3. Sensor for CO2 detection with Y2O3-doped CeO2 sensitive material

The ion conductivity of CeO2 can be significantly improved upon substitution with some trivalent oxides of lanthanides like Y2O3, Sm2O3 and Gd2O3, because the number of oxygen vacancy will be considerably increased for charge compensation. The electrical conductivity in doped ceria is influenced by factors such as: the dopant ion, the dopant concentration, the oxygen vacancy concentration and the defect association enthalpy. An example is constituted by combination Y2O3-doped CeO2 which has been used usually as the solid electrolyte for moderate temperature solid oxide fuel cells [40]. In our case, we used the Y2O3-doped CeO2 as sensitive material for CO2 detection. For Y2O3-CeO2 synthesis, it utilizes several methods such as hydrothermal [41], electrospinning [23], thermolysis [42] and sol gel [43].

#### 3.1. Synthesis method

Sol gel method applied for synthesis of Y2O3-doped CeO2 sensitive material, is in accord with ref. [44] and used as starting reagents Ce(SO4)2 � 4H2O (97% purity, Merck) and Y (NO3)3 � 3H2O (98% purity Karlsrushe GmbH in molar ratio CeO2/Y2O3 = 4:1). The salts were dissolved in deionized water. To 100 ml salt solution, 25 ml solution of 1 M citric acid as chelating agent was added. To obtain gel, the salt solution was heated to 70�C under constant stirring. To this solution, 40 ml ethylene glycol was added to promote citrate polymerization and heated at 90�C. The gel formed was filtered, washed and heat treated in oven at 100�C. The powder obtained was calcined at 800�C for 2 hours. The powder was pressed to disc form using 10 ton force/cm<sup>2</sup> , with dimensions diameter 4 mm, height 1 mm and then sinterized at 1100�C for 6 hours [44].

Figure 9. CO2 sensor, component parts: 1. Ce2O2-Y2O3 disc; 2. Gold electrode, thin film deposition of Au, in the form of the disc; 3. Ag micro-wire connections; 4. The positioning piece; 5. TO-8 package plated base; 6. The pins; 7. A, B terminals of 18 turns heating resistance.

releases two endothermic peaks at 159.2 and 225.1C. The last one has a correspondent in DTG curve at 219C, the total mass loss was 86.95% from initial mass. Other thermal transformation appears at 430C which represents on in a TG curve a loss of 4.91% which correspond to the decomposition of precursors, consisting in cerium sulfate and yttrium nitrate and in the end

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The X-ray diffraction patterns of the CeO2-Y2O3 oxides powder calcinations at 800C for 2 hours are shown in Figure 11. For comparison, Figure 12 shows the X-ray diffraction for commercial CeO2 powder. For this oxides system, the XRD pattern reveals the formation of well crystallized phases, CeO2 indexed with the cubic fluorite structure and Y2O3 with cubic structure. Also, a secondary phase with cubic structure and composition Ce0.6Y0.4O1.8 was

Table 2 presents X-ray parameters for Y2O3-doped CeO2, cell parameters and crystallite sizes

The morphological structure of Y2O3-doped CeO2 was investigate by SEM measurements using FESEM-FIB type Auriger model Carl Zeiss SMT GmbH at a high voltage acceleration of 2 and 3 kV. The SEM sample morphology was investigated trough SESI (combined detector in SEM chamber–Evernhart Thornley type with Faraday cup). Figure 13 shows the SEM image for disc CeO2-Y2O3 sintered, where it can be seen as a relative homogeneous structure and the crystallite sizes of CeO2 and Y2O3 were in range of 26–54 nm in good accord with X-ray diffraction analysis. Figure 14 shows the SEM images for CeO2-Y2O3 powder calcined at

only Ce-Y-related oxides are obtained [44].

Figure 10. Thermal analysis TG, DTA and DTG curves for dried gel of CeO2-Y2O3.

determined with Scherrer formula.

identified [44].

## 3.2. The construction of the sensor for CO2 detection designed with Y2O3-doped CeO2 sensitive material

On both sides of disc, gold electrodes in circular form with diameter of 1 mm was deposed. The gold was deposed by e-beam evaporation method using Baltzer equipment with conditions: pressure P = 10<sup>5</sup> Torr and current I = 8 mA, for 60 s time deposition. The disc with electrodes deposed was mounted on a 12 pin TO-8 package base. Below the base, the heater element composed of Ni wire with a diameter of 0.1 mm was placed, the winding is composed from 18 turns with a diameter of d = 3 mm. Figure 9 shows how it built the CO2 sensor [44].

### 3.3. Structural and morphological characterization of sensitive material Y2O3-doped CeO2

Thermal analysis was performed with NETZSCH STA 409 simultaneous thermogravimetric balance, in the analysis conditions: inert atmosphere of argon, heating rate of 10C/min in alumina crucible and the mass sample was 15.7 mg. Figure 10 presents the thermal analysis TG, DTA and differential thermogravimetry (DTG) curves for the dried gel. The DTA curve Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems http://dx.doi.org/10.5772/intechopen.80801 71

Figure 10. Thermal analysis TG, DTA and DTG curves for dried gel of CeO2-Y2O3.

3.2. The construction of the sensor for CO2 detection designed with Y2O3-doped CeO2

On both sides of disc, gold electrodes in circular form with diameter of 1 mm was deposed. The gold was deposed by e-beam evaporation method using Baltzer equipment with conditions: pressure P = 10<sup>5</sup> Torr and current I = 8 mA, for 60 s time deposition. The disc with electrodes deposed was mounted on a 12 pin TO-8 package base. Below the base, the heater element composed of Ni wire with a diameter of 0.1 mm was placed, the winding is composed from 18 turns with a diameter of d = 3 mm. Figure 9 shows how it built the CO2 sensor [44].

Figure 9. CO2 sensor, component parts: 1. Ce2O2-Y2O3 disc; 2. Gold electrode, thin film deposition of Au, in the form of the disc; 3. Ag micro-wire connections; 4. The positioning piece; 5. TO-8 package plated base; 6. The pins; 7. A, B terminals

3.3. Structural and morphological characterization of sensitive material Y2O3-doped CeO2

Thermal analysis was performed with NETZSCH STA 409 simultaneous thermogravimetric balance, in the analysis conditions: inert atmosphere of argon, heating rate of 10C/min in alumina crucible and the mass sample was 15.7 mg. Figure 10 presents the thermal analysis TG, DTA and differential thermogravimetry (DTG) curves for the dried gel. The DTA curve

sensitive material

of 18 turns heating resistance.

70 Cerium Oxide - Applications and Attributes

releases two endothermic peaks at 159.2 and 225.1C. The last one has a correspondent in DTG curve at 219C, the total mass loss was 86.95% from initial mass. Other thermal transformation appears at 430C which represents on in a TG curve a loss of 4.91% which correspond to the decomposition of precursors, consisting in cerium sulfate and yttrium nitrate and in the end only Ce-Y-related oxides are obtained [44].

The X-ray diffraction patterns of the CeO2-Y2O3 oxides powder calcinations at 800C for 2 hours are shown in Figure 11. For comparison, Figure 12 shows the X-ray diffraction for commercial CeO2 powder. For this oxides system, the XRD pattern reveals the formation of well crystallized phases, CeO2 indexed with the cubic fluorite structure and Y2O3 with cubic structure. Also, a secondary phase with cubic structure and composition Ce0.6Y0.4O1.8 was identified [44].

Table 2 presents X-ray parameters for Y2O3-doped CeO2, cell parameters and crystallite sizes determined with Scherrer formula.

The morphological structure of Y2O3-doped CeO2 was investigate by SEM measurements using FESEM-FIB type Auriger model Carl Zeiss SMT GmbH at a high voltage acceleration of 2 and 3 kV. The SEM sample morphology was investigated trough SESI (combined detector in SEM chamber–Evernhart Thornley type with Faraday cup). Figure 13 shows the SEM image for disc CeO2-Y2O3 sintered, where it can be seen as a relative homogeneous structure and the crystallite sizes of CeO2 and Y2O3 were in range of 26–54 nm in good accord with X-ray diffraction analysis. Figure 14 shows the SEM images for CeO2-Y2O3 powder calcined at

Figure 11. X-ray diffraction of Y2O3-doped CeO2 synthesized by sol gel method.

Figure 12. X-ray diffraction for commercial CeO2.

800C for 2 hours, where it can be see a nonhomogeneous structure composed by agglomerates [44].

3.4. The CO2 gas sensing mechanism and gas sensors testing

Figure 13. SEM images for sintering disc CeO2-Y2O3.

The improved sensing response at CO2 can be attributed to synergistic effects between Y2O3 doped CeO2. In certain conditions such as high temperature, reduced state or pure CeO2, lose

When CO2 comes in contact with CeO2-activated surface, this forms carbonates as a product

2CeO<sup>2</sup> ! Ce2O<sup>3</sup> þ O�: (7)

indexes hkl

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems

28.536 111 48.1

29.121 222 27.0

28.639 111 49.2

Crystallites-size (nm)

73

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some amount of oxygen and generate oxygen vacancies in accord with Eq. (7),

Phase Crystal structure Unit cell parameter (Å) a = b = c 2Θ Crystalline face

CeO2 Cubic 5.41325 5.41100

Y2O3 Cubic 10.61131 10.61060

Y0.4Ce0.6O1.8 Cubic 5.39449 5.39300

Table 2. X-ray parameters for Y2O3-doped CeO2.

Experimental Theoretic card no.:

CeO2 Merck Cubic 5.384 111 154.9

PDF 01-071-4199

PDF 01-076-8044

PDF 01-075-0177

through the participation of surface oxide ions in accordance with Eq. (8),

N2 adsorption desorption isotherms were performed with the AUTOSORB-1, Quantachrome Instruments, United Kingdom in the following conditions: working gas N2, measured temperature: 196C and relative pressure range P/Po = 0.001–0.99. For binary oxides CeO2-Y2O3, powder calcined at 800C for 2 hours, BET analysis revealed the results: the specific surface area was 3.13 m2 /g, the total volume of the pores was 1.066x10<sup>3</sup> cm3 /g and pore sizes of 8.93 Å. There is a specific ratio P/Po = 0.02898 for the pores with diameters smaller than 6.9 Å [44].

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems http://dx.doi.org/10.5772/intechopen.80801 73


Table 2. X-ray parameters for Y2O3-doped CeO2.

Figure 13. SEM images for sintering disc CeO2-Y2O3.

800C for 2 hours, where it can be see a nonhomogeneous structure composed by agglomer-

N2 adsorption desorption isotherms were performed with the AUTOSORB-1, Quantachrome Instruments, United Kingdom in the following conditions: working gas N2, measured temperature: 196C and relative pressure range P/Po = 0.001–0.99. For binary oxides CeO2-Y2O3, powder calcined at 800C for 2 hours, BET analysis revealed the results: the specific surface area

/g and pore sizes of 8.93 Å. There

/g, the total volume of the pores was 1.066x10<sup>3</sup> cm3

Figure 11. X-ray diffraction of Y2O3-doped CeO2 synthesized by sol gel method.

is a specific ratio P/Po = 0.02898 for the pores with diameters smaller than 6.9 Å [44].

ates [44].

Figure 12. X-ray diffraction for commercial CeO2.

72 Cerium Oxide - Applications and Attributes

was 3.13 m2

#### 3.4. The CO2 gas sensing mechanism and gas sensors testing

The improved sensing response at CO2 can be attributed to synergistic effects between Y2O3 doped CeO2. In certain conditions such as high temperature, reduced state or pure CeO2, lose some amount of oxygen and generate oxygen vacancies in accord with Eq. (7),

$$\text{C}\text{C}\text{e}\text{O}\_2 \rightarrow \text{C}\text{e}\_2\text{O}\_3 + \text{O}^-.\tag{7}$$

When CO2 comes in contact with CeO2-activated surface, this forms carbonates as a product through the participation of surface oxide ions in accordance with Eq. (8),

Figure 14. SEM images for oxidic powder CeO2-Y2O3 calcined at 800�C for 2 hours.

$$\rm{CO}^{2-} + \rm{CO}\_2 \rightarrow \rm{CO}\_3^{2-}.\tag{8}$$

Figure 15. The sensor voltage function with CO2 concentration, for T = 20C and two relative humidity testing 40% RH

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Figure 16. The electronic module for signal conditioning provided by sensing element, designing with ADA4627-1

and 80% RH, respectively.

analog devices.

The carbonates disappear when they are exposed to oxidizing conditions [31]. The sensor characteristic was performed using test installation presented in Figure 4. The sensor was exposed at CO2 atmospheres in the concentration range of 0–5000 ppm CO2 in the climatic conditions: T = 20�C and two relative humidity testing 40% RH and 80% RH, respectively. The sensor functions at 135�C, temperature provided by the heating resistance (Figure 9, Pos. 7). Figure 15 shows the variation of sensor voltage with the gas concentration. The characteristics show a slow linear decreasing of voltage with CO2 concentration which allows an easy signal conditioning. In the concentration range 0–5000 ppm CO2, the sensor presents a voltage variation as follows: 378.17–377.32 mV for T = 20�C, RH 40% and 377.11–376.61 mV for T = 20�C, RH 80%. The sensor data show a little dependence of voltage with relative humidity that makes usable in environment with high relative humidity. The sensitivity of the sensor was 0.3 V/ppm and the response time was less than 30 s [44].
