Highly sensitive CO₂ sensor based on microrods-like La₂O₃ thin film electrode

Abstract

In this paper, microrods-like La₂O₃ thin films are successfully prepared by a chemical bath deposition method. The structural, morphological, wettability and compositional properties of La₂O₃ thin films are studied using X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) and X-ray photo-electron spectrum (XPS) techniques. The La₂O₃ thin film shows highest selectivity 48% toward CO₂ gas as compared to other gases with response and recovery time periods of 50 and 73 s, respectively at a concentration of 350 ppm.

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1. Introduction

In the past few years, great efforts have been made to reduce and prevent global warming and climate change. Among them, the monitoring and control of carbon dioxide (CO₂) gas emission from motor vehicles, industries, and other sources of pollutants are of great importance. It is one of the principle gas among other green house gases and its increased emission in the atmosphere leads to the phenomenon of global warming.¹ The industrial revolution has played a critical role towards the increase of CO₂ concentration in environment, hence, it is very important to detect and control the level of CO₂ gas in the atmosphere.² A wide range of metal oxides especially; ZnO,² SnO₂,³ Fe₂O₃,⁴ CuO,⁵ TiO₂,⁶ Co₃O₄,⁷ WO₃,⁸ etc. have been used as active sensor materials for the various gases and vapors like CO, NO_x, SO_x, NH₃, alcohols etc. A number of pure and mixed metal oxides, such as ZnO, SnO₂, CdO, BaTiO₃–CuO, etc. have been investigated as CO₂ sensing materials.^3,9

The p-type oxides, such as Cr₂O₃,¹⁰ CuO,⁵ NiO,¹¹ La₂O₃,¹² Co₂O₃ (ref. 13) etc. can be good alternatives to n-type sensors. Among them, La₂O₃ shows good sensing properties towards various gases like CO_x, NO_x and ethanol.¹⁴ The band gap of La₂O₃ is 5.5 eV and a dielectric constant (k) between 8 and 23, which have motivated its application in high-k dielectric device, flash memories, incorporating as a trapping layer and in the development of luminescent materials.^15–19 In the area of chemical sensors, Jinesh et al.²⁰ reported CO₂ sensing response of a device composed by the superposition of metal/La₂O₃/Si layers. More investigations on structural, morphological and sensing studies of p-type oxides are necessary, in order to gain insight on their sensing mechanism.

In the present work, chemical bath deposition a versatile, economic and simple method, is used for the large area deposition of La₂O₃ thin film. The sensing properties of La₂O₃ thin film are studied for CO₂ gas in the temperature range 473–573 K and concentration range of 100 to 350 ppm. La₂O₃ thin film are characterized by X-ray powder diffraction (XRD), Fourier Transform Raman (FT-Raman) spectroscopy, X-ray photo-electron spectrum (XPS), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques.

2. Experimental

The La₂O₃ thin film was synthesized by chemical bath deposition (CBD) method. The analytical grade lanthanum nitrate (La(NO₃)₃) and urea ((NH₂)₂CO) were dissolved in 50 ml distilled water with slowly stirring until a clear homogeneous solution was obtained. The properly cleaned glass substrate was immersed in the mixed solution and then kept in constant temperature water bath at 358 K. After 5 h, La₂O₃ thin film deposited on the glass substrate was air annealed at temperature 773 K.²¹ The reaction mechanism of film formation is represented as follows.and

In case of CBD method, the film is formed when supersaturated solution transformed in to the saturated state. The transformation includes the step such as the nucleation, coalescence and growth by aggregation of particles, which are shown in the Fig. 1.²¹ During nucleation, clusters of precursor molecule are decomposed and nucleation sites are formed. The basic structure of ground work is formed by coalescence of aggregated particle.

Fig. 1 Schematic diagram for growth of rod-like La₂O₃ thin film on SS substrate. (a) Nucleation, (b) aggregation, (c) coalescence and (d) growth of particles.

X-ray diffraction (XRD) data of La₂O₃ thin film was collected to characterize the structural and phase identification using X-ray diffractometer (Bruker AXS D8 Advance) with Cu-K radiation. The Fourier Transform Raman (FT-Raman) spectrum was recorded using FT-Raman spectrometer (Bruker Multi RAM, Germany). The surface morphology was visualized with the help of field emission scanning electron microscopy (SEM, JSM-6700F, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurement was performed on Thermo Scientific, K-alpha set up by using monochromatic Al Kα X-ray source. Active surface area was measured by Brunauer–Emmett–Teller (BET) (Quantachrome Nova Win) measurement. The CO₂ gas sensing properties of La₂O₃ films were studied in a computerized gas sensor assembly as described in ref. 22. The electrical resistance of La₂O₃ film in air (R_a) and in the presence of CO₂ gas (R_g) was measured to evaluate the CO₂ gas response (S) defined as follows:

where, R_a and R_g are the electrical resistances of La₂O₃ thin film in air and in CO₂ gas, respectively. The response and recovery time periods are defined as the time required for a change in the resistance to reach 90% of the equilibrium value after injecting and that for removing the CO₂, respectively.

3. Results and discussion

The crystal structure of La₂O₃ thin film is identified using XRD pattern as shown in Fig. 2(a). The La₂O₃ thin film shows orientations along (100), (101), (002) and (110) planes of hexagonal crystal structure La₂O₃ (JCPDS card no. 00-002-0618). Yadav et al.²³ observed hexagonal phase for chemically deposited La₂O₃ thin film. Similarly, Michel et al.²⁴ also observed the hexagonal phase for La₂O₃ thin film synthesized by co-precipitation method. The Scherrer's relation is used to calculate the grain size of La₂O₃ thin film. The crystallite size of 35.48 nm is calculated for (002) plane of La₂O₃ thin film.²⁵

Fig. 2 (a) The XRD pattern and (b) FT-Raman spectrum of La₂O₃ thin film, and (c) XPS spectra of Lanthanum (La³⁺) region and oxygen (O²⁻) regions.

The FT-Raman study showed nature of the bonding present in the La₂O₃ thin film. The Raman spectrum in the range of wave number for the 200–800 cm⁻¹ is shown in Fig. 2(b). In the present study, bands at 400, 284, and 241 cm⁻¹ are observed at all excitations. These bands have been identified as Raman bands. The band at 400 cm⁻¹ is assigned to the E_g ν1 mode and is consistent with band observed for La₂O₃ by Denning and Ross.²⁶

The composition analysis of La₂O₃ is done using XPS measurement. The XPS spectra confirmed the presence of La and O. The La 3d states in the XPS spectra as shown in Fig. 2(c) possess doublets and the peaks appearing on the high energy side of the 3d_5/2 peak is a satellite peak. The doublet of La 3d_5/2 having peak maxima at 833.35 and 838.0 eV are related to La₂O₃.²⁷ The state of O 1s indicates that two distinct oxygen species for O 1s showing that oxygen is present in the metal oxide. The main peak at 530.0 eV in the O 1s spectrum corresponds to oxygen ions.²⁸

The morphology of La₂O₃ thin film is observed with SEM micrograph at 5000× magnification. Fig. 3(a) shows that La₂O₃ thin film has rod-like morphology. Such morphology provides more surface area due to which the desorption of gas a increases. Gurav et al.²⁹ observed similar result for the chemically deposited ZnO films.

Fig. 3 (a) The SEM image at magnification 5000×, (b) TEM image and (c) selected area electron diffraction pattern (SEAD) of La₂O₃ thin film.

The La₂O₃ thin film was further investigated by transmission electron microscopy (TEM). Fig. 3(b) shows the typical TEM image of La₂O₃ thin film. TEM observation revealed that La₂O₃ thin film exhibits rod-like morphology. The crystallinity of La₂O₃ thin film is further studied by selected-area electron diffraction (SAED) pattern as shown in Fig. 3(c). The SAED pattern shows lattice fringes, which are oriented randomly with respect to each other. The high crystallinity of the powder leads to its corresponding well-pronounced Debye–Scherrer's diffraction rings in the selected-area electron diffraction (SAED) pattern.²⁹

The specific surface area and porosity of microstructure are investigated using BET technique. Nitrogen adsorption and desorption isotherms of La₂O₃ thin film are shown in Fig. 4. The isotherms can be categorized as the type IV isotherm with distinct hysteresis loop. The curves of pore volume distribution versus pore diameter exhibit that La₂O₃ thin film has mesoporous structures. The relatively large pores may have been originated from the interconnected network structures. This unique morphology could enable to fast penetration of target gas to the layer of sensing materials resulting in a considerable improvement in their sensing performance.²³

Fig. 4 Nitrogen adsorption–desorption isotherm of La₂O₃.

4. CO₂ gas sensing properties

Before exposing to CO₂ gas, La₂O₃ films were allowed to be stable for electrical resistance for 30 min and the stabilized resistance was taken as R_a. In the present study, initially the gas response to 350 ppm of CO₂ gas was measured as a function of operating temperature for La₂O₃ film and the result is shown in Fig. 5(a). From the figure, it is found that the sensor response reaches maximum at 523 K (gas response = 48%) and then decreases at 623 K (gas response = 29%). The optimum temperature for La₂O₃ based CO₂ gas sensor is lower than that of the CO₂ sensor based on CO-doped ZnO.⁹ Therefore, the temperature 523 K was taken as an optimum operating temperature for further study. The sensitivity of these sensor increases with increasing operating temperature obtaining maximum value and then decreases with further increasing in operating temperature. These behavior based on the adsorption and desorption kinetics on the surface of La₂O₃. If the operating temperature is relatively low, the chemical activity of La₂O₃ is low which leads the low response. At high temperature, some of the gas molecules escape from the surface before reaction because of strong thermal motion due to high temperature, and thus response decreases.¹⁰ Therefore, the optimum temperature is required for good response.

Fig. 5 The variation of CO₂ gas response of La₂O₃ thin film with (a) different operating temperatures under exposure of 350 ppm CO₂ gas, (b) CO₂ gas response at different concentrations of CO₂, at operating temperature at 523 K. (c) Variation of response and recovery time periods at different concentrations of CO₂ gas and (d) variation of CO₂ gas response at different relative humidity.

Once the operating temperature was fixed the sensor response was studied to different CO₂ gas concentrations. Fig. 5(b) shows the response of La₂O₃ film as a function of CO₂ concentration at 523 K. The figure reveals that the response increased from 4.8 to 48% as CO₂ concentration increased from 100 to 350 ppm. Further increase in concentration the increase in CO₂ gas response was gradual and saturated at concentrations more than 350 ppm. The response of a sensor depends on removal of adsorbed oxygen molecules by reaction with a target gas and generation of electrons. For a small concentration of gas, exposed to a fixed surface area of a sample, there is a lower coverage of gas molecules on the surface and hence lower surface reaction occurs. An increase in gas concentration, increases the surface reaction due to a larger surface coverage.²⁹ A further increase in surface reaction is gradual when the saturation point on the coverage of molecules is reached. The response and recovery plots of La₂O₃ thin film upon exposure of different concentrations of CO₂ gas at 523 K are represented in Fig. 5(c). La₂O₃ thin film shows the response and recovery time periods of 50 and 73 s, respectively.

During practical operating conditions of CO₂ sensor in both indoor and outdoor applications, it is in contact with a humid environment. The presence of humidity may interact strongly with the gas detection. Although it is not always the case, generally humidity leads to a degradation of metal oxides-based sensor performance. In fact, the water molecules could react with the chemisorbed oxygen species or adsorb on the metal oxide surface in both cases, limiting the availability of active sites for the adsorption of the gas molecules. If these active sites are the ones involved in the sensing mechanism of the analyte species to detect, their limited availability leads to a reduced sensitivity.⁹ The behavior of mixed gas (CO₂ gas and humidity) is shown in Fig. 5(d). The sensitivity of CO₂ gas is reduced with increasing relative humidity (RH), the sensor response S_mix% is given by,

where, R_CO2 is the sensor resistance at a given RH and CO₂ level, and R_Wet is the sensor resistance at the same RH level, but in an atmosphere containing 0 ppm CO₂. The S_mix represents the resistance change due to the response of the sensor to CO₂ gas, when it is added to an already humid atmosphere. The CO₂ sensitivities at 350 ppm are seen to reduce in an approximately linear fashion as RH increases. This may arise if the humidity is preferentially adsorbed at the sensor interface, progressively inhibiting access to CO₂ sites.⁹

The selectivity study of CO₂ sensor with respect to other gasses such as N₂, LPG, ethyl alcohol (C₂H₂OH) and acetone ((CH₂)₃CO) at 523 K is shown in Fig. 6(a). The response for CO₂ gas is greater than that of the other gasses at the same concentration of 350 ppm. To confirm the stability of La₂O₃ sensor the response was studied at 300 ppm over a month. Stability of this material is good enough to detect CO₂ over a long time as shown in Fig. 6(b). The CO₂ sensor also shows good response to N₂ gas. The N₂ gas is considered as chemically inert gas at high temperature. While, CO₂ is an oxidizing gas and it reacts with oxides at elevated temperatures.³⁰ Therefore, La₂O₃ thin film shows more selectivity towards CO₂ gas.

Fig. 6 (a) The gas responses of La₂O₃ films to various gases with concentrations of 350 ppm at 523 K, and (b) stability of La₂O₃ and Pd–La₂O₃ thin film at 523 K and 500 ppm for 30 days.

The mechanism of CO₂ gas sensing of La₂O₃ explained with help of schematic diagram as shown in Fig. 7. La₂O₃ is a p-type semiconductor metal oxide in which holes are majority charge carriers as Fig. 7(a). At the higher temperature, the excited electrons from the valance band come at the surface due to which the resistance of La₂O₃ surface decreases [Fig. 7(b)]. When this surface is exposed to air, the oxygen from the air is adsorbed on La₂O₃ surface by trapping electrons from surface [Fig. 7(c)] and oxygen is ionized in to O⁻ and O²⁻. Due to the accumulation on the surface, the resistance of surface decreases.³¹ When reducing CO₂ gas is introduced, it reacts with the adsorbed oxygen ions and chemisorbed by donating electrons to the oxygen. The electrons are increased and the recombination of electrons–holes takes place which results in the increase of sensor resistance [Fig. 7(d)].³² The absorbed CO₂ acts as an acceptor in accordance with the following reversible process,

CO₂ + e⁻ → CO₂⁻, (5)

and

CO₂ + O₂⁻ + 2e⁻ → CO₂⁻ + 2O⁻ (6)

Fig. 7 Schematics diagrams of La₂O₃ showing the corresponding CO₂ gas sensing mechanism. (a) The conduction of thermally exited electrons, (b) donation of electron metal oxide to oxygen, (c) chemisorption of oxygen on the surface of metal oxide, (d) under the exposure of CO₂ gas molecule, reacts with the chemisorbed oxygen, and (e) under the exposure of fresh air, CO₂ gas is removed and oxygen are chemisorbed on metal oxide surface.

In desorption of CO₂ gas molecules, the charge carriers in the accumulation layer are restored and cause decrease in the sensor resistance as shown in Fig. 7(e). The activation energy of adsorption and desorption depends on the sensor temperature on which the response and recovery times of the sensor signal depend.³¹ From the literature, it is seen that the surface reaction mechanisms at the low and high operating temperature are different. This depends on the interaction between the surface molecule and gas molecule. At the high temperature surface is sufficiently active, so probability of CO₂ gas molecule regeneration is low.^33,34 At higher temperature, the required activation energy is gained thermally and hence catalytic conversion takes place.

5. Conclusions

The rod-like La₂O₃ thin films are synthesized using chemical bath deposition method. The synthesis of La₂O₃ thin film is confirmed by the XRD, FT Raman, XPS, SEM, TEM and BET studies. La₂O₃ thin film is used as a sensing layer for chemoresistive CO₂ gas sensor. The sensor response showed a linear relationship with CO₂ gas concentration. The best sensing performance with a maximum response of 48% is obtained at 523 K and 350 ppm. The experimental results indicate the good potential of using La₂O₃ thin film for CO₂ gas sensing.

Acknowledgements

This work was supported by the Human Resources Development program (No. 20124010203180) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea government Ministry of Trade, Industry and Energy and supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A2A2A01006856).

References

1. R. N. Bulakhe and C. D. Lokhande, Sens. Actuators, B, 2014, 200, 245–250.
2. M. Morio, T. Hyodo, Y. Shimizu and M. Egashir, Sens. Actuators, B, 2009, 139, 563–569.
3. Y. Sadaok, Sens. Actuators, B, 2007, 121, 194–199.
4. H. J. Kim, K. I. Choi, A. Pan, I. D. Kim, H. R. Kim, K. M. Kim, C. W. Na, G. Cao and J. H. Lee, J. Mater. Chem., 2011, 39, 6549–6555.
5. D. Gopalakrishna, K. Vijayalakshmi and C. Ravidhas, J. Mater. Sci.: Mater. Electron., 2013, 13, 1004–1011.
6. I. D. Kim, A. Rothschild, B. H. Lee, D. Y. Kim, S. M. Jo and H. L. Tuller, Nano Lett., 2006, 6, 2009–2013.
7. C. Sun, X. Su, F. Xiao, C. Niu and J. Wang, Sens. Actuators, B, 2011, 157, 681–685.
8. X. L. Li, T. J. Lou, X. M. Sun and Y. D. Li, Inorg. Chem., 2004, 43, 5442–5449.
9. R. Dhahri, S. G. Leonardi, M. Hjiri, L. E. Mir, A. Bonavita, N. Donato, D. Iannazzo and G. Neri, Sens. Actuators, B, 2017, 239, 36–44.
10. V. B. Kamble and A. M. Umarji, J. Mater. Chem. C, 2013, 1, 8167–8176.
11. N. G. Cho, I. S. Hwang, H. G. Kim, J. H. Lee and I. D. Kim, Sens. Actuators, B, 2011, 155, 366–371.
12. S. J. Huang, A. B. Walters and M. A. Venice, J. Catal., 2000, 194, 29–47.
13. Y. Liu, G. Zhu, B. Ge, H. Zhou, A. Yuan and X. Shen, CrystEngComm, 2012, 14, 6264–6270.
14. S. J. Huang, A. B. Walters and M. A. Vannice, Appl. Catal., B, 2000, 26, 101–118.
15. M. Y. Yang, D. S. Yu and A. Chin, J. Electrochem. Soc., 2004, 151, 162–165.
16. S. Ohmi, C. Kobayashi, I. Kashiwagi, C. Ohshima, H. Ishiwara and H. Iwai, J. Electrochem. Soc., 2003, 150, 134–140.
17. Y. H. Lin, C. H. Chien, T. Y. Yang and T. F. Lei, J. Electrochem. Soc., 2007, 154, 619–622.
18. G. Mao, H. Zhang, H. Li, J. Jin and S. Niu, J. Electrochem. Soc., 2012, 159, 48–53.
19. F. He, P. Yang, D. Wang, C. Li, N. Niu, S. Gai and M. Zhang, Langmuir, 2011, 27, 5616–5623.
20. K. B. Jinesh, V. A. Dam, J. Swerts, C. Nooijer, S. Elshocht, S. H. Brongersm and M. Crego-Calama, Sens. Actuators, B, 2011, 156, 276–282.
21. M. F. Vignolo, S. Duhalde, M. Bormioli, G. Quintana, M. Cerver and J. Tocho, Appl. Surf. Sci., 2002, 198, 522–526.
22. U. T. Nakate, R. N. Bulakhe, C. D. Lokhande and S. N. Kale, Appl. Surf. Sci., 2016, 331, 224–230.
23. A. A. Yadav, A. C. Lokhande and C. D. Lokhande, Mater. Lett., 2015, 108, 500–502.
24. C. R. Michel and A. H. Martinez, Sens. Actuators, B, 2015, 208, 355–362.
25. A. A. Yadav, V. S. Kumbhar, S. J. Patil, N. R. Chodankar and C. D. Lokhande, Ceram. Int., 2016, 42, 2079–2084.
26. J. H. Denning and S. D. Ross, J. Phys. C: Solid State Phys., 1972, 5, 1123–1133.
27. C. Yang, H. Fan, S. Qiu, Y. Xi and Y. Fu, J. Non-Cryst. Solids, 2011, 355, 33–37.
28. T. Honma, Y. Benino, T. Fujiwara, T. Komatsu, R. Sato and V. Dimitrov, J. Appl. Phys., 2002, 91, 2942–2950.
29. K. V. Gurav, U. M. Patil, S. W. Shin, S. M. Pawar, J. H. Kim and C. D. Lokhande, J. Alloys Compd., 2012, 524, 1–7.
30. S. Kar, B. N. Pal, S. Chaudhuri and D. Chakravorty, J. Phys. Chem. B, 2006, 110, 4605–4609.
31. C. Sun, G. Xiao, H. Li and L. Chen, J. Am. Ceram. Soc., 2007, 90, 2576–2581.
32. M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto and T. Watanabe, Langmuir, 2000, 16, 5754–5760.
33. S. E. Mason, I. Grinberg and A. M. Rappe, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 69, 161–401.
34. J. W. Yoon, J. K. Choi and J. H. Lee, Sens. Actuators, B, 2012, 161, 570–577.

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