Preparation of Sr-Doped Lafeo 3 by Thermal Decomposition of Metal Organic Complex and their Gas-Sensing Properties

Sr-doped LaFeO3 powders, named herein as La1-xSrxFeO3 (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 La1-xSrxFeO3 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 La1xSrxFeO3 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 La0.5Sr0.5FeO3.


Introduction
Within the past decades, perovskite-type oxides with a general formula ABO 3 (A = rare-earth element, B = 3d transition metal) have been known as one of the promising materials which provide a wide variety of applications.LaFeO 3 , 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 LaFeO 3 had been applied to modify structural properties andthe non-stoichiometric materials had been investigated showing the applicable uses ins catalysts [1][2][3][4][5][6], electrode materialsfor solid oxide fuel cell [7][8][9][10], magnetic materials [11][12][13] and chemical sensors [14][15][16][17].Due to their superb catalytic and electronic properties, a number of studies have focused on the sensing properties of LaFeO 3 and the substituted LaFeO 3 for toxic and combustible gases such as carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO 2 ), methane (CH 4 ) and liquefied petroleum gas (LPG) [14,[18][19][20].According to the literatures, a partial substitution at La site in LaFeO 3 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 LaFeO 3 exhibits improved sensing performances and requires low power consumption [23].
LaFeO 3 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][25][26][27][28][29][30][31][32].According to the dependency of physical and gas sensing properties of LaFeO 3 -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 LaFeO 3 powders by thermal decomposition of innovative metal organic complexes [34][35][36][37][38][39][40][41][42][43][44][45].Note that, the notation La 1-x Sr x FeO 3 (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 LaFeO 3 were characterized.Moreover, the La 1-x SrxFeO 3 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 CH 4 and H 2 gases were selected as the test molecules for preliminary sensing study and the possibility to use as sensors are discussed in detail below.
Metal-organic complexes were converted to La 1-x Sr x FeO 3 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, CuK α 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).

Fabrication and gas-sensing measurement of La 1-x Sr x FeO 3based sensors
The prepared La 1-x Sr x FeO 3 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-x Sr x FeO 3 films were annealed at 400 o C for 3 hours to remove organic contents.Characterizations of the La 1-x Sr x FeO 3 films fabricated on substrate by XRD and SEM was carried out.
The gas-sensing characteristics of La 1-x Sr x FeO 3 films were systematically characterized towards reducing gases including ethanol, acetone, CH 4 and H 2 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-x Sr x FeO 3 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 (R a ) of the films were measured in dry air.The resistances of La 1-x Sr x FeO 3 sensors were continuously measured for the different concentrations of the probe gases (R g ) 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.

Results And Discussion
FTIR and TGA measurements of the metal organic complex (Figure 1) shows a comparison of FTIR spectra between the metal organic complex (LFO precursor) and N (CH 2 CH 2 OH) 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 (-CH 2 rocking).The small peak at 518 cm -1 is assigned to M-O or M-N stretching of metal coordinated with N(CH 2 CH 2 OH) 3 ligand.The characteristic bands appeared in the LFO pattern Figure 1(b) are slightly changed and shifted from those of the free N (CH 2 CH 2 OH) 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

Preparation of Sr-Doped Lafeo 3 by Thermal Decomposition of Metal Organic Complex and their Gas-Sensing Properties
Copyright: © 2018 Koonsaeng N, et al.  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.
XRD, FTIR, SEM and BET results of the La 1-x Sr x FeO 3 (x = 0.0, 0.1, 0.3 and 0.5)powders (Figure 3) shows the XRD patterns of the obtained products calcined at 850°C for 4 hours.XRD diffraction patterns of LaFeO 3 (LFO), La 0.1 Sr 0.9 FeO 3 (LSFO10), La 0.3 Sr 0.7 FeO 3 (LSFO30) and La 0.5 Sr 0.5 FeO 3 (LSFO50) are similar which can be assigned to the diffraction lines of LaFeO 3 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 SrLaFeO 3 [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 La1-xSrxFeO3 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, LaFeO 3 , into the nonstoichiometric La 1-x Sr x FeO 3 -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.

Preparation of Sr-Doped Lafeo 3 by Thermal Decomposition of Metal Organic Complex and their Gas-Sensing Properties
Copyright: © 2018 Koonsaeng N, et al.
absorption bands around 1565 -1440 cm -1 is ascribed to vibration of CO 3 2 -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 CO 2 .(Figure 5) shows SEM micrograph of the La 1-x Sr x FeO 3 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 m 2 /g.In the cases of Sr-doped LFO, SBET initially decreases to ~8 m 2 /g (LFSO10) but then increases as Sr (II) increases further (SSBET of LSFO30 and LFSO50 = ~10 and ~12 m 2 /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.
XRD and SEM results of the La 1-x Sr x FeO 3 (x = 0.0, 0.1, 0.3 and 0.5) thick films The thickness of La 1-x Sr x FeO 3 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 LaFeO 3 at the 2-theta angle of about 32 and 57° in addition to the high intense peaks of substrate materials, i.e., Al 2 O 3 (JCPDS file no.82-1467), and Au (JCPDS file no.04-0784).This confirms the presence of LaFeO 3 perovskite phase in all sensing films.(Figure 7) illustrates SEM micrographs of La 1-x Sr x FeO 3 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 LaFeO 3 film displays lots of cavity on the surface.

Preparation of Sr-Doped Lafeo 3 by Thermal Decomposition of Metal Organic Complex and their Gas-Sensing Properties
Copyright: © 2018 Koonsaeng N, et al.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.8 (a) -(c) while the response towards H 2 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.

Gas-sensing propertiesof the La1-xSrxFeO3thick films
The dependence on gas concentration of responses of La 1-x Sr x FeO 3 -based sensors with different Sr (II) contents towards ethanol, acetone, CH 4 and H 2 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 Organic Complex and their Gas-Sensing Properties.SOJ Mater Sci Eng 6(1): 1-9.

Preparation of Sr-Doped Lafeo 3 by Thermal Decomposition of Metal Organic Complex and their Gas-Sensing Properties
Copyright: © 2018 Koonsaeng N, et al.  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 LaFeO 3 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 LaFeO 3 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 CH 4 while LSFO30 sensor gives the best performance for H 2 .Additionally, the responses of La 1-x Sr x FeO 3 -based sensors display a similar concentration dependence behavior towards CH 4 and H 2 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 CH 4 and H 2 gases.This is evident from the very low responses, for CH 4 (3.2 at 20000 ppm) and H 2 (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-x Sr x FeO 3based 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-x Sr x FeO 3 (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 LaFeO 3 is preferable for oxygen adsorption [52].The 2p orbital of absorbed oxygen atoms consumed the 3d valence electron from Fe ion in La 1-x Sr x FeO 3 and became chemisorbed oxygen ions, Oor O 2-.In this case and ethanol is oxidized by the pre-adsorbed oxygen species on the surface of La 1-x Sr x FeO 3 , according to: Where the subscripts (g) and (ads) denote the state of gas and adsorbate, respectively The reaction generates CO 2 , H 2 O 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.

Conclusion
The La 1-x Sr x FeO 3 -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-x Sr x FeO 3 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-x Sr x FeO 3 -based sensors towards ethanol, acetone, CH 4 and H 2 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 ) indicates three weight loss steps in thermo gram profile.The first Citation: Koonsaeng N, Thaweechai, Wisitsoraat A, et al. (2018) Preparation of Sr-Doped Lafeo 3 by Thermal Decomposition of Metal Organic Complex and their Gas-Sensing Properties.SOJ Mater Sci Eng 6(1): 1-9.

Figure 3 :
Figure 3: XRD patterns of un-doped and Sr (II) doped LaFeO 3 powders calcined at 850 o C for 4 h The FTIR spectra of La 1-x Sr x FeO 3 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 ABO 3 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

Figure 8 (
Figure 8 (a)-(d) presents the relationship between operating temperature and response of La 1-x Sr x FeO 3 -based sensors with different Sr(II) contents towards the vapors of ethanol (500 ppm) and acetone (2000 ppm), CH 4 (20000 ppm) and H 2 (30000 ppm) gases.It is observed that the sensor responses towards ethanol, acetone and CH 4 monotonically increase with increasing operating temperature up to 350°C Figure8 (a) -(c) while the response towards H 2 only increases with increasing operating temperature up to 300°C but then decreases as the temperature increases further Figure8 (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.

Figure 8 :
Figure 8: The effect of operating temperature on the response of La 1-x Sr x FeO 3 -based sensors to different gases; (a) 500 ppm ethanol, (b) 2000 ppm acetone, (c) 20000 ppm methane and (d) 30000 ppm hydrogen

Figure 9 :
Figure 9: TThe effects of gas concentration on the response of La 1-x Sr x FeO 3 -based sensors to different gases at appropriate operating temperature; (a) ethanol at 350 °C, (b) acetone at 350 °C , (c) methane at 350 °C and (d) hydrogen at 350 °C