Gas Sensor and Sensitivity

*Manar Lo Dayekh and Saleem Azara Hussain*

### **Abstract**

Gas sensors help to detect toxic and flammable gases in the atmosphere, and the use of these devices can reduce or prevent severe consequences for people and the environment. Metal oxides are one of the best materials used in the preparation of gas sensors, and they have proven in general that they have resistance to high temperatures Also, they are characterized by optical transparency at visible wavelengths, and they have a wide band gap. Whereas the interactive properties of metal oxides are the applications key chemical sensor. One of the characteristics of sensitivity is sensitivity, which is defined as the rate of change in the resistance of the thin film in the presence and absence of gas. Sensitivity is affected by several factors, including the relative humidity, the temperature of the sensors, the response time of the sensitivity, the time of exposure of the membranes to the gas, the background of the gas composition, and the thickness of the film.The chapter includes an explanation of the sensor parts and measurement sensitivity.

**Keywords:** gas sensing, sensitivity, gas sensitive materials, factors on gas, (Tio2/rGo) nanocomposite, NH3 gas sensor

### **1. Introduction**

According to the scientific and technological developments, researchers has witnessed big dealing in the study of the applications of sensor including gas sensors and for its fundamental applications in the various fields of life like industrial industries, power generation, food, and beverages, medical, and therapeutic as well as agricultural industries. As a result of current industries possesses that increasingly involve the use and manufacture of highly hazardous materials especially toxic gases and flammable create a potential hazard to industrial facilities and their employees even the people who live near them. Moreover, all events that take place in the world represent as reminder for this problem [1].

### **2. Classification of gas-sensitive materials**

According to the base of gas sensing, gas-sensitive materials are divided into two kinds based on electrochemical components and other principles, as shown in **Figure 1** [2].

There are three materials utilized as sensing components: metal oxide semiconductor, conductive polymer composites, and carbon nanomaterial [2].

#### **Figure 1.**

*Classification of gas-sensitive materials [2].*

Metal oxides are among the best materials used in the preparation of gas sensors as shown in **Figure 2** and it proves generally that it resists high temperature. It is optically transparent at visible wavelengths and has a wide band gap [1, 3]. Both thin film and bulk semiconducting metal oxide materials have been widely used for the detection of a wide range of chemicals such as H2, CO, NO2, H2S, ethanol, acetone, and human breath [4].

The reactive properties of metal oxide are the key to chemical sensing applications and when exposed to oxidizing gases including (O2) and gas (NO2) where the common denominator in the reactions of these gases is that they tend to form oxygen ions (O) or (O2 ) that electrically active in order for the oxygen ion to be stabilized, it needs to diffuse into the vacant levels formed as a result of crystal defects within the composition of the substance in the following equations [3].

**Figure 2.** *Metal oxides are used in sensors [1].*

**Figure 3.**

*Components of an electrochemical cell [4].*

$$\bullet \bullet \mathrm{2} + \mathrm{e}^- \to \bullet \mathrm{^{\cdot}\_2} \tag{1}$$

$$\text{O}^-\text{ }\_2 + \text{e}^- \rightarrow 2\text{O}^-\tag{2}$$

Electrochemical cells contain individual electrodes. They are sites of chemical reactions that involve electron transfer with charged species (ions) that are transferred by electric current as it is showing in **Figure 3** where the anode is the electrode where the oxidation reaction takes place (the loss of electrons to the electrical circuit) while the cathode takes the results of the reduction reaction (gaining electrons from electric circuit) where electrical circuits permit the balance of the charge by electron transfer. It also provides the necessary electrical voltage and where the increase and decrease in resistance begin when exposed to one of the gases; therefore, the chemical sensors depend on the change in the resistance. The semiconductors are from the type (n-type) give a change in the resistance from the highest value to the lowest value if there is gas while (p-type) the change in resistance is in opposite way (from the lowest value to the highest value) in addition to the impurities here play a vital role in improving the interactive properties of the sensor modifying the reaction pathway sensitization and selectivity and increasing the detection limits of gas. Adding some kind of impurities is a catalyst and activator for these properties in addition to particle size and surface porosity are all activating factors to improve the sensitivity of gas and it is one of the necessary principles that must be noticed at the beginning of choosing materials for the chemical sensors [4].

Recent studies showed that it could improve the properties of sensors including selectivity and others by using a mixture of oxides of nanomaterials. The properties of the sensor are influenced greatly by granular size and structure porosity of materials. The particle boundary can be increased by decreasing the particle size and that lead to increase in the sensitivity and reduce the operating temperature which saves energy as well as recent researchers have turned to materials with nano-crystalline structure since it increases the improvement of sensors properties like eclectic and response time and this is achieved by providing a massive increase in the surface area [5]. **Figure 4** shows the gas component system.

### **3. Sensing properties**

1.Sensitivity: Sensitivity can be defined as the percentage change in the resistance of the thin film in the existence of gas and lack thereof. Sensitivity is affected by factors including relative humidity, temperature of the sensors, the response time of the sensitivity, the time of exposure of the films to the gas composition,

**Figure 4.** *Gas sensing system [6].*

and the thickness of the film are symbolized by the symbol for (S) and can be shown in the following relation [1, 7, 8].

$$\mathbf{S} = \frac{|\Delta \mathbf{R}|}{\mathbf{R}\_{\circ}} \times \mathbf{100\%} = \frac{\mathbf{R}\_{\text{gas}} - \mathbf{R}\_{\text{air}}}{\mathbf{R}\_{\text{air}}} \times \mathbf{100\%} \tag{3}$$

Where:

S: Sensitivity.

(RgasÞ: Electrical resistance.

(RairÞ: Resistance in dry air.

(RoÞ: Resistance when entering the analytical gas

