**Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3**

Carlos R. Michel1, Narda L. López Contreras1, Edgar R. López-Mena1, Juan Carlos Ibarra1, Arturo Chávez-Chávez1 and Mauricio Ortiz-Gutiérrez2 *1Universidad de Guadalajara, CUCEI, 2Universidad Michoacana de San Nicolás de Hidalgo, México* 

## **1. Introduction**

168 Advances in Chemical Sensors

Watanabe, Y. & Sakai, T. (1971). Application of a thick anode film to semiconductor devices.

Wolkin M.V. et al. (1999). Electronic States and Luminescence in Porous Silicon Quantum

0029-067X.

0031-9007.

*Reviews of the Electrical Communications Laboratories*, Vol. 19, No. 7-8, pp. 899, ISSN:

Dots: The Role of Oxygen. *Physical Review Letters*, Vol. 82, No. 1, pp. 197-200, ISSN

Global warming has become one of the most important issues worldwide (Kerr, 2007). The emission of large amounts of CO2 has been identified as its main cause (Karl et al., 2003; Parry et al., 2008). In order to determine the concentration of this and other polluting gases, researchers around the world have developed solid state chemical sensors. Even though SnO2, ZnO, TiO2 and WO3 have been some of the most studied gas sensor materials, other oxides, with unique physical and chemical properties are also appropriate for this application (Yamazoe, 2005). Ternary and quaternary oxides, whose crystal structures are the perovskite or spinel, have notable electrical and catalytical properties, useful for gas sensing purposes (Brosha et al., 2000; Dutta et al., 2004; Kong et al., 1996; Kosacki et al., 1998; Post et al., 1999; Suo et al., 1997).

GdCoO3 is a semiconductor material, having the perovskite-type structure. It possess outstanding physical and physicochemical properties like magnetotransport, thermoelectricity, mixed ionic-electronic conductivity for solid oxide fuel cells, photocatalytical activity for the decomposition of dyes and phenols, and gas sensing activity (Rey-Cabezudo et al., 2002; Moon et al., 2000; Takeda et al., 1996; Wienhöfer et al., 2004; Mahata et al., 2007; Michel et al., 2009).

In this work, samples of Gd1-xSrxCoO3 (x = 0, 0.1) were prepared by two different methods. The main goals were to study the effect of chemical composition and microstructure on the gas sensing properties. The effect of the chemical composition was approached through strontium doping, and the effect of the microstructure was analyzed by preparing samples with different particle size. The preparation of Gd1-xSrxCoO3 powders was made in aqueous media, using the solution and solution-polymerization methods.

In recent years, the development of nanomaterials has attracted the attention of many research groups around the world. In order to decrease the particle size of ceramic materials, the use of polymerizing agents, such as polyvinyl alcohol and polyethylene glycol has been previously reported in the literature (Lee et al., 1998; Gülgün et al., 1999). A wide variety of nanostructured materials, such as cordierite, monoclinic yttrium

Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3 171

(TEM, Jeol JEM-1010) was used. Thick films of GdCoO3 and Gd0.9Sr0.1CoO3 were prepared by depositing a suspension of the powders on alumina discs. In this process, ~0.5 g of each powder was dispersed in 2 ml of isopropyl alcohol, using ultrasonic vibration. Then, the suspension was poured on alumina substrates, producing films with 3 mm diameter and ~400 μm thickness. The electrical contacts were made with high-purity silver wires

Electrical characterization was carried out by the two-point probe method, using a tube-type furnace having programmable temperature control. The direct current measurements (DC) were obtained by using a digital data acquisition unit (Agilent 34970A), having a multiplexer module. A digital voltmeter (Agilent 34401A) was also used. The polarization curves were recorded from -5V to 5V, using a Solartron 1285A potentiost/galvanostat. The alternating current characterization (AC) was performed by measuring the magnitude of the impedance (|Z|), with a LCR meter (Agilent 4263B). The graphs were obtained using the

The gases (CO2, O2 and extra dry compressed air) were supplied by using a mass flow controller (647C, MKS Instruments). Fig. 3 shows a diagram of the experimental setup and Fig. 4 shows an image of the actual instruments. It is important to mention that in this work the experimental setup was not pressurized; then, the pressure of the test chamber was

(diameter 0.2 mm). Fig. 2 shows a scheme of the gas sensor device.

Fig. 2. Schematic illustration of the gas sensor device.

LabView 8.6 software (National Instruments).

slightly above 1013 milibars.

aluminate, portland cement components, hydroxyapatite, and many more, have been synthesized by this method.

On the other hand, nanostructured materials provide large specific surface areas; which increases the interaction between the sensor surface and the surrounding gases. Therefore, in this work, the gas sensing properties of Gd1-xSrxCoO3 powders were evaluated. This characterization was done by using direct and alternating current on Gd1-xSrxCoO3 thick films.

## **2. Experimental**

GdCoO3 was synthesized by a solution-polymerization method, using stoichiometric amounts of Gd(NO3)36H2O (Alfa Aesar) and Co(NO3)36H2O (J.T. Baker). The reagents were dissolved in 100 ml of deionized water. To promote the polymerization effect, 0.44 g of polyvinyl alcohol (PVA, Alfa Aesar) and 3.2 g of sucrose (Alfa Aesar) were dissolved in 100 ml of deionized water. Due to the poor solubility of PVA in cool water, the water was preheated at 60oC. All the solutions were mixed under strong stirring for 2 h (60oC); Fig. 1A shows a typical solution obtained after this process. Gd0.9Sr0.1CoO3 was prepared by the solution and solution-polymerization methods. Similarly to the previous synthesis, stoichiometric amounts of gadolinium, cobalt and strontium nitrates (Sr(NO3)2 (Alfa Aesar)) were dissolved in 100 ml of deionized water. By the solution method, 0.1 mol of citric acid (Alfa Aesar) was added to the nitrate solution. In the case of the solution-polymerization, the process previously described was used. Then, water evaporation was done by using microwave radiation; a domestic microwave oven (Panasonic) was used. The materials obtained after the microwave–assisted evaporation are shown in: Fig. 1B (solution method) and Fig. 1C (solution-polymerization). In the latter, a dry solid having extensive porosity was produced.

Fig. 1. (A) Aqueous solution obtained by solution-polymerization. Materials obtained after water evaporation: (B) solution and (C) solution-polymerization.

In order to obtain single-phase GdCoO3 and Gd0.9Sr0.1CoO3 powders, the precursors were calcined in the range of 550 to 800°C. The resulting powders were analyzed by X-ray powder diffraction (XRD), at room temperature, using a Rigaku Miniflex apparatus. Cu Kα<sup>1</sup> radiation (1.5405 Å) was used. The surface morphology of the powders was observed by scanning electron microscopy (SEM), using a Jeol JSM-5400LV microscope, in secondary electron mode. For the identification of nanoparticles, transmission electron microscopy

aluminate, portland cement components, hydroxyapatite, and many more, have been

On the other hand, nanostructured materials provide large specific surface areas; which increases the interaction between the sensor surface and the surrounding gases. Therefore, in this work, the gas sensing properties of Gd1-xSrxCoO3 powders were evaluated. This characterization was done by using direct and alternating current on Gd1-xSrxCoO3 thick

GdCoO3 was synthesized by a solution-polymerization method, using stoichiometric amounts of Gd(NO3)36H2O (Alfa Aesar) and Co(NO3)36H2O (J.T. Baker). The reagents were dissolved in 100 ml of deionized water. To promote the polymerization effect, 0.44 g of polyvinyl alcohol (PVA, Alfa Aesar) and 3.2 g of sucrose (Alfa Aesar) were dissolved in 100 ml of deionized water. Due to the poor solubility of PVA in cool water, the water was preheated at 60oC. All the solutions were mixed under strong stirring for 2 h (60oC); Fig. 1A shows a typical solution obtained after this process. Gd0.9Sr0.1CoO3 was prepared by the solution and solution-polymerization methods. Similarly to the previous synthesis, stoichiometric amounts of gadolinium, cobalt and strontium nitrates (Sr(NO3)2 (Alfa Aesar)) were dissolved in 100 ml of deionized water. By the solution method, 0.1 mol of citric acid (Alfa Aesar) was added to the nitrate solution. In the case of the solution-polymerization, the process previously described was used. Then, water evaporation was done by using microwave radiation; a domestic microwave oven (Panasonic) was used. The materials obtained after the microwave–assisted evaporation are shown in: Fig. 1B (solution method) and Fig. 1C (solution-polymerization). In the latter, a dry solid having extensive porosity

water evaporation: (B) solution and (C) solution-polymerization.

Fig. 1. (A) Aqueous solution obtained by solution-polymerization. Materials obtained after

**A B C** 

In order to obtain single-phase GdCoO3 and Gd0.9Sr0.1CoO3 powders, the precursors were calcined in the range of 550 to 800°C. The resulting powders were analyzed by X-ray powder diffraction (XRD), at room temperature, using a Rigaku Miniflex apparatus. Cu Kα<sup>1</sup> radiation (1.5405 Å) was used. The surface morphology of the powders was observed by scanning electron microscopy (SEM), using a Jeol JSM-5400LV microscope, in secondary electron mode. For the identification of nanoparticles, transmission electron microscopy

synthesized by this method.

films.

**2. Experimental** 

was produced.

(TEM, Jeol JEM-1010) was used. Thick films of GdCoO3 and Gd0.9Sr0.1CoO3 were prepared by depositing a suspension of the powders on alumina discs. In this process, ~0.5 g of each powder was dispersed in 2 ml of isopropyl alcohol, using ultrasonic vibration. Then, the suspension was poured on alumina substrates, producing films with 3 mm diameter and ~400 μm thickness. The electrical contacts were made with high-purity silver wires (diameter 0.2 mm). Fig. 2 shows a scheme of the gas sensor device.

Fig. 2. Schematic illustration of the gas sensor device.

Electrical characterization was carried out by the two-point probe method, using a tube-type furnace having programmable temperature control. The direct current measurements (DC) were obtained by using a digital data acquisition unit (Agilent 34970A), having a multiplexer module. A digital voltmeter (Agilent 34401A) was also used. The polarization curves were recorded from -5V to 5V, using a Solartron 1285A potentiost/galvanostat. The alternating current characterization (AC) was performed by measuring the magnitude of the impedance (|Z|), with a LCR meter (Agilent 4263B). The graphs were obtained using the LabView 8.6 software (National Instruments).

The gases (CO2, O2 and extra dry compressed air) were supplied by using a mass flow controller (647C, MKS Instruments). Fig. 3 shows a diagram of the experimental setup and Fig. 4 shows an image of the actual instruments. It is important to mention that in this work the experimental setup was not pressurized; then, the pressure of the test chamber was slightly above 1013 milibars.

Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3 173

replaced by strontium is small, because their ionic radii are 0.94Å (Gd) and 1.16Å (Sr). The calcination at 700oC increased the reaction rate between Gd2O3 and Co3O4, producing a larger amount of Gd0.9Sr0.1CoO3. Firing at 750oC yielded near single-phase Gd0.9Sr0.1CoO3, as it can be observed in the corresponding pattern. The Miller indices of each plane were

(113)

(022)

(211)

(202)

(021)

(200)

(112)

(020)

(221)

(004)

(220)

(131)

550°C

600°C

650°C

700°C

750°C

20 30 40 50 60

Fig. 5. X-ray powder diffraction patterns of GdCoO3 obtained by solution-polymerization

Figure 6 shows the crystal structure evolution as temperature increased from 600 to 800oC. The calcination at 600oC produced a mixture Gd2O3, Co3O4 and Gd0.9Sr0.1CoO3, being the latter the main crystalline phase. At 700oC, a significant increase of Gd0.9Sr0.1CoO3 can be noticed; however, the main diffraction line of Gd2O3, placed at 2θ = 28.6o is present. The calcination at 800oC produced almost single-phase Gd0.9Sr0.1CoO3. Comparing these results with those obtained for GdCoO3, Gd0.9Sr0.1CoO3 appears at a lower calcination temperature; Fig. 7 shows X-ray diffraction patterns obtained from precursor powders of Gd0.9Sr0.1CoO3 (solution-polymerization), calcined at different temperatures. The calcination at 550oC produced an amorphous solid; however, at 600oC nearly single-phase Gd0.9Sr0.1CoO3 was produced. Increasing the calcination temperature to 700oC produced no perceptible changes in the XRD patterns. Comparing the sequence of XRD patterns of Fig. 7, with those displayed in Fig. 6, the solution-polymerization method decreased the temperature at which Gd0.9Sr0.1CoO3 can be formed. Besides, Fig. 7 does not display diffraction lines of Gd2O3 and Co3O4, indicating that the formation and further reaction of these oxides does not occur. The interaction, at molecular scale, of gadolinium, strontium and cobalt ions, could be an explanation of the formation of Gd0.9Sr0.1CoO3 at lower temperature. Evidently the solutionpolymerization method provides a notable save of energy in the preparation of ceramics, by

2θ (°)

assigned according to the JCPDF card.

(002)

(111)

Co3 O4 Gd2 O3

method, calcined at various temperatures.

using an inexpensive polymeric precursor.

Intensity (a.u.)

Fig. 3. Diagram of the experimental setup used for the gas sensing characterization.

Fig. 4. Image of the instruments used for the gas sensing measurements.

## **3. Results**

## **3.1 X-ray powder diffraction**

Fig. 5 shows the XRD patterns of the precursor powder of GdCoO3 calcined from 550 to 750oC in air. The calcination at 550oC produced no crystalline materials; however, at 600oC the main diffraction lines of Gd2O3 and Co3O4 were identified by using the JCPDF files 012- 0797 and 043-1003 respectively. The calcination at 650 oC revealed that the diffraction lines of Gd2O3 and Co3O4 are still present; however, the main diffraction line of Gd0.9Sr0.1CoO3 can be observe at 2θ = 33.8o. The identification of Gd0.9Sr0.1CoO3 was made by using the JCPDF file 25-1057, which indeed corresponds to GdCoO3; however, these oxides have the same crystal structure. However, it should be consider that the amount of gadolinium that can be

Fig. 3. Diagram of the experimental setup used for the gas sensing characterization.

Fig. 4. Image of the instruments used for the gas sensing measurements.

Fig. 5 shows the XRD patterns of the precursor powder of GdCoO3 calcined from 550 to 750oC in air. The calcination at 550oC produced no crystalline materials; however, at 600oC the main diffraction lines of Gd2O3 and Co3O4 were identified by using the JCPDF files 012- 0797 and 043-1003 respectively. The calcination at 650 oC revealed that the diffraction lines of Gd2O3 and Co3O4 are still present; however, the main diffraction line of Gd0.9Sr0.1CoO3 can be observe at 2θ = 33.8o. The identification of Gd0.9Sr0.1CoO3 was made by using the JCPDF file 25-1057, which indeed corresponds to GdCoO3; however, these oxides have the same crystal structure. However, it should be consider that the amount of gadolinium that can be

**3. Results** 

**3.1 X-ray powder diffraction** 

replaced by strontium is small, because their ionic radii are 0.94Å (Gd) and 1.16Å (Sr). The calcination at 700oC increased the reaction rate between Gd2O3 and Co3O4, producing a larger amount of Gd0.9Sr0.1CoO3. Firing at 750oC yielded near single-phase Gd0.9Sr0.1CoO3, as it can be observed in the corresponding pattern. The Miller indices of each plane were assigned according to the JCPDF card.

Fig. 5. X-ray powder diffraction patterns of GdCoO3 obtained by solution-polymerization method, calcined at various temperatures.

Figure 6 shows the crystal structure evolution as temperature increased from 600 to 800oC. The calcination at 600oC produced a mixture Gd2O3, Co3O4 and Gd0.9Sr0.1CoO3, being the latter the main crystalline phase. At 700oC, a significant increase of Gd0.9Sr0.1CoO3 can be noticed; however, the main diffraction line of Gd2O3, placed at 2θ = 28.6o is present. The calcination at 800oC produced almost single-phase Gd0.9Sr0.1CoO3. Comparing these results with those obtained for GdCoO3, Gd0.9Sr0.1CoO3 appears at a lower calcination temperature;

Fig. 7 shows X-ray diffraction patterns obtained from precursor powders of Gd0.9Sr0.1CoO3 (solution-polymerization), calcined at different temperatures. The calcination at 550oC produced an amorphous solid; however, at 600oC nearly single-phase Gd0.9Sr0.1CoO3 was produced. Increasing the calcination temperature to 700oC produced no perceptible changes in the XRD patterns. Comparing the sequence of XRD patterns of Fig. 7, with those displayed in Fig. 6, the solution-polymerization method decreased the temperature at which Gd0.9Sr0.1CoO3 can be formed. Besides, Fig. 7 does not display diffraction lines of Gd2O3 and Co3O4, indicating that the formation and further reaction of these oxides does not occur. The interaction, at molecular scale, of gadolinium, strontium and cobalt ions, could be an explanation of the formation of Gd0.9Sr0.1CoO3 at lower temperature. Evidently the solutionpolymerization method provides a notable save of energy in the preparation of ceramics, by using an inexpensive polymeric precursor.

Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3 175

Fig. 8 shows the typical surface morphology (SEM) of powders of GdCoO3 calcined at: (A) 550, (B) 600, (C) 650, (D) 700 and (E) 750oC. The microstructure of samples calcined from 550 to 700oC was of thin laminas having smooth surface and semispherical cavities. The entire solids possess extensive porosity. The sample calcined at 750oC shows a slight different microstructure, because a granular shape can be noticed. This feature can be explained by the sintering process occurred at higher temperature. The observation of this sample by TEM will provide a better insight of the microstructure of this sample, as will be shown later

**A B** 

**C D** 

**E** 

Fig. 8. SEM images of GdCoO3 powder calcined at: A) 550°C, B) 600°C, C) 650°C, D) 700°C

**3.2 Scanning electron microscopy** 

in this chapter.

and E) 750°C, in air.

Fig. 6. Crystal structure evolution with temperature of Gd0.9Sr0.1CoO3 prepared by the solution method.

Fig. 7. X-ray powder diffraction patterns of Gd0.9Sr0.1CoO3 obtained by solutionpolymerization, calcined at various temperatures.

## **3.2 Scanning electron microscopy**

174 Advances in Chemical Sensors

(211) (022) (202)

(200) (021)

(020)

(111)

(002)

Intensity (a.u.)

solution method.

(112)

> (113)

(231)

(131)

800°C

600°C

700°C

(221) (004)

> (131)

(220)

(004)

(221)

700°C

650°C

600°C

550°C

(220)

20 30 40 50 60

Fig. 6. Crystal structure evolution with temperature of Gd0.9Sr0.1CoO3 prepared by the

(020) (200)

(021)

(211) (022) (202)

(113)

(112)

(002) (111)

polymerization, calcined at various temperatures.

Intensity (a.u.)

20 30 40 50 60

2θ (°)

Fig. 7. X-ray powder diffraction patterns of Gd0.9Sr0.1CoO3 obtained by solution-

2θ (°)

Fig. 8 shows the typical surface morphology (SEM) of powders of GdCoO3 calcined at: (A) 550, (B) 600, (C) 650, (D) 700 and (E) 750oC. The microstructure of samples calcined from 550 to 700oC was of thin laminas having smooth surface and semispherical cavities. The entire solids possess extensive porosity. The sample calcined at 750oC shows a slight different microstructure, because a granular shape can be noticed. This feature can be explained by the sintering process occurred at higher temperature. The observation of this sample by TEM will provide a better insight of the microstructure of this sample, as will be shown later in this chapter.

Fig. 8. SEM images of GdCoO3 powder calcined at: A) 550°C, B) 600°C, C) 650°C, D) 700°C and E) 750°C, in air.

Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3 177

**C D** 

**A B** 

 Fig. 10. SEM images of Gd0.9Sr0.1CoO3 prepared by solution-polymerization: A) 550°C, B)

TEM in bright-field mode was used to examine samples of single phase GdCoO3 and Gd0.9Sr0.1CoO3 (both methods). Fig. 11 shows TEM images of: A) GdCoO3, synthesized by the solution-polymerization method, calcined at 750°C, B) Gd0.9Sr0.1CoO3 prepared by solution method, calcined at 800°C, and C) Gd0.9Sr0.1CoO3 prepared by solutionpolymerization (650°C). Fig. 11A shows interconnected, rounded or semispherical particles, with size larger than 100 nm. The formation of necks between these submicron particles was identified throughout the sample. Moreover, the interconnection between particles produced a rigid solid structure, with high porosity, which increases the contact surface

Fig. 11B shows a typical TEM image of Gd0.9Sr0.1CoO3 powder prepared by solution method. It exhibits a similar microstructure than that observed for GdCoO3, having extensive connection among particles, forming a continuous solid. Fig. 11C displays the microstructure of Gd0.9Sr0.1CoO3 made by solution-polymerization. Compared to previous samples, a notable smaller particle size can be noticed; an average particle size of 45 nm was measured. Even though, nanoparticle agglomeration is present, abundant nanoporosity can be observed. It can be concluded that the solution-polymerization method reduced the energy expenditure in the preparation of Gd0.9Sr0.1CoO3, and produced a significant reduction in particle size. High

porosity and high specific surface areas can be obtained by using this method.

600°C, C) 650°C and D) 700°C.

**3.3 Transmission electron microscopy** 

between gaseous species and the perovskite.

Fig. 9 shows the surface microstructure of Gd0.9Sr0.1CoO3 samples (solution method), calcined at A) 600°C, B) 700°C and C) 800°C. These SEM images revealed that the increase in the calcination temperature had little effect on the surface microstructure. All the samples show no relevant features; however, in order to determine if this material is composed by nanoparticles, further observation by TEM was performed.

Fig. 9. SEM image of Gd0.9Sr0.1CoO3 (solution method) calcined at: A) 600°C, B) 700°C and C) 800°C, in air.

Fig. 10 shows SEM photos of Gd0.9Sr0.1CoO3 synthesized by the solution-polymerization route, calcined at: A) 550°C, B) 600°C, C) 650°C and D) 700°C. A highly porous material, having smooth surfaces was produced. By increasing the calcination temperature, the microstructure of the samples was not altered. Comparing these images with those obtained for Gd0.9Sr0.1CoO3 (the solution method), significant differences can be noticed. The use of a polymerizing agent in aqueous solution produced at least a solid with high porosity. Due to Gd0.9Sr0.1CoO3 prepared by solution-polymerization can be indeed formed by nanoparticles its observation by TEM was also done.

Fig. 9 shows the surface microstructure of Gd0.9Sr0.1CoO3 samples (solution method), calcined at A) 600°C, B) 700°C and C) 800°C. These SEM images revealed that the increase in the calcination temperature had little effect on the surface microstructure. All the samples show no relevant features; however, in order to determine if this material is composed by

C

**A B** 

Fig. 9. SEM image of Gd0.9Sr0.1CoO3 (solution method) calcined at: A) 600°C, B) 700°C and C)

Fig. 10 shows SEM photos of Gd0.9Sr0.1CoO3 synthesized by the solution-polymerization route, calcined at: A) 550°C, B) 600°C, C) 650°C and D) 700°C. A highly porous material, having smooth surfaces was produced. By increasing the calcination temperature, the microstructure of the samples was not altered. Comparing these images with those obtained for Gd0.9Sr0.1CoO3 (the solution method), significant differences can be noticed. The use of a polymerizing agent in aqueous solution produced at least a solid with high porosity. Due to Gd0.9Sr0.1CoO3 prepared by solution-polymerization can be indeed formed by nanoparticles

800°C, in air.

its observation by TEM was also done.

nanoparticles, further observation by TEM was performed.

Fig. 10. SEM images of Gd0.9Sr0.1CoO3 prepared by solution-polymerization: A) 550°C, B) 600°C, C) 650°C and D) 700°C.

#### **3.3 Transmission electron microscopy**

TEM in bright-field mode was used to examine samples of single phase GdCoO3 and Gd0.9Sr0.1CoO3 (both methods). Fig. 11 shows TEM images of: A) GdCoO3, synthesized by the solution-polymerization method, calcined at 750°C, B) Gd0.9Sr0.1CoO3 prepared by solution method, calcined at 800°C, and C) Gd0.9Sr0.1CoO3 prepared by solutionpolymerization (650°C). Fig. 11A shows interconnected, rounded or semispherical particles, with size larger than 100 nm. The formation of necks between these submicron particles was identified throughout the sample. Moreover, the interconnection between particles produced a rigid solid structure, with high porosity, which increases the contact surface between gaseous species and the perovskite.

Fig. 11B shows a typical TEM image of Gd0.9Sr0.1CoO3 powder prepared by solution method. It exhibits a similar microstructure than that observed for GdCoO3, having extensive connection among particles, forming a continuous solid. Fig. 11C displays the microstructure of Gd0.9Sr0.1CoO3 made by solution-polymerization. Compared to previous samples, a notable smaller particle size can be noticed; an average particle size of 45 nm was measured. Even though, nanoparticle agglomeration is present, abundant nanoporosity can be observed. It can be concluded that the solution-polymerization method reduced the energy expenditure in the preparation of Gd0.9Sr0.1CoO3, and produced a significant reduction in particle size. High porosity and high specific surface areas can be obtained by using this method.

Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3 179

1.4

390ºC

O2

260ºC 380ºC

1.6

Sensitivity (Rgas/Rair)

100 200 300 400 500 600

*(Rgas* / *Rair)* O2 CO2 GdCoO3 580°C 410°C

(solution-polymerization) 255°C 390°C 260°C 380°C

Fig. 13 shows resistance *vs.* time graphs recorded on GdCoO3, in: (A) air/100 ppm of O2 (in air), and (B) air/100 ppm of CO2 (in air). These tests were done at a fixed temperature, which correspond to the maximum sensitivity values shown in Table 1. Fig. 13 shows a decrease of R after the introduction of O2; in CO2 the opposite behavior was registered. The variation of resistance in O2 was -6 Ω; whereas, in CO2 was ~40 Ω. However, in CO2 the

(solution) 429°C 600°C 445°C 579°C

Temperature (°C)

255ºC

Fig. 12. Sensitivity *vs.* temperature plots recorded in O2 and CO2 for A) GdCoO3, B) Gd0.9Sr0.1CoO3 (solution method), and C) Gd0.9Sr0.1CoO3 (solution-polymerization).

Table 1. Temperatures of maximum sensitivity values recorded in O2 and CO2 for

1.8

410ºC

**A B** 

CO2

O **C** 

2.0

300 400 500 600 700

CO2

580ºC

O2

Temperature (°C)

300 400 500 600 700

Temperature (°C)

Sensitivity **(** Rgas / Rair

Sensitivity

Gd0.9Sr0.1CoO3

Gd0.9Sr0.1CoO3

**3.4.2 Dynamic gas sensing response** 

resistance did not stabilize.

Gd1-xSrxCoO3.

 **)** 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 579ºC

CO2

O2

429ºC

445ºC

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Sensitivity **(** Rgas / Rair

 **)**

Fig. 11. TEM images of (A) GdCoO3 (750°C), (B) Gd0.9Sr0.1CoO3 prepared by solution method (800°C), and (C) Gd0.9Sr0.1CoO3 prepared by solution-polymerization (650°C).

The abundant porosity of Gd0.9Sr0.1CoO3 (solution-polymerization) is associated to the evolution of carbon dioxide and steam during the thermal decomposition of the polymerized metal ion-chelated complex (Das, 2001). A large amount of heat is also produced in this process. Theoretically, one mole of sucrose delivers 23 moles of gases, which avoid particle agglomeration and produce extensive porosity and fine particles in the final product. The thermal decomposition of sucrose occurs by the following reaction:

$$\rm C\_{12}H\_{22}O\_{11} + 12O\_2 \to 12CO\_2 + 11H\_2O \tag{1}$$

Another role of sucrose in the preparation of Gd0.9Sr0.1CoO3 (solution-polymerization) is the formation of sacaric acid; which is obtained after its contact with citric acid. Sacaric acid is known to be a good chelating agent for metal ions, which produces a uniform distribution of cations throughout the polymeric network.

#### **3.4 Gas sensing characterization**

#### **3.4.1 Gas sensitivity curves**

About the gas sensing properties of GdCoO3 and Gd0.9Sr0.1CoO3, Fig. 12 shows the sensitivity curves obtained in O2 and CO2. Fig. 12A displays the graphs obtained from GdCoO3, whereas Fig. 12B corresponds to Gd0.9Sr0.1CoO3 (solution), and Fig. 12C to Gd0.9Sr0.1CoO3 (solution-polymerization). The sensitivity curves were obtained from the ratio: *Rgas* / *Rair*; where *Rgas* is the resistance measured in the test gas (O2 or CO2), and *Rair* the resistance measured in dry air. Table 1 shows the temperatures at which the maximum sensitivity values were detected. From these results, sensitivity maxima occur at lower temperature in Gd0.9Sr0.1CoO3 (solution polymerization). Moreover, sensitivity maxima in CO2 are sharper than those obtained in O2; in the latter, broad curves were recorded instead. By comparing these results, with those reported from other materials such as ZnO and SnO2, Gd0.9Sr0.1CoO3 displays moderate gas sensitivity values.

**C** 

**A B** 

Fig. 11. TEM images of (A) GdCoO3 (750°C), (B) Gd0.9Sr0.1CoO3 prepared by solution method

The abundant porosity of Gd0.9Sr0.1CoO3 (solution-polymerization) is associated to the evolution of carbon dioxide and steam during the thermal decomposition of the polymerized metal ion-chelated complex (Das, 2001). A large amount of heat is also produced in this process. Theoretically, one mole of sucrose delivers 23 moles of gases, which avoid particle agglomeration and produce extensive porosity and fine particles in the final product. The thermal decomposition of sucrose occurs by the following reaction:

Another role of sucrose in the preparation of Gd0.9Sr0.1CoO3 (solution-polymerization) is the formation of sacaric acid; which is obtained after its contact with citric acid. Sacaric acid is known to be a good chelating agent for metal ions, which produces a uniform distribution of

About the gas sensing properties of GdCoO3 and Gd0.9Sr0.1CoO3, Fig. 12 shows the sensitivity curves obtained in O2 and CO2. Fig. 12A displays the graphs obtained from GdCoO3, whereas Fig. 12B corresponds to Gd0.9Sr0.1CoO3 (solution), and Fig. 12C to Gd0.9Sr0.1CoO3 (solution-polymerization). The sensitivity curves were obtained from the ratio: *Rgas* / *Rair*; where *Rgas* is the resistance measured in the test gas (O2 or CO2), and *Rair* the resistance measured in dry air. Table 1 shows the temperatures at which the maximum sensitivity values were detected. From these results, sensitivity maxima occur at lower temperature in Gd0.9Sr0.1CoO3 (solution polymerization). Moreover, sensitivity maxima in CO2 are sharper than those obtained in O2; in the latter, broad curves were recorded instead. By comparing these results, with those reported from other materials such as ZnO and SnO2,

C H O 12O 12CO 11H O 12 22 11 2 +→ +2 2 (1)

**200 nm 100 nm** 

**100 nm** 

cations throughout the polymeric network.

Gd0.9Sr0.1CoO3 displays moderate gas sensitivity values.

**3.4 Gas sensing characterization** 

**3.4.1 Gas sensitivity curves** 

(800°C), and (C) Gd0.9Sr0.1CoO3 prepared by solution-polymerization (650°C).

Fig. 12. Sensitivity *vs.* temperature plots recorded in O2 and CO2 for A) GdCoO3, B) Gd0.9Sr0.1CoO3 (solution method), and C) Gd0.9Sr0.1CoO3 (solution-polymerization).


Table 1. Temperatures of maximum sensitivity values recorded in O2 and CO2 for Gd1-xSrxCoO3.

#### **3.4.2 Dynamic gas sensing response**

Fig. 13 shows resistance *vs.* time graphs recorded on GdCoO3, in: (A) air/100 ppm of O2 (in air), and (B) air/100 ppm of CO2 (in air). These tests were done at a fixed temperature, which correspond to the maximum sensitivity values shown in Table 1. Fig. 13 shows a decrease of R after the introduction of O2; in CO2 the opposite behavior was registered. The variation of resistance in O2 was -6 Ω; whereas, in CO2 was ~40 Ω. However, in CO2 the resistance did not stabilize.

Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3 181

50

51

Air

58 57

R**(**Ω**)**

56

0 3 6 9 12 15 18 21 24 27

Time (min)

O2 CO2 O2 CO2

Air Air Air Air

Fig. 15. Resistance *vs.* time plots acquired in: (A) air, CO2 and O2; and (B) different

Fig. 15B displays the ability of Gd0.9Sr0.1CoO3 to detect variations in the concentration of CO2. In this experiment, dry air was supplied for about 10 min, until stable resistance measurements were obtained. Then, CO2 with a concentration of 50 ppm was injected; this produced an increase of R of about 2 Ω. Later, the concentration of CO2 was increased to 100 ppm, which increased R in ~2.5 Ω. This result agrees with that shown in Fig. 14B; however, when the concentration of CO2 was increased to 150 and 200 ppm, smaller increments of R were registered. The latter suggests **that the concentration limit at which Gd0.9Sr0.1CoO3 can satisfactorily detect CO2 is 100 ppm**. **However,** for large concentrations, a saturated state was **observed**. **This behavior can be attributed to several factors, which include surface area, gas diffusivity and temperature, among others. On the other hand,** this kind of experiment was also performed to detect variations in the concentration of O2; however, unreliable results were produced. Future improvements on the gas sensor device may result in a better performance in oxygen. Therefore, polarization curves were focused on the ability of Gd0.9Sr0.1CoO3 to detect

Polarization curves (or I-V curves), were recorded to test the ability to detect changes in the concentration of CO2 (static mode). These curves were obtained at 380°C, using CO2 at concentrations of 50, 100, 150 and 200 ppm. Measurements in air were also performed to obtain a reference curve. Fig. 16 shows typical I -V curves recorded from -5 V to 5 V. It is worth to mention that beyond this voltage range, reliable measurements were not acquired. These curves show that the presence of CO2 produced an abrupt decrease of current. This is in agreement with the results presented in the previous section, where CO2 increased the resistance. By increasing the concentration of CO2, smaller values of current were

On the other hand, the nonlinear behavior of I-V curves of Fig. 16 is similar to that reported for varistor gas sensors. According to Lin et al., the increase of nonlinearity of I-V curves of varistor gas sensors is caused by the decrease of grain size (Lin et al., 1995). Due to the small

48 49 50

Air

concentrations of CO2 (380°C).

variations in the concentration of CO2.

registered; at both cathodic and anodic voltages.

**3.4.3 Polarization curves** 

R**(**Ω**)**

**A B** 

200 ppm CO2 150 ppm CO2

0 3 6 9 12 15 18 21 24 27

Time (min)

100 ppm CO2

50 ppm CO2

Fig. 13. Resistance *vs.* time graphs registered in air/O2 (580°C), and air/CO2 (410°C), for GdCoO3.

Fig. 14 shows R *vs.* time graphs recorded in: (A) air/100 ppm of O2 (in air), and (B) air/100 ppm of CO2 (in air), for Gd0.9Sr0.1CoO3 prepared by solution and solution-polymerization. These measurements were performed at 380°C, which is an intermediate sensitivity value. This temperature was chosen in order to compare the results obtained from different Gd0.9Sr0.1CoO3 samples. Compared to the graphs of Fig. 13, the resistance decreased about one order of magnitude. Gd0.9Sr0.1CoO3 (solution-polymerization) displayed the largest variation of resistance; however, the stabilization of resistance with time was not observed. Gd0.9Sr0.1CoO3 (solution method), displays stabilization of resistance in both gases; however, further experiments demonstrated that Gd0.9Sr0.1CoO3 (solution-polymerization) exhibited a better gas sensing performance.

Fig. 14. Resistance *vs.* time graphs recorded in: (A) air/O2, and (B) air/CO2, for Gd0.9Sr0.1CoO3 prepared by solution and solution-polymerization methods (380°C).

Fig. 15A shows the variation of resistance with time measured when air, CO2 and O2 were alternatively supplied. This experiment was made in order to test the gas selectivity of Gd0.9Sr0.1CoO3 prepared by solution-polymerization. Fig. 15A shows a combination of the results displayed in Fig. 14; having a decrease of R in oxygen, and the opposite behavior in carbon dioxide. The variation of resistance registered in each gas was nearly the same to that observed in Fig. 14. The response pattern of Fig. 15 was highly reproducible, revealing also that the gas detection involves reversible processes.

Fig. 15. Resistance *vs.* time plots acquired in: (A) air, CO2 and O2; and (B) different concentrations of CO2 (380°C).

Fig. 15B displays the ability of Gd0.9Sr0.1CoO3 to detect variations in the concentration of CO2. In this experiment, dry air was supplied for about 10 min, until stable resistance measurements were obtained. Then, CO2 with a concentration of 50 ppm was injected; this produced an increase of R of about 2 Ω. Later, the concentration of CO2 was increased to 100 ppm, which increased R in ~2.5 Ω. This result agrees with that shown in Fig. 14B; however, when the concentration of CO2 was increased to 150 and 200 ppm, smaller increments of R were registered. The latter suggests **that the concentration limit at which Gd0.9Sr0.1CoO3 can satisfactorily detect CO2 is 100 ppm**. **However,** for large concentrations, a saturated state was **observed**. **This behavior can be attributed to several factors, which include surface area, gas diffusivity and temperature, among others. On the other hand,** this kind of experiment was also performed to detect variations in the concentration of O2; however, unreliable results were produced. Future improvements on the gas sensor device may result in a better performance in oxygen. Therefore, polarization curves were focused on the ability of Gd0.9Sr0.1CoO3 to detect variations in the concentration of CO2.

#### **3.4.3 Polarization curves**

180 Advances in Chemical Sensors

Fig. 13. Resistance *vs.* time graphs registered in air/O2 (580°C), and air/CO2 (410°C), for

Fig. 15A shows the variation of resistance with time measured when air, CO2 and O2 were alternatively supplied. This experiment was made in order to test the gas selectivity of Gd0.9Sr0.1CoO3 prepared by solution-polymerization. Fig. 15A shows a combination of the results displayed in Fig. 14; having a decrease of R in oxygen, and the opposite behavior in carbon dioxide. The variation of resistance registered in each gas was nearly the same to that observed in Fig. 14. The response pattern of Fig. 15 was highly reproducible, revealing also

Fig. 14. Resistance *vs.* time graphs recorded in: (A) air/O2, and (B) air/CO2, for Gd0.9Sr0.1CoO3 prepared by solution and solution-polymerization methods (380°C).

that the gas detection involves reversible processes.

Fig. 14 shows R *vs.* time graphs recorded in: (A) air/100 ppm of O2 (in air), and (B) air/100 ppm of CO2 (in air), for Gd0.9Sr0.1CoO3 prepared by solution and solution-polymerization. These measurements were performed at 380°C, which is an intermediate sensitivity value. This temperature was chosen in order to compare the results obtained from different Gd0.9Sr0.1CoO3 samples. Compared to the graphs of Fig. 13, the resistance decreased about one order of magnitude. Gd0.9Sr0.1CoO3 (solution-polymerization) displayed the largest variation of resistance; however, the stabilization of resistance with time was not observed. Gd0.9Sr0.1CoO3 (solution method), displays stabilization of resistance in both gases; however, further experiments demonstrated that Gd0.9Sr0.1CoO3 (solution-polymerization) exhibited a

O2

**A B** 

R **(**Ω**)**

345

Time (min)

Air

567

Air

Time (min)

CO2

better gas sensing performance.

R **(**Ω**)**

GdCoO3.

Polarization curves (or I-V curves), were recorded to test the ability to detect changes in the concentration of CO2 (static mode). These curves were obtained at 380°C, using CO2 at concentrations of 50, 100, 150 and 200 ppm. Measurements in air were also performed to obtain a reference curve. Fig. 16 shows typical I -V curves recorded from -5 V to 5 V. It is worth to mention that beyond this voltage range, reliable measurements were not acquired. These curves show that the presence of CO2 produced an abrupt decrease of current. This is in agreement with the results presented in the previous section, where CO2 increased the resistance. By increasing the concentration of CO2, smaller values of current were registered; at both cathodic and anodic voltages.

On the other hand, the nonlinear behavior of I-V curves of Fig. 16 is similar to that reported for varistor gas sensors. According to Lin et al., the increase of nonlinearity of I-V curves of varistor gas sensors is caused by the decrease of grain size (Lin et al., 1995). Due to the small

Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3 183

Fig. 17. |Z| *vs.* time plots recorded at different exposition times to 50 ppm O2 (100 kHz,

Even so, we can conclude that a better performance in CO2 was obtained.

oxygen gas sensor material, a rather limited performance was observed.

2 4 6 8 10 12

Time (min)

47.0

(100 kHz, 380°C).

47.5

48.0

48.5

⏐Z⏐ (Ω) 49.0

49.5

Air

50 ppm CO2

Fig. 18 shows typical results obtained when a film of Gd0.9Sr0.1CoO3 was exposed to air and 50 ppm CO2 (100 kHz, 380°C). Analogously to the previous results, the difference between the figures 18A and 18B is the exposition time to CO2. Notable repeatibility in the detection of CO2 can be observed in Fig. 18A. Full recovery of the original |Z| value can also be noticed. Fig. 18B revealed a lack of a saturated state, similar to that observed in oxygen.

Fig. 19 shows |Z| *vs.* time graphs acquired at different concentrations of: (A) oxygen and (B) carbon dioxide. It is worth to mention that these experiments were also performed at direct current; however, the results were far to be satisfactory, and they were not included in this work. Fig. 19 reveals that the increase of the gas concentration in 50 ppm produced a proportional variation of |Z|. In oxygen, a continuous decrease of |Z| with time can be observed; which is more attenuated in carbon dioxide. In summary, based in the preceding results, Gd0.9Sr0.1CoO3 can be considered as an alternativey CO2 sensor material. As an

> 47.0 47.5 48.0 48.5 49.0 49.5 50.0

⏐Z⏐ (Ω)

Fig. 18. |Z| *vs.* time graphs acquired using two different exposition times to 50 ppm CO2

42.4

42.8

43.2

⏐Z⏐(Ω)

43.6

15 20 25

air

B

50 ppm O2

Time (min)

3456789

50 ppm CO2

**A B** 

Air

Time (min)

A

4 6 8 10 12 14 16

50 ppm O2

Time (min)

42.2

380°C).

42.4

air

42.6

42.8

⏐Z⏐(Ω)

43.0

43.2

Fig. 16. Polarization curves obtained at different concentrations of CO2, on Gd0.9Sr0.1CoO3 films (380°C).

particle size of Gd0.9Sr0.1CoO3 (solution-polymerization), the nonlinearity of I-V curves is similar to that reported by Lin and coworkers.

#### **3.4.4 Dynamic gas sensing response in alternating current**

Another useful approach to analyze the gas sensing properties of materials, is by means of the use of alternating current. One of the advantages of using alternating current is that some materials respond satisfactorily to gases, at high frequencies, but not at direct current. In the present experiments, the magnitude of the impedance (|Z|) was measured using several frequencies, from 100 Hz to 100 kHz. The amplitude of the signal was 1 V. From several results it was observed that the best graphs were recorded at 100 kHz.

Fig. 17 shows |Z| *vs.* time graphs obtained after the successive injection of air and 50 ppm O2 (100 kHz, 380°C). The difference between the graphs 17A and 17B is the exposition time to the test gas. One of the reasons to perform these experiments was to investigate if the stabilization of |Z| can be obtained after a prolonged exposition to the test gas. Fig. 17B shows that the longer exposition to 50 ppm O2 produced a continuous decrease of |Z|; which is in agreement with Fig. 14A. **The incomplete desorption of oxygen may be explained by the fact that Gd0.9Sr0.1CoO3 possess oxygen vacancies in its crystal structure, which is commom in transition metal oxides with the perovskitetype structure. Is possible that part of the oxygen supplied in these tests occupies the oxygen vacancies of Gd0.9Sr0.1CoO3. About this issue, it is well known that the oxygenation of YBa2Cu3O7-**δ **produces the high Tc superconducting phase; which indicates the ability of perovskites to retain oxygen. The stabilization of |Z| with time was not observed in these experiments, but some improvements in the gas sensor device may enhace the detection of O2. These improvements include the increase in the surface contact between the metal electrode and the oxide powder. Another relevant issue is the use of gold or platinum as electrodes.** 


V (V)

Fig. 16. Polarization curves obtained at different concentrations of CO2, on Gd0.9Sr0.1CoO3

particle size of Gd0.9Sr0.1CoO3 (solution-polymerization), the nonlinearity of I-V curves is

Another useful approach to analyze the gas sensing properties of materials, is by means of the use of alternating current. One of the advantages of using alternating current is that some materials respond satisfactorily to gases, at high frequencies, but not at direct current. In the present experiments, the magnitude of the impedance (|Z|) was measured using several frequencies, from 100 Hz to 100 kHz. The amplitude of the signal was 1 V. From

Fig. 17 shows |Z| *vs.* time graphs obtained after the successive injection of air and 50 ppm O2 (100 kHz, 380°C). The difference between the graphs 17A and 17B is the exposition time to the test gas. One of the reasons to perform these experiments was to investigate if the stabilization of |Z| can be obtained after a prolonged exposition to the test gas. Fig. 17B shows that the longer exposition to 50 ppm O2 produced a continuous decrease of |Z|; which is in agreement with Fig. 14A. **The incomplete desorption of oxygen may be explained by the fact that Gd0.9Sr0.1CoO3 possess oxygen vacancies in its crystal structure, which is commom in transition metal oxides with the perovskitetype structure. Is possible that part of the oxygen supplied in these tests occupies the oxygen vacancies of Gd0.9Sr0.1CoO3. About this issue, it is well known that the oxygenation of YBa2Cu3O7-**δ **produces the high Tc superconducting phase; which indicates the ability of perovskites to retain oxygen. The stabilization of |Z| with time was not observed in these experiments, but some improvements in the gas sensor device may enhace the detection of O2. These improvements include the increase in the surface contact between the metal electrode and the oxide powder. Another relevant** 

several results it was observed that the best graphs were recorded at 100 kHz.

200 ppm CO2

150 ppm CO2 100 ppm CO2

50 ppm CO2

Air


similar to that reported by Lin and coworkers.

**issue is the use of gold or platinum as electrodes.** 

**3.4.4 Dynamic gas sensing response in alternating current** 

films (380°C).


0.0

0.5

I (A)

1.0

1.5

Fig. 17. |Z| *vs.* time plots recorded at different exposition times to 50 ppm O2 (100 kHz, 380°C).

Fig. 18 shows typical results obtained when a film of Gd0.9Sr0.1CoO3 was exposed to air and 50 ppm CO2 (100 kHz, 380°C). Analogously to the previous results, the difference between the figures 18A and 18B is the exposition time to CO2. Notable repeatibility in the detection of CO2 can be observed in Fig. 18A. Full recovery of the original |Z| value can also be noticed. Fig. 18B revealed a lack of a saturated state, similar to that observed in oxygen. Even so, we can conclude that a better performance in CO2 was obtained.

Fig. 19 shows |Z| *vs.* time graphs acquired at different concentrations of: (A) oxygen and (B) carbon dioxide. It is worth to mention that these experiments were also performed at direct current; however, the results were far to be satisfactory, and they were not included in this work. Fig. 19 reveals that the increase of the gas concentration in 50 ppm produced a proportional variation of |Z|. In oxygen, a continuous decrease of |Z| with time can be observed; which is more attenuated in carbon dioxide. In summary, based in the preceding results, Gd0.9Sr0.1CoO3 can be considered as an alternativey CO2 sensor material. As an oxygen gas sensor material, a rather limited performance was observed.

Fig. 18. |Z| *vs.* time graphs acquired using two different exposition times to 50 ppm CO2 (100 kHz, 380°C).

Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3 185

Where R is the electrical resistance, κ is the dielectric constant, ω is the angular frequency and Co is the capacitance in air (Halliday, et al., 2001). The decrease of κ produces the increase of |Z|; then, when air is reintroduced, |Z| returns to its original value. Some authors have studied the formation of bicarbonate, bidentate or monodentate on MgO, CaO and ZrO2 (Tsuji, et al., 2003); however, the identification of the specific type of carbonate

On the other hand, some authors have reported that the process by which charge carriers are transported through a material, depends on the particle size (Bochenkov, et al., 2005). First, the thickness of the charged layer (LS) depends on the surface potential (VS), according

> s D eV L =L

Where LD is the Debye length, e is the electron charge, k is the Boltzmann´s constant and T is the temperature. LD depends on other physical parameters through the following relation:

D 2

2. D = 2LS, the conductivity is controlled by the necks formed among particles

Fig. 20. Scheme of two mechanisms of conductance in semiconductor materials.

Where κ is the dielectric constant and Nd is the concentration of donor impurity. Typical values of LS are in the range 1 to 100 nm. If D is the crystallite size, three possible

3. D < 2LS, in this case the entire volume of each crystallite participates in the charge

Fig. 20 shows a schematic illustration of the last two mechanisms. The dark green area represents the volume through which charge carriers move. The third mechanism may be a possible explanation of the improved gas sensing behavior in nanostructured Gd0.9Sr0.1CoO3

<sup>κ</sup>kT L =

2 S

d

kT (6)

2πe N (7)

D = 2Ls

D < 2Ls

formed on Gd0.9Sr0.1CoO3 is currently investigated.

to the equation:

mechanisms are usually proposed:

transport

(solution-polymerization).

1. D >> 2LS, the conductance is limited by Schottky barriers

Fig. 19. |Z| *vs.* time graphs acquired at different concentrations of (A) O2 and (B) CO2 (100 kHz, 380°C).

#### **3.5 Gas sensing mechanisms**

The explanation of the gas sensing mechanism in semiconductor oxides is frequently based in the change of resistance (or impedance) produced by adsorption processes. It has been reported in the literature that gas adsorption depends, among others, on the operation temperature, particle size, specific surface area and gas partial pressure.

Oxide semiconductors can be classified as p-type or n-type, according to the variation of conductivity caused by a reducing or oxidizing gas (Moseley, et al., 1991). In an oxidizing atmosphere, a p-type semiconductor material adsorbs oxygen molecules. These molecules capture electrons, increasing the number of charge carriers (holes). As a result, the electrical conductivity increases (Baraton, et al., 2003). The oxygen adsorption can occur by means of one of the following reactions:

$$\bullet \bullet\_2 \text{(gas)} \text{+ e'} \rightarrow \bullet \text{O}\_2 \text{(ads)} \tag{2}$$

$$\text{CO}\_2\text{(gas)} + 2\text{e}^\cdot \rightarrow 2\text{O}^\cdot\text{(ads)}\tag{3}$$

$$\text{CO}\_2\text{(gas)} + 4\text{e}^\cdot \rightarrow 2\text{O}^{2-} \text{(ads)}\tag{4}$$

About the CO2 gas sensing mechanism, the increase of |Z| suggests a change in the dielectric constant (κ) of the film. CO2 can not be adsorbed on the surface of Gd0.9Sr0.1CoO3 because is a stable molecule. The formation of a thin layer of carbonate decreases κ, due to the dielectric constant of carbonates is frequently smaller than the oxides (Ishihara, et al., 1991). The decrease of |Z| with κ can be explained in terms of the following equation:

$$|Z| = \sqrt{\mathbb{R}^2 + \left(\frac{1}{\text{ко}\mathbb{C}\_o}\right)^2} \tag{5}$$

40.5

41.0

⏐Z⏐ (Ω)

Fig. 19. |Z| *vs.* time graphs acquired at different concentrations of (A) O2 and (B) CO2 (100

The explanation of the gas sensing mechanism in semiconductor oxides is frequently based in the change of resistance (or impedance) produced by adsorption processes. It has been reported in the literature that gas adsorption depends, among others, on the operation

Oxide semiconductors can be classified as p-type or n-type, according to the variation of conductivity caused by a reducing or oxidizing gas (Moseley, et al., 1991). In an oxidizing atmosphere, a p-type semiconductor material adsorbs oxygen molecules. These molecules capture electrons, increasing the number of charge carriers (holes). As a result, the electrical conductivity increases (Baraton, et al., 2003). The oxygen adsorption can occur by means of

About the CO2 gas sensing mechanism, the increase of |Z| suggests a change in the dielectric constant (κ) of the film. CO2 can not be adsorbed on the surface of Gd0.9Sr0.1CoO3 because is a stable molecule. The formation of a thin layer of carbonate decreases κ, due to the dielectric constant of carbonates is frequently smaller than the oxides (Ishihara, et al., 1991). The decrease of |Z| with κ can be explained in terms of the following equation:

2

<sup>1</sup> Z= R +

41.5

42.0

5 10 15 20

Time (min)

Air




2

o

κωC ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ 50 ppm O2

**A B** 

100 ppm O2

150 ppm O2

(5)

5 10 15 20

150 ppm CO2

temperature, particle size, specific surface area and gas partial pressure.

2

2

2

Time (min)

100 ppm CO2

50 ppm CO2

**3.5 Gas sensing mechanisms** 

one of the following reactions:

200 ppm CO2

kHz, 380°C).

Air

⏐Z⏐ (Ω) Where R is the electrical resistance, κ is the dielectric constant, ω is the angular frequency and Co is the capacitance in air (Halliday, et al., 2001). The decrease of κ produces the increase of |Z|; then, when air is reintroduced, |Z| returns to its original value. Some authors have studied the formation of bicarbonate, bidentate or monodentate on MgO, CaO and ZrO2 (Tsuji, et al., 2003); however, the identification of the specific type of carbonate formed on Gd0.9Sr0.1CoO3 is currently investigated.

On the other hand, some authors have reported that the process by which charge carriers are transported through a material, depends on the particle size (Bochenkov, et al., 2005). First, the thickness of the charged layer (LS) depends on the surface potential (VS), according to the equation:

$$\mathbf{L}\_{\rm s} = \mathbf{L}\_{\rm D} \sqrt{\frac{\mathbf{e} \mathbf{V}\_{\rm S}^{2}}{\mathbf{kT}}} \tag{6}$$

Where LD is the Debye length, e is the electron charge, k is the Boltzmann´s constant and T is the temperature. LD depends on other physical parameters through the following relation:

$$\mathbf{L\_D = \sqrt{\frac{\kappa \mathbf{k} \mathbf{T} \mathbf{T}}{2 \text{me}^2 \mathbf{N\_d}}}} \tag{7}$$

Where κ is the dielectric constant and Nd is the concentration of donor impurity. Typical values of LS are in the range 1 to 100 nm. If D is the crystallite size, three possible mechanisms are usually proposed:


Fig. 20 shows a schematic illustration of the last two mechanisms. The dark green area represents the volume through which charge carriers move. The third mechanism may be a possible explanation of the improved gas sensing behavior in nanostructured Gd0.9Sr0.1CoO3 (solution-polymerization).

Fig. 20. Scheme of two mechanisms of conductance in semiconductor materials.

Improvement of the Gas Sensing Properties in Nanostructured Gd0.9Sr0.1CoO3 187

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1719-1723.

173-176.

2301-2306.

85.

130, pp. 152-153.

pp. 1187-1190.

## **4. Conclusions**

The solution-polymerization process is a simple and cost effective method for preparing nanostructured Gd0.9Sr0.1CoO3. Due to PVA and sucrose are inexpensive materials, this method can be used to obtain other nanostructured compounds; which can be used in fields like gas sensors, gas separation membranes, electrodes for solid oxide fuel cells and catalysis, to mention a few. In this work, single phase Gd0.9Sr0.1CoO3 prepared by solutionpolymerization was obtained from 600oC. However, by the solution method, it was obtained at 800oC. Extensive porosity caused by the decomposition of PVA and sucrose, as well as lower calcination temperatures, produced a nanostructured network of Gd0.9Sr0.1CoO3. The latter was observed by transmission electron microscopy.

Through the analysis of the gas sensing results it was possible to conclude that Gd0.9Sr0.1CoO3 (solution-polymerization), detects oxygen and carbon dioxide at lower operation temperature. **The stability and repeatability observed in the detection of CO2 was notably enhanced when alternating current was used. However, the stability in the detection of O2 needs improvements in the gas sensor device. The response and recovery times as a function of operation temperature are important parameters and are under investigation.** 

According to recent publications, the decrease of particle size, at the nanometer scale, causes that charge carriers are transported through the entire volume of the crystallites. This is relevant in solid state gas sensor materials because small resistance changes can be detected using standard instrumentation.

## **5. Acknowledgment**

Financial support from the Coordinación General Académica of the Universidad de Guadalajara and COECYTJAL (Grant PS-2008-847) is greatly acknowledged. The authors are grateful to CONACYT for two doctorate scholarships (N.L.L.C. and E.R.L.M.).

## **6. References**


The solution-polymerization process is a simple and cost effective method for preparing nanostructured Gd0.9Sr0.1CoO3. Due to PVA and sucrose are inexpensive materials, this method can be used to obtain other nanostructured compounds; which can be used in fields like gas sensors, gas separation membranes, electrodes for solid oxide fuel cells and catalysis, to mention a few. In this work, single phase Gd0.9Sr0.1CoO3 prepared by solutionpolymerization was obtained from 600oC. However, by the solution method, it was obtained at 800oC. Extensive porosity caused by the decomposition of PVA and sucrose, as well as lower calcination temperatures, produced a nanostructured network of Gd0.9Sr0.1CoO3. The

Through the analysis of the gas sensing results it was possible to conclude that Gd0.9Sr0.1CoO3 (solution-polymerization), detects oxygen and carbon dioxide at lower operation temperature. **The stability and repeatability observed in the detection of CO2 was notably enhanced when alternating current was used. However, the stability in the detection of O2 needs improvements in the gas sensor device. The response and recovery times as a function of operation temperature are important parameters and are under** 

According to recent publications, the decrease of particle size, at the nanometer scale, causes that charge carriers are transported through the entire volume of the crystallites. This is relevant in solid state gas sensor materials because small resistance changes can be detected

Financial support from the Coordinación General Académica of the Universidad de Guadalajara and COECYTJAL (Grant PS-2008-847) is greatly acknowledged. The authors are

Baraton, M.I. & Merhari L. (2003). Electrical Behavior of Semiconducting Nanopowders

Bochenkov, V.E. & Sergeev G.B. (2005). Preparation and Chemiresistive Properties of Nanostructured Materials, *Adv. Colloid. Interface Sci.* Vol. 116, pp. 245-254. Brosha, E.L.; Mukindan, R.; Brown, D.R.; Garzon, F.H.; Visser, J.H.; Zanini, M.; Zhou, Z. &

Dutta, A.; Ishihara, T.; Nishiguchi, H. & Takita Y. (2004). Amperometric Solid-State Gas

Hydrocarbon in Exhaust Gas, *J. Electrochem. Soc.* Vol. 151, pp. H122-H127. Halliday, D.; Resnick, R. & Walker, J. (2001). *Fundamentals of Physics*, Wiley, New York, USA.

La0.8Sr0.2CoO3-δ Metal Oxides, *Sens. Actuators B*, Vol. 69, pp. 171-182. Das, R.N. (2001). Nanocrystalline Ceramics from Sucrose Process, *Mater. Lett.* Vol. 47, pp.

Logothetis, E.M. (2000). CO/HC Sensors Based on Thin Films of LaCoO3 and

Sensor using LaGaO3 Based Perovskite Oxide Electrolyte for Detecting

grateful to CONACYT for two doctorate scholarships (N.L.L.C. and E.R.L.M.).

versus Environment, *Rev. Adv. Mater. Sci.* Vol. 4, pp. 15-24.

latter was observed by transmission electron microscopy.

**4. Conclusions** 

**investigation.** 

using standard instrumentation.

**5. Acknowledgment** 

344-350.

**6. References** 


**9** 

 *Iran* 

**Survey of the Application** 

Mahboubeh Masrournia and Zahra Ahmadabadi

**Nanoscale Material in Chemical Sensors** 

*Department of Chemistry, Faculty of Science, Mashhad Branch, Islamic Azad University,* 

Sensors and sensor arrays for the detection of chemical and biological substances have attracted much attention in recent years. The ultimate goal is to fabricate sensors that can determine the presence of a wide range of substances at relevant concentration levels with sufficient selectivity and sensitivity. Such research would ultimately produce technology that could be applicable in many segments including food processing, environment alremediation, agriculture, medical diagnostics and defense. The main requirements besides selectivity and sensitivity are fast response, low fabrication costs, robustness and portability. Hence intensive research activities around the world are focused on developing new sensing materials and technologies (Fam et al., 2011, Sinh et al., 2006). With the development of nanotechnology, there is a growing demand for advanced electronics based on functional nanomaterials. To date, various nanomaterials have been investigated from both fundamental and practical perspectives, and their unique chemical and physical characteristics have been continuously discovered. Important characteristics and the quality parameters of nanosensors can be improved over the case of classically modeled sensors

Since discovery, carbon nanotubes (CNTs) (in 1991) have been extensively explored for numerous applications. Of all the nanomaterials reported, carbon-based nanomaterials like fullerenes, graphene and carbon nanotubes (CNTs) show a huge potential in bringing sensor technology to the next level. Of these carbonaceous materials, CNT stand out as the most promising material for deployment in electronic sensing platforms due to its superior chemical and electronic properties. Furthermore, CNT possess a great potential for being employed both as a part of the transducer element as well as a functional receptor element in an electronic device. Despite the potential associated with CNT-based sensors, the efforts devoted to commercialization are still being limited by the challenges involved in CNT synthesis and device fabrication. Over the years, several attempts have been made to address these issues, for example, controlled synthesis of CNT. Although the scientific breakthroughs are numerous in this area, much of the CNT sensor research still remains at a

**1. Introduction** 

merely reduced in size.

**2. Nanostructure sensors 2.1 Carbon nanotube sensors** 

