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Effect of different solvents in the synthesis of LaCoO3 nanopowders prepared by the co-precipitation method
Advanced Powder Technology 25 (2014) 1834–1838
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Effect of different solvents in the synthesis of LaCoO3 nanopowders prepared by the co-precipitation method J. Chandradass a,⇑, Hern Kim b,⇑, Francis W.Y. Momade c a
Center for Nanotechnology, PRIST University, Trichy Campus, Trichy 620 009, India Energy and Environment Fusion Technology Center, Department of Environmental Engineering and Biotechnology, Myongji University, Yongin, Kyonggi-do 449-728, Republic of Korea c Department of Materials Engineering, Kwame Nkrumah University of Science and Technology (KNUST), Kumasi, Ghana b
a r t i c l e
i n f o
Article history: Received 28 June 2012 Received in revised form 26 October 2013 Accepted 26 July 2014 Available online 5 August 2014 Keywords: Nanopowders Chemical synthesis LaCoO3 Electron microscopy Magnetic properties
a b s t r a c t This paper describes the obtention of LaCoO3 nanopowders by the co-precipitate method with ammonium hydroxide from solvent such as water, ethyl alcohol and ethylene glycol. The crystalline structure and average particle size are dependent of type-solvent. The XRD patterns indicated that the LaCoO3 nanopowders prepared with water and ethyl alcohol exhibit a pure perovskite-type LaCoO3 in the rhombohedra structure. The average diameter of the particles prepared with ethyl alcohol and water are 27 ± 4.49 nm and 64.4 ± 12.92 nm respectively. High resolution transmission electron microcopy revealed an oriented attachment mechanism for the growth of aggregated LaCoO3 nanocrystals. Room temperature magnetization results of the heat treated LaCoO3 nanopowders exhibited a paramagnetic behavior. The average particle size and formation temperature of LaCoO3 obtained in this study is comparatively lower than those reported in the literature. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Lanthanum cobaltite (LaCoO3-) perovskite-type oxides are attractive materials due to their potential applications as electrode material in intermediate temperature solid oxide fuel cells [1,2] and cathode catalyst for membranous fuel cell [3]. LaCoO3 exhibit interesting electrical, electro catalytic properties, high electronic conductivity, good ionic conductivity [4], unique magnetic transitions from nonmagnetic to paramagnetic behavior at about 100 K and a broad transition at around 500 K accompanied by an insulator-metal transition [5]. The physical-chemistry properties of this material are strongly dependent on the preparation method which affects its numerous applications. Several methods have been reported for the synthesis of LaCoO3 ultrafine powders, such as: reactive grinding [6], microwave-assisted solid-state decomposition [4], sol–gel method [5], co-precipitation [7,8], combustion [9] and pechini-type polymerizable complex [10,11]. In terms of simplicity of the synthesis, co-precipitation is the preferred route. Aqueous media are mostly used in the synthesis of LaCoO3 oxide, since water as solvent presents special characteristics such as high ⇑ Corresponding authors. Tel.: +82 31 330 6688; fax: +82 31 336 6336. E-mail addresses: [email protected] (J. Chandradass), [email protected] (H. Kim).
dielectric constant and permanent polarization [12,13]. Depending on the experimental conditions, alcohols may behave as weak acids or weak bases, with ethanol being the most important amphoteric solvent. As a weak base, it is able to accept protons from strong acids in reversible ways, producing (ROH2)+. As weak acids (ethanol Ka (acidity constant) = 10 19) alcohols dissociate slightly producing alcoxide ions (RO ). Another reason for alcohol usage as solvents is its low dielectric constant that facilitates the production of ions in low concentrations [14]. Ethylene glycol acts as a barrier for the fast movement of solutes due to its viscous nature and form a loosely-bound paint on the individual particle surface which could perform as nano-scaled reactors [15–17]. The synthesis of LaCoO3 by co-precipitation method, characterization of these powders and the effect of small amount of surfactant on the morphological change from spherical to rod shaped particles have been previously reported [7,8]. Although some work have been published on the preparation of LaCoO3 nanopowders, to the best of our knowledge none of these works have been devoted to the influence of solvent on the synthesis of LaCoO3 nanopowders by co-precipitation route. Therefore, in this work, we report the preparation of LaCoO3- nanopowders by the co-precipitation method and study the effect of different solvents in structure and average grain size. The solvents used were water (er = 78.5), ethanol (er = 24.3) and ethylene glycol (EG) (er = 37) [18]. The
http://dx.doi.org/10.1016/j.apt.2014.07.014 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
J. Chandradass et al. / Advanced Powder Technology 25 (2014) 1834–1838
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presence of solvent in co-precipitation route carries great importance because it plays a vital role in controlling the phase and size of the crystal. 2. Experimental details 2.1. Chemicals All the reagents and solvents used were analytical grate (Aldrich Co.) and were used without any further purification. 2.2. Preparation of LaCoO3 nanopowders In the synthesis, 0.04 M of lanthanum (III) nitrate hydrate (Sigma Aldrich, 99.99%) and 0.04 M cobalt (II) nitrate hexahydrate (Sigma Aldrich, 98%) were dissolved in 20 ml of different solvents (water, ethyl alcohol and ethylene glycol (99.8%, Sigma Adrich) to prepare a precursor solution. The ammonium hydroxide (Sigma Aldrich, 33%) was added to the precursor solution with constant stirring at room temperature. The final pH of the solution was adjusted to 9 by using NH4OH. The resulting solution was stirred continuously for 2 h with a magnetic stirrer. The precipitate was separated by centrifugation and washed several times with ethanol to remove the unreacted molecules. Subsequently the precipitate was dried at 80 °C for 48 h and calcined at 600 °C for 2 h to obtain crystalline phase.
Fig. 1. X-ray diffraction patterns of LaCoO3 nanopowders prepared with water as solvent and calcined at different temperature for 2 h (a) 400 °C; (b) 500 °C; (c) 600 °C; (d) 700 °C; (d) 800 °C.
2.3. Characterization The powder X-ray diffraction (XRD) patterns were recorded by X-ray diffractometer (Panalytical X’pert MPD). The transmission electron microscopy was taken with JEOL-JEM-3010 operating at 300 kV. Samples for transmission electron microscopy (TEM) and high resolution (HT)-TEM were prepared by dispersing a small amount of powder into ethanol solvent and sonicated for 10 min. The diluted suspension of the sample powders was dropped on to a standard carbon coated copper grid. The average size of the particle estimated from the TEM micrographs using standard software (IMAGE J). The magnetic measurements were conducted with a superconducting quantum interference device magnetometer (Quantum design, MPMS XL-7).
Fig. 2. X-ray diffraction patterns of LaCoO3 nanopowders prepared with different solvent and calcined at 600 °C for 2 h (a) water; (b) ethanol; (c) ethylene glycol.
3. Results and discussion The presence of diffraction peaks can be used to evaluate the structural order at long range or periodicity of the material [19]. Fig. 1 illustrates the XRD patterns of LaCoO3 nanopowders heat treated at different temperatures. The as-synthesized powders are structurally disordered or amorphous calcined at 400 °C. The characteristic peaks of LaCoO3 appeared at 500 °C with low intensity, which signified the transformation of amorphous structure to LaCoO3 crystalline structure. Heating the precursor to 600 °C yields well crystallized pure perovskite-type LaCoO3 (JCPDS: 48-0123), in the rhombohedra structure. It is obvious that the width and intensity of the diffraction peaks changed markedly with calcinations temperature; the crystallite size became larger and the crystallinity became better with increasing calcinations temperature. Fig. 2 shows the X-ray diffraction pattern of the powders prepared by different solvents at 600 °C. It is seen that the XRD pattern of samples prepared with water and ethanol show the formation of a single phase and perfectly match with the JCPDS X-ray pattern file (48-0123 for LaCoO3) demonstrating the formation of perovskitetype structure. On the other hand with ethylene glycol as solvent, pure Co3O4 intermediate phase is present. As reported elsewhere [15], ethylene glycol acts as a barrier due to its viscous nature
and form a loosely bound paint around each individual particle which may inhibit the phase transformation to LaCoO3 at low temperature (600 °C). Armelao et al. [20] prepared LaCoO3 by two process namely combustion synthesis and molten chloride flux process at 800 °C for 17 h and 950 °C for 24 h respectively. Ivanova et al. [21] have also successfully synthesized LaCoO3 powders by two techniques: (i) pechini route from the aqueous solution of citric acid to a suspension of CoCO3 in aqueous solution of La(NO3)3.6H2O and (ii) conventional solid state reaction between La2O3 and CoCO3. In the pechini process, pure LaCoO3 phase was obtained by calcining at 600 °C for 20 h whereas in conventional solid state process, pure LaCoO3 was obtained by annealing at 800 °C for 20 h. Gaki et al. [11] pointed that the polymerizable complex method can be successfully applied for the preparation of LaCoO3. The formation requires 3 h calcination at 600 °C. Zhou et al. [5] prepared pure LaCoO3 by sol–gel process at 600 °C for 6 h. Fig. 3 shows the TEM images of LaCoO3 powders prepared with water and ethanol solvent. The particles prepared from water consist of large agglomerates of average size 64.4 ± 12.92 nm. The agglomeration of particles depends on the processing condition and calcinations temperatures. The presence of agglomerated particles is detrimental to the final state of sintering and introduces
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J. Chandradass et al. / Advanced Powder Technology 25 (2014) 1834–1838
Fig. 3. TEM micrographs and particle size distribution of LaCoO3 nanopowders prepared with different solvent and calcined at 600 °C for 2 h (a) water; (b) ethanol.
Table 1 Comparison of LaCoO3 particle size and formation temperature. Methods
Particle size (TEM)
Formation temperature (°C)
References
Pechini Water based sol–gel method Freeze dried Glycine nitrate combustion
80 nm 50 nm 65–75 Not mentioned Not mentioned 31–60 nm Not mentioned Not mentioned 80 nm Not mentioned Not mentioned 60–450 nm Not mentioned 25 nm
900 °C 600 °C 600 °C 900 °C
[16] [20] [16] [2]
Coprecipitation Aqueous gel casting Polymer precursor Mechanochemical Low temperature route Combustion using a-alanine fuel Combustion using urea fuel Sol–gel method Combustion using citric acid fuel Surfactant assisted coprecipitation Molten salt method Sol–gel method
Fig. 4. (a) HR-TEM images of LaCoO3 nanopowders calcined at 600 °C for 2 h; (b) HR-TEM showing OA attachment between two nanocrystals.
Co-precipitation (ethanol solvent)
Not mentioned Not mentioned 27 nm
(20 h) (6 h) (20 h) (3 h)
900 °C (3 h)
[2]
650 °C (3 h) 600 °C (3 h)
[1] [11]
600 °C (1 h)
[21]
450 °C (1 h) 600 °C (3 h)
[7] [9]
800 °C (3 h)
[9]
600 °C (6 h) 600 °C (6 h)
[5] [23]
600 °C (2 h)
[8]
500–850 °C (2 h) 600 °C (2 h)
[24]
600 °C (2 h)
Present work
[3]
J. Chandradass et al. / Advanced Powder Technology 25 (2014) 1834–1838
Fig. 5. SAED pattern of LaCoO3 nanopowders calcined at 600 °C for 2 h.
heterogeneities in the microstructure of the sintered ceramics that cannot be eliminated readily. These agglomerated particles act as defect centers [22] and therefore the agglomerates need to be eliminated by grinding. The wet grinding process is effective in reducing agglomeration [2], whereas for the nanoparticles obtained from the ethanol solvent possess an average size of 27 ± 4.49 nm. The size obtained from XRD is generally larger than the size obtained from TEM because nanocrystals have grown by an oriented attachment (OA) process where large nanostructures are formed from adjacent small nanostructures (Fig. 4(b) [19]. The particle size determined from TEM in the present study is compared with other methods reported in the literature (Table 1). The particle size obtained in this study (ethanol as solvent) is smaller than those reported in the literature. The low dielectric constant
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of ethanol can alter the thermodynamics of reaction system and nucleation kinetics, which would result in reducing the particle size and size distribution of resulting particles [25]. Though the formation temperature of low temperature route reported by Jadhav et al. [7] was 450 °C, the particle size was too high (80 nm) than our study. Junwu et al. [8] reported spherical nanoparticles with an average diameter of 25 nm were obtained by co-precipitation method by adding small amount of surfactant. However, the particles were highly agglomerated. HRTEM micrograph (Fig. 4) shows clear lattice fringes, which allowed the identification of crystallographic spacing and indicated the prepared perovskite LaCoO3 nanocrystals was well crystalline. The lattice spacing of 0.272 nm were corresponding to the crystallographic plane (1 1 0) of the rhombohedral LaCoO3 structure. The lattice spacing of crystallographic plane (1 1 0) were calculated to be 0.275 nm, which matched the standard value (0.272 nm). The corresponding SAED patterns (Fig. 5) shows spotty ring patterns revealing their crystalline perovskite structure. Measured interplanar spacing (dhkl) from SAED patterns shown in Fig. 6 are in good agreement with the values in the standard data (JCPDS: 48-0123). The diffraction rings are identified as the (1 1 0), (2 0 2) (0 2 4) and (2 1 4) planes. This concurs with the result of XRD presented in Fig. 1. Fig. 6 shows the magnetization versus applied field curve for LaCoO3 nanocrystals synthesized using water and ethanol solvent. It exhibits hysteresis behavior indicating ferromagnetic property at room temperature (300 K). It can be seen that the saturation magnetization value of LaCoO3 are 1.134 emu/g (Water) and 0.189 emu/g (ethanol solvent) respectively. As reported elsewhere [26] saturation magnetization increases with increase in particle size. The particle size of LaCoO3 nanoparticles synthesized using water as solvent is larger than using ethanol solvent. Hence the saturation magnetization value of LaCoO3 nanocrystals synthesized using water solvent is larger than other solvent. 4. Conclusion The study demonstrates that ethyl alcohol could be used as a solvent for the production of LaCoO3 nanocrystals of fine particle size (27 ± 4.49 nm) compared to those reported in the literature. This method is very practical, economical, environmental friendly, because it involves inexpensive and less toxic metal salts and organic solvent. XRD results show the formation of rhombohedral structure at 600 °C. HR-TEM images revealed that LaCoO3 nanocrystals grows by oriented attached mechanism. The resultant nanocrystalline LaCoO3 exhibited ferromagnetic behavior at room temperature. Acknowledgements This study was supported by National Research Foundation of Korea (NRF) – Grants funded by the Ministry of Science, ICT and Future Planning (2014R1A2A2A01004352) and the Ministry of Education (2009-0093816), Republic of Korea. References
Fig. 6. Magnetization curves of LaCoO3 nanopowders prepared with different solvent and calcined at 600 °C for 2 h (a) water; (b) ethanol. Inset shows zoom part of central magnetization curve.
[1] C.S. Cheng, L. Zhang, Y.J. Zhang, S.P. Jiang, Solid State Ion. 179 (2008) 282. [2] M. Kumar, S. Srikanth, B. Ravikumar, T.C. Alex, S.K. Das, Mater. Chem. Phys. 113 (2009) 803. [3] Y. Liu, J. Ma, J. Lai, Y. Liu, J. Alloys Compd. 488 (2009) 204. [4] S. Farhadi, S. Sepahvand, J. Alloys Compd. 489 (2010) 586. [5] S. Zhou, L. He, S. Zhao, Y. Guo, J. Zhao, L. Shi, J. Phys. Chem. C 113 (2009) 13522. [6] L. Huang, M. Bassir, S. Kaliaguine, Mater. Chem. Phys. 101 (2007) 259. [7] A.D. Jadhav, A.B. Gaikwad, V. Samuel, V. Ravi, Mater. Lett. 61 (2007) 2030. [8] Z. Junwu, S. Xiaojie, W. Yanping, W. Xin, Y. Xujie, L. Lude, J. Rare Earths 25 (2007) 601. [9] D. Berger, C. Matei, F. Papa, G. Voicu, V. Fruth, Prog. Solid State Ch. 35 (2007) 183.
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[10] M. Popa, J.M. Calderon-Moreno, J. Eur. Ceram. Soc. 29 (2009) 2281. [11] A. Gaki, O. Anagnostaki, D. Kioupis, T. Perraki, D. Gakis, G. Kakali, J. Alloys Compd. 451 (2008) 305. [12] W. Stumn, J.J. Morgan, Aquatic Chemistry, John Wiley, New York, 1999. [13] J. Jolivet, Metal Oxide Chemistry and Synthesis, John Wiley, New York, 2000. [14] Encyclopedia of Science and Technology, vol. 5, McGraw-Hill, USA, 1960. [15] G. Murugadoss, B. Rajamannan, V. Ramasamy, G. Viruthagiri, J. Ovonic Res. 5 (2009) 107. [16] V.S. Marques, L.S. Cavalcante, J.C. Sczancoski, A.F.P. Alcantara, M.O. Orlandi, E. Moraes, E. Longo, J.A. Varela, M. Siu Li, M.R.M.C. Santos, Cryst. Growth Des. 10 (2012) 4752. [17] M.A.P. Almeida, L.S. Cavalcante, C. Morilla-Santos, C.J. Dalmaschio, S. Rajagopal, M. Siu Li, E. Longo, Cryst. Eng. Commun. 14 (2012) 7127–7132. [18] Dielectric Constant Chart,.
[19] L.S. Cavalcante, V.S. Marques, J.C. Sczancoski, M.T. Escote, M.R. Joya, J.A. Varela, M.R.M.C. Santos, P.S. Pizani, E. Longo, Chem. Eng. J. 143 (2008) 299–307. [20] L. Armelao, G. Bandoli, D. Barreca, M. Bettinelli, G. Bottaro, A. Caneschi, Surf. Interface Anal. 34 (2002) 112. [21] S. Ivanova, A. Senyshyn, E. Zhecheva, K. Tenchev, V. Nikolov, R. Stoyanova, H. Fuess, J. Alloys Compd. 480 (2009) 279. [22] S. Ramanathan, M.B. Kalkade, S.K. Roy, K.K. Kutty, Ceram. Int. 29 (2003) 477. [23] S. Ayyappan, S. Mahadevan, P. Chandramohan, M.P. Srinivasan, J. Philip, Baldev Raj, J. Phys. Chem. C 114 (2010) 6334. [24] L. Predoana, B. Malic, M. Kosec, M. Carata, M. Caldararu, M. Zaharescu, J. Eur. Ceram. Soc. 27 (2007) 4407. [25] H.-I. Chen, H.-Y. Chang, Colloids Surf., A: Physicochem. Eng. Aspects 242 (2004) 61. [26] S. Maensiri, M. Sangmanee, A. Wiengmoon, Nanoscale Res. Lett. 4 (2009) 221.
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Effect of different solvents in the synthesis of LaCoO3 nanopowders prepared by the co-precipitation method J. Chandradass a,⇑, Hern Kim b,⇑, Francis W.Y. Momade c a
Center for Nanotechnology, PRIST University, Trichy Campus, Trichy 620 009, India Energy and Environment Fusion Technology Center, Department of Environmental Engineering and Biotechnology, Myongji University, Yongin, Kyonggi-do 449-728, Republic of Korea c Department of Materials Engineering, Kwame Nkrumah University of Science and Technology (KNUST), Kumasi, Ghana b
a r t i c l e
i n f o
Article history: Received 28 June 2012 Received in revised form 26 October 2013 Accepted 26 July 2014 Available online 5 August 2014 Keywords: Nanopowders Chemical synthesis LaCoO3 Electron microscopy Magnetic properties
a b s t r a c t This paper describes the obtention of LaCoO3 nanopowders by the co-precipitate method with ammonium hydroxide from solvent such as water, ethyl alcohol and ethylene glycol. The crystalline structure and average particle size are dependent of type-solvent. The XRD patterns indicated that the LaCoO3 nanopowders prepared with water and ethyl alcohol exhibit a pure perovskite-type LaCoO3 in the rhombohedra structure. The average diameter of the particles prepared with ethyl alcohol and water are 27 ± 4.49 nm and 64.4 ± 12.92 nm respectively. High resolution transmission electron microcopy revealed an oriented attachment mechanism for the growth of aggregated LaCoO3 nanocrystals. Room temperature magnetization results of the heat treated LaCoO3 nanopowders exhibited a paramagnetic behavior. The average particle size and formation temperature of LaCoO3 obtained in this study is comparatively lower than those reported in the literature. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Lanthanum cobaltite (LaCoO3-
dielectric constant and permanent polarization [12,13]. Depending on the experimental conditions, alcohols may behave as weak acids or weak bases, with ethanol being the most important amphoteric solvent. As a weak base, it is able to accept protons from strong acids in reversible ways, producing (ROH2)+. As weak acids (ethanol Ka (acidity constant) = 10 19) alcohols dissociate slightly producing alcoxide ions (RO ). Another reason for alcohol usage as solvents is its low dielectric constant that facilitates the production of ions in low concentrations [14]. Ethylene glycol acts as a barrier for the fast movement of solutes due to its viscous nature and form a loosely-bound paint on the individual particle surface which could perform as nano-scaled reactors [15–17]. The synthesis of LaCoO3 by co-precipitation method, characterization of these powders and the effect of small amount of surfactant on the morphological change from spherical to rod shaped particles have been previously reported [7,8]. Although some work have been published on the preparation of LaCoO3 nanopowders, to the best of our knowledge none of these works have been devoted to the influence of solvent on the synthesis of LaCoO3 nanopowders by co-precipitation route. Therefore, in this work, we report the preparation of LaCoO3-
http://dx.doi.org/10.1016/j.apt.2014.07.014 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
J. Chandradass et al. / Advanced Powder Technology 25 (2014) 1834–1838
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presence of solvent in co-precipitation route carries great importance because it plays a vital role in controlling the phase and size of the crystal. 2. Experimental details 2.1. Chemicals All the reagents and solvents used were analytical grate (Aldrich Co.) and were used without any further purification. 2.2. Preparation of LaCoO3 nanopowders In the synthesis, 0.04 M of lanthanum (III) nitrate hydrate (Sigma Aldrich, 99.99%) and 0.04 M cobalt (II) nitrate hexahydrate (Sigma Aldrich, 98%) were dissolved in 20 ml of different solvents (water, ethyl alcohol and ethylene glycol (99.8%, Sigma Adrich) to prepare a precursor solution. The ammonium hydroxide (Sigma Aldrich, 33%) was added to the precursor solution with constant stirring at room temperature. The final pH of the solution was adjusted to 9 by using NH4OH. The resulting solution was stirred continuously for 2 h with a magnetic stirrer. The precipitate was separated by centrifugation and washed several times with ethanol to remove the unreacted molecules. Subsequently the precipitate was dried at 80 °C for 48 h and calcined at 600 °C for 2 h to obtain crystalline phase.
Fig. 1. X-ray diffraction patterns of LaCoO3 nanopowders prepared with water as solvent and calcined at different temperature for 2 h (a) 400 °C; (b) 500 °C; (c) 600 °C; (d) 700 °C; (d) 800 °C.
2.3. Characterization The powder X-ray diffraction (XRD) patterns were recorded by X-ray diffractometer (Panalytical X’pert MPD). The transmission electron microscopy was taken with JEOL-JEM-3010 operating at 300 kV. Samples for transmission electron microscopy (TEM) and high resolution (HT)-TEM were prepared by dispersing a small amount of powder into ethanol solvent and sonicated for 10 min. The diluted suspension of the sample powders was dropped on to a standard carbon coated copper grid. The average size of the particle estimated from the TEM micrographs using standard software (IMAGE J). The magnetic measurements were conducted with a superconducting quantum interference device magnetometer (Quantum design, MPMS XL-7).
Fig. 2. X-ray diffraction patterns of LaCoO3 nanopowders prepared with different solvent and calcined at 600 °C for 2 h (a) water; (b) ethanol; (c) ethylene glycol.
3. Results and discussion The presence of diffraction peaks can be used to evaluate the structural order at long range or periodicity of the material [19]. Fig. 1 illustrates the XRD patterns of LaCoO3 nanopowders heat treated at different temperatures. The as-synthesized powders are structurally disordered or amorphous calcined at 400 °C. The characteristic peaks of LaCoO3 appeared at 500 °C with low intensity, which signified the transformation of amorphous structure to LaCoO3 crystalline structure. Heating the precursor to 600 °C yields well crystallized pure perovskite-type LaCoO3 (JCPDS: 48-0123), in the rhombohedra structure. It is obvious that the width and intensity of the diffraction peaks changed markedly with calcinations temperature; the crystallite size became larger and the crystallinity became better with increasing calcinations temperature. Fig. 2 shows the X-ray diffraction pattern of the powders prepared by different solvents at 600 °C. It is seen that the XRD pattern of samples prepared with water and ethanol show the formation of a single phase and perfectly match with the JCPDS X-ray pattern file (48-0123 for LaCoO3) demonstrating the formation of perovskitetype structure. On the other hand with ethylene glycol as solvent, pure Co3O4 intermediate phase is present. As reported elsewhere [15], ethylene glycol acts as a barrier due to its viscous nature
and form a loosely bound paint around each individual particle which may inhibit the phase transformation to LaCoO3 at low temperature (600 °C). Armelao et al. [20] prepared LaCoO3 by two process namely combustion synthesis and molten chloride flux process at 800 °C for 17 h and 950 °C for 24 h respectively. Ivanova et al. [21] have also successfully synthesized LaCoO3 powders by two techniques: (i) pechini route from the aqueous solution of citric acid to a suspension of CoCO3 in aqueous solution of La(NO3)3.6H2O and (ii) conventional solid state reaction between La2O3 and CoCO3. In the pechini process, pure LaCoO3 phase was obtained by calcining at 600 °C for 20 h whereas in conventional solid state process, pure LaCoO3 was obtained by annealing at 800 °C for 20 h. Gaki et al. [11] pointed that the polymerizable complex method can be successfully applied for the preparation of LaCoO3. The formation requires 3 h calcination at 600 °C. Zhou et al. [5] prepared pure LaCoO3 by sol–gel process at 600 °C for 6 h. Fig. 3 shows the TEM images of LaCoO3 powders prepared with water and ethanol solvent. The particles prepared from water consist of large agglomerates of average size 64.4 ± 12.92 nm. The agglomeration of particles depends on the processing condition and calcinations temperatures. The presence of agglomerated particles is detrimental to the final state of sintering and introduces
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J. Chandradass et al. / Advanced Powder Technology 25 (2014) 1834–1838
Fig. 3. TEM micrographs and particle size distribution of LaCoO3 nanopowders prepared with different solvent and calcined at 600 °C for 2 h (a) water; (b) ethanol.
Table 1 Comparison of LaCoO3 particle size and formation temperature. Methods
Particle size (TEM)
Formation temperature (°C)
References
Pechini Water based sol–gel method Freeze dried Glycine nitrate combustion
80 nm 50 nm 65–75 Not mentioned Not mentioned 31–60 nm Not mentioned Not mentioned 80 nm Not mentioned Not mentioned 60–450 nm Not mentioned 25 nm
900 °C 600 °C 600 °C 900 °C
[16] [20] [16] [2]
Coprecipitation Aqueous gel casting Polymer precursor Mechanochemical Low temperature route Combustion using a-alanine fuel Combustion using urea fuel Sol–gel method Combustion using citric acid fuel Surfactant assisted coprecipitation Molten salt method Sol–gel method
Fig. 4. (a) HR-TEM images of LaCoO3 nanopowders calcined at 600 °C for 2 h; (b) HR-TEM showing OA attachment between two nanocrystals.
Co-precipitation (ethanol solvent)
Not mentioned Not mentioned 27 nm
(20 h) (6 h) (20 h) (3 h)
900 °C (3 h)
[2]
650 °C (3 h) 600 °C (3 h)
[1] [11]
600 °C (1 h)
[21]
450 °C (1 h) 600 °C (3 h)
[7] [9]
800 °C (3 h)
[9]
600 °C (6 h) 600 °C (6 h)
[5] [23]
600 °C (2 h)
[8]
500–850 °C (2 h) 600 °C (2 h)
[24]
600 °C (2 h)
Present work
[3]
J. Chandradass et al. / Advanced Powder Technology 25 (2014) 1834–1838
Fig. 5. SAED pattern of LaCoO3 nanopowders calcined at 600 °C for 2 h.
heterogeneities in the microstructure of the sintered ceramics that cannot be eliminated readily. These agglomerated particles act as defect centers [22] and therefore the agglomerates need to be eliminated by grinding. The wet grinding process is effective in reducing agglomeration [2], whereas for the nanoparticles obtained from the ethanol solvent possess an average size of 27 ± 4.49 nm. The size obtained from XRD is generally larger than the size obtained from TEM because nanocrystals have grown by an oriented attachment (OA) process where large nanostructures are formed from adjacent small nanostructures (Fig. 4(b) [19]. The particle size determined from TEM in the present study is compared with other methods reported in the literature (Table 1). The particle size obtained in this study (ethanol as solvent) is smaller than those reported in the literature. The low dielectric constant
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of ethanol can alter the thermodynamics of reaction system and nucleation kinetics, which would result in reducing the particle size and size distribution of resulting particles [25]. Though the formation temperature of low temperature route reported by Jadhav et al. [7] was 450 °C, the particle size was too high (80 nm) than our study. Junwu et al. [8] reported spherical nanoparticles with an average diameter of 25 nm were obtained by co-precipitation method by adding small amount of surfactant. However, the particles were highly agglomerated. HRTEM micrograph (Fig. 4) shows clear lattice fringes, which allowed the identification of crystallographic spacing and indicated the prepared perovskite LaCoO3 nanocrystals was well crystalline. The lattice spacing of 0.272 nm were corresponding to the crystallographic plane (1 1 0) of the rhombohedral LaCoO3 structure. The lattice spacing of crystallographic plane (1 1 0) were calculated to be 0.275 nm, which matched the standard value (0.272 nm). The corresponding SAED patterns (Fig. 5) shows spotty ring patterns revealing their crystalline perovskite structure. Measured interplanar spacing (dhkl) from SAED patterns shown in Fig. 6 are in good agreement with the values in the standard data (JCPDS: 48-0123). The diffraction rings are identified as the (1 1 0), (2 0 2) (0 2 4) and (2 1 4) planes. This concurs with the result of XRD presented in Fig. 1. Fig. 6 shows the magnetization versus applied field curve for LaCoO3 nanocrystals synthesized using water and ethanol solvent. It exhibits hysteresis behavior indicating ferromagnetic property at room temperature (300 K). It can be seen that the saturation magnetization value of LaCoO3 are 1.134 emu/g (Water) and 0.189 emu/g (ethanol solvent) respectively. As reported elsewhere [26] saturation magnetization increases with increase in particle size. The particle size of LaCoO3 nanoparticles synthesized using water as solvent is larger than using ethanol solvent. Hence the saturation magnetization value of LaCoO3 nanocrystals synthesized using water solvent is larger than other solvent. 4. Conclusion The study demonstrates that ethyl alcohol could be used as a solvent for the production of LaCoO3 nanocrystals of fine particle size (27 ± 4.49 nm) compared to those reported in the literature. This method is very practical, economical, environmental friendly, because it involves inexpensive and less toxic metal salts and organic solvent. XRD results show the formation of rhombohedral structure at 600 °C. HR-TEM images revealed that LaCoO3 nanocrystals grows by oriented attached mechanism. The resultant nanocrystalline LaCoO3 exhibited ferromagnetic behavior at room temperature. Acknowledgements This study was supported by National Research Foundation of Korea (NRF) – Grants funded by the Ministry of Science, ICT and Future Planning (2014R1A2A2A01004352) and the Ministry of Education (2009-0093816), Republic of Korea. References
Fig. 6. Magnetization curves of LaCoO3 nanopowders prepared with different solvent and calcined at 600 °C for 2 h (a) water; (b) ethanol. Inset shows zoom part of central magnetization curve.
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