Enhanced sensitivity and selectivity of CO2 gas sensor based on modified La2O3 nanorods

Accepted Manuscript Enhanced sensitivity and selectivity of CO2 gas sensor based on modified La2O3 nanorods A.A. Yadav, A.C. Lokhande, J.H. Kim, C.D. Lokhande PII:

S0925-8388(17)32222-3

DOI:

10.1016/j.jallcom.2017.06.223

Reference:

JALCOM 42292

To appear in:

Journal of Alloys and Compounds

Received Date: 15 April 2017 Revised Date:

20 June 2017

Accepted Date: 21 June 2017

Please cite this article as: A.A. Yadav, A.C. Lokhande, J.H. Kim, C.D. Lokhande, Enhanced sensitivity and selectivity of CO2 gas sensor based on modified La2O3 nanorods, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.223. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Enhanced sensitivity and selectivity of CO2 gas sensor based on modified La2O3 nanorods A.A. Yadava, A.C. Lokhandeb, J.H. Kim *b, C. D. Lokhande*c

Kolhapur - 416004 (M.S), India

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a Thin Film Physics Laboratory, Department of Physics, Shivaji University,

b Photonic and Electronic Thin Film Laboratory, Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea

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c Centre for Interdiscipleean studies, D. Y. Patil University, Kolhapur, (M S) India Abstract

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La2O3 thin films are synthesized successfully by simple microwave assisted chemical method and characterized with structural and morphological charactrition techniques. CO2 gas sensing properties of La2O3 thin films are enhanced significantly by palladium (Pd) sensitization. La2O3 thin film shows maximum sensitivity of 25% at temperature of 723K and concentration of

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400 ppm CO2 gas, which enhanced to 64% at temperature of 523 K after Pd sensitization. Keywords: CO2 sensor, La2O3, Nanorods, Pd: La2O3, Thin films. CORRESPONDING AUTHOR FOOTNOTE

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*Prof. C. D. Lokhande

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Tel.: +91 231 2609225, Fax: +91 231 2609233 E-mail address: [email protected] Prof. J. H. Kim

Tel.: +82 62 530 1702, fax: +82 62 530 1699. E-mail address: [email protected]

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1. Introduction

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In the recent years, global warming and climate change are the alarming problems. Therefore, to reduce global warming and control the climate change, monitoring of responsible gases emission is necessary. Carbon dioxide (CO2) gas is one of the greenhouse gas and it’s

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emission from the industries, motor vehicles etc causes global warming. More even, industrial revolution has played a critical role for increased CO2 gas emission in the environment [1].

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Hence, it is very important to detect and control the level of CO2 gas in the atmosphere. The monitoring of CO2 gas is useful in the food control [2], breath analysis [3] etc. Various nanostructured materials have been proposed as sensing material to develop CO2 sensor. These materials are widely used for the fabrication of gas sensors because of their

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enhanced surface area which increases adsorption of gas molecules and promotes surface reaction with gaseous species [4]. Metal oxides, such as CdO, BaTiO3-CuO, SnO2 etc. have been investigated as CO2 sensing materials [5-10].

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La2O3 is used in the various applications such as gas sensor, supercapacitor, catalyst, photoelectrochemical cell, and Li ion- batteries [11–15]. Various methods including

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hydrothermal, chemical bath deposition, chemical vapor deposition, and pulsed laser deposition have been used to develop nanostructures [16-19]. Among these, microwave assisted chemical bath deposition (MW-CBD) synthesizes uniform nanoporous structures [18]. In microwaves assisted chemical synthesis, volumetric heating of materials causes the conventional heat transmission of energy to reaction bath by conduction and convection. Energy and process time conservation are optimized in MW-CBD method due to the transformation of microwave energy

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into heat inside the material [17]. Also MW-CBD method provides advantages such as controlled thickness, adherent film formation, short deposition time, and low temperature deposition [19,20].

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The enhancement in gas sensing performance is achieved by the addition of other phases to the metal oxide to increase physical and chemical interactions between gas and host material surface [21]. Such surface modification, known as decoration or sentanaization includes the addition of

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noble metal (Au, Ag, Pt, Pd) nanoparticles which act as catalysts, influence chemical and electronic sensitization [22]. The sensitization of noble metal nanoparticles has led to large

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increase in sensor response of different oxides [5–8].

In present work, La2O3 thin films are prepared by microwave assisted chemical bath deposition method. CO2 gas sensing performance of La2O3 thin film is improved using sensitization of palladium (Pd) metal. The Pd sensitized La2O3 thin films are characterized by X-

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ray powder diffraction (XRD), X-ray photo-electron spectrum (XPS), Brunauer-Emmer-Teller (BET), transmission electron microscopy (TEM) and field emission scanning electron

are studied. 2. Experimental

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microscopy (FE-SEM) techniques. CO2 gas sensing properties of La2O3 and Pd: La2O3 thin films

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La2O3 thin film was prepared using microwave assisted chemical bath deposition (MW-CBD) method with working frequency of 2.45 GHz. The analytical grade lanthanum nitrate (La (NO3)3) and urea ((NH2)2CO) were dissolved in 50 ml distilled water to obtain homogenous and clear precursor solution. The reaction bath containing glass substrate placed in the reaction beaker. This system was placed at the center of microwave oven. The gray colored lanthanum oxycarbonate (La2O2CO3) thin film was formed, which further annealed at 773 K to form La2O3

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[23]. For Pd sensitization, La2O3 film was dipped in a methanolic solution of 10 mM PdCl2 for 5 s and then dried by air-flow. This process was repeated for 15 times. The Pd sensitized La2O3 films was heated at 473 K to remove the chloride from Pd: La2O3 thin film [19].

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2.1 La2O3 thin film characterizations

The structure and phase identification of La2O3 and Pd: La2O3 thin films were carried out using X-ray diffractometer (Bruker AXS D8 Advance) with Cu-Kα radiation. The surface

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morphology was visualized with the help of field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurement was

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performed on Thermo Scientific, K-Alpha set up using monochromatic Al Kα X-ray source. The active surface area was measured by Brunauer-Emmer-Teller (BET) (Quantachrome Nova Win) measurement. 2.2 CO2 gas sensing measurements

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The detailed gas sensing experimental process by using computerized gas sensor unit can be found in the previous report [24]. The silver contact was printed on surface of sample and two probes pressed on the sample by adjusting their position in the system. The sensor was preheated

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at various operating temperatures to obtain the stable resistance (Ra), and target gas is injected into the target chamber. The sensor resistance reaches to the new constant value (Rg). The gas

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response calculated as,




% =  X 100

(1)



where, Rg is resistance measured in the presence of the target gas and Ra in the presence of air. Response and recovery time periods are defined as the time needed to reach 90% of the total resistance change on exposure to the gas and the air.

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3. Results and discussion 3.1 Structural study Fig. 1(a) shows XRD pattern of La2O3 films which shows (100), (101), (102), (110), (103),

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(112), and (204) planes of hexagonal phase of La2O3 (JCPDS 00-024-0554). Fig. 1 (b) shows XRD pattern of Pd: La2O3 which contains low intense peaks for Pd (fcc phase) corresponding to (111) plane at 40.15° and (200) at 46.27° (JCPDS No. 01-1201) along with peaks of La2O3. The

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crystallite size of La2O3 and Pd: La2O3 film particles calculated using (101) plane are 55 and 35 nm, respectively for La2O3 and Pd: La2O3. The incorporation of Pd content shows reduction

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in crystallite size which rises surface area [25].

The information about valance states of La and O in La2O3 is gained from XPS measurements. The narrow scan spectrum for the lanthanum (Fig. 2 (a) shows characteristic peaks centered at 834.20, and 855.84 eV assigned to La3d5/2 and La3d3/2 core levels,

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respectively. The doublet is formed due to hybridization of O2p and La4f orbitals [26]. In agreement with previous XPS measurements La3d5/2, and La3d3/2 peaks related to La2O3 are positioned over the binding energy range of 835.2 to 837.0 eV [27-29]. The XPS spectrum for

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Pd3d is shown in Fig. 2 (b). The peaks at 336.5 and 338.04 eV of Pd 3d5/2 are attributed to metallic Pd0 and Pd+2 (PdO), respectively. The peak at 343.03 eV of Pd 3d3/2 is due to presence

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of PdO [27]. The O1s core level is shown in Fig. 2 (c). Two peak at 528.5, and 531.2 eV are connected to oxygen atoms in La2O3 and carbonate groups, and hydroxyl impurities, respectively [30,31].

3.2 Morphological study

Fig. 3 (a and b) displays FESEM images of La2O3 and Pd: La2O3 thin films, respectively at magnification of 25,000 X. La2O3 thin film shows porous rod-like morphology (Fig. 3 (a). When

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La2O3 thin film is immersed in a methanolic solution of 10 mM PdCl2 for 5 s, the chloride ions are adsorbed on La2O3 surface and react with lanthanum ions to form a complex [23]. The surface oxygen atoms of La2O3 attract the palladium ions in solution by electrostatic interaction.

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Pd nanoparticles attached on La2O3 surface are shown in Fig. 3 (b). The existance of disk-like nanoparticles on La2O3 rods are shown in Fig. 3 (c) at magnification of 50,000 X. Fig. 4 (a and b) shows TEM images of La2O3 and Pd: La2O3, respectively. Fig. 4 (a) confirms rod-like

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morphology of La2O3 thin film and the presence of Pd nanoparticles are shown in Fig 4 (b). 3.3 Brunauer-Emmett-Teller study

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The specific surface area and porosity of La2O3 and Pd: La2O3 films are investigated using BET technique. Nitrogen adsorption and desorption isotherms of La2O3 and Pd: La2O3 thin films are shown in Fig. 5 (a and b). The isotherms are characterized as type IV isotherm through the different hysteresis loop. Using these specific area of La2O3 of 45.56 and 55.95 m2g-1 are

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calculated for La2O3 and Pd: La2O3 films, respectively. Such morphology enables fast diffusion of target gas to a layer of sensing materials resulting in a significant development in their sensing performance [32, 8].

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4. Response to CO2 gas sensor

4.1 Effect of operating temperature

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At an exposure of 400 ppm CO2 gas concentration, CO2 gas response of La2O3 and Pd: La2O3 films as function of working temperature is shown in Fig. 6 (A). CO2 gas response for La2O3 film improved up to 25% at temperature of 723 K and decreased afterwards. The maximum response for Pd: La2O3 film is 64% at 523 K, much higher than La2O3 film. At a low operating temperature, a low response can be predictable because CO2 gas molecules do not have enough thermal energy to react with the surface adsorbed oxygen species. The electrons are drained from

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the conduction band of La2O3 by the adsorbed oxygen, and a potential barrier to charge transport is produced [33]. At higher temperatures, the thermal energy obtained is high enough to overcome the potential barrier, and a significant increase in electron concentration results from

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the sensing reaction. The response of a metal oxide gas sensor to the presence of a given gas depends on the speed of the chemical reaction on the surface, and the speed of diffusion of the gas molecules to the surface which are both activation processes. The activation energy of the

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chemical reaction is higher at high temperature. In this case, at low temperature, the sensor response is restricted by the speed of the chemical reaction, and at higher temperature, it is

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restricted by the speed of diffusion of gas molecules. At some intermediate temperature, the speed values of the two processes becomes equal, and at that point, the sensor response reaches its maximum [34]. Thus, in the present case, the optimum operating temperatures for La2O3 and Pd: La2O3 thin films are 723 and 523 K, respectively at which La2O3 and Pd-La2O3 sensor

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response attains it’s peak value. The temperature, which corresponds to a certain peak value is a function of the kind of target gases, the chemical composition of oxide, including the additives and the catalysts. Therefore, the high operating temperature is necessary for La2O3 films to

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interact with CO2 gas [35].

4.2 Effect of CO2 gas concentration

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Fig. 6 (B) shows the correlation between gas response and CO2 gas concentration from increased from 50 to 450 ppm. For La2O3 and Pd: La2O3 film sensors, as CO2 gas concentration increased from 50 to 450 ppm, the response increased up to 25 and 64%, respectively. A higher CO2 gas concentrations (>450 ppm), the increase in CO2 gas response is gradual and saturated. The response of a sensor depends on the removal of adsorbed oxygen molecules by reaction with CO2 gas and generation of electrons. For a low concentration of gas, exposed on a fixed surface

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area of a sample, there is a lower coverage of gas molecules on the surface and hence lower surface reaction occurs [36]. An increase in gas concentration increases the surface reaction due to a larger surface coverage. Fig. 6 (C) shows the response recovery time periods of Pd: La2O3

periods of Pd: La2O3 film at 400 ppm are 80 and 50 s, respectively. 4.3 Effect of humidity on response

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film due to CO2 gas concentrations ranging from 250 to 450 ppm. The response recovery time

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The presence of humidity may affect strongly the gas detection. Humidity leads to a degradation of metal oxide based sensor performance. H2O molecules could react with

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chemisorbed oxygen species or adsorb on Pd: La2O3 surface, 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 [37]. The behavior of mixed gas (CO2 gas and humidity) is shown in Fig. 6 (D). The

Smix is calculated by,

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sensitivity of CO2 gas is reduced with increasing relative humidity (RH). The sensor response


 =

 



(2)

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where, RCO2 is the sensor resistance at a given RH and CO2 level, and Rwet is the sensor

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resistance at the same RH level without CO2. The Smix represents resistance change due to response of sensor to CO2 gas, it adds to the previously humid atmosphere. CO2 sensitivity at 400 ppm is reduced as RH rises. This is due to adsorbed water molecules at sensor boundary, which increasingly constrain to the entry of CO2 gas molecules [38]. 4.4 Selectivity and stability study Variations in the electrical resistance due to adsorption of various gases is measured on Pd: La2O3 sensor. Nitrogen (N2), acetone (C3H6O), ethyl alcohol (C2H5 OH), nitrogen oxide (NO2),

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liquefied petroleum gases, and ammonia (NH3) gases are used as probe molecules to investigate the cross-sensitivity of the sensor at an operating temperature of 523 K. Fig. 7 (a) reveals that Pd: La2O3 do not show any notable response to other gases, confirming the high CO2 selectivity.

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The response of Pd: La2O3 sensor measured at 523 K and 400 ppm CO2 gas for 30 days shows 87% stability (Fig 7 (b). 4.5 CO2 gas sensing mechanism

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At a temperature of 573 K, the oxygen ions on a surface of La2O3 appear to be highly active. An appropriate amount of Pd metal shows higher catalytic activity and leads to the consumption

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of larger amount of oxygen adsorbate at lower temperature [30]. The net electron transfer from the semiconductor to the metal will occur, if Pd and La2O3 come into contact, forming a Schottky junction at the metal surface and acting as an “electron bath”, which enhances the catalytic performance of Pd [27-31]. The amount of oxygen ion species adsorbed on the surface of an

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oxide depends on the temperature of the operation [32]. In the case of La2O3, it is assumed that the surface oxygen vacancies play a role of chemisorption sites in the presence of oxygen. At low temperature, the oxygen molecules are physisorbed on the oxide surface, and the bond is

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weak, leading to a comparatively small response to CO2 gas. The oxygen species dissociate into

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more active molecules and atomic ions as follows [33]. O2(gas) ↔ O2(ads)

(3)

O2(ads) + e- ↔ O2-(ads)

(4)

O2-(ads) + e- ↔ 2O-(ads)

(5)

where, O2 gas is a gaseous oxygen molecule in an ambient atmosphere. The ionization of oxygen molecules occurs due to the capture of electrons from the conduction band of La2O3. These oxygen molecules act as electron acceptors, resulting in a deep electron depletion region

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with reduced electron mobility near the surface of the oxide. This phenomenon enhances the Schottky barrier potential as shown in (Fig. 8 (a). It is seen that when La2O3 sensor is exposed to CO2, the surface oxygen ions react with the

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dissociated oxygen ions and create a number of carriers. Pd atoms are weakly bonded with the oxygen gas, and the resulting complex is readily dissociated at relatively low temperature and the oxygen atoms are produced. Pd nanoparticles would induce a well-known spillover effect which

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dramatically promotes dissociation of oxygen molecules in direct contact with Pd catalysts, and thus largely increases the ionosorption of the dissociated oxygen species on the surface of La2O3

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thin film [38]. Moreover, the presence of Pd nanoparticles may also activate another mechanism called as back-spillover effect, which captures and catalytically dissociates the oxygen molecules briefly adsorbed on La2O3. Thus, these oxygen atoms capture electrons from the surface layer and acceptor surface states are formed. The reducing gases react with surface oxygen and reduce the

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resistance [39]. As shown in Fig. 8 (b), when CO2 gas flows in, the CO2 molecules are dissociated on the surface by Pd catalyst. As the inflow of CO2 ceases, the density of the surface oxygen increases and the resultant CO2 molecules are removed in the form of CO2 vapor. The

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electrons return to La2O3 molecules, varying the resistance of the sensor. Enhanced response of Pd: La2O3 can be attributed to the formation of highly reactive species according to the reaction,

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O2 +Pd2→ 2 Pd : O

(6)

The created atoms migrate along the surface of the metal oxide. Thus, these oxygen atoms capture electrons from the surface layer and acceptor surface states are formed [40]. By removing electrons from the semiconductor surface, adsorbed oxygen causes Schottky potential barriers at the grain boundaries. The reducing gases react with surface oxygen and reduce the resistance. Thus as the number of oxygen species increases, more reaction at low temperature

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occurs and sensitivity increases. Therefore, presence of oxygen on surface is important in case of CO2 sensor. 5. Conclusions

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CO2 gas sensing properties of La2O3 and Pd: La2O3 thin film are studied. Nanorods-like morphology of La2O3 thin film is obtained by simple microwave assisted chemical method. The morphology of Pd: La2O3 consists of disk-like nanoparticle of palladium on surface of La2O3 thin

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film. Maximum response of 25 and 64% are observed for La2O3 and Pd: La2O3 thin films at operating temperatures of 723 and 523 K, respectively. The Pd: La2O3 sensor shows stability of

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87%. Acknowledgments

This work was supported by the Human Resources Development program (No. 20124010203180) of the Korea Institute of Energy Technology Evaluation and Planning

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(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-

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2015R1A2A2A01006856).

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Figure captions

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Fig. 1 XRD patterns of (a) La2O3 and (b) Pd: La2O3 thin films.

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Fig. 2 XPS spectra of (a) lanthanum (La+3), (b) oxygen (O-2) and (c) palladium (Pd+2) regions. Fig. 3 FE-SEM images of (a) La2O3 and (b) Pd: La2O3 thin films, respectively at magnification

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of 25,000 X (c) Pd: La2O3 thin film at magnification of 50,000 X. Fig. 4 TEM images of (a) La2O3 and, (b) Pd/La2O3 thin films. Fig. 5 Nitrogen adsorption-desorption isotherms of (a) La2O3 and (b) Pd: La2O3 thin films. Fig. 6 The variation of CO2 gas response of La2O3 and Pd: La2O3 thin films with (A) different operating temperatures under exposure of 400 ppm CO2 gas, and (B) CO2 gas response at different concentrations of CO2 gas, at operating temperatures of 723 and 523 K, respectively. 14

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(C) Variation of response and recovery time periods of Pd: La2O3 thin film at different concentrations of CO2 gas and (D) variation of CO2 gas response of Pd: La2O3 thin film at different relative humidity.

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Fig. 7 (a) The gas response of Pd:La2O3 thin film to various gases with concentrations of 400 ppm at 523 K, and (b) stability of Pd: La2O3 thin film at 523 K and 400 ppm for 30 days.

Fig. 8 Schematic diagram illustrating CO2 gas sensing mechanism for Pd: La2O3 in presence of

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(a) air and, (b) CO2 gas.

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Fig. 6 The variation of CO2 gas response of La2O3 and Pd: La2O3 thin films with (A) different

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operating temperatures under exposure of 400 ppm CO2 gas, and (B) CO2 gas response at different concentrations of CO2 gas, at operating temperatures of 723 and 523 K, respectively.

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(C) Variation of response and recovery time periods of Pd: La2O3 thin film at different concentrations of CO2 gas and (D) variation of CO2 gas response of Pd: La2O3 thin film at different relative humidity.

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Highlights Novel synthesis of Pd: La2O3 thin film by MW-CBD method.

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Improved CO2 gas sensing performance of La2O3 by Pd catalyst.

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Higher stability and fast response and recovery time periods of Pd: La2O3.

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