Synthesis and Characterization of Al₂o₃-Doped Lafeo₃ Thick Film Ceramics for Ethanol Gas Sensing Application

Introduction

Gas sensors are electro-chemical sensors which provide an electrical signal in response to chemical interaction with the gas.¹ A gas sensor enables to detect environmental changes by changing the amount of chemical into electrical quantities.^2,3 Ethanol gas sensor (C₂ H₆O) is one of the gas sensors needed for daily activities such as in the perfume industry,⁴ food and beverages.⁵

Perovskite-type oxides with an ABO₃ perovskite structure (A: alkali metal cations or alkaline-earth metal; B: transition metal cations) has been widely used as raw material for gas sensor for it has high temperature stability and selectivity to different gases.^6,7,8 The example of perovskite-type oxides is lanthanum ferrite (LaFeO₃) which is derived from synthesis of La₂O₃ and Fe₂O₃. Metal oxides can be obtained from purified yarosit mineral. This yarosit mineral are easily found in Indonesia, on the north coast of the Java, Sumatra, Kalimantan and Bangka Island.⁹

The purified yarosit mineral contains Fe₂O₃ amounting to 91.30% by weight, 3.30% of Al₂O_3, 2.05% of SiO_2, 3.02% of TiO₂, 0.16% of CaO and 0.12% of MnO [10]. Since it has a large content of iron, the purified yarosit mineral has potential as an industrial material that is used as the base material for thick film ceramics to apply gas sensors. Research on the LaFeO₃ ethanol gas sensor with Fe₂O₃ as the basic materials, the yarosit purification result has been performed and the results show that the material is sensitive to the presence of ethanol gas .^5.11

The effect of each ingredient contained in yarosit purification result has not been recognized. To determine the effect of the minerals, the addition of purified yarosit Al₂O₃ was done as much as 3.75% mole or equal to 1.6% by weight of the main ingredient of LaFeO₃. This concentration is a concentration below the concentration of Al2O₃ contained in yarosit mineral purification results. The materials used in the manufacture of Al₂O₃-dopped LaFeO₃ are pure materials so that there are no impurities other than in ceramics. Crystal characteristics, morphology and electrical characteristics of thick film ceramics will be analyzed and compared with Al₂O₃-dopped LaFeO₃. If the addition of 3.75% mole of  Al₂O₃ shows a positive influence on LaFeO₃, the content of Al₂O₃ in the purified yarosit shall not be eliminated so it will reduce the cost of the purification process.

Materials and Methods

Materials

The materials used in this study are, Lanthanum (III) chloride heptahydrate (Iron, (III) oxide (Fe₂O₃) and aluminum chloride (LaCl₃). These materials are determined severity based chemical calculations. The powder of LaFeO₃ is synthesized from powder and mole ratio of 1: 1. The powder of Al₂O₃-dopped LaFeO₃ was synthesizedfrom LaCl₃, 7H₂O₃ and laCl₃ comparing with each mole were 13:13;1 respectively.

Preparation of Powder

LaFeO₃ and Al₂O₃-dopped LaFeO₃  powder were made by using co-precipitation method. First, all ingredients were dissolved with each solvent. LaCl₃, 7H₂O, and AlO₃ were dissolved by using distilled water (aquades) and Fe₂O₃ was dissolved with  HCl 10 M. After each ingredient was dissolved, all solution was mixed and was added by NH₄OH solution so that the precipitate of LaFeO₃ and Al₂O₃-dopped LaFeO₃ was formed. The precipitate was dried at a temperature of 100°C and was calcined at a temperature of 800°C for 2 hours. The results of calcination were crushed and screened to the same nano-sized powder.

Preparation Thick Film

Each LaFeO₃ and Al₂O₃-dopped LaFeO₃ powder was mixed with Organic Vehicle (OV) with a ratio of 70% and 30% in order to form a paste. The pasta was coated on a substrate of alumina which has been coated with silver using screen printing method and baked at a temperature of 600°C for 2 hours to produce LaFeO₃ and Al₂O₃-dopped LaFeO₃  thick film ceramics.

Characterization of Thick Film

Crystal characteristics of LaFeO₃ and Al₂O₃-dopped LaFeO₃ thick film ceramics were analyzed using X-Ray Difractrometer (XRD PRO series, λ = 1.540598Å, PAN), thick film ceramic surface morphology was analyzed using Scanning Electron Microscope (SEM, JEOL-6360LA) and electrical characteristics of thick film ceramics were measured by using a set of tools chamber gas. The measurement of the electrical properties was done by heating the thick film ceramics from room temperature to a temperature of 360°C. When the resistance value was read, every increase in temperature of 5°C, the temperature and the resistance value were recorded. Electrical characteristic testing was done without using gas (ambient state) and using ethanol gas at a concentration of 100 ppm, 200 ppm to 300 ppm.

Results and Discussion

Characteristics of Crystals  

Graph of the XRD LaFeO₃ and Al₂O₃-dopped LaFeO₃ thick film ceramics is shown in Figure 1. From the graph it can be seen the diffraction angle and orientation of the area corresponding to the database crystallographic open database (COD) No.96-154-2033. The formed phase crystals are the same phase that is cubic with lattice parameters a = b = c = 3,926Å. It can be seen from the graph that the relationship between the intensity and 2θ  had the same diffraction pattern. The addition of 3.75% mole of Al₂O₃ did not change the phase and the lattice parameter of LaFeO₃ thick film ceramics.

Figure 1: XRD pattern of LaFeO3 and Al2O3 dopped LaFeO3 Click here to view figure

 

The average size of the crystals can be calculated using the Debye-Scherrer equation as follows⁷:

D is the size of the crystal in the (nm), λ is the wavelength used in the test XRD (nm), and B is the peak half width in radians, and θ is the angle position crests. The average of crystallite size of LaFeO₃ and Al₂O₃-dopped LaFeO₃ thick film ceramics are respectively 52.17nm and 44.52nm. Crystallite sizes shrink when added to. doping.

The Surface Morphology

SEM test results of LaFeO₃ and Al₂O₃-dopped LaFeO₃ thick film ceramics can be seen in Figure 2.

Figure 2: Morphology structure of thick films of ceramics (a) LaFeO3 and (b) Al2O3-doped LaFeO3. Click here to view figure

 

The effect of the addition can be seen from the grain size of each ingredient. The surface of LaFeO₃ and Al₂O₃-dopped LaFeO₃ thick film ceramics has almost the same (uniform) grain sizes with the size 0.40 μm and 0.25 μm each. The grain size decreases when added by Al₂O₃. This is due to the addition of 3.75% mole of Al₂O₃ causing the particles of Al₂O₃ were segregated at the border of the grain so that the grain size of can be reduced resulting in contact among the grains become mutually bounding. Figure 2 also shows the pores of La  doped by Al₂O₃ that seem smaller and few.

Electrical Properties

Figure 3 shows a graph of the temperature with a thick film ceramic resistance value. The resistance value decreases as the operating temperature increases. This is in accordance with semiconductor characteristic that is the semiconductor is lowering barriers exponentially when the ambient temperature increases.¹²

Figure 3: Graph of functional resistance of temperature of thick films ceramics (a) LaFeO3 and (b) Al2O3 doped LaFeO3. Click here to view figure

 

Figure 4: The band gap energy structure of semiconductor (a) p-type and (b) n-type. Click here to view figure

 

For the p-type semiconductor, at low temperatures electrons in the valence band will move to the level of the acceptor producing holes in the valence band, whereas the conduction band electrons are not free because it requires a fairly high energy (thermal energy). Concentration of electric charge carriers was unbalanced, the amount of holes are much larger than free electrons. For the n-type semiconductor, at low temperatures, the electrons at donor level can be to the level of conduction into free electrons, while the electrons in the valence band cannot move into the conduction band since it requires considerable energy. In these circumstances, the concentration of electric charge was not balanced; the amounts of free electrons are much larger than the hole. Along with the increase of temperature on semiconductor material the chance of occurrence process of atom ionization of semiconductor crystal compiler will increase, thus there will be electrons at valence band which moved to conduction band so the free electron concentration will increase as well. The process of electron excitation to the conduction band will leave a hole in the valence band.¹³ The more electrons move to the conduction band, the easier thick film ceramics in conducting the electric current so that the resistance value will be smaller.

The Sensitivity and the Sensor Mechanism

Thick film ceramic sensitivity values ​​are calculated using equation (2)¹⁴:

where S is the sensitivity of the sensor, R_g, is the resistance when the gas contained ethanol (MΩ), and R_g, is the resistance at ambient circumstances (MΩ).

Figure 5: Graph of sensitivity function of temperature of thick film ceramics (a) LaFeO3 and (b) Al2O3 doped LaFeO3. Click here to view figure

 

At the same temperature, the resistance value increases along with increasing the concentration of ethanol gas. Since ethanol gas is a reducing gas^14,15 it is known that LaFeO₃ and Al₂O₃-dopped LaFeO₃ thick film ceramics has the characteristics as a p-type semiconductor (hole majority carrier) for materials with excess holes will react towards reduction with ethanol gas resulting in decreasing the gas flow characterized by an increased resistance of the material. The increase of the value of this resistance occurs because the more gas is adsorbed, the number of atoms / ions of oxygen in the thick film ceramics increases and the number of electrons decreases. The more the concentration of ethanol gas, the higher the potential barrier which were formed so the electron is hard to be excited from the valence band to the conduction band.^16,17

The influence of operating temperature on the sensitivity of LaFeO₃ and Al₂O₃-dopped LaFeO₃ thick film ceramics at 100, 200 and 300ppm ethanol gas was shown in Figure 5 (a) and (b). The gas response increased along with increasing temperature of operation and achieved maximum sensitivity at the operating temperature 330°C for LaFeO₃ thick film ceramics and 350°C for Al₂O₃-dopped LaFeO₃ and then decreased along with increasing operating temperature.

At high temperatures the oxygen molecule was adsorbed the surface of thick film. The order of the processes involved in the absorption of oxygen can be described by the following equation¹⁵:

O₂(gas) ↔ O₂(ads)                                          (3)

O₂(ads) + e¯ ↔ O₂¯  (ads)                           (4)

O₂¯  (ads) + e¯ ↔ 20¯ (lattice)                   (5)

in this state the electrons present in the valence band obtain a number of thermal energy that can be excited into the conduction band and leave holes in the valence band (Figure 6 (a)).

Figure 6: Adsorption -oxidation-desorption process (a) ambient and (b) ethanol exposed gases. Click here to view figure

 

In Figure 6 (a) the oxygen molecules were adsorbed on the surface of the thick film ceramics will be ionized because it binds to electrons in the conduction band that are shown by equations 4 and 5. In addition to the formation of ion O¯ this absorption of oxygen causes forming a depletion layer around the grain boundaries. Depletion layer thickness is determined by the amount of oxygen and intrinsic electrons contained in the material surface. Layer depletion at grain boundaries generate potential barrier so that the grain boundaries impede the movement of electrons.¹⁷

Figure 7: The potential barrier of the p-type semiconductor in various condition. Click here to view figure

 

At the time of LaFeO₃ and Al₂O₃-dopped LaFeO₃ thick film ceramics exposure to ethanol gas, the ethanol gas molecules will react with oxygen gas molecules on the surface of thick film ceramics that has been ionized into O¯ and O²¯ produced 2CO₂ and 3H₂O in accordance with Figure 6 (b). The mechanism of the gas sensor is basically an oxidation-reduction reaction on the surface of the material. O²¯ was absorbed accelerating this following reaction¹⁴:

C₂H₅OH + 6O^n- → 2CO₂ + 3H₂O +6ne^–                     (6)

This reaction caused the increase in resistance value that was indicated by the higher potential barrier (Figure 7 (b)) as the detached electrons return to the valence band so that the number of holes in the valence band was reduced.^14,15

Acknowledgments

This work was financially supported by “Hibah Penelitian Terapan Unggulan Perguruan Tinggi” Research Grants in the fiscal year 2018.

Conclusion

The powder of LaFeO₃ and Al₂O₃-dopped LaFeO₃ has been made by employing co-precipitation method. Thick film ceramics were generated through screen printing method. The XRD results showed LaFeO₃ and Al₂O₃-dopped LaFeO₃ thick film ceramicshas a cubic phase with a lattice parameter a = b = c = 39.52nm. The crystallite size before and after the addition of doping was shrunk from 52.17nm to 44.52nm.  The addition of Al₂O₃ minimized the size of grain from 0:40 μm and 0.25 μm, increased the value of electric resistance and increase the operating temperature. Gas sensors doped by Al₂O₃have a higher sensitivity of the gas sensor without the addition of doping.

References

1. Yadav, AK; Singh, RK; Singh, P. Sensors and Actuators B: Chemical. 2016, 229, 25-30.
   CrossRef
2. Hiskia. BATAN. 2007, 7-18.
   CrossRef
3. Sujitno, T., Atmono, T., Sayono, S., & Susita, L. BATAN. 2006, 9(2).
   CrossRef
4. Rahimah S.; Sharif DG; Suhendi E. BATAN. 2011, 329-322.
5. Suhendi, E.; Witra; Hasanah, L.; Sharif, DG AIP. 2017, 1848.
6. Cyza, A .; Kopia, A.; Cieniek, lbs.; Kusiński, J. Materials Today. 2016, 3(8), 2707-2712.
   CrossRef
7. Haron, W.; Wisitsoraat, A.; Wongnawa, S. Prepared Oxide Nanocrystalline perovskite LaFeO3 at Lower Temperature with Improved Ethanol Gas Sensoring. ICBNE. 2015, 10-14.
8. Khetre, SM; Chopade, AU; Khilare, CJ; Kulal, SR.; Jadhav, HV; Jagadale, PN; Bamane, SR, J of Shivaji Union (Sci & Tech). 2014, 41(2), 250-5347.
9. Taufik, D.; Sharif, DG; Karim, S. BATAN. 2007.
10. Suhendi, E.; Novia, H.; Sharif, DG; Prajitno, D. H, Proceedings of the National Seminar on Research, Education and Application of Mathematics, Faculty of Science, University of Yogyakarta, 2010.
11. Ariyani, NI; Sharif, DG; Suhendi, E. IOP. 2018, 384.
12. Yacoobi, BG Plenum. 2003.
13. Barsoum, MW Institute of Physics. 2003.
14. Thuy, NT; Minh, DL; Giang, HT; Toan, NN Hindawi. 2014.
    CrossRef
15. Benali, A.; Azizi, S.; Bejar, M.; Dhahri, E .; Graça, MFP Ceramics International. 2014, 40(9), 14367-14373.
16. Shankar, P.; Rayappan, JBB Sci. Lett. J, 2015. 4, 126.
17. Balaguru, RJB.; Jeyaprakash, BG Tamil Nadu. 2012.

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