Improvement of the gas sensing response of nanostructured LaCoO3 by the addition of Ag nanoparticles

Sensors and Actuators B 246 (2017) 181–189

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Improvement of the gas sensing response of nanostructured LaCoO3 by the addition of Ag nanoparticles Carlos R. Michel a,∗ , Alma H. Martínez-Preciado b , Edgar R. López-Mena c , c ˜ Alex Elías-Zuniga , Nicolás Cayetano-Castro d , Oscar Ceballos-Sanchez e a

Departamento de Física, Universidad de Guadalajara CUCEI, Guadalajara, Jalisco 44410, Mexico Departamento de Ingeniería Química, Universidad de Guadalajara CUCEI, Guadalajara, Jalisco 44410, Mexico c Escuela de Ingenieria y Ciencias, Tecnologico de Monterrey, Campus Monterrey, Monterrey, Nuevo León 64849, Mexico d Centro de Nanociencias y Micro y Nanotecnologías, Instituto Politécnico Nacional, Ciudad de México 13440, Mexico e Departamento de Ingeniería de Proyectos, Universidad de Guadalajara CUCEI, Zapopan, Jalisco 45100, Mexico b

a r t i c l e

i n f o

Article history: Received 27 July 2016 Received in revised form 16 January 2017 Accepted 8 February 2017 Keywords: LaCoO3 Silver nanoparticles Polymerization method Impedance Gas sensors

a b s t r a c t LaCoO3 decorated with nanoparticles of noble metals has been proposed as catalyst in Suzuki reactions, automotive catalytic converters and gas sensors. In this work, LaCoO3 decorated with silver nanoparticles (AgNPs) was prepared by the solution-polymerization method, using polyvinyl alcohol (PVA) as polymerizing agent. For comparative purposes, LaCoO3 was also synthesized by this method. Calcination at 500 ◦ C produced LaCoO3 ; however, better crystallinity was obtained at 700 ◦ C. To attach AgNPs to LaCoO3 , a simple wet impregnation process was used. The microstructure of samples displayed interconnected nanoparticles and extensive porosity. Ag-LaCoO3 shows AgNPs with average size of 10 nm. Gas sensing characterization performed at 250 ◦ C revealed high reproducibility for detecting CO, CO2 and excess O2 (in air). The results obtained from Ag-LaCoO3 exhibit better stability and the quantitative detection of the test gases. In conclusion, surface decoration of LaCoO3 with AgNPs resulted in an effective and low-cost approach to improve the gas sensing properties of LaCoO3. This process can also be used to enhance the gas response of other oxide perovskites. © 2017 Elsevier B.V. All rights reserved.

1. Introduction LaCoO3 has been studied since time ago for its outstanding physicochemical properties, and it has been applied in heterogeneous catalysis and gas sensors [1–7]. The high catalytic activity of this oxide is associated to the high concentration of tetravalent cobalt cations (Co4+ ), which is larger than that measured in Co3 O4 [8,9]. LaCoO3 possess the perovskite-type structure and was synthesized for the first time by Askham et al. in 1950 [10]. Since then, it has been obtained by numerous methods, such as high energy ball milling, co-precipitation, molten salts, citrate, solution-polymerization and sol-gel, to name a few [11–15]. For the synthesis of ceramic materials, the solution-polymerization method has demonstrated to be an inexpensive process to obtain nanostructured materials [16]. When polyvinyl alcohol (PVA) is used, the metal cations can be uniformly distributed in the solid, by

∗ Corresponding author at: Departamento de Física CUCEI, Universidad de Guadalajara, M. García Barragán 1421 Guadalajara, Jalisco, 44410, Mexico. E-mail addresses: [email protected], [email protected] (C.R. Michel). http://dx.doi.org/10.1016/j.snb.2017.02.045 0925-4005/© 2017 Elsevier B.V. All rights reserved.

means of the hydroxyl ( OH) groups of this compound. After calcination, a nanostructured and highly porous material is obtained. Compared to other synthesis methods, the calcination temperature is lower because the combination among ions takes place at molecular level. To enhance the catalytic properties of LaCoO3 , the effect of decorating it with noble metals has been investigated. For instance, LaCoO3 decorated with Pd (Pd-LaCoO3 ) has been successfully tested as catalyst in Suzuki reactions [17]. Turnover numbers above 150,000 were reported, whereas the reaction with LaCoO3 was unsuccessful. More recently, Surendar et al. studied the production of hydrogen from glycerol, using LaCoO3 decorated with platinum particles (Pt-LaCoO3 ) as catalyst [18]. In the field of automotive catalytic converters, Pd-LaCoO3 catalysts have been successfully tested [19–21]. The improvement of the gas sensing properties of SnO2 by incorporating metal particles was investigated years ago by Yamazoe [22,23]. He found that the amount and size of the metal particles play a key role in the gas sensing performance; where particles with nanometric size displayed the best response. Since then, the effect of adding metal particles on the gas sensing properties

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3. Results 3.1. Crystal structure and morphology

Fig. 1. Drawing of the gas sensor device based on an Ag-LaCoO3 thick-film.

of binary oxides has been broadly studied. However, for ternary oxides significantly less information is available. For instance, Ding et al. reported the detection of carbon monoxide by using Pd-LaCoO3 [24]. However, since information about this topic is scarce, in this work the gas sensing properties of LaCoO3 decorated with silver nanoparticles (AgNPs) were investigated. Silver was chosen because its significant lower price compared to other precious metals (Pt, Au, Pd), and the ability to produce electronic sensitization when added to metal oxides. In order to produce nanostructured LaCoO3 with a uniform distribution of AgNPs, the solution-polymerization method was explored. For comparative purposes, the same synthesis method and gas sensing characterization were used for LaCoO3 . 2. Experimental Ag-LaCoO3 was prepared by dissolving stoichiometric amounts of La(NO3 )3 ·6H2 O (99.9%, Alfa-Aesar) and Co(NO3 )2 ·6H2 O (99%, J.T.Baker), in 10 ml of deionized water containing 2 wt% PVA (Aldrich). Then, AgNO3 (98%, J.T.Baker) was added to the latter to obtain 2 wt% of AgNPs. After stirring for 24 h, the solvent was evaporated by microwave irradiation, using a home microwave oven. At the end of this process, a spontaneous ignition occurred, caused by the exothermic reaction between the nitrates and PVA. The resulting material was annealed from 400 to 700 ◦ C, in air for 5 h. The same synthesis method (in absence of AgNO3 ) was used for the preparation of LaCoO3 . X-ray powder diffraction (XRD) patterns were obtained from calcined samples, using a Rigaku Miniflex diffractometer (Cu K␣1 radiation). The surface morphology of samples was observed by field emission scanning electron microscope (FESEM, Tescan, Mira). Elemental chemical analysis was done by X-ray energy dispersive spectroscopy (EDS, Brucker), attached to the FESEM. High resolution transmission electron microscopy (HRTEM, Jeol, JEM ARM 200CF) was used to observe the nanostructures. The oxidation state of elements was determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha), using a monochromatic Al K␣ source (1486 eV). The gas sensing characterization was performed on thick films, made with as-prepared LaCoO3 and Ag-LaCoO3 powders. The films were prepared by depositing a suspension of each powder, into an alumina ring, using a syringe. Gold wires (99%, Aldrich) attached to the ceramic ring were used as electrical contacts. Fig. 1 shows a drawing of the sensor device. The films had 3 mm diameter and ∼1 mm thickness. Alternating current measurements (AC) were carried out using a LCR meter (Agilent 4263B), which operates at six constant frequencies, in the range 100 Hz–100 kHz. The polarization curves (I–V) were recorded using a potentiostat/galvanostat (Solartron 1285). CO2 (99%), CO (99.9%), O2 (99%) and compressed air (base gas), all extra dry, were used in this characterization. The gases were mixed by using mass flow controllers and a readout instrument (MKS Instruments, 647C). Fig. 2 shows a scheme of the experimental setup.

Fig. 3 shows XRD patterns of powders annealed from 400 to 700 ◦ C: (A) LaCoO3 and (B) Ag-LaCoO3 . At 400 ◦ C most of the material was amorphous, but the main diffraction lines of LaCoO3 (2␪ ∼ 23, 33, 47 and 59◦ ) can be noticed. These lines matched with those of the JCPDF file No. 48-0123. Calcination at 500 and 600 ◦ C produced an improvement of the crystallinity. However, the diffraction lines of planes (110) and (104), located at 2␪ = 32.9 and 33.3◦ , were overlapped. Conversely, annealing at 700 ◦ C produced single-phase LaCoO3 , in which all the diffraction lines are present. For Ag-LaCoO3 samples (Fig. 3B), in addition to the diffraction lines of LaCoO3 , those corresponding to silver (2␪ = 38 and 44◦ ; JCPDF file No. 04-0783) are observed. This result indicates that silver was not introduced into the crystal lattice of LaCoO3 and remained as metal. This is because the ionic radius and oxidation states of cobalt and silver differ significantly: Co3+ (0.61 Å) and Ag1+ (1.15 Å) [25]. Fig. 4A and B shows FESEM images of LaCoO3 calcined at 700 ◦ C. The morphology corresponds to a highly porous material, formed by layered networks of particles. This morphology is typical of samples prepared by the solution-polymerization method, in which a large amount of gases are emitted during calcination. Fig. 4C displays an EDS spectrum of this sample, showing peaks of lanthanum, cobalt and oxygen. The unidentified peaks of this pattern correspond to the sample holder. The morphology of Ag-LaCoO3 is shown in Fig. 5A and B, which reveals similar characteristics than those observed for LaCoO3 . Fig. 5C shows an EDS spectrum of AgLaCoO3 , in which the L␣ transition peak of Ag located at 2.96 keV can be noticed. This result confirms the presence of silver, previously detected by XRD. To confirm the deposition of AgNPs on LaCoO3 , HRTEM was used. Fig. 6 shows typical HRTEM images of: (A) and (B) LaCoO3 , and (C) and (D) Ag-LaCoO3 , annealed at 700 ◦ C. Similar to that observed by FESEM, Fig. 6A exhibits interconnected LaCoO3 nanoparticles, with size in the range 20–150 nm. The crystallinity of the latter can be observed in Fig. 6B, through the presence of the numerous crystalline planes. For Ag-LaCoO3 (Fig. 6C), a similar morphology and particle size was obtained. Besides, AgNPs attached to them were identified. AgNPs displayed an average size of 10 nm and were firmly attached to LaCoO3 , as can be observed in Fig. 6D. The inset of the latter shows a typical EDS pattern of an Ag-LaCoO3 particle, which shows the corresponding peak of silver, placed at 3 keV. Furthermore, the inspection of AgNPs like that shown in Fig. 6D revealed that they were polycrystalline, because of the presence of randomly oriented crystalline planes. The uniform particle size of AgNPs can be attributed to the effect of the polymerizing agent (PVA) during solution. According to the literature, PVA produces a strong ordering of metal ions in a ceramic material, which has been supported by FTIR analyses [16]. Fig. 7A and B shows XPS spectra obtained from samples LaCoO3 and Ag-LaCoO3 , respectively. These results correspond to wide scans, which show the presence of lanthanum (La 3d), cobalt (Co 2p) and oxygen (O 1s) for LaCoO3 and the same elements for AgLaCoO3 , besides silver (Ag 3d). Fig. 8A displays the Ag 3d peaks corresponding to a Ag-LaCoO3 sample; which were fitted using two doublet peaks with a spin-orbit separation of 6.0 eV. The peak centered at 367.6 eV corresponds to Ag1+ (Ag2 O), while the smaller peak centered at 368.6 eV is attributed to metallic silver (Ag0 ) [26]. On the other hand, Fig. 8B shows a comparison of the O 1s spectra of LaCoO3 (upper graph) and Ag-LaCoO3 (bottom graph), where it is possible to observe a strong contribution of oxygen bonded to silver. The peaks centered at 528.5 and 529.3 eV were associated to La-O and Co-O bonds, respectively [27,28]. No significant changes were observed in the spectra of La 3d and Co 2p after the incor-

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Fig. 2. Scheme of the experimental setup used for gas sensing characterization.

Fig. 3. XRD patterns of samples calcined from 400 to 700 ◦ C: (A) LaCoO3 and (B) Ag-LaCoO3 .

poration of silver, indicating that the perovskite structure was not altered.

ization curves were also recorded. Since LaCoO3 is a semiconductor material, increasing the operating temperature causes a decrease on the electrical signal, which may affect the detection limit.

3.2. Gas sensing properties Since gas species can easily flow through the porosity of samples, the microstructure of LaCoO3 and Ag-LaCoO3 (700 ◦ C), resulted appropriate for gas sensing characterization. Besides, the corresponding XRD patterns show single-phase and well crystallized samples. Therefore, the response of these materials to CO, CO2 and O2 was evaluated. The measurements were started at room temperature (26 ◦ C); however, reliable and reproducible results were obtained at 250 ◦ C, being this temperature at which the characterization was done. The latter was focused on measuring the variation of the impedance magnitude (|Z|) with time, but polar-

3.2.1. Carbon monoxide sensing Fig. 9A shows |Z| vs. time graphs registered when 100 ppm CO (in air) were introduced for 1 min, using an applied frequency (f) of 100 kHz. The increase of |Z| displayed by both materials agrees with the fact that LaCoO3 is a p-type semiconductor material. Moreover, when air was injected, a fast return of |Z| to the baseline can be noticed. It can also be observed that the graph obtained for Ag-LaCoO3 displays a larger variation of |Z|. The variation of |Z| produced by the test gas, named |Z| hereafter, was determined from Fig. 9A, resulting in 17.3 and 21.3 , for LaCoO3 and Ag-LaCoO3 , respectively. This means that |Z| increased 25% in Ag-LaCoO3 . In

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Fig. 4. (A) and (B) FESEM images showing the surface morphology of LaCoO3 calcined at 700 ◦ C; (C) corresponding EDS analysis.

Fig. 5. (A) and (B) FESEM photos of an Ag-LaCoO3 sample, annealed at 700 ◦ C. (C) corresponding EDS analysis.

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Fig. 6. HRTEM images of: (A) and (B) LaCoO3 and (C) and (D) Ag-LaCoO3 , calcined at 700 ◦ C. The inset of the latter displays its EDS analysis.

reference to the applied frequency it is important to mention that measurements done at lower frequencies displayed more noise and instability, being at 100 kHz where the best results were obtained. Fig. 9B shows a |Z| vs. time graph obtained when the exposure time to CO and air increased to 5 min. This graph displays a monotonic increase of |Z| in CO, which indicates the absence of a steady state. Conversely, when air was injected, an average recovery time (trec ) of 8 s was determined. The latter corresponds to 90% of the time elapsed to reach the baseline. These results qualitatively agree with those reported by Ding et al. for LaCoO3 [5]. However, the use of an AC signal in this work, produced |Z| values of ∼25 , whereas the electrical resistance (R) reported by those authors was in the range 104 –107 . The CO gas sensing mechanism can be explained through Eq. No. (1): − 2CO(gas) + O− 2 (ads) → 2CO2 + e

(1)

Where the electrons produced by this reaction combine with the holes of the conduction band of LaCoO3 , increasing the impedance. The adsorption of O− 2 molecules occurs by the electrostatic interaction between them and the holes of LaCoO3 , due to their opposite charges [29]. The role of the addition of AgNPs on the gas sensing properties of SnO2 was explained years ago by Yamazoe [22,23]. He found that a stable thin layer of Ag2 O was formed on Ag particles after their deposition in air. Using XPS, he discovered that Ag2 O produces an electron-depleted layer inside SnO2 . Moreover, Ag2 O can be easily reduced to Ag in presence of CO, which in turn removes the electron-depleted layer. Since SnO2 is an n-type semiconductor material, the latter provides additional electrons to the conduction

band of SnO2 , increasing its conductivity. In this work, the formation Ag2 O around AgNPs was detected by XPS. This explains the improvement in the detection of CO, as well as the other test gases, as it will be presented later. Furthermore, since LaCoO3 is a p-type semiconductor material, the results obtained in this work displayed the opposite behavior than that reported for SnO2 . This means that for Ag-LaCoO3 , a larger number of electrons are supplied to the conduction band (compared to LaCoO3 ), producing an additional increase of |Z|. It is important to mention that according to Yamazoe there is an optimal amount of metal loading, which is between 1 and 3 wt%. That was the reason in this work the amount of AgNPs was 2 wt%. Besides, by increasing the amount of AgNO3 in solution (source of AgNPs) a larger agglomeration of AgNPs was observed. The quantitative detection of CO was also evaluated; Fig. 10 shows a typical graph obtained when the concentration of this gas increased from 100 to 400 ppm (250 ◦ C, f = 100 kHz). Both materials display a non-linear increase of |Z| with CO concentration, although the response of Ag-LaCoO3 was larger. It can also be observed a good response to small concentrations, whereas at high CO concentrations |Z| tends to a plateau. The calibration curves (inset of Fig. 10) display these characteristics in more detail. The improvement of the quantitative detection of CO in Ag-LaCoO3 can be associated to the extent of reduction of Ag2 O to Ag, which depends on the concentration of CO. The preceding results were compared with those reported by Brosha et al., who used an Au/LaCoO3 /Y2 O3 -ZrO2 /Au sensor array, operated at 600 ◦ C. They found a non-linear variation of the sensor response with CO partial pressure [3]. The response patterns obtained by these authors are similar to our results; however, the quantitative differences can be attributed to different operating

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Fig. 7. XPS spectra (wide scan) of: (A) LaCoO3 and (B) Ag-LaCoO3 samples, calcined at 700 ◦ C.

Fig. 9. |Z| vs. time graphs recorded using 100 ppm CO and air (250 ◦ C, 100 kHz). The exposure time to gases was: (A) 1 min and (B) 5 min.

Fig. 10. Quantitative detection of CO, measured in the range 100–400 ppm (250 ◦ C, 100 kHz). The inset shows calibration curves.

temperatures and sensor characteristics. In another work, Ghasdi et al. analyzed the effect of the synthesis method of LaCoO3 in detecting CO [4]. They concluded that samples prepared by high energy ball milling exhibited the best CO sensing properties, which was attributed to the smaller crystallite size. In an alternative strategy to improve the CO sensing response of LaCoO3 , Salker et al. synthesized it adding 2 wt% of Bi2 O3 , PdO and In2 O3 [30]. They concluded that the best results corresponded to PdO-LaCoO3 , operated at 300 ◦ C.

Fig. 8. XPS spectra of: (A) silver (Ag 3d) and (B) oxygen (O 1s) of LaCoO3 (upper graph) and Ag-LaCoO3 (bottom graph).

3.2.2. Response to carbon dioxide Fig. 11A displays the response pattern obtained when 100 ppm CO2 and air were alternatively supplied (100 kHz, 250 ◦ C). Similarly to the results obtained in CO, the introduction of CO2 increased |Z|;

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Fig. 11. |Z| vs. time plots obtained in CO2 , with concentrations: (A) 100 ppm and (B) 100–400 ppm (100 kHz, 250 ◦ C). The inset of (B) displays the calibration curves.

however, |Z| was significantly smaller. The average |Z| values determined from these curves were 0.07 and 0.09 , for LaCoO3 and Ag-LaCoO3 , respectively. Even though, these results represent ∼0.4% of |Z| registered in CO, Ag-LaCoO3 exhibited an increase of 28%, compared to LaCoO3 . Besides, better measurement stability was observed for Ag-LaCoO3 , which can be observed by the dotted lines drawn at the bottom of Fig. 11A. The CO2 sensing mechanism can be explained by the formation of a thin carbonate layer on the surface of LaCoO3 , which alters the dielectric constant (␬) of the film [31–33]. According to Eq. No. (2), the increase of |Z| in CO2 suggests a reduction of ␬, due to ␬ of the carbonates is smaller than that of the oxides [34].

 |Z| =

R2 +

 1 2 ωCo

(2)

Where R is the electrical resistance, ␻ is the angular frequency and Co is the capacitance in air [35]. To support this explanation, the surface carbonation of BaCoO3 by CO2 was analyzed [36]. The results revealed the formation of BaCO3 , which was identified by Raman spectroscopy and XRD. This suggests the growth of a thin layer of La2 (CO3 )3 on LaCoO3 , which produces the increase of |Z|. The improvement of the response of Ag-LaCoO3 can be attributed to the catalytic properties of Ag2 O, but not to the formation of Ag2 CO3 , because the melting point of this carbonate (218 ◦ C) is below the operating temperature. Fig. 11B shows the variation of |Z| with time, measured while the concentration of CO2 increased from 100 to 400 ppm. For LaCoO3 , a similar response pattern than that displayed in Fig. 11A can be noticed. This reveals the lack of sensitivity to detect variations in the concentration of CO2 . Conversely, the results obtained from AgLaCoO3 revealed a non linear increase of |Z| with CO2 concentration. The inset of Fig. 11B shows the corresponding calibration curves.

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Fig. 12. Oxygen response pattern obtained in air (as baseline), with additional O2 of: (A) 1% and (B) 1–4% (100 kHz, 250 ◦ C). The inset displays the calibration curve of Ag-LaCoO3 .

The ability of Ag-LaCoO3 to detect variations in the concentration of CO2 has not been reported before, in our knowledge. 3.2.3. Oxygen sensing properties Fig. 12A shows the variation of |Z| produced by increasing the concentration of O2 in air, in 1% (250 ◦ C, 100 kHz). This gas mixture had approximately 77% N2 , 22% O2 , and 1% corresponding to inert gases. The injection of additional 1% O2 provoked a decrease of |Z| in both, LaCoO3 and Ag-LaCoO3 . However, the measurement drift observed for LaCoO3 was significantly large. Conversely, AgLaCoO3 exhibited a more stable response over time, having an average |Z| value of approximately 0.05 . Even though |Z| was notably smaller than that measured in CO, the response pattern was reproducible and reliable. The increase of conductivity measured in excess O2 can be interpreted as a consequence of the depletion of electrons in the valence band of LaCoO3 , which occurs during the adsorption of O2 molecules. Since LaCoO3 is a p-type semiconductor, the latter produced an increase on the number of charge carriers, decreasing |Z|. Depending on the operating temperature, the adsorption of oxygen in metal oxides occurs by one of the following reactions [37]: O2 (gas) + e− → O− 2 (ads)

(3)

O2 (gas) + 2e− → O2− 2 (ads)

(4)

−

−

O2 (gas) + 2e → 2O (ads)

(5)

−

2−

(6)

O2 (gas) + 4e → 2O

(ads)

The detection of changes in the concentration of O2 in air was also analyzed. Fig. 12B shows the results obtained when that concentration increased from 1 to 4%. For LaCoO3 , a gradual decrease of |Z| by increasing the O2 concentration can be noticed. How-

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Acknowledgement This work was supported by the Coordinación General Académica of Universidad de Guadalajara through the PRO-SNI 2015 program. References

Fig. 13. Polarization curves acquired from Ag-LaCoO3 in air, air with additional 1% O2 , 100 ppm CO2 and 100 ppm CO (250 ◦ C). The inset shows the current range: −0.1 to 0.1 mA.

ever, the large measurement instability observed in this curve becomes an obstacle for quantitative oxygen detection. Conversely, the response obtained from Ag-LaCoO3 displays a proportional decrease of |Z| with O2 concentration. Besides, the stability was notably better. The inset of Fig. 12B shows the corresponding calibration curve, which indicates a nearly linear variation of |Z| with O2 concentration. The improved O2 sensing response of Ag-LaCoO3 can be explained through the chemical sensitization around the interface AgNPs–LaCoO3 . In an oxygen atmosphere, Ag-LaCoO3 respond as a better oxygen dissociation catalyst than LaCoO3 , increasing the amount of adsorbed oxygen. Moreover, Fig. 12B reveals that the amount of adsorbed oxygen molecules depends on the oxygen concentration. On the other hand, the ability of Ag2 O catalysts to adsorb oxygen molecules has been previously used for the oxidation of organic compounds [38–40]. 3.2.4. Polarization curves Fig. 13 shows electrical current vs. voltage curves (I–V curves) for Ag-LaCoO3 recorded in air, air with additional 1% O2 , 100 ppm CO2 and 100 ppm CO (250 ◦ C). Since the electrical current in CO was notably smaller, the inset of this figure shows the same curves in the range: −0.1 to 0.1 mA. The results display a non linear variation of the current with voltage, except for carbon monoxide. The non linear behavior in the first three gases is attributed to Schottky barriers, which are formed by the contact between the gold electrode and the Ag-LaCoO3 film [41,42]. Conversely, a nearly linear response (ohmic) was observed in CO. The I–V curves are in agreement with the impedance results, in the sense that greater current values (lower |Z| values) were recorded in air, and air with extra oxygen. In CO2 , a small decrease of current can be observed, which is associated to the low reactivity of this gas. However, when 100 ppm CO was used, a drastic decrease of the current occurred. 4. Conclusions Research about gas sensor materials has been mainly focused on n-type semiconductor materials (ZnO and SnO2 ); however, ptype semiconductor oxides also have interesting properties [29]. LaCoO3 decorated with Pd or Pt particles has been successfully used in the field of automotive catalytic converters, as well as in catalysis. Decorating LaCoO3 with AgNPs resulted in an attractive alternative because of their low cost, and the easy deposition by wet impregnation. In this work was demonstrated that the detection of CO, CO2 and O2 (in air) was significantly improved by the addition of AgNPs.

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Biographies Carlos R. Michel is professor-researcher in the Department of Physics of the Centro Universitario de Ciencias Exactas e Ingenierías (CUCEI), Universidad de Guadalajara, México. He received his M.Eng. in manufacturing engineering in the Tecnologico de Monterrey, Campus Monterrey, México, and his Doctoral degree in materials science in 1997 from the Universitat Autònoma de Barcelona/Institut de Ciència de Materials de Barcelona, Spain. His main interests are focused on the preparation of inorganic materials and their evaluation as chemical sensors. Alma H. Martínez-Preciado obtained her M.Sc. in biotechnology in 2004 and her Ph.D. in chemical engineering in 2012, both from CUCEI, Universidad de Guadalajara. She is professor-researcher in the Department of Chemical Engineering, CUCEI. Her research interests are the synthesis of materials by soft chemistry routes and their characterization. Edgar R. López-Mena received his M.Sc. in physics in 2007 and his Ph.D. in physics in 2012, both from CUCEI, Universidad de Guadalajara. He is professor-researcher in the Escuela de Ingenieria y Ciencias, Tecnologico de Monterrey, Campus Monterrey. His major research interests are the preparation of semiconductor nanomaterials (thin films and powders) and their applications. ˜ is professor-researcher in the Escuela de Ingenierias y Ciencias, Alex Elías-Zuniga Tecnologico de Monterrey, Campus Monterrey. He received his M.Sc. in mechanical engineering from Tecnologico de Monterrey, Campus Monterrey, México, and his Ph. D. in mechanical engineering in 1994 from University of Nebraska-Lincoln. His research interests are focused in mechanical properties and nanotechnology. Nicolas Cayetano Castro obtained his Ph.D. in materials science from Instituto Politécnico Nacional (IPN) in 2008. Since 2013, is professor-researcher in the Centro de Nanociencias y Micro y Nanotecnologías of the IPN. His research interests are the characterization of nanostructured materials. Oscar Ceballos-Sanchez received his M.Sc. in materials science from CinvestavQueretaro, México, in 2010. In 2015, he obtained his Ph.D. in materials science from Cinvestav-Queretaro and Grenoble Alpes University, France, as a result of a dual degree program. His research interest is the analysis by electronic spectroscopies of multifunctional materials.