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Mixture of fuels approach for the solution combustion synthesis of LaAlO3 nanopowders
Advanced Powder Technology 21 (2010) 100–105
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Mixture of fuels approach for the solution combustion synthesis of LaAlO3 nanopowders J. Chandradass *, Ki Hyeon Kim * Department of Physics, Yeungnam University, Gyeongsan, Gyeongsangbuk-do 712-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 May 2009 Received in revised form 7 August 2009 Accepted 30 October 2009
Keywords: Perovskite Chemical synthesis Characterization Crystallite size
a b s t r a c t A modified solution combustion approach was used in the preparation of nanosize LaAlO3 (23.6 nm) using mixture of citric acid and oxalic acid as fuels with corresponding metal nitrates. The synthesized and calcined powders were characterized by Fourier transform infra red spectrometry (FTIR), Differential thermal analysis-Thermogravimetry analysis (DTA–TGA), X-ray diffractometry (XRD) and Transmission electron microscopy (TEM). The FTIR spectra show the lower frequency bands at 656 and 442 cm 1corresponds to metal–oxygen bonds (possible La–O and Al–O stretching frequencies) vibrations for the perovskite structure compound. DTA confirms the formation temperature of LaAlO3 varies between 830–835 °C. XRD results show that mixture of fuels ratio is influential on the crystallite size of the resultant powders. The average particle size of LaAlO3-1 as determined from TEM was about 41 nm, whereas for LaAlO3-2 and LaAlO3-3 samples, particles are seriously aggregated. Ó 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction LaAlO3 with a perovskite-type structure is drawing considerable attention with applications in various fields, such as dielectric resonators [1], substrate material for thin-film high-temperature superconductors [2], catalyst for oxidative coupling of methane and hydrogenation and hydrogenolysis of hydrocarbons [3] electrolyte material for solid oxide fuel cells [4]. Traditionally, LaAlO3 has been prepared by conventional solid-state reaction of Al2O3 and La2O3 in the temperature range 1500–1700 °C [5,6]. This conventional solid-state reaction process suffers from many inherent shortcomings, such as high reaction temperature, large particle size, limited chemical homogeneity and low sinterability, which have a detrimental effect on the microwave dielectric properties. Among the large number of techniques employed for the synthesis of LaAlO3, solution combustion synthesis is unique and highly versatile [7]. A very wide variety of materials ranging from high-Tc superconductors to insulators has been prepared by solution combustion method [8]. It is an easy and fast process, which yields high-purity, homogenous, crystalline oxides in a short time and with less energy [8,9]. The surface area, size-distribution and agglomeration of the particles in the final product mainly depend on the flame-temperature, which in turn is related to the nature of fuel and fuel to-oxidant ratio. Among the various control param-
* Corresponding authors. Tel.: +82 53 810 2334; fax: +82 53 810 4616. E-mail addresses: [email protected] (J. Chandradass), [email protected] (K.H. Kim).
eters in a combustion process, fuel plays an important role in determining the morphology, phase and particulate properties of the final product [10]. LaAlO3 has been synthesized by combustion technique using various fuels such as citric acid [11], urea [12]; polyvinyl alcohol [13]; Ethylenediaminetetracetic [14]; triethanolamine [15]; glycine [16] and oleic acid [17]. Aruna and Rajam, first proposed and illustrated the proof of concept of the mixture of fuels approach for the synthesis of alumina and zirconia toughened alumina [18] and CeO2–CeAlO3 system [10]. This approach facilitated the reduction of particle size compared to the product formed using single fuel. In conclusion, the mixture of fuels approach gives a good control over the flame-temperature [18]. To the best of our knowledge, reports related to the use of mixture of fuels (using commercially available chemicals) concept for the preparation of LaAlO3 by solution combustion route have never been published. The optimum concentration of citric and oxalic acid required for preparing LaAlO3 nanopowders with smallest crystallite size was fixed by carrying out a series of experiment. 2. Experimental Aluminium nitrate nonhydrate (Sigma–Aldrich, USA), Lanthanum(III) nitrate hexahydrate (Aldrich, USA) was used as the metallic precursors for the powder. Mixture of fuels such as citric acid (CA) monohydrate (Sigma–Aldrich, USA) and oxalic acid (Sigma–Aldrich, USA) were used as the sacrificial organic fuel in this combustion synthesis process. First, each metallic source was dissolved in distilled water separately. The La-containing
0921-8831/$ - see front matter Ó 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2009.10.014
J. Chandradass, K.H. Kim / Advanced Powder Technology 21 (2010) 100–105
solution was added to the Al-containing solution in the molar ratio 1:1. Lanthanum aluminate (LaAlO3-1) was prepared using mixture of fuels containing 0.0025 M concentration of citric acid and 0.0075 M concentration of oxalic acid (0.0025MC + 0.0075MO). Total concentration of the fuel was 0.01 M. Fuels were added with La–Al mixture solution and stirred vigorously. The initial fuel/oxidant molar ratio in the reaction mixture was unity. The resultant sol was continuously stirred for several hours and kept at a temperature of 60 °C until it turned into a yellowish sol. Then the stabilized sol was rapidly heated to 80 °C. Viscosity and color changed as the sol turned into a transparent stick gel. The gel was heat treated at 200 °C for 2 h and a fluffy, polymeric citrate precursor was gained. Finally, the prepared precursor was ground to a fine powder and calcined at different temperature for 2 h in a furnace. Similar experiments were carried out with the following fuel compositions: 0.005MC + 0.005MO, 0.0075MC + 0.0025MO to prepare LaAlO3-2 and LaAlO3-3 samples, respectively. Fourier transform infrared spectra (FTIR) were measured on a Nicolet Impact 410 DSP spectrophotometer using the KBr pellet method. The thermal decomposition behavior of the dried precursor was studied by TG–DTA analysis (STA 1500). The phase identification of calcined powders was recorded by an X-ray diffractometer (Philips X’pert MPD 3040) with Cu Ka radiation. Transmission electron microscopy (TEM) was performed at an accelerating voltage of 200 kV by placing the powder on a copper grid to observe the morphology and size of the powders. The average size of the particle was estimated from the TEM micrograph using standard software (IMAGE J). The crystallite size of the calcined particle was calculated from full width at half-maximum (FWHM) using Scherer’s formula [19] for (1 1 2) crystal face of LaAlO3 nanoparticles.
3. Results and discussion The IR spectrum for the charred solid precursor at 200 °C is shown in Fig. 1. A broad absorption in the spectrum of the
101
precursor powders at around 3431 cm 1 indicates the presence of adsorbed water [20]. The peak at 2376 cm 1 can be attributed to the absorption of CO2 [21]. The absorption band at 1640 cm 1 is attributed to free water in the sample [22]. The transmittance band at 1330 cm 1 correspond to CH3 bending mode [21].The transmittance bands for carbonate ion complexes CO3 (including bridging) was observed at 1471, 1043, 822 and 739 cm 1 [23]; the transmittance band at 447 cm 1 should be ascribed to the M–O (M = Al and/or La) bonding in the oxide. After calcining the sample at 750 °C for 2 h (Fig. 2) new transmittance bands detected in the specimens were those for LaAlO3 (the characteristic bands at 656 and 442 cm 1) [24]. The transmittance bands corresponding to the adsorbed water, free water and organic species are present with reduced intensity in calcined sample. The presence of adsorbed water and free water in the calcined samples may be due to absorption of moisture. The presence of organics may inhibit the growth of the particle size. TG/DTA curves showed that in all cases (LaAlO3-1, LaAlO3-3) the thermal decomposition proceeded in a similar way. TG/DTA profiles for two representative samples during heating at a rate of 5 °C min 1 from room temperature to 1000 °C in an air atmosphere are shown in Figs. 3 and 4. Decomposition started below 200 °C with a weight loss of 6.2% and 2% for LaAlO3-1 and LaAlO3-3 samples. This weight loss corresponds to the removal of adsorbed water. In the temperature region between 200 and 600 °C, the main decomposition occurs with a weight loss of 28% and 31%, respectively. The thermal decomposition behavior is associated with endothermic and exothermic effects in the DTA curve. The first decomposition step assignable to removal of adsorbed and chemisorbed water is indicated by broad endothermic peak from 30 °C to 100 °C on the DTA curves. The exothermic peak at 345 °C and endothermic peaks at 450 °C in the temperature range between 200 and 600 °C in the DTA curve are due to the decomposition and pyrolysis processes involving the rapid formation of large quantity of gaseous products like NO2, CO, CO2 and H2O and oxalate decomposition. The final weight loss of 6.6% and 5.6% on the TG curve was observed in the temperature range
Fig. 1. FTIR analysis of dried LaAlO3 powders synthesized with mixture of fuels (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
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J. Chandradass, K.H. Kim / Advanced Powder Technology 21 (2010) 100–105
Fig. 2. FTIR analysis of as-synthesized LaAlO3 powders synthesized with mixture of fuels and calcined at 750 °C for 2 h (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
Fig. 3. DTA analysis of as-synthesized powders with mixture of fuels: (a) LaAlO3-1; (b) LaAlO3-3.
>600 °C are due to the thermal decomposition of carbonate phases and the completion of the crystallization reaction. A clear intense exothermic peak is observed at 830 and 835 °C for samples LaAlO3-1 and LaAlO3-3 corresponding to the decomposition of intermediate oxycarbonate La2O2CO3 to the oxide [25]. The XRD patterns of the precursor calcined in air at 700–800 °C for 2 h are shown in Figs. 5–7. After calcined at 700 °C for 2 h, the
product is still amorphous, as is characterized by the broad continuum. The powders calcined at 750 °C for 2 h shows good crystallinity. These XRD peaks correspond to reflections from rhombohedral LaAlO3 with a perovskite structure (JCPDS card 31-0022). The difference in the crystallization temperature of LaAlO3 as observed from DTA and XRD could be because of the difference in heating schedule for the samples. While XRD pattern
J. Chandradass, K.H. Kim / Advanced Powder Technology 21 (2010) 100–105
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Fig. 4. TGA analysis of as-synthesized powders with mixture of fuels: (a) LaAlO3-1; (b) LaAlO3-3.
Fig. 5. XRD patterns of as-synthesized powders with mixture of fuels (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
was recorded on samples, which were held for 2 h at 750 °C, the DTA was done without any isothermal hold. Thus, the isothermal hold at 750 °C has accelerated the transformation to LaAlO3 at lower temperature. Further heating only increased the intensity of the X-ray peaks reflecting greater crystallization and no other phases are observed. The crystallite size was calculated from the peak width of XRD profiles. The results were shown in Table 1. The crystallite size calculated for LaAlO3-1, LaAlO3-2 and LaAlO3-
3 are 23.6, 32.2 and 26.8 nm, respectively. Recently Yu et al. [26] prepared pure LaAlO3 nanoparticles using a citrate precursor technique with an average crystallite size of 29–35 nm. Thus, mixture of fuels (citric acid and oxalic acid) approach facilitated the reduction of crystallite size compared to the product formed using single fuel (citric acid). Fig. 8 shows TEM micrographs of the LaAlO3 powders obtained at 750 °C for 2 h with mixture of fuels. The TEM micrographs show
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Fig. 6. XRD patterns of as-synthesized powders calcined at 750 °C with mixture of fuels (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
Fig. 7. XRD patterns of as-synthesized powders calcined at 800 °C with mixture of fuels (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
that the powders had faceted polyhedral morphologies. The average particle size of LaAlO3-1 was about 41 nm, whereas for LaAlO3-2 and LaAlO3-3 samples, particles are seriously aggregated. The average particle size obtained from TEM technique is bigger than that observed from X-ray line broadening techniques. This may be due to the agglomeration of fine particles.
Table 1 Crystallite size of LaAlO3 prepared by mixture of fuels. Sample name
Fuel
Crystallite size (750 °C) (XRD) (nm)
LaAlO3-1 LaAlO3-2 LaAlO3-3
0.0025C + 0.0075O 0.005C + 0.005O 0.0075C + 0.0025O
23.6 35.2 26.8
J. Chandradass, K.H. Kim / Advanced Powder Technology 21 (2010) 100–105
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Fig. 8. TEM micrographs of LaAlO3 powders synthesized with mixture of fuels and calcined at 750 °C for 2 h (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
4. Conclusions The optimum concentration of citric and oxalic acid required for preparing LaAlO3 nanopowders with smallest crystallite size was fixed by carrying out a series of experiment. The results show that mixture of fuels ratio is influential on the morphology and size of the resultant powders. LaAlO3-1 prepared using 0.0025 M concentration of citric acid and 0.0075 M concentration of oxalic acid had a crystallite size of 23.6 nm. Thus, it is concluded that by using mixture of fuels one cannot only reduce the exothermicity of the combustion reaction but also reduce the crystallite size largely. The average particle size of LaAlO3-1 as determined from TEM was about 41 nm, whereas for LaAlO3-2 and LaAlO3-3 samples, particles are seriously aggregated. References [1] S.Y. Cho, I.T. Kim, K.S. Hong, Microwave dielectric properties and applications of rare-earth aluminates, J. Mater. Res. 14 (1999) 114–119. [2] R.W. Simon, C.E. Platt, A.E. Lee, G.S. Lee, K.P. Daly, M.S. Wire, J.A. Luine, M. Urbanik, Low-loss substrate for epitaxial growth of high-temperature superconductor thin films, Appl. Phys. Lett. 53 (1988) 2677. [3] R. Spinicci, P. Marini, S.D. Rossi, M. Faticanti, P. Porta, Oxidative coupling of methane on LaAlO3 perovskites partially substituted with alkali or alkali-earth ions, J. Mol. Catal. A: Chem. 176 (2001) 253–265. [4] T. Takahashi, H. Iwahara, Ionic conduction in perovskite-type oxide solid. Solution and its application to solid electrolyte fuel cell, Energy Convers. 11 (1971) 105–111. [5] B. Jancar, D. Suvorov, M. Valant, G. Drazic, Characterization of CaTiO3–NdAlO3 dielectric ceramics, J. Eur. Ceram. Soc. 23 (2003) 1391–1400. [6] I. Zvereva, Y. Smirnov, V. Gusarov, V. Popova, J. Choisnet, Complex aluminates RE2SrAl2O7 (RE = La, Nd, Sm-Ho): cation ordering and stability of the double perovskite slab-rock salt layer P-2/RS inter-growth, Solid State Sci. 5 (2003) 343–349. [7] J.J. Kingsley, K.C. Patil, A novel combustion process for the synthesis of fine particle a-alumina and related oxide materials, Mater. Lett. 6 (1988) 427–432. [8] K.C. Patil, S.T. Aruna, S. Ekambaram, Combustion synthesis, Curr. Opin. Solid State Mater. Sci. 2 (1997) 158–165. [9] K.C. Patil, S.T. Aruna, T. Mimani, Combustion synthesis: an update, Curr. Opin. Solid State Mater. Sci. 6 (2002) 507–512.
[10] S.T. Aruna, N.S. Kini, K.S. Rajam, Solution combustion synthesis of CeO2– CeAlO3 nano-composites by mixture-of-fuels approach, Mater. Res. Bull. 44 (2009) 728–733. [11] M.D.S. Kumar, T.M. Srinivasan, P. Ramasamy, C. Subramanian, Synthesis of lanthanum aluminate by a citrate-combustion route, Mater. Lett. 25 (1995) 171–174. [12] E. Taspinar, A.C. Tas, Low-temperature chemical synthesis of lanthanum monoaluminate, J. Am. Ceram. Soc. 80 (1997) 133–141. [13] A.K. Adak, P. Pramanik, Synthesis and characterization of lanthanum aluminate powder at relatively low temperature, Mater. Lett. 30 (1997) 269– 273. [14] D. Zhou, G. Huang, X. Chena, J. Xua, S. Gonga, Synthesis of LaAlO3 via ethylenediaminetetraacetic acid precursor, Mater. Chem. Phys. 84 (2004) 33– 36. [15] S. Ran, L. Gao, Synthesis of LaAlO3 powder using triethanolamine, Ceram. Int. 34 (2008) 443–446. [16] Z. Tian, H.-T. Yu, Z.-L. Wang, Combustion synthesis and characterization of nanocrystalline LaAlO3 powders, Mater. Chem. Phys. 106 (2007) 126–129. [17] J. Chandradass, Ki Hyeon Kim, Synthesis and characterization of LaAlO3 by emulsion combustion synthesis, J. Alloys Compd. 481 (2009) L31–L34. [18] S.T. Aruna, K.S. Rajam, Mixture of fuels approach for the solution combustion synthesis of Al2O3–ZrO2 nanocomposite, Mater. Res. Bull. 39 (2004) 157–167. [19] D.M. Moore, R.C. Reynolds Jr., X-ray Diffraction and The Identification and Analysis of Clay Minerals, 2nd edition., Oxford University press, Oxford, 1997. p. 87. [20] A. Barabauskas, D. Jasaitis, A. Kareiva, Characterization of sol–gel process in the Y–Ba–Cu–O acetate–tartrate system using IR spectroscopy, Vib. Spectrosc. 28 (2002) 263–275. [21] G. Socrates, Infrared and Raman Characteristic Group Frequencies Tables and Charts, third ed., Wiley, Chichester, 2001. [22] V. Saraswathi, G.V.N. Rao, G.V. Rama Rao, Structural evolution in alumina gel, J. Mater. Sci. 22 (1987) 2529–2534. [23] P. Jeevanandam, Y. Koltypin, O. Palchik, A. Gedanken, Synthesis of morphologically controlled lanthanum carbonate particles using ultrasound irradiation, J. Mater. Chem. 11 (2001) 869–873. [24] B. Schrader (Ed.), Infrared Raman Spectroscopy: Methods and Applications, VCH, Weinheim, 1995. [25] M. Chroma, J. Pinkas, I. Pakutinskiene, A. Beganskiene, A. Kareiva, Processing and characterization of sol–gel fabricated mixed metal aluminates, Ceram. Int. 31 (2005) 1123–1130. [26] H.-F. Yu, J. Wang, S.-S. Wang, Y.-M. Kuo, Thermochemical behavior of metallic citrate precursors for the production of pure LaAlO3, J. Phys. Chem. Solids 70 (2009) 218–223.
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Mixture of fuels approach for the solution combustion synthesis of LaAlO3 nanopowders J. Chandradass *, Ki Hyeon Kim * Department of Physics, Yeungnam University, Gyeongsan, Gyeongsangbuk-do 712-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 11 May 2009 Received in revised form 7 August 2009 Accepted 30 October 2009
Keywords: Perovskite Chemical synthesis Characterization Crystallite size
a b s t r a c t A modified solution combustion approach was used in the preparation of nanosize LaAlO3 (23.6 nm) using mixture of citric acid and oxalic acid as fuels with corresponding metal nitrates. The synthesized and calcined powders were characterized by Fourier transform infra red spectrometry (FTIR), Differential thermal analysis-Thermogravimetry analysis (DTA–TGA), X-ray diffractometry (XRD) and Transmission electron microscopy (TEM). The FTIR spectra show the lower frequency bands at 656 and 442 cm 1corresponds to metal–oxygen bonds (possible La–O and Al–O stretching frequencies) vibrations for the perovskite structure compound. DTA confirms the formation temperature of LaAlO3 varies between 830–835 °C. XRD results show that mixture of fuels ratio is influential on the crystallite size of the resultant powders. The average particle size of LaAlO3-1 as determined from TEM was about 41 nm, whereas for LaAlO3-2 and LaAlO3-3 samples, particles are seriously aggregated. Ó 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction LaAlO3 with a perovskite-type structure is drawing considerable attention with applications in various fields, such as dielectric resonators [1], substrate material for thin-film high-temperature superconductors [2], catalyst for oxidative coupling of methane and hydrogenation and hydrogenolysis of hydrocarbons [3] electrolyte material for solid oxide fuel cells [4]. Traditionally, LaAlO3 has been prepared by conventional solid-state reaction of Al2O3 and La2O3 in the temperature range 1500–1700 °C [5,6]. This conventional solid-state reaction process suffers from many inherent shortcomings, such as high reaction temperature, large particle size, limited chemical homogeneity and low sinterability, which have a detrimental effect on the microwave dielectric properties. Among the large number of techniques employed for the synthesis of LaAlO3, solution combustion synthesis is unique and highly versatile [7]. A very wide variety of materials ranging from high-Tc superconductors to insulators has been prepared by solution combustion method [8]. It is an easy and fast process, which yields high-purity, homogenous, crystalline oxides in a short time and with less energy [8,9]. The surface area, size-distribution and agglomeration of the particles in the final product mainly depend on the flame-temperature, which in turn is related to the nature of fuel and fuel to-oxidant ratio. Among the various control param-
* Corresponding authors. Tel.: +82 53 810 2334; fax: +82 53 810 4616. E-mail addresses: [email protected] (J. Chandradass), [email protected] (K.H. Kim).
eters in a combustion process, fuel plays an important role in determining the morphology, phase and particulate properties of the final product [10]. LaAlO3 has been synthesized by combustion technique using various fuels such as citric acid [11], urea [12]; polyvinyl alcohol [13]; Ethylenediaminetetracetic [14]; triethanolamine [15]; glycine [16] and oleic acid [17]. Aruna and Rajam, first proposed and illustrated the proof of concept of the mixture of fuels approach for the synthesis of alumina and zirconia toughened alumina [18] and CeO2–CeAlO3 system [10]. This approach facilitated the reduction of particle size compared to the product formed using single fuel. In conclusion, the mixture of fuels approach gives a good control over the flame-temperature [18]. To the best of our knowledge, reports related to the use of mixture of fuels (using commercially available chemicals) concept for the preparation of LaAlO3 by solution combustion route have never been published. The optimum concentration of citric and oxalic acid required for preparing LaAlO3 nanopowders with smallest crystallite size was fixed by carrying out a series of experiment. 2. Experimental Aluminium nitrate nonhydrate (Sigma–Aldrich, USA), Lanthanum(III) nitrate hexahydrate (Aldrich, USA) was used as the metallic precursors for the powder. Mixture of fuels such as citric acid (CA) monohydrate (Sigma–Aldrich, USA) and oxalic acid (Sigma–Aldrich, USA) were used as the sacrificial organic fuel in this combustion synthesis process. First, each metallic source was dissolved in distilled water separately. The La-containing
0921-8831/$ - see front matter Ó 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2009.10.014
J. Chandradass, K.H. Kim / Advanced Powder Technology 21 (2010) 100–105
solution was added to the Al-containing solution in the molar ratio 1:1. Lanthanum aluminate (LaAlO3-1) was prepared using mixture of fuels containing 0.0025 M concentration of citric acid and 0.0075 M concentration of oxalic acid (0.0025MC + 0.0075MO). Total concentration of the fuel was 0.01 M. Fuels were added with La–Al mixture solution and stirred vigorously. The initial fuel/oxidant molar ratio in the reaction mixture was unity. The resultant sol was continuously stirred for several hours and kept at a temperature of 60 °C until it turned into a yellowish sol. Then the stabilized sol was rapidly heated to 80 °C. Viscosity and color changed as the sol turned into a transparent stick gel. The gel was heat treated at 200 °C for 2 h and a fluffy, polymeric citrate precursor was gained. Finally, the prepared precursor was ground to a fine powder and calcined at different temperature for 2 h in a furnace. Similar experiments were carried out with the following fuel compositions: 0.005MC + 0.005MO, 0.0075MC + 0.0025MO to prepare LaAlO3-2 and LaAlO3-3 samples, respectively. Fourier transform infrared spectra (FTIR) were measured on a Nicolet Impact 410 DSP spectrophotometer using the KBr pellet method. The thermal decomposition behavior of the dried precursor was studied by TG–DTA analysis (STA 1500). The phase identification of calcined powders was recorded by an X-ray diffractometer (Philips X’pert MPD 3040) with Cu Ka radiation. Transmission electron microscopy (TEM) was performed at an accelerating voltage of 200 kV by placing the powder on a copper grid to observe the morphology and size of the powders. The average size of the particle was estimated from the TEM micrograph using standard software (IMAGE J). The crystallite size of the calcined particle was calculated from full width at half-maximum (FWHM) using Scherer’s formula [19] for (1 1 2) crystal face of LaAlO3 nanoparticles.
3. Results and discussion The IR spectrum for the charred solid precursor at 200 °C is shown in Fig. 1. A broad absorption in the spectrum of the
101
precursor powders at around 3431 cm 1 indicates the presence of adsorbed water [20]. The peak at 2376 cm 1 can be attributed to the absorption of CO2 [21]. The absorption band at 1640 cm 1 is attributed to free water in the sample [22]. The transmittance band at 1330 cm 1 correspond to CH3 bending mode [21].The transmittance bands for carbonate ion complexes CO3 (including bridging) was observed at 1471, 1043, 822 and 739 cm 1 [23]; the transmittance band at 447 cm 1 should be ascribed to the M–O (M = Al and/or La) bonding in the oxide. After calcining the sample at 750 °C for 2 h (Fig. 2) new transmittance bands detected in the specimens were those for LaAlO3 (the characteristic bands at 656 and 442 cm 1) [24]. The transmittance bands corresponding to the adsorbed water, free water and organic species are present with reduced intensity in calcined sample. The presence of adsorbed water and free water in the calcined samples may be due to absorption of moisture. The presence of organics may inhibit the growth of the particle size. TG/DTA curves showed that in all cases (LaAlO3-1, LaAlO3-3) the thermal decomposition proceeded in a similar way. TG/DTA profiles for two representative samples during heating at a rate of 5 °C min 1 from room temperature to 1000 °C in an air atmosphere are shown in Figs. 3 and 4. Decomposition started below 200 °C with a weight loss of 6.2% and 2% for LaAlO3-1 and LaAlO3-3 samples. This weight loss corresponds to the removal of adsorbed water. In the temperature region between 200 and 600 °C, the main decomposition occurs with a weight loss of 28% and 31%, respectively. The thermal decomposition behavior is associated with endothermic and exothermic effects in the DTA curve. The first decomposition step assignable to removal of adsorbed and chemisorbed water is indicated by broad endothermic peak from 30 °C to 100 °C on the DTA curves. The exothermic peak at 345 °C and endothermic peaks at 450 °C in the temperature range between 200 and 600 °C in the DTA curve are due to the decomposition and pyrolysis processes involving the rapid formation of large quantity of gaseous products like NO2, CO, CO2 and H2O and oxalate decomposition. The final weight loss of 6.6% and 5.6% on the TG curve was observed in the temperature range
Fig. 1. FTIR analysis of dried LaAlO3 powders synthesized with mixture of fuels (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
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J. Chandradass, K.H. Kim / Advanced Powder Technology 21 (2010) 100–105
Fig. 2. FTIR analysis of as-synthesized LaAlO3 powders synthesized with mixture of fuels and calcined at 750 °C for 2 h (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
Fig. 3. DTA analysis of as-synthesized powders with mixture of fuels: (a) LaAlO3-1; (b) LaAlO3-3.
>600 °C are due to the thermal decomposition of carbonate phases and the completion of the crystallization reaction. A clear intense exothermic peak is observed at 830 and 835 °C for samples LaAlO3-1 and LaAlO3-3 corresponding to the decomposition of intermediate oxycarbonate La2O2CO3 to the oxide [25]. The XRD patterns of the precursor calcined in air at 700–800 °C for 2 h are shown in Figs. 5–7. After calcined at 700 °C for 2 h, the
product is still amorphous, as is characterized by the broad continuum. The powders calcined at 750 °C for 2 h shows good crystallinity. These XRD peaks correspond to reflections from rhombohedral LaAlO3 with a perovskite structure (JCPDS card 31-0022). The difference in the crystallization temperature of LaAlO3 as observed from DTA and XRD could be because of the difference in heating schedule for the samples. While XRD pattern
J. Chandradass, K.H. Kim / Advanced Powder Technology 21 (2010) 100–105
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Fig. 4. TGA analysis of as-synthesized powders with mixture of fuels: (a) LaAlO3-1; (b) LaAlO3-3.
Fig. 5. XRD patterns of as-synthesized powders with mixture of fuels (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
was recorded on samples, which were held for 2 h at 750 °C, the DTA was done without any isothermal hold. Thus, the isothermal hold at 750 °C has accelerated the transformation to LaAlO3 at lower temperature. Further heating only increased the intensity of the X-ray peaks reflecting greater crystallization and no other phases are observed. The crystallite size was calculated from the peak width of XRD profiles. The results were shown in Table 1. The crystallite size calculated for LaAlO3-1, LaAlO3-2 and LaAlO3-
3 are 23.6, 32.2 and 26.8 nm, respectively. Recently Yu et al. [26] prepared pure LaAlO3 nanoparticles using a citrate precursor technique with an average crystallite size of 29–35 nm. Thus, mixture of fuels (citric acid and oxalic acid) approach facilitated the reduction of crystallite size compared to the product formed using single fuel (citric acid). Fig. 8 shows TEM micrographs of the LaAlO3 powders obtained at 750 °C for 2 h with mixture of fuels. The TEM micrographs show
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Fig. 6. XRD patterns of as-synthesized powders calcined at 750 °C with mixture of fuels (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
Fig. 7. XRD patterns of as-synthesized powders calcined at 800 °C with mixture of fuels (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
that the powders had faceted polyhedral morphologies. The average particle size of LaAlO3-1 was about 41 nm, whereas for LaAlO3-2 and LaAlO3-3 samples, particles are seriously aggregated. The average particle size obtained from TEM technique is bigger than that observed from X-ray line broadening techniques. This may be due to the agglomeration of fine particles.
Table 1 Crystallite size of LaAlO3 prepared by mixture of fuels. Sample name
Fuel
Crystallite size (750 °C) (XRD) (nm)
LaAlO3-1 LaAlO3-2 LaAlO3-3
0.0025C + 0.0075O 0.005C + 0.005O 0.0075C + 0.0025O
23.6 35.2 26.8
J. Chandradass, K.H. Kim / Advanced Powder Technology 21 (2010) 100–105
105
Fig. 8. TEM micrographs of LaAlO3 powders synthesized with mixture of fuels and calcined at 750 °C for 2 h (a) LaAlO3-1; (b) LaAlO3-2; (c) LaAlO3-3.
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