SOJ Materials Science & Engineering

Sr-doped LaFeO₃ powders, named herein as La_1-xSr_xFeO₃ (x represents the Sr-doping molar ratio with respect to La used at the starting point of the synthesis), were prepared by thermal decomposition of metal organic complexes. The obtained products show the characteristic orthorhombic structure of La_1-xSr_xFeO₃ perovskite with grain sizes of ~2-5 μm. SEM imagesindicate that the growth of the grain is restrained with the increasing concentration of Sr (II) dopant and leads to the small particle size. The specific surface area of 8-12 m₂/g was observed, and the variation of specific surface area is related to the different particle size. The sensitivity of La_{1- x}Sr_xFeO₃ films was tested for ethanol, acetone, methane and hydrogen at different concentrations and temperatures. The films were found to be selective and applicable for detecting ethanol gas. The highest response to 1000 ppm ethanol is ~31.8 at 350°C for the La_0.5Sr_0.5FeO₃.

Keywords: LaFeO₃; Ethanol sensing, gas sensor; Metal-organic complex; La_1-xSr_xFeO₃ thick film;

Within the past decades, perovskite-type oxides with a general formula ABO₃ (A = rare-earth element, B = 3d transition metal) have been known as one of the promising materials which provide a wide variety of applications. LaFeO₃, one of the most common and well-known perovskite oxides, has received much attention due to its excellent thermal and chemical stability. Recently, partial substitution of other metallic ionsat La and/or Fe sites of LaFeO₃ had been applied to modify structural properties andthe non-stoichiometric materials had been investigated showing the applicable uses ins catalysts [1-6], electrode materialsfor solid oxide fuel cell [7-10], magnetic materials [11- 13] and chemical sensors [14-17]. Due to their superb catalytic and electronic properties, a number of studies have focused on the sensing properties of LaFeO₃ and the substituted LaFeO₃ for toxic and combustible gases such as carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO₂), methane (CH₄) and liquefied petroleum gas (LPG) [14,18-20]. According to the literatures, a partial substitution at La site in LaFeO₃ by Pb (II) showed the enhancement of sensitivity and selectivity to ethanol [21], while Sr (II) dopant was found to improve the response time [22]. Regarding to the state-of-the-art, a micro-structured ethanol gas sensor based on the Sr (II) substituted LaFeO₃ exhibits improved sensing performances and requires low power consumption [23].

LaFeO₃ has been successfully prepared by various methods, for examples solid-state reaction, wet combustion synthesis, hydrothermal synthesis, thermal decompositions of hetero nuclear complex, glycine combustion, citrate, sol-gel, coprecipitation and microwave-assisted decomposition, etc [24- 32]. According to the dependency of physical and gas sensing properties of LaFeO₃-based sensors on the characteristics of the materials, the study on preparation methods to optimise and control over morphological structure, purity and composition of the materials is crucial [33]. In this recent work, we indicate the effective syntheses of Sr-doped LaFeO₃ powders by thermal decomposition of innovative metal organic complexes [34-45]. Note that, the notation La_1-xSr_xFeO₃ (x = 0, 0.1, 0.3 and 0.5) used along the whole manuscript represents the doping ratio of Sr with respect to La from the starting point of the synthetic procedure, which could imply the molar ratio within the perovskite structure of the obtained powders however not precisely indicates. The effects of Sr (II) doping concentration on the structure of LaFeO₃ were characterized. Moreover, the La_1-xSr_xFeO₃ thick films were fabricated by spin coating method on an alumina substrate (3 mm x 2 mm size) contacted with gold electrode in order to investigate the sensing properties. Herein, the vapours of ethanol and acetone, as well as the CH₄ and H₂ gases were selected as the test molecules for preliminary sensing study and the possibility to use as sensors are discussed in detail below.

Lanthanum (III) nitrate hexahydrate (La (NO₃)₃.6H₂O), iron (III) nitrate nonahydrate (Fe (NO₃)₃.9H₂O) and strontium (II) nitrate (Sr (NO₃)₂) were purchased from Fisher Scientific, Ajax Finechem and HIMEDIA, respectively. Triethanolamine (TEA, N (CH₂CH₂OH)₃) and ethylene glycol (EG,HOCH₂CH₂OH) were obtained from UNILAB. Triton X-100 and acetyl acetone were acquired from Fluka. The substances were used without further purification except EG was distilled before use.

The La_1-xSr_xFeO₃ (x = 0, 0.1, 0.3 and 0.5) powders were prepared by thermal decomposition of metal organic complexes. First of all, the metal-organic complexes were prepared by mixing La(NO₃)₃.6H₂O, Sr(NO₃)₂, Fe(NO₃)₃.9H₂O and TEA with the La(III): Sr (II):Fe(III):TEA molar ratio of 1:0:1:3, 0.9:0.1:1:3, 0.7:0.3:1:3 and 0.5:0.5:1:3, in separated round bottom flasks which contained EG solvent (each composition denoted as a complex precursor for La_1-xSr_xFeO₃, where x = 0, 0.1,0.3and 0.5, respectively). After mixing, the solution was distilled at 190_oC for 6 hours with continuous stirring to obtain the precipitates. The complexes were separated by filtration, washed twice with ethanol and dried at 80_oC. The as-prepared complexes were characterized by FTIR (Perkin Elmer system 2000, Fourier Transform Infrared Spectrometer). The thermal decomposition of the obtained complexes was studied by Thermo gravimetric Analysis (TGA, 761 Connecticut 06859 Perkin Elmer, heating rate of 10°C/min over 50oC to 1000°C temperature range).

Metal-organic complexes were converted to La_1-xSr_xFeO₃ powders by calcinations at 850°C for 4 hours. Phase identification of the obtained powders was performed by X-ray diffraction (XRD, Bruker-AXS Advance D8 X-ray powder diffractometer, CuKα radiation, operated at 40 kV and scan step was 0.04°). The infrared spectra were recorded using Fourier Transform Infrared (FTIR) Spectrometer (Perkin Elmer system 2000). The specific surface areas of powders were measured by Brunauer-Emmett- Teller (BET) nitrogen gas absorption method. The particle size and morphology of the synthesized powders were identified by scanning electron microscope (SEM, 145 OVP LEO).

The prepared La_1-xSr_xFeO₃ powder, Triton X-100 binder and acetyl acetone solvent were mixed and grounded in a mortar to form a paste. To fabricate a film, the paste was dropped on alumina substrates (3 mm x 2 mm size) equipped with inter digitated gold electrodes, and then spun by spin coater at a rate of 3000 rounds/ second for 30 seconds. The La_1-xSr_xFeO₃ films were annealed at 400oC for 3 hours to remove organic contents. Characterizations of the La_1-xSr_xFeO₃ films fabricated on substrate by XRD and SEM was carried out.

The gas-sensing characteristics of La_1-xSr_xFeO₃ films were systematically characterized towards reducing gases including ethanol, acetone, CH₄ and H₂ using the standard flow through technique operated at various temperatures ranging from 200- 350°C. Note that, the operating temperature was limited to 350°C due the constraints of test apparatus. Specifically, the electrodes of La_1-xSr_xFeO₃ sensors were probed on a stage equipped with Ni- Cr heating coils in a stainless steel gas-testing chamber. In each measurement, a constant flux (2 L/min) of synthetic air used as a carrier gas was mixed with the desired concentration of the target gases dispersed in synthetic dry air through the chamber. Regarding to the set-up, the gas flow rates were precisely manipulated using a computer-controlled multi-channel mass flow controllers and the operating temperature was controlled using PID controller and adjustable DC power supply. For gas testing, the resistances (Ra) of the films were measured in dry air. The resistances of La_1-xSr_xFeO₃ sensors were continuously measured for the different concentrations of the probe gases (Rg) with a computerized system by voltage-amperometric technique with 10 V DC bias and current measurement using a picoammeter. The sensors were exposed to a gas sample for 10 minutes and the air flux was then restored for 25 minutes at each gas concentration. The gas-sensing performances are characterized in terms of sensor response (S), which is defined as the resistance ratio R_a/R_g, where Ra is the resistance in dry air, and Rg is the resistance in a tested gas.

(Figure 1) shows a comparison of FTIR spectra between the metal organic complex (LFO precursor) and N (CH₂CH₂OH)₃ ligand. The LFO precursor exhibits characteristic bands assigned as follow; broad band at around 3382 cm^-1 (O-H stretching), very weak bands around 2950-2870 cm^-1 (C-H stretching), 1640 cm^-1 (O-H overtone), 1316 cm^-1 (C-H bending), 1074 cm^{- 1}(C-N stretching), 935 cm^-1(C-O stretching) and 788 cm^-1 (-CH₂ rocking). The small peak at 518 cm^-1 is assigned to M-O or M-N stretching of metal coordinated with N(CH₂CH₂OH)₃ ligand. The characteristic bands appeared in the LFO pattern Figure 1(b) are slightly changed and shifted from those of the free N (CH₂CH₂OH)₃ ligand Figure 1(a) due to the coordination bonds with the metal. In particular, the strong C-O (1035 cm^-1) and the medium C-N bands (1152 cm^-1) are changed to weak absorption bands and shifted to lower frequencies at 935 cm^-1 and 1074 cm_-1, respectively.

The TGA curve of the LFO precursor shown in (Figure 2) indicates three weight loss steps in thermo gram profile. The first

one in the temperature range of 50-200°C is attributed to the evaporation of physically adsorbed water and some remaining organic solvent. The next step showed at the temperature range from 200°C to 500°C is mainly attributed to the decomposition of organic ligand and nitrate. The final weight loss between 500 and 700°C corresponds to the decomposition of carbon residues [46- 47]. Above 700°C, no weight loss is observed indicating that all organic contents have been removed; resulting in approximately 50% remain mass of the oxide powder. Based on TGA result, the metal organic complex precursors were calcined at 850°C for 4 hours in order to obtain only the desired oxide powder.

(Figure 3) shows the XRD patterns of the obtained products calcined at 850°C for 4 hours. XRD diffraction patterns of LaFeO₃(LFO), La_0.1Sr_0.9FeO₃(LSFO10), La0_.3Sr_0.7FeO₃(LSFO30) and La_0.5Sr_0.5FeO₃(LSFO50) are similar which can be assigned to the diffraction lines of LaFeO₃ perovskite phase with orthorhombic structure (JCPDS file no. 37-1493). In addition, the peak intensities decrease and the peak positions slightly shift to higher 2-theta angle values with respect to the increasing of Sr (II) doping concentration. Moreover, the peaks are comparatively widened, indicating a decrease in crystallite size according to Scherrer Equation. Additional peaks of minor phase identified as SrLaFeO₃[48] are found in the obtained powders synthesized with high doping ratio of Sr (II) i.e. LSFO30 and LSFO50. The lattice parameters including unit cell volume and crystallite size of La_{1- x}Sr_xFeO₃ powders are reported in Table 1. It is seen that the unit cell volume and crystallite size slightly decrease with increasing Sr (II) content. The minor change to the unit cell parameters may be attributed to the slight difference between the ionic radii of La (III) and Sr (II) (La (III) = 150 pm, Sr (II) = 158 pm) [49]. As the La (III) is partly substituted by Sr (II) ion, an oxygen vacancy is created in order to maintain charge neutrality and consequently causes the transformation of the stoichiometric perovskite, LaFeO₃, into the nonstoichiometric La_1-xSr_xFeO₃-δ structure. In the case of high Sr (II) content, a large number of oxygen vacancies are produced, resulting in the shrinkage of unit cell volume in order to maintain the normal structure of perovskite phase [15]. As seen in Table 1, the higher content of Sr (II) dopant leads to the smaller unit cell volume and crystallite size, suggesting that the substitution of La (III) site by Sr (II) ion may retard the growth of crystallite size.

The FTIR spectra of La_1-xSr_xFeO₃ perovskites with different Sr contents are displayed in (Figure 4). All samples show a strong absorption band at around 591- 613 cm^-1 corresponding to Fe-O stretching vibration in the BO6 octahedral unit of the perovskite oxides ABO₃ structure [1, 24, 50-51]. In addition, the Fe-O peak is slightly shifted towards higher frequency as the Sr (II) amount increases, implying that Fe-O bond strength is affected by Sr (II) substitution consistent with the change in lattice parameters and unit cell volume observed by XRD. The appearance of weak absorption bands around 1565 - 1440 cm^-1 is ascribed to vibration of CO₃²- group, denoting carbonate contamination in the prepared samples. In addition, the observed peak at around 2366 cm^-1 can be attributed to the physically surface-adsorbed CO2. (Figure 5)

shows SEM micrograph of the La_1-xSr_xFeO₃ powders with different Sr (II) contents. The morphologies of all calcined powders are similar and reveal rather homogeneous micron-size aggregates of fine nanoparticles. However, the particle size of the obtained powders are different in the three doping concentration which is clearly seen when spun on the substrate to form the film (see the following section). The BET specific surface area (SSBET) of LFO is found to be ~11 m²/g. In the cases of Sr-doped LFO, SBET initially decreases to ~8 m²/g (LFSO10) but then increases as Sr (II) increases further (SSBET of LSFO30 and LFSO50 = ~10 and ~12 m²/g, respectively). Since the inorganic oxide perovskite structures are known to be the dense phases, the micropores are not observed. Their BET surface area rather depends on the outer surface of the particles and the packing of nanoparticles, which corresponds to the particle size. Hence, the LSFO30 and LSFO50 show higher BET surface area because of the smaller particle size.

The thickness of La_1-xSr_xFeO₃ films fabricated on alumina substrate with gold electrode were estimated to be approximately 30-40 μm by an optical microscope at the magnefication of 50x. The fabricated films were characterized byXRD and SEM prior togas-sensing measurement. The diffraction patterns of all the films as demonstrated in (Figure 6) clearly exhibit two main peaks of LaFeO₃ at the 2-theta angle of about 32 and 57° in addition to the high intense peaks of substrate materials, i.e., Al₂O₃ (JCPDS file no. 82-1467), and Au (JCPDS file no. 04-0784). This confirms the presence of LaFeO₃ perovskite phase in all sensing films. (Figure 7) illustrates SEM micrographs of La_1-xSr_xFeO₃ films with different Sr contents. It is seen that the surface morphologies of films with different Sr doping levels are considerably different. The un-doped LaFeO₃ film displays lots of cavity on the surface.

In the cases of Sr (II)-doped films, particle size observed in the top-view SEM images observably decreases with respect to an increasing of Sr(II) concentration. In addition, LSFO10 exhibits the most loosely aggregated grains while LSFO50 has more uniform structure of fine nanoparticles with no surface cracking.

Figure 8 (a)-(d) presents the relationship between operating temperature and response of La_1-xSr_xFeO₃-based sensors with different Sr(II) contents towards the vapors of ethanol (500 ppm) and acetone (2000 ppm), CH₄ (20000 ppm) and H₂ (30000 ppm) gases. It is observed that the sensor responses towards ethanol, acetone and CH₄ monotonically increase with increasing operating temperature up to 350°C Figure 8 (a) - (c) while the response towards H₂ only increases with increasing operating temperature up to 300°C but then decreases as the temperature increases further Figure 8 (d). Note that, we decided to use the operating temperature of 350°C for the further experiments because of the interesting feature of the significant increase in ethanol response by the LSFO50 sensor with respect to the temperature rising from 300 to 350°C.

The dependence on gas concentration of responses of La_1-xSr_xFeO₃-based sensors with different Sr (II) contents towards ethanol, acetone, CH₄ and H₂ at 350°C is shown in (Figure 9). It is evident that LSFO-50 sensor exhibits the highest response to ethanol. In addition, the ethanol response is nonlinear with a rapid rise at low concentration and a slow but steady increase at a concentration higher than 200 ppm. On the contrary, LSFO10, LSFO30 and un-doped LFO sensors show a steady response increase over the whole concentration range and the responses are considerably lower than that of LSFO50 Figure 9(a). The significant enhancement in ethanol response of LSFO50 may be attributed to more active sites for the ethanol adsorption created due to extensive substitution of La (III) lattice sites with Sr (II) ionic do pants. Note that, these catalytic active sites particularly enhance the reducing reaction with ethanol [1, 51] and may explains the enhancement of ethanol sensitivity. The

more Sr-doping ratio at the starting point of the synthesis, the more possibility to incorporate the Sr ions into the perovskite structure and consequently generate more active sites for ethanol adsorption. Another important contribution can be the difference in surface morphology. From SEM and BET results, LSFO50 sensor possesses favorable surface morphology with high surface homogeneity, small grain size and relatively large specific surface area. From both perspectives, the use of La:Sr with 1:1 molar ratio for the synthesis of Sr-doped LaFeO₃ by thermal decomposition method of metal-organic complex is the optimized condition to get the suitable particles for the fabrication as film by spin coating which lead to the highest sensing performance.

For acetone sensing, the sensor response increases quite linearly with increasing acetone concentration. However, all the Sr (II)-doped LaFeO₃ sensors exhibit considerably lower acetone responses compared with un-doped one Figure 9(b). The result indicates that Sr (II) sites may inhibit reducing reaction with acetone. Nevertheless, the heavily-doped LSFO50 sensor still shows a moderate acetone response of 9.43 at 2000 ppm, which is considerably higher than those of sensors with lower doping concentrations (LSFO30 and LSFO10). The behavior may be explained based on the observed results that LSFO50 possesses larger BET specific surface area than LSFO30 and LSFO10, respectively. Regarding flammable gas sensing, LSFO50 sensor shows the highest response to CH₄ while LSFO30 sensor gives the best performance for H₂. Additionally, the responses of La_1-xSr_xFeO₃-based sensors display a similar concentration dependence behavior towards CH₄ and H₂ which show an increasing response by an increasing concentration at lower concentration until reaching the optimal point and after that the response is decayed Figure 9(c) - (d). Comparing all the tests, LSFO50 shows the highest response to ethanol (31.8 at 1000 ppm), indicating a very good sensitivity to ethanol. All the prepared films showed the remarkably poor sensitivity to CH₄ and H₂ gases. This is evident from the very low responses, for CH₄ (3.2 at 20000 ppm) and H₂ (1.9 at 30000 ppm). As for acetone, the highest response was reached from the un-doped sensor (9.43 at 2000 ppm), whereas the lowest signal was obtained in the case of ethanol. Therefore, the un-doped and the LSFO50 films exhibited different selectivity toward acetone and ethanol. This evidence shows that the high-concentration Sr doping in LFO material system significantly improves ethanol-sensing selectivity, which could be a promising candidate for further development for practical uses in sensing applications.

(Figure 10) illustrates dynamic response of La_1-xSr_xFeO₃- based sensors with different Sr (II) contents to 500 ppm of ethanol at 350°C. It is evident that the sensor resistances readily increase when exposed to the ethanol vapor. The increase of sensor resistance to reducing gas indicates that La_1-xSr_xFeO₃(x = 0.0, 0.1, 0.3 and 0.5) is a p-type metal oxide semiconductor [18, 20]. In addition, it can be observed that the baseline resistance of LSFO material tends to decrease, signifying increased whole concentration by Sr (II), and a p-type dopant. Due to the Fe site in LaFeO₃ is preferable for oxygen adsorption [52]. The 2p orbital of absorbed oxygen atoms consumed the 3d valence electron from

O₂ (g) + 2e^- ↔ 2O^-(ads) (1)
C₂H₅OH (g) + 6O₂-(ads) → 2CO₂ + 3H₂O + 12e^- (2)

Where the subscripts (g) and (ads) denote the state of gas and adsorbate, respectively

The reaction generates CO₂, H₂O and electrons released from oxygen species back into conduction band, which are then transported into the material and recombined with existing holes, leading to the decrease of whole concentration and increase of resistance. The LSFO sensors with different Sr (II) contents exhibits distinct characteristics both in magnitude and rate of resistance change, which may be quantitatively compared by relative response and response time to 500 ppm of ethanol at 350°C as listed in The relative response and response time are defined by the ratio of response of doped sensor to that of undoped sensor and the time to reach 90% at the final resistance [53]. It is clear that the LSFO50 sensor exhibits the highest relative response of 4.29 and the shortest response time of 120 seconds while the LFO (undoped) sensor has the lowest relative response of 1 and LSFO30 has the longest response time of 250 seconds.

The La_1-xSr_xFeO₃-based sensors (x = 0.0, 0.1, 0.3 and 0.5) prepared by thermal decomposition of metal organic complexes were systematically investigated for gas-sensing applications. XRD patterns indicated that all prepared powders were perovskite phase with orthorhombic structure and the crystallite sizes of La_1-xSr_xFeO₃ powders decreased from 42 to 23 nm as Sr (II) content (x) increased from 0 to 0.5, indicating that Sr (II) doping led to grain size refinement. Gas-sensing characterizations of La_1-xSr_xFeO₃-based sensors towards ethanol, acetone, CH₄ and H₂ showed that Sr (II) content (x) of 0.5 yields a promising sensor for ethanol detection with a high response of ~31 and a short response time of 120 s to 500 ppm of ethanol at 350°C. Moreover, this sensor also exhibited rather good ethanol selectivity against acetone, CH₄ and H₂.

This work was supported by Kasetsart University Research and Development Institute (KURDI), Kasetsart University and Thailand Graduate Institute of Science and Technology (TGIST), National Science and Technology Development Agency (NSTDA).

1. Barbero BP, Gamboa JA, CadÚs LE. Synthesis and characterization of La1-xCaxFeO3 perovskite-type oxide catalysts for total oxidation of volatile organic compounds. Applied Catalysis B: Environmental. 2006;65(1-2):21-30.
2. Taran OP, Ayusheev AB, Ogorodnikova OL, Prosvirin IP, Isupova LA, Parmon VN. Perovskite-like catalysts LaBO3 (B=Cu, Fe, Mn, Co, Ni) for wet peroxide oxidation of phenol. Applied Catalysis B: Environmental. 2016;180:86-93.
3. Gao B, Deng J, Liu Y, Zhao Z, Li X, Wang Y, et al. Mesoporous LaFeO3 catalysts for the oxidation of toluene and carbón monóxido. Chinese Journal of Catalysis. 2013;34:2223-2229.
4. Phan TTN, Nikoloski AN, Bahri PA, Li D. Heterogeneous photo-Fenton degradation of organics using highly efficient Cu-doped LaFeO3 under visible light. Journal of Industrial and Engineering Chemistry. 2018;61:53-64.
5. Markova-Velichkova M, Lazarova T, Tumbalev V, Ivanov G, Kovacheva D, Stefanov P, et al. Complete oxidation of hydrocarbons on YFeO3 and LaFeO3 catalysts. Chemical Engineering Journal. 2013;231:236-244.
6. Qin Y, Sun L, Zhang D, Huang L. Role of ceria in the improvement of SO2 resistance of LaxCe1−xFeO3 catalysts for catalytic reduction of NO with CO. Catalysis Communications. 2016;79:53-57.
7. Bidrawn F, Kim G, Aramrueang N, Vohs JM, Gorte RJ. Dopants to enhance SOFC cathodes based on Sr-doped LaFeO3 and LaMnO3. Journal of Power Sources. 2010;195(3):720-728.
8. Mat MD, Liu X, Zhu Z, Zhu B. Development of cathodes for methanol and ethanol fuelled low temperature (300-600 °C) solid oxide fuel cells. International Journal of Hydrogen Energy. 2007;32(7):796-801.
9. Buamann FS, Fleig J, Habermeier HU, Maier J. Impedance spectroscopic study on well-defined (La, Sr)(Co, Fe)O3-δ model electrodes. Solid State Ionics. 2006;177(11-12):1071-1081.
10. Murata K, Fukui T, Abe H, Naito M, Nogi K. Morphology control of La(Sr)Fe(Co)5O3-δ cathodes for IT-SOFCs. Journal of Power Sources. 2005;145(2):257-261.
11. Świerczek K, Dabrowski B, Suescun L, Kolesnik S. Crystal structure and magnetic properties of high-oxygen pressure annealed Sr1-xLaxCo0.5Fe0.5O3-δ (0≤ x ≤ 0.5). J Solid State Chem. 2009;182:280-288.
12. Gateshki M, Suescun L, Kolesnik S, Mais J, Świerczek K, Short S, et al. Structural, magnetic and electronic properties of LaNi0.5Fe0.5O3 in the temperature range 5-1000 K. Journal of Solid State Chemistry. 2008;181(8):1833-1839.
13. Achara S, Mondal J, Ghosh S, Roy SK, Chakrabarti PK. Multiferroic behaviour of lanthanum orthoferrite (LaFeO3). Materials Letters. 2010;64(3):415-418.
14. Toan NN, Saukko S, Lantto V. Gas sensing with semiconducting perovskite oxide LaFeO3. Physica B: Condensed Matter. 2003;327(2-4):279-282.
15. Song P, Hu J, Qin H, Zhang L, An K. Preparation and ethanol sensitivity of nanocrystalline La0.7Pb0.3FeO3- based gas sensor. Materials Letters. 2004;58(21):2610-2613.
16. Aono H, Traversa E, Sakamoto M, Sadaoka Y. Crystallographic characterization and NO2 gas sensing property of LnFeO3 prepared by thermal decomposition of Ln-Fe hexacyanocomplexes, Ln[Fe(CN)6]n.H2O, Ln = La, Nd, Sm, Gd and Dy. Sensors and Actuators B-chemical. 2003;94:132-139.
17. Kong LB, Shen YS. Gas-sensing property and mechanism of CaxLa1-xFeO3 ceramics. Sensors and Actuators B: Chemical. 1996;30(3):217-221.
18. Liu X, Ji H, Gu Y, Xu M. Preparation and acetone sensitive characteristics of nano-LaFeO3 semiconductor thin films by polymerization complex method. Materials Science and Engineering: B. 2006;133(1-3):98-101.
19. Martinelli G, Carotta MC, Ferroni M, Sadaoka Y, Traversa E. Screen-printed perovskite-type thick films as gas sensors for environmental monitoring. Sensors and Actuators B: Chemical. 1999;55(2-3):99-110.
20. Chaudhari GN, Jagtap SV, Gedam NN, Pawar MJ, Sangwar VS. Sol-gel synthesized semiconducting LaCo0.8Fe0.2O3 based-powder for thick film NH3 gas sensor. Talanta. 2009;78(3):1136-1140.
21. Song P, Qin H, Zhang L, An K, Lin Z, Hu J, et al. The structure, electrical and ethanol-sensing properties of La1-xPbxFeO3 perovskite ceramics with x ≤ 0.3. Sensors and Actuators B: Chemical. 2005;104(2):312-316.
22. Suo H, Wang J, Wu F, Liu G, Xu B, Zhao M. Influence of Sr Content on the Ethanol Sensitivity of Nanocrystalline La1-xSrxFeO3. Journal of Solid State Chemistry. 1997;130(1):152-153.
23. Liu L, Zhang T, Qi Q, Zhang L, Chen W, Xu B. A novel micro-structure ethanol gas sensor with low power consumption based on La0.7Sr0.3FeO3. Solid State Electronics. 2007;51(7):1029-1033.
24. Gosavi PV, Biniwale RB. Pure phase LaFeO3 perovskite with improved surface area synthesized using different routes and its characterization. Materials Chemistry and Physics. 2010;119(1-2):324-329.
25. Liou YC, Chen YR. Synthesis and microstructure of (LaSr)MnO3 and (LaSr)FeO3 ceramics by a reaction-sintering process. Ceramics International.  2008;34(2):273-278.
26. Popa M, Hong LV, Kakihana M. Nanopowders of LaMeO3 perovskites obtained by a solution-based ceraic processing technique. Physica B: Condensed Matter. 2003;327(2-4):233-236.
27. Qi X, Zhou J, Yue Z, Gui Z, Li L. A simple way to prepare nanosized LaFeO3 powders at room temperature. Ceramics International. 2003;29(3):347-349.
28. Xiangfengand C, Siciliano P. CH3SH-sensing characteristics of LaFeO3 thick-film prepared by co-precipitation method. Sensors and Actuators B: Chemical. 2003;94(2):197-200.
29. Jadhav AD, Gaikawad AB, Samuel V, Ravi V. A low temperature route to prepare LaFeO3 and LaCoO3. Materials Letters. 2007;61(10):2030-2032.
30. Shabbir G, Qureshi AH, Saeed K. Nano-crystalline LaFeO3 powders synthesized by the citric-gel method. Materials Letters. 2006;60(29):3706-3709.
31. Wang Y, Zhu J, Xujie L, Lude Y, Wang X. Preparation and characterization of perovskite LaFeO3 nanocrystals. Materials Letters. 2006;60(13-14):1767-1770.
32. Farhadi S, Momeni Z, Taherimehr M. Rapid synthesis of perovskite-type LaFeO3 nanoparticles by microwave-assisted decomposition of bimetallic La[Fe(CN)6]×5H2O compound.  Journal of Alloys and Compounds. 2009;471(1-2):L5-L8.
33. Li F, Zheng H, Jia D, Xin X, Xue Z. Synthesis of perovskite-type composite oxides nanocrystals by solid-state reactions. Materials Letters. 2002;53(4):282-286.
34.  Laobuthee A, Wongkasemjit S, Traversa E, Laine RM. MgAl2O4spinel powders from oxide one pot synthesis (OOPS) process for ceramic humidity sensors.  Journal of the European Ceramic Society. 2000;20(2):91-97.
35. Thaweechai T, Wisitsoraat A, Laobuthee A, Koonsaeng N. Ethanol sensing of La1-xSrxFeO3 (x= 0, 0.1 and 0.3) prepared by metal-organic complex decomposition. Kasetsart Journal - Natural Science. 2009;43(5):218-223.
36. Haron W, Thaweechai T, Wattanathana W, Laobuthee A, Manaspiya H, Veranitisagul C, et al. Structural Characteristics and Dielectric Properties of La1-xCoxFeO3 and LaFe1-xCoxO3 Synthesized via Metal Organic Complexes. Energy Procedia. 2013;34: 791-800.
37. Laobuthee A, Veranitisagul C, Koonsaeng N, Bhavakul V, Laosiripojana N. Catalytic activity of ultrafine CexGdySmzO2 synthesized by metal organic complex method toward steam reforming of methane. Catalysis Communications. 2010;12(1):25-29.
38. Wattanathana W, Lakkham A, Kaewvilai A, Koonsaeng N, Laobuthee A, Veranitisagul C. Preliminary study of Pd/CeO2 derived from cerium complexes as solid support catalysts for hydrogenation reaction in a micro-reactor. Energy Procedia. 2011;9:568-574.
39. Veranitisagul C, Kaewvilai A, Wattanathana W, Koonsaeng N, Traversa E, Laobuthee A. Electrolyte materials for solid oxide fuel cells derived from metal complexes: Gadolinia-doped ceria. Ceramics International. 2012;38(3):2403-2409.
40. Veranitisagul C, Koonsaeng N, Laosiripojana N, Laobuthee A. Preparation of gadolinia doped ceria via metal complex decomposition method: Its application as catalyst for the steam reforming of ethane. Journal of Industrial and Engineering Chemistry. 2012;18(3):898-903.
41. Laobuthee A, Veranitisagul C, Wattanathana W, Koonsaeng N, Laosiripojana N. Activity of Fe supported by Ce1-xSmxO2-δ derived from metal complex decomposition toward the steam reforming of toluene as biomass tar model compound. Renewable Energy. 2015;74:133-138.
42. Wattanathana W, Nootsuwan N, Veranitisagul C, Koonsaeng N, Laosiripojana N, Laobuthee A. Simple cerium-triethanolamine complex: Synthesis, characterization, thermal decomposition and its application to prepare ceria support for platinum catalysts used in methane steam reforming. Journal of Molecular Structure. 2015;1089:9-15.
43. C Veranitisagul, W Wattanathana, W Nantharak, P Jantaratana, A Laobuthee, N Koonsaeng. BaFe12O19from thermal decomposition of bimetallic triethanolamine complex as magnetic filler for bioplastics. Materials Chemistry and Physics.  2016;177(1):48-55.
44. Nantharak W, Wattanathana W, Klysubun W, Rimpongpisarn T, Veranitisagul C, Koonsaeng N, et al. Effect of local structure of Sm3+ inMgAl2O4:Sm3+ phosphors prepared by thermal decomposition of triethanolamine complexes on their luminescence property. Journal of Alloys and Compounds. 2017;701:1019–1026.
45. Wattanathana W, Veranitisagul C, Wannapaiboon C, Klysubun W, Koonsaeng N, Laobuthee A. Samarium doped ceria (SDC) synthesized by a metal triethanolamine complex decomposition method: Characterization and an ionic conductivity study. Ceramics International. 2017;43(13):9823–9830.
46. Kazak C, Hamamci S, Topcu Y, Yilmaz VT. An eight-coordinate strontium complex with two tetradentate triethanolamine ligand: synthesis, IR spectra, thermal analysis and crystal structure of bis (triethanolamine)strontium(II). Journal of Molecular Structure. 2003;657(1-3):351-356.
47. Biswas SK, Pathak A, Pramanik NK, Dhak D, Pramanik P. Codoped Cr and W rutile nanosized powders obtained by pyrolysis of triethanolamine complexes. Ceramics International. 2008;34(8):1875-1883.
48. Berger D, Matei C, Voicu G, Bobaru A. Synthesis of La1-xSrxMO3 (M = Mo, Fe, Co, Ni) nanopowder by alanine-combustion technique, Journal of the European Ceramic Society. 2010;30:617-622.
49. Hueey JE, Keiter EA, Keiter RL. Inorganic chemistry: principles of structure and reactivity. 4thEdn. HarperCollines College Publishers. 1993.
50. Shivakumaru C. Low temperature synthesis and characterization of rare earth orthoferrites LnFeO3 (Ln = La, Pr, and Nd) from molten NaOH flux. Solid State Communications. 2006;139(4):165-169.
51.  Dai X, Yu C, Wu Q. Comparison of LaFeO3, La0.8Sr0.2FeO3 and La0.8Sr0.2Fe0.9Co0.1O3 perovskite oxides as oxygen carrier for partial oxidation of methane. Journal of Natural Gas Chemistry. 2008;17(4):415-418.
52. Lu X, Hu J, Cheng B, Qin H, Zhao M, Yang C. First-principles study of O2 adsorption on the LaFeO3 (010) surface. Sensors & Actuators: B. Chemical. 2009;139(2):520-526.
53. Wang Y, Wang Y, Cao J, Kong F, Xia H, Zhang J, et al. Low-temperature H2S sensors based on Ag doped α-Fe2O3 nanoparticles. Sensors and Actuators B: Chemical. 2008;131(1):183-189.