination at a high temperature of 900 °C, as shown in Fig. 2. We examined the particle size distribution of LS-ol and LS-cit precursors in the solution to explain these results (Fig. 4). The profile of the LS-ol precursor in Fig. 4 shows a single sharp peak, suggesting a small particle size of several tens of nanometers and a narrow size distribution. Conversely, the profile of the LS-cit sample was split into two peaks, suggesting a much larger particle size than LS-ol. These results indicate that oleic acid on the particle surface functioned as a surfactant in the case of LS-ol to promote the dispersibility of the precursor in the solution and helped to attain high homogeneity. We believe that the high dispersibility and homogeneity enabled the formation of single-phase LSCF even at a low calcination temperature of 500 °C. Another possibility is that LSCF crystallization was also assisted by combustion heat from oleic acid during the calcination, as shown in Fig. 5. Table 1 ICP results of LS-ol calcined at 800 °C. A11 data were averaged values for 10 times measurements. Sample weight 10.34 mg.
Concentration (mg l1) R S Cv Molar ratio
Fig. 1. XRD profiles of LS-ol samples calcined at various temperatures.
Fe
Co
Sr
La
40.26 0.65 0.20 0.49 4.38
9.70 0.17 0.06 0.57 1.00
31.36 0.53 0.17 0.54 2.18
72.65 1.13 0.46 0.63 3.18
R – resolution of the each element. S – standard delation of measurements. Cv – coefficient of the variation.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006
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Fig. 5. TG/DTA profile of sodium oleate.
Fig. 3. Lattice parameters of LS-ol samples plotted as a function of calcination temperature.
Fig. 6. Zeta potential of LS-ol and LS-cit precursor samples measured at 25 °C.
Fig. 4. Particle size distribution of LS-ol and LS-cit precursor samples measured at pH 10 and 25 °C.
Fig. 7 shows the dependence of calculated crystallite size on the calcination temperature. The crystallite size almost linearly decreased as the calcination temperature decreased to 500 °C. The crystallite sizes of around 15–20 nm for the samples calcined below 800 °C were much smaller than those of 20–40 nm for samples prepared by combustion syntheses using glycine
We examined the zeta potential of the precursor solutions to investigate the high dispersibility of LS-ol. The LS-ol precursor exhibited a more negative zeta potential than LS-cit at pH > 7, as shown in Fig. 6. This result can be explained by considering the ionization of the side-chain carboxyl group of oleic acid. Because the carboxyl group is deprotonated in alkaline conditions, the precursor is negatively charged at high pH. Ignoring steric repulsion, electrostatic repulsion increased as the negative charge on the nanoparticle surface increased, resulting in the LS-ol precursor being well-dispersed in the solution (pH 10), as shown in Fig. 4. 3.2. Size and morphology The crystallite sizes of the calcined samples were calculated using the XRD results in Fig. 1 and following equation:
D ¼ Kk=B cos h; where D is the volume mean diameter, K is a constant (=1.33), B is the integral width, k is the wavelength (=1.54055 Å), and h is the diffraction angle [19].
Fig. 7. Crystallite size of LS-ol samples plotted as a function of calcination temperature.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006
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(calcined at 850 °C) [14] and EDTA–citrate (calcined at 900 °C) [15], and the polymerization complex process using citric acid and ethylene glycol (calcined at 900 °C) [16]. This is because crystallite growth is suppressed at lower temperatures. Furthermore, oleic acid surrounding LSCF nanoparticles and/or its derived substance around the precursor might interfere with the direct contact of the particles and suppress particle growth during calcination. Fig. 8 shows TEM images of the LS-ol sample calcined at various temperatures. The crystallite sizes were slightly smaller for lower calcination temperatures, and they were much smaller than those of LS-cit (>100 nm, Fig. 9). The results can be explained as follows. In the case of LS-cit, without inhibitor between the LSCF precursor
Fig. 9. TEM image of LS-cit sample calcined at 900 °C.
particles, volume diffusion and particle growth were promoted because of the larger contact area between the particles compared to the LS-ol. The approximate crystallite sizes of LS-ol observed by TEM were around 20 nm, and are consistent with the calculated sizes (Fig. 7). The specific surface areas of LS-ol calcined at 500 and 800 °C were 10.73 and 7.95 m2/g, respectively, which are much larger than the reported values of 3.4–5.4 m2/g [14,16,17]. This result can be attributed to the smaller crystallite sizes, as shown in Figs. 4, 7 and 8. 3.3. TG/DTA Fig. 10 shows the TG/DTA spectra of the LS-ol precursor measured up to 900 °C. The weight loss up to 150 °C was assigned to the dehydration of the absorbed water on the precursors. The broad bands in the range 150–500 °C were assumed to be the decompositions of citric acid [18] and oleic acid, as shown in Fig. 5. The two sharp peaks observed between 500 and 600 °C in Fig. 5 are ascribed to oleic acid decomposition. da Conceição et al. reported that weight loss at around 800 °C can be attributed to LSCF phase formation [18]. However, we confirmed that the LSCF phase was obtained even at a calcination temperature of 500 °C, and suggest that the weight loss at around 800 °C might be because of the burning of residual organic molecules surrounding the nanoparticle surfaces, because a similar weight loss was observed for sodium oleate. Raman spectra of the LS-ol samples calcined at 600–800 °C are shown in Fig. 11. Although the graphite has high
Fig. 8. TEM images of LS-ol samples calcined at (a) 500, (b) 800, and (c) 900 °C.
Fig. 10. TG/DTA profile of LS-ol precursor sample.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006
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References
Fig.11. Raman spectra of LS-ol samples calcined at 600, 700 and 800 °C.
Raman activity, no D band structure around 1350 cm1 reflecting the defects and distortion was observed in each spectrum. From these results, we thought that the amount of residual carbon was surprisingly low in LS-ol samples calcined at 600–800 °C. 4. Conclusions LSCF precursor was synthesized by the citrate complex method adding oleic acid as a surfactant. From the XRD results, we succeeded in obtaining crystalline LSCF powders by calcining the precursor at a temperature as low as 500 °C in air. This temperature is much lower than that required for the crystallization of LSCF powders synthesized by the conventional citrate complex method. DLS results showed that the oleic acids around the precursor nanoparticles improve their dispersibility, enabling the formation of a homogeneous fine precipitate that promotes the crystallization of LSCF at low temperature. The crystallite sizes of the calcined samples were around 20 nm by TEM, which is consistent with the sizes calculated by Scherer’s equation. The specific surface area increased from 7.9 to 10.95 m2/g as the calcination temperature was decreased from 800 to 500 °C. These results indicate that a low calcination temperature suppresses the growth and agglomeration of LSCF particles and enabled the formation of nanosized particles.
[1] H.J. Hwang, J.W. Moon, S. Lee, E.A. Lee, Electrochemical performance of LSCFbased composite cathodes for intermediate temperature SOFCs, J. Power Sources 145 (2005) 243–248. [2] C.C. Chen, M.M. Nasrallah, H.U. Anderson, M.A. Alim, Immittance response of La0.6Sr0.4Co0.2Fe0.8O3 based electrochemical cells, J. Electrochem. Soc. 142 (1995) 491–496. [3] G. DiGiuseppe, L. Sun, Electrochemical performance of a solid oxide fuel cell with an LSCF cathode under different oxygen concentrations, Int. J. Hydrogen Energy 36 (2011) 5076–5087. [4] M. Kumar, S. Srikantha, B. Ravikumar, T.C. Alex, S.K. Das, Synthesis of pure and Sr-doped LaGaO3, LaFeO3 and LaCoO3 and Sr, Mg-doped LaGaO3 for ITSOFC application using different wet chemical routes, Mater. Chem. Phys. 113 (2009) 803–815. [5] Z. Shao, W. Zhou, Z. Zhu, Advanced synthesis of materials for intermediatetemperature solid oxide fuel cells, Prog. Mater Sci. 57 (2012) 804–874. [6] B. Fan, X. Liu, A-deficit LSCF for intermediate temperature solid oxide fuel cells, Solid State Ionics 180 (2009) 973–977. [7] M. Ghouse, Y. Al-Yousef, A. Al-Musa, M.F. Al-Otaibi, Preparation of La0.6Sr0.4Co0.2Fe0.8O3 nanoceramic cathode powders for solid oxide fuel cell (SOFC) application, Int. J. Hydrogen Energy 35 (2010) 9411–9419. [8] L. Baqué, A. Caneiro, M.S. Moreno, Adriana Serquis, High performance nanostructured IT-SOFC cathodes prepared by novel chemical method, Electrochem. Commun. 10 (2008) 1905–1908. [9] T. Taniguchi, K. Katsumata, S. Omata, K. Okada, N. Matsushita, Tuning growth modes of ceria-based nanocubes by a hydrothermal method, Cryst. Growth Des. 11 (9) (2011) 3754–3760. [10] F. Izum, K. Momma, Three-dimensional visualization in powder diffraction, Solid State Phenom. 130 (2007) 15–20. [11] T. Itoh, M. Nakayama, Using in situ X-ray absorption spectroscopy to study the local structure and oxygen ion conduction mechanism in (La0.6Sr0.4)(Co0.2Fe0.8)O3d, J. Solid State Chem. 192 (2012) 38–46. [12] A. Dutta, J. Mukhopadhyay, R.N. Basu, Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells, J. Eur. Ceram. Soc. 29 (2009) 2003–2011. [13] S. Hashimoto, Y. Fukuda, M. Kuhn, K. Sato, K. Yashiro, J. Mizusaki, Oxygen nonstoichiometry and thermo-chemical stability of La0.6Sr0.4Co1yFeyO3d (y = 0.2, 0.4, 0.6, 0.8), Solid State Ionics 181 (2010) 1713–1719. [14] M. Zawadzki, H. Grabowska, J. Trawczyn´ski, Effect of synthesis method of LSCF perovskite on its catalytic properties for phenol methylation, Solid State Ionics 181 (2010) 1131–1139. [15] C. Nityanand, W.B. Nalin, B.S. Rajkumar, C.M. Chandr, Synthesis and physicochemical characterization of nanocrystalline cobalt doped lanthanum strontium ferrite, Solid State Sci. 13 (2011) 1022–1030. [16] L. Ge, Z. Zhu, Z. Shao, S. Wang, S. Liu, Effects of preparation methods on the oxygen nonstoichiometry, B-site cation valences and catalytic efficiency of perovskite La0.6Sr0.4Co0.2Fe0.8O3d, Ceram. Int. 35 (2009) 3201–3206. [17] M. Zawadzki, J. Trawczyn´ski, Synthesis, characterization and catalytic performance of LSCF perovskite for VOC combustion, Catal. Today 176 (2011) 449–452. [18] L. Conceição, A.M. Silva, N.F.P. Ribeiro, M.M.V.M. Souza, Combustion synthesis of La0.7Sr0.3Co0.5Fe0.5O3 (LSCF) porous materials for application as cathode in IT-SOFC, Mater. Res. Bull. 46 (2011) 308–314. [19] T. Ida, S. Shimazaki, H. Hibino, H. Toraya, Diffraction peak profiles from spherical crystallites with lognormal size distribution, J. Appl. Crystallogr. 36 (2003) 1107–1115.
Please cite this article in press as: S. Mimuro et al., Low-temperature synthesis of lanthanum strontium cobalt ferrite by the citrate complex method adding oleic acid, Advanced Powder Technology (2015), http://dx.doi.org/10.1016/j.apt.2015.06.006