4.4. The NO2 gas sensing mechanism

In metal oxide semiconductor gas sensors, the resistance is measured as a function of the gas concentration. Generally, this devices function at elevated temperature between 200 and 600C in air. The grain of metal oxide is covered by adsorbed oxygen molecules. Oxygen molecules present the character of electronegativity, they extract electrons from the conduction band of

Figure 21. The NO2 gas sensing element structure (a) PCB substrate with interdigitated Ag array electrode; (b) the sensing element made with PCB substrate and sensitive material deposed on surface electrode.

metal oxide causing the formation of oxygen ions O� <sup>2</sup> , <sup>O</sup>�, <sup>O</sup><sup>2</sup>�, adsorbed at the surface of metal oxide. Since electrons are removed from the metal oxide, the concentration of free charge carriers is reduced forming a depletion layer at grain boundaries. The surface reactions can be written according with Eqs. (10–12):

$$O\_{2(gas)} + e^- \to O\_2^-(ads) \tag{10}$$

$$\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft} + e^- \to \text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblright}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblright}\text{\textquright}\end{\text{\textquotedblleft}\text{\textquotedblleft}\text{\textquotedblright}}\text{\textquotedbl$$

$$\bullet \bullet^{-} + \text{e}^{-} \rightarrow \bullet^{2-} (\text{ads}) \tag{12}$$

As is it known, nitrogen oxides specify as NOx have the character of oxidizing gases with very high electron affinity 2.28 eV as compared with oxygen 0.43 eV. The NOx molecules interact with the surface of metal oxide through surface adsorbed oxygen ions, thus increasing the potential barrier at grain boundaries. The redox reactions taking place on the surface of a metal oxide can be written according with Eqs. (13–14) [70].

$$\rm NO/NO\_{2(gas)} + e^- \rightarrow NO^-/NO\_2^- \text{(ads)}\tag{13}$$

$$\rm NO/NO\_2(gas) + O\_2^-(ads) \rightarrow NO^-/NO\_2^-(ads) + O\_2(gas) \tag{14}$$

As result, the thickness and resistance of the depletion layer increase and resistance change is reversible at operating temperature [70]. The oxygen vacancies can significantly enhance the adsorption of oxygen molecules and electrons will transfer from the oxygen vacancies from CeO2 to the oxygen molecules, resulting in more oxygen species (especially O2�). These oxygen species will react with NO2, resulting in an abrupt change in the conductivity of the sensor [71]. The graphene sheets by their good properties as: high surface area 2630 m2 /g, thermal conductivity in the range of 3000–5000 W/mK at room temperature carrier mobility up to 200,000 cm<sup>2</sup> /Vs [72], electrical conductivity of 7200 S/m [73], coming from their structure two-dimensional (2D) single atom layer is used in gas sensing and in the composite leads to increase of the electrical conductivity of CeO2 and thus improve the performance to gas sensing room temperature [71].

#### 4.5. The NO2 gas sensors testing and sensing characteristics

4.3. The construction of the sensing element for NO2 gas detection designed with

The sensor module is constituted from printed circuit board (PCB), substrate with interdigitated Ag array electrode deposed by photolitografic technology and the sensitive material in amounts 15–20 mg was deposited on surface electrode. The active area for sensitive material

In metal oxide semiconductor gas sensors, the resistance is measured as a function of the gas concentration. Generally, this devices function at elevated temperature between 200 and 600C in air. The grain of metal oxide is covered by adsorbed oxygen molecules. Oxygen molecules present the character of electronegativity, they extract electrons from the conduction band of

rGO-doped CeO2 and CeO2-doped rGO/ZnO sensitive material

Figure 20. SEM images for: (a) CeO2; (b) CeO2/rGO; (c) CeO2/rGO/ZnO.

was 10 mm 0.5 mm, Figure 21(a) and (b).

4.4. The NO2 gas sensing mechanism

80 Cerium Oxide - Applications and Attributes

The sensors with sensitive materials 1%rGO-doped CeO2, and 1% CeO2/1%rGO-doped ZnO were tested in NO2 atmosphere in concentrations 5 and 10 ppm. The gas testing was effected with testing installation presented in Figure 4. The gas testing was performed in order to establishment of the sensitivity sensors and response time. The sensor sensitivity was expressed in accord with Eq. (15), as the ratio of resistance in air to that in target gas, in this case NO2,

$$S = \frac{Ra}{Rg} \tag{15}$$

Analyzing the obtained results, it can be concluded that the both sensitive materials show the good performance at NO2 exposure at room temperature. However, the sensitive material composed by 1%rGO/CeO2 presents very good sensitivity at NO2 exposure for 5 and 10 ppm concentrations of 2000 and 1818 and very short response time of 2.5and 3.5 s. Thus, sensitive materials with CeO2 in majority concentration in matrix with reduced oxide graphene presents the best performance at NO2 detection, face to sensitive materials 1%CeO2/1%rGO/ZnO where

Resistance in air, [kΩ]

1%rGO/CeO2 5 1060 0.53 2000 2.5 1%CeO2/1%rGO/ZnO 5 885 21.43 41.29 2.8 1%rGO/CeO2 10 2800 1.54 1818 3.5 1%CeO2/1%rGO/ZnO 10 3180 9.9 321.2 2.2

Table 5. The characteristics of sensors with sensitive materials 1%rGO/CeO2 and 1%CeO2/1%rGO/ZnO.

Resistance in gas, [kΩ] (after 3600 s exposure)

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Response time, [s]

ZnO is majority and are a promising sensitive materials for NO2 detection.

rGO-doped CeO2 and CeO2-doped rGO/ZnO sensitive material

Figure 23. The sensitivity variation function with time.

NO2, [ppm]

Sensitive material Concentration

4.6. Signal conditioning of the sensing element for NO2 gas detection designed with

Resistance of sensor sensing element ES, R + ΔR, Figure 24 may vary from less than 10 kΩ to several hundred kΩ, depending on the design of the sensor and the physical environment to

where Ra is the resistance of sensor in air and Rg is the resistance of sensor in gas.

The response time is expressed by formula:

$$Ra - 90\% \text{x} (Ra - Rg) \tag{16}$$

Notations are the same with Eq. (15) [28]. Having the resistance values, from the graph, the response time can be determined. Figure 22 shows the resistance variation with time exposure gas and Figure 23 shows the sensitivity (response) for sensing element with time exposure gas for two sensitive material: 1%rGO/CeO2 and 1%CeO2/1%rGO/ZnO. All the characteristics are considered for the 1 hour time exposure. Since the resistance of sensors decreases sharply, for a good view we opted for a semilogarithmic scale representation of resistance and sensors response with exposure time. The decreases of resistance denotes a character of type p semiconductors for both sensitive materials in oxidant gas like NO2, character given by reduced graphene oxide which is a semiconductor type p.

The sensors performances can be resumed in Table 5.

Figure 22. Resistance variation function with time.

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Figure 23. The sensitivity variation function with time.

with testing installation presented in Figure 4. The gas testing was performed in order to establishment of the sensitivity sensors and response time. The sensor sensitivity was expressed in accord with Eq. (15), as the ratio of resistance in air to that in target gas, in this

<sup>S</sup> <sup>¼</sup> Ra

Notations are the same with Eq. (15) [28]. Having the resistance values, from the graph, the response time can be determined. Figure 22 shows the resistance variation with time exposure gas and Figure 23 shows the sensitivity (response) for sensing element with time exposure gas for two sensitive material: 1%rGO/CeO2 and 1%CeO2/1%rGO/ZnO. All the characteristics are considered for the 1 hour time exposure. Since the resistance of sensors decreases sharply, for a good view we opted for a semilogarithmic scale representation of resistance and sensors response with exposure time. The decreases of resistance denotes a character of type p semiconductors for both sensitive materials in oxidant gas like NO2, character given by reduced

where Ra is the resistance of sensor in air and Rg is the resistance of sensor in gas.

The response time is expressed by formula:

graphene oxide which is a semiconductor type p.

Figure 22. Resistance variation function with time.

The sensors performances can be resumed in Table 5.

Rg (15)

Ra � 90%x Ra ð Þ � Rg (16)

case NO2,

82 Cerium Oxide - Applications and Attributes


Table 5. The characteristics of sensors with sensitive materials 1%rGO/CeO2 and 1%CeO2/1%rGO/ZnO.

Analyzing the obtained results, it can be concluded that the both sensitive materials show the good performance at NO2 exposure at room temperature. However, the sensitive material composed by 1%rGO/CeO2 presents very good sensitivity at NO2 exposure for 5 and 10 ppm concentrations of 2000 and 1818 and very short response time of 2.5and 3.5 s. Thus, sensitive materials with CeO2 in majority concentration in matrix with reduced oxide graphene presents the best performance at NO2 detection, face to sensitive materials 1%CeO2/1%rGO/ZnO where ZnO is majority and are a promising sensitive materials for NO2 detection.

### 4.6. Signal conditioning of the sensing element for NO2 gas detection designed with rGO-doped CeO2 and CeO2-doped rGO/ZnO sensitive material

Resistance of sensor sensing element ES, R + ΔR, Figure 24 may vary from less than 10 kΩ to several hundred kΩ, depending on the design of the sensor and the physical environment to

Figure 24. Schematic of the electronic block for signal conditioning generated by the sensing element.

be measured. The sensing element ES of the NO2 gas sensor is disposed in one of the Wheatstone bridge arms and shows the resistance R for a NO2 concentration of zero ppm. The resistances of resistors disposed in all of other branches of the bridge show the same value, namely R. A DC voltage excitation source U1 is connected to one of the bridge diagonals [74].

If the gas concentration of NO2 is zero ppm, the sensing element ES shows the resistance R. The Wheatstone bridge is in this case at equilibrium so that the voltage measured on the other diagonal of the bridge is 0 V. Variation of NO2 gas concentration in the range from zero ppm to 10 ppm causes a voltage variation with ΔU0, which can be measured on the other diagonal of the bridge. The voltage variation up to ΔU0 is given by the relation (17):

$$
\Delta U l\_0 = \frac{U \mathbf{1}}{2} \left[ \frac{\frac{\Delta R}{2}}{R + \frac{\Delta R}{2}} \right] \tag{17}
$$

4.6.2. Bridge-linearization electronic circuit

operational in-amp instrumentation amplifier.

or AD 734 analog multiplier is written [76, 77]:

W ¼ A<sup>0</sup>

Figure 25. Schematic of the electronic block for the U1 excitation voltage source.

Schematic of the electronic linearization block of the signal generated by the Wheatstone bridge, via an operational amplifier, in-amp uses an analog multiplier [75], AD 534 or AD 734, produced by analog devices (Figure 26). The transfer function associated with the AD 534

ð Þ X1 � X2 ð Þ Y1 � Y2

Figure 26. Schematic of the electronic linearization block of the signal generated by the Wheatstone bridge, via an

SF � ð Þ <sup>Z</sup><sup>1</sup> � <sup>Z</sup><sup>2</sup>

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(19)

The operational amplifier that can be used with the best performance is instrumentation type amplifier (in-amp), "resistor programmable" (Figure 24). Considering the transfer function of the electronic amplifier module and taking into account the relation (17), we obtain [74]:

$$\text{LII2} = \frac{\text{LI1}}{\text{2}} \left[ \frac{\frac{\Delta R}{2}}{R + \frac{\Delta R}{2}} \right] \\ A = K \left[ \frac{\frac{\Delta R}{2}}{R + \frac{\Delta R}{2}} \right], \tag{18}$$

where A is the amplification factor, depending on the Rg resistance value and <sup>K</sup> <sup>¼</sup> <sup>U</sup><sup>1</sup> <sup>2</sup> A, is a constant. In-amps such as the AD620 family, the AD623 and AD627, Analog Devices type can be used in single (or dual) supply bridge applications.

#### 4.6.1. Realization of the continuous U1 excitation voltage source

The continuous U1 excitation voltage source is made using a D/A digital/analog converter, a Uref reference voltage and an operational amplifier (OA) (Figure 25). Thus, depending on the values set for the least significant bit (LSB) up to the most significant bit (MSB), the resulting word can establish a desired U1 continuous excitation voltage. Figure 25 shows the schematic of the electronic block for the U1 excitation voltage source.

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Figure 25. Schematic of the electronic block for the U1 excitation voltage source.

#### 4.6.2. Bridge-linearization electronic circuit

be measured. The sensing element ES of the NO2 gas sensor is disposed in one of the Wheatstone bridge arms and shows the resistance R for a NO2 concentration of zero ppm. The resistances of resistors disposed in all of other branches of the bridge show the same value, namely R. A DC voltage excitation source U1 is connected to one of the bridge diagonals [74]. If the gas concentration of NO2 is zero ppm, the sensing element ES shows the resistance R. The Wheatstone bridge is in this case at equilibrium so that the voltage measured on the other diagonal of the bridge is 0 V. Variation of NO2 gas concentration in the range from zero ppm to 10 ppm causes a voltage variation with ΔU0, which can be measured on the other diagonal of

Figure 24. Schematic of the electronic block for signal conditioning generated by the sensing element.

84 Cerium Oxide - Applications and Attributes

the bridge. The voltage variation up to ΔU0 is given by the relation (17):

<sup>U</sup><sup>2</sup> <sup>¼</sup> <sup>U</sup><sup>1</sup> 2

be used in single (or dual) supply bridge applications.

4.6.1. Realization of the continuous U1 excitation voltage source

of the electronic block for the U1 excitation voltage source.

<sup>Δ</sup>U<sup>0</sup> <sup>¼</sup> <sup>U</sup><sup>1</sup> 2

> ΔR 2 <sup>R</sup> <sup>þ</sup> <sup>Δ</sup><sup>R</sup> 2

where A is the amplification factor, depending on the Rg resistance value and <sup>K</sup> <sup>¼</sup> <sup>U</sup><sup>1</sup>

constant. In-amps such as the AD620 family, the AD623 and AD627, Analog Devices type can

The continuous U1 excitation voltage source is made using a D/A digital/analog converter, a Uref reference voltage and an operational amplifier (OA) (Figure 25). Thus, depending on the values set for the least significant bit (LSB) up to the most significant bit (MSB), the resulting word can establish a desired U1 continuous excitation voltage. Figure 25 shows the schematic

" #

The operational amplifier that can be used with the best performance is instrumentation type amplifier (in-amp), "resistor programmable" (Figure 24). Considering the transfer function of the electronic amplifier module and taking into account the relation (17), we obtain [74]:

ΔR 2 <sup>R</sup> <sup>þ</sup> <sup>Δ</sup><sup>R</sup> 2

A ¼ K

ΔR 2 <sup>R</sup> <sup>þ</sup> <sup>Δ</sup><sup>R</sup> 2

" #

(17)

<sup>2</sup> A, is a

, (18)

" #

Schematic of the electronic linearization block of the signal generated by the Wheatstone bridge, via an operational amplifier, in-amp uses an analog multiplier [75], AD 534 or AD 734, produced by analog devices (Figure 26). The transfer function associated with the AD 534 or AD 734 analog multiplier is written [76, 77]:

$$W = A\_0 \left\{ \frac{(X1 - X2)(Y1 - Y2)}{SF} - (Z1 - Z2) \right\} \tag{19}$$

Figure 26. Schematic of the electronic linearization block of the signal generated by the Wheatstone bridge, via an operational in-amp instrumentation amplifier.

where A0 is the open loop gain, X1, X2, Y1, Y2, Z1 and Z2 represent the inputs of the analog multiplier, SF a scale factor, typically SF = 10 V and W = OUT, according to Figure 26.

Since A0 ! 72 dB can be considered as W/A0 ! 0 and the relation (19) becomes:

$$\mathbf{F}(X1-X2)(Y1-Y2) = \mathbf{SF}(Z1-Z2) \tag{20}$$

Since Z1 = W it is obtained:

$$W = \frac{(X1 - X2)(Y1 - Y2)}{SF} + Z2\tag{21}$$

Since Z2 = U2, Y1-Y2 = βU2, 0 ≤ β < 1, X2 = 0 and X1 = Z1 = W=U3, according to Figure 26. Finally,

$$\text{UZ} = W = \frac{\text{UZ}}{1 - \frac{\text{f}\Omega \text{L}}{\text{SF}}} \tag{22}$$

the electronic block for signal conditioning generated by the sensing element, single supply

Figure 28. Schematic of the electronic block for signal conditioning generated by the sensing element, dual supply bridge

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It is possible to reconfigure circuits so as to improve the performance in terms of reduces the dc common-mode voltage to zero. Figure 28 shows how the use of split U1 tension in order to

An isolation amplifier can be useful for this application, with respect to the signal-conditioning, so that it does not exist galvanic connections between the bridge and grounded instrumenta-

states Ce3+ and Ce4+ allowing a redox reaction between them which gives CeO2 excellent chemical and physical properties, is used in many applications, like as: three-way catalytic reactions to eliminate toxic automobile exhaust, the low-temperature water gas shift reaction, oxygen permeation membrane systems for fuel cells as well as gas sensors. For gas sensing applications, several sensitive elements based on CeO2 were tested to determine both this

• By doping the CeO2 with oxides semiconductor, for example, Nb2O5 introduced in CeO2 structure, the following mechanism is triggered: Nb5+ ions initiate the reduction of Ce4+ to Ce3+ resulting in the formation of oxygen vacancies with consequences in increasing the sensitivity.

• The ionic conductivity of CeO2 is improved by doping with rare earth oxides such as Sm2O3, Gd2O3 and Y2O3. The size of conductivity for doped ceria depends on the ionic

6s<sup>2</sup>

) and by the two common valence

bridge applications.

applications.

tion circuitry.

5. Conclusions

reduce the dc common-mode voltage to zero.

Cerium, by its unique electronic configuration ([Xe] 4f2

detection function as well as this performances:

is obtained

The relation (6) together with the relation (2) represents the calculation method regarding the linearization of the signal generated by the Wheatstone bridge, via an operational in-amp instrumentation amplifier.

### 4.6.3. Resulting structures for the electronic block for signal conditioning generated by the sensing element

By considering the three previously analyzed electronic blocks, the electronic block for signal conditioning generated by the sensing element is obtained. Figure 27 shows the schematic of

Figure 27. Schematic of the electronic block for signal conditioning generated by the sensing element, single supply bridge applications.

Prototyping a Gas Sensors Using CeO2 as a Matrix or Dopant in Oxide Semiconductor Systems http://dx.doi.org/10.5772/intechopen.80801 87

Figure 28. Schematic of the electronic block for signal conditioning generated by the sensing element, dual supply bridge applications.

the electronic block for signal conditioning generated by the sensing element, single supply bridge applications.

It is possible to reconfigure circuits so as to improve the performance in terms of reduces the dc common-mode voltage to zero. Figure 28 shows how the use of split U1 tension in order to reduce the dc common-mode voltage to zero.

An isolation amplifier can be useful for this application, with respect to the signal-conditioning, so that it does not exist galvanic connections between the bridge and grounded instrumentation circuitry.
